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Rule2024-06920

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

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Published

April 18, 2024

Effective

June 17, 2024

Issuing agencies

Labor DepartmentMine Safety and Health Administration

Abstract

The Mine Safety and Health Administration (MSHA) is amending its existing standards to better protect miners against occupational exposure to respirable crystalline silica, a significant health hazard, and to improve respiratory protection for miners from exposure to airborne contaminants. MSHA's final rule also includes other requirements to protect miner health, such as exposure sampling, corrective actions to be taken when a miner's exposure exceeds the permissible exposure limit, and medical surveillance for metal and nonmetal mines.

Full Text

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[Federal Register Volume 89, Number 76 (Thursday, April 18, 2024)]
[Rules and Regulations]
[Pages 28218-28485]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2024-06920]

[[Page 28217]]

Vol. 89

Thursday,

No. 76

April 18, 2024

Part III

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; Final Rule

Federal Register / Vol. 89 , No. 76 / Thursday, April 18, 2024 /
Rules and Regulations

[[Page 28218]]

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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: Final rule.

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SUMMARY: The Mine Safety and Health Administration (MSHA) is amending
its existing standards to better protect miners against occupational
exposure to respirable crystalline silica, a significant health hazard,
and to improve respiratory protection for miners from exposure to
airborne contaminants. MSHA's final rule also includes other
requirements to protect miner health, such as exposure sampling,
corrective actions to be taken when a miner's exposure exceeds the
permissible exposure limit, and medical surveillance for metal and
nonmetal mines.

DATES:
Effective date: The final rule is effective June 17, 2024, except
for amendments 21, 22, 25, 26, 27, 30, 31, 34, 35, 36, 38, 39, 42, 43,
46, 47, 50, 51, 54, 55, 59, 60, 63, 64, 68, 69, 73, 74, 77, 78, 81, 82,
83, 86, 87, 90, 91, 94, 95, 98, 99, 102, 103, 106, 107, 110, and 111,
which are effective April 14, 2025, and amendments 4, 5, 8, 9, 13, 14,
17, and 18, which are effective April 8, 2026.
Incorporation by reference date: The incorporation by reference of
certain materials listed in the rule is approved by the Director of the
Federal Register beginning June 17, 2024, except for the material in
amendment 60, which is approved beginning April 14, 2025, and the
material in amendments 9 and 18, which is approved beginning April 8,
2026. The incorporation by reference of certain other material listed
in the rule was approved by the Director of the Federal Register as of
July 10, 1995.
Compliance dates: Compliance with this final rule is required April
14, 2025 for coal mine operators and April 8, 2026 for metal and
nonmetal mine operators.

FOR FURTHER INFORMATION CONTACT: S. Aromie Noe, Director, Office of
Standards, Regulations, and Variances, MSHA, at:
<a href="/cdn-cgi/l/email-protection#eb98828782888a9a9e8e989f82848598ab8f8487c58c849d"><span class="__cf_email__" data-cfemail="02716b6e6b616373776771766b6d6c7142666d6e2c656d74">[email&#160;protected]</span></a> (email); 202-693-9440 (voice); or 202-693-9441
(facsimile). These are not toll-free numbers.

SUPPLEMENTARY INFORMATION:
The preamble to the final standard follows this outline:

I. Executive Summary
II. Pertinent Legal Authority
III. Regulatory History
IV. Background
V. Health Effects Summary
VI. Final Risk Analysis Summary
VII. Feasibility
VIII. Summary and Explanation of the Final Rule
IX. Summary of Final Regulatory Impact Analysis and Regulatory
Alternatives
X. Final Regulatory Flexibility Analysis
XI. Paperwork Reduction Act
XII. Other Regulatory Considerations
XIII. References
XIV. Appendix

Acronyms and Abbreviations

COPD chronic obstructive pulmonary disease
ESRD end-stage renal disease
FEV forced expiratory volume
FRA final risk analysis
FRIA final regulatory impact analysis
FVC forced vital capacity
L/min liters 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
MRE Mining Research Establishment
NMRD nonmalignant respiratory disease
PEL permissible exposure limit
PMF progressive massive fibrosis
PRA preliminary risk analysis
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. Executive Summary

A. Purpose of the Regulatory Action

The purpose of this final rule is to reduce occupational disease in
miners and to improve respiratory protection against airborne
contaminants. The rule sets the permissible exposure limit (PEL) 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 (TWA) for all mines. This rule also establishes an
action level for respirable crystalline silica of 25 [micro]g/m\3\ for
a full-shift exposure, calculated as an 8-hour TWA for all mines. In
addition to the PEL and action level, the rule includes provisions for
methods of compliance, exposure monitoring, corrective actions,
respiratory protection, medical surveillance for metal and nonmetal
(MNM) mines, and recordkeeping.
The statutory authority for this rule is provided by the Mine Act
under sections 101(a), 103(h), and 508. 30 U.S.C. 811(a), 813(h), and
957. A full discussion of Mine Act legal requirements can be found in
Section II. Pertinent Legal Authority. MSHA implements and administers
the provisions of the Mine Act to prevent death, illness, and injury
from mining and promote safe and healthful workplaces for miners.
Respirable crystalline silica is classified by the International
Agency for Research on Cancer (IARC) as a human carcinogen.
Occupational exposure to respirable crystalline silica results in
adverse health effects and increases risk of death. The adverse health
effects include silicosis (i.e., acute silicosis, accelerated
silicosis, chronic silicosis, and progressive massive fibrosis),
nonmalignant respiratory diseases (e.g., emphysema and chronic
bronchitis), lung cancer, and kidney disease. Each of these effects is
chronic, irreversible, and potentially disabling or fatal. Occupational
exposure to respirable crystalline silica at mines occurs most commonly
from respirable dust generated during mining activities, such as
cutting, sanding, drilling, crushing, grinding, sawing, scraping,
jackhammering, excavating, and hauling of materials that contain
silica.
Existing standards pertaining to respirable crystalline silica for
both MNM and coal mines have been in place since the early 1970s. For
MNM mines, the existing standards, established by the Department of
Interior, Bureau of Mines, in 1974, helped protect miners from the most
dangerous levels of exposure to respirable crystalline silica. The
existing MNM PELs for the three polymorphs of respirable crystalline
silica are: 0.1 mg/m\3\ or 100 micrograms per cubic meter of air
([micro]g/m\3\) for quartz; 0.05 mg/m\3\ or 50 [micro]g/m\3\ for
cristobalite; and 0.05 mg/m\3\ or 50 [micro]g/m\3\ for tridymite.
Existing standards for coal mines, first established by the Federal
Coal Mine Health and Safety Act of 1969 as interim standards in 1970,
control miners' exposures to respirable crystalline silica indirectly
by reducing the respirable coal mine dust standard when quartz is
present. 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.
However, since the promulgation of these existing standards, the
National Institute for Occupational Safety and Health (NIOSH) has
recommended a

[[Page 28219]]

lower respirable crystalline silica exposure level of 50 [micro]g/m\3\
for all workers, including miners. In 2016, the Occupational Safety and
Health Administration (OSHA) established a PEL of 50 and an action
level of 25 [micro]g/m\3\ as an 8-hour TWA in the general and
construction industries and maritime sector that it regulates. In the
mining industry, however, the higher PELs have remained in place for
miners in both the MNM sector and the coal sector.
To better protect miners' health, therefore, with this final rule
MSHA is lowering its existing exposure limits for quartz or respirable
crystalline silica to 50 [micro]g/m\3\ and setting an action level of
25 [mu]g/m\3\ for all miners. As discussed in Section V. Health Effects
Summary and Section VI. Final Risk Analysis Summary, lowering the PEL
will substantially reduce health risks to miners. This final rule also
provides a uniform, streamlined regulatory framework to ensure
consistent protection across mining sectors and make compliance more
straightforward. As discussed in Section VII. Feasibility and Section
IX. Summary of Final Regulatory Impact Analysis and Regulatory
Alternatives, compliance with the final rule is technologically and
economically feasible, and the final rule has quantified benefits in
terms of avoided deaths and illnesses that greatly outweigh the costs,
as well as other important unquantified benefits.

B. Summary of Major Provisions

MSHA amends its existing standards on respirable crystalline silica
or quartz, after considering all the testimonies and written comments
the Agency received from a variety of stakeholders, including miners,
mine operators, labor unions, industry trade associations, government
officials, and public health professionals, in response to its notice
of proposed rulemaking. Below is a summary of major provisions in the
final rule. Section VIII. Summary and Explanation of the Final Rule
discusses each provision in the final rule.
This final rule:
1. Establishes a uniform permissible exposure limit (PEL) and
action level for all mines. The rule sets a PEL for respirable
crystalline silica at 50 micrograms per cubic meter of air ([micro]g/
m\3\) over a full shift, calculated as an 8-hour TWA and an action
level at 25 [micro]g/m\3\ over a full shift, calculated as an 8-hour
TWA for all mines.
2. Requires exposure monitoring for respirable crystalline silica.
Mine operators are required to conduct sampling to assess miners'
exposures to respirable crystalline silica. Mine operators are also
required to evaluate the impact of mining production, processes,
equipment, engineering controls, and geological condition changes on
respirable crystalline silica exposures.
3. Updates the standard for respirable crystalline silica sampling.
ISO 7708:1995(E), Air quality--Particle size fraction definitions for
health-related sampling, First Edition, 1995-04-01 (ISO 7708:1995), is
incorporated by reference. The final rule requires mine operators to
conduct sampling for respirable crystalline silica using respirable
particle size-selective samplers that conform to ISO 7708:1995, which
is the international consensus standard that defines sampling
conventions for particle size fractions used in assessing possible
health effects of airborne particles in the workplace and ambient
environment.
4. Requires immediate reporting and corrective action to remedy
overexposures. Whenever an overexposure is identified, mine operators
must immediately report to MSHA and take corrective action to lower the
concentration of respirable crystalline silica to at or below the PEL,
resample to determine the efficacy of the corrective action taken, and
make a record of all sampling and corrective actions that were taken.
5. Specifies methods of controlling respirable crystalline silica.
All mines are required to install, use, and maintain feasible
engineering controls as the primary means of controlling respirable
crystalline silica; administrative controls may be used, when
necessary, as a supplementary control.
6. Requires temporary use of respirators at metal and nonmetal
mines when miners must work in concentrations above the PEL. When MNM
miners must work in concentrations of respirable crystalline silica
above the PEL while engineering controls are being developed and
implemented or it is necessary by nature of the work involved, the mine
operator shall use respiratory protection as a temporary measure.
7. Updates the respiratory protection standard. ASTM F3387-19,
Standard Practice for Respiratory Protection, approved August 1, 2019
(ASTM F3387-19), is incorporated by reference. When approved
respirators are used, the mine operator must have a written respiratory
protection program to protect miners from airborne contaminants,
including respirable crystalline silica, in accordance with ASTM
requirements.
8. Requires medical surveillance at MNM mines. Metal and nonmetal
mine operators are required to provide to all miners, including those
who are new to the mining industry, periodic medical examinations
performed by a physician or other licensed health care professional
(PLHCP) or specialist, at no cost to the miner. Like coal miners, MNM
miners will be able to monitor their health and detect early signs of
respiratory illness.
The requirements in the new part 60 will take effect on June 17,
2024. For coal mine operators, compliance with part 60 is required by
12 months after the publication date; for MNM operators, compliance is
required by 24 months after the publication date. The delayed
compliance is to strike a balance between meeting the urgent need to
protect miners from this health hazard and giving mining operators
adequate preparation time to allow them to comply effectively with the
new requirements.
In addition, conforming amendments to parts 56, 57, 70, 71, 72, 75,
and 90 will take effect on June 17, 2024. Compliance with conforming
amendments to parts 56 and 57 is required by 24 months after the
publication date; and compliance with conforming amendments to parts
70, 71, 72, 75, and 90 is required by 12 months after the publication
date.

C. Summary of Final Regulatory Impact Analysis

MSHA's economic analysis estimates that the final rule would cost
approximately an average of $89 million per year in 2022 dollars at an
undiscounted rate, $90 million at a 3 percent discount rate, and $92
million at a 7 percent discount rate. Based on the results of the Final
Regulatory Impact Analysis (FRIA), MSHA estimates that this final
rule's monetized benefits would exceed its costs, with or without
discount rates. Monetized benefits are estimated from avoidance of 531
deaths related to NMRD, silicosis, ESRD, and lung cancer and 1,836
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 $294 million at an undiscounted
rate, $157 million at a 3 percent discount rate, and $40 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.'' The Office of
Management and Budget has determined that the final rule is significant
under E.O. 12866 Section 3(f)(1).

[[Page 28220]]

In summary, this final rule will strengthen MSHA's existing
regulatory framework and improve health protections for the nation's
miners. It establishes a uniform PEL that aligns respirable crystalline
silica exposure limits for MNM and coal miners with workers in other
industries. Moreover, the final rule updates 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. It
also requires all MNM operators to provide medical surveillance in the
form of a medical examination regime similar to the one that already
covers coal miners. Cumulatively, the final rule will lower miners'
risks 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. Pertinent Legal Authority

The statutory authority for this final rule 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
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 mandatory health standards to address
toxic materials or harmful physical agents. Under Section 101(a), a
standard must protect lives and prevent injuries in mines and be
``improved'' over any standard that it replaces or revises.
The Secretary must set standards to assure, based on the best
available evidence, that no miner 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. As a result, courts
have found it ``appropriate to `give an extreme degree of deference' ''
to MSHA `` `when it is evaluating scientific data within its technical
expertise.' '' Nat'l Mining Ass'n, 812 F.3d at 866 (quoting Kennecott
Greens Creek Mining Co. v. MSHA, 476 F.3d 946, 954 (D.C. Cir. 2007)).
Consequently, 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. Thus, ``when
MSHA itself weighs the evidence before it, it does so in light of its
congressional mandate'' in favor of protecting miners' health. Id.
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 Mine Act at 30 U.S.C. 811(a) with the OSH
Act at 29 U.S.C. 652 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'').
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). Additionally, section 103(h) requires that every mine operator
establish and maintain records, make reports, and provide this
information as required by the Secretary. Id. 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 final rule to lower the exposure limits for respirable
crystalline silica adopts an integrated monitoring approach across all
mining sectors and updates the existing respiratory protection
requirements. The final rule fulfills Congress' direction to protect
miners from material impairments of health or functional capacity
caused by exposure to respirable crystalline silica and other airborne
contaminants.

III. Regulatory History

On August 29, 2019, MSHA published a Request for Information (RFI)
in the Federal Register to solicit information and data on a variety of
topics concerning silica (quartz) in respirable dust (84 FR 45452). In
the RFI, MSHA requested data and information on technologically and
economically feasible best practices to protect MNM and coal miners'
health from exposure to quartz, including a lowered permissible
exposure limit (PEL), new or developing protective technologies, and/or
effective technical and educational assistance (84 FR 45456).
Specifically, MSHA requested input from industry, labor, and other
interested parties on the following four topics: (1) new or developing
technologies and best practices that can be used to protect miners from
exposure to quartz dust; (2) how engineering controls, administrative
controls, and personal protective equipment can be used, either alone
or concurrently, to protect miners from exposure to quartz dust; (3)
additional feasible dust-control methods that could be used by mining
operations to reduce miners' exposures to respirable quartz during
high-silica cutting situations, such as on development sections, shaft
and slope work, and cutting overcasts; and (4) any other experience,
data, or information that may be useful to MSHA in evaluating miners'
exposures to quartz (84 FR 45456).
The Agency received 57 comments from citizens, labor, industry, and
public health stakeholders in response to the RFI. Stakeholders
expressed various and differing opinions on how and to what extent MSHA
should address the protection of miners' health from exposure to
silica. Many of these stakeholders also commented on MSHA's proposed
rulemaking, summarized below.
On June 30, 2023, MSHA made an informal copy of the proposed rule
available on the Agency's website, prior to publication in the Federal
Register, so the public and stakeholders could

[[Page 28221]]

review it in advance of the comment period.
On July 13, 2023, MSHA published the proposed rule, Lowering
Miners' Exposure to Respirable Crystalline Silica and Improving
Respiratory Protection, in the Federal Register (88 FR 44852). The
standalone documents ``Health Effects of Respirable Crystalline
Silica,'' ``Preliminary Risk Analysis,'' and ``Preliminary Regulatory
Impact Analysis'' were also made publicly available at that time. MSHA
proposed to set the PEL of respirable crystalline silica at 50
micrograms \1\ per cubic meter of air ([micro]g/m\3\) for a full-shift
exposure, calculated as an 8-hour time-weighted average. MSHA's
proposal included other requirements for sampling, qualitative
evaluations, corrective actions, and medical surveillance for MNM
mines. Finally, the proposal included requirements for respiratory
protection, including the incorporation by reference of ASTM F3387-19
Standard Practice for Respiratory Protection.
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\1\ One microgram is equal to one-thousandth of a milligram (1
milligram = 1000 micrograms).
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On July 26, 2023, MSHA published a notice in the Federal Register
scheduling three public hearings on the proposed rule (88 FR 48146).
Hearings were held on: (1) August 3, 2023, in Arlington, Virginia; (2)
August 10, 2023, in Beckley, West Virginia; and (3) August 21, 2023, in
Denver, Colorado. Speakers and attendees could participate in-person or
online. There were 14 speakers and over 150 attendees at the Arlington
hearing; 24 speakers and over 200 attendees at the Beckley hearing; and
10 speakers and over 175 attendees at the Denver hearing. Speakers
included active and retired miners and representatives from the mining
industry, unions, the health care profession, advocacy groups, industry
groups, trade associations, and law firms. Transcripts from the public
hearings are available at <a href="http://www.regulations.gov">www.regulations.gov</a> and on the MSHA website.
On August 14, 2023, in response to requests from the public, MSHA
published a notice in the Federal Register extending the comment period
by changing the closing date from August 28, 2023, to September 11,
2023 (88 FR 54961).
During the comment period, MSHA received 157 written comments on
the proposed rule from miners, mine operators, individuals, government
officials, labor organizations, advocacy groups, industry groups, trade
associations, and health organizations. Some commenters supported
various aspects of the proposal. Other commenters opposed aspects of
the proposal and offered recommendations for suggested changes to the
proposed rule. All public comments and supporting documentation are
available at <a href="http://www.regulations.gov">www.regulations.gov</a> and on the MSHA website. MSHA
carefully reviewed and considered the written comments on the proposed
rule and the speakers' testimonies from the hearings and addresses them
in the relevant sections below.

IV. Background

A. 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, meaning several different structures
with the same chemical composition. The most common form of crystalline
silica found in nature is quartz, but cristobalite and tridymite also
occur 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 bands and veins are 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 can occur 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 therefore may lead to miner exposure.
Inhaled small particles of silica dust can be deposited throughout
the lungs. Because of their small size, many of these particles can
reach and remain in the deep lung (i.e., alveolar region), although
some can be 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
result in a variety of cellular responses that may lead to pulmonary
diseases, such as silicosis and lung cancer. 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 IV-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.\2\ Table IV-1 shows
that a majority of MNM mines produce sand and gravel, while the largest
number of MNM miners work at metal mines, not including MNM contract
workers (i.e.,

[[Page 28222]]

independent contractors and employees of independent contractors who
are engaged in mining operations).
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\2\ Commodities such as sand, gravel, silica, and/or stone are
used in road building, concrete construction, the 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] TR18AP24.131

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, and other components (IARC, 1997).
These activities include the general mining activities previously
mentioned (e.g., cutting, sanding, drilling, crushing, hauling, etc.),
as well as roof bolter operations, continuous mining machine
operations, longwall mining, and other activities. Table IV-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).

B. Existing Standards

Since the early 1970s, MSHA has maintained health standards to
protect MNM and coal miners from excessive exposure to airborne
contaminants, including respirable crystalline silica. These standards
require mine operators 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 miners with
respiratory protection in limited situations for a short period. The
existing standards for MNM and coal mines differ in some respects,
including exposure limits and monitoring requirements. 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 the standards in recent
years.
1. Existing Standards--Metal and Nonmetal Mines
MSHA's existing standards for exposure to airborne contaminants in
MNM mines, including respirable crystalline silica, are found in 30 CFR
56 subpart D (Air Quality and Physical Agents) and 30 CFR 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 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 micrograms
per cubic meter of air ([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\.
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, examination of dust control system and
ventilation system maintenance, and

[[Page 28223]]

review of 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 and the effectiveness of existing controls in reducing
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 its adjustments to
control measures were successful. Re-surveying should be done as
frequently as necessary to ensure that the sampling results comply with
the PEL and the implemented control measures remain adequate.
Exposure Controls. MSHA's existing standards for controlling a
miner's exposure to harmful airborne contaminants in Sec. Sec. 56.5005
and 57.5005 require, if feasible, prevention of contamination, removal
by exhaust ventilation, or dilution with uncontaminated air. These
requirements to use feasible engineering controls, supplemented by
administrative controls, are consistent with widely accepted industrial
hygiene principles and NIOSH's recommendations (NIOSH, 1974).
Engineering controls designed to remove or reduce the hazard at the
source are the most effective. Although administrative controls are
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.
The use of respiratory protective equipment is also allowed under
specified circumstances, such as where engineering controls are not yet
developed or when it is necessary due to the nature of the work--for
example, while establishing controls or during occasional entry into
hazardous atmospheres to perform maintenance or investigation.
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 respirable crystalline silica and other airborne
contaminants. When respiratory protective equipment is used, MNM mine
operators must implement a respiratory protection program consistent
with the requirements of American National Standards Practices for
Respiratory Protection ANSI Z88.2-1969 (ANSI Z88.2-1969).
2. Existing Standards--Coal Mines
Under the existing coal mine standards, there is no separate
standard for respirable crystalline silica. 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 Part 90 miners.\3\ Coal
miners' exposures to respirable quartz are indirectly regulated through
reductions in the overall respirable dust standards.
---------------------------------------------------------------------------

\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.
---------------------------------------------------------------------------

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.\4\ Therefore, the terms 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.
---------------------------------------------------------------------------

\4\ 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.\5\ The 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\. Various sections within a mine may have different
reduced respirable coal mine dust (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. Because respirable crystalline silica is a percentage of
RCMD, 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\.
---------------------------------------------------------------------------

\5\ As defined in 30 CFR 70.2, an MRE instrument is a
gravimetric dust sampler with a four channel horizontal elutriator
developed by the Mining Research Establishment of the National Coal
Board, London, England. MSHA inspectors use Dorr-Oliver 10-mm nylon
cyclones operated at a 2.0 L/min flow rate (reported as MRE-
equivalent concentrations) for coal mine sampling.
---------------------------------------------------------------------------

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 dust
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. Respirable
dust sampling must be representative of respirable dust exposures
during a normal production shift and 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, in their normal work locations, from the start of their work
day to the end of their work day.
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. 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
RCMD standard, they are required to take corrective

[[Page 28224]]

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.
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, 70.209, 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

Under the existing standards, MSHA collects respirable dust samples
at mines and analyzes them for respirable crystalline silica to
determine whether the respirable crystalline silica exposure limits are
exceeded and whether exposure controls are adequate. MSHA's inspection
and respirable dust sampling were discussed in detail in the proposal
(88 FR 44862). This section, for ease of reference, briefly summarizes
the process for MSHA's inspection and respirable dust sampling.
1. Respirable Dust Sample Collection
Under the existing standards, MSHA inspectors arrive at mines,
determine which miners and which areas of the mine to select for
respirable dust sampling, and place gravimetric samplers on the
selected miners and at the selected locations. The gravimetric samplers
capture air from the breathing zone of each selected miner and from
each selected work area for the entire duration of the work shift.
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,
MSHA inspectors send cassettes containing the full-shift respirable
dust samples to the MSHA Laboratory for analysis.
2. Respirable Dust Sample Analysis
The MSHA Laboratory analyzes respirable dust samples following the
standard operating procedures summarized below.\6\ Any samples that are
broken, torn, or visibly wet are voided and removed before analysis.
Samples are weighed and then examined for validity based on mass gain.
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.\7\
---------------------------------------------------------------------------

\6\ 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.
\7\ 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)
in respirable dust samples: X-ray diffraction (XRD) for samples from
MNM mines and Fourier transform infrared spectroscopy (FTIR) for
samples from coal mines.\8\ 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. 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.
---------------------------------------------------------------------------

\8\ 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> (last accessed Jan. 10, 2024). 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> (last accessed Jan.
10, 2024).
---------------------------------------------------------------------------

MSHA calculates full-shift exposures to respirable crystalline
silica (and other airborne contaminants) in the same way for MNM and
coal miners when the miner works an 8-hour shift, but the calculated
exposures differ for longer shifts. For work shifts that last longer
than 8 hours, a coal miner's full-shift exposure is calculated using
the entire duration of the coal miner's shift. For the MNM miner, by
contrast, MSHA calculates extended full-shift exposure for respirable
dust samples using 480 minutes (8 hours) as the sampling time, meaning
that contaminants collected over extended shifts (e.g., 600-720
minutes) are calculated as if they had been collected over 480 minutes.

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

MSHA's respirable crystalline silica sampling results for MNM mines
were discussed in detail in the proposal (88 FR 44863). This section,
for ease of reference, summarizes 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 met the
minimum mass gain criteria and were analyzed for respirable crystalline
silica. The vast majority of the 46,585 valid samples that were
excluded from the analysis did not meet the mass gain criteria. Further
information on the valid respirable dust samples that were excluded
from the analysis can be found in Appendix A of the preamble.
1. Annual Results of MNM Respirable Crystalline Silica Samples
Table IV-2 below shows the variation between 2005 and 2019 in: (1)
the number 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.
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[GRAPHIC] [TIFF OMITTED] TR18AP24.132

BILLING CODE 4520-43-C
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-3 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).

[[Page 28226]]

[GRAPHIC] [TIFF OMITTED] TR18AP24.133

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-4 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-4 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
higher rates, ranging up to more than 19 percent (in the case of stone
cutting operators).
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[[Page 28227]]

[GRAPHIC] [TIFF OMITTED] TR18AP24.134

BILLING CODE 4520-43-C
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.
While stone-cutting operators have historically had high exposures to
respirable dust and respirable crystalline silica \10\ and continue to
experience the highest overexposures of any MNM occupation, about 80
percent of samples taken from stone cutting operators did not exceed
the existing PEL. 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).
---------------------------------------------------------------------------

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'
exposures at or below the existing limit of 100 [micro]g/m\3\.

E. Respirable Crystalline Silica Sampling Results--Coal Mines

MSHA's respirable crystalline silica sampling results for coal
mines were discussed in detail in the proposal (88 FR 44866). This
section, for ease of reference, summarizes the results of 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 MSHA 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 (referred to throughout the
preamble as the 2014 RCMD Standard) (79 FR 24813) went into effect. At
that time, the exposure limits for RCMD 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 the valid samples, only those collected from the breathing
zones of miners were used in the analysis for this rulemaking; no
environmental dust

[[Page 28228]]

samples were included.\11\ Of the valid breathing zone samples, there
were 63,127 samples that met the minimum mass gain criteria and were
analyzed for respirable quartz. 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 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-5, 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] TR18AP24.135

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-6, 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. 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.

[[Page 28229]]

[GRAPHIC] [TIFF OMITTED] TR18AP24.136

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 job category of
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-7 displays the number and percent of respirable coal
mine dust samples with quartz greater than the existing exposure limit
for each occupational category.

[[Page 28230]]

[GRAPHIC] [TIFF OMITTED] TR18AP24.137

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.

Figure IV-1: Percent of RCMD Samples With Respirable Crystalline Silica
Concentration Greater Than 100 MRE [micro]g/m\3\ (MRE) by Occupational
Category *

[[Page 28231]]

[GRAPHIC] [TIFF OMITTED] TR18AP24.076

* For Crusher Operators (Surface), only one sample with a quartz
concentration greater than 100 [micro]g/m\3\ MRE occurred (in 2018);
and for Mobile Workers (Surface), only nine samples with a quartz
concentration greater than 100 [micro]g/m\3\ MRE occurred (three in
2017, five in 2018 and one in 2021). Source: MSHA MSIS respirable
crystalline silica data for the Coal Industry, August 1, 2016,
through July 31, 2021 (version 20220617).

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 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 of the
health effects literature is contained in the standalone document,
entitled ``Effects of Occupational Exposure to Respirable Crystalline
Silica on the Health of Miners'' (referred to as the standalone Health
Effects document throughout the preamble), which is placed in the
rulemaking docket for the MSHA silica rulemaking (RIN 1219-AB36, Docket
No. MSHA-2023-0001). MSHA reviewed a wide range of health effects
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. The purpose of this summary is to briefly present MSHA's
findings on the nature of the hazards of exposure to respirable
crystalline silica. Based on its review of the health effects
literature and the weight-of-evidence approach, MSHA makes the
following conclusions:
1. Miners in MNM and coal mines exposed to respirable crystalline
silica at MSHA's existing exposure limits are subject to material
impairment of health or functional capacity. The illnesses associated
with exposure to respirable crystalline silica develop independent of
other exposures.
2. Occupational exposure to respirable crystalline silica (as
quartz and/or cristobalite) causes silicosis,

[[Page 28232]]

nonmalignant respiratory disease (NMRD) (e.g., emphysema and chronic
bronchitis), lung cancer, and renal disease. Each of these health
effects outcomes is exposure-dependent, potentially chronic,
irreversible, potentially disabling, and can be fatal.
3. Exposure to respirable crystalline silica contributes to the
development of autoimmune disorders through inflammatory pathways.
4. The development of silicosis, NMRD, lung cancer, renal disease,
and autoimmune disorders is largely dependent upon cumulative
respirable crystalline silica exposure.
These conclusions are the basis of MSHA's Final Risk Analysis (FRA)
on miners' exposure to respirable crystalline silica. In the FRA, MSHA
quantifies risks associated with the five specific health outcomes
mentioned above. The FRA summary is presented in Section VI. Final Risk
Analysis Summary and a standalone document, entitled ``Final Risk
Analysis'' (referred to as the standalone FRA document throughout the
preamble), has been placed in the rulemaking docket for the MSHA silica
rulemaking (RIN 1219-AB36, Docket No. MSHA-2023-0001).
From its health effects literature review and FRA, MSHA determines
that miners exposed to respirable crystalline silica continue to face a
risk of material impairment of health or functional capacity under
MSHA's existing exposure limits. Thus, MSHA also makes the following
conclusions:
(1) The rate of silicosis and other diseases caused by respirable
crystalline silica exposure would decrease with reduction in
occupational exposures, which is the most effective way to prevent
these types of diseases.
(2) Regulatory action is necessary to reduce these occupational
exposures and protect miners' health. Section 101(a)(6)(A) of the
Federal Mine Safety and Health Act of 1977, as amended (Mine Act),
requires MSHA 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 even if such miner has
regular exposure to the hazards dealt with by such standard for the
period of his working life.'' 30 U.S.C. 811(a)(6)(A).
Regulatory action to protect miners' health is required by section
101(a)(6)(A) of the Mine Act, and MSHA's statutory authority and
mission has been recognized and upheld by reviewing courts. ``[T]he
Mine Act evinces a clear bias in favor of miner health and safety.''
Nat'l Min. Ass'n v. Sec'y, U.S. Dep't of Lab., 812 F.3d 843, 866 (11th
Cir. 2016). Courts interpret MSHA's obligation to promulgate standards
to protect the health of the nation's miners to include ``
`prevent[ing],' not merely reduc[ing] the incidence of, `occupational
diseases originating in . . . mines.' '' Id. at 883 (quoting 30 U.S.C.
801(c)). Where occupational disease ``incidence has not been reduced to
zero . . . MSHA has not completely fulfilled its mission to `protect
the health . . . of the Nation's coal or other miners.' '' Id. (quoting
30 U.S.C. 801(g)). Case law instructs that MSHA must demonstrate risk
before regulating: ``[B]efore promulgating a health or safety standard
under the Mine Act, MSHA must show that the substance being regulated
presents a risk of `material impairment of health or functional
capacity' for miners who are regularly exposed to the substance.''
Kennecott Greens Creek Min. Co. v. Mine Safety & Health Admin., 476
F.3d 946, 952 (D.C. Cir. 2007) (quoting 30 U.S.C. 811(a)(6)(A)).
Although the Mine Act requires MSHA to consider the best available
evidence, the ``duty to use the best available evidence . . . cannot be
wielded as a counterweight to MSHA's overarching role to protect the
life and health of workers in the mining industry.'' Nat'l Min. Ass'n,
812 F.3d at 866. With this regulatory action, MSHA is addressing this
urgent need. See 30 U.S.C. 801(c).
On July 13, 2023, MSHA published a notice of proposed rulemaking,
entitled ``Lowering Miners' Exposure to Respirable Crystalline Silica
and Improving Respiratory Protection'', along with supplemental
documents. The Agency specifically sought comments on its preliminary
determination from the literature review that miners' exposure to
respirable crystalline silica presents a risk of material health
impairment or functional capacity. MSHA also requested input on any
additional adverse health effects that should be included or more
recent literature that offers a different perspective. MSHA received
numerous comments in response to this request and considered them in
preparing the final standalone Health Effects document and the final
rule.
This section will describe how MSHA conducted its review of the
health effects literature on respirable crystalline silica and what the
Agency has found about the toxicity of respirable crystalline silica.
This section will also present the findings on the following health
effects: (1) Silicosis; (2) Non-malignant respiratory disease (NMRD),
excluding silicosis; (3) Lung cancer and cancer at other sites; (4)
Renal disease; and (5) Autoimmune diseases. Public comments received
are reflected throughout this section.

A. General Approach to Health Effects Literature Review

MSHA reviewed a wide range of health effects literature totaling
over 600 studies that explore the relationship between respirable
crystalline silica exposure and resultant adverse health effects in
miners and other workers across various industries. The health effects
literature reviewed by MSHA included both studies reviewed by OSHA for
its 2016 respirable crystalline silica standard and many other newer
studies and studies that focused specifically on the mining industry.
OSHA's ``Health Effects Analysis and Preliminary Quantitative Risk
Assessment'' (2013b) included studies that were identified from
previously published scientific reviews, such as the IARC (1997) and
NIOSH (2002), and from newer evaluations of scientific literature,
literature searches, and contact with experts and stakeholders. That
document underwent extensive peer review by a panel of nationally
recognized experts in occupational epidemiology, biostatistics and risk
assessment, animal and cellular toxicology, and occupational medicine
who had no conflict of interest (COI) or apparent bias in performing
the review. These experts were asked to consider the strengths,
weaknesses, interpretations, and inclusion of studies used to support
the findings, and OSHA revised the document based on their feedback.
To ensure that its literature review was thorough and up to date,
MSHA reviewed a large body of additional evidence beyond the studies
considered by OSHA. It added many studies focused on miners' exposures
to respirable crystalline silica, as well as newer studies published
over the past decade. MSHA 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, analyses of morbidity (having a disease
or a symptom of disease) and mortality (disease resulting in death),
progression and pathology evaluations, death certificate and autopsy
reviews, medical surveillance data, health hazard assessments, in vivo
(animal) and in vitro (cell-based) toxicity data, and other
toxicological reviews. These studies are cited throughout this summary
and are listed in the References section of MSHA's standalone Health
Effects

[[Page 28233]]

document. Additionally, these studies appear in the rulemaking docket.
MSHA received some comments from industry stakeholders who
disagreed with MSHA's selection of studies for its literature review
and therefore with its findings. The Nevada Mining Association (NVMA)
and the Sorptive Minerals Institute (SMI) stated that not all relevant
studies were discussed in the Health Effects literature review
(Document ID 1441; 1446). NVMA also stated that the studies referenced
are outdated. The National Stone, Sand, & Gravel Association (NSSGA)
stated that MSHA's review is overly reliant on OSHA's review (2013b)
(Document ID 1448, Attachment 3). The state mining association stated
that the studies MSHA considered do not recognize that the likelihood
of prolonged exposure to respirable crystalline silica has been
dramatically reduced over the years, noting improvements to
respirators, equipment, and engineering controls (Document ID 1441).
However, commenters from health and labor organizations stated that
MSHA's review was thorough, was consistent with the scientific
consensus, and addressed the primary health effects of concern. These
commenters agreed with MSHA's findings and conclusions related to
health risks from exposure to respirable crystalline silica (Document
ID 1398; 1405; 1410; 1416). The American Public Health Association
(APHA) also noted the inclusion of several recent peer-reviewed
publications included in MSHA's review (Document ID 1416). The American
College of Occupational and Environmental Medicine (ACOEM) commented
that there has been an explosion of new information about the molecular
basis for silica's adverse effects since OSHA's comprehensive summary
of the medical literature in its preamble to the 2016 revisions to the
silica standard (Document ID 1405). This commenter stressed that this
new information only adds to the urgency of establishing and enforcing
MSHA's proposed standard and applauded the Agency's review of the
medical and epidemiologic literature on the health effects of silica
exposure.
MSHA has taken several steps to ensure that its review of health
effects literature represents the current understanding of health risks
related to exposures to respirable crystalline silica. In its initial
standalone Health Effects document, which was published alongside the
proposed rule, MSHA included several recent publications (published as
late as 2022), and since then, it has added more recent publications
(through 2023) in its final standalone Health Effects document.
Examples of recent literature included in the standalone Health Effects
document are: Carrington and Hershberger (2022), Cohen et al. (2022),
Descatha et al. (2022), Hall et al. (2022), and Keles et al. (2022).
Furthermore, many of the more recent studies included miners regulated
under the existing MSHA PEL of 100 [micro]g/m\3\ (e.g., Almberg et al.,
2017, 2018a; Graber et al., 2017; Blackley et al., 2018a; Cohen et al.,
2022). In response to the comment that the initial standalone Health
Effects document did not take into account improved mining conditions
or contemporary engineering controls, the Agency notes that it
considered several studies featuring miners in a larger range of
exposure groups, including some that had lower exposure levels (e.g.,
Mannetje et al., 2002b; Park et al., 2002; Buchanan et al., 2003;
Attfield and Costello, 2004; Chen et al., 2012).
Two commenters (an industry trade association and a training
consulting company) stated that MSHA presented a significant amount of
data showing the consequences of the various chronic health effects
that silica can and does have on the human body but no viable data on
mortality and morbidity among MNM miners (Document ID 1442; 1392).
As discussed elsewhere, MSHA is not required to prove a risk of
death due to silica exposure to justify regulating to reduce a silica
health risk. But the evidence shows that respirable silica exposure
causes death as well as chronic disease. MSHA reviewed and discussed
multiple studies that reported an increase in mortality rates
throughout the standalone Health Effects document (e.g., Bang et al.,
2005; Mazurek and Wood, 2008a; Liu et al., 2017a; Wang et al., 2020a).
Examples of MNM morbidity studies included are Mamuya et al. (2007),
Tse et al. (2007a), Rego et al. (2008), Reynolds et al. (2016), and
Wang et al. (2020b); while MNM specific mortality studies include
Attfield and Costello (2004), Chen et al. (2005, 2012), Schubauer-
Berigan et al. (2009), and Vacek et al. (2011), among others. MSHA
considered the best available evidence for MNM and concludes that MNM
miners have an increased mortality and morbidity due to exposure to
respirable crystalline silica.
Commenters from health and labor organizations suggested additional
studies for MSHA to include in the final standalone Health Effects
document (Document ID 1405; 1373; 1449). These studies included topics
such as new information regarding the molecular basis for silica's
adverse health effects or related to engineered stone workers. One
commenter stated that MSHA should include studies from outside of the
mining industry (Document ID 1448, Attachment 3).
MSHA thoroughly reviewed these studies and did not find sufficient
evidence to alter MSHA's overall conclusions of health risk, as
discussed in detail in the sections that follow. However, MSHA did add
many of the recommended studies to its final standalone Health Effects
document (e.g., Chilosi et al., 2003; Chen et al., 2018; Cao et al.,
2020). MSHA also reviewed other suggested literature, including
promising animal studies exploring novel drug treatments for diseases
caused by exposure to respirable crystalline silica; however, it
determined that these studies are not sufficiently developed for
inclusion at this time (e.g., Guo et al., 2019; Huang et al., 2019; Jia
et al., 2022). MSHA has already included several studies related to
non-mining occupations throughout its standalone Health Effects
document. Examples of other occupational studies include studies of
health effects on granite workers (e.g., Davis et al., 1983; Attfield
and Costello, 2004), brick workers (e.g., Merlo et al., 1991), agate
stone grinders (Rastogi et al., 1991), pottery workers (e.g., McDonald
et al., 1995; Cherry et al., 1998), industrial sand workers (e.g.,
McDonald et al., 2001; Rando et al., 2001), concrete workers (e.g.,
Meijers et al., 2001), ceramic workers (e.g., Forastiere et al., 2002),
and foundry workers (e.g., Hertzberg et al., 2002; Vihlborg et al.,
2017), among others. Occupations such as granite, industrial sand, or
concrete workers, represent similar job tasks and exposures which may
overlap with mining occupations. Others such as brick, pottery, and
ceramic workers involve processing of mined materials into a commercial
product.
To analyze the extensive literature that it considered, MSHA used
the widely accepted weight-of-evidence (WoE) approach. Under this
approach, studies with varied methodologies and conclusions 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. This
approach is a well-accepted method of conducting health hazard
assessments (NRC, 2009; NIOSH, 2019a). Additionally, it was used by
OSHA in its review of health effects literature (2013b) for its 2016
respirable crystalline silica standard. Factors that MSHA considered in
its WoE analysis include: (1) size of the cohort studied and power of
the study to detect a

[[Page 28234]]

sufficiently low level of disease risk; (2) duration of follow-up of
the study population; (3) potential for study bias, such as selection
bias or healthy worker effects, and (4) adequacy of underlying exposure
information for examining exposure-response relationships. Of the
studies examined in the standalone Health Effects document, studies
were deemed suitable for inclusion in the FRA if they provided adequate
quantitative information on exposure and disease risks and were judged
to be of sufficiently high quality according to the above criteria.
MSHA's literature review expanded upon OSHA's (2013b) review of the
health effects literature to support its final respirable crystalline
silica rule (81 FR 16286), reviewing pertinent new research. MSHA's
assessment of the literature is consistent with OSHA's conclusion from
its silica literature review.
MSHA received one comment from the NSSGA challenging the validity
of MSHA's literature review methodology (Document ID 1448, Attachment
3). This commenter submitted a report analyzing MSHA's health effects
literature review, arguing that MSHA's review cannot be replicated or
fully evaluated for its scientific validity and claiming that it is
unclear whether MSHA's interpretations are sufficiently reliable as a
basis for decision-making. The commenter asserted the need for
literature reviews to be done pursuant to Lynch et al.'s (2022)
framework of a ``systematic review,'' a review method that seeks to
eliminate bias by adhering to a transparent, a priori protocol. The
commenter also expressed concerns that MSHA's methodology is
inadequately explained and possibly dated. The commenter suggested
further studies to be included in MSHA's review and provided specific
responses to some of MSHA's statements in its literature review.
On the other hand, the APHA provided a different perspective on the
methodology (Document ID 1416). This commenter stated that MSHA
thoroughly describes the health risks, which include developing chronic
silicosis, accelerated silicosis, progressive massive fibrosis, chronic
obstructive pulmonary disease, lung cancer and kidney disease. Further,
the commenter noted that MSHA's review of the health effects literature
included more than three dozen peer-reviewed papers published in just
the last few years. This commenter concurred with MSHA's determination
that miners' exposure to respirable crystalline silica presents a risk
of material impairment of health or functional capacity.
MSHA disagrees with the comment challenging MSHA's methodology.
Although the ``systematic review'' framework outlined in Lynch et al.
(2022) is increasingly used in review publications, it is not the only
valid method of conducting a literature review of the current science.
As explained in the standalone Health Effects document, MSHA's review
of the scientific literature on respirable crystalline silica used a
widely accepted WoE approach.
The term, ``weight-of-evidence'' was coined as early as 40 years
ago by the NRC (1983) in their seminal publication ``Risk Assessment in
the Federal Government: Managing the Process''. It has become a
fundamental element of the risk assessment process (NRC, 2009; EPA,
1986; Martin et al., 2018; Lee et al., 2023). MSHA selected this
approach for use in its respirable crystalline silica risk analysis for
a variety of reasons. First, it has withstood the scrutiny of
scientists throughout the world (Suter et al., 2020). Second, it has
been used successfully throughout the world for conducting a wide
variety of risk assessments and analyses involving a wide range of
exposures in both occupational and environmental settings (e.g., drugs,
pesticides, industrial chemicals) (EPA, 1986, 2016; National Research
Council (NRC), 2009; Suter et al., 2020; Government of Canada, 2022).
Third, it continues to be a solid and accepted approach that is still
used today (EPA, 1986, 2016; National Research Council (NRC), 2009;
Martin et al., 2018; Suter et al., 2020; Government of Canada, 2022;
Lee et al., 2023). Current searches of the scientific literature (e.g.,
using search engines such as PubMed or Google Scholar) continue to
identify studies in which the WoE approach has been employed. Finally,
numerous courts have approved of federal agencies relying on this
methodology in rulemaking for over 40 years. See Mississippi v. E.P.A.,
744 F.3d 1334, 1344-45 (D.C. Cir. 2013) (upholding the ``weight of
evidence approach'' because ``one type of study might be useful for
interpreting ambivalent results from another type . . . and though a
new study does little besides confirm or quantify a previous finding,
such incremental (and arguably duplicative) studies are valuable
precisely because they confirm or quantify previous findings or
otherwise decrease uncertainty'') (citing Ethyl Corp. v. EPA, 541 F.2d
1, 26 (D.C. Cir. 1976) (en banc)); N. Am.'s Bldg. Trades Unions v.
OSHA, 878 F.3d 271, 284 (D.C. Cir. 2017) (rejecting challenges to
OSHA's ``weight of evidence'' approach supporting its silica
rulemaking). Thus, MSHA finds that the WoE approach is appropriate for
use in its respirable crystalline silica rulemaking.
In summary, MSHA's weight-of-evidence analysis is based on OSHA's
extensive literature review and peer review process; includes a
substantial number of studies and data published after the OSHA
rulemaking; and has received support from NIOSH experts.\16\
---------------------------------------------------------------------------

\16\ MSHA's review benefitted from feedback and review from
experts at NIOSH, both informally and through the interagency review
process organized by OMB, during the literature review process and
preparation of the standalone Health Effects document.
---------------------------------------------------------------------------

As described in greater detail in MSHA's standalone Health Effects
document, the scientific 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. 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 or
functional capacity. Regulatory action to reduce occupational exposures
that cause these diseases is necessary to ensure no miner suffers
material impairment of health or functional capacity, as required by
section 101(a)(6)(A) of the Mine Act.
Based on an extensive review of health effects literature, MSHA
determines that occupational exposure to respirable crystalline silica
causes silicosis (acute silicosis, accelerated silicosis, chronic
silicosis, and progressive massive fibrosis (PMF)), NMRD (including
COPD), lung cancer, and end-stage renal disease (ESRD). Each of these
effects is exposure-dependent, potentially chronic, irreversible,
potentially disabling, and can be fatal. In addition, MSHA's review of
the health effects literature has shown that respirable crystalline
silica exposure is causally related to the development of some
autoimmune disorders through inflammatory pathways. Current health
information cited in the final standalone Health Effects document
indicates that miners are suffering material impairment of health or
functional capacity due to their occupational exposures to respirable
crystalline silica. MSHA's review of respirable crystalline silica
health effects concludes that the final rule, which lowers the exposure
limits in MNM and coal mining to 50 [micro]g/m\3\ and establishes an
action level of 25 [micro]g/m\3\ for a full-shift exposure, calculated
as an 8-hour TWA, will reduce the risk

[[Page 28235]]

of miners developing silicosis, NMRD, lung cancer, and renal disease.

B. 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 they are deposited in mines or mills. Respirable crystalline
silica particles may be irregularly shaped and variable in size. These
particles may be inhaled by miners and can be deposited throughout the
lungs. Some pulmonary clearance of particles deposited in the alveolar
region (deep lung) may occur, but many 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
are not metabolized into less toxic compounds. This is important
because 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, both of which 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
(ATSDR, 2019).
Inflammatory 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
(Castranova and Vallyathan, 2000; Castranova, 2004; Hamilton et al.,
2008). The elevated production of ROS/RNS could result in oxidative
stress and lung injury that stimulate alveolar macrophages, ultimately
resulting in fibroblast activation and pulmonary fibrosis (Li et al.,
2018; Feng et al., 2020). The prolonged recruitment of macrophages and
PMN causes persistent inflammation, regarded as a primary step in the
development of silicosis.
The strong immune response in the lung following exposure to
respirable crystalline silica may also be linked to a variety of extra-
pulmonary adverse effects such as hypergammaglobulinemia
(overproduction of more than one class of immunoglobulins by plasma
cells), production of rheumatoid factor, anti-nuclear antibodies, and
release of other immune complexes (Haustein and Anderegg, 1998; Green
and Vallyathan, 1996; Parks et al., 1999). Respirable crystalline
silica exposure has also been associated with ESRD 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 (Nolan et
al., 1981; Shi et al., 1989, 1998; Brown and Donaldson, 1996;
Castranova, 2004; Fubini et al., 2004).
Three commenters expressed concerns about the findings of the
health effects literature review and their relevance to the sorptive
minerals industry (Document ID 1446, Attachment 1; 1442; 1419). The SMI
and Essential Minerals Association (EMA) stated that MSHA has an
incomplete understanding of the latest available scientific research
(Document ID 1446, Attachment 1; 1442). Asserting that occluded quartz
in sorptive clays is not fractured (either in the clay formation in
which it exists or during the mining and processing of the material to
form sorptive mineral-based products), the SMI concluded that occluded
quartz in sorptive clays does not pose the health risk posed by
fractured quartz (Document ID 1446, Attachment 1). Discussing at length
studies it recommended MSHA include in its health effects literature
review, SMI and EMA said that much of this research was previously
considered by OSHA (2013b) and that it had led to OSHA's decision to
exempt sorptive clays from coverage under OSHA's silica standard. SMI
also noted that additional research since OSHA's revised silica
standard was promulgated has advanced the question of how quartz causes
disease and the difference in risk potential between fractured and
unfractured and occluded quartz. Asserting that, without consideration
of the additional research provided, the proposed standard would not be
based on the best available evidence and would not reflect the latest
available scientific data in the field, this commenter discussed Mine
Act statutory provisions and case law that it asserted demonstrate the
high level of scientific evidence and scrutiny required of MSHA when
setting health and safety standards.
A more detailed response to SMI's overall comment can be found in
Section VIII.A. General Issues of this preamble. In response to the
suggestion to consider additional studies, MSHA reviewed the suggested
references and added some to the final standalone Health Effects
document (Creutzenberg et al., 2008; Borm et al., 2018; Pavan et al.,
2019). MSHA also notes that some of these studies were already cited in
the version of the standalone Health Effects document published
alongside the proposed rule (e.g., Donaldson and Borm, 1998; Fubini,
1998; Bruch et al., 2004; Fubini et al., 2004). Overall, many of the
studies suggested by the commenter have argued that occluded or aged
quartz is less toxic but have not suggested that occluded or aged
quartz is not toxic or carries no risk of disease. MSHA agrees that
there is some evidence to suggest that occluded silica is less toxic
than unoccluded silica (Wallace et al., 1996), but there is no evidence
that occlusion and the initial reduced toxicity persist following
deposition and retention of the crystalline silica particles in the
lungs. Similarly, animal studies involving respirable crystalline
silica suggest that the aged form has lower toxicity than the freshly
fractured form; however, the aged form still retains toxicity
(Shoemaker et al., 1995; Vallyathan et al., 1995; Porter et al.,
2002c). From these studies, MSHA concludes that

[[Page 28236]]

exposure to the crystalline silica present in sorptive minerals poses a
risk of material impairment of health or functional capacity to miners.
Others appeared to be irrelevant to the scope of the rule, such as
those focused on amorphous silica, microscopy techniques, or workshop
discussions (e.g., Mercer et al., 2018; Weber et al., 2018; Driscoll
and Borm, 2020). MSHA notes that none of the suggested animal studies
included acute or chronic inhalation exposures to aged or occluded
respirable crystalline silica. One suggested review, Poland et al.
(2023) described a 2020 animal inhalation study (nose-only) which did
not include exposures to aged or occluded respirable crystalline
silica; the 2020 study was conducted using amorphous silica and the
data were compared to a 1988 animal study that included whole-body (as
opposed to nose-only) exposures to respirable crystalline silica.\17\
Since this 2020 surface area comparison study described by Poland et
al. (2023) focused on amorphous silica, which is not a part of this
rulemaking, it was deemed unsuitable for inclusion in MSHA's final
standalone Health Effects document. Other animal studies discussing
aged or occluded respirable crystalline silica suggested used either
intratracheal instillation or oropharyngeal aspiration, which do not
reflect the behavior of particles that enter the lungs via inhalation,
including lung clearance (Foster et al., 2001; Wong, 2007; Driscoll and
Borm, 2020). Section VIII.A. General Issues of this preamble responds
more fully to these comments. In its response, MSHA notes that several
studies of occluded or fractured quartz discussed their methods,
including careful handling of occluded samples, but did not include
analysis of occluded quartz that was analyzed with less than careful
handling. This is not applicable to real-world conditions; MSHA's
experience with mining and processing of sorptive minerals includes the
use of grinding and milling processes.
---------------------------------------------------------------------------

\17\ These two studies (1988 and 2020) described by Poland et
al. (2023) had limited comparability for a variety of reasons; they
differ in: (1) rat strains (types of rats), (2) exposure durations,
(3) recovery periods, as well as (4) types of inhalation exposure,
among others.
---------------------------------------------------------------------------

After reviewing the available literature, MSHA concludes that
miners working in the sorptive minerals industry are exposed to
respirable crystalline silica. OSHA (2013b) concluded that while there
was considerable evidence that several environmental influences can
modify surface activity to either enhance or diminish the toxicity of
silica, the available information was insufficient to determine in any
quantitative way how these influences may affect disease risk to
workers in any particular workplace setting (81 FR at 16311). MSHA
agrees with OSHA (2013b) that there is evidence to support that surface
activity of respirable crystalline silica may play a role in producing
disease. However, mining is significantly different from other
industries regulated by OSHA, for instance, in that it involves
milling, grinding and removal of overburden. While the available
information is insufficient to determine how these influences may
affect disease risk to miners in any quantitative way and in any mining
sector. MSHA is permitted `` `to err on the side of overprotection by
setting a fully adequate margin of safety.' '' Kennecott Greens Creek
Min. Co. v. Mine Safety & Health Admin., 476 F.3d 946, 952 (D.C. Cir.
2007) (quoting Nat'l Min. Ass'n v. Mine Safety & Health Admin., 116
F.3d 520, 528 (D.C. Cir. 1997)).

C. Diseases

1. Silicosis
Silicosis is a material impairment of health or functional
capacity, as defined by the Mine Act, and refers to a group of lung
diseases caused by the inhalation of respirable crystalline silica. See
30 U.S.C. 811(a)(6)(A). Silicosis is a progressive, occupational
disease, in which accumulation of respirable crystalline silica
particles causes an inflammatory reaction in the lung. This reaction
leads to lung damage and scarring and, in some cases, progresses to
disability and death. Respirable crystalline silica has long been
identified as a cause of lung diseases in miners, and adverse health
effects were noted and described as early as 1550 by Georgius Agricola
(Agricola, as translated by Banner in 1950). Based on the review of the
literature, MSHA has determined that exposure to respirable crystalline
silica causes silicosis in MNM and coal miners and that it is a
significant cause of premature morbidity and mortality (Mazurek and
Attfield, 2008; Mazurek and Wood, 2008a,b; Mazurek et al., 2015, 2018).
When respirable crystalline silica accumulates in the lungs, it
causes an inflammatory reaction, leading to lung damage and scarring.
Silicosis can continue to develop even after silica exposure has ceased
(Hughes et al., 1982; Ng et al., 1987a; Hessel et al., 1988; Kreiss and
Zhen, 1996; Miller et al., 1998; Yang et al., 2006). 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 miners can lead to all three
forms of silicosis (acute, accelerated, and chronic). These forms
differ in the rate of exposure, pathology (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, an 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 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 leads to
the impairment of gas exchange (oxygen) in the lungs and respiratory
distress of the patient. The X-ray appearance and results of
microscopic examination of acute silicosis are like those of idiopathic
(having an unknown cause) pulmonary alveolar proteinosis.
Accelerated silicosis includes both inflammation and fibrosis and
is associated with intense respirable crystalline silica exposure.
Accelerated silicosis usually manifests over a period of three to ten
years (Cowie and Becklake, 2016), but it can develop in as little as
two to five years if exposure is sufficiently intense (Davis, 1996).
Accelerated silicosis may have features of both chronic and acute
silicosis, with alveolar proteinosis in addition to X-ray evidence of
fibrosis, seen as small opacities or the large opacities of PMF.
Although the symptoms are like 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. Accelerated silicosis
is frequently fatal.
Chronic silicosis is the most frequently observed form of silicosis
in the United States today (Banks, 2005; OSHA, 2013b; Cowie and
Becklake,

[[Page 28237]]

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 ten 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 histopathologically 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
identification of small and large opacity disease on chest X-ray films
usually underestimate the true prevalence of silicosis (Craighead and
Vallyathan, 1980; Hnizdo et al., 1993; Rosenman et al., 1997; Cohen and
Velho, 2002). 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
chronic cough, sputum production, shortness of breath, and reduced
pulmonary function.
Among coal miners, silicosis is usually found in conjunction with
simple coal workers' pneumoconiosis (CWP) because of the miners'
exposures to RCMD that also contains respirable crystalline silica
(Castranova and Vallyathan, 2000). 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; 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 (Davis et al., 1979; Ruckley et al., 1981, 1984; Douglas et al.,
1986; Fernie and Ruckley, 1987; Green et al., 1989, 1998b; Attfield et
al., 1994; Vallyathan et al., 2011; Cohen et al., 2016, 2019, 2022).
Autopsy studies in British coal miners indicated that the more advanced
the disease, the more mixed-RCMD 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 (Cohen et al., 2016, 2022), sandblasters
(Hughes et al., 1982; Abraham and Wiesenfeld, 1997), 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).
a. Classifying Radiographic Findings of Silicosis
The studies reviewed by MSHA used one of two established methods
for identifying findings of pneumoconiosis: the International Labour
Office (ILO) Classification System or the Chinese categorization
system, each of which is described below. In addition, the NIOSH case
definition of silicosis used in surveillance systems relies on the ILO
system.
The ILO developed a standardized system to classify the
radiographic appearances of pneumoconiosis identified in chest X-rays
films or digital chest radiographic images (ILO, 1980, 2002, 2011,
2022). One aspect of the ILO system involves grading the size, shape,
and profusion (density) of opacities in the lungs. The density of
opacities is classified on a four-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 consistent
with pneumoconiosis and categories 1 to 3 reflect increasing profusion
of opacities and a concomitant increase in severity of disease.
However, some studies in MSHA's literature review used the Chinese
system of X-ray classification based on the ``Radiological Diagnostic
Criteria of Pneumoconiosis and Principles for Management of
Pneumoconiosis'' (GB5906-86). This includes four categories of
pneumoconiosis findings: a suspected case (0+), stage I, stage II, or
stage III. Under this scheme, a panel of three radiologists determines
the presence and severity of radiographic changes consistent with
pneumoconiosis. 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).
MSHA's analysis of silicosis studies uses NIOSH's surveillance case
definition to determine the presence of silicosis. As described further
in the final standalone Health Effects document, 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 in which 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). MSHA interprets
``minimal effects'' to mean an X-ray ILO profusion score of category 1/
0 or greater. This is also consistent with Hnizdo et al. (1993), which
recommended that, due to the low sensitivity of chest x-rays for
detecting silicosis, radiographs consistent with an ILO category of 0/1
or greater be considered indictive of silicosis among workers exposed
to a high concentration of silica-containing dust.
b. Progression and Associated Impairment
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
literature not previously reviewed by OSHA (2013b) (Mohebbi and
Zubeyri, 2007; Wade et al., 2011; Dumavibhat et al., 2013).
Progression of silicosis is recognized when there are changes or
worsening of the opacities in the lungs, and sequential chest
radiographs are

[[Page 28238]]

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 variability in
film technique and classification of films, some investigators count
progression as advancing two or more subcategories, such as 1/0 to 1/2.
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 (Cochrane et al., 1956; Maclaren and Soutar, 1985; Hurley et
al., 1987; Kimura et al., 2010; Almberg et al., 2020; Hall et al.,
2020b). In addition, although lung function impairment is highly
correlated with chest X-ray films indicating silicosis, researchers
caution 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 (Ng et al., 1987a study
of granite miners; Hessel et al., 1988 study of gold miners; Miller et
al., 1998 study of coal miners; Miller and MacCalman, 2010 study of
coal 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 (the product of the
concentration times duration of exposure, which is summed over time)
(Ng et al., 1987a; Hessel et al., 1988; Miller et al., 1998; Miller and
MacCalman, 2010). 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. Duration of
employment 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 in 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\
and for a brief period, concentrations exceeded 10,000 [micro]g/m\3\
for one job. Some of these high exposures were associated with
accelerated disease progression in these miners.
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 [micro]g/m\3\) increased the silicosis
risk by three-fold (compared to the risk of cumulative exposure alone)
(see the standalone FRA document).
The risks of increased rate of progression predicted by Buchanan et
al. (2003) have been seen in coal miners (Miller et al., 1998; Laney et
al., 2010, 2017; Cohen et al., 2016), 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 (NIOSH, 2000a,b; Ogawa et al., 2003a; Mohebbi and
Zubeyri, 2007). Accordingly, it is important to limit higher exposures
to respirable crystalline silica to minimize the risk of rapid
progressive pneumoconiosis (RPP) in miners. RPP is the development of
progressive massive fibrosis (PMF) and/or an increase in small opacity
profusion greater than one subcategory over five years or less
(Ant[atilde]o et al., 2005).
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 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.
Many studies found an association between pulmonary function
decrements and ILO profusion category 2 or 3. Additionally, the review
of the literature indicated a decreased lung function among workers who
were exposed to respirable crystalline silica. MSHA therefore concludes
that respirable crystalline silica exposure may impair lung function in
some instances before silicosis can be detected by chest X-rays.
c. Occupation-Based Epidemiological Studies
MSHA reviewed the occupation-based epidemiological literature,
which examines health outcomes among workers and their potential
association with conditions in the workplace. In addition, MSHA
reviewed additional occupation-based literature specific to respirable
crystalline silica exposure in MNM and coal miners and 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 two months.
MSHA's review included the occupation-based literature cited by
OSHA (2013b) in developing its respirable crystalline silica standard
(OSHA, 2016a). Overall, MSHA found substantial evidence suggesting that
occupational exposure to respirable crystalline silica increases the
risk of silicosis. This conclusion is consistent with OSHA's
conclusion.
In a population of granite quarry workers (mean length of
employment:

[[Page 28239]]

23.4 years) exposed to an average respirable crystalline silica
concentration of 480 [micro]g/m\3\, 45 percent of those diagnosed with
simple silicosis showed radiological progression of disease two to ten
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).
The risk of silicosis, and particularly its progression, carries
with it an increased risk of reduced lung function. 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). Additionally, 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 chronic 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 FEV1/FVC).
Accordingly, MSHA concludes that respirable crystalline silica
exposure increases the risk of silicosis morbidity and mortality among
miners. This conclusion is consistent with OSHA's conclusion that there
is substantial evidence that occupational exposure to respirable
crystalline silica increases the risk of silicosis.
d. Surveillance Data
In addition to occupation-based epidemiological studies, MSHA
reviewed surveillance studies, including those submitted by commenters,
which provide and interpret data to facilitate the prevention and
control of disease, and ultimately MSHA finds that the prevalence of
silicosis generally increases with duration of exposure (work tenure).
This is evident from the statistically significant proportional
mortality ratios (PMRs) reported in the National Occupational Mortality
System (NORMS) data previously reviewed by OSHA and reported by MSHA in
its standalone Health Effects document. Several small and ad hoc
surveillance reports reported in the standalone Health Effects document
also found a prevalence of silicosis of up to 50 percent among working
and retired miners (Hnizdo and Sluis-Cremer, 1993; Ng and Chan, 1994;
Kreiss and Zhen, 1996; Finkelstein, 2000).
However, the available statistics may underestimate silicosis-
related morbidity and mortality in miners. It has been widely reported
that statistics underestimate silicosis cases due to: (1)
misclassification of causes of death (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, reliance on 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 it 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
In addition to the relationship between silica exposure and
silicosis, studies indicate a relationship between silica exposure,
silicosis, and pulmonary TB. MSHA reviewed these studies and concluded
that silica exposure and silicosis increase the risk of pulmonary TB
(Cowie, 1994; Hnizdo and Murray, 1998; teWaterNaude et al., 2006),
concurring with the conclusion reached by OSHA in its review.
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 (active) pulmonary TB
(Sherson and 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'' (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 studies published since OSHA's
(2013b) review (Oni and Ehrlich, 2015; Ndlovu et al., 2019). 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 seven
percent of men (n=54) and three percent of women (n=4) were found to
have pulmonary silicosis.
Overall, MSHA finds, consistent with OSHA's conclusion, that silica
exposure increases the risk of pulmonary TB, and that pulmonary TB can
be a complication of chronic silicosis.
2. Nonmalignant Respiratory Disease (Excluding Silicosis)
In addition to causing silicosis, exposure to respirable
crystalline silica causes other NMRD. NMRD is an umbrella term that
includes chronic obstructive pulmonary disease (COPD). Emphysema and
chronic bronchitis are two lung diseases included within COPD. In
patients with COPD, either chronic bronchitis or emphysema may be
present or both conditions may be present together (ATS, 2010a).
Based on its review of the literature, MSHA 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 results in the destruction of lung architecture in the
alveolar region, causing airway obstruction and impaired gas exchange.
Based on its health effects literature review, MSHA concludes that
exposure to respirable crystalline silica can increase the risk of
emphysema, regardless of whether silicosis is present. In addition,
MSHA concludes that this is the case for

[[Page 28240]]

smokers and that smoking amplifies the effects of respirable
crystalline silica exposure, increasing the risk of emphysema. MSHA's
conclusions are consistent with those drawn by OSHA (2013b). The
reviewed studies 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 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 lifelong 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
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 that previously reported by Becklake et al. (1987). A study by
Cowie et al. (1993) found that the presence and grade of emphysema were
statistically significant in Black underground gold miners.
B[eacute]gin et al. (1995) found that respirable crystalline silica-
exposed smokers without silicosis had a higher prevalence of emphysema
than a group of asbestos-exposed workers with a similar smoking
history.
Several of the studies 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 (Becklake et al., 1987;
Hnizdo et al., 1994; B[eacute]gin et al., 1995). 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 tend to support human studies that respirable crystalline
silica-induced emphysema can occur absent signs of silicosis.
OSHA (2013b) and others have concluded that there is a relationship
between respirable crystalline silica exposure and emphysema. 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. Additionally, NIOSH (2002b)
concluded in its Hazard Review that occupational exposure to respirable
crystalline silica is associated with emphysema; however, it noted some
epidemiological studies that suggested that this effect might be less
frequent or absent in non-smokers.
Overall, MSHA concludes that exposure to respirable crystalline
silica causes emphysema even in the absence of silicosis. Thus, MSHA
concurs with the conclusions previously reached by OSHA (2013b).
b. Chronic Bronchitis
MSHA considered many studies that examined the association between
respirable crystalline silica exposure and chronic bronchitis and
concluded 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.
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, specifically
the presence of a productive cough with sputum production for at least
three months of the year for at least two consecutive years (ATS,
2010b). MSHA's conclusions are supported by OSHA's review of the
literature.
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 four 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\ (Hurley et al., 2002).
Cowie and Mabena (1991) found that chronic bronchitis was present
in 742 of 1,197 (62 percent) 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. They did not
find as clear a relationship as did the above studies and concluded
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
(1975) found that the prevalence of chronic bronchitis rose
significantly with increasing dust concentration and cumulative dust
exposure in South African gold miners who were 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 the grinders' respirable
crystalline silica exposure durations were very short, and control
workers may also have been exposed to respirable crystalline silica
(Rastogi et al., 1991).
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 who had worked in the mines for

[[Page 28241]]

over one 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 five months when the study began, so miners were
not exposed at the time of the study.
Some reviews concluded that respirable crystalline silica exposure
causes the development of bronchitis. The American Thoracic Society
(ATS) (1997) published a review that found chronic bronchitis to be
common among worker groups exposed to dusty environments contaminated
with respirable crystalline silica. NIOSH (2002b) also published a
review demonstrating 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.
Additionally, Hnizdo et al. (1990) re-analyzed data from an earlier
investigation (Wiles and Faure, 1975) and 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. The
authors also found that for miners with the most severe impairment, the
effects of smoking and dust were synergistic (more than additive)
(Hnizdo et al., 1990).
Overall, MSHA concludes that exposure to respirable crystalline
silica causes chronic bronchitis, regardless of whether silicosis is
present, and that an exposure-response relationship may exist. This
conclusion is consistent with the findings of OSHA's Health Effects
document (2013b).
c. Pulmonary Function Impairment
Pulmonary function impairment is a common feature of NMRD and may
be assessed via spirometry (lung volumes, flows) and gas diffusion
tests. MSHA has reviewed the studies cited by OSHA and agrees with
their conclusions. 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.
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
(termed ``dropouts'') and consequently did not voluntarily participate
in the last of a series of annual pulmonary function tests. This group
experienced steeper declines in lung function compared to the subset of
workers who remained at work (termed ``survivors'') and participated in
all tests, 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
five-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 its 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 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 standards (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 exposure measurement surrogates such as tenure). The results of
these studies were like those of the longitudinal studies previously
discussed. In several studies, respirable crystalline silica exposure
was found to reduce lung function of:
(1) White South African gold miners (Hnizdo et al., 1990),
(2) Black South African gold miners (Irwig and Rocks, 1978; Cowie
and Mabena, 1991),
(3) Respirable crystalline silica-exposed workers in Quebec
(B[eacute]gin et al., 1995),
(4) Rock drilling and crushing workers in Singapore (Ng et al.,
1992b),
(5) Granite shed workers in Vermont (Theriault et al., 1974a,b),
(6) Aggregate quarry workers and coal miners in Spain (Montes et
al., 2004a,b),
(7) Concrete workers in the Netherlands (Meijers et al., 2001),
(8) Chinese refractory brick manufacturing workers in an iron-steel
plant (Wang et al., 1997),
(9) Chinese gemstone workers (Ng et al., 1987b),
(10) Hard-rock miners in Manitoba, Canada (Manfreda et al., 1982)
and in Colorado (Kreiss et al., 1989),
(11) Pottery workers in France (Neukirch et al., 1994),
(12) Potato sorters in the Netherlands (Jorna et al., 1994),
(13) Slate workers in Norway (Suhr et al., 2003), and
(14) Men in a Norwegian community with years of occupational
exposure to respirable crystalline silica (quartz) (Humerfelt et al.,
1998).
OSHA (2013b) 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 described by OSHA (2013b) have also

[[Page 28242]]

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 concludes that there is an exposure-
response relationship between respirable crystalline silica and the
impairment of lung function. MSHA also concludes that 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. MSHA's conclusions are consistent with OSHA's findings from
its literature review.
3. Lung Cancer
Commenters from United Steelworkers (USW), American Industrial
Hygiene Association (AIHA), and Vanderbilt Minerals, agreed with MSHA's
conclusion that miners exposed to respirable crystalline silica have an
increased risk of lung cancer (Document ID 1447; 1351; 1419). The AIHA
also cited research by the International Agency for Research on Cancer
(IARC) as documenting the health risks from inhalation of respirable
crystalline silica, specifically cancers of the lung, stomach, and
esophagus (Document ID 1351). MSHA agrees with this comment for the
reasons discussed below.
a. Lung Cancer
Lung cancer, an irreversible and usually fatal disease, is a type
of cancer that forms in lung tissue. MSHA has 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 ATS (1997),
the IARC (1997, 2012), the NTP (2000, 2016), NIOSH (2002b), and the
ACGIH (2010), which have classified respirable crystalline silica as a
``known human carcinogen.'' The Agency's determination also 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
(Gu[eacute]nel et al., 1989a,b; Costello et al., 1995; Carta et al.,
2001; Attfield and Costello, 2004), 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 (Hessel et al., 1986,
1990; Hnizdo and Sluis-Cremer, 1991; Meijers et al., 1991; Chen et al.,
1992, 2006, 2012; McLaughlin et al., 1992; Hua et al., 1994; Roscoe et
al., 1995; Steenland and Brown, 1995a; Reid and Sluis-Cremer, 1996;
Hnizdo et al., 1997; deKlerk and Musk, 1998; Finkelstein, 1998; Chen
and Chen, 2002; Schubauer-Berigan et al., 2009; Liu et al., 2017a; Wang
et al., 2020a,b, 2021), coal mining (Meijers et al., 1988; Miyazaki and
Une, 2001; Miller et al., 2007; Miller and MacCalman, 2010; Tomaskova
et al., 2012, 2017, 2020, 2022; Graber et al., 2014a,b; 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).
One commenter stated that the work of Steenland and Sanderson
should not be ``discounted'' and that Miller and MacCalman ``did not
report on occupational exposure monitoring concentrations'' reported by
Steenland and Sanderson (Document ID 1351).
MSHA chose Miller and MacCalman (2010) rather than the Steenland et
al. (2001a) pooled cohort study for its lung cancer mortality risk
model but has not discounted the study of Steenland and Sanderson. MSHA
has cited the Steenland and Sanderson (2001) study at multiple points
in the final standalone Health Effects document and has also cited
other investigations from both researchers. The Miller and MacCalman
(2010) study contained detailed time-exposure measurements of both
respirable crystalline silica (quartz) and total mine dust, detailed
individual work histories, and individual smoking histories. Further
discussion regarding the selection of the risk model of Miller and
MacCalman (2001) is located in the standalone FRA document.
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.
This evidence is confirmed by the ten-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 and 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; Attfield and Kuempel, 2008; Tomaskova et al.,
2012, 2017, 2020, 2022; Graber et al., 2014a,b; NIOSH, 2019a; Kurth et
al., 2020). These studies also discuss the associations between RCMD
and respirable crystalline silica exposures with lung cancer in coal
mining populations. Furthermore, the findings of 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 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 health effects literature,
MSHA has 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).
Other toxicity studies (non-animal) provide additional evidence of
the carcinogenic potential of respirable crystalline silica. Studies
using DNA exposed directly to freshly fractured respirable crystalline
silica demonstrate that respirable crystalline silica directly
increases DNA breakage. Cell culture research has investigated the
processes by which respirable crystalline silica disrupts 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 genotoxicity
support MSHA's determination that respirable crystalline silica is an
occupational carcinogen (Borm and Driscoll, 1996; Schins et al., 2002).
b. Cancers of Other Sites
In addition to examining studies on lung cancer, MSHA has reviewed
studies examining the relationship between respirable crystalline
silica exposure and cancers at other sites. MSHA has reviewed the
studies

[[Page 28243]]

examined by OSHA, together with additional studies focusing on miners'
exposure, and has concluded (as OSHA did) that there is insufficient
evidence to demonstrate a causal relationship between respirable
crystalline silica exposure and other (non-lung) cancer mortality. MSHA
notes that OSHA reviewed mortality studies, on 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. In addition, NIOSH (2002b) in their respirable
crystalline silica review concluded that no association has been
established between respirable crystalline silica exposure and excess
mortality from cancer at other sites. The following summarizes the
studies reviewed with inconclusive findings.
(1) Laryngeal Cancer
MSHA reviewed three lung cancer studies also discussed by OSHA
(2013b) which suggested an association between respirable crystalline
silica exposure and increased mortality from laryngeal cancer (Davis et
al., 1983; Checkoway et al., 1997; McDonald et al., 2001). However, a
small number of cases were reported in those studies, and the
researchers were unable to determine a statistically significant
effect. Therefore, MSHA found that there was little evidence of an
association based on these studies. OSHA also reached this conclusion.
(2) Gastric (Stomach) Cancer
MSHA reviewed the literature discussed by OSHA (2013b) to assess a
potential relationship between respirable crystalline silica exposures
and stomach cancers. OSHA concurred with observations made previously
by Cocco et al. (1996) and in the NIOSH (2002b) respirable crystalline
silica hazard review, 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 (Selikoff, 1978; Stern et al., 2001; Moshammer and
Neuberger, 2004; Finkelstein and Verma, 2005) or did not demonstrate a
statistically significant dose-response relationship (Tsuda et al.,
2001; Calvert et al., 2003). For these reasons, MSHA determined these
studies were inconclusive in the context of this rulemaking.
(3) Esophageal Cancer
MSHA has reviewed studies that focused on miners and concludes that
the literature does not support attributing increased esophageal cancer
mortality with exposure to respirable crystalline silica. The studies
by Meijers et al. (1991) and Swaen et al. (1995) assessed mortality
from esophageal cancer in Dutch underground coal miners. Meijers et al.
(1991) reported an elevated standardized mortality ratio (SMR) of 396,
which was not statistically significant. The SMR was based on two cases
out of 334 confirmed pneumoconiosis cases followed through the end of
1983 (case selection based on health screening between 1956-1960).
Swaen et al. (1995) reported a SMR of 62 (95 percent CI: 25-127) based
on seven cases out of 3,790 underground coal miners who were diagnosed
with pneumoconiosis between 1956 and 1960. This result was not
statistically significant.
MSHA reviewed the studies presented by OSHA (2013b) and agrees with
OSHA's conclusion that the literature does not support attributing
increased esophageal cancer mortality to exposure to respirable
crystalline silica. 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;
Yu et al., 2005; Wernli et al., 2006). Other studies also indicated
elevated rates of esophageal cancer mortality with respirable
crystalline silica exposure (Xu et al., 1996a; Tsuda et al., 2001).
However, OSHA (2013b) identified that in all studies, confounding 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).
(4) Other Sites
MSHA's review of additional studies specific to miners further
establishes that respirable crystalline silica exposure increases the
risk of lung cancer, although there is insufficient evidence to
demonstrate a causal relationship between respirable crystalline silica
exposure and other (non-lung) cancer mortalities. Specifically, MSHA
concludes that the epidemiological literature is not sufficient to
conclude that there is an association between respirable crystalline
silica exposures and increased cancer of the larynx, gastric cancer
mortality, or esophageal cancer mortality.
MSHA's conclusion is consistent with OSHA's conclusion. Overall,
OSHA concluded that there was insufficient evidence of an association
between silica exposure and cancer at sites other than the lungs. OSHA
included a health literature review by NIOSH (2002b) that examined
effects potentially associated with respirable crystalline silica
exposure; that review identified only infrequent reports of
statistically significant excesses of deaths for other cancers. Cancer
studies have been reported on 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.
MSHA has reviewed the studies cited by OSHA and agrees with OSHA's
conclusion. MSHA's review of additional studies specific to miners
further establishes that respirable crystalline silica exposure
increases the risk of lung cancer, though there is insufficient
evidence to demonstrate a causal relationship between respirable
crystalline silica exposure and other (non-lung) cancer mortalities.
4. Renal Disease
MSHA received two comments related to MSHA's conclusions related to
renal disease. The AIHA agreed that silica probably causes renal
disease, quoting a paper by Steenland (2005b) (Document ID 1351). In
contrast, the NSSGA stated that it was unclear whether renal disease is
causally related to occupational crystalline silica exposure, citing a
2017 German Federal Institute for Occupational Safety and Health
systematic review that conducted a meta-analysis on respirable
crystalline silica and non-malignant renal disease (M[ouml]hner et al.,
2017) (Document ID 1448).

[[Page 28244]]

MSHA acknowledges that some studies have not found associations
between respirable crystalline silica exposures and renal disease;
however, those studies are generally statistically underpowered,
meaning that their sample sizes are too small to detect even some
substantial health effects. In contrast, as discussed below, studies
with large cohort sizes and well-documented, validated job-exposure
matrices found statistically significant effects on renal disease. MSHA
reviewed the study by M[ouml]hner et al. (2017) and found that it was
not suitable for inclusion in the literature review. The selection
terms used by M[ouml]hner et al. (2017) appear to be overly limiting
and did not appear to capture many of the studies that were included in
MSHA's previous standalone Health Effects document published with its
proposed silica rule (e.g., Gregorini et al., 1993; Hotz et al., 1995;
Fenwick and Main, 2000; Rosenman et al., 2000; Kurth et al., 2020).
MSHA also notes that several studies included in the review by
M[ouml]hner et al. (2017) were already cited in MSHA's previous
standalone Health Effects document published with its proposed silica
rule (e.g., Koskela et al., 1987; Brown et al., 1997; Checkoway et al.,
1997; Calvert et al., 2003; Brown and Rushton, 2005b).
Renal disease is characterized by the loss of kidney function and,
in the case of ESRD, a permanent loss of kidney function leading to the
need for a regular course of long-term dialysis or a kidney transplant
to maintain life. 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 concludes that there
is substantial evidence in the literature suggesting that occupational
exposures to respirable crystalline silica exposure increases the risk
of morbidity and 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 (now
known as granulomatosis with polyangiitis, GPA), which is severe injury
to the glomeruli that, if untreated, rapidly leads to renal failure
(Nuyts et al., 1995). 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 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 determines that respirable crystalline silica
exposure in mining increases the risk of renal disease.
5. Autoimmune Disease
Two commenters--AIHA and National Coalition of Black Lung and
Respiratory Disease Clinics (hereafter referred to as ``Black Lung
Clinics'')--agreed with MSHA's finding that there is evidence of a
relationship between respirable crystalline silica exposure and
autoimmune diseases (Document ID 1351; 1410). The Black Lung Clinics
also qualified that there is insufficient data to model the risk of
disease (Document ID 1410). This is consistent with MSHA's conclusion
that there is a casual association between occupational exposure to
respirable crystalline silica and the development of systematic
autoimmune diseases in miners; however, there are no studies available
to date that can be used to model respirable crystalline silica-
exposure risk of autoimmune diseases in the Agency's risk analysis.
Autoimmune diseases occur when the immune system mistakenly attacks
healthy tissues within the body, causing inflammation, swelling, pain,
and tissue damage. Examples of autoimmune diseases include autoimmune
rheumatic diseases, sarcoidosis and seropositive rheumatoid arthritis
(RA), Crohn's disease (CD), ulcerative colitis (UC), systemic lupus
erythematosus (SLE), scleroderma, and systemic sclerosis (SSc). Some
studies reviewed by MSHA suggest a casual association between
occupational exposure to respirable crystalline silica and the
development of systematic autoimmune diseases, particularly RA.
Wallden et al. (2020) found that respirable crystalline silica
exposure is correlated with an increased risk of developing UC, and
that the risk 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. Vihlborg et al. (2017) found a significant
increased risk of seropositive RA with high exposure (>48 [micro]g/
m\3\) to respirable crystalline silica when compared to rates for
individuals with lower or no exposure. They examined detailed exposure-
response relationships across four different groups, each of which was
exposed to a different concentration of respirable crystalline silica
(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 (a condition in which groups of
cells in the immune system form granulomas in various organ systems)
and seropositive RA in relation to respirable crystalline silica
exposure using models that could be used in MSHA's risk analysis. 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 that could be used in the risk analysis.
Based on its literature review, MSHA concludes that there is a
causal association between occupational exposure to respirable
crystalline silica and the development of systemic autoimmune diseases
in miners, but that no studies are available to date that can be used
to model respirable crystalline silica-exposure risk in a risk
analysis.

D. Conclusion

MSHA concludes that exposure to respirable crystalline silica
causes silicosis (acute, accelerated, chronic, and PMF), NMRD
(including COPD), lung cancer, and renal disease. Each of these effects
is exposure-dependent, potentially chronic, irreversible, potentially
disabling, and can be fatal.

[[Page 28245]]

Respirable crystalline silica exposure is also linked to the
development of some autoimmune disorders through inflammatory pathways.
The health effects 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 exposure limits of 100
[micro]g/m\3\ still have an unacceptable amount of excess risk, for
developing and dying from diseases related to their occupational
respirable crystalline silica exposures.
MSHA is entrusted with ensuring that ``no miner will suffer
material impairment of health or functional capacity even if such miner
has regular exposure to the hazards dealt with by such standard for the
period of his working life'' (30 U.S.C. 811(a)(6)(A)). The Agency
believes that when the final rule is implemented and enforced
effectively, it will reduce the rate of silicosis and other diseases
caused by respirable crystalline silica exposure and will substantially
improve miners' lives.

VI. Final Risk Analysis Summary

MSHA's FRA quantifies risks associated with five specific health
outcomes identified in the standalone Health Effects document:
silicosis morbidity and mortality, and mortality from NMRD, lung
cancer, and ESRD. This section serves as a summary of the standalone
FRA document, which is placed into the rulemaking docket for the MSHA
respirable crystalline silica rulemaking (RIN 1219-AB36, Docket No.
MSHA-2023-0001) and is available at <a href="http://Regulations.gov">Regulations.gov</a>.
MSHA developed an FRA to support its risk determinations and to
quantify the health risk to miners exposed to respirable crystalline
silica under the existing exposure limits for MNM and coal miners, at
the new PEL of 50 [micro]g/m\3\, and at the action level of 25
[micro]g/m\3\.
This analysis addresses three questions related to the final 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 final rule will reduce those risks.
To answer these questions, MSHA relied on the large body of
research on the health effects of respirable crystalline silica and
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
quantitative risk assessments are based on several studies of
occupational cohorts in a variety of industrial sectors. The underlying
studies are described in the standalone Health Effects document and are
summarized in Section V. Health Effects Summary.
Based on its analysis, MSHA found that, once the current mining
workforce is replaced with new entrants to the mining industry so that
all working miners and retired miners have been exposed only under the
new PEL, the final rule will decrease lifetime excess deaths by at
least 1,067 and will decrease lifetime excess cases of non-fatal
silicosis by at least 3,746 among the working and future retired miner
population. In the FRA, MSHA also increases its estimate of the number
of miners who will benefit from this rule to include future retired
miners. While the Preliminary Risk Analysis (PRA) did consider
reductions in excess risk during years of retirement, the PRA did not
account for the fact that future retired miners are among the
population that will benefit from the rule. Once the entire mining
workforce, including future retired miners, has worked only under the
new PEL (i.e., 60 years after the start of implementation of the rule),
both the retired and working miners will experience fewer deaths and
illnesses. The FRA updates benefit estimates to account for all
lifetime excess cases that will be avoided among all working miners and
future retired miners. It is important to note that the FRA (as well as
the FRIA, discussed below in Section IX) only monetizes benefits to
future retired miners. The FRA methodology does not attribute any
health benefits to individuals who retired before the start of
implementation of the final rule.
This summary highlights the main findings from the FRA, briefly
describes how they were derived, and directs readers interested in more
detailed information to corresponding sections of the standalone FRA
document.

A. Summary of MSHA's Final 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 working and the future population of retired MNM and coal
miners based on real exposure conditions, as indicated by the samples
in the compliance sampling datasets.
MSHA's FRA is largely based on the methodology and findings from
OSHA's 2013 preliminary quantitative risk assessment (PQRA), OSHA's
2016 final quantitative risk assessment (QRA), and the 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 FRA 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
new PEL, and (3) the action level. As in past MSHA rulemakings, MSHA
estimated and compared lifetime excess risks associated with exposures
at the existing and new PEL (and at the action level) over a miner's
full working life of 45 years and 15 years of retirement.
MSHA's FRA 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 (i.e., 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
action level (25 [micro]g/m\3\) to above the existing standards (100
[micro]g/m\3\ in MNM standards and 100 [micro]g/m\3\ MRE in coal
standards,

[[Page 28246]]

which is approximately 85.7 [micro]g/m\3\ ISO).<SUP>18 19</SUP>

\18\ As discussed in the FRA, 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 FRA converts the MRE full-shift TWA concentrations experienced
by coal miners to ISO 8-hour TWA concentrations. (See Section 4 of
the standalone FRA document for a full explanation.) The equation
used to convert MRE full-shift TWA concentrations into ISO 8-hour
TWA concentrations is:
[GRAPHIC] [TIFF OMITTED] TR18AP24.077

Exposures at TWA 100 [micro]g/m\3\ MRE and SWA 85.7 [micro]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 [micro]g/m\3\ as 85.7
[micro]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 new 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.
\19\ A sample-specific exposure limit is calculated for each
sample based on the polymorphs present. For samples with >1% quartz
by mass, the formula is:
[GRAPHIC] [TIFF OMITTED] TR18AP24.078

When quartz is the only respirable crystalline silica polymorph
in the sample, the existing MNM standard limits respirable
crystalline silica exposures to 100 [micro]g/m\3\ or less in MNM
operations. Cristobalite exposures are currently limited to 50
[micro]g/m\3\ or less when cristobalite is the only polymorph
present, and the same is true for tridymite \19\. When more than one
polymorph is present in the same sample, then a Threshold Limit
Value for mixtures is used.

One commenter (a safety compliance consultant) stated that the \20\
2005-2019 MNM respirable dust samples analyzed for respirable
crystalline silica show a downward trend in average annual rates of
overexposure and requested access to data for 2020-2022 (Document ID
1383). In response, MSHA notes that the 2020-2022 data may be skewed by
the reduction in mining during the COVID-19 pandemic and would
therefore bias the analysis. Further, 2019 is recent enough to
adequately capture the current exposure profile of working miners.
---------------------------------------------------------------------------

\20\
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In addition, commenters from the United Mine Workers of America
(UMWA), the Black Lung Clinics, and the Appalachian Citizens' Law
Center (ACLC) expressed concern that MSHA used coal mine dust data from
2016-2021, a historically low period for quartz levels in coal mining,
according to the commenters (Document ID 1398; 1410; 1445). The ACLC
asserted that, as a result, the estimate of avoided illnesses and
deaths in MSHA's PRA is low and urged the Agency to include a longer
history of coal dust sampling data when estimating miners' future
exposures (Document ID 1445). As discussed below, MSHA chose this time
period to account for the 2014 RCMD Standard, which came into full
effect in 2016. The ACLC also stated that, because the 2014 RCMD
Standard does not directly regulate respirable crystalline silica,
there is no justification for excluding prior sampling data (Document
ID 1445).
MSHA believes the 2014 RCMD Standard impacted respirable
crystalline silica exposures, in part because (a) the coal dust
exposure limit is based on a formula that reduces the limit when the
respirable crystalline silica content exceeds 100 [micro]g/m\3\, and
(b) measures that coal mine operators may have taken to reduce
exposures to coal dust under that rule would have also reduced
exposures to other respirable hazards including crystalline silica.
Using more recent coal exposure data from 2016-2021 thus avoids
possibly attributing benefits from the 2014 RCMD Standard to this rule.
However, MSHA agrees that if respirable crystalline silica
concentrations were to rise in the future--while remaining within the
limits of the 2014 RCMD Standard and complying with all existing
regulations--there would be additional unquantified benefits from the
final rule.\21\ For example, some researchers have attributed the
increase in pneumoconiosis prevalence among miners since the 1990s to
respirable crystalline silica (Cohen et al., 2022; Hall et al., 2020b).
Cohen et al. (2022) states that respirable crystalline silica has
become more concentrated due to improvements in mining equipment and
processing technology, which allow ``recovery of thin coal seams, which
involves the extraction of large quantities of surrounding rock strata
that can contain crystalline silica.'' The possibility that respirable
crystalline silica exposure could increase in the future in the absence
of this rule underscores the rule's importance.
---------------------------------------------------------------------------

\21\ In the analyzed coal compliance data from 2016 through
2021, only 6 percent of samples are above the new PEL of 50 [mu]g/
m\3\. Currently regulation provides protections to keep samples
below 85.7 [mu]g/m\3\, but it is insufficient to prevent increases
in the proportion of concentrations in the range of 50 to 85.7
[mu]g/m\3\. The possibility of such an increase further necessitates
this rule.
---------------------------------------------------------------------------

The primary results of the FRA are the calculated number of deaths
and illnesses avoided assuming full compliance after implementation of
MSHA's final 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 excess illnesses and deaths currently
occurring in the industry, assuming mines fully comply with the
previous standards (100 [micro]g/m\3\ for MNM and 85.7 [micro]g/m\3\
ISO for coal) and (b) the number of excess deaths and illnesses
expected to occur following implementation of the final rule, which
includes a new PEL of 50 [micro]g/m\3\ for a full-shift exposure,
calculated as an 8-hour TWA.
Excess 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 new PEL 50
[mu]g/m\3\ scenario where all risks were capped at the new 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 new PEL.

[[Page 28247]]

To calculate excess 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 miner (excluding contract miner) 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 miners (excluding contract miners), 60,275 for MNM
contract miners, 51,573 for coal miners (excluding contract miners),
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 uses weighted average FTE ratios to account for the fact that
contract miners may experience lower exposures per year from mining.
However, this underestimates the cumulative exposures that miners
(excluding contract miners) experience. The average coal miner
(excluding contract miners), for example, works approximately 2,280
hours per year, which equates to an average shift of over 9.1 hours
when assuming 250 working days per year.\22\ Additionally, the studies
the FRA relied on to model excess risks define a full working year as
1,740 hours, in instances where such a definition is given (Buchanan et
al., 2003; Miller and MacCalman, 2010). Based on these studies'
definition of a year, MNM miners (excluding contract miners) have an
FTE ratio of 1.13 and coal miners (excluding contract miners) have an
FTE ratio of 1.31. Additionally, the contract miner FTE ratios likely
have some negative bias since any individual who works for multiple
contracting companies is counted multiple times in the data, inflating
the denominator in the FTE ratio calculation. MSHA also notes that the
contract miner FTE ratios may underrepresent the true overall
cumulative exposures since contract miners may have other jobs
involving exposure to respirable crystalline silica (e.g., in
construction or the oil and gas industry).
---------------------------------------------------------------------------

\22\ The fact that miners work over 8-hour shifts is also
supported by MSHA's compliance data, which show an average shift
duration of approximately 9.2 hours for MNM (MSHA, 2022a) and 9.6
hours for coal (MSHA, 2022b). These values differ from the average
hours per day implied by the FTE ratios because the compliance data
is only a sample of full shifts, whereas the FTE data is based on
comprehensive reporting of all full-time and part-time shifts.
---------------------------------------------------------------------------

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 \23\ 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 enter the workforce at the
start of age 21, retire at the end of age 65, and do not live past the
end of age 80. From the life tables, MSHA acquired the lifetime excess
risk of mortality by summing the miner cohort's excess 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).
---------------------------------------------------------------------------

\23\ 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 New PEL 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 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 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 of working and future retired
miners 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 new PEL, MSHA compared the lifetime excess risks and
lifetime excess cases across the two scenarios (Baseline and New PEL 50
[mu]g/m\3\).
In the PRA, MSHA underestimated the number of miners who will
benefit from the proposed rule. Based on the 2019 Quarterly Employment
Production Industry Profile (MSHA, 2019a) and the 2019 Quarterly
Contractor Employment Production Report (MSHA, 2019b), the current
number of working miner FTEs is estimated to be 184,615 for MNM and
72,768 for coal. In the PRA, MSHA assumed excess cases of disease would
be reduced only among these working miners. However, once the current
mining workforce is replaced with new entrants to the mining industry
so that the entire workforce has worked only under the new PEL for
their 45 years of working life, the future mining workforce will
experience fewer excess deaths and illnesses from exposure to
respirable crystalline silica. The PRA's methodology did not include
the number of future retired miners who will experience lower exposure
for their working lives under the final rule and will continue to
benefit during retirement, and therefore, the PRA underestimated the
number of avoided lifetime excess cases attributable to the rule. In
the FRA, the estimates are updated to account for all excess cases that
will be avoided among not only working miners but also future retired
miners. As discussed in greater detail in the FRA, the number of future
retired miners who are expected to benefit from the rule can be
calculated from the survival rates (which are computed in the life
tables) and from the assumption that the mining workforces in MNM and
coal will remain the same size as they are today.
On the related question raised by the ACLC about whether new
clinical data suggests that the PRA underestimated benefits of the
lower PEL, MSHA

[[Page 28248]]

determines that the approach in the PRA is the appropriate one
(Document ID 1445). The risk models that MSHA uses are exposure-
response models, originally selected through OSHA's peer review process
and silica rulemaking, based on past clinical data on patients whose
exposure history was known. Newer data from Black Lung Clinics can
provide suggestive evidence of the risks, but because it is not yet
incorporated into these peer-reviewed risk models, it cannot be
included in this analysis as this commenter recommends.

B. Overview of Epidemiologic Studies

MSHA reviewed extensive research on the health effects of
respirable crystalline silica and quantitative risk assessments
published in the peer-reviewed scientific literature regarding
occupational exposure risks of illness and death from silicosis, NMRD,
lung cancer, and ESRD. The standalone 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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[[Page 28250]]

[GRAPHIC] [TIFF OMITTED] TR18AP24.139

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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 working and future retired miner populations based on real
exposure conditions. Table VI-2 summarizes key characteristics of the
models presented in the 13 studies from OSHA's PQRA, including the
cohort that was investigated, the specific health endpoint (e.g., chest
X-ray of category 2/1+), whether a lag between exposure and excess risk
was included, and key model parameters. 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 [micro]g/m\3\. Thorough
evaluation has led MSHA to determine that the studies OSHA selected
still provide the best available epidemiological models (with the
exception of lung cancer mortality). However, MSHA utilized the Miller
and MacCalman (2010) study to estimate risks for lung cancer mortality.
This study was included in OSHA's health effects assessment and PQRA
but was published after OSHA completed much of its modeling for the
PQRA. The following lists the studies 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).
As explained in detail in the standalone FRA document, MSHA
developed its risk estimates based on recent mortality data and certain
assumptions that differed from those used by OSHA. 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).\24\
---------------------------------------------------------------------------

\24\ FTEs were used to adjust the cumulative exposure over a
year based on the average number of hours that miners work.

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[[Page 28251]]

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
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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[[Page 28253]]

[GRAPHIC] [TIFF OMITTED] TR18AP24.141

[[Page 28254]]

[GRAPHIC] [TIFF OMITTED] TR18AP24.142

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Two commenters (SMI and NVMA) expressed concern that not all
relevant studies were considered in MSHA's analysis of the health
effects literature on occupational exposure to respirable crystalline
silica (Document ID 1446; 1441). For example, the NVMA commented that
the studies referenced in the health effects literature review are

[[Page 28255]]

outdated and do not recognize the changing conditions in mines that
reduce the likelihood of prolonged exposure to respirable crystalline
silica, such as the updates made by mines in response to the diesel
particulate matter standard published in the early 2000s (Document ID
1441). Similarly, the Pennsylvania Coal Alliance stated that the
majority of research MSHA relied on did not account for significant
technological advancements in mining and dust control technology
(Document ID 1378). This commenter further asserted that the rule
cannot be justified until the effects of the 2014 RCMD Standard are
better understood (Document ID 1378).
MSHA reviewed the relevant literature, including recent
publications. Additionally, in response to comments on the PRA, MSHA
read and reviewed studies suggested by commenters. MSHA selected the
studies which provide the best available epidemiological models to
develop the estimates of lifetime excess risks and lifetime excess
cases. These mod

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