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Proposed Rule2023-14199

Lowering Miners' Exposure to Respirable Crystalline Silica and Improving Respiratory Protection

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Published

July 13, 2023

Issuing agencies

Labor DepartmentMine Safety and Health Administration

Abstract

The Mine Safety and Health Administration (MSHA) proposes to amend its existing standards to better protect miners against occupational exposure to respirable crystalline silica, a carcinogenic hazard, and to improve respiratory protection for all airborne hazards. MSHA has preliminarily determined that under the Agency's existing standards, miners at metal and nonmetal mines and coal mines face a risk of material impairment of health or functional capacity from exposure to respirable crystalline silica. MSHA proposes to set the permissible exposure limit of respirable crystalline silica at 50 micrograms per cubic meter of air ([micro]g/m\3\) for a full shift exposure, calculated as an 8-hour time-weighted average, for all miners. MSHA's proposal would also include other requirements to protect miner health, such as exposure sampling, corrective actions to be taken when miner exposure exceeds the permissible exposure limit, and medical surveillance for metal and nonmetal miners. Furthermore, the proposal would replace existing requirements for respiratory protection and incorporate by reference ASTM F3387-19 Standard Practice for Respiratory Protection. The proposed uniform approach to respirable crystalline silica occupational exposure and improved respiratory protection for all airborne hazards would significantly improve health protections for all miners and lower the risk of material impairment of health or functional capacity.

Full Text

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[Federal Register Volume 88, Number 133 (Thursday, July 13, 2023)]
[Proposed Rules]
[Pages 44852-45019]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2023-14199]

[[Page 44851]]

Vol. 88

Thursday,

No. 133

July 13, 2023

Part II

Department of Labor

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Mine Safety and Health Administration

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30 CFR Parts 56, 57, 60, et al.

Lowering Miners' Exposure to Respirable Crystalline Silica and
Improving Respiratory Protection; Proposed Rule

Federal Register / Vol. 88 , No. 133 / Thursday, July 13, 2023 /
Proposed Rules

[[Page 44852]]

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DEPARTMENT OF LABOR

Mine Safety and Health Administration

30 CFR Parts 56, 57, 60, 70, 71, 72, 75, and 90

[Docket No. MSHA-2023-0001]
RIN 1219-AB36

Lowering Miners' Exposure to Respirable Crystalline Silica and
Improving Respiratory Protection

AGENCY: Mine Safety and Health Administration (MSHA), Department of
Labor.

ACTION: Proposed rule; request for comments; notice of public hearings.

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SUMMARY: The Mine Safety and Health Administration (MSHA) proposes to
amend its existing standards to better protect miners against
occupational exposure to respirable crystalline silica, a carcinogenic
hazard, and to improve respiratory protection for all airborne hazards.
MSHA has preliminarily determined that under the Agency's existing
standards, miners at metal and nonmetal mines and coal mines face a
risk of material impairment of health or functional capacity from
exposure to respirable crystalline silica. MSHA proposes to set the
permissible exposure limit of respirable crystalline silica at 50
micrograms per cubic meter of air ([micro]g/m\3\) for a full shift
exposure, calculated as an 8-hour time-weighted average, for all
miners. MSHA's proposal would also include other requirements to
protect miner health, such as exposure sampling, corrective actions to
be taken when miner exposure exceeds the permissible exposure limit,
and medical surveillance for metal and nonmetal miners. Furthermore,
the proposal would replace existing requirements for respiratory
protection and incorporate by reference ASTM F3387-19 Standard Practice
for Respiratory Protection. The proposed uniform approach to respirable
crystalline silica occupational exposure and improved respiratory
protection for all airborne hazards would significantly improve health
protections for all miners and lower the risk of material impairment of
health or functional capacity.

DATES: Written comments. Written comments, including comments on the
information collection requirements described in this preamble, must be
received or postmarked by midnight Eastern Time on August 28, 2023.
Public Hearings. MSHA will hold two public hearings on August 3,
2023 in Arlington, Virginia and August 21, 2023 in Denver, Colorado.
For more information on the public hearings, see SUPPLEMENTARY
INFORMATION.

ADDRESSES: All submissions must include RIN 1219-AB36 or Docket No.
MSHA-2023-0001. You should not include personal or proprietary
information that you do not wish to disclose publicly. If you mark
parts of a comment as ``business confidential'' information, MSHA will
not post those parts of the comment. Otherwise, MSHA will post all
comments without change, including any personal information provided.
MSHA cautions against submitting personal information.
You may submit comments and informational materials, clearly
identified by RIN 1219-AB36 or Docket Id. No. MSHA-2023-0001, by any of
the following methods:
Federal E-Rulemaking Portal: <a href="https://www.regulations.gov">https://www.regulations.gov</a>. Follow
the online instructions for submitting comments.
Email: <a href="/cdn-cgi/l/email-protection#b9c3c3f4eaf1f894dad6d4d4dcd7cdcaf9ddd6d597ded6cf"><span class="__cf_email__" data-cfemail="85ffffc8d6cdc4a8e6eae8e8e0ebf1f6c5e1eae9abe2eaf3">[email&#160;protected]</span></a>. Include ``RIN 1219-AB36'' in the
subject line of the message.
Regular Mail: MSHA, Office of Standards, Regulations, and
Variances, 201 12th Street South, Suite 4E401, Arlington, Virginia
22202-5450.
Hand Delivery or Courier: MSHA, Office of Standards, Regulations,
and Variances, 201 12th Street South, Suite 4E401, Arlington, Virginia,
between 9:00 a.m. and 5:00 p.m. Monday through Friday, except Federal
holidays. Before visiting MSHA in person, call 202-693-9440 to make an
appointment. Special health precautions may be required.
Facsimile: 202-693-9441. Include ``RIN 1219-AB36'' in the subject
line of the message.
Information Collection Requirements. Comments concerning the
information collection requirements of this proposed rule must be
clearly identified with ``RIN 1219-AB36'' or ``Docket No. MSHA-2023-
0001,'' and sent to MSHA by one of the methods previously explained.
Docket. For access to the docket to read comments and background
documents, go to <a href="https://www.regulations.gov">https://www.regulations.gov</a>. The docket can also be
reviewed in person at MSHA, Office of Standards, Regulations, and
Variances, 201 12th Street South, Arlington, Virginia, between 9 a.m.
and 5 p.m. Monday through Friday, except Federal holidays. Before
visiting MSHA in person, call 202-693-9440 to make an appointment.
Special health precautions may be required.
Email Notification. To subscribe to receive an email notification
when MSHA publishes rulemaking documents in the Federal Register, go to
<a href="https://public.govdelivery.com/accounts/USDOL/subscriber/new">https://public.govdelivery.com/accounts/USDOL/subscriber/new</a>.

FOR FURTHER INFORMATION CONTACT: S. Aromie Noe, Director, Office of
Standards, Regulations, and Variances, MSHA, at:
<a href="/cdn-cgi/l/email-protection#f6859f9a9f959787839385829f999885b692999ad8919980"><span class="__cf_email__" data-cfemail="186b7174717b79696d7d6b6c7177766b587c7774367f776e">[email&#160;protected]</span></a> (email); 202-693-9440 (voice); or 202-693-9441
(facsimile). These are not toll-free numbers.

SUPPLEMENTARY INFORMATION:
MSHA will hold two public hearings to provide industry, labor, and
other interested parties with an opportunity to present oral
statements, written comments, and other information on the proposed
rule. The public hearings will begin at 9 a.m. local time and end after
the last presenter speaks on the following dates:

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Date Location Contact number
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August 3, 2023.............. Mine Safety and Health 202-693-9440
Administration, 201 12th
Street South, Room 7W202,
Arlington, VA 22202.
August 21, 2023............. Denver Federal Center, 202-693-9440
Building 25 Lecture Hall,
West 6th Avenue and
Kipling Street, Denver,
CO 80225.
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The public hearings will begin with an opening statement from MSHA,
followed by an opportunity for members of the public to make oral
presentations. Speakers and other attendees may present information to
MSHA for inclusion in the rulemaking record. The hearings will be
conducted in an informal manner. Formal rules of evidence or cross
examination will not apply.
A verbatim transcript of each of the proceedings will be prepared
and made a part of the rulemaking record. Copies of the transcripts
will be available to the public. MSHA will make the transcript of the
hearings available at <a href="http://www.regulations.gov">http://www.regulations.gov</a> and on MSHA's website
at <a href="https://arlweb.msha.gov/currentcomments.asp">https://arlweb.msha.gov/currentcomments.asp</a>.
MSHA will accept post-hearing written comments and other
appropriate information for the record from any interested party,
including those not presenting oral statements, received by

[[Page 44853]]

midnight (Eastern Time) on August 28, 2023.
Pre-registration is not required to attend the hearings. Interested
parties may attend the hearings virtually or in person. Interested
parties who intend to present testimony at the hearings are asked to
register in advance on MSHA's website (<a href="http://www.msha.gov">http://www.msha.gov</a>). Speakers
will be called in the order in which they signed up. Those who do not
register in advance will have an opportunity to speak after all those
who pre-registered have spoken. You may submit hearing testimony and
documentary evidence, identified by docket number (MSHA-2023-0001), by
any of the methods previously identified. Additional information on how
to access the public hearings will be posted when available at <a href="https://www.msha.gov/regulations/rulemaking">https://www.msha.gov/regulations/rulemaking</a>.
The preamble to the proposed standard follows this outline:

I. Introduction
II. Request for Comments
III. Background
IV. Existing Standards and Implementation
V. Health Effects Summary
VI. Preliminary Risk Analysis Summary
VII. Section-by-Section Analysis
VIII. Technological Feasibility
IX. Summary of Preliminary Regulatory Impact Analysis and Regulatory
Alternatives
X. Initial Regulatory Flexibility Analysis
XI. Paperwork Reduction Act
XII. Other Regulatory Considerations
XIII. References Cited in the Preamble
XIV. Appendix

Acronyms and Abbreviations

COPD chronic obstructive pulmonary disease
ESRD end-stage renal disease
FEV forced expiratory volume
FVC forced vital capacity
L/min liter per minute
mg milligram
mg/m\3\ milligrams per cubic meter
mL milliliter
[micro]g/m\3\ micrograms per cubic meter
MNM metal and nonmetal
NMRD nonmalignant respiratory disease
PEL permissible exposure limit
PMF progressive massive fibrosis
RCMD respirable coal mine dust
REL recommended exposure limit
SiO<INF>2</INF> silica
TB tuberculosis
TLV[supreg] Threshold Limit Value
TWA time-weighted average

I. Introduction

With the passage of the Federal Mine Safety and Health Act of 1977
(Mine Act), Congress declared that ``the first priority and concern of
all in the coal or other mining industry must be the health and safety
of its most precious resource--the miner[.]'' 30 U.S.C. 801(a). In
furtherance of that clear guiding principle, this proposed rule
promotes MSHA's mission and statutory mandate to prevent death,
illness, and injury from mining and promote safe and healthful
workplaces for U.S. miners. This proposal provides the public with the
opportunity to comment on the Agency's proposed uniform and streamlined
regulatory approach to lowering miners' exposure to respirable
crystalline silica and improving respiratory protection.
Exposure to silica dust causes adverse health effects, including
silicosis (acute silicosis, accelerated silicosis, simple chronic
silicosis, and progressive massive fibrosis (PMF)), nonmalignant
respiratory diseases (NMRD) (e.g., emphysema and chronic bronchitis),
lung cancer, and renal diseases. Each of these effects is chronic,
irreversible, and potentially disabling or fatal. Silica dust is
generated in most mining activities, including cutting, sanding,
drilling, crushing, grinding, sawing, scraping, jackhammering,
excavating, and hauling materials that contain silica, and is found in
all mines--underground and surface metal and nonmetal (MNM) and coal
mines. In a mining context, silica exposures may occur in respirable
dust together with exposures to other airborne contaminants and
combustion biproducts.
MSHA's existing standards, established in the early 1970s, help
protect miners from the most dangerous levels of exposure to respirable
crystalline silica. However, since their promulgation, scientific
understanding of respirable crystalline silica toxicity has advanced,
and the National Institute for Occupational Safety and Health (NIOSH)
has recommended a respirable crystalline silica exposure level of 50
[micro]g/m\3\ for workers. In 2016, the Occupational Safety and Health
Administration (OSHA) established a permissible exposure limit (PEL) of
50 [micro]g/m\3\ in many industry sectors that it regulates.
To provide miners with exposure limits consistent with workers in
other industries and NIOSH's recommendation, and to improve miners'
health, MSHA proposes to lower its existing exposure limits to 50
[micro]g/m\3\ for respirable crystalline silica in MNM and coal mines.
MSHA considered exposure limits below 50 [micro]g/m\3\. However, MSHA
believes, based on a review of the Agency's available silica sample
data, that an exposure limit of 25 [micro]g/m\3\ may not be achievable
for all mines. The proposed PEL would be expressed as a full-shift
exposure, calculated as an 8-hour time-weighted average (TWA).
Importantly, a uniform proposed PEL for all mines would make compliance
simpler--especially for coal mines by eliminating the existing
respirable dust standard when quartz is present.
To meet the requirements of the proposed PEL, mine operators would
have to implement engineering controls, followed by administrative
controls if supplementary protection is needed. Engineering controls,
which are most effective, are designed to remove or reduce the hazard
at the source and could include the installation of proper ventilation
systems, use of water sprays or wetting agents to suppress airborne
contaminants, installation of machine-mounted dust collectors to
capture respirable crystalline silica and other contaminants, and the
installation of control booths or environmental cabs to enclose
equipment operators. Administrative controls, which are often less
effective than engineering controls, are designed to change the way
miners work. One example would be ensuring that miners safely clean
dust off their work clothes so that they are not exposed to respirable
dust after their shift ends.
MSHA's proposed rule would further protect all miners by requiring
exposure sampling and corrective actions when miners' exposures exceed
the proposed PEL, as well as periodic sampling when miners' exposure
levels meet or exceed the proposed action level. The proposed rule also
includes medical surveillance requirements for MNM miners (medical
surveillance requirements already exist for coal miners). Proposed
medical examinations would include chest X-rays, spirometry, symptom
assessment, and occupational history and would be provided at no cost
to the miner.
Finally, the proposed rule would incorporate by reference an
updated respiratory protection standard, ASTM F3387-19, ``Standard
Practice for Respiratory Protection'' (ASTM F3387-19), for respirable
crystalline silica and all other regulated airborne contaminants. This
voluntary consensus standard represents up-to-date advancements in
respiratory protection technologies, practices, and techniques,
including proper selection, use, and maintenance of respirators. The
proposed incorporation of ASTM F3387-19 by reference would better
protect all miners from airborne hazards. However, respiratory
protection should only be relied upon as an exposure control measure in
limited situations and on a temporary basis, and to supplement
engineering controls, followed by administrative controls.
Taken together, all elements of the proposed rule are
technologically and economically feasible. MSHA's 2014

[[Page 44854]]

final rule, Lowering Miners' Exposure to Respirable Coal Mine Dust,
Including Continuous Personal Dust Monitors (Coal Dust Rule) improved
health protections for coal miners by lowering exposure limits to
respirable coal mine dust and establishing sampling requirements that
included the use of a Continuous Personal Dust Monitor (79 FR 24813,
May 1, 2014). Coal mine operators have generally achieved compliance
with the respirable dust standards primarily by implementing or
adjusting existing engineering controls. Coal mine operators' sampling
data and MSHA's compliance data show that operators have lowered coal
miners' exposures to respirable coal mine dust and to respirable
crystalline silica. Data show that average exposures in coal mines are
below the proposed PEL of 50 [mu]g/m\3\, and therefore, corrective
measures would often not be needed. Similarly, for MNM miners, MSHA
data also show that most exposures to respirable crystalline silica are
below the proposed PEL. However, at MNM and coal mines where elevated
exposures are found, operators will be able to reduce exposures to the
proposed PEL through some combination of properly maintaining existing
engineering controls, implementing new engineering controls, and
requiring safe work practices. Mines and laboratories will be able to
meet exposure monitoring requirements with existing validated and
widely used sampling and analytical methods. The proposed revision to
the respiratory protection standard is technologically feasible because
MSHA's existing respiratory protection requirements for selecting,
fitting, using, and maintaining respiratory protection include similar
requirements.
MSHA's Preliminary Risk Analysis (PRA) suggests that exposure
consistent with a lower proposed PEL of 50 [micro]g/m\3\ would deliver
many health benefits to miners who currently experience exposures above
the proposed PEL by reducing the likelihood of respirable crystalline
silica-related diseases. For those miners working only under the
proposed PEL, MSHA estimates that the proposed rule would result in a
total of 799 lifetime avoided deaths (63 in coal and 736 in MNM mines)
and 2,809 lifetime avoided morbidity cases (244 in coal and 2,566 in
MNM mines) over a 60-year period. MSHA expects full implementation and
compliance to reduce lifetime mortality risk due specifically to silica
exposures by 9.5 percent and to reduce silicosis morbidity risk by 41.9
percent. The latter statistic is particularly important to coal miners
given surveillance findings noted by the National Academies of
Sciences, Engineering, and Medicine that severe pneumoconiosis where
respirable crystalline silica is likely an important contributor is
presenting in relatively young miners, sometimes in their late 30's and
early 40's.
MSHA's economic analysis estimates that the proposed respirable
crystalline silica rule would cost an average of $56.1 million per year
in 2021 dollars at an undiscounted rate, $57.6 million at a 3 percent
discount rate, and $59.9 million at a 7 percent discount rate. Based on
the results of the Preliminary Regulatory Impact Analysis (PRIA), MSHA
estimates that the proposed rule's benefits would exceed its costs,
with or without discount rates. Monetized benefits are estimated from
avoidance of 410 deaths related to NMRD, silicosis, ESRD, and lung
cancer and 1,420 cases of silicosis associated with silica exposure
over the first 60-year period after the promulgation of the final rule.
The estimated annualized net benefit is approximately $212.8 million at
an undiscounted rate, $118.2 million at a 3 percent discount rate, and
$36.3 million at a 7 percent discount rate.
A rule is significant under Executive Order 12866 Section 3(f)(1),
as amended by E.O. 14094, if it is likely to result in ``an annual
effect on the economy of $200 million or more or . . . adversely affect
in a material way the economy, a sector of the economy, productivity,
competition, jobs, the environment, public health or safely, or State,
local, or tribal governments or communities.'' The Office of Management
and Budget has determined that the proposed rule is significant within
the meaning of E.O. 12866 Section 3(f)(1).
The proposed rule would strengthen MSHA's existing regulatory
framework. It would establish a uniform proposed PEL that provides all
MNM and coal miners with the same exposure limits for respirable
crystalline silica consistent with exposure limits that other U.S.
workers currently receive in non-mining industries. It would update the
existing respiratory protection standard to require mine operators to
provide miners with NIOSH-approved respiratory equipment that has been
fitted, selected, maintained, and used in accordance with recent
consensus standards. The proposed rule would also include requirements
for all MNM operators to provide medical surveillance in the form of a
medical examination regime similar to what coal miners already receive.
Cumulatively, the proposed provisions would lower miners' risk of
developing chronic, irreversible, disabling, and potentially fatal
health conditions, consistent with MSHA's mission and statutory mandate
to prevent occupational diseases and protect U.S. miners from suffering
material health impairments.

II. Request for Comments

MSHA requests comments on the proposed rule and all relevant
issues, including the review and conclusions of the health effects
discussion, preliminary risk analysis, feasibility analysis,
preliminary regulatory impact analysis and regulatory alternatives, and
preliminary regulatory flexibility analysis. While MSHA invites
comments on any aspect of its proposed rule and related documents, the
Agency particularly seeks information and data in response to questions
posed in this section and any other aspect of this proposed rule.
Instructions for submitting and viewing comments are provided under the
DATES heading. MSHA will consider all timely comments and may change
the proposed rule based on such comments.
MSHA requests that commenters organize their comments, to the
extent possible, around the following numbered questions. The Agency is
interested in receiving responses to the listed questions and any
information or data supporting the responses.

Health Effects

1. In the standalone, background document entitled ``Health Effects
of Respirable Crystalline Silica'' and as summarized in Section V.
Health Effects Summary of this preamble, MSHA has made a preliminary
determination that miners' exposure to respirable crystalline silica
presents a risk of material health impairment due to the risk of
developing silicosis, NMRD, lung cancer, and renal disease, based on
its extensive review of the health effects literature. MSHA requests
comments on this preliminary determination and its literature review,
which draws heavily from the review conducted by OSHA for its 2016
rulemaking. Are there additional adverse health effects that should be
included or more recent literature that offers a different perspective?
MSHA requests that commenters submit information, data, or additional
studies or their citations. Please be specific regarding the basis for
any recommendation to include additional adverse health effects.

Preliminary Risk Analysis

2. In the standalone, background document entitled ``Preliminary
Risk Analysis'' and as summarized in Section VI. Preliminary Risk
Analysis Summary

[[Page 44855]]

of this preamble, MSHA relied on risk models that OSHA used in support
of its 2016 respirable crystalline silica final rule. Does the context
of the MSHA rule suggest that the model would benefit from changes? If
so, please describe both the justification for those changes and the
likely impact on the final risk estimates. Are there additional studies
or sources of data that MSHA should consider? What is the rationale for
recommending the use of these additional studies or data?
3. MSHA's risk analysis of lung cancer mortality uses the exposure-
response model from Miller and MacCalman (2010) instead of Steenland et
al. (2001a), on which OSHA's risk assessment of lung cancer mortality
was based. MSHA uses Miller and MacCalman (2010) for several reasons.
First, it covers coal mining-specific cohort large enough (with 45,000
miners) to provide adequate statistical power to detect low levels of
risk, and it covers an extended follow-up period (1959-2006). Second,
the study provided data on cumulative exposure of cohort members and
adjusted for or addressed confounders such as smoking and exposure to
other carcinogens. Finally, it developed quantitative assessments of
exposure-response relationships using appropriate statistical models or
otherwise provided sufficient information that permitted MSHA to do so.
The Agency is requesting comment on MSHA's reliance on the Miller and
MacCalman (2010) study in assessing lung cancer mortality. Please
provide any other studies or information that MSHA should take into
account in determining the risk of lung cancer mortality among miners.

Technological Feasibility of the Proposed Rule

4. As discussed in Section VIII. Technological Feasibility of this
preamble, MSHA has preliminarily determined that it is technologically
feasible for mine operators to conduct air sampling and analysis and to
achieve the proposed PEL using commercially available samplers. MSHA
has also determined that these technologically feasible samplers are
widely available, and a number of commercial laboratories provide the
service of analyzing dust containing respirable crystalline silica. In
addition, MSHA has determined that technologically feasible engineering
controls are readily available, can control crystalline silica-
containing dust particles at the source, provide reliable and
consistent protection to all miners who would otherwise be exposed to
respirable dust, and can be monitored. MSHA has also determined that
administrative controls, used to supplement engineering controls, can
further reduce and maintain exposures at or below the proposed PEL.
Moreover, MSHA has preliminarily determined the proposed respiratory
protection practices for respirator use are technologically feasible
for mine operators to implement. MSHA requests comments on these
preliminary conclusions. What methods have you used that proved
effective in reducing miners' exposure to respirable crystalline silica
in mining operations? Please explain how those methods were effective
in reducing miners' exposures. To what extent do existing controls that
reduce exposure to other airborne hazards (e.g., coal dust, diesel
particulate matter) already reduce exposures to respirable crystalline
silica below the proposed PEL? To what extent does the proposed rule
including the PEL facilitate MSHA's workplace health and safety goals?
Please provide supporting information, such as quantitative data if
available.
5. MSHA has determined that the proposed medical surveillance
requirements for MNM are technologically feasible. MSHA requests
comments on this preliminary conclusion. Please provide supporting
information, such as quantitative data if available.

Preliminary Regulatory Impact Analysis and Regulatory Alternatives

6. In the standalone background document entitled ``Preliminary
Regulatory Impact Analysis'' and as summarized in Section IX. Summary
of Preliminary Regulatory Impact Analysis and Regulatory Alternatives
of this preamble, MSHA developed estimated costs of compliance with the
proposed rule and estimated monetized benefits associated with averted
cases of respirable crystalline silica-related diseases. MSHA requests
comments on the methodologies, baseline, assumptions, and estimates
presented in the Preliminary Regulatory Impact Analysis. Please provide
any data or quantitative information that may be useful in evaluating
the estimated costs and benefits associated with the proposed rule.
7. MSHA considered two regulatory alternatives in developing the
proposed rule discussed in Section IX. Summary of Preliminary
Regulatory Impact Analysis and Regulatory Alternatives. In the
regulatory alternatives presented, MSHA discussed alternatives to the
proposed PEL, action level, sampling requirements, and semi-annual
evaluations. MSHA requests comments on these and other regulatory
alternatives and information on any other alternatives that the Agency
should consider, including different average working-life spans and
different average shift lengths. Please provide supporting information
about how these alternatives could affect miners' protection from
respirable crystalline silica exposure and affect mine operators'
costs.

Initial Regulatory Flexibility Analysis

8. As summarized in Section X. Initial Regulatory Flexibility
Analysis of this preamble, MSHA examined the impact of the proposed
rule on small mines in accordance with the Regulatory Flexibility Act.
MSHA estimated that small-entity controllers would be expected to
incur, on average, additional regulatory costs equaling approximately
0.122 percent of their revenues (or $1,220 for every $1 million in
revenues). MSHA is interested in how the proposed rule would affect
small mines, including their ability to comply with the proposed
requirements. Please provide information and data that supports your
response. If you operate a small mine, please provide any projected
impacts of the proposal on your mine, including the specific rationale
supporting your projections.

Scope and Effective Date

9. MSHA is proposing a unified regulatory and enforcement framework
for controlling miners' exposures to respirable crystalline silica for
the mining industry. MSHA requests comments on this unified regulatory
and enforcement framework. MSHA requests the views and recommendations
of stakeholders regarding the scope of proposed part 60, which would
include all surface and underground MNM and coal mines. MSHA requests
comments on whether separate standards should be developed for the MNM
mining industry and the coal mining industry. Please provide supporting
information.
10. MSHA is proposing that the final rule would be effective 120
days after its publication in the Federal Register. This period is
intended to provide mine operators time to evaluate existing
engineering and administrative controls, update their respiratory
protection programs, and prepare to comply with other provisions of the
rule including recordkeeping requirements. Please provide your views on
the proposed effective date. In your response, please include the
rationale for your position.

[[Page 44856]]

Definitions

11. MSHA requests comments on the proposed action level.
Stakeholders should provide specific information and data in support of
or against a proposed action level. Stakeholders should include a
discussion of how the use of a proposed action level would impact their
mines, including the cost of monitoring respirable crystalline silica
above the proposed action level, and other relevant information. Please
provide supporting information.
12. MSHA requests comments on the proposed definition for
``objective data.'' Is it appropriate to allow mine operators to use
objective data instead of a second baseline sample? Please provide
supporting information.

Proposed Permissible Exposure Limit

13. MSHA is proposing a PEL for respirable crystalline silica of 50
[mu]g/m\3\ for a full-shift exposure, calculated as an 8-hour TWA for
MNM and coal miners. MSHA has made a preliminary determination that the
proposed PEL would reduce miners' risk of suffering material impairment
of health or functional capacity over their working lives. MSHA seeks
the views and recommendations of stakeholders on the proposed PEL. MSHA
solicits comments on the approach of having a standalone PEL and
whether to eliminate the reduced standard for total respirable dust
when quartz is present at coal mines. Please provide evidence to
support your response.
14. MSHA is proposing a PEL of 50 ug/m\3\ and an action level of 25
[mu]g/m\3\ for respirable crystalline silica exposure. Which proposed
requirements should be triggered by exposure at, above, or below the
proposed action level? Please provide supporting information.

Methods of Compliance

15. MSHA requests comments on the proposed prohibition against
rotation of miners as an administrative control. Please include a
discussion of the potential effectiveness of this non-exposure approach
and its impact on miners at specific mines. Please provide supporting
information.
16. MSHA requests comments on the proposed requirement that mine
operators must install, use, and maintain feasible engineering and
administrative controls to keep miners' exposures to respirable
crystalline silica below the proposed PEL. Please provide supporting
information.

Proposed Exposure Monitoring

17. MSHA requests comments and information from stakeholders
concerning the proposed approaches to monitoring exposures, and other
approaches to accurately monitor miner exposure to respirable
crystalline silica in MNM and coal mines. Please provide supporting
information and data.
18. MSHA proposes to require mine operators to collect a respirable
crystalline silica sample for a miner's regular full shift during
typical mining activities. Many potential sources of respirable
crystalline silica are present only when the mine is operating under
typical conditions. MSHA requests comments on this requirement and
whether to specify environmental conditions under which samples should
be taken to ensure that samples accurately reflect actual levels of
respirable crystalline silica exposure. In MSHA's experience, for
example, environmental conditions such as precipitation (e.g., rain or
snow) or wind could affect the actual levels of respirable crystalline
silica exposure at miners' normal or regular workplaces throughout
their typical workday. Please provide supporting information and data.
19. MSHA recognizes that some mining facilities operate seasonally
or intermittently and that cumulative exposures for miners at these
facilities may be lower than that of miners working at year-round
operations. MSHA requests comments on the exposure monitoring approach
under proposed Sec. 60.12, including the frequency of exposure
monitoring necessary to safeguard the health of miners at seasonal or
intermittent operations. Please provide supporting information and
data.
20. MSHA is proposing that each mine operator perform baseline
sampling within 180 days after the rule becomes effective to assess the
respirable crystalline silica exposure of each miner who is or may
reasonably be expected to be exposed to respirable crystalline silica.
MSHA requests comments on this proposed baseline sampling requirement.
MSHA also requests comment on the ability of service providers used by
mines such as industrial hygiene suppliers and consultants, and
accredited laboratories that conduct respirable crystalline silica
analysis, to meet the demand created by the baseline sampling
requirements within the proposed timeline. Please include alternative
approaches that might be equally protective of miners that should be
implemented for assessing a miner's initial exposure to respirable
crystalline silica.
21. MSHA is proposing a requirement that mine operators
qualitatively evaluate every 6 months any changes in production,
processes, engineering controls, personnel, administrative controls, or
other factors, beginning 18 months after the effective date. MSHA
requests comments on the timing of the proposed semi-annual evaluation
requirements, and in particular, whether miners would possibly be
exposed unnecessarily to respirable crystalline silica levels above the
PEL due to the gap between the effective date and the proposed
requirements. Please provide supporting information.
22. MSHA has determined that most occupations related to extraction
and processing would meet the ``reasonably be expected'' threshold for
baseline sampling. MSHA recognizes that some miners may work in areas
or perform tasks where exposure is not reasonably expected, if at all.
MSHA solicits comments on the assumption that most miners are exposed
to at least some level of respirable crystalline silica, and on the
proposed requirement that these miners should be subject to baseline
sampling. Please provide supporting information.
23. MSHA is proposing that mine operators would not be required to
conduct periodic sampling if the baseline sampling result, together
with another sampling result or objective data, as defined in proposed
Sec. 60.2, confirms miners' exposures are below the proposed action
level. MSHA seeks comments on this proposal. Please provide supporting
information and data.
24. MSHA is proposing that mine operators conduct periodic sampling
within 3 months where the most recent sampling indicates miner
exposures are at or above the proposed action level but at or below the
proposed PEL and continue to sample within 3 months of the previous
sampling until two consecutive samplings indicate that miner exposures
are below the action level. MSHA solicits comments on the proposed
frequency for periodic sampling, including whether the consecutive
samples should be at least 7 days apart. Please provide supporting
information and data.
25. MSHA is proposing that mine operators may discontinue periodic
sampling when two consecutive samples indicate that miner exposures are
below the proposed action level. MSHA requests comments on this
proposal. Please provide supporting information and data.
26. MSHA is proposing that mine operators conduct semi-annual
evaluations to evaluate whether any changes in production, processes,
engineering controls, personnel, administrative controls, or other
factors may reasonably be expected to result in

[[Page 44857]]

new or increased respirable crystalline silica exposures. Please
provide comments on this proposal, as well as alternative approaches
that would be appropriate for evaluating any potential new or increased
respirable crystalline silica exposures. Please provide supporting
information and data.
27. MSHA is proposing that miners' exposures are measured using
personal breathing-zone air samples for MNM operations and occupational
environmental samples collected in accordance with Sec. Sec.
70.201(c), 71.201(b), or 90.201(b) for coal operations. MSHA requests
comments on this proposal. Please provide supporting information and
data.
28. MSHA is proposing the use of representative sampling. Where
several miners perform the same task on the same shift and in the same
work area, the mine operator may sample a representative fraction of
miners to meet the proposed exposure monitoring requirements. MSHA
seeks comments on the use of representative sampling. Please provide
supporting information and data.
29. MSHA is proposing that mine operators use laboratories
accredited to ISO/IEC 17025 ``General requirements for the competence
of testing and calibration laboratories,'' where the accreditation has
been issued by a body that is compliant with ISO/IEC 17011 ``Conformity
assessment--requirements for accreditation bodies accrediting
conformity assessment bodies.'' MSHA solicits comments on this
proposal. Are there additional requirements that should be incorporated
into this proposal to ensure accurate sample analysis methods? Please
provide supporting information and data.
30. MSHA seeks comments on the proposal that mine operators ensure
that laboratories evaluate all respirable crystalline silica samples
using respirable crystalline silica analytical methods specified by
MSHA, NIOSH, or OSHA. Are there additional requirements that should be
incorporated into this proposal to ensure accurate sample analysis?
Please provide supporting information and data.
31. MSHA seeks comments and information on mine operator and
stakeholder experience using NIOSH's rapid field-based quartz
monitoring (RQM) monitors for determining miners' exposures to
respirable crystalline silica. Please provide any information and data.

Proposed Medical Surveillance for Metal and Nonmetal Miners

32. MSHA is proposing to require medical surveillance for MNM
miners. Medical surveillance is already required for coal miners under
30 CFR 72.100 and has played an important role in tracking the burden
of pneumoconiosis in coal miners but is not currently required for MNM
miners. MSHA's proposal would require MNM mine operators to provide
each miner new to the mining industry with an initial medical
examination and a follow-up examination no later than 3 years after the
initial examination, at no cost to the miner. It would also require MNM
mine operators to provide examinations for all miners at least every 5
years, which would be voluntary for miners. Is there an alternative
strategy or schedule, such as voluntary initial or follow-up
examinations, tying the medical surveillance requirement to miners
reasonably expected to be exposed to any level of silica or to the
action level that would be more appropriate for new MNM miners? Should
the rule make each 5-year examination mandatory? Should the 5-year
examination be mandatory for coal mine operators as well? Please
provide data or cite references to support your position.
33. MSHA's proposed medical surveillance requirements for MNM
miners do not include some requirements that are in MSHA's existing
medical surveillance requirements for coal mine operators in 30 CFR
72.100. For example, Sec. 72.100 requires coal mine operators to use
NIOSH-approved facilities for medical examinations. Should MNM
operators be required to use NIOSH-approved facilities for medical
examinations? Coal mine operators also are required to submit for
approval to NIOSH a plan for providing miners with the examinations
specified. This is because NIOSH administers medical surveillance for
coal miners with requirements for coal operators, but not MNM
operators, in NIOSH standards (42 CFR part 37). Should the plan
requirements be extended to MNM operators? However, the proposed
requirements also include some requirements for MNM operators that are
not included for coal operators. For example, the proposed provisions
require operators of MNM mines to provide MNM miners with periodic
medical examinations performed by physicians or other licensed health
care professionals (PLHCP) or specialists including a history and
physical examination focused on the respiratory system, a chest X-ray,
and a spirometry test. The proposed rule also requires a written
medical opinion be provided by the PLHCP or specialist to the mine
operator regarding the miner's ability to wear a respirator. MSHA seeks
comment on the differences between the medical surveillance
requirements for MNM operators in this proposed rule and the existing
medical surveillance requirements for coal mine operators in Sec.
72.100. MSHA also seeks comment on how best to collect health
surveillance data from PLHCPs and specialists to track MNM miners'
health, for example how to know when pneumoconiosis cases occur. MSHA
seeks comments on alternative approaches to scheduling periodic medical
surveillance. MSHA proposes to require operators to keep medical
surveillance information for the duration of a miner's employment plus
6 months. The Agency seeks comments on this proposed requirement and on
any alternative recordkeeping schedules that would be appropriate.
Please provide supporting information.
34. MSHA's proposed medical surveillance requirements for MNM
miners would require operators of MNM mines to provide miners with
periodic medical examinations performed by PLHCP or specialists,
including a history and physical examination focused on the respiratory
system, a chest X-ray, and a spirometry test. MSHA seeks comment on
whether use of any new diagnostic technology (e.g., high-resolution
computed tomography) for the purposes of medical surveillance should be
used.
35. MSHA's proposed medical surveillance requirements would require
that the MNM mine operator provide a mandatory follow-up examination to
the miner no later than 3 years after the miner's initial medical
examination. If a miner's 3-year follow-up examination shows evidence
of a respirable crystalline silica-related disease or decreased lung
function, the operator would be required to provide the miner with
another mandatory follow-up examination with a specialist within 2
years. For examinations that show evidence of disease or decreased lung
function, MSHA seeks comment on how, and to whom, test results should
be communicated.
36. MSHA requests comments as to whether the proposed provisions
should include a medical removal option for MNM miners who have
developed evidence of silica-related disease that is equivalent to the
transfer rights and exposure monitoring provided to coal miners in 30
CFR part 90 (part 90). Under part 90, any coal miner who has evidence
of the development of pneumoconiosis based on a chest X-ray or other
medical examinations has the

[[Page 44858]]

option to work in an area of the mine where the average concentration
of respirable dust in the mine atmosphere during each shift to which
that miner is exposed is continuously maintained at or below the
applicable standard. Under part 90, coal miners are entitled to
retention of pay rate, future actual wage increases, and future work
assignment, shift and respirable dust protection. MSHA seeks comment on
whether this medical removal option should be provided to MNM miners.
What would be the economic impact of providing MNM miners a medical
removal option? Please provide supporting information and data.

Proposed Respiratory Protection Standard

37. MSHA requests comments concerning the temporary, non-routine
use of respirators and whether there are other instances or occupations
in which the Agency should allow the use of respirators as a
supplemental control. Please discuss any impacts on particular mines
and mining conditions and the cost of air-purifying respirators, if
applicable. MSHA also solicits comments on the proposed requirement
that affected miners wear respiratory protection to maintain protection
during temporary and non-routine use of respirators. Please provide
supporting information.
38. MSHA is proposing to incorporate by reference ASTM F3387-19,
published in 2019. Whenever respiratory protective equipment is needed,
mine operators would be required to follow practices for program
administration, standard operating procedures, medical evaluations,
respirator selection, training, fit testing, and maintenance,
inspection, and storage in accordance with the requirements of ASTM
F3387-19. Beyond these elements, MSHA is proposing to provide operators
the flexibility to select the elements in ASTM F3387-19 that are
applicable to their practices of respirator use at their mines. Should
mine operators have the flexibility to choose the ASTM F3387-19
elements that are appropriate for their mine-specific hazards because
the need for respirators may vary due to the variability of mining
processes, activities, airborne hazards, and commodities mined? What,
specifically, do you think should factor into the determination of what
is applicable? MSHA seeks comments on its proposed approach and the
impact it would have on mine operators and on miners' life and health.
39. ASTM F3387-19 identifies a variety of respiratory protection
practice elements. MSHA proposes to require certain minimally
acceptable program elements: program administration; standard operating
procedures; medical evaluations; respirator selection; training; fit
testing; and maintenance, inspection, and storage. Please comment on
whether these are the appropriate elements to require, or if there are
any other elements of ASTM F3387-19 that should be minimally included
in any respiratory protection program. MSHA also welcomes comments on
whether it would be appropriate to require the standard in its
entirety. Please identify those elements that would ensure that
approved respirators are selected, fitted, used, cleaned, and
maintained so that the life and health of miners are safeguarded. MSHA
also seeks data and information on the impact these changes would have
on mine operators, especially smaller operators. What would be the
economic impact if all or parts of ASTM F3387-19 were required
respirator program elements? Please be specific with your response and
provide details on respirator use at your mine to include information
and data on mining processes and environmental conditions; level of
exposures to airborne contaminants; frequency and duration of
exposures; type and amount of work or physical labor, including
frequency and duration; and medical evaluation on respirator use, if
applicable.

Recordkeeping Requirements

40. MSHA is proposing to require recordkeeping for records of
evaluations, records of samplings, records of corrective actions, and
written determination records received from a PLHCP. The proposed
rule's recordkeeping requirements are discussed in the Section-by-
Section Analysis section of this Preamble. MSHA seeks comment on the
utility of these recordkeeping requirements as well as the costs of
making and maintaining these records. Please provide supporting
information.

Training Requirements

41. MSHA requests the views and recommendations of stakeholders
regarding whether training requirements for miners should be included
in proposed part 60. Please provide supporting information and data.

Conforming Changes

42. MSHA requests comments on the proposed conforming changes to
remove the reduced coal dust standard from 30 CFR and the potential
impact on coal mines and miners and on whether to retain the reduced
standard for part 90 miners. Please provide supporting information.
43. MSHA is not proposing to adopt a similar approach as the OSHA
Table 1 for the construction industry, where MSHA would prescribe
specific exposure control methods for task-based work practices when
working with materials containing respirable crystalline silica. See 29
CFR 1926.1153(c)(1). MSHA requests comments on specific tasks and
exposure control methods appropriate for a Table 1-approach for the
mining industry that also would adequately protect miners from risk of
exposure to respirable crystalline silica. Please provide specific
rationale and supporting information, including data on how such an
approach would be implemented.

III. Background

The purpose of this proposed rule is to reduce miners' risk of
developing occupational lung disease and other diseases caused by
exposure to respirable crystalline silica and to better protect all
miners from occupational exposure to airborne hazards. In promulgating
mandatory standards dealing with toxic materials or harmful physical
agents, MSHA is required to ``set standards which most adequately
assure on the basis of the best available evidence that no miner will
suffer material impairment of health or functional capacity . . .'' 30
U.S.C. 811(a)(6)(A).

A. Statutory Authority

The statutory authority for this proposal is provided by the Mine
Act under sections 101(a), 103(h), and 508. 30 U.S.C. 811(a), 813(h),
and 957. MSHA implements the provisions of the Mine Act to prevent
death, illness, and injury from mining and promote safe and healthful
workplaces for miners. The Mine Act requires the Secretary of Labor
(Secretary) to develop and promulgate improved mandatory health or
safety standards to prevent hazardous and unhealthy conditions and
protect the health and safety of the nation's miners. 30 U.S.C. 811(a).
Congress passed the Mine Act to address these dangers, finding ``an
urgent need to provide more effective means and measures for improving
the working conditions and practices in the Nation's coal or other
mines in order to prevent death and serious physical harm, and in order
to prevent occupational diseases originating in such mines.'' 30 U.S.C.
801(c). Congress concluded that ``the existence of unsafe and
unhealthful conditions and practices in the Nation's coal or other

[[Page 44859]]

mines is a serious impediment to the future growth of the coal or other
mining industry and cannot be tolerated.'' 30 U.S.C. 801(d).
Accordingly, ``the Mine Act evinces a clear bias in favor of miner
health and safety.'' Nat'l Mining Ass'n v. Sec'y, U.S. Dep't of Lab.,
812 F.3d 843, 866 (11th Cir. 2016).
Section 101(a) of the Mine Act gives the Secretary the authority to
develop, promulgate, and revise, as appropriate, mandatory health
standards to address toxic materials or harmful physical agents. Under
Section 101(a), standards must protect lives and prevent injuries in
mines and be ``improved'' over any standard that it replaces or
revises. Moreover, ``the Mine Act does not contain the `significant
risk' threshold requirement . . . from the OSH Act.'' Nat'l Mining
Ass'n v. United Steel Workers, 985 F.3d 1309, 1319 (11th Cir. 2021);
see also Nat'l Min. Ass'n v. Mine Safety & Health Admin., 116 F.3d 520,
527-28 (D.C. Cir. 1997) (contrasting the OSH Act at 29 U.S.C. 652 with
the Mine Act at 30 U.S.C. 811(a) and noting that ``[a]rguably, this
language does not mandate the same risk-finding requirement as OSHA''
and holding that ``[a]t most, . . . . [MSHA] was required to identify a
significant risk associated with having no oxygen standard at all''
(emphasis in original)).
The Secretary must set standards to assure, based on the best
available evidence, that no miners will suffer material impairment of
health or functional capacity from exposure to toxic materials or
harmful physical agents over their working lives. 30 U.S.C.
811(a)(6)(A). In developing standards that attain the ``highest degree
of health and safety protection for the miner,'' the Mine Act requires
that the Secretary consider the latest available scientific data in the
field, the feasibility of the standards, and experience gained under
the Mine Act and other health and safety laws. Id. However, MSHA's
``duty to use the best evidence and to consider feasibility . . .
cannot be wielded as counterweight to MSHA's overarching role to
protect the life and health of workers in the mining industry.'' Nat'l
Mining Ass'n, 812 F.3d at 866. Instead, ``when MSHA itself weighs the
evidence before it, it does so in light of its congressional mandate.''
Id.
Section 103(h) of the Mine Act gives the Secretary the authority to
promulgate standards involving recordkeeping and reporting. 30 U.S.C.
813(h). In general, section 103(h) requires that every mine operator
establish and maintain records, make reports, and provide this
information, if required by the Secretary. Id. Also, section 508 of the
Mine Act gives the Secretary the authority to issue regulations to
carry out any provision of the Mine Act. 30 U.S.C. 957.
MSHA's proposal to lower the exposure limits for respirable
crystalline silica and adopt an integrated monitoring approach across
all mining sectors and to update the existing respiratory protection
requirements would fulfill Congress' direction by preventing miners
from suffering material impairment of health or functional capacity
caused by exposure to respirable crystalline silica and other airborne
contaminants.

B. Respirable Crystalline Silica Hazard and Mining

Silica is a common component of rock composed of silicon and oxygen
(chemical formula SiO<INF>2</INF>), existing in amorphous and
crystalline states. Silica in the crystalline state is the focus of
this rulemaking. Respirable crystalline silica consists of small
particles of crystalline silica that can be inhaled and reach the
alveolar region of the lungs, where they can accumulate and cause
disease. In crystalline silica, the silicon and oxygen atoms are
arranged in a three-dimensional repeating pattern. The crystallization
pattern varies depending on the circumstances of crystallization,
resulting in a polymorphic state--several different structures with the
same chemical composition. The most common form of crystalline silica
found in nature is quartz, but cristobalite and tridymite may also be
found in limited circumstances. Quartz accounts for the overwhelming
majority of naturally occurring crystalline silica. In fact, quartz
accounts for almost 12 percent of the earth's crust by volume. All
soils contain at least trace amounts of quartz and it is present in
varying amounts in almost every type of mineral. Quartz is also
abundant in most rock types, including granites, sandstones, and shale.
Moreover, quartz is commonly found in limestone formations, although
limestone itself does not contain quartz. Because of its abundance,
crystalline silica in the form of quartz is present in nearly all
mining operations.
Cristobalite and tridymite are formed at very high temperatures and
are associated with volcanic activity. Naturally occurring cristobalite
and tridymite are rare, but they can be found in volcanic ash and in a
relatively small number of rock types limited to specific geographic
regions. Although rare, exposure to cristobalite occurs when volcanic
deposits are mined. In addition, when other materials are mined, miners
can potentially be exposed to cristobalite during certain processing
steps (e.g., heating silica-containing materials) and contact with
refractory materials (e.g., replacing fire bricks in mine processing
facility furnaces). Tridymite is rarely found in nature and miner
exposure to tridymite is much more infrequent.
Most mining activities generate silica dust because silica is often
contained in the ore being mined or in the overburden (i.e., the soil
and surface material surrounding the commodity being mined). Such
activities include, but are not limited to, cutting, sanding, drilling,
crushing, grinding, sawing, scraping, jackhammering, excavating, and
hauling materials that contain silica. These activities can generate
respirable crystalline silica and may therefore lead to miner exposure.
Inhaled small particles of silica dust can be deposited throughout
the lungs. A large number of crystalline silica particles can reach and
remain in the deep lung (i.e., alveolar region), although some small
particles are cleared from the lungs. Because respirable crystalline
silica particles are not water-soluble and do not undergo metabolism
into less toxic compounds, those particles remaining in the lungs for
prolonged periods result in a variety of cellular responses that may
lead to pulmonary disease. The respirable crystalline silica particles
that are cleared from the lungs can be distributed to lymph nodes,
blood, liver, spleen, and kidneys, potentially accumulating in those
other organ systems and causing renal disease and other adverse health
effects.
In the U.S. in 2021, a total of 12,162 mines produced a variety of
commodities. As shown in Table III-1, of those 12,162 total mines,
11,231 mines were MNM mines and 931 mines were coal mines. MNM mines
can be broadly divided into five commodity groups: metal, nonmetal,
stone, crushed limestone, and sand and gravel. These broad categories
encompass approximately 98 different commodities.\1\ Table III-1 shows
that a majority of MNM mines produce sand and gravel, while the largest
number of MNM miners work at metal mines (not

[[Page 44860]]

including MNM contract workers (i.e., independent contractors and
employees of independent contractors who are engaged in mining
operations)).
---------------------------------------------------------------------------

\1\ Commodities such as sand, gravel, silica, and/or stone for
example are used in road building, concrete construction,
manufacture of glass and ceramics, molds for metal castings in
foundries, abrasive blasting operations, plastics, rubber, paint,
soaps, scouring cleansers, filters, hydraulic fracturing, and
various architectural applications. Some commodities naturally
contain high levels of crystalline silica, such as high-quartz
industrial and construction sands and granite dimension stone and
gravel (both produced for the construction industry).
[GRAPHIC] [TIFF OMITTED] TP13JY23.000

The 931 coal mines--underground and surface--produce bituminous,
subbituminous, anthracite, and lignite coal. Coal mining activities
generate mixed coal mine dust that contains respirable silicates such
as kaolinite, oxides such as quartz, as well as other components (IARC,
1997). These activities include the general mining activities
previously mentioned (e.g., cutting, sanding, drilling, crushing, and
hauling materials), as well as roof bolter operations, continuous
mining machine operations, longwall mining, and other activities. Table
III-1 shows that there are more surface coal mines than underground
coal mines, but more miners are working in underground coal mines than
surface coal mines (not including coal contract workers).

IV. Existing Standards and Implementation

MSHA has maintained health standards to protect MNM and coal miners
from excessive exposure to respirable crystalline silica for decades.
MSHA's existing standards, established in the early 1970s, limit
miners' exposures to respirable crystalline silica. These standards
require mine operators to monitor occupational exposures to respirable
crystalline silica and to use engineering controls as the primary means
of suppressing, diluting, or diverting dust generated by mining
activities. They also require mine operators to provide respiratory
protection in limited situations and on a temporary basis. The existing
standards for MNM and coal mines differ in some respects, including
exposure limits and monitoring. This section describes MSHA's existing
standards for respirable crystalline silica and presents respirable
crystalline silica sampling data to show how MNM and coal mine
operators have complied with them in recent years.

A. Existing Standards--Metal and Nonmetal Mines

MSHA's existing standards for exposure to airborne contaminants,
including respirable crystalline silica, in MNM mines are found in 30
CFR part 56, subpart D (Air Quality and Physical Agents), and 30 CFR
part 57, subpart D (Air Quality, Radiation, Physical Agents, and Diesel
Particulate Matter). These standards include PELs for airborne
contaminants (Sec. Sec. 56.5001 and 57.5001), exposure monitoring
(Sec. Sec. 56.5002 and 57.5002), and control of exposure to airborne
contaminants (Sec. Sec. 56.5005 and 57.5005).
Permissible Exposure Limits. The existing PELs for the three
polymorphs of respirable crystalline silica are based on the
TLVs[supreg] Threshold Limit Values for Chemical Substances in Workroom
Air Adopted by the American Conference of Governmental Industrial
Hygienists (ACGIH) for 1973, incorporated by reference in 30 CFR
56.5001 and 57.5001 (ACGIH, 1974). The 1973 TLV[supreg] establishes
limits for respirable dust containing 1 percent quartz or greater and
is calculated in milligrams per cubic meter of air (mg/m\3\) for each
respirable dust sample. The TLV[supreg] for quartz is calculated by
dividing the percent of respirable quartz plus 2, into the number 10.
The TLV[supreg] for cristobalite and the TLV[supreg] for tridymite,
respectively, are calculated by multiplying the same mass formula by
one-half using the percentages of either cristobalite or tridymite
found in the sample. Thus, the resulting TLVs[supreg] for respirable
dust containing 1 percent respirable crystalline silica or greater are
designed to limit exposures to less than 0.1 mg/m\3\ or 100 [micro]g/
m\3\ for quartz, to less than 0.05 mg/m\3\ or 50 [micro]g/m\3\ for
cristobalite, and to less than 0.05 mg/m\3\ or 50 [micro]g/m\3\ for
tridymite. Throughout the remainder of this preamble, the
concentrations of respirable dust and respirable crystalline silica are
expressed in [micro]g/m\3\.

[[Page 44861]]

Exposure Monitoring. Under 30 CFR 56.5002 and 57.5002, MNM mine
operators must conduct respirable dust ``surveys . . . as frequently as
necessary to determine the adequacy of control measures.'' Mine
operators can satisfy the survey requirement through various
activities, such as respirable dust sampling and analysis, walk-through
inspections, wipe sampling, examining dust control system and
ventilation system maintenance, and reviewing information obtained from
injury, illness, and accident reports.
MSHA encourages MNM mine operators to conduct sampling for airborne
contaminants to ensure a healthy and safe work environment for miners
because sampling provides more accurate information about miners'
exposures to harmful airborne contaminants and the effectiveness of
existing controls in reducing such exposures. When a mine operator's
respirable dust survey indicates that miners have been overexposed to
any airborne contaminant, including respirable crystalline silica, the
operator is expected to adjust its control measures (e.g., exhaust
ventilation) to reduce or eliminate the identified hazard. After doing
so, the mine operator is expected to conduct additional surveys to
determine whether these efforts were successful. Re-surveying should be
done as frequently as necessary to ensure that the implemented control
measures remain adequate. MSHA's determination of whether a mine
operator has surveyed frequently enough is based on several factors,
including whether sampling results comply with the permissible exposure
limit, whether there have been changes in the mining operation or
process, and whether controls such as local exhaust ventilation systems
need routine or special maintenance.
Exposure Controls. MSHA's existing standards for controlling a
miner's exposure to harmful airborne contaminants (Sec. Sec. 56.5005
and 57.5005) require, if feasible, prevention of contamination, removal
by exhaust ventilation, or dilution with uncontaminated air. The use of
respiratory protective equipment is also allowed under specified
circumstances such as when engineering controls are being developed or
are not feasible. When respiratory protective equipment is used, the
operator must have a respiratory protection program consistent with the
requirements of American National Standards Practices for Respiratory
Protection ANSI Z88.2-1969.
Consistent with widely accepted industrial hygiene principles and
NIOSH's recommendations, MSHA requires the use of engineering controls,
supplemented by administrative controls, in its enforcement for the
control of occupational exposure to respirable crystalline silica and
other airborne contaminants (NIOSH, 1974). Engineering controls
designed to remove or reduce the hazard at the source are the most
effective. Examples of engineering controls include the installation of
proper ventilation systems, use of water sprays or wetting agents to
suppress airborne contaminants, installation of machine-mounted dust
collectors to capture respirable crystalline silica and other
contaminants, and the installation of control booths or environmental
cabs to enclose equipment operators.
Although considered a supplementary or secondary measure to
engineering controls, mine operators may use administrative controls to
further reduce miners' exposures to respirable crystalline silica and
other airborne contaminants. In applying administrative controls, mine
operators can direct miners to perform certain activities in specific
manners. For instance, as an administrative control, operators can
specify adequate housekeeping procedures for miners to clean spills or
handle contaminated clothing which could reduce occupational exposure
to airborne contaminants, including respirable crystalline silica.
In addition, respiratory protective equipment can be used in
controlling miners' exposures to airborne contaminants, including
respirable crystalline silica, on a temporary basis or under non-
routine, limited conditions. The use of respiratory protection is,
however, considered to be a supplement, not an alternative to any
engineering or administrative control, in reducing or eliminating a
miner's exposure to airborne contaminants including respirable
crystalline silica.
Under the existing standards in Sec. Sec. 56.5005 and 57.5005, in
circumstances where engineering controls are not yet developed or where
it is necessary for miners to enter hazardous atmospheres to establish
controls or to perform non-routine maintenance or investigation, a
miner using appropriate respiratory protection ``may work for
reasonable periods of time'' in concentrations of airborne contaminants
which exceed exposure limits. Respirators approved by NIOSH and
suitable for their intended purpose must be provided by mine operators
at no cost to the miner and must be used by miners to protect
themselves against the health and safety hazards of airborne
contaminants. Whenever respiratory protection is used, MNM mine
operators are required to have a respirator program consistent with the
requirements specified in ANSI Z88.2-1969.

B. Existing Standards--Coal Mines

Under existing standards, there is no separate standard for
respirable crystalline silica for coal mines. MSHA's existing standards
for exposure to respirable quartz in coal mines, found in 30 CFR 70.101
and 71.101, establish a respirable dust standard when quartz is present
for underground and surface coal mines, respectively. Under 30 CFR part
90 (Mandatory Health Standards--Coal Miners Who Have Evidence of the
Development of Pneumoconiosis), Sec. 90.101 also sets the respirable
dust standard when quartz is present for coal miners. Under these
respirable dust standards, coal miners' exposures to respirable quartz
are indirectly regulated through reductions in the overall respirable
dust standard.
Under its existing respirable coal mine dust standards, MSHA
defines quartz as crystalline silicon dioxide (SiO<INF>2</INF>), which
includes not only quartz but also two other polymorphs, cristobalite
and tridymite.\2\ Therefore, quartz and respirable crystalline silica
are used interchangeably in the discussions of MSHA's existing
standards for controlling exposures to respirable crystalline silica in
coal mines.
---------------------------------------------------------------------------

\2\ Quartz is defined in 30 CFR 70.2, 71.2, and 90.2 as
crystalline silicon dioxide (SiO<INF>2</INF>) not chemically
combined with other substances and having a distinctive physical
structure. Crystalline silicon dioxide is most commonly found in
nature as quartz but sometimes occurs as cristobalite or, rarely, as
tridymite. Quartz accounts for the overwhelming majority of
naturally occurring crystalline silica and is present in varying
amounts in almost every type of mineral.
---------------------------------------------------------------------------

Exposure Limits. The exposure limit for respirable crystalline
silica during a coal miner's shift is 100 [micro]g/m\3\, reported as an
equivalent concentration as measured by the Mining Research
Establishment (MRE) instrument. This equivalent concentration of
respirable crystalline silica must not be exceeded during the miner's
entire shift, regardless of duration. When the equivalent concentration
of respirable quartz exceeds 100 [micro]g/m\3\, under Sec. Sec.
70.101, 71.101, and 90.101, MSHA imposes a reduced respirable dust
standard designed to ensure that respirable quartz will not exceed 100
[micro]g/m\3\. The applicable dust standard, when the equivalent
concentration of respirable crystalline silica exceeds 100 [micro]g/
m\3\, is computed by dividing the percent of quartz into the number 10.

[[Page 44862]]

The result of this calculation becomes the exposure limit for
respirable coal mine dust (RCMD), for the sections of the mine
represented by the sample. Various sections within a mine may have
different reduced RCMD exposure limits. Therefore, when a respirable
dust sample collected by MSHA indicates that the average concentration
of respirable quartz dust exceeds the exposure limit, the mine operator
is required to comply with the applicable dust standard. By reducing
the amount of respirable dust to which miners are exposed during their
shifts, the miners' exposures to respirable crystalline silica are
reduced to a level at or below the exposure limit of 100 [micro]g/m\3\.
Exposure Monitoring. Under Sec. Sec. 70.208, 70.209, 71.206, and
90.207, coal mine operators are required to sample for respirable dust
on a quarterly basis for specified occupations and work areas. The
occupations and work areas specified in the existing coal standards are
the occupations and work areas at a coal mine that are expected to have
the highest concentrations of respirable dust--typically in locations
where respirable dust is generated. In addition, respirable dust
sampling must be representative of respirable dust exposures during a
normal production shift. Also, sampling must occur while miners are
performing routine, day-to-day activities. Part 90 miners must be
sampled for the air they breathe while performing their normal work
duties, from the start of their work day to the end of their work day,
in their normal work locations.\3\
---------------------------------------------------------------------------

\3\ A ``Part 90 miner'' is defined in 30 CFR 90.3 as a miner
employed at a coal mine who shows evidence of having contracted
pneumoconiosis based on a chest X-ray or based on other medical
examinations, and who is afforded the option to work in an area of a
mine where the average concentration of respirable dust in the mine
atmosphere during each shift to which that miner is exposed is
continuously maintained at or below the applicable standard.
---------------------------------------------------------------------------

Exposure Controls. Under Sec. Sec. 70.208, 70.209, 71.206, and
90.207, coal mine operators are required to use engineering or
environmental controls as the primary means of complying with the
respirable dust standards. Similar to the MNM standards, engineering
and environmental controls include the use of dust collectors, water
sprays, and ventilation controls. For many underground coal mines,
providing adequate ventilation is the primary engineering control for
respirable dust, ensuring that dust concentrations are continuously
diluted with fresh air and exhausted away from miners.
When a respirable dust sample exceeds the exposure limit of 100
[micro]g/m\3\ for respirable quartz, the operator must reduce the
average concentration of RCMD to a level designed to maintain the
quartz level at or below 100 [micro]g/m\3\. If operators exceed the
reduced RCMD standard, they are required to take corrective action to
reduce exposure and comply with the reduced standard. Corrective
actions that lower respirable coal mine dust, thus lowering respirable
quartz exposures, are selected after evaluating the cause or causes of
the overexposure. Corrective actions can include increasing air flow,
improving ventilation controls, repairing and maintaining existing dust
suppression controls, adding water sprays or other controls, cleaning
dust filters or collectors more frequently, or repositioning the miner
away from the dust source.
When taking corrective actions to reduce the exposure to respirable
dust, coal mine operators must make approved respiratory equipment
available to miners under Sec. Sec. 70.208 and 71.206. Whenever
respiratory protection is used, Sec. 72.700 requires coal mine
operators to comply with requirements specified in ANSI Z88.2-1969.

C. MSHA Inspection and Respirable Dust Sampling

MSHA collects respirable dust samples at mines and analyzes them
for respirable crystalline silica to determine whether the respirable
crystalline silica exposure limits are met and whether exposure
controls are adequate. This section describes the respirable dust
samples collected at MNM and coal mines in recent years and presents
the results of the sample data analyses.
1. Respirable Dust Sample Collection
This subsection offers a brief description of how MSHA samples for
respirable crystalline silica under the existing standards. Upon their
arrival at mines, MSHA inspectors determine which areas of the mine and
which miners to select for respirable dust sampling. At MNM mines, the
MSHA inspector often determines sampling locations based on sample
results from previous inspections and on the inspector's onsite
observations of work practices and work areas. At coal mines, the MSHA
inspector conducts sampling among the occupations or from the work
areas that are specified for operator sampling under 30 CFR parts 70,
71, and 90. Generally speaking, MSHA inspectors collect respirable dust
samples from the common occupations during typical and normal
activities at the mine and from the positions that are commonly known
to have the highest concentration of respirable dust.
After identifying which miners and which areas at the mine will be
sampled for respirable dust, MSHA inspectors place gravimetric samplers
on the selected miners or at the selected locations. Gravimetric
samplers consist of a portable air-sampling pump connected to a
particle-size separator (i.e., cyclone) and collection medium (i.e.,
filter). MSHA inspectors use Dorr-Oliver 10-mm nylon cyclones operated
at a 1.7 liters per minute (L/min) flow rate for MNM mine sampling and
at a 2.0 L/min flow rate (reported as MRE-equivalent concentrations)
for coal mine sampling.\4\ For the entire duration of the work shift,
the gravimetric sampler captures air from the breathing zone of each
selected miner or occupation and from each selected work area.
---------------------------------------------------------------------------

\4\ This type of sampling equipment was developed to separate
the airborne particles by size in a manner similar to the size-
selective deposition and retention characteristics of the human
respiratory system. It is important to note that size-selective
sampling does not measure the deposition of respirable particles in
the lung. Rather, it provides a measure of the particulate mass
available for deposition to the deep lung during breathing (Raabe
and Stuart, 1999).
---------------------------------------------------------------------------

MSHA inspectors use the full-shift sampling approach. When miners
work longer than an 8-hour shift, which is common, those miners are
sampled continuously throughout the extended work shifts. Full-shift
sampling is used to minimize errors associated with fluctuations in
airborne contaminant concentrations during the miners' work shifts and
to avoid any speculation about the miners' exposures during unsampled
periods of the work shift. Once sampling is completed, the inspectors
send the cassettes containing the full-shift respirable dust samples to
the MSHA Laboratory for analysis.

[[Page 44863]]

2. Respirable Dust Sample Analysis
The MSHA Laboratory analyzes inspectors' respirable dust samples,
following its standard operating procedures (SOPs) summarized below.\5\
Any samples that are broken, torn, or visibly wet are voided and
removed before analysis. Once weighing of the samples is completed,
samples are again screened based on mass gain and examined for
validity. All valid samples that meet the minimum mass gain criteria
per the associated MSHA analytical method are then analyzed for
respirable crystalline silica and for the compliance determination.\6\
---------------------------------------------------------------------------

\5\ The MSHA Laboratory has fulfilled the requirements of the
AIHA Laboratory Accreditation Programs (AIHA-LAP), LLC accreditation
to the ISO/IEC 17025:2017 international standard for industrial
hygiene.
\6\ The minimum mass gain criteria used by the MSHA Laboratory
for the different samples are:
<bullet> MNM mine respirable dust samples: greater than or equal
to 0.100 mg;
<bullet> Underground coal mine respirable dust samples: greater
than or equal to 0.100 mg; and
<bullet> Surface coal mine respirable dust samples: greater than
or equal to 0.200 mg.
Exception: For six surface occupations that have been deemed
``high risk,'' the laboratory uses a minimum mass gain criterion of
greater than or equal to 0.100 mg.
If cristobalite analysis is requested for MNM mine respirable
dust samples, filters having a mass gain of 0.05 mg or more are
analyzed. In the rare instance when tridymite analysis is requested,
a qualitative analysis for the presence of the polymorph is
conducted concurrently with the cristobalite analysis.
---------------------------------------------------------------------------

The MSHA Laboratory uses two analytical methods to determine the
concentration of quartz (and cristobalite and tridymite, if requested):
X-ray diffraction (XRD) for respirable dust samples from MNM mines, and
Fourier transform infrared spectroscopy (FTIR) for respirable coal mine
dust samples.\7\ The XRD method uses X-rays to distinguish and measure
the structure, composition, and physical properties of a sample. The
FTIR method relies on the absorption of infrared light to determine the
composition of a sample. The percentage of silica in the MNM mine dust
sample is calculated using the mass of quartz or cristobalite
determined from the XRD analysis and the measured mass of respirable
dust. The percentage of silica is used to calculate MSHA's PELs for
quartz and cristobalite, in accordance with Sec. Sec. 56.5001 and
57.5001. Similarly, in the respirable coal mine dust sample, the
percentage of quartz is calculated using the quartz mass determined
from the FTIR analysis and the sample's mass of dust. Current FTIR
methods, however, cannot quantify quartz and cristobalite, and/or
tridymite, in the same sample. For coal mines, the percentage of quartz
is used to calculate the reduced dust standard when the quartz
concentration exceeds 100 [micro]g/m\3\ (MRE).
---------------------------------------------------------------------------

\7\ Details on MSHA's analytical procedures for respirable
crystalline silica analysis can be found in ``MSHA P-2: X-Ray
Diffraction Determination of Quartz and Cristobalite in Respirable
Metal/Nonmetal Mine Dust'' and ``MSHA P-7: Determination of Quartz
in Respirable Coal Mine Dust by Fourier Transform Infrared
Spectroscopy.''
Department of Labor, Mine Safety and Health Administration,
Pittsburgh Safety and Health Technology Center, X-Ray Diffraction
Determination of Quartz and Cristobalite in Respirable Metal/
Nonmetal Mine Dust. <a href="https://arlweb.msha.gov/Techsupp/pshtcweb/MSHA%20P2.pdf">https://arlweb.msha.gov/Techsupp/pshtcweb/MSHA%20P2.pdf</a>. Department of Labor, Mine Safety and Health
Administration, Pittsburgh Safety and Health Technology Center, MSHA
P-7: Determination of Quartz in Respirable Coal Mine Dust By Fourier
Transform Infrared Spectroscopy. <a href="https://arlweb.msha.gov/Techsupp/pshtcweb/MSHA%20P7.pdf">https://arlweb.msha.gov/Techsupp/pshtcweb/MSHA%20P7.pdf</a>.
---------------------------------------------------------------------------

It is worth noting how MSHA calculates full-shift exposure to
respirable crystalline silica (and other airborne contaminants). When a
miner who works an 8-hour shift is sampled, the miner's 8-hour TWA
exposure is calculated as follows:
[GRAPHIC] [TIFF OMITTED] TP13JY23.001

However, for work shifts that last longer than 8 hours, a coal
miner's full-shift exposure is calculated differently than an MNM
miner's full-shift exposure. In accordance with Sec. 70.2, the coal
miner's extended full-shift exposure has, since 2014, been calculated
in the following way:
[GRAPHIC] [TIFF OMITTED] TP13JY23.002

For the MNM miner, MSHA calculates extended full-shift exposure
according to the following formula:
[GRAPHIC] [TIFF OMITTED] TP13JY23.003

For respirable dust samples from MNM mines, 480 minutes is used in
the denominator regardless of the actual sampling time. Contaminants
collected over extended shifts (e.g., 600-720 minutes) are calculated
as if they had been collected over 480 minutes. MSHA has used this
calculation approach (also known as ``shift-weighted average'') since
the 1970s.
Under the shift-weighted average approach, exposures for work
schedules greater than 8 hours are proportionately adjusted to allow
direct comparison with the 8-hour PEL. The ACGIH TLVs[supreg] adopted
by MSHA are based on exposure periods of no more than 8 hours per day
and 40 hours per week, with 16 hours of recovery time between shifts.

D. Respirable Crystalline Silica Sampling Results--Metal and Nonmetal
Mines

This section presents the results of respirable dust samples that
were collected by MSHA inspectors at MNM mines from 2005 to 2019. From
January 1, 2005, to December 31, 2019, a total of 104,354 valid samples
were collected. Of this total, 57,769 samples that met the minimum mass
gain criteria were analyzed for respirable crystalline silica.

[[Page 44864]]

The vast majority of the 46,585 valid samples that were excluded from
the analysis in this rulemaking did not meet the mass gain criteria
described earlier and therefore the lab did not determine their silica
concentration. Further information on the valid respirable dust samples
that are excluded from the analysis in this rulemaking can be found in
Appendix A of the preamble.
The respirable crystalline silica concentration is calculated using
the measured mass of each of the polymorphs and the air sampling
volume. As discussed above, the existing PEL for quartz in MNM mines is
approximately equivalent to 100 [micro]g/m\3\ for a full-shift
exposure, calculated as an 8-hour TWA, while the existing PELs for
cristobalite and tridymite, respectively, are approximately equivalent
to 50 [micro]g/m\3\ for a full-shift exposure, calculated as an 8-hour
TWA.\8\
---------------------------------------------------------------------------

\8\ If more than one polymorph is present the equation used to
calculate the TLV[supreg] for respirable dust containing quartz is
modified per Appendix C of the 1973 ACGIH TLV[supreg] Handbook, and
the equation is modified as follows: 10/[(% quartz + 2) + 2 (%
cristobalite + 2)].
---------------------------------------------------------------------------

1. Annual Results of MNM Respirable Crystalline Silica Samples
Table IV-1 below shows the variation between 2005 and 2019 in: (1)
the numbers of MNM respirable dust samples analyzed for respirable
crystalline silica; and (2) the number and percentage of samples that
had concentrations of respirable crystalline silica greater than 100
[micro]g/m\3\. Of the 57,769 MNM respirable dust samples analyzed for
respirable crystalline silica over the 15-year period, about 6 percent
(3,539 samples) had respirable crystalline silica concentrations
exceeding the existing PEL of 100 [micro]g/m\3\. The average annual
rates of overexposure ranged from a maximum of approximately 10 percent
in 2006 (the second year) to a minimum of approximately 4 percent in
2019 (the last year of the time series). Compared with the rates in
2005-2008, overexposure rates were substantially lower in 2009-2017,
with a further drop in 2018-19.
BILLING CODE 4520-43-P
[GRAPHIC] [TIFF OMITTED] TP13JY23.004

[[Page 44865]]

2. Analysis of MNM Respirable Crystalline Silica Samples by Commodity
Because the MNM mining industry produces commodities that contain
varying degrees of respirable crystalline silica, it is important to
examine each commodity separately. MNM mines can be grouped by five
commodities: metal, sand and gravel, stone, crushed limestone, and
nonmetal (where nonmetal includes all other materials that are not
metals, besides sand, gravel, stone, and limestone). This grouping is
based on the mine operator-reported mining products and the North
American Industry Classification System (NAICS) codes. (Appendix B of
the preamble provides a list of the NAICS codes relevant for MNM mining
and how each code is assigned to one of the five commodities.)
Table IV-2 shows the distribution of the respirable dust samples
analyzed for respirable crystalline silica by mine commodity. The
percentage of samples with respirable crystalline silica concentrations
greater than the existing exposure limit of 100 [micro]g/m\3\ varies
across the different commodities. It is highest for the metal, sand and
gravel, and stone commodities (at approximately 11, 7, and 7 percent,
respectively), and lowest for the nonmetal and crushed limestone
commodities (at approximately 4 and 3 percent, respectively).
[GRAPHIC] [TIFF OMITTED] TP13JY23.005

3. Analysis of MNM Respirable Crystalline Silica Samples by Occupation
To examine how miners who perform different tasks differ in
occupational exposure to respirable crystalline silica, MSHA grouped
MNM mining jobs into 11 occupational categories. These categories
include jobs that are similar in terms of tasks performed, equipment
used, and engineering or administrative controls used to control
miners' exposure. For example, backhoe operators, bulldozer operators,
and tractor operators were grouped into ``operators of large powered
haulage equipment,'' whereas belt crew, belt cleaners, and belt
vulcanizers were grouped into ``conveyer operators.'' The 121 MNM job
codes used by MSHA inspectors were grouped into the following
occupational categories: \9\
---------------------------------------------------------------------------

\9\ For a full crosswalk of job codes included in each of these
11 Occupational Categories, please see Appendix C of the preamble.
Also, note that the order of the presentation of the 11 Occupational
Categories here follows the general sequence of mining activities:
first development and production, then ore/mineral processing, then
loading, hauling, and dumping, and finally all others.
---------------------------------------------------------------------------

(1) Drillers (e.g., Diamond Drill Operator, Wagon Drill Operator,
and Drill Helper),
(2) Stone Cutting Operators (e.g., Jackhammer Operator, Cutting
Machine Operator, and Cutting Machine Helper),
(3) Kiln, Mill, and Concentrator Workers (e.g., Ball Mill Operator,
Leaching Operator, and Pelletizer Operator),
(4) Crushing Equipment and Plant Operators (e.g., Crusher Operator/
Worker, Scalper Screen Operator, and Dry Screen Plant Operator),
(5) Packaging Equipment Operators (e.g., Bagging Operator and
Packaging Operations Worker),
(6) Conveyor Operators (e.g., Belt Cleaner, Belt Crew, and Belt
Vulcanizer),
(7) Truck Loading Station Tenders (e.g., Dump Operator and Truck
Loader),
(8) Operators of Large Powered Haulage Equipment (e.g., Tractor
Operators, Bulldozer Operator, and Backhoe Operators),
(9) Operators of Small Powered Haulage Equipment (e.g., Bobcat
Operator, Scoop-Tram Operator, and Forklift Operator),
(10) Mobile Workers (e.g., Laborers, Electricians, Mechanics, and
Supervisors), and
(11) Miners in Other Occupations (e.g., Welder, Dragline Operator,
Ventilation Crew and Dredge/Barge Operator).
Table IV-3 shows sample numbers and overexposure rates by MNM
occupation. Operators of large powered haulage equipment accounted for
the largest number of samples analyzed for silica (17,016 samples),
whereas conveyor operators accounted for the fewest (215 samples).
Table IV-3 also shows the number and percentage of the samples
exceeding the existing respirable crystalline silica PEL of 100
[micro]g/m\3\. In every occupational category, some MNM miners were
exposed to respirable crystalline silica levels above the existing PEL.
In 9 out of the 11 occupational categories, the percentage of samples
exceeding the existing PEL is less than 10 percent, although two have

[[Page 44866]]

higher rates, ranging up to more than 19 percent (in the case of stone
cutting operators).
[GRAPHIC] [TIFF OMITTED] TP13JY23.006

4. Conclusion
This analysis of MSHA inspector sampling data shows that MNM
operators have generally met the existing standard. Of the 57,769
respirable dust samples from MNM mines, approximately 6 percent
exceeded the existing respirable crystalline silica PEL of 100
[micro]g/m\3\, although there are several outliers with much higher
overexposures. For 9 of the 11 occupational categories, less than 10
percent of the respirable dust samples had concentrations over the
existing PEL of 100 [micro]g/m\3\ for respirable crystalline silica. In
addition, about 80 percent of samples taken from stone cutting
operators did not exceed the existing PEL, which historically has had
high exposures to respirable dust and respirable crystalline silica;
\10\ nevertheless, this occupation continues to experience the highest
overexposures relative to other MNM occupations. For the categories of
drillers, miners in other occupations, and operators of large powered
haulage equipment, approximately 5 percent or less of the respirable
dust samples showed concentrations over the existing exposure limit.
---------------------------------------------------------------------------

\10\ Analysis of MSHA respirable dust samples from 2005 to 2010
showed that stone and rock saw operators had approximately 20
percent of the sampled exposures exceeding the PEL. Watts et al.
(2012).
---------------------------------------------------------------------------

MSHA believes that improved technology, engineering controls, and
better training contributed to the reductions in exposures for miners
who work in occupations exposed to the highest levels of respirable
crystalline silica. In summary, the analysis of MSHA inspector sampling
data indicates that the controls that MNM mine operators are using,
together with MSHA's enforcement, have generally been effective in
keeping miners' exposure at or below the existing limit of 100
[micro]g/m\3\.

E. Respirable Crystalline Silica Sampling Results--Coal Mines

To examine coal mine operators' compliance with existing respirable
crystalline silica standards, MSHA analyzed RCMD samples collected by
MSHA inspectors from 2016 to 2021. (The data analyses for this
rulemaking do not include any respirable dust samples collected by coal
mine operators.) The analysis below is based on the samples collected
by inspectors starting on August 1, 2016, when Phase III of MSHA's 2014
Lowering Miners' Exposure to Respirable Coal Mine Dust, Including
Continuous Personal Dust Monitors (Coal Dust Rule) (79 FR 24813, May 1,
2014) went into effect. At that time, the exposure limits for RCMD

[[Page 44867]]

were lowered from 2.0 mg/m\3\ to 1.5 mg/m\3\ (MRE equivalent) at
underground and surface coal mines, and from 1.0 mg/m\3\ to 0.5 mg/m\3\
(MRE equivalent) for intake air at underground coal mines and for Part
90 miners. From August 1, 2016, to July 31, 2021, MSHA inspectors
collected a total of 113,607 valid RCMD samples. Of these valid
samples, only those collected from the breathing zones of miners were
used in the analysis for this rulemaking; no environmental dust samples
were included.\11\ Of those samples, 63,127 samples that met the
minimum mass gain criteria and had no other disqualifying issues were
analyzed for respirable quartz and quartz concentrations were
determined. The majority of the non-environmental valid samples
excluded from this rulemaking analysis were excluded due to
insufficient mass. Further information on the valid respirable dust
samples that are not included in the rulemaking analysis can be found
in Appendix A of the preamble.
---------------------------------------------------------------------------

\11\ Environmental samples were not included in the analysis to
be consistent with the proposed sampling requirements to determine
individual miner exposure.
---------------------------------------------------------------------------

Of the 63,127 valid samples analyzed for respirable crystalline
silica and used for this analysis, about 1 percent (777 samples) were
over the existing quartz exposure limit of 100 [micro]g/m\3\ (MRE
equivalent) for a full shift, calculated as a TWA.\12\ Overexposure
rates (the percent of samples above the exposure limit, on average
across all coal mining occupations) decreased by nearly a quarter
between the first half and the second half of the 2016-2021 period. As
in MNM mines, different miner occupations had different overexposure
rates. Using broader groupings, surface mines experienced higher rates
of overexposure than underground mines (2.4 percent versus 1.0 percent,
respectively).
---------------------------------------------------------------------------

\12\ The conversion between ISO values and MRE values uses the
NIOSH conversion factor of 0.857. In the 1995b Criteria Document,
NIOSH presented an empirically derived conversion factor of 0.857
for comparing current (MRE) and recommended (ISO) respirable dust
sampling criteria using the 10 mm Dorr-Oliver nylon cyclone operated
at 2.0 and 1.7 L/min, respectively (i.e., 1.5 mg/m\3\ BMRC-MRE =
1.29 mg/m\3\ ISO).
---------------------------------------------------------------------------

1. Annual Results of Coal Respirable Crystalline Silica Samples
In examining trends from one year to the next, the discussion below
focuses on the samples collected in the 6 calendar years from 2016 to
2021. The number of samples per year was stable from 2017 to 2019
before decreasing in 2020.\13\ The overexposure rate decreased across
the entire 2016 to 2021 period, from 1.41 percent in 2016 to 0.95
percent in 2021. As shown in Table IV-4, a review of the 6 calendar
years reveals that the overexposure rate decreased by nearly a quarter
from 2016-2018 (1.38 percent) to 2019-2021 (1.07 percent).
---------------------------------------------------------------------------

\13\ The coal samples for 2016 begin in August of that year and
the coal samples for 2021 end in July of that year.
[GRAPHIC] [TIFF OMITTED] TP13JY23.007

2. Analysis of Coal Respirable Crystalline Silica Samples by Location
Coal mining activities differ depending on the characteristics and
locations of coal seams. When coal seams are several hundred feet below
the surface, miners tunnel into the earth and use underground mining
equipment to extract coal, whereas miners at surface coal mines remove
topsoil and layers of rock to expose coal seams. Due to these
differences, it is important to examine the respirable crystalline
silica data by location to determine how underground and surface coal
miners differ in occupational exposure to respirable crystalline
silica.
Table IV-5, which presents the overexposure rate by type of mine
where respirable coal mine dust samples were collected, shows that
samples from surface coal mines reflected higher rates of overexposure
than samples from underground mines.

[[Page 44868]]

Out of the 53,095 respirable coal mine dust samples from underground
mines, 1 percent (537 samples) were over the existing exposure limit.
By contrast, there were 10,032 samples from surface coal mines, and
approximately 2.4 percent (240 samples) of those samples were over the
existing exposure limit.
[GRAPHIC] [TIFF OMITTED] TP13JY23.008

3. Analysis of Coal Respirable Crystalline Silica Samples by Occupation
To assess the exposure to respirable crystalline silica of miners
in different occupations, MSHA has consolidated the 220 job codes for
coal mines into 9 occupational categories (using a similar process to
the one it used for the MNM mines, but with different job codes and
categories). For the coal mine occupational categories,\14\ a
distinction is made between occupations based on whether the job tasks
are being performed at the surface of a mine or underground. For
example, bulldozer operators are assigned to the operators of large
powered haulage equipment grouping and then sorted into separate
occupational categories based on whether they are working at the
surface of a mine or underground.
---------------------------------------------------------------------------

\14\ For a full crosswalk of which job codes were included in
each of these nine Occupational Categories, please see Appendix C of
the preamble.
---------------------------------------------------------------------------

Of the nine occupational categories used for coal miners, the five
underground categories are:
(1) Continuous Mining Machine Operators (e.g., Coal Drill Helper
and Coal Drill Operator),
(2) Longwall Workers (e.g., Headgate Operator and Jack Setter
(Longwall)),
(3) Roof Bolters (e.g., Roof Bolter and Roof Bolter Helper),
(4) Operators of Large Powered Haulage Equipment (e.g., Shuttle Car
Operator, Tractor Operator/Motorman, Scoop Car Operator), and
(5) All Other Underground Miners (e.g., Electrician, Mechanic, Belt
Cleaner and Laborer, etc.).
The four surface occupational categories are:
(1) Drillers (e.g., Coal Drill Operator, Coal Drill Helper, and
Auger Operator),
(2) Crusher Operators (e.g., Crusher Attendant, Washer Operator,
and Scalper-Screen Operator),
(3) Operators of Large Powered Haulage Equipment (e.g., Backhoe
Operator, Forklift Operator, and Bulldozer Operator), and
(4) Mobile Workers (e.g., Electrician, Mechanic, Blaster, Laborer,
etc.).
The most sampled occupational category was operators of large
powered haulage equipment (underground), representing approximately 34
percent of the samples taken. The least sampled occupational category
was crusher operators (surface), consisting of 1 percent of the samples
taken. Table IV-6 displays the number and percent of respirable coal
mine dust samples with quartz greater than the existing exposure limit
for each occupational category.

[[Page 44869]]

[GRAPHIC] [TIFF OMITTED] TP13JY23.009

Looking at trends, every occupational category shows a decrease in
overexposure rates over time. See Figure IV-1. Most of the nine
categories had lower rates of overexposure in the 2019-2021 period than
in the 2016-2018 period.

[[Page 44870]]

[GRAPHIC] [TIFF OMITTED] TP13JY23.010

BILLING CODE 4520-43-C
In all occupational categories, coal miners were sometimes exposed
to respirable crystalline silica levels above the existing exposure
limit. But the sampling data showed that coal mine operators can
generally comply with the existing exposure limit. For example,
although mining tasks performed by the occupational category of roof
bolters (underground) historically resulted in high levels of
overexposure to quartz, the low levels of overexposure for that
occupation in 2016-2021 (i.e., 1 percent) suggest that roof bolters now
benefit from the improved respirable dust standard, improved
technology, and better training.\15\ Over the 2016-2021 period, coal
miners in the occupational category drillers (surface) were the most
frequently overexposed, with approximately 6 percent of samples over
the existing quartz limit; they were followed by longwall workers
(underground) (about 4 percent), operators of large powered haulage
equipment (surface) (about 3 percent), and continuous mining machine
operators (underground) (about 2 percent). For all other occupational
categories, the overexposure rate was less than 1 percent.
---------------------------------------------------------------------------

\15\ The drilling operation in the roof bolting process,
especially in hard rock, generates excessive respirable coal and
quartz dusts, which could expose the roof bolting operator to
continued health risks (Jiang and Luo, 2021).
---------------------------------------------------------------------------

4. Conclusion
This analysis of MSHA inspector sampling data shows that coal mine
operators can generally comply with the existing standards related to
quartz. Of the 63,127 valid respirable dust samples from coal mines
over the most recent 5-year period, 1.2 percent had respirable quartz
over the existing exposure limit of 100 [micro]g/m\3\ (MRE equivalent)
for a full-shift exposure, calculated as a TWA. Seven of the nine
occupational categories had overexposure rates of 2.5 percent or less.
Roof bolters (underground), which historically have had high exposures
to respirable dust and respirable crystalline silica, had overexposure
rates of 1 percent over this recent period. The data demonstrates that
the controls that coal mine operators are using, together with MSHA's
enforcement, have generally been effective in keeping miners' exposure
to respirable crystalline silica at or below the existing exposure
limit.

V. Health Effects Summary

This section summarizes the health effects from occupational
exposure to respirable crystalline silica. MSHA's full analysis is
contained in the standalone document, entitled Effects of Occupational
Exposure to Respirable Crystalline Silica on the Health of Miners
(Health Effects document), which has been placed in the rulemaking
docket for the MSHA silica rulemaking (RIN 1219-AB36, Docket ID no.
MSHA-2023-0001) and is available on MSHA's website.
The purpose of the Agency's scientific review is to present MSHA's
preliminary findings on the nature of the hazards presented by exposure
to respirable crystalline silica and to present the basis for the
Preliminary

[[Page 44871]]

Risk Analysis (PRA) to follow. (A PRA summary is presented in Section
VI of this preamble and a standalone document entitled Preliminary Risk
Analysis has been placed in the rulemaking docket for the MSHA silica
rulemaking (RIN 1219-AB36, Docket ID no. MSHA-2023-0001) and is
available on MSHA's website.) MSHA reviewed a wide range of health
research literature that included more than 600 studies exploring the
relationship between respirable crystalline silica exposure and
resultant health effects in miners and other workers across various
industries. After discussing the toxicity of respirable crystalline
silica, MSHA's review of the literature covers the following topics:
(1) Silicosis;
(2) NMRD, excluding silicosis;
(3) Lung cancer and cancer at other sites;
(4) Renal disease; and
(5) Autoimmune diseases.
To develop this literature review, MSHA expanded upon OSHA's
(2013b) review of the health effects literature to support its final
respirable crystalline silica rule (81 FR 16286, March 25, 2016). MSHA
also drew upon numerous studies conducted by NIOSH, the International
Agency for Research on Cancer (IARC), the National Toxicology Program
(NTP), and other researchers. These studies provided epidemiological
data, morbidity (having a disease or a symptom of disease) and
mortality (disease resulting in death) analyses, progression and
pathology evaluations, death certificate and autopsy reviews, medical
surveillance data, health hazard assessments, in vivo (animal) and in
vitro toxicity data, and other toxicological reviews. These sources are
cited throughout this summary and are listed in the References section
of the Health Effects document. Additionally, these sources appear in
the rulemaking docket.
MSHA's literature review is based on a weight-of-evidence approach,
in which studies are evaluated for their overall quality. Causal
inferences are drawn based on a determination of whether there is
substantial evidence that exposure increases the risk of a particular
adverse health effect. Factors MSHA considered in this weight-of-
evidence analysis include: size of the cohort studied and power of the
study to detect a sufficiently low level of disease risk, duration of
follow-up of the study population, potential for study bias (such as
selection bias or healthy worker effects), and adequacy of underlying
exposure information for examining exposure-response relationships. Of
the studies examined in the Health Effects document, studies were
deemed suitable for inclusion in the PRA if there was adequate
quantitative information on exposure and disease risks and the study
was judged to be of sufficiently high quality according to the above
criteria.
The understanding of how respirable crystalline silica causes
adverse health effects has evolved greatly in the more than 45 years
since the Mine Act was passed in 1977. Based on its extensive review of
health research literature, MSHA has preliminarily determined that
occupational exposure to respirable crystalline silica causes silicosis
(acute silicosis, accelerated silicosis, simple chronic silicosis, and
PMF), NMRD (including COPD), and lung cancer, and it also causes end-
stage renal disease (ESRD). In addition, MSHA believes that respirable
crystalline silica exposure is causally related to the development of
some autoimmune disorders through inflammation pathways. Each of these
effects is exposure-dependent, chronic, irreversible, and potentially
disabling or fatal. MSHA's review of the literature indicates that
under the existing standards found in 30 CFR parts 56, 57, 70, 71, and
90, miners are still developing preventable diseases that are material
impairments of health and functional capacity. Based on the assessment
of health effects of respirable crystalline silica, MSHA preliminarily
concludes that the proposed rule, which would lower the exposure limits
in MNM and coal mining to 50 [micro]g/m\3\ and establish an action
level of 25 [micro]g/m\3\ for a full-shift exposure, calculated as an
8-hour TWA, would reduce the risk of miners developing silicosis, NMRD,
lung cancer, and renal disease.

A. Toxicity of Respirable Crystalline Silica

Respirable crystalline silica is released into the environment
during mining or milling processes, thus creating an airborne hazard.
The particles may be freshly generated or re-suspended from surfaces on
which it is deposited in mines or mills. Respirable crystalline silica
particles may be irregularly shaped and variable in size. Inhaled
respirable crystalline silica can be deposited throughout the lungs.
Some pulmonary clearance of particles deposited in the deep lung (i.e.,
alveolar region) may occur, but a large number of particles can be
retained and initiate or advance the disease process. The toxicity of
these retained particles is amplified because the particles are not
water-soluble and do not undergo metabolism into less toxic compounds.
This is important biologically and physiologically, as insoluble dusts
may remain in the lungs for prolonged periods, resulting in a variety
of cellular responses that can lead to pulmonary disease (ATSDR, 2019).
Respirable crystalline silica particles that are cleared from the lungs
by the lymphatic system are distributed to the lymph nodes, blood,
liver, spleen, and kidneys, potentially accumulating in these other
organ systems and causing renal disease and other adverse health
effects (ATSDR, 2019).
Physical characteristics relevant to the toxicity of respirable
crystalline silica primarily relate to its size and surface
characteristics. Researchers believe that the size and surface
characteristics play important roles in how respirable crystalline
silica causes tissue damage. Any factor that influences or modifies
these physical characteristics may alter the toxicity of respirable
crystalline silica by affecting the mechanistic processes (OSHA, 2013b;
ATSDR, 2019).
Inflammation pathways affect disease development in various systems
and tissues in the human body. For instance, it has been proposed that
lung fibrosis caused by exposure to respirable crystalline silica
results from a cycle of cell damage, oxidant generation, inflammation,
scarring, and ultimately fibrosis. This has been reported by Nolan et
al. (1981), Shi et al. (1989, 1998), Lapp and Castranova (1993), Brown
and Donaldson (1996), Parker and Banks (1998), Castranova and
Vallyathan (2000), Castranova (2004), Fubini et al. (2004), Hu et al.
(2017), Benmerzoug et al. (2018), and Yu et al. (2020).
Respirable crystalline silica entering the lungs could cause damage
by a variety of mechanisms, including direct damage to lung cells. In
addition, activation or stimulation by respirable crystalline silica of
alveolar macrophages (after phagocytosis) and/or alveolar epithelial
cells may lead to: (1) release of cytotoxic enzymes, reactive oxygen
species (ROS), reactive nitrogen species (RNS), inflammatory cytokines
and chemokines, (2) eventual cell death with the release of respirable
crystalline silica, and (3) recruitment and activation of
polymorphonuclear leukocytes (PMNs) and additional alveolar
macrophages. The elevated production of ROS/RNS would result in
oxidative stress and lung injury that stimulates alveolar macrophages,
ultimately resulting in fibroblast activation and pulmonary fibrosis.
The prolonged recruitment of macrophages and PMN causes a persistent
inflammation, regarded as a primary step in the development of
silicosis.
The strong immune response in the lung following exposure to
respirable

[[Page 44872]]

crystalline silica may also be linked to a variety of extra-pulmonary
adverse effects such as hypergammaglobulinemia, production of
rheumatoid factor, anti-nuclear antibodies, and release of other immune
complexes (Parks et al., 1999, Haustein and Anderegg, 1988; Green and
Vallyathan, 1996). Respirable crystalline silica exposure has also been
associated with nonmalignant renal disease through the initiation of
immunological injury to the glomerulus of the kidney (Calvert et al.,
1997).
Proposed mechanisms involved in respirable crystalline silica-
induced carcinogenesis have included: direct DNA damage, inhibition of
the p53 tumor suppressor gene, loss of cell cycle regulation;
stimulation of growth factors, and production on oncogenes (Brown and
Donaldson, 1996; Castranova, 2004; Fubini et al., 2004; Nolan et al.,
1981; Shi et al., 1989, 1998).

B. Diseases

1. Silicosis
Silicosis is a progressive occupational disease that has long been
identified as a cause of lung disease in miners. Based on its review of
the literature, MSHA has preliminarily determined that exposure to
respirable crystalline silica causes silicosis (acute silicosis,
accelerated silicosis, simple chronic silicosis, and PMF) in MNM and
coal miners, which is a significant cause of serious morbidity and
early mortality in this occupational cohort (Mazurek and Attfield,
2008; Mazurek and Wood, 2008a, 2008b; Mazurek et al., 2015, 2018).
When respirable crystalline silica particles accumulate in the
lungs, they cause an inflammatory reaction, leading to lung damage and
scarring. Silicosis can continue to develop even after silica exposure
has ceased. It is not reversible, and there is only symptomatic
treatment, including bronchodilators to maintain open airways, oxygen
therapy, and lung transplants in the most severe cases (Cochrane et
al., 1956; Ng et al., 1987a; Lee et al., 2001; Mohebbi and Zubeyri,
2007; Kimura et al., 2010; Laney et al., 2017; Almberg et al., 2020;
Hall et al., 2022).
Respirable crystalline silica exposure in MNM miners can lead to
all three forms of silicosis (acute, accelerated, and chronic). These
forms differ in the rate of exposure, pathology (i.e., the structural
and functional changes produced by the disease), and latency period
from exposure to disease onset. Acute silicosis is an aggressive
inflammatory process following intense exposure to respirable
crystalline silica for ``periods measured in months rather than years''
(Cowie and Becklake, 2016). It causes alveolar proteinosis
(accumulation of lipoproteins in the alveoli of the lungs). This
restructuring of the lungs leads to symptoms such as coughing and
difficult or labored breathing, and it often progresses to profound
disability and death due to respiratory failure or infectious
complications. In addition, symptoms often advance even after exposure
has stopped, primarily due to the massive amount of protein debris and
fluid that collects in the alveoli, which can suffocate the patient.
The radiographic (X-ray) appearance and results of microscopic
examination of acute silicosis are like those of idiopathic pulmonary
alveolar proteinosis.
Chronic silicosis is the most frequently observed form of silicosis
in the United States today (Banks, 2005; OSHA, 2013b; Cowie and
Becklake, 2016). It is also the most common form of silicosis diagnosed
in miners. Chronic silicosis is a fibrotic process that typically
follows less intense respirable crystalline silica exposure of 10 or
more years (Becklake, 1994; Balaan and Banks, 1998; NIOSH, 2002b,
Kambouchner and Bernaudin, 2015; Cowie and Becklake, 2016; Rosental,
2017; ATSDR, 2019; Barnes et al., 2019; Hoy and Chambers, 2020). It is
identified by the presence of the silicotic islet or nodule that is an
agent-specific fibrotic lesion and is recognized by its pathology
(Balaan and Banks, 1998). Chronic silicosis develops slowly and creates
rounded whorls of scar tissue that progressively destroy the normal
structure and function of the lungs. In addition, the scar tissue
opacities become visible by chest X-ray or computerized tomography (CT)
only after the disease is well established and the lesions become large
enough to view. As a result, surveys based on chest X-ray films usually
underestimate the true prevalence of silicosis (Craighead and
Vallathol, 1980; Hnizdo et al., 1993; Rosenman et al., 1997; Cohen and
Velho, 2002). However, the lesions eventually advance and result in
lung restriction, reduced lung volumes, decreased pulmonary compliance,
and reduction in the gas exchange capabilities of the lungs (Balaan and
Banks, 1998). As the disease progresses, affected miners may have a
chronic cough, sputum production, shortness of breath, and reduced
pulmonary function.
Accelerated silicosis includes both inflammation and fibrosis and
is associated with intense respirable crystalline silica exposure.
Accelerated silicosis usually manifests over a period of 3 to 10 years
(Cowie and Becklake, 2016), but it can develop in as little as 2 to 5
years if exposure is sufficiently intense (Davis, 1996). Accelerated
silicosis may have features of both chronic and acute silicosis (i.e.,
alveolar proteinosis in addition to X-ray evidence of fibrosis).
Although the symptoms are similar to those of chronic silicosis, the
clinical and radiographic progression of accelerated silicosis evolves
more rapidly, and often leads to PMF, severe respiratory impairment,
and respiratory failure. Accelerated silicosis can progress with
associated morbidity and mortality, even if exposure ceases.
Among coal miners, silicosis is usually found in conjunction with
simple coal worker's pneumoconiosis (CWP) (Castranova and Vallyathan,
2000) because of their exposures to RCMD that contains respirable
crystalline silica. Coal miners also face an added risk of developing
mixed-dust pneumoconiosis (MDP) (includes the presence of coal dust
macules), mixed-dust fibrosis (MDF), and/or silicotic nodules (Honma et
al., 2004, see Figure 2, Green 2019). The autopsy studies on coal
miners that MSHA reviewed support a pathological relationship between
mixed-RCMD or respirable crystalline silica exposures and PMF,
silicosis, and CWP (Attfield et al., 1994; Cohen et al., 2016, 2019,
2022; Davis et al., 1979; Douglas et al., 1986; Fernie and Ruckley,
1987; Green et al., 1989, 1998b; Ruckley et al., 1981, 1984; Vallyathan
et al., 2011). Autopsy studies in British coal miners indicated that
the more advanced the disease, the more mixed coal mine dust components
were retained in the lung tissue (Ruckley et al., 1984; Douglas et al.,
1986). Green et al. (1998b) determined that of 4,115 coal miners with
pneumoconiosis autopsied as part of the National Coal Workers' Autopsy
Study (NCWAS), 39 percent had mixed dust nodules and 23 percent had
silicotic nodules.
PMF or ``complicated silicosis'' has been diagnosed in both coal
and MNM miners exposed to dusts containing respirable crystalline
silica. Recent literature on the pathophysiology of PMF supports the
importance of crystalline silica as a cause of PMF in silica-exposed
workers such as coal miners from the United States (Cohen et al., 2016,
2022), sandblasters (Abraham and Wiesenfeld, 1997; Hughes et al.,
1982), industrial sand workers (Vacek et al., 2019), hard rock miners
(Verma et al., 1982, 2008), and gold miners (Carneiro et al., 2006a;
Tse et al., 2007b).

[[Page 44873]]

a. Classifying Radiographic Findings of Silicosis
Two classification methods used to characterize the radiographic
findings of silicosis in chest X-rays are described in this literature
review: the International Labour Office (ILO) Standardized System and
the Chinese categorization system.\16\
---------------------------------------------------------------------------

\16\ The ``Radiological Diagnostic Criteria of Pneumoconiosis
and Principles for Management of Pneumoconiosis'' (GB5906-86) (Chen
et al., 2001; Yang et al., 2006).
---------------------------------------------------------------------------

To describe the presence and severity of pneumoconiosis from chest
X-rays or digital radiographic images, the ILO developed a standardized
system to classify the opacities identified (ILO, 1980, 2002, 2011,
2022). The ILO system grades the size, shape, and profusion (frequency)
of opacities in the lungs. The density of opacities is classified on a
4-point major category scale (category 0, 1, 2, or 3), with each major
category divided into three subcategories, giving a 12-point scale
between 0/- and 3/+. Differences between ILO categories are subtle. For
each subcategory, the top number indicates the major category that the
profusion most closely resembles, and the bottom number indicates the
major category that was given secondary consideration. For example,
film readers may assign classifications such as 1/0, which means the
reader classified it as category 1, but category 0 (normal) was also
considered (ILO, 2022). Major category 0 indicates the absence of
visible opacities and categories 1 to 3 reflect increasing profusion of
opacities and a concomitant increase in severity of disease.
MSHA's analysis of silicosis studies uses NIOSH's surveillance case
definition to determine the presence of silicosis. NIOSH defines the
presence of silicosis in terms of the ILO system and considers a small
opacity profusion score of 1/0 or greater to indicate pneumoconiosis
(NIOSH, 2014b). This definition originated from testimony before
Congress regarding the 1969 Coal Act where the Public Health Service
recommended that miners be removed from dusty environments as soon as
they showed ``minimal effects'' of dust exposure on a chest X-ray
(i.e., pinpoint, dispersed micro-nodular lesions).\17\ MSHA interprets
``minimal effects'' to mean an X-ray ILO profusion score of category 1/
0 or greater.
---------------------------------------------------------------------------

\17\ On March 26, 1969, Charles C. Johnson, Jr., Administrator,
Consumer Protection and Environmental Health Service, PHS, U.S.
Department of Health, Education, and Welfare, testified before the
General Subcommittee on Labor and presented remarks of the Surgeon
General. They are referenced in the 91st Congress House of
Representatives Report, 1st Session No. 91-563, Federal Coal Mine
Health and Safety Act, October 13, 1969 (<a href="https://arlweb.msha.gov/SOLICITOR/COALACT/69hous.htm">https://arlweb.msha.gov/SOLICITOR/COALACT/69hous.htm</a>).
---------------------------------------------------------------------------

However, some studies in MSHA's literature review use the Chinese
categorization scheme, which includes four categories of silicosis: a
suspected case (0+), stage I, stage II, or stage III. The four
categories correspond to ILO profusion category 0/1, category 1,
category 2, and category 3, respectively. A suspected case of silicosis
(0+) in a dust-exposed worker refers to a dust response in the lung and
its corresponding lymph nodes, or a scale and severity of small
opacities that fall short of the level observed in a stage I case of
silicosis (Chen et al., 2001; Yang et al., 2006). Under this scheme, a
panel of three radiologists determines the presence and severity of
radiographic changes consistent with pneumoconiosis.
b. Progression and Associated Impairment
Progression of silicosis is shown when there are changes or
worsening of the opacities in the lungs, and sequential chest
radiographs are classified higher by one or more subcategories (e.g.,
from 1/0 to 1/1) because of changes in the location, thickness, or
extent of lung abnormalities and/or the presence of calcifications. The
higher the category number, the more severe the disease. Due to the
uncertainty in scoring films, some investigators count progression as
advancing two or more subcategories, such as 1/0 to 1/2.
MSHA reviewed studies referenced by OSHA (2013b) that examined the
relationship between exposure and progression, as well as between X-ray
findings and pulmonary function. Additionally, MSHA considered more
recent literature (Dumavibhat et al., 2013; Mohebbi and Zubeyri, 2007;
Wade et al., 2011) not previously reviewed by OSHA (2013b).
Overall, the studies indicate that progression is more likely with
continued exposure, especially high average levels of exposure.
Progression is also more likely for miners with higher ILO profusion
classifications. As discussed previously, progression of disease may
continue after miners are no longer exposed to respirable crystalline
silica (Almberg et al., 2020; Cochrane et al., 1956; Hall et al.,
2020b; Hurley et al., 1987; Kimura et al., 2010; Maclaren et al.,
1985). In addition, although lung function impairment is highly
correlated with chest X-ray films indicating silicosis, researchers
cautioned that respirable crystalline silica exposure could impair lung
function before it is detected by X-ray.
Of the studies in which silicosis progression was documented in
populations of workers, four included quantitative exposure data that
were based on either existing exposure levels or historical
measurements of respirable crystalline silica (Hessel et al., 1988
study of gold miners; Miller and MacCalman, 2010 study of coal miners;
Miller et al., 1998 study of coal miners; Ng et al., 1987a study of
granite miners). In some studies, episodic exposures to high average
concentrations were documented and considered in the analysis. These
exposures were strong predictors of more rapid progression beyond that
predicted by cumulative exposure alone. Otherwise, the variable most
strongly associated in these studies with progression of silicosis was
cumulative respirable crystalline silica exposure (i.e., the product of
the concentration times duration of exposure, which is summed over
time) (Hessel et al., 1988; Ng et al., 1987a; Miller and MacCalman,
2010; Miller et al., 1998). In the absence of concentration
measurements, duration of employment in specific occupations known to
involve exposure to high levels of respirable dust has been used as a
surrogate for cumulative exposure to respirable crystalline silica. It
has also been found to be associated with the progression of silicosis
(Ogawa et al., 2003a).
Miller et al. (1998) examined the impact of high quartz exposures
on silicosis disease progression on 547 British coal miners from 1990
to 1991 and evaluated chest X-ray changes after the mines closed in
1981. The study reviewed chest X-rays taken during health surveys
conducted between 1954 and 1978 and data from extensive exposure
monitoring conducted between 1964 and 1978. For some occupations,
exposure was high because miners had to dig through a sandstone stratum
to reach the coal. For example, quarterly mean respirable crystalline
silica (quartz) concentrations ranged from 1,000 to 3,000 [micro]g/m\3\
(1-3 mg/m\3\), and for a brief period, concentrations exceeded 10,000
[micro]g/m\3\ (10 mg/m\3\) for one job. Some of these high exposures
were associated with accelerated disease progression.
Buchanan et al. (2003) reviewed the exposure history and chest X-
ray progression of 371 retired miners and found that short-term
exposures (i.e., ``a few months'') to high concentrations of respirable
crystalline silica (e.g., >2,000 [mu]g/m\3\, >2 mg/m\3\) increased the
silicosis risk by three-fold (compared to the risk of cumulative
exposure alone) (see the

[[Page 44874]]

separate Preliminary Risk Analysis document).
The risks of increased rate of progression, predicted by Buchanan
et al. (2003) have been seen in coal miners (e.g., Cohen et al., 2016;
Laney et al., 2010, 2017; Miller et al., 1998), metal (Hessel et al.,
1988; Hnizdo and Sluis-Cremer, 1993; Nelson, 2013), and nonmetal miners
such as silica plant and ground silica mill workers, whetstone cutters,
and silica flour packers (Mohebbi and Zubeyri, 2007; NIOSH 2000a,b;
Ogawa et al., 2003a). Accordingly, it is important to limit higher
exposures to respirable crystalline silica in order to minimize the
risk of rapid progressive pneumoconiosis (RPP) in miners.
The results of many surveillance studies conducted by NIOSH as part
of the Coal Workers' Health Surveillance Program indicate that the
pathology of pneumoconiosis in coal miners has changed over time, in
part due to increased exposure to respirable crystalline silica. The
studies of Cohen et al. (2016, 2022) indicate that a RPP develops due
to increased exposure to respirable crystalline silica among
contemporary coal miners as compared to historical coal miners. Through
the examination of pathologic materials from 23 contemporary (born in
or after 1930) and 62 historical coal miners (born between 1910 and
1930) with severe pneumoconiosis, who were autopsied as part of NCWAS,
Cohen et al. (2022) found a significantly higher proportion of silica-
type PMF among contemporary miners (57 percent vs. 18 percent, p
<0.001). They also found that mineral dust alveolar proteinosis (MDAP)
was more common in the current generation of miners and that the lung
tissues of contemporary coal miners contained a significantly greater
percentage and concentration of silica particles than those of past
generations of miners.
c. Occupation-Based Epidemiological Studies
MSHA reviewed the occupation-based epidemiological literature
(i.e., studies that examine health outcomes among workers and their
potential association with conditions in the workplace). MSHA's review
included the occupation-based literature OSHA cited in developing its
respirable crystalline silica standard (OSHA, 2013b). Overall, OSHA
found substantial evidence suggesting that occupational exposure to
respirable crystalline silica increases the risk of silicosis, and MSHA
concurs with this conclusion. MSHA also reviewed additional occupation-
based literature specific to respirable crystalline silica exposure in
MNM and coal miners and preliminarily concludes that respirable
crystalline silica exposure increases the risk of silicosis morbidity
and early mortality. One study examined the acute and accelerated
silicosis outbreak that occurred during and after construction of
Hawk's Nest Tunnel in West Virginia from 1930 to 1931. There, an
estimated 2,500 men worked in a tunnel drilling rock consisting of 90
percent silica or more. The study later estimated that at least 764 of
the 2,500 workers (30.6 percent) died from acute or accelerated
silicosis (Cherniack, 1986). There was also high turnover among the
tunnel workers, with an average length of employment underground of
only about 2 months.
In a population of granite quarry workers (mean length of
employment: 23.4 years) exposed to an average respirable crystalline
silica concentration of 480 [micro]g/m\3\ (0.48 mg/m\3\), 45 percent of
those diagnosed with simple silicosis showed radiological progression
of disease 2 to 10 years after diagnosis (Ng et al., 1987a). Among a
population of gold miners, 92 percent showed progression after 14 years
(Hessel et al., 1988). Chinese factory workers and miners who were
categorized under the Chinese system of X-ray classification as
``suspected'' silicosis cases (analogous to ILO 0/1) had a progression
rate to stage I (analogous to ILO major category 1) of 48.7 percent,
with an average interval of about 5.1 years (Yang et al., 2006).
Strong evidence has shown that lung function deteriorates more
rapidly in miners exposed to respirable crystalline silica, especially
in those with silicosis (Hughes et al., 1982; Ng and Chan, 1992;
Malmberg et al., 1993; Cowie, 1998). The rates of decline in lung
function are greater where disease shows evidence of radiologic
progression (B[eacute]gin et al., 1987; Ng et al., 1987a; Ng and Chan,
1992; Cowie, 1998). The average deterioration of lung function exceeds
that in smokers (Hughes et al., 1982).
Blackley et al. (2015) found progressive lung function impairment
across the range of radiographic profusion of simple CWP in a cohort of
8,230 coal miners that participated in the Enhanced Coal Workers'
Health Surveillance Program from 2005 to 2013. There, 269 coal miners
had category 1 or 2 simple CWP. This study also found that each
increase in profusion score was associated with decreases in various
lung function parameters: 1.5 percent (95 percent CI, 1.0 percent-1.9
percent) in forced expiratory volume in one second (FEV<INF>1</INF>)
percent predicted, 1.0 percent (95 percent CI, 0.6 percent-1.3 percent)
forced vital capacity (FVC) percent predicted, and 0.6 percent (95
percent CI, 0.4 percent-0.8 FEV<INF>1</INF>/FVC).
Overall, MSHA preliminarily agrees with OSHA's conclusion that
substantial evidence suggests that occupational exposure to respirable
crystalline silica increases the risk of silicosis. MSHA also
preliminarily concludes that respirable crystalline silica exposure
increases the risk of silicosis morbidity and early mortality among
miners.
d. Surveillance Data
In addition to occupation-based epidemiological studies, MSHA
reviewed surveillance studies, which provide and interpret data to
facilitate the prevention and control of disease, and preliminarily
finds that the prevalence of silicosis generally increases with
duration of exposure (work tenure). However, the available statistics
may underestimate silicosis-related morbidity and mortality in miners.
For example, the following have been reported: (1) misclassification of
causes of death (e.g., as TB, chronic bronchitis, emphysema, or cor
pulmonale); (2) errors in recording occupation on death certificates;
and (3) misdiagnosis of disease (Windau et al., 1991; Goodwin et al.,
2003; Rosenman et al., 2003, Blackley et al., 2017). Furthermore, chest
X-ray findings may lead to missed silicosis cases when fibrotic changes
in the lung are not yet visible on chest X-rays. In other words,
silicosis may be present but not yet detectable by chest X-ray, or may
be more severe than indicated by the assigned profusion score
(Craighead and Vallyathan, 1980; Hnizdo et al., 1993; Rosenman et al.,
1997).
e. Pulmonary Tuberculosis
Finally, in addition to the relationship between silica exposure
and silicosis, studies indicate a relationship between silica exposure,
silicosis, and pulmonary TB. OSHA reviewed these and concluded that
silica exposure and silicosis increase the risk of pulmonary TB (Cowie,
1994; Hnizdo and Murray, 1998; teWaterNaude et al., 2006). MSHA agrees
with this conclusion.
Although early descriptions of dust diseases of the lung did not
distinguish between TB and silicosis and most fatal cases described in
the first half of the 20th century were likely a combination of
silicosis and TB (Castranova et al., 1996), more recent findings have
demonstrated that respirable crystalline silica exposure, even without
silicosis, increases the risk of infectious (i.e., active) pulmonary TB
(Sherson and

[[Page 44875]]

Lander, 1990; Cowie, 1994; Hnizdo and Murray, 1998; teWaterNaude et
al., 2006). These co-morbid conditions hasten the development of
respiratory impairment and increased mortality risk even beyond the
risk in unexposed persons with active TB (Banks, 2005).
Ng and Chan (1991) hypothesized that silicosis and TB ``act
synergistically'' (i.e., are more than additive) to increase fibrotic
scar tissue (leading to massive fibrosis) or to enhance susceptibility
to active mycobacterial infection. The authors found that lung fibrosis
is common to both diseases, and that both diseases decrease the ability
of alveolar macrophages to aid in the clearance of dust or infectious
particles.
These findings are also supported by new studies (Ndlovu et al.,
2019; Oni and Ehrlich, 2015) published since OSHA's review (2013b). Oni
and Ehrlich (2015) reviewed a case of silico-TB in a former gold miner
with ILO category 2/2 silicosis. Ndlovu et al. (2019) found that in a
study sample of South African gold miners who had died from causes
other than silicosis between 2005 and 2015, 33 percent of men (n = 254)
and 43 percent of women (n = 29) at autopsy were found to have TB,
whereas 7 percent of men (n = 54) and 3 percent of women (n = 4) were
found to have pulmonary silicosis.
Overall, MSHA agrees with OSHA's conclusion that silica exposure
increases the risk of pulmonary TB and that pulmonary TB is a
complication of chronic silicosis.
2. Nonmalignant Respiratory Disease (Excluding Silicosis)
In addition to causing silicosis (acute silicosis, accelerated
silicosis, simple chronic silicosis, and PMF), exposure to respirable
crystalline silica causes other NMRD. NMRD includes emphysema and
chronic bronchitis, which are both diagnoses within the category of
COPD. Patients with COPD may have chronic bronchitis, emphysema, or
both (ATS, 2010a).
Based on its review of the literature, MSHA preliminarily concludes
that exposure to respirable crystalline silica increases the risk for
mortality from NMRD. The following summarizes MSHA's review of the
literature.
a. Emphysema
Emphysema involves the destruction of lung architecture in the
alveolar region, causing airway obstruction and impaired gas exchange.
In its literature review, OSHA (2013b) concluded that exposure to
respirable crystalline silica can increase the risk of emphysema,
regardless of whether silicosis is present. OSHA also concluded that
this is the case for smokers and that smoking amplifies the effects of
respirable crystalline silica exposure, increasing the risk of
emphysema. MSHA reviewed the studies cited by OSHA and agrees with its
conclusion. The studies reviewed are summarized below.
Becklake et al. (1987) determined that a miner who had worked in a
high dust environment for 20 years had a greater chance of developing
emphysema than a miner who had never worked in a high dust environment.
In a retrospective cohort study, Hnizdo et al. (1991a) used autopsy
lung specimens from 1,553 white gold miners to investigate the types of
emphysema caused by respirable crystalline silica and found that the
occurrence of emphysema was related to both smoking and dust exposure.
This study also found a significant association between emphysema (both
panacinar and centriacinar emphysema types) and length of employment
for miners working in high dust occupations. A separate study by Hnizdo
et al. (1994) on life-long non-smoking South African gold miners found
that the degree of emphysema was significantly associated with the
degree of hilar gland nodules, which the authors suggested might serve
as a surrogate for respirable crystalline silica exposure. While Hnizdo
et al. (2000) conversely found that emphysema prevalence was decreased
in relation to dust exposure, the authors suggested that selection bias
was responsible for this finding.
The findings of several cross-sectional and case-control studies
discussed in the OSHA (2013b) Health Effects Literature were more
mixed. For example, de Beer et al. (1992) found an increased risk for
emphysema; however, the reported odds ratio (OR) was smaller than
previously reported by Becklake et al. (1987).
The OSHA (2013b) Health Effects Literature also recognized that
several of the referenced studies (Becklake et al., 1987 Hnizdo et al.,
1994) found that emphysema might occur in respirable crystalline
silica-exposed workers who did not have silicosis and suggested a
causal relationship between respirable crystalline silica exposure and
emphysema. Experimental (animal) studies found that emphysema occurred
at lower respirable crystalline silica exposure concentrations than
fibrosis in the airways or the appearance of early silicotic nodules
(Wright et al., 1988). These findings tended to support human studies
that respirable crystalline silica-induced emphysema can occur absent
signs of silicosis.
Green and Vallyathan (1996) reviewed several studies of emphysema
in workers exposed to silica and found an association between
cumulative dust exposure and death from emphysema. The IARC (1997) also
reviewed several studies and concluded that exposure to respirable
crystalline silica increases the risk of emphysema. Finally, NIOSH
(2002b) concluded in its Hazard Review that occupational exposure to
respirable crystalline silica is associated with emphysema. However,
some epidemiological studies suggested that this effect might be less
frequent or absent in non-smokers.
Overall, MSHA agrees with OSHA that exposure to respirable
crystalline silica causes emphysema even in the absence of silicosis.
b. Chronic Bronchitis
Chronic bronchitis is long-term inflammation of the bronchi,
increasing the risk of lung infections. This condition develops slowly
by small increments and ``exists'' when it reaches a certain stage
(i.e., the presence of a productive cough sputum production for at
least 3 months of the year for at least 2 consecutive years) (ATS,
2010b).
OSHA considered many studies that examined the association between
respirable crystalline silica exposure and chronic bronchitis,
concluding the following: (1) exposure to respirable crystalline silica
causes chronic bronchitis regardless of whether silicosis is present;
(2) an exposure-response relationship may exist; and (3) smokers may be
at an increased risk of chronic bronchitis compared to non-smokers.
MSHA has reviewed the literature and agrees with OSHA's conclusions.
Miller et al. (1997) reported a 20 percent increased risk of
chronic bronchitis in a British mining cohort compared to the disease
occurrence in the general population. Using British pneumoconiosis
field research data, Hurley et al. (2002) calculated estimates of
mixed-RCMD-related disease in British coal miners at exposure levels
that were common in the late 1980s and related their lung function and
development of chronic bronchitis with their cumulative dust exposure.
The authors estimated that by the age of 58, 5.8 percent of these men
would report breathlessness for every 100 gram-hour/m\3\ dust exposure.
The authors also estimated the prevalence of chronic bronchitis at age
58 would be 4 percent per 100 gram-hour/m\3\ of dust exposure. These
miners averaged over 35 years of tenure in mining and a cumulative
respirable dust exposure of 132 gram-hour/m\3\.
Cowie and Mabena (1991) found that chronic bronchitis was present
in 742 of

[[Page 44876]]

1,197 (62 percent) black South African gold miners, and Ng et al.
(1992b) found a higher prevalence of respiratory symptoms, independent
of smoking and age, in Singaporean granite quarry workers exposed to
high levels of dust (rock drilling and crushing) compared to those
exposed to low levels of dust (maintenance and transport workers).
However, Irwig and Rocks (1978) compared symptoms of chronic bronchitis
in silicotic and non-silicotic South African gold miners and did not
find as clear a relationship as did the above studies, concluding that
the symptoms were not statistically more prevalent in the silicotic
miners, although prevalence was slightly higher.
Sluis-Cremer et al. (1967) found that dust-exposed male smokers had
a higher prevalence of chronic bronchitis than non-dust exposed smokers
in a gold mining town in South Africa. Similarly, Wiles and Faure
(1977) found that the prevalence of chronic bronchitis rose
significantly with increasing dust concentration and cumulative dust
exposure in South African gold miners of smokers, nonsmokers, and ex-
smokers. Rastogi et al. (1991) found that female grinders of agate
stones in India had a significantly higher prevalence of acute
bronchitis, but they had no increase in the prevalence of chronic
bronchitis compared to controls matched by socioeconomic status, age,
and smoking. However, the study noted that respirable crystalline
silica exposure durations were very short, and control workers may also
have been exposed to respirable crystalline silica.
Studies examining the effect of years of mining on chronic
bronchitis risk were mixed. Samet et al. (1984) found that prevalence
of symptoms of chronic bronchitis was not associated with years of
mining in a population of underground uranium miners, even after
adjusting for smoking. However, Holman et al. (1987) studied gold
miners in West Australia and found that the prevalence of chronic
bronchitis, as indicated by ORs (controlled for age and smoking), was
significantly increased in those that had worked in the mines for over
1 year, compared to lifetime non-miners. In addition, while other
studies found no effect of years of mining on chronic bronchitis risk,
those studies often qualified this result with possible confounding
factors. For example, Kreiss et al. (1989) studied 281 hard-rock
(molybdenum) miners and 108 non-miner residents of Leadville, Colorado.
They did not find an association between the prevalence of chronic
bronchitis and work in the mining industry (Kreiss et al., 1989);
however, it is important to note that the mine had been temporarily
closed for 5 months when the study began, so miners were not exposed at
the time of the study.
The American Thoracic Society (ATS) (1997) published a review
finding chronic bronchitis to be common among worker groups exposed to
dusty environments contaminated with respirable crystalline silica.
NIOSH (2002b) also published a review finding that occupational
exposure to respirable crystalline silica has been associated with
bronchitis; however, some epidemiological studies suggested this effect
might be less frequent or absent in non-smokers.
Finally, Hnizdo et al. (1990) found an independent exposure-
response relationship between respirable crystalline silica exposure
and impaired lung function. For miners with less severe impairment, the
effects of smoking and dust together were additive. However, for miners
with the most severe impairment, the effects of smoking and dust were
synergistic (i.e., more than additive).
Overall, MSHA agrees with OSHA's conclusion that exposure to
respirable crystalline silica causes chronic bronchitis regardless of
whether silicosis is present and that an exposure-response relationship
may exist.
c. Pulmonary Function Impairment
Pulmonary function impairment, generally defined as reduction below
the lower limit of normal predicted by reference equations (and in
older literature as less than 80 percent predicted) of diffusion
capacity for carbon monoxide (DLCOcSB), total lung capacity (TLC), FVC,
or FEV<INF>1</INF> is also a common condition of NMRD. Based on its
review of the evidence in numerous longitudinal and cross-sectional
studies and reviews, OSHA concluded that there is an exposure-response
relationship between respirable crystalline silica and the development
of impaired lung function. OSHA also concluded that the effect of
tobacco smoking on this relationship may be additive or synergistic,
and workers who were exposed to respirable crystalline silica but did
not show signs of silicosis may also have pulmonary function
impairment. MSHA has reviewed the studies cited by OSHA and agrees with
their conclusions.
OSHA reviewed several longitudinal studies regarding the
relationship between respirable crystalline silica exposure and
pulmonary function impairment. To evaluate whether exposure to silica
affects pulmonary function in the absence of silicosis, the studies
focused on workers who did not exhibit progressive silicosis.
Among both active and retired Vermont granite workers exposed to an
average quartz dust exposure level of 60 [micro]g/m\3\, researchers
found no exposure-related decreases in pulmonary function (Graham et
al., 1981, 1994). However, Eisen et al. (1995) found significant
pulmonary decrements among a subset of granite workers who left work
and consequently did not voluntarily participate in the last of a
series of annual pulmonary function tests (termed ``dropouts''). This
group experienced steeper declines in lung function compared to the
subset of workers who remained at work and participated in all tests
(termed ``survivors''), and these declines were significantly related
to dust exposure. Exposure-related changes in lung function were also
reported in a 12-year study of granite workers (Malmberg et al., 1993),
in two 5-year studies of South African miners (Hnizdo, 1992; Cowie,
1998), and in a study of foundry workers whose lung function was
assessed between 1978 and 1992 (Hertzberg et al., 2002). Similar
reductions in FEV<INF>1</INF> (indicating an airway obstruction) were
linked to respirable crystalline silica exposure.
Each of these studies reported their findings in terms of rates of
decline in any of several pulmonary function measures (e.g.,
FEV<INF>1</INF>, FVC, FEV<INF>1</INF>/FVC). To put these declines in
perspective, Eisen et al. (1995) reported that the rate of decline in
FEV<INF>1</INF> seen among the dropout subgroup of Vermont granite
workers was 4 ml per 1,000 [micro]g/m\3\-year (4 ml per mg/m\3\-year)
of exposure to respirable granite dust. By comparison, FEV<INF>1</INF>
declines at a rate of 10 ml/year from smoking one pack of cigarettes
daily. From their study of foundry workers, Hertzberg et al. (2002)
reported a 1.1 ml/year decline in FEV<INF>1</INF> and a 1.6 ml/year
decline in FVC for each 1,000 [micro]g/m\3\-year (1 mg/m\3\-year) of
respirable crystalline silica exposure after controlling for ethnicity
and smoking. From these rates of decline, they estimated that exposure
to 100 [micro]g/m\3\ of respirable crystalline silica for 40 years
would result in a total loss of FEV<INF>1</INF> and FVC that was less
than, but still comparable to, smoking a pack of cigarettes daily for
40 years. Hertzberg et al. (2002) also estimated that exposure to the
existing MSHA standard (100 [micro]g/m\3\) for 40 years would increase
the risk of developing abnormal FEV<INF>1</INF> or FVC by factors of
1.68 and 1.42, respectively.
OSHA reviewed cross-sectional studies that described relationships
between lung function loss and respirable crystalline silica exposure
or

[[Page 44877]]

exposure measurement surrogates (e.g., tenure). The results of these
studies were similar to those longitudinal studies already discussed.
In several studies, respirable crystalline silica exposure was found to
reduce lung function of:
<bullet> White South African gold miners (Hnizdo et al., 1990),
<bullet> Black South African gold miners (Cowie and Mabena, 1991;
Irwig and Rocks, 1978),
<bullet> Respirable crystalline silica-exposed workers in Quebec
(B[eacute]gin et al., 1995),
<bullet> Rock drilling and crushing workers in Singapore (Ng et
al., 1992b),
<bullet> Granite shed workers in Vermont (Theriault et al., 1974a,
1974b),
<bullet> Aggregate quarry workers and coal miners in Spain (Montes
et al., 2004a, 2004b),
<bullet> Concrete workers in the Netherlands (Meijer et al., 2001),
<bullet> Chinese refractory brick manufacturing workers in an iron-
steel plant (Wang et al., 1997),
<bullet> Chinese gemstone workers (Ng et al., 1987b),
<bullet> Hard-rock miners in Manitoba, Canada (Manfreda et al.,
1982) and in Colorado (Kreiss et al., 1989),
<bullet> Pottery workers in France (Neukirch et al., 1994),
<bullet> Potato sorters in the Netherlands (Jorna et al., 1994),
<bullet> Slate workers in Norway (Suhr et al., 2003), and
<bullet> Men in a Norwegian community with years of occupational
exposure to respirable crystalline silica (quartz) (Humerfelt et al.,
1998).
The OSHA (2013b) Health Effects Literature recognized that many of
these studies found that pulmonary function impairment: (1) can occur
in respirable crystalline silica-exposed workers without silicosis, (2)
was still observable when controlling for silicosis in the analysis,
and (3) was related to the magnitude and duration of respirable
crystalline silica exposure, rather than to the presence or severity of
silicosis. Many other studies in the OSHA (2013b) Health Effects
Literature have also found a relationship between respirable
crystalline silica exposure and lung function impairment, including
IARC (1997), the ATS (1997), and Hnizdo and Vallyathan (2003).
MSHA reviewed the studies and agrees with OSHA's finding that there
is an exposure-response relationship between respirable crystalline
silica and the impairment of lung function. MSHA also agrees with
OSHA's finding that the effect of tobacco smoking on this relationship
may be additive or synergistic, and that workers who were exposed to
respirable crystalline silica, but did not show signs of silicosis, may
also have pulmonary function impairment.
3. Carcinogenic Effects
a. Lung Cancer
Lung cancer, an irreversible and usually fatal disease, is a type
of cancer that forms in lung tissue. Agreeing with the conclusion of
other government and public health organizations that respirable
crystalline silica is a ``known human carcinogen,'' MSHA has
preliminarily found that the scientific literature supports that
respirable crystalline silica exposure significantly increases the risk
of lung cancer mortality among miners. This determination is consistent
with the conclusions of other government and public health
organizations, including the IARC (1997b, 2012), the NTP (2000, 2016),
NIOSH (2002b), the ATS (1997), and the American Conference of
Governmental Industrial Hygienists (ACGIH[supreg], (2010)). The
Agency's determination is supported by epidemiological literature,
encompassing more than 85 studies of occupational cohorts from more
than a dozen industrial sectors including: granite/stone quarrying and
processing (Carta et al., 2001; Attfield and Costello, 2004; Costello
et al., 1995; Gu[eacute]nel et al., 1989a,b), industrial sand
(Sanderson et al., 2000; Hughes et al., 2001; McDonald et al., 2001,
2005; Rando et al., 2001; Steenland and Sanderson, 2001), MNM mining
(Steenland and Brown, 1995a; deKlerk and Musk, 1998; Roscoe et al.,
1995; Hessel et al., 1986, 1990; Hnizdo and Sluis-Cremer, 1991; Reid
and Sluis-Cremer, 1996; Hnizdo et al., 1997; Chen et al., 1992;
McLaughlin et al., 1992; Chen and Chen, 2002; Chen et al., 2006;
Schubauer-Berigan et al., 2009; Hua et al., 1994; Meijers et al., 1991;
Finkelstein 1998; Chen et al., 2012; Liu et al., 2017a; Wang et al.,
2020a,b; Wang et al., 2021), coal mining (Meijers et al., 1988; Miller
et al., 2007; Miller and MacCalman, 2010; Miyazaki and Une, 2001;
Graber et al., 2014a,b; Tomaskova et al., 2012, 2017, 2020, 2022; Kurth
et al., 2020), pottery (Winter et al., 1990; McLaughlin et al., 1992;
McDonald et al., 1995), ceramic industries (Starzynski et al., 1996),
diatomaceous earth (Checkoway et al., 1993, 1996, 1997, 1999; Seixas et
al., 1997; Rice et al., 2001), and refractory brick industries
(cristobalite exposures) (Dong et al., 1995).
The strongest evidence comes from the worldwide cohort and case-
control studies reporting excess lung cancer mortality among workers
exposed to respirable crystalline silica in various industrial sectors,
confirmed by the 10-cohort pooled case-control analysis by Steenland et
al. (2001a), the more recent pooled case-control analysis of seven
European countries by Cassidy et al. (2007), and two national death
certificate registry studies (Calvert et al., 2003 in the United
States; Pukkala et al., 2005 in Finland).
Recent studies examined lung cancer mortality among coal and non-
coal miners (Meijers et al., 1988, 1991; Starzynski et al., 1996;
Miyazaki and Une, 2001; Tomaskova et al., 2012, 2017, 2020, 2022;
Attfield and Kuempel, 2008; Graber et al., 2014a, 2014b; Kurth et al.,
2020; NIOSH, 2019a). These studies also discuss the associations
between RCMD and respirable crystalline silica exposures with lung
cancer in coal mining populations. Furthermore, these newer studies are
consistent with the conclusion of OSHA's final Quantitative Risk
Assessment (QRA) (2016a) that respirable crystalline silica is a human
carcinogen. MSHA preliminarily concludes that miners, both MNM and coal
miners, are at risk of developing lung cancer due to their occupational
exposure to respirable crystalline silica.
In addition, based on its review of the literature, MSHA has
preliminarily determined that radiographic silicosis is a marker for
lung cancer risk. Reducing exposure to levels that lower the silicosis
risk would reduce the lung cancer risk to exposed miners (Finkelstein,
1995, 2000; Brown, 2009). MSHA has also found that, based on the
available epidemiological and animal data, respirable crystalline
silica causes lung cancer (IARC, 2012; RTECS, 2016; ATSDR, 2019).
Miners who inhale respirable crystalline silica over time are at
increased risk of developing silicosis and lung cancer (Greaves, 2000;
Erren et al., 2009; Tomaskova et al., 2017, 2020, 2022).
Toxicity studies provide additional evidence of the carcinogenic
potential of respirable crystalline silica. Studies using DNA exposed
directly to freshly fractured respirable crystalline silica demonstrate
the direct effect respirable crystalline silica had on DNA breakage.
Cell culture research has investigated the processes by which
respirable crystalline silica disrupt normal gene expression and
replication. Studies have demonstrated that chronic inflammatory and
fibrotic processes resulting in oxidative and cellular damage may lead
to neoplastic changes in the lung (Goldsmith, 1997). In addition, the
biologically damaging physical characteristics of respirable
crystalline silica and its direct and indirect

[[Page 44878]]

genotoxicity (Schins et al., 2002; Borm and Driscoll, 1996) support
MSHA's preliminary determination that respirable crystalline silica is
an occupational carcinogen.
b. Cancers of Other Sites
In addition to lung cancer, OSHA reviewed studies examining the
relationship between silica exposure and cancers at other sites. MSHA
notes that OSHA reviewed these mortality studies (e.g., cancer of the
larynx and the digestive system, including the stomach and esophagus)
and found that studies suggesting a dose-response relationship were too
limited in terms of size, study design, or potential for confounding
variables to be conclusive. OSHA also pointed to the NIOSH (2002b)
silica (respirable crystalline silica) hazard review, which concluded
that no association has been established between respirable crystalline
silica exposure and excess mortality from cancer at other sites. MSHA
has reviewed these studies and agrees with OSHA's conclusion. The
following summarizes the studies reviewed with inconclusive findings.
(1) Laryngeal Cancer
Three lung cancer studies (Checkoway et al., 1997; Davis et al.,
1983; McDonald et al., 2001) included in OSHA's health literature
review suggest an association between respirable crystalline silica
exposure and increased mortality from laryngeal cancer. However, a
small number of cases were reported and researchers were unable to
determine a statistically significant effect. Therefore, there is
little evidence of an association based on these studies.
(2) Gastric (Stomach) Cancer
OSHA reviewed several studies in its 2013b health literature review
to assess a potential relationship between respirable crystalline
silica exposures and stomach cancers. OSHA's literature review noted
observations made previously by Cocco et al. (1996) and in the NIOSH
respirable crystalline silica hazard review (2002b), which found that
most epidemiological studies of respirable crystalline silica and
stomach cancer did not sufficiently adjust for the effects of
confounding factors. In addition, some of these studies were not
properly designed to assess a dose-response relationship (e.g.,
Finkelstein and Verma, 2005; Moshammer and Neuberger, 2004; Selikoff,
1978; Stern et al., 2001) or did not demonstrate a statistically
significant dose-response relationship (e.g., Calvert et al., 2003;
Tsuda et al., 2001). For these reasons, MSHA determined these studies
were inconclusive in the context of this rulemaking.
(3) Esophageal Cancer
OSHA considered several studies that examined the relationship
between respirable crystalline silica exposures and esophageal cancer
and found that the studies were limited in terms of size, study design,
or potential for confounding variables. Three nested case-control
studies of Chinese workers demonstrated a dose-response association
between increased risk of esophageal cancer mortality and respirable
crystalline silica exposure (Pan et al., 1999; Wernli et al., 2006; Yu
et al., 2005). Other studies (Tsuda et al., 2001; Xu et al., 1996a)
also indicated elevated rates of esophageal cancer mortality with
respirable crystalline silica exposure. However, OSHA noted that
confounding factors due to other occupational exposures was possible.
Additionally, two large national mortality studies in Finland and the
United States did not show a positive association between respirable
crystalline silica exposure and esophageal cancer mortality (Calvert et
al., 2003; Weiderpass et al., 2003). MSHA agrees with OSHA's conclusion
that the literature does not support attributing increased esophageal
cancer mortality to exposure to respirable crystalline silica.
(4) Other Sites
NIOSH (2002b) conducted a health literature review of the health
effects potentially associated with respirable crystalline silica
exposure, which identified only infrequent reports of statistically
significant excesses of deaths for other cancers. Cancer studies have
been reported in the following organs/systems: salivary gland, liver,
bone, pancreas, skin, lymphopoietic or hematopoietic, brain, and
bladder (see NIOSH, 2002b for full bibliographic references). However,
the findings were not observed consistently among epidemiological
studies, and NIOSH (2002b) concluded that no association has been
established between these cancers and respirable crystalline silica
exposure. OSHA concurred with NIOSH that these isolated reports of
excess cancer mortality were insufficient to determine the role of
respirable crystalline silica exposure.
Overall, OSHA concluded that evidence of an association between
silica exposure and cancer at sites other than the lungs is not
sufficient. MSHA agrees with OSHA's conclusion.
4. Renal Disease
Renal disease is characterized by the loss of kidney function, and
in the case of ESRD, the need for a regular course of long-term
dialysis or a kidney transplant. MSHA reviewed a wide variety of
longitudinal and mortality epidemiological studies, including case
series, case-control, and cohort studies, as well as case reports, and
preliminarily concludes that respirable crystalline silica exposure
increases the risk of morbidity and/or mortality related to ESRD.
However, MSHA notes that the available literature on respirable
crystalline silica exposures and renal disease in coal miners is less
conclusive than the literature related to MNM miners.
Epidemiological studies have found statistically significant
associations between occupational exposure to respirable crystalline
silica and chronic renal disease (e.g., Calvert et al., 1997), sub-
clinical renal changes, including proteinuria and elevated serum
creatinine (e.g., Ng et al., 1992a; Hotz et al., 1995; Rosenman et al.,
2000), ESRD morbidity (e.g., Steenland et al., 1990), ESRD mortality
(Steenland et al., 2001b, 2002a), and Wegener's granulomatosis (Nuyts
et al., 1995) (severe injury to the glomeruli that, if untreated,
rapidly leads to renal failure). The pooled analysis conducted by
Steenland et al. (2002a) is particularly convincing because it involved
a large number of workers from three combined cohorts and had well-
documented, validated job exposure matrices. Steenland et al. (2002a)
found a positive and monotonic exposure-response trend for both
multiple-cause mortality and underlying cause data. MSHA has
preliminarily determined that the underlying data from Steenland et al.
(2002a) are sufficient to provide useful estimates of risk.
Possible mechanisms suggested for respirable crystalline silica-
induced renal disease include: (1) a direct toxic effect on the kidney,
(2) a deposition in the kidney of immune complexes (e.g.,
Immunoglobulin A (IgA), an antibody blood protein) in the kidney
following respirable crystalline silica-related pulmonary inflammation,
and (3) an autoimmune mechanism (Gregorini et al., 1993; Calvert et
al., 1997). Steenland et al. (2002a) demonstrated a positive exposure-
response relationship between respirable crystalline silica exposure
and ESRD mortality.
Overall, MSHA preliminarily determines that respirable crystalline
silica exposure in mining increases the risk of renal disease.

[[Page 44879]]

5. Autoimmune Disease
Autoimmune diseases occur when the immune system mistakenly attacks
healthy tissues within the body, causing inflammation, swelling, pain,
and tissue damage. Examples include rheumatoid arthritis (RA), systemic
lupus erythematosus (SLE), scleroderma, and systemic sclerosis (SSc).
Based on its literature review, MSHA preliminarily concludes that there
is a causal association between occupational exposure to respirable
crystalline silica and the development of systemic autoimmune diseases
in miners. However, no studies are available to date that can be used
to model respirable crystalline silica-exposure risk in a formal
quantitative risk analysis.
Wallden et al. (2020) found that respirable crystalline silica
exposure is correlated with an increased risk of developing ulcerative
colitis, which increases with duration of exposure (work tenure) and
the level of exposure. This effect was especially significant in men.
Schmajuk et al. (2019) found that RA was significantly associated with
coal mining and other non-coal occupations exposed to respirable
crystalline silica. Finally, Vihlborg et al. (2017) found a significant
increased risk of seropositive RA with high exposure (>0.048 mg/m\3\)
to respirable crystalline silica dust when compared to individuals with
no or lower exposure by examining detailed exposure-response
relationships across four different respirable crystalline silica dose
groups (quartiles): <23 [micro]g/m\3\, 24 to 35 [micro]g/m\3\, 36 to 47
[micro]g/m\3\, and >48 [micro]g/m\3\. However, these researchers did
not report the risk of sarcoidosis and seropositive RA in relation to
respirable crystalline silica exposure using logistic regressions
resulting in models that could be used in the risk assessment. In
addition, the meta-analysis of 19 published case-control and cohort
studies on scleroderma by Rubio-Rivas et al. (2017) found statistically
significant risks among individuals exposed to respirable crystalline
silica, solvents, silicone, breast implants, epoxy resins, pesticides,
and welding fumes, but did not provide detailed quantitative exposure
information.
C. Conclusion
MSHA preliminarily concludes that occupational exposure to
respirable crystalline silica causes silicosis (acute silicosis,
accelerated silicosis, simple chronic silicosis, and PMF), NMRD
(including COPD), lung cancer, and kidney disease. Each of these
effects is exposure-dependent, chronic, irreversible, potentially
disabling, and can be fatal. MSHA suspects that respirable crystalline
silica exposure is also linked to the development of some autoimmune
disorders through inflammation pathways.
The scientific literature (including peer-reviewed medical,
toxicological, public health, and other related disciplinary
publications) is robust and compelling. It shows that miners exposed to
the existing respirable crystalline silica limit of 100 [mu]g/m\3\
still have an unacceptable amount of excess risk for developing and
dying from diseases related to occupational respirable crystalline
silica exposures and still suffer material impairments of health or
functional capacity.

VI. Preliminary Risk Analysis Summary

MSHA's preliminary risk analysis (PRA) quantifies risks associated
with five specific health outcomes identified in the separate,
standalone Health Effects document: silicosis morbidity and mortality,
and mortality from NMRD, lung cancer, and ESRD. The standalone
document, entitled Preliminary Risk Analysis (PRA document), has been
placed into the rulemaking docket for the MSHA respirable crystalline
silica rulemaking (RIN 1219-AB36, Docket ID no. MSHA-2023-0001) and is
available on MSHA's website.
MSHA developed a PRA to support the risk determinations required to
set an exposure limit for a toxic substance under the Mine Act. MSHA's
PRA quantifies the health risk to miners exposed to respirable
crystalline silica under the existing exposure limits for MNM and coal
miners, at the proposed PEL of 50 [mu]g/m\3\, and at the proposed
action level of 25 [mu]g/m\3\.
This analysis addresses three questions related to the proposed
rule:
(1) whether potential health effects associated with existing
exposure conditions constitute material impairment to any miner's
health or functional capacity;
(2) whether existing exposure conditions place miners at risk of
incurring any material impairment if regularly exposed for the period
of their working life; and
(3) whether the proposed rule would reduce those risks.
To answer these questions, MSHA relied on the large body of
research on the health effects of respirable crystalline silica and
several published, peer-reviewed, quantitative risk assessments that
describe the risk of exposed workers to silicosis mortality and
morbidity, NMRD mortality, lung cancer mortality, and ESRD mortality.
These assessments are based on several studies of occupational cohorts
in a variety of industrial sectors. The underlying studies are
described in the Health Effects document and are summarized in Section
V. Health Effects Summary of this preamble.
This summary highlights the main findings from the PRA, briefly
describes how they were derived, and directs readers interested in more
detailed information to corresponding sections of the standalone PRA
document.

A. Summary of MSHA's Preliminary Risk Analysis Process and Methods

MSHA evaluated the literature and selected an exposure-response
model for each of the five health endpoints--silicosis morbidity,
silicosis mortality, NMRD mortality, lung cancer mortality, and ESRD
mortality. The selected exposure-response models were used to estimate
lifetime excess risks and lifetime excess cases among the current
population of MNM and coal miners based on real exposure conditions, as
indicated by the samples in the compliance sampling datasets.
MSHA's PRA is largely based on the methodology and findings from a
peer-reviewed January 2013 OSHA preliminary quantitative risk
assessment (PQRA) and associated analysis of health effects in
connection with OSHA's promulgation of a rule setting PELs for
workplace exposure to respirable crystalline silica. OSHA's PQRA
presented quantitative relationships between respirable crystalline
silica exposure and multiple health endpoints. Following multiple legal
challenges, the U.S. Court of Appeals for the D.C. Circuit rejected
challenges to OSHA's risk assessment methodology and its findings on
different health risks. N. Am.'s Bldg. Trades Unions v. OSHA, 878 F.3d
271, 283-89 (D.C. Cir. 2017).
MSHA's PRA presents detailed quantitative analyses of health risks
over a range of exposure concentrations that have been observed in MNM
and coal mines. MSHA applied exposure-response models to estimate the
respirable crystalline silica-related risk of material impairment of
health or functional capacity of miners exposed to respirable
crystalline silica at three levels--(1) the existing standards, (2) the
proposed PEL, and (3) the proposed action level. As in past MSHA
rulemakings, MSHA estimated and compared lifetime excess risks
associated with exposures at the existing and proposed PEL (and at the
proposed action level) over a miner's full working life of 45 years.

[[Page 44880]]

MSHA's PRA is also based on a compilation of miner exposure data to
respirable crystalline silica. For the MNM sector, MSHA evaluated
57,769 valid respirable dust samples collected between January 2005 and
December 2019; and for the coal sector, MSHA evaluated 63,127 valid
respirable dust samples collected between August 2016 and July 2021.
The compiled data set characterizes miners' exposures to respirable
crystalline silica in various locations (e.g., underground, surface),
occupations (e.g., drillers, underground miners, equipment operators),
and commodities (e.g., metal, nonmetal, stone, crushed limestone, sand
and gravel, and coal). MSHA enforcement sampling indicates a wide range
of exposure concentrations. These include exposures from below the
proposed action level (25 [mu]g/m\3\) to above the existing standards
(100 [mu]g/m\3\ in MNM standards, 100 [mu]g/m\3\ MRE in coal standards,
which is approximately 85.7 [mu]g/m\3\ ISO).\18\
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\18\ As discussed in the PRA, the existing PEL for coal is 100
[mu]g/m\3\ MRE, measured as a full-shift time-weighted average
(TWA). To calculate risks consistently for both coal and MNM miners,
the PRA converts the MRE full-shift TWA concentrations experienced
by coal miners to ISO 8-hour TWA concentrations. (See Section 4 of
the PRA document for a full explanation.) The equation used to
convert MRE full-shift TWA concentrations into ISO 8-hour TWA
concentrations is:
ISO 8-hour TWA concentration = (MRE TWA) x (original sampling
time)/(480 minutes) x 0.857
Exposures at TWA 100 [mu]g/m\3\ MRE and SWA 85.7 [mu]g/m\3\ ISO
are only equivalent when the sampling duration is 480 minutes (eight
hours). However, for the sake of simplicity and for comparison
purposes, the risk analysis approximates exposures at the existing
coal exposure limit of 100 MRE [mu]g/m\3\ as 85.7 [mu]g/m\3\ ISO.
Thus, ISO concentration values (measured as an 8-hour TWA) were used
as the exposure metric when (a) calculating risk under the
assumption of full compliance with the existing standards and (b)
calculating risk under the assumption that no exposure exceeds the
proposed PEL of 50 [mu]g/m\3\. To simulate compliance among coal
miners at the existing exposure limit, exposures were capped at 85.7
[mu]g/m\3\ measured as an ISO 8-hour TWA.
---------------------------------------------------------------------------

The primary results of the PRA are the calculated number of deaths
and illnesses avoided assuming full compliance after implementation of
MSHA's proposed rule. These calculations were performed for non-fatal
silicosis illnesses (morbidity) and for deaths (mortality) due to
silicosis, lung cancer, NMRD, and ESRD. For each health outcome, the
reduced number of illnesses or deaths is calculated as the difference
between (a) the number of illnesses and deaths currently occurring in
the industry, assuming mines fully comply with the existing standards
(100 [mu]g/m\3\ for MNM and 85.7 [mu]g/m\3\ ISO for coal) and (b) the
number of deaths and illnesses expected to occur following
implementation of the proposed rule, which includes a proposed PEL of
50 [mu]g/m\3\ for a full shift exposure, calculated as an 8-hour TWA.
Risks and cases were estimated under two scenarios: (a) a Baseline
scenario where all exposures were capped at 100 [mu]g/m \3\ for MNM
miners and at 85.7 [mu]g/m \3\ for coal miners, and (b) a proposed 50
[mu]g/m \3\ scenario where all risks were capped at the proposed PEL of
50 [mu]g/m \3\ for both MNM and coal miners. The difference between the
two scenarios yields the estimated reduction in lifetime excess risks
and in lifetime excess cases due to the proposed PEL.
To calculate risks, MSHA grouped MNM miners into the following
exposure intervals: <=25, >25 to <=50, >50 to <=100, >100 to <=250,
>250 to <=500, and >500 [mu]g/m \3\. MSHA grouped coal miners into the
following exposure intervals: <=25, >25 to <=50, >50 to <=85.7, >85.7
to <=100, >100 to <=250, >250 to <=500, and >500 [mu]g/m \3\. MSHA
calculated the median of all exposure samples in each exposure interval
and assumed the population of miners is distributed across the exposure
intervals in proportion to the number of exposure samples from the
compliance dataset in each interval. Then, miners were assumed to
encounter constant exposure at the median value of their assigned
exposure interval. MSHA adjusted the annual cumulative exposure by a
full-time equivalency (FTE) factor to account for the fact that miners
may experience more or less than 2,000 hours of exposure per year. MSHA
calculated the FTE adjustment factor as the weighted average of the
production employee FTE ratio (0.99 for MNM and 1.14 for coal) and the
contract miner FTE ratio (0.59 for MNM and 0.64 for coal), where the
weights are the number of miners (150,928 for MNM production employees,
60,275 for MNM contract miners, 51,573 for coal production employees,
and 22,003 for coal contract miners). For example, the weighted average
FTE ratio for MNM is (0.987 x 150,928 + 0.591 x 60,275)/(150,928 +
60,275) = 0.87 and is (1.139 x 51,573 + 0.636 x 22,003)/(51,573 +
22,003) = 0.99 for coal.
MSHA calculated excess risk, which refers to the additional risk of
disease and death attributable to exposure to respirable crystalline
silica. For silicosis morbidity, MSHA used an exposure-response model
that directly yields the accumulated or lifetime excess risk of
silicosis morbidity, assuming there is no background rate \19\ of
silicosis in an unexposed (i.e., non-miner) group. For the four
mortality endpoints (silicosis mortality, lung cancer mortality, NMRD
mortality, and ESRD mortality), MSHA used cohort life tables to
calculate excess risks, assuming all miners begin working at age 21,
retire at the end of age 65, and do not live past age 80. From the life
tables, MSHA acquired the lifetime mortality risk by summing the miner
cohort's mortality risks in each year from age 21 through age 80. Life
tables were also constructed for unexposed (i.e., non-miner) groups
assumed to die from a given disease at typical rates for the U.S. male
population. MSHA used 2018 data for all males in the U.S. (published by
the National Center for Health Statistics, 2020b) to estimate (a) the
disease-specific mortality rates among unexposed males and (b) the all-
cause mortality rates among both groups (exposed miners and unexposed
non-miners).
---------------------------------------------------------------------------

\19\ Here, the ``background'' risk (or rate) refers to the risk
of disease that the exposed person would have experienced in the
absence of exposure to respirable crystalline silica. These
background morbidity and mortality rates are measured using the
disease-specific rates among the general population, which is not
exposed to respirable crystalline silica.
---------------------------------------------------------------------------

For a given scenario (either Baseline or Proposed 50 [mu]g/m\3\),
MSHA constructed life tables in the manner described above, both for a
miner cohort exposed to respirable crystalline silica and for an
unexposed non-miner cohort. MSHA calculated excess risk of the disease
as the difference between the two cohorts' disease-specific mortality
risk (due to silicosis, lung cancer, NMRD, or ESRD). MSHA determined
the lifetime excess cases by multiplying the lifetime excess risk by
the number of exposed miner FTEs (including both production employee
FTEs and contract miner FTEs). Risks and cases were calculated
separately for each exposure interval listed above. Then, the lifetime
excess cases were aggregated across all exposure intervals. MSHA
calculated the final lifetime excess risks per 1,000 miners in the full
population by dividing the total number of lifetime excess cases by the
total number of miners in the population (exposed at any interval).
Finally, to estimate the risk reductions and avoided cases of illness
due to the proposed PEL, MSHA compared the lifetime excess risks and
lifetime excess cases across the two scenarios (Baseline and Proposed
50 [mu]g/m\3\).

B. Overview of Epidemiologic Studies

MSHA reviewed extensive research on the health effects of
respirable crystalline silica and several quantitative risk assessments
published in the peer-reviewed scientific literature

[[Page 44881]]

regarding occupational exposure risks of illness and death from
silicosis, NMRD, lung cancer, and ESRD. The Health Effects document
describes the specific studies reviewed by MSHA. Of the many studies
evaluated, MSHA believes that the 13 studies used by OSHA (2013b) to
estimate risks provide reliable estimates of the disease risk posed by
miners' exposure to respirable crystalline silica. These studies are
summarized in Table VI-1.
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Of these 13 studies, OSHA selected one per health endpoint for
final modeling and estimation of lifetime excess risk and cases.
Combining the five selected studies with the observed exposure data
yields estimates of actual lifetime excess risks and lifetime excess
cases among worker populations based on real exposure conditions. Table
VI-2 presents the 13 studies from OSHA's PQRA, which MSHA has also
considered. MSHA evaluated the evidence of OSHA's analysis of the 13
studies and the accompanying risks associated with exposure at 25, 50,
100, 250, and 500 [mu]g/m\3\. Thorough evaluation has led MSHA to
determine that the studies OSHA selected still provide the best
available epidemiological models. However, MSHA utilized the Miller and
MacCalman (2010) study to estimate risks. This study was published
after OSHA completed much of its modeling for their 2013 PRA (OSHA,
2013b). The study was included in OSHA's health effects assessment and
its PQRA. The following lists the study used by MSHA for each health
endpoint:
Silicosis morbidity: Buchanan et al. (2003);
Silicosis mortality: Mannetje et al. (2002b);
NMRD mortality: Park et al. (2002);
Lung cancer mortality: Miller and MacCalman (2010); and
ESRD mortality: Steenland et al. (2002a).
MSHA developed its risk estimates based on recent mortality data
and using certain assumptions that differed from those used by OSHA, as
explained in the standalone PRA document. Examples of these MSHA
assumptions include a lifetime that ends at age 80, updated background
mortality data and all-cause mortality, miner population sizes, and
miner-specific full-time equivalents (FTEs).\20\
---------------------------------------------------------------------------

\20\ FTEs were used to adjust the cumulative exposure over a
year based on the average number of hours that miners work.
---------------------------------------------------------------------------

MSHA's modeling has been done using life tables, in a manner
consistent with OSHA's PQRA. In general, the life table is a technique
that allows estimation of excess risk of disease-specific mortality
while factoring in the probability of surviving to a particular age
assuming no exposure to respirable crystalline silica. This analysis
accounts for competing causes of death, background mortality rates of
the disease, and the effect of the accumulation of risk due to elevated
mortality rates in each year of a working life. For each cause of
mortality, the selected study was used in the life table analysis to
compute the increase in miners' disease-specific mortality rates
attributable to respirable crystalline silica exposure.
MSHA uses cumulative exposure (i.e., cumulative dose) to
characterize the total exposure over a 45-year working life. Cumulative
exposure is defined as the product of exposure duration and exposure
intensity (i.e., exposure level). Cumulative exposure is the predictor
variable in the selected exposure-response models.

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For each health endpoint, MSHA generated two sets of risk
estimates--one representing a scenario of full compliance with the
existing standards (herein referred to as the ``Baseline'' scenario)
and another representing a scenario wherein no samples exceed the
proposed PEL (herein referred to as the ``Proposed 50 [mu]g/m\3\''
scenario). In the Baseline scenario, MNM miners in the >100-250, >250-
500, and >500 [mu]g/m\3\ groups were assigned exposure intensities of
100 [mu]g/m\3\ ISO. Coal miners in the 85.7-100, >100-250, >250-500,
and >500 [mu]g/m\3\ groups were assigned exposure intensities of 85.7
[mu]g/m\3\ ISO, calculated as an 8-hour TWA. Exposure intensities were
not changed for miners with lower exposure concentrations, because
their exposures were considered compliant with the existing standards.
A similar procedure was used for the Proposed 50 [mu]g/m\3\ scenario,
except that each miner group whose exposure exceeded the proposed PEL
was assigned a new exposure of 50 [mu]g/m\3\ ISO (for both MNM and
coal). This process--of creating an exposure profile based on actual
exposure data and modifying it based on the existing standards or the
proposed PEL--allowed MSHA to estimate real exposure conditions that
miners would encounter under each scenario, thereby enabling estimates
of the actual excess risks the current population of miners would
experience under each scenario (Baseline and Proposed 50 [mu]g/m\3\).
For purposes of calculating risk in the PRA, both for MNM and coal
miners, MSHA estimated excess risks by using the concentration
collected over the full shift and calculating it as a full-shift, 8-
hour TWA expressed in ISO standards. This metric of exposure
intensity--the 8-hour TWA concentration of respirable crystalline
silica in ISO standards--was used consistently across all sets of
estimates (both MNM and coal sectors, and both the Baseline and
Proposed 50 [mu]g/m\3\ scenarios), thereby facilitating meaningful
comparison. MSHA acknowledges that this metric does not correspond to
the manner in which coal exposure concentrations are calculated for
purposes of evaluating compliance under the existing standard.
Nonetheless, MSHA believes that a full-shift, 8-hour TWA concentration
accurately represents risks to miners and thus is the most appropriate
cumulative exposure metric for computing risk given that FTEs were used
to scale exposure durations relative to the assumption of 250 8-hour
workdays per year.

C. Summary of Studies Selected for Modeling

1. Silicosis Morbidity
Due to the long latency periods associated with chronic silicosis,
OSHA's respirable crystalline silica standard relied on the subset of
studies that were able to contact and evaluate many workers through
retirement. MSHA agrees that relying on studies that included retired
workers comes closest to characterizing lifetime risk of silicosis
morbidity.
The health endpoint of interest in these studies was the appearance
of opacities on chest radiographs indicative of pulmonary
pneumoconiosis (a group of lung diseases caused by the lung's reaction
to inhaled dusts). The most reliable estimates of silicosis morbidity,
as detected by chest X-rays, come from the studies that evaluated those
X-rays over time, included radiographic evaluation of workers after
they left employment, and derived cumulative or lifetime estimates of
silicosis disease risk.
To describe the presence and severity of pneumoconiosis, including
silicosis, the International Labour Organization (ILO) developed a
standardized system to classify lung opacities identified on chest
radiographs (X-rays) (ILO, 1980, 2002, 2011, 2022). The ILO system

[[Page 44886]]

grades the size, shape, and profusion of opacities. Although silicosis
is defined and categorized based on chest X-ray, the X-ray is an
imprecise tool for detecting pulmonary pneumoconiosis (Craighead and
Vallyathan, 1980; Hnizdo et al., 1993; Rosenman et al., 1997; Cohen and
Velho, 2002). Hnizdo et al. (1993) recommended that an ILO category 0/1
(or greater) should be considered indicative of silicosis among workers
exposed to high respirable crystalline silica concentrations. They
noted that the sensitivity of the chest X-ray as a screening test
increases with disease severity and to maintain high specificity,
category 1/0 (or 1/1) chest X-rays should be considered as a positive
diagnosis of silicosis for miners who work in low dust occupations
(Hnizdo et al., 1993). MSHA, consistent with NIOSH's use of chest X-
rays in their occupational respiratory disease surveillance program
(NIOSH 2014b), agrees that a small opacity profusion score of 1/0 is
consistent with chronic silicosis stage 1. Most of the studies reviewed
by MSHA considered a finding consistent with an ILO category of 1/1 or
greater to be a positive diagnosis of silicosis, although some also
considered an X-ray classification of 1/0 or 0/1 to be positive. The
low sensitivity of chest radiography to detect minimal silicosis
suggests that risk estimates derived from radiographic evidence likely
underestimate the true risk of this disease (Craighead and Vallyathan,
1980; Hnizdo et al., 1993; Rosenman et al., 1997; Cohen and Velho,
2002).
OSHA summarized the Miller et al. (1995, 1998) and Buchanan et al.
(2003) papers in their final respirable crystalline silica standard in
2016 (OSHA 2016a, 81 FR 16286, 16316). These researchers reported on a
1991 follow-up study of 547 survivors of a 1,416-member cohort of
Scottish coal workers from a single mine. These men had all worked in
the mine during the period between early 1971 and mid-1976, during
which time they had experienced ``unusually high concentrations of
freshly cut quartz in mixed coal mine dust.'' The population's
exposures to quartz dust had been measured in unique detail for a
considerable proportion of the men's working lives (OSHA 2013b, page
333).
The 1,416 men had previous chest X-rays dating from before, during,
or just after this high respirable crystalline silica exposure period.
Of these 1,416 men, 384 were identified as having died by 1990/1991. Of
the 1,032 remaining men, 156 were untraced, and, of the 876 who were
traced and replied, 711 agreed to participate in the study. Of these,
the total number of miners who were surveyed was 551. Four of these
were omitted, two because of a lack of an available chest X-ray. The
547 surviving miners (age range: 29-85 years, average = 59 years) were
interviewed and received their follow-up chest X-rays between November
1990 and April 1991. The interviews consisted of questions on current
and past smoking habits and occupational history since leaving the coal
mine, which closed in 1981. They were also asked about respiratory
symptoms and were given a spirometry test (OSHA 2013b, pages 333-334).
Exposure characterization was based on extensive respirable dust
sampling; samples were analyzed for quartz content by IR spectroscopy.
Between 1969 and 1977, two coal seams were mined. One had produced
quarterly average concentrations of respirable crystalline silica much
less than 1,000 [mu]g/m\3\ (only 10 percent exceeded 300 [mu]g/m\3\).
The other more unusual seam (mined between 1971 and 1976) lay in
sandstone strata and generated respirable crystalline silica levels
such that quarterly average exposures exceeded 1,000 [mu]g/m\3\ (10
percent of the quarterly measurements were over 10,000 [mu]g/m\3\).
Thus, this cohort study allowed evaluation of the effects of both
higher and lower respirable crystalline silica concentrations and
exposure-rate effects on the development of silicosis (OSHA 2013b, page
334).
Three physicians read each chest film taken during the current
survey as well as films from the surveys conducted in 1974 and 1978.
Films from an earlier 1970 survey were read only if no films were
available from the subsequent two surveys. Silicosis cases were
identified if the median classification of the three readers indicated
an ILO category of 1/1 or greater (Miller et al, 1995, page 24), plus a
progression from the earlier reading. Of the 547 men, 203 (38 percent)
showed progression of at least 1 ILO category from the 1970s' surveys
to the 1990-91 survey; in 128 of these (24 percent) there was
progression of 2 or more ILO categories. In the 1970s' surveys, 504 men
had normal chest X-rays; of these 120 (24 percent) acquired an abnormal
X-ray consistent with ILO category 1/0 or greater at the follow-up. Of
the 36 men whose X-rays were consistent with ILO category 1/0 or
greater in the 1970s' surveys, 27 (75 percent) exhibited further
progression at the 1990/1991 follow-up. Only one subject showed a
regression from any earlier reading, and that was slight, from 1/0 to
0/1. The earlier Miller et al. (1995) report presented results for
cases classified as having X-ray films consistent with either 1/0+ and
2/1+ degree of profusion; the Miller et al. (1998) analysis and the
Buchanan et al. (2003) re-analyses emphasized the results from cases
having X-rays classified as 2/1+ (OSHA 2013b, page 334).
MSHA modeled the exposure-response relationship by using cumulative
exposure expressed as gram/m\3\-hours, assuming 2,000 work hours per
year and a 45-year working life (after adjusting for full-time
equivalents, including production employees and contract workers). MSHA
estimated risk at the existing standard assuming cumulative exposure to
100 [micro]g/m\3\ ISO for MNM miners and 85.7 [micro]g/m\3\ ISO (100
[micro]g/m\3\ MRE) for coal miners. Respirable crystalline silica
exposures were calculated by commodity, and median exposure values were
used within a variety of exposure intervals. Risks were computed using
a life table methodology which iteratively updated the survival, risk,
and mortality rates each year based on the results of the preceding
year. Covariates in the regression included smoking, age, amount of
coal dust, and percent of quartz in the coal dust during various
previous survey periods.
Both Miller et al. papers (1995, 1998) presented the results of
numerous regression models, and they compared the results of the
partial regression coefficients using Z statistics of the coefficient
divided by the standard error. Also presented were the residual
deviances of the models and the residual degrees of freedom. In the
introduction to the results section, Miller et al. (1995) stated that,
``in none of the models fitted was there a significant effect of
smoking habit (current, ex-smoker, and never smoker), nor was there any
evidence of any difference between smoking groups in their relationship
of response with age.'' They therefore presented the results of the
regression analyses without terms for smoking effects (i.e., without
including smoking effects as a variable in the final regression
analysis, because they found that smoking did not affect the modeling
results). The logistic regression models developed by Miller et al.
(1995) included terms for cumulative exposure and age. In their later
publication, Miller et al. (1998) presented models similar to their
1995 report, but without the age variable. Their logistic regression
model A from Table 7 of their report (page 56) included only an
intercept (-4.32) and the respirable crystalline silica (quartz)
cumulative exposure variable (0.416). They estimated that respirable
crystalline silica exposure at an average

[[Page 44887]]

concentration of 100 [micro]g/m\3\ for 15 years (2.6 gram/m\3\-hr
assuming 1,750 hours worked per year) would result in an increased risk
of silicosis (ILO > 2/1) of 5 percent (OSHA 2013b, page 334).
OSHA had a high degree of confidence in the estimates of silicosis
morbidity risk from this Scotland coal mine study. This was mainly
because of highly detailed and extensive exposure measurements,
radiographic records, and detailed analyses of high exposure-rate
effects. However, in another paper, Soutar et al. (2004) noted that:
``If the effects of silica vary according to the conditions of
exposure, these risks are probably towards the high end of the risk
spectrum, since the silica was freshly frac

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