Proposed Rule2023-00269
Reconsideration of the National Ambient Air Quality Standards for Particulate Matter
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Published
January 27, 2023
Issuing agencies
Environmental Protection Agency
Abstract
Based on the Environmental Protection Agency's (EPA's) reconsideration of the air quality criteria and the national ambient air quality standards (NAAQS) for particulate matter (PM), the EPA proposes to revise the primary annual PM<INF>2.5</INF> standard by lowering the level. The Agency proposes to retain the current primary 24-hour PM<INF>2.5</INF> standard and the primary 24-hour PM<INF>10</INF> standard. The Agency also proposes not to change the secondary 24-hour PM<INF>2.5</INF> standard, secondary annual PM<INF>2.5</INF> standard, and secondary 24-hour PM<INF>10</INF> standard at this time. The EPA also proposes revisions to other key aspects related to the PM NAAQS, including revisions to the Air Quality Index (AQI) and monitoring requirements for the PM NAAQS.
Full Text
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[Federal Register Volume 88, Number 18 (Friday, January 27, 2023)]
[Proposed Rules]
[Pages 5558-5719]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2023-00269]
[[Page 5557]]
Vol. 88
Friday,
No. 18
January 27, 2023
Part III
Environmental Protection Agency
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40 CFR Parts 50, 53, and 58
Reconsideration of the National Ambient Air Quality Standards for
Particulate Matter; Proposed Rule
Federal Register / Vol. 88 , No. 18 / Friday, January 27, 2023 /
Proposed Rules
[[Page 5558]]
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ENVIRONMENTAL PROTECTION AGENCY
40 CFR Parts 50, 53, and 58
[EPA-HQ-OAR-2015-0072; FRL-8635-01-OAR]
RIN 2060-AV52
Reconsideration of the National Ambient Air Quality Standards for
Particulate Matter
AGENCY: Environmental Protection Agency (EPA).
ACTION: Proposed rule.
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SUMMARY: Based on the Environmental Protection Agency's (EPA's)
reconsideration of the air quality criteria and the national ambient
air quality standards (NAAQS) for particulate matter (PM), the EPA
proposes to revise the primary annual PM<INF>2.5</INF> standard by
lowering the level. The Agency proposes to retain the current primary
24-hour PM<INF>2.5</INF> standard and the primary 24-hour
PM<INF>10</INF> standard. The Agency also proposes not to change the
secondary 24-hour PM<INF>2.5</INF> standard, secondary annual
PM<INF>2.5</INF> standard, and secondary 24-hour PM<INF>10</INF>
standard at this time. The EPA also proposes revisions to other key
aspects related to the PM NAAQS, including revisions to the Air Quality
Index (AQI) and monitoring requirements for the PM NAAQS.
DATES: Comments must be received on or before March 28, 2023.
Public Hearings: The EPA will hold a virtual public hearing on this
proposed rule. This hearing will be announced in a separate Federal
Register document that provides details, including specific dates,
times, and contact information for these hearings.
ADDRESSES: You may submit comments, identified by Docket ID No. EPA-HQ-
OAR-2015-0072, by any of the following means:
<bullet> Federal eRulemaking Portal: <a href="https://www.regulations.gov/">https://www.regulations.gov/</a>
(our preferred method). Follow the online instructions for submitting
comments.
<bullet> Email: <a href="/cdn-cgi/l/email-protection#99f8b4f8f7fdb4ebb4ddf6faf2fcedd9fce9f8b7fef6ef"><span class="__cf_email__" data-cfemail="86e7abe7e8e2abf4abc2e9e5ede3f2c6e3f6e7a8e1e9f0">[email protected]</span></a>. Include the Docket ID No.
EPA-HQ-OAR-2015-0072 in the subject line of the message.
<bullet> Mail: U.S. Environmental Protection Agency, EPA Docket
Center, Air and Radiation Docket, Mail Code 28221T, 1200 Pennsylvania
Avenue NW, Washington, DC 20460.
<bullet> Hand Delivery or Courier (by scheduled appointment only):
EPA Docket Center, WJC West Building, Room 3334, 1301 Constitution
Avenue NW, Washington, DC 20004. The Docket Center's hours of
operations are 8:30 a.m.-4:30 p.m., Monday-Friday (except Federal
Holidays).
Instructions: All submissions received must include the Docket ID
No. for this document. Comments received may be posted without change
to <a href="https://www.regulations.gov">https://www.regulations.gov</a>, including any personal information
provided. For detailed instructions on sending comments and additional
information on the rulemaking process, see the SUPPLEMENTARY
INFORMATION section of this document.
FOR FURTHER INFORMATION CONTACT: Dr. Lars Perlmutt, Health and
Environmental Impacts Division, Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency, Mail Code C539-04,
Research Triangle Park, NC 27711; telephone: (919) 541-3037; fax: (919)
541-5315; email: <a href="/cdn-cgi/l/email-protection#433326312f2e3637376d2f223130032633226d242c35"><span class="__cf_email__" data-cfemail="225247504e4f5756560c4e435051624752430c454d54">[email protected]</span></a>.
SUPPLEMENTARY INFORMATION:
General Information
Preparing Comments for the EPA
Follow the online instructions for submitting comments. Once
submitted to the Federal eRulemaking Portal, comments cannot be edited
or withdrawn. The EPA may publish any comment received to its public
docket. Do not submit electronically any information you consider to be
Confidential Business Information (CBI) or other information whose
disclosure is restricted by statute. Multimedia submissions (audio,
video, etc.) must be accompanied by a written submission. The written
comment is considered the official comment and should include
discussion of all points you wish to make. The EPA will generally not
consider comments or comment contents located outside of the primary
submission (i.e., on the web, the cloud, or other file sharing system).
For additional submission methods, the full EPA public comment policy,
information about CBI or multimedia submissions, and general guidance
on making effective comments, please visit <a href="https://www.epa.gov/dockets/commenting-epa-dockets">https://www.epa.gov/dockets/commenting-epa-dockets</a>.
When submitting comments, remember to:
<bullet> Identify the action by docket number and other identifying
information (subject heading, Federal Register date and page number).
<bullet> Explain why you agree or disagree, suggest alternatives,
and substitute language for your requested changes.
<bullet> Describe any assumptions and provide any technical
information and/or data that you used.
<bullet> Provide specific examples to illustrate your concerns and
suggest alternatives.
<bullet> Explain your views as clearly as possible, avoiding the
use of profanity or personal threats.
<bullet> Make sure to submit your comments by the comment period
deadline identified.
Availability of Information Related to This Action
All documents in the dockets pertaining to this action are listed
on the <a href="http://www.regulations.gov">www.regulations.gov</a> website. This includes documents in the
docket for the proposed decision (Docket ID No. EPA-HQ-OAR-2015-0072)
and a separate docket, established for the Integrated Science
Assessment (ISA) (Docket ID No. EPA-HQ-ORD-2014-0859) that has been
adopted by reference into the docket for this proposed decision.
Although listed in the index, some information is not publicly
available, e.g., CBI or other information whose disclosure is
restricted by statute. Certain other material, such as copyrighted
material, is not placed on the internet and may be viewed with prior
arrangement with the EPA Docket Center. Additionally, a number of the
documents that are relevant to this proposed decision are available
through the EPA's website at <a href="https://www.epa.gov/naaqs/particulate-matter-pm-air-quality-standards">https://www.epa.gov/naaqs/particulate-matter-pm-air-quality-standards</a>. These documents include the Integrated
Science Assessment for Particulate Matter (U.S. EPA, 2019a), available
at <a href="https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=347534">https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=347534</a>, the
Supplement to the 2019 Integrated Science Assessment for Particulate
Matter (U.S. EPA, 2022a), available at <a href="https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=354490">https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=354490</a>, and the Policy Assessment for the
Reconsideration of the National Ambient Air Quality Standards for
Particulate Matter (U.S. EPA, 2022b), available at <a href="https://www.epa.gov/naaqs/particulate-matter-pm-standards-integrated-science-assessments-current-review">https://www.epa.gov/naaqs/particulate-matter-pm-standards-integrated-science-assessments-current-review</a>.
Table of Contents
The following topics are discussed in this preamble:
Executive Summary
I. Background
A. Legislative Requirements
B. Related PM Control Programs
C. Review of the Air Quality Criteria and Standards for
Particulate Matter
1. Reviews Completed in 1971 and 1987
2. Review Completed in 1997
3. Review Completed in 2006
4. Review Completed in 2012
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5. Review Completed in 2020
6. Reconsideration of the 2020 PM NAAQS Final Action
a. Decision To Initiate a Reconsideration
b. Process for Reconsideration of the 2020 PM NAAQS Decision
D. Air Quality Information
1. Distribution of Particle Size in Ambient Air
2. Sources and Emissions Contributing to PM in the Ambient Air
3. Monitoring of Ambient PM
4. Ambient Concentrations and Trends
a. PM<INF>2.5</INF> Mass
b. PM<INF>2.5</INF> Components
c. PM<INF>10</INF>
d. PM<INF>10-2.5</INF>
e. UFP
5. Characterizing Ambient PM<INF>2.5</INF> Concentrations for
Exposure
a. Predicted Ambient PM<INF>2.5</INF> and Exposure Based on
Monitored Data
b. Comparison of PM<INF>2.5</INF> Fields in Estimating Exposure
and Relative to Design Values
6. Background PM
II. Rationale for Proposed Decisions on the Primary PM<INF>2.5</INF>
Standards
A. General Approach
1. Background on the Current Standards
a. Considerations Regarding the Adequacy of the Existing
Standards in the 2020 Review
2. General Approach and Key Issues in This Reconsideration of
the 2020 Final Decision
B. Overview of the Health Effects Evidence
1. Nature of Effects
a. Mortality
b. Cardiovascular Effects
c. Respiratory Effects
d. Cancer
e. Nervous System Effects
f. Other Effects
2. Public Health Implications and At-Risk Populations
3. PM<INF>2.5</INF> Concentrations in Key Studies Reporting
Health Effects
a. PM<INF>2.5</INF> Exposure Concentrations Evaluated in
Experimental Studies
b. Ambient PM<INF>2.5</INF> Concentrations in Locations of
Epidemiologic Studies
4. Uncertainties in the Health Effects Evidence
C. Summary of Exposure and Risk Estimates
1. Key Design Aspects
2. Key Limitations and Uncertainties
3. Summary of Risk Estimates
D. Proposed Conclusions on the Primary PM<INF>2.5</INF>
Standards
1. CASAC Advice in This Reconsideration
2. Evidence- and Risk-Based Considerations in the Policy
Assessment
a. Evidence-Based Considerations
b. Risk-Based Considerations
3. Administrator's Proposed Conclusions on the Primary
PM<INF>2.5</INF> Standards
a. Adequacy of the Current Primary PM<INF>2.5</INF> Standards
b. Consideration of Alternative Primary Annual PM<INF>2.5</INF>
Standard Levels
E. Proposed Decisions on the Primary PM<INF>2.5</INF> Standards
III. Rationale for Proposed Decisions on the Primary PM<INF>10</INF>
Standard
A. General Approach
1. Background on the Current Standard
i. Considerations Regarding the Adequacy of the Existing
Standard in the 2020 Review
2. General Approach and Key Issues in This Reconsideration of
the 2020 Final Decision
B. Overview of Health Effects Evidence
1. Nature of Effects
a. Mortality
i. Long-Term Exposures
ii. Short-Term Exposures
b. Cardiovascular Effects
i. Long-Term Exposures
ii. Short-Term Exposures
c. Respiratory Effects--Short-Term Exposures
d. Cancer--Long-Term Exposures
e. Metabolic Effects--Long-Term Exposures
f. Nervous System Effects--Long-Term Exposures
C. Proposed Conclusions on the Primary PM<INF>10</INF> Standard
1. CASAC Advice in This Reconsideration
2. Evidence-Based Considerations in the Policy Assessment
3. Administrator's Proposed Decision on the Current Primary
PM<INF>10</INF> Standard
IV. Communication of Public Health
A. Air Quality Index Overview
B. Air Quality Index Category Breakpoints for PM<INF>2.5</INF>
1. Air Quality Index Values of 50, 100 and 150
2. Air Quality Index Values of 200 and 300
3. Air Quality Index Value of 500
C. Air Quality Index Category Breakpoints for PM<INF>10</INF>
D. Air Quality Index Reporting
V. Rationale for Proposed Decisions on the Secondary PM Standards
A. General Approach
1. Background on the Current Standards
a. Non-Visibility Effects
i. Considerations Regarding Adequacy of the Existing Standards
for Non-Visibility Effects in the 2020 Review
b. Visibility Effects
i. Considerations Regarding Adequacy of the Existing Standards
for Visibility Effects in the 2020 Review
2. General Approach and Key Issues in This Reconsideration of
the 2020 Final Decision
B. Overview of Welfare Effects Evidence
1. Nature of Effects
a. Visibility
b. Climate
c. Materials
C. Summary of Air Quality and Quantitative Information
1. Visibility Effects
a. Target Level of Protection in Terms of a PM<INF>2.5</INF>
Visibility Index
b. Relationship Between the PM<INF>2.5</INF> Visibility Index
and the Current Secondary 24-Hour PM<INF>2.5</INF> Standard
2. Non-Visibility Effects
D. Proposed Conclusions on the Secondary PM Standards
1. CASAC Advice in This Reconsideration
2. Evidence- and Quantitative Information-Based Considerations
in the Policy Assessment
3. Administrator's Proposed Decision on the Current Secondary PM
Standards
VI. Interpretation of the NAAQS for PM
A. Proposed Amendments to Appendix K: Interpretation of the
NAAQS for Particulate Matter
1. Updating Design Value Calculations To Be on a Site-Level
Basis
2. Codifying Site Combinations To Maintain a Continuous Data
Record
3. Clarifying Daily Validity Requirements for Continuous
Monitors
B. Proposed Amendments to Appendix N: Interpretation of the
NAAQS for PM<INF>2.5</INF>
1. Updating References to the Proposed Revision(s) of the
Standards
2. Codifying Site Combinations To Maintain a Continuous Data
Record
VII. Proposed Amendments to Ambient Monitoring and Quality Assurance
Requirements
A. Proposed Amendment in 40 CFR Part 50 (Appendix L): Reference
Method for the Determination of Fine Particulate Matter as
PM<INF>2.5</INF> in the Atmosphere--Addition of the Tisch Cyclone as
an Approved Second Stage Separator
B. Issues Related to 40 CFR Part 53 (Reference and Equivalent
Methods)
1. Update to Program Title and Delivery Address for FRM and FEM
Application and Modification Requests
2. Requests for Delivery of a Candidate FRM or FEM Instrument
3. Amendments to Requirements for Submission of Materials in
Sec. 53.4(b)(7) for Language and Format
4. Amendment to Designation of Reference and Equivalent Methods
5. Amendment to One Test Field Campaign Requirement for Class
III PM<INF>2.5</INF> FEMs
6. Amendment to Use of Monodisperse Aerosol Generator
7. Corrections to 40 CFR Part 53 (Reference and Equivalent
Methods)
C. Proposed Changes to 40 CFR Part 58 (Ambient Air Quality
Surveillance)
1. Quality Assurance Requirements for Monitors Used in
Evaluations for National Ambient Air Quality Standards
a. Quality System Requirements
b. Measurement Quality Check Requirements
c. Calculations for Data Quality Assessments
d. References
2. Quality Assurance Requirements for Prevention of Significant
Deterioration (PSD) Air Monitoring
a. Quality System Requirements
b. Measurement Quality Check Requirements
c. Calculations for Data Quality Assessments
d. References
3. Proposed Amendments to PM Ambient Air Quality Methodology
a. Proposal To Revoke Approved Regional Methods (ARMs)
b. Proposal for Calibration of PM Federal Equivalent Methods
(FEMs)
4. Proposed Amendment to the PM<INF>2.5</INF> Monitoring Network
Design Criteria To Address At-Risk Communities
5. Proposed Revisions To Probe and Monitoring Path Siting
Criteria
a. Providing Separate Section for Open Path Monitoring
Requirements
[[Page 5560]]
b. Amending Distance Precision for Spacing Offsets
c. Clarifying Summary Table of Probe Siting Criteria
d. Adding Flexibility for the Spacing From Minor Sources
e. Amendments and Clarification for the Spacing From
Obstructions and Trees
f. Reinstating Minimum 270-Degree Arc and Clarifying 180-Degree
Arc in Regulatory Text
g. Clarification on Obstacles That Act as an Obstruction
h. Amending and Clarifying the 10-Meter Tree Dripline
Requirement
i. Amending Spacing Requirement for Microscale Monitoring
j. Amending Waiver Provisions
k. Broadening of Acceptable Probe Materials
D. Taking Comment on Incorporating Data From Next Generation
Technologies
1. Background on Use of FRM and FEM Monitors
2. Next Generation Technologies: Data Considerations
3. PM<INF>2.5</INF> Continuous FEMs
4. PM<INF>2.5</INF> Satellite Products
5. Use of Air Sensors
6. Summary
VIII. Clean Air Act Implementation Requirements for the PM NAAQS
A. Designation of Areas
B. Section 110(a)(1) and (2) Infrastructure SIP Requirements
C. Implementing Any Revised PM<INF>2.5</INF> NAAQS in
Nonattainment Areas
D. Implementing the Primary and Secondary PM<INF>10</INF> NAAQS
E. Prevention of Significant Deterioration and Nonattainment New
Source Review Programs for the Proposed Revised Primary Annual
PM<INF>2.5</INF> NAAQS
F. Transportation Conformity Program
G. General Conformity Program
IX. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
B. Paperwork Reduction Act (PRA)
C. Regulatory Flexibility Act (RFA)
D. Unfunded Mandates Reform Act (UMRA)
E. Executive Order 13132: Federalism
F. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
G. Executive Order 13045: Protection of Children From
Environmental Health and Safety Risks
H. Executive Order 13211: Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution or Use
I. National Technology Transfer and Advancement Act (NTTAA)
J. Executive Order 12898: Federal Actions To Address
Environmental Justice in Minority Populations and Low-Income
Populations
References
Executive Summary
This document presents the Administrator's proposed decisions for
the reconsideration of the 2020 final decision on the primary (health-
based) and secondary (welfare-based) National Ambient Air Quality
Standards (NAAQS) for Particulate Matter (PM). More specifically this
document summarizes the background and rationale for the
Administrator's proposed decisions to revise the primary annual
PM<INF>2.5</INF> standard by lowering the level from 12.0 [micro]g/m\3\
to within the range of 9.0 to 10.0 [micro]g/m\3\ while taking comment
on alternative annual standard levels down to 8.0 [micro]g/m\3\ and up
to 11.0 [micro]g/m\3\; to retain the current primary 24-hour
PM<INF>2.5</INF> standard (at a level of 35 [micro]g/m\3\) while taking
comment on revising the level as low as 25 [micro]g/m\3\; to retain the
primary 24-hour PM<INF>10</INF> standard, without revision; and, not to
change the secondary PM standards at this time, while taking comment on
revising the level of the secondary 24-hour PM<INF>2.5</INF> standard
as low as 25 [micro]g/m\3\. In reaching his proposed decisions, the
Administrator has considered the currently available scientific
evidence in the 2019 Integrated Science Assessment (2019 ISA) and the
Supplement to the 2019 ISA (ISA Supplement), quantitative and policy
analyses presented in the Policy Assessment (PA), and advice from the
Clean Air Scientific Advisory Committee (CASAC). The EPA solicits
comment on the proposed decisions described here and on the array of
issues associated with the reconsideration of these standards,
including the judgments of public health, public welfare and science
policy inherent in the proposed decisions, and requests commenters also
provide the rationales upon which views articulated in submitted
comments are based.
The EPA has established primary and secondary standards for
PM<INF>2.5</INF>, which includes particles with diameters generally
less than or equal to 2.5 [micro]m, and PM<INF>10</INF>, which includes
particles with diameters generally less than or equal to 10 [micro]m.
The standards include two primary PM<INF>2.5</INF> standards, an annual
average standard, averaged over three years, with a level of 12.0
[micro]g/m\3\ and a 24-hour standard with a 98th percentile form,
averaged over three years, and a level of 35 [micro]g/m\3\. It also
includes a primary PM<INF>10</INF> standard with a 24-hour averaging
time, and a level of 150 [micro]g/m\3\, not to be exceeded more than
once per year on average over three years. Secondary PM standards are
set equal to the primary standards, except that the level of the
secondary annual PM<INF>2.5</INF> standard is 15.0 [micro]g/m\3\.
The last review of the PM NAAQS was completed in December 2020. In
that review, the EPA retained the primary and secondary NAAQS, without
revision (85 FR 82684, December 18, 2020). Following publication of the
2020 final action, several parties filed petitions for review and
petitions for reconsideration of the EPA's final decision.
In June 2021, the Agency announced its decision to reconsider the
2020 PM NAAQS final action.\1\ The EPA is reconsidering the December
2020 decision because the available scientific evidence and technical
information indicate that the current standards may not be adequate to
protect public health and welfare, as required by the Clean Air Act.
The EPA noted that the 2020 PA concluded that the scientific evidence
and information called into question the adequacy of the primary
PM<INF>2.5</INF> standards and supported consideration of revising the
level of the primary annual PM<INF>2.5</INF> standard to below the
current level of 12.0 [micro]g/m\3\ while retaining the primary 24-hour
PM<INF>2.5</INF> standard (U.S. EPA, 2020a). The EPA also noted that
the 2020 PA concluded that the available scientific evidence and
information did not call into question the adequacy of the primary
PM<INF>10</INF> or secondary PM standards and supported consideration
of retaining the primary PM<INF>10</INF> standard and secondary PM
standards without revision (U.S. EPA, 2020a).
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\1\ The press release for this announcement is available at:
<a href="https://www.epa.gov/newsreleases/epa-reexamine-health-standards-harmful-soot-previous-administration-left-unchanged">https://www.epa.gov/newsreleases/epa-reexamine-health-standards-harmful-soot-previous-administration-left-unchanged</a>.
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The proposed decisions presented in this document on the primary
PM<INF>2.5</INF> standards have been informed by key aspects of the
available health effects evidence and conclusions contained in the 2019
ISA and ISA Supplement, quantitative exposure/risk analyses and policy
evaluations presented in the PA, advice from the CASAC \2\ and public
comment received as part of this reconsideration.\3\ The health effects
evidence available in this reconsideration, in conjunction with the
full body of evidence critically evaluated in the 2019 ISA, supports a
causal relationship between long- and
[[Page 5561]]
short-term exposures and mortality and cardiovascular effects, and the
evidence supports a likely to be a causal relationship between long-
term exposures and respiratory effects, nervous system effects, and
cancer. The longstanding evidence base, including animal toxicological
studies, controlled human exposure studies, and epidemiologic studies,
reaffirms, and in some cases strengthens, the conclusions from past
reviews regarding the health effects of PM<INF>2.5</INF> exposures.
Epidemiologic studies available in this reconsideration demonstrate
generally positive, and often statistically significant,
PM<INF>2.5</INF> health effect associations. Such studies report
associations between estimated PM<INF>2.5</INF> exposures and non-
accidental, cardiovascular, or respiratory mortality; cardiovascular or
respiratory hospitalizations or emergency room visits; and other
mortality/morbidity outcomes (e.g., lung cancer mortality or incidence,
asthma development). The scientific evidence available in this
reconsideration, as evaluated in the 2019 ISA and ISA Supplement,
includes a number of epidemiologic studies that use various methods to
characterize exposure to PM<INF>2.5</INF> (e.g., ground-based monitors
and hybrid modeling approaches) and to evaluate associations between
health effects and lower ambient PM<INF>2.5</INF> concentrations. There
are a number of recent epidemiologic studies that use varying study
designs that reduce uncertainties related to confounding and exposure
measurement error. The results of these analyses provide further
support for the robustness of associations between PM<INF>2.5</INF>
exposures and mortality and morbidity. Moreover, the Administrator
notes that recent epidemiologic studies strengthen support for health
effect associations at lower PM<INF>2.5</INF> concentrations, with
these new studies finding positive and significant associations when
assessing exposure in locations and time periods with lower mean and
25th percentile concentrations than those evaluated in epidemiologic
studies available at the time of previous reviews. Additionally, the
experimental evidence (i.e., animal toxicological and controlled human
exposure studies) strengthens the coherence of effects across
scientific disciplines and provides additional support for potential
biological pathways through which PM<INF>2.5</INF> exposures could lead
to the overt population-level outcomes reported in epidemiologic
studies for the health effect categories for which a causal
relationship (i.e., short- and long-term PM<INF>2.5</INF> exposure and
mortality and cardiovascular effects) or likely to be causal
relationship (i.e., short- and long-term PM<INF>2.5</INF> exposure and
respiratory effects; and long-term PM<INF>2.5</INF> exposure and
nervous system effects and cancer) was concluded.
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\2\ In 2021, the Administrator announced his decision to
reestablish the membership of the CASAC. The Administrator selected
seven members to serve on the chartered CASAC, and appointed a PM
CASAC panel to support the chartered CASAC's review of the draft ISA
Supplement and the draft PA as a part of this reconsideration (see
section I.C.6.b below for more information).
\3\ More information regarding the CASAC review of the draft ISA
Supplement and the draft PA, including opportunities for public
comment, can be found in the following Federal Register notices: 86
FR 54186, September 30, 2021; 86 FR 52673, September 22, 2021; 86 FR
56263, October 8, 2021; 87 FR 958, January 7, 2022.
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The available evidence in the 2019 ISA continues to provide support
for factors that may contribute to increased risk of PM<INF>2.5</INF>-
related health effects including lifestage (children and older adults),
pre-existing diseases (cardiovascular disease and respiratory disease),
race/ethnicity, and socioeconomic status. For example, the 2019 ISA and
ISA Supplement conclude that there is strong evidence that Black and
Hispanic populations, on average, experience higher PM<INF>2.5</INF>
exposures and PM<INF>2.5</INF>-related health risk than non-Hispanic
White populations. In addition, studies evaluated in the 2019 ISA and
ISA Supplement also provide evidence indicating that communities with
lower socioeconomic status (SES), as assessed in epidemiologic studies
using indicators of SES including income and educational attainment
are, on average, exposed to higher concentrations of PM<INF>2.5</INF>
compared to higher SES communities.
The quantitative risk assessment, as well as policy considerations
in the PA, also inform the proposed decisions on the primary
PM<INF>2.5</INF> standards. The risk assessment in this consideration
focuses on all-cause or nonaccidental mortality associated with long-
and short-term PM<INF>2.5</INF> exposures. The primary analyses focus
on exposure and risk associated with air quality that might occur in an
area under air quality conditions that just meet the current and
potential alternative standards. The risk assessment estimates that the
current primary PM<INF>2.5</INF> standards could allow a substantial
number of PM<INF>2.5</INF>-associated premature deaths in the United
States, and that public health improvements would be associated with
just meeting all of the alternative (more stringent) annual and 24-hour
standard levels modeled. Additionally, the results of the risk
assessment suggest that for most of the U.S., the annual standard is
the controlling standard and that revision to that standard has the
most potential to reduce PM<INF>2.5</INF> exposure related risk.
Further analyses comparing the reductions in average national
PM<INF>2.5</INF> concentrations and risk rates within each demographic
population estimate that the average percent PM<INF>2.5</INF>
concentrations and risk reductions are slightly greater in the Black
population than in the White population when meeting a revised annual
standard with a lower level. The analyses are summarized in this
document and described in detail in the PA.
In its advice to the Administrator, the CASAC concurred with the
draft PA that the currently available health effects evidence calls
into question the adequacy of the primary annual PM<INF>2.5</INF>
standard. With regard to the primary annual PM<INF>2.5</INF> standard,
the majority of the CASAC concluded that the level of the standard
should be revised within the range of 8.0 to 10.0 [micro]g/m\3\, while
the minority of the CASAC concluded that the primary annual
PM<INF>2.5</INF> standard should be revised to a level of 10.0 to 11.0
[micro]g/m\3\. With regard to the primary 24-hour PM<INF>2.5</INF>
standard, the majority of the CASAC concluded that the primary 24-hour
PM<INF>2.5</INF> was not adequate and that the level of the standard
should be revised to within the range of 25 to 30 [micro]g/m\3\, while
the minority of the CASAC concluded that the primary 24-hour
PM<INF>2.5</INF> standard was adequate and should be retained, without
revision.
In considering how to revise the suite of standards to provide the
requisite degree of protection, the Administrator recognizes that the
current annual standard and 24-hour standard, together, are intended to
provide public health protection against the full distribution of
short- and long-term PM<INF>2.5</INF> exposures. Further, he recognizes
that changes in PM<INF>2.5</INF> air quality designed to meet either
the annual or the 24-hour standard would likely result in changes to
both long-term average and short-term peak PM<INF>2.5</INF>
concentrations. Based on the current evidence and quantitative
information, as well as consideration of CASAC advice and public
comment thus far in this reconsideration, the Administrator proposes to
conclude that the current primary PM<INF>2.5</INF> standards are not
adequate to protect public health with an adequate margin of safety.
The Administrator also notes that the CASAC was unanimous in its
advice regarding the need to revise the annual standard. In considering
the appropriate level for a revised annual standard, the Administrator
provisionally concludes that a standard set within the range of 9.0 to
10.0 [micro]g/m\3\ would reflect his placing the most weight on the
strongest available evidence while appropriately weighing the
uncertainties. In addition, the Administrator recognizes that some
members of CASAC advised, and the PA concluded, that the available
scientific information provides support for considering a range that
extends up to 11.0 [micro]g/m\3\ and down to 8.0 [micro]g/m\3\.
With regard to the primary 24-hour PM<INF>2.5</INF> standard, the
Administrator finds it is less clear whether the available scientific
evidence and quantitative
[[Page 5562]]
information calls into question the adequacy of the public health
protection afforded by the current 24-hour standard. He notes that a
more stringent annual standard is expected to reduce both average
(annual) concentrations and peak (daily) concentrations. Furthermore,
he notes that the CASAC did not reach consensus on whether revisions to
the primary 24-hour PM<INF>2.5</INF> standard were warranted at this
time. The majority of the CASAC recommended that the level of the
current primary 24-hour PM<INF>2.5</INF> should be revised to within
the range of 25 to 30 [micro]g/m\3\, while the minority of the CASAC
recommended retaining the current standard. The Administrator proposes
to conclude that the 24-hour standard should be retained, particularly
when considered in conjunction with the protection provided by the
suite of standards and the proposed decision to revise the annual
standard to a level of 9.0 to 10.0 [micro]g/m\3\.
The EPA solicits comment on the Administrator's proposed
conclusions, and on the proposed decision to revise the primary annual
PM<INF>2.5</INF> standard and retain the primary 24-hour
PM<INF>2.5</INF> standard, without revision. The Administrator is
conscious of his obligation to set primary standards with an adequate
margin of safety and preliminarily determines that the proposed
decision balances the need to provide protection against uncertain
risks with the obligation to not set standards that are more stringent
than necessary. The requirement to provide an adequate margin of safety
was intended to address uncertainties associated with inconclusive
scientific and technical information and to provide a reasonable degree
of protection against hazards that research has not yet identified.
Reaching decisions on what standards are appropriate necessarily
requires judgments of the Administrator about how to consider the
information available from the epidemiologic studies and other relevant
evidence. In the Administrator's judgment, the proposed suite of
primary PM<INF>2.5</INF> standards reflects the appropriate
consideration of the strength of the available evidence and other
information and their associated uncertainties and the advice of the
CASAC. The final rulemaking will reflect the Administrator's ultimate
judgments as to the suite of primary PM<INF>2.5</INF> standards that
are requisite to protect the public health with an adequate margin of
safety. Consistent with these principles, the EPA also solicits public
comment on alternative annual standard levels down to 8.0 [micro]g/m\3\
and up to 11.0 [micro]g/m\3\, on an alternative 24-hour standard level
as low as 25 [micro]g/m\3\ and on the combination of annual and 24-hour
standards that commenters may believe is appropriate, along with the
approaches and scientific rationales used to support such levels. For
example, the EPA solicits comments on the uncertainties in the reported
associations between daily or annual average PM<INF>2.5</INF> exposures
and mortality or morbidity in the epidemiologic studies, the
significance of the 25th percentile of ambient concentrations reported
in studies, the relevance and limitations of international studies, and
other topics discussed in section II.D.3.b.
The primary PM<INF>10</INF> standard is intended to provide public
health protection against health effects related to exposures to
PM<INF>10-2.5</INF>, which are particles with a diameter between 10
[micro]m and 2.5 [micro]m. The proposed decision to retain the current
24-hour PM<INF>10</INF> standard has been informed by key aspects of
the available health effects evidence and conclusions contained in the
2019 ISA, the policy evaluations presented in the PA, advice from the
CASAC and public comment received as part of this reconsideration.
Specifically, the health effects evidence for PM<INF>10-2.5</INF>
exposures is somewhat strengthened since past reviews, although the
strongest evidence still only provides support for a suggestive of, but
not sufficient to infer, causal relationship with long- and short-term
exposures and mortality and cardiovascular effects, short-term
exposures and respiratory effects, and long-term exposures and cancer,
nervous system effects, and metabolic effects. In reaching his proposed
decision, the Administrator recognizes that, while the available health
effects evidence has expanded, recent studies are subjected to the same
types of uncertainties that were judged to be important in previous
reviews. He also recognizes that the CASAC generally agreed with the
draft PA that it was reasonable to retain the primary 24-hour
PM<INF>10</INF> standard given the available scientific evidence,
including PM<INF>10</INF> as an appropriate indicator. He proposes to
conclude that the newly available evidence does not call into question
the adequacy of the current primary PM<INF>10</INF> standard, and he
proposes to retain that standard, without revision.
This reconsideration of the secondary PM standards focuses on
visibility, climate, and materials effects.\4\ The Administrator's
proposed decision to not change the current secondary standards at this
time has been informed by key aspects of the currently available
welfare effects evidence as well as the conclusions contained in the
2019 ISA and ISA Supplement; quantitative analyses of visibility
impairment; policy evaluations presented in the PA; advice from the
CASAC; and public comment received as part of this reconsideration.
Specifically, the welfare effects evidence available in this
reconsideration is consistent with the evidence available in previous
reviews and supports a causal relationship between PM and visibility,
climate, and materials effects. With regard to climate and materials
effects, while the evidence has expanded since previous reviews,
uncertainties remain in the evidence and there are still significant
limitations in quantifying potential adverse effects from PM on climate
and materials for purposes of setting a standard. With regard to
visibility effects, the results of quantitative analyses of visibility
impairment are similar to those in previous reviews, and suggest that
in areas that meet the current secondary 24-hour PM<INF>2.5</INF>
standard that estimated light extinction in terms of a 3-year
visibility metric would be at or well below the upper end of the range
for the target level of protection (i.e., 30 deciviews (dv)). The CASAC
generally agreed with the draft PA that substantial uncertainties
remain in the scientific evidence for climate and materials effects. In
considering the available scientific evidence for climate and materials
effects, along with CASAC advice, the Administrator proposes to
conclude that it is appropriate to retain the existing secondary
standards and that it is not appropriate to establish any distinct
secondary PM standards to address non-visibility PM-related welfare
effects. With regard to visibility effects, while the Administrator
notes that the CASAC did not recommend revising either the target level
of protection for the visibility index or the level of the current
secondary 24-hour PM<INF>2.5</INF> standard, the Administrator
[[Page 5563]]
recognizes that, should an alternative level be considered for the
visibility index, that the CASAC recommends also considering revisions
to the secondary 24-hour PM<INF>2.5</INF> standard. In considering the
available evidence and quantitative information, with its inherent
uncertainties and limitations, the Administrator proposes not to change
the secondary PM standards at this time, and solicits comment on this
proposed decision. In addition, the Administrator additionally solicits
comment on the appropriateness of a target level of protection for
visibility below 30 dv and down as low as 25 dv, and of revising the
level of the current secondary 24-hour PM<INF>2.5</INF> standard to a
level as low as 25 [micro]g/m\3\.
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\4\ Consistent with the 2016 Integrated Review Plan (U.S. EPA,
2016), other welfare effects of PM, such as ecological effects, are
being considered in the separate, on-going review of the secondary
NAAQS for oxides of nitrogen, oxides of sulfur and PM. Accordingly,
the public welfare protection provided by the secondary PM standards
against ecological effects such as those related to deposition of
nitrogen- and sulfur-containing compounds in vulnerable ecosystems
is being considered in that separate review. Thus, the
Administrator's conclusion in this reconsideration of the 2020 final
decision will be focused only and specifically on the adequacy of
public welfare protection provided by the secondary PM standards
from effects related to visibility, climate, and materials and
hereafter ``welfare effects'' refers to those welfare effects.
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Any proposed revisions to the PM NAAQS, if finalized, would trigger
a process under which states (and tribes, if they choose) make
recommendations to the Administrator regarding designations,
identifying areas of the country that either meet or do not meet the
new or revised PM NAAQS. Those areas that do not meet the PM NAAQS will
need to develop plans that demonstrate how they will meet the
standards. As part of these plans, states have the opportunity to use
tools to advance environmental justice, in this case for overburdened
communities in areas with high PM concentrations above the NAAQS, as
provided in current PM NAAQS implementation guidance to meet
requirements (80 FR 58010, 58136, August 25, 2016). The EPA is not
proposing changes to any of the current PM NAAQS implementation
programs in this proposed rulemaking, and therefore is not requesting
comment on any specific proposals related to implementation or
designations.
On other topics, the EPA proposes to make two sets of changes to
the PM<INF>2.5</INF> sub-index of the AQI. First, the EPA proposes to
continue to use the approach used in the revisions to the AQI in 2012
(77 FR 38890, June 29, 2012) of setting the lower breakpoints (50, 100
and 150) to be consistent with the levels of the primary
PM<INF>2.5</INF> annual and 24-hour standards and proposes to revise
the lower breakpoints to be consistent with any changes to the primary
PM<INF>2.5</INF> standards that are part of this reconsideration. In so
doing, the EPA proposes to revise the AQI value of 50 within the range
of 9.0 and 10.0 [micro]g/m\3\ and proposes to retain the AQI values of
100 and 150 at 35.4 [micro]g/m\3\ and 55.4 [micro]g/m\3\, respectively.
Second, the EPA proposes to revise the upper AQI breakpoints (200 and
above) and to replace the linear-relationship approach used in 1999 (64
FR 42530, August 4, 1999) to set these breakpoints, with an approach
that more fully considers the PM<INF>2.5</INF> health effects evidence
from controlled human exposure and epidemiologic studies that has
become available in the last 20 years. The EPA also proposes to revise
the AQI values of 200, 300 and 500 to 125.4 [micro]g/m\3\, 225.4
[micro]g/m\3\, and 325.4 [micro]g/m\3\, respectively. The EPA proposes
to finalize these changes to the PM<INF>2.5</INF> AQI in conjunction
with the Agency's final decisions on the primary annual and 24-hour
PM<INF>2.5</INF> standards, if proposed revisions to such standards are
promulgated. The EPA is soliciting comment on the proposed revisions to
the AQI. In addition, the EPA also proposes to revise the daily
reporting requirement from 5 days per week to 7 days per week, while
also reformatting appendix G and providing clarifications.
With regard to monitoring-related activities, the EPA proposes
revisions to data calculations and ambient air monitoring requirements
for PM to improve the usefulness of and appropriateness of data used in
regulatory decision making and to better characterize air quality in
communities that are at increased risk of PM<INF>2.5</INF> exposure and
health risk. These proposed changes are found in 40 CFR part 50
(appendices K, L, and N), part 53, and part 58 with associated
appendices (A, B, C, D, and E). These proposed changes include
addressing updates in data calculations, approval of reference and
equivalent methods, updates in quality assurance statistical
calculations to account for lower concentration measurements, updates
to support improvements in PM methods, a revision to the
PM<INF>2.5</INF> network design to account for at-risk populations, and
updates to the Probe and Monitoring Path Siting Criteria for NAAQS
pollutants.
In setting the NAAQS, the EPA may not consider the costs of
implementing the standards. This was confirmed by the Supreme Court in
Whitman v. American Trucking Associations, 531 U.S. 457, 465-472, 475-
76 (2001), as discussed in section II.A of this document. As has
traditionally been done in NAAQS rulemaking, the EPA prepared a
Regulatory Impact Analysis (RIA) to provide the public with information
on the potential costs and benefits of attaining several alternative
PM<INF>2.5</INF> standard levels. In NAAQS rulemaking, the RIA is done
for informational purposes only, and the proposed decisions on the
NAAQS in this rulemaking are not based on consideration of the
information or analyses in the RIA. The RIA fulfills the requirements
of Executive Orders 13563 and 12866. The RIA estimates the costs and
monetized human health benefits of attaining three alternative annual
PM<INF>2.5</INF> standard levels and one alternative 24-hour
PM<INF>2.5</INF> standard level. Specifically, the RIA examines the
proposed annual and 24-hour alternative standard levels of 10/35
[micro]g/m\3\ and 9/35 [micro]g/m\3\, as well as the following two more
stringent alternative standard levels: (1) An alternative annual
standard level of 8 [micro]g/m\3\ in combination with the current 24-
hour standard (i.e., 8/35 [micro]g/m\3\), and (2) an alternative 24-
hour standard level of 30 [micro]g/m\3\ in combination with the
proposed annual standard level of 10 [micro]g/m\3\ (i.e., 10/30
[micro]g/m\3\). The RIA presents estimates of the costs and benefits of
applying illustrative national control strategies in 2032 after
implementing existing and expected regulations and assessing emissions
reductions to meet the current annual and 24-hour particulate matter
NAAQS (12/35 [micro]g/m\3\).
I. Background
A. Legislative Requirements
Two sections of the Clean Air Act (CAA) govern the establishment
and revision of the NAAQS. Section 108 (42 U.S.C. 7408) directs the
Administrator to identify and list certain air pollutants and then to
issue air quality criteria for those pollutants. The Administrator is
to list those pollutants ``emissions of which, in his judgment, cause
or contribute to air pollution which may reasonably be anticipated to
endanger public health or welfare''; ``the presence of which in the
ambient air results from numerous or diverse mobile or stationary
sources''; and for which he ``plans to issue air quality criteria. . .
.'' (42 U.S.C. 7408(a)(1)). Air quality criteria are intended to
``accurately reflect the latest scientific knowledge useful in
indicating the kind and extent of all identifiable effects on public
health or welfare which may be expected from the presence of [a]
pollutant in the ambient air. . . .'' (42 U.S.C. 7408(a)(2)).
Section 109 [42 U.S.C. 7409] directs the Administrator to propose
and promulgate ``primary'' and ``secondary'' NAAQS for pollutants for
which air quality criteria are issued [42 U.S.C. 7409(a)]. Section
109(b)(1) defines primary standards as ones ``the attainment and
maintenance of which in the judgment of the Administrator, based on
such criteria and allowing an adequate margin of safety, are requisite
to protect the public health.'' \5\ Under
[[Page 5564]]
section 109(b)(2), a secondary standard must ``specify a level of air
quality the attainment and maintenance of which, in the judgment of the
Administrator, based on such criteria, is requisite to protect the
public welfare from any known or anticipated adverse effects associated
with the presence of [the] pollutant in the ambient air.'' \6\
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\5\ The legislative history of section 109 indicates that a
primary standard is to be set at ``the maximum permissible ambient
air level . . . which will protect the health of any [sensitive]
group of the population,'' and that for this purpose ``reference
should be made to a representative sample of persons comprising the
sensitive group rather than to a single person in such a group.'' S.
Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970).
\6\ Under CAA section 302(h) (42 U.S.C. 7602(h)), effects on
welfare include, but are not limited to, ``effects on soils, water,
crops, vegetation, manmade materials, animals, wildlife, weather,
visibility, and climate, damage to and deterioration of property,
and hazards to transportation, as well as effects on economic values
and on personal comfort and well-being.''
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In setting primary and secondary standards that are ``requisite''
to protect public health and welfare, respectively, as provided in
section 109(b), the EPA's task is to establish standards that are
neither more nor less stringent than necessary. In so doing, the EPA
may not consider the costs of implementing the standards. See generally
Whitman v. American Trucking Associations, 531 U.S. 457, 465-472, 475-
76 (2001). Likewise, ``[a]ttainability and technological feasibility
are not relevant considerations in the promulgation of national ambient
air quality standards.'' American Petroleum Institute v. Costle, 665
F.2d 1176, 1185 (D.C. Cir. 1981); accord Murray Energy Corporation v.
EPA, 936 F.3d 597, 623-24 (D.C. Cir. 2019).
The requirement that primary standards provide an adequate margin
of safety was intended to address uncertainties associated with
inconclusive scientific and technical information available at the time
of standard setting. It was also intended to provide a reasonable
degree of protection against hazards that research has not yet
identified. See Lead Industries Association v. EPA, 647 F.2d 1130, 1154
(D.C. Cir 1980); American Petroleum Institute v. Costle, 665 F.2d at
1186; Coalition of Battery Recyclers Ass'n v. EPA, 604 F.3d 613, 617-18
(D.C. Cir. 2010); Mississippi v. EPA, 744 F.3d 1334, 1353 (D.C. Cir.
2013). Both kinds of uncertainties are components of the risk
associated with pollution at levels below those at which human health
effects can be said to occur with reasonable scientific certainty.
Thus, in selecting primary standards that include an adequate margin of
safety, the Administrator is seeking not only to prevent pollution
levels that have been demonstrated to be harmful but also to prevent
lower pollutant levels that may pose an unacceptable risk of harm, even
if the risk is not precisely identified as to nature or degree. The CAA
does not require the Administrator to establish a primary NAAQS at a
zero-risk level or at background concentration levels, see Lead
Industries Ass'n v. EPA, 647 F.2d at 1156 n.51, Mississippi v. EPA, 744
F.3d at 1351, but rather at a level that reduces risk sufficiently so
as to protect public health with an adequate margin of safety.
In addressing the requirement for an adequate margin of safety, the
EPA considers such factors as the nature and severity of the health
effects involved, the size of the sensitive population(s), and the kind
and degree of uncertainties. The selection of any particular approach
to providing an adequate margin of safety is a policy choice left
specifically to the Administrator's judgment. See Lead Industries Ass'n
v. EPA, 647 F.2d at 1161-62; Mississippi v. EPA, 744 F.3d at 1353.
Section 109(d)(1) of the Act requires the review every five years
of existing air quality criteria and, if appropriate, the revision of
those criteria to reflect advances in scientific knowledge on the
effects of the pollutant on public health and welfare. Under the same
provision, the EPA is also to review every five years and, if
appropriate, revise the NAAQS, based on the revised air quality
criteria.
Section 109(d)(2) addresses the appointment and advisory functions
of an independent scientific review committee. Section 109(d)(2)(A)
requires the Administrator to appoint this committee, which is to be
composed of ``seven members including at least one member of the
National Academy of Sciences, one physician, and one person
representing State air pollution control agencies.'' Section
109(d)(2)(B) provides that the independent scientific review committee
``shall complete a review of the criteria . . . and the national
primary and secondary ambient air quality standards . . . and shall
recommend to the Administrator any new . . . standards and revisions of
existing criteria and standards as may be appropriate. . . .'' Since
the early 1980s, this independent review function has been performed by
the Clean Air Scientific Advisory Committee (CASAC) of the EPA's
Science Advisory Board.
As previously noted, the Supreme Court has held that section 109(b)
``unambiguously bars cost considerations from the NAAQS-setting
process.'' Whitman v. Am. Trucking Associations, 531 U.S. 457, 471
(2001). Accordingly, while some of these issues regarding which
Congress has directed the CASAC to advise the Administrator are ones
that are relevant to the standard setting process, others are not.
Issues that are not relevant to standard setting may be relevant to
implementation of the NAAQS once they are established.\7\
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\7\ Some aspects of the CASAC's advice may not be relevant to
the EPA's process of setting primary and secondary standards that
are requisite to protect public health and welfare. Indeed, were the
EPA to consider costs of implementation when reviewing and revising
the standards ``it would be grounds for vacating the NAAQS.''
Whitman, 531 U.S. at 471 n.4. At the same time, the CAA directs the
CASAC to provide advice on ``any adverse public health, welfare,
social, economic, or energy effects which may result from various
strategies for attainment and maintenance'' of the NAAQS to the
Administrator under section 109(d)(2)(C)(iv). In Whitman, the Court
clarified that most of that advice would be relevant to
implementation but not standard setting, as it ``enable[s] the
Administrator to assist the States in carrying out their statutory
role as primary implementers of the NAAQS.'' Id. at 470 (emphasis in
original). However, the Court also noted that the CASAC's ``advice
concerning certain aspects of `adverse public health . . . effects'
from various attainment strategies is unquestionably pertinent'' to
the NAAQS rulemaking record and relevant to the standard setting
process. Id. at 470 n.2.
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B. Related PM Control Programs
States are primarily responsible for ensuring attainment and
maintenance of ambient air quality standards once the EPA has
established them. Under section 110 and Part D, Subparts 1, 4 and 6 of
the CAA, and related provisions and regulations, states are to submit,
for the EPA's approval, state implementation plans (SIPs) that provide
for the attainment and maintenance of such standards through control
programs directed to sources of the pollutants involved. The states, in
conjunction with the EPA, also administer the prevention of significant
deterioration of air quality program that covers these pollutants (see
42 U.S.C. 7470-7479). In addition, Federal programs provide for or
result in nationwide reductions in emissions of PM and its precursors
under Title II of the Act, 42 U.S.C. 7521-7574, which involves controls
for motor vehicles and nonroad engines and equipment; the new source
performance standards under section 111 of the Act, 42 U.S.C. 7411; and
the national emissions standards for hazardous pollutants under section
112 of the Act, 42 U.S.C. 7412.
C. Review of the Air Quality Criteria and Standards for Particulate
Matter
1. Reviews Completed in 1971 and 1987
The EPA first established NAAQS for PM in 1971 (36 FR 8186, April
30, 1971), based on the original Air Quality
[[Page 5565]]
Criteria Document (AQCD) (DHEW, 1969).\8\ The Federal reference method
(FRM) specified for determining attainment of the original standards
was the high-volume sampler, which collects PM up to a nominal size of
25 to 45 [micro]m (referred to as total suspended particulates or TSP).
The primary standards were set at 260 [micro]g/m\3\, 24-hour average,
not to be exceeded more than once per year, and 75 [micro]g/m\3\,
annual geometric mean. The secondary standards were set at 150
[micro]g/m\3\, 24-hour average, not to be exceeded more than once per
year, and 60 [micro]g/m\3\, annual geometric mean.
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\8\ Prior to the review initiated in 2007 (see below), the AQCD
provided the scientific foundation (i.e., the air quality criteria)
for the NAAQS. Beginning in that review, the Integrated Science
Assessment (ISA) has replaced the AQCD.
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In October 1979 (44 FR 56730, October 2, 1979), the EPA announced
the first periodic review of the air quality criteria and NAAQS for PM.
Revised primary and secondary standards were promulgated in 1987 (52 FR
24634, July 1, 1987). In the 1987 decision, the EPA changed the
indicator for particles from TSP to PM<INF>10</INF>, in order to focus
on the subset of inhalable particles small enough to penetrate to the
thoracic region of the respiratory tract (including the
tracheobronchial and alveolar regions), referred to as thoracic
particles.\9\ The level of the 24-hour standards (primary and
secondary) was set at 150 [micro]g/m\3\, and the form was one expected
exceedance per year, on average over three years. The level of the
annual standards (primary and secondary) was set at 50 [micro]g/m\3\,
and the form was annual arithmetic mean, averaged over three years.
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\9\ PM<INF>10</INF> refers to particles with a nominal mean
aerodynamic diameter less than or equal to 10 [micro]m. More
specifically, 10 [micro]m is the aerodynamic diameter for which the
efficiency of particle collection is 50 percent.
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2. Review Completed in 1997
In April 1994, the EPA announced its plans for the second periodic
review of the air quality criteria and NAAQS for PM, and in 1997 the
EPA promulgated revisions to the NAAQS (62 FR 38652, July 18, 1997). In
the 1997 decision, the EPA determined that the fine and coarse
fractions of PM<INF>10</INF> should be considered separately. This
determination was based on evidence that serious health effects were
associated with short- and long-term exposures to fine particles in
areas that met the existing PM<INF>10</INF> standards. The EPA added
new standards, using PM<INF>2.5</INF> as the indicator for fine
particles (with PM<INF>2.5</INF> referring to particles with a nominal
mean aerodynamic diameter less than or equal to 2.5 [micro]m). The new
primary standards were as follows: (1) an annual standard with a level
of 15.0 [micro]g/m\3\, based on the 3-year average of annual arithmetic
mean PM<INF>2.5</INF> concentrations from single or multiple community-
oriented monitors;\10\ and (2) a 24-hour standard with a level of 65
[micro]g/m\3\, based on the 3-year average of the 98th percentile of
24-hour PM<INF>2.5</INF> concentrations at each monitor within an area.
Also, the EPA established a new reference method for the measurement of
PM<INF>2.5</INF> in the ambient air and adopted rules for determining
attainment of the new standards. To continue to address the health
effects of the coarse fraction of PM<INF>10</INF> (referred to as
thoracic coarse particles or PM<INF>10-2.5</INF>; generally including
particles with a nominal mean aerodynamic diameter greater than 2.5
[micro]m and less than or equal to 10 [micro]m), the EPA retained the
primary annual PM<INF>10</INF> standard and revised the form of the
primary 24-hour PM<INF>10</INF> standard to be based on the 99th
percentile of 24-hour PM<INF>10</INF> concentrations at each monitor in
an area. The EPA revised the secondary standards by setting them equal
in all respects to the primary standards.
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\10\ The 1997 annual PM<INF>2.5</INF> standard was compared with
measurements made at the community-oriented monitoring site
recording the highest concentration or, if specific constraints were
met, measurements from multiple community-oriented monitoring sites
could be averaged (i.e., ``spatial averaging''). In the last review
(completed in 2012) the EPA replaced the term ``community-oriented''
monitor with the term ``area-wide'' monitor. Area-wide monitors are
those sited at the neighborhood scale or larger, as well as those
monitors sited at micro- or middle-scales that are representative of
many such locations in the same core-based statistical area (CBSA)
(78 FR 3236, January 15, 2013).
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Following promulgation of the 1997 PM NAAQS, petitions for review
were filed by several parties, addressing a broad range of issues. In
May 1999, the U.S. Court of Appeals for the District of Columbia
Circuit (D.C. Circuit) upheld the EPA's decision to establish fine
particle standards, holding that ``the growing empirical evidence
demonstrating a relationship between fine particle pollution and
adverse health effects amply justifies establishment of new fine
particle standards.'' American Trucking Associations, Inc. v. EPA, 175
F. 3d 1027, 1055-56 (D.C. Cir. 1999). The D.C. Circuit also found
``ample support'' for the EPA's decision to regulate coarse particle
pollution, but vacated the 1997 PM<INF>10</INF> standards, concluding
that the EPA had not provided a reasonable explanation justifying use
of PM<INF>10</INF> as an indicator for coarse particles. American
Trucking Associations v. EPA, 175 F. 3d at 1054-55. Pursuant to the
D.C. Circuit's decision, the EPA removed the vacated 1997
PM<INF>10</INF> standards, and the pre-existing 1987 PM<INF>10</INF>
standards remained in place (65 FR 80776, December 22, 2000). The D.C.
Circuit also upheld the EPA's determination not to establish more
stringent secondary standards for fine particles to address effects on
visibility. American Trucking Associations v. EPA, 175 F. 3d at 1027.
The D.C. Circuit also addressed more general issues related to the
NAAQS, including issues related to the consideration of costs in
setting NAAQS and the EPA's approach to establishing the levels of
NAAQS. Regarding the cost issue, the court reaffirmed prior rulings
holding that in setting NAAQS the EPA is ``not permitted to consider
the cost of implementing those standards.'' American Trucking
Associations v. EPA, 175 F. 3d at 1040-41. Regarding the levels of
NAAQS, the court held that the EPA's approach to establishing the level
of the standards in 1997 (i.e., both for PM and for the ozone NAAQS
promulgated on the same day) effected ``an unconstitutional delegation
of legislative authority.'' American Trucking Associations v. EPA, 175
F. 3d at 1034-40. Although the court stated that ``the factors EPA uses
in determining the degree of public health concern associated with
different levels of ozone and PM are reasonable,'' it remanded the rule
to the EPA, stating that when the EPA considers these factors for
potential non-threshold pollutants ``what EPA lacks is any determinate
criterion for drawing lines'' to determine where the standards should
be set.
The D.C. Circuit's holding on the cost and constitutional issues
were appealed to the United States Supreme Court. In February 2001, the
Supreme Court issued a unanimous decision upholding the EPA's position
on both the cost and constitutional issues. Whitman v. American
Trucking Associations, 531 U.S. 457, 464, 475-76. On the constitutional
issue, the Court held that the statutory requirement that NAAQS be
``requisite'' to protect public health with an adequate margin of
safety sufficiently guided the EPA's discretion, affirming the EPA's
approach of setting standards that are neither more nor less stringent
than necessary.
The Supreme Court remanded the case to the D.C. Circuit for
resolution of any remaining issues that had not been addressed in that
court's earlier rulings. Id. at 475-76. In a March 2002 decision, the
D.C. Circuit rejected all remaining challenges to the standards,
holding that the EPA's PM<INF>2.5</INF> standards were reasonably
supported by the administrative record and were not ``arbitrary and
capricious.'' American
[[Page 5566]]
Trucking Associations v. EPA, 283 F. 3d 355, 369-72 (D.C. Cir. 2002).
3. Review Completed in 2006
In October 1997, the EPA published its plans for the third periodic
review of the air quality criteria and NAAQS for PM (62 FR 55201,
October 23, 1997). After the CASAC and public review of several drafts,
the EPA's National Center for Environmental Assessment (NCEA) finalized
the AQCD in October 2004 (U.S. EPA, 2004a). The EPA's Office of Air
Quality Planning and Standards (OAQPS) finalized a Risk Assessment and
Staff Paper in December 2005 (Abt Associates, 2005; U.S. EPA,
2005).\11\ On December 20, 2005, the EPA announced its proposed
decision to revise the NAAQS for PM and solicited public comment on a
broad range of options (71 FR 2620, January 17, 2006). On September 21,
2006, the EPA announced its final decisions to revise the primary and
secondary NAAQS for PM to provide increased protection of public health
and welfare, respectively (71 FR 61144, October 17, 2006). With regard
to the primary and secondary standards for fine particles, the EPA
revised the level of the 24-hour PM<INF>2.5</INF> standards to 35
[micro]g/m\3\, retained the level of the annual PM<INF>2.5</INF>
standards at 15.0 [micro]g/m\3\, and revised the form of the annual
PM<INF>2.5</INF> standards by narrowing the constraints on the optional
use of spatial averaging. With regard to the primary and secondary
standards for PM<INF>10</INF>, the EPA retained the 24-hour standards,
with levels at 150 [micro]g/m\3\, and revoked the annual standards.\12\
The Administrator judged that the available evidence generally did not
suggest a link between long-term exposure to existing ambient levels of
coarse particles and health or welfare effects. In addition, a new
reference method was added for the measurement of PM<INF>10-2.5</INF>
in the ambient air in order to provide a basis for approving Federal
equivalent methods (FEMs) and to promote the gathering of scientific
data to support future reviews of the PM NAAQS.
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\11\ Prior to the review initiated in 2007, the Staff Paper
presented the EPA staff's considerations and conclusions regarding
the adequacy of existing NAAQS and, when appropriate, the potential
alternative standards that could be supported by the evidence and
information. More recent reviews present this information in the
Policy Assessment.
\12\ In the 2006 proposal, the EPA proposed to revise the 24-
hour PM<INF>10</INF> standard in part by establishing a new
PM<INF>10-2.5</INF> indicator for thoracic coarse particles (i.e.,
particles generally between 2.5 and 10 [micro]m in diameter). The
EPA proposed to include any ambient mix of PM<INF>10-2.5</INF> that
was dominated by resuspended dust from high density traffic on paved
roads and by PM from industrial sources and construction sources.
The EPA proposed to exclude any ambient mix of PM<INF>10-2.5</INF>
that was dominated by rural windblown dust and soils and by PM
generated from agricultural and mining sources. In the final
decision, the existing PM<INF>10</INF> standard was retained, in
part due to an ``inability . . . to effectively and precisely
identify which ambient mixes are included in the
[PM<INF>10-2.5</INF>] indicator and which are not'' (71 FR 61197,
October 17, 2006).
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Several parties filed petitions for review following promulgation
of the revised PM NAAQS in 2006. These petitions addressed the
following issues: (1) Selecting the level of the primary annual
PM<INF>2.5</INF> standard; (2) retaining PM<INF>10</INF> as the
indicator of a standard for thoracic coarse particles, retaining the
level and form of the 24-hour PM<INF>10</INF> standard, and revoking
the PM<INF>10</INF> annual standard; and (3) setting the secondary
PM<INF>2.5</INF> standards identical to the primary standards. On
February 24, 2009, the D.C. Circuit issued its opinion in the case
American Farm Bureau Federation v. EPA, 559 F. 3d 512 (D.C. Cir. 2009).
The court remanded the primary annual PM<INF>2.5</INF> NAAQS to the EPA
because the Agency had failed to adequately explain why the standards
provided the requisite protection from both short- and long-term
exposures to fine particles, including protection for at-risk
populations. Id. at 520-27. With regard to the standards for
PM<INF>10,</INF> the court upheld the EPA's decisions to retain the 24-
hour PM<INF>10</INF> standard to provide protection from thoracic
coarse particle exposures and to revoke the annual PM<INF>10</INF>
standard. Id. at 533-38. With regard to the secondary PM<INF>2.5</INF>
standards, the court remanded the standards to the EPA because the
Agency failed to adequately explain why setting the secondary PM
standards identical to the primary standards provided the required
protection for public welfare, including protection from visibility
impairment. Id. at 528-32. The EPA responded to the court's remands as
part of the next review of the PM NAAQS, which was initiated in 2007
(discussed below).
4. Review Completed in 2012
In June 2007, the EPA initiated the fourth periodic review of the
air quality criteria and the PM NAAQS by issuing a call for information
(72 FR 35462, June 28, 2007). Based on the NAAQS review process, as
revised in 2008 and again in 2009,\13\ the EPA held science/policy
issue workshops on the primary and secondary PM NAAQS (72 FR 34003,
June 20, 2007; 72 FR 34005, June 20, 2007), and prepared and released
the planning and assessment documents that comprise the review process
(i.e., integrated review plan (IRP) (U.S. EPA, 2008), ISA (U.S. EPA,
2009a), REA planning documents for health and welfare (U.S. EPA, 2009a,
U.S. EPA, 2009c), a quantitative health risk assessment (U.S. EPA,
2009a, U.S. EPA, 2009c), a quantitative health risk assessment (U.S.
EPA, 2010b) and an urban-focused visibility assessment (U.S. EPA,
2010a), and PA (U.S. EPA, 2011). In June 2012, the EPA announced its
proposed decision to revise the NAAQS for PM (77 FR 38890, June 29,
2012).
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\13\ The history of the NAAQS review process, including
revisions to the process, is discussed athttps://<a href="http://www.epa.gov/naaqs/historical-information-naaqs-review-process">www.epa.gov/naaqs/historical-information-naaqs-review-process</a>.
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In December 2012, the EPA announced its final decisions to revise
the primary NAAQS for PM to provide increased protection of public
health (78 FR 3086, January 15, 2013). With regard to primary standards
for PM<INF>2.5</INF>, the EPA revised the level of the annual
PM<INF>2.5</INF> standard \14\ to 12.0 [micro]g/m\3\ and retained the
24-hour PM<INF>2.5</INF> standard, with its level of 35 [micro]g/m\3\.
For the primary PM<INF>10</INF> standard, the EPA retained the 24-hour
standard to continue to provide protection against effects associated
with short-term exposure to thoracic coarse particles (i.e.,
PM<INF>10-2.5</INF>). With regard to the secondary PM standards, the
EPA generally retained the 24-hour and annual PM<INF>2.5</INF>
standards \15\ and the 24-hour PM<INF>10</INF> standard to address
visibility and non-visibility welfare effects.
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\14\ The EPA also eliminated the option for spatial averaging.
\15\ Consistent with the primary standard, the EPA eliminated
the option for spatial averaging with the annual standard.
---------------------------------------------------------------------------
As with previous reviews, petitioners challenged the EPA's final
rule. Petitioners argued that the EPA acted unreasonably in revising
the level and form of the annual standard and in amending the
monitoring network provisions. On judicial review, the revised
standards and monitoring requirements were upheld in all respects. NAM
v EPA, 750 F.3d 921 (D.C. Cir. 2014).
5. Review Completed in 2020
In December 2014, the EPA announced the initiation of the current
periodic review of the air quality criteria for PM and of the
PM<INF>2.5</INF> and PM<INF>10</INF> NAAQS and issued a call for
information (79 FR 71764, December 3, 2014). On February 9 to 11, 2015,
the EPA's NCEA and OAQPS held a public workshop to inform the planning
for the review of the PM NAAQS (announced in 79 FR 71764, December 3,
2014). Workshop participants, including a wide range of external
experts as well as the EPA staff representing a variety of areas of
expertise (e.g., epidemiology, human and animal toxicology, risk/
[[Page 5567]]
exposure analysis, atmospheric science, visibility impairment, climate
effects), were asked to highlight significant new and emerging PM
research, and to make recommendations to the Agency regarding the
design and scope of the review. This workshop provided for a public
discussion of the key science and policy-relevant issues around which
the EPA structured the review of the PM NAAQS and of the most
meaningful new scientific information that would be available in the
review to inform understanding of these issues.
The input received at the workshop guided the EPA staff in
developing a draft IRP, which was reviewed by the CASAC Particulate
Matter Panel and discussed on public teleconferences held in May 2016
(81 FR 13362, March 14, 2016) and August 2016 (81 FR 39043, June 15,
2016). Advice from the CASAC, supplemented by the Particulate Matter
Panel, and input from the public were considered in developing the
final IRP (U.S. EPA, 2016). The final IRP discusses the approaches to
be taken in developing key scientific, technical, and policy documents
in the review and the key policy-relevant issues that frame the EPA's
consideration of whether the primary and/or secondary NAAQS for PM
should be retained or revised.
In May 2018, the Administrator issued a memorandum describing a
``back-to-basics'' process for reviewing the NAAQS (Pruitt, 2018). This
memo announced the Agency's intention to conduct the review of the PM
NAAQS in such a manner as to ensure that any necessary revisions were
finalized by December 2020. Following this memo, on October 10, 2018,
the Administrator additionally announced that the role of reviewing the
key assessments developed as part of the ongoing review of the PM NAAQS
(i.e., drafts of the ISA and PA) would be performed by the seven-member
chartered CASAC (i.e., rather than the CASAC Particulate Matter Panel
that reviewed the draft IRP).\16\
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\16\ Announcement available at: <a href="https://www.regulations.gov/document/EPA-HQ-OAR-2015-0072-0223">https://www.regulations.gov/document/EPA-HQ-OAR-2015-0072-0223</a>.
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The EPA released the draft ISA in October 2018 (83 FR 53471,
October 23, 2018). The draft ISA was reviewed by the chartered CASAC at
a public meeting held in Arlington, VA, in December 2018 (83 FR 55529,
November 6, 2018) and was discussed on a public teleconference in March
2019 (84 FR 8523, March 8, 2019). The CASAC provided its advice on the
draft ISA in a letter to the EPA Administrator dated April 11, 2019
(Cox, 2019a). The EPA took steps to address these comments in the final
ISA, which was released in December 2019 (U.S. EPA, 2019a).
The EPA released the draft PA in September 2019 (84 FR 47944,
September 11, 2019). The draft PA was reviewed by the chartered CASAC
and discussed in October 2019 at a public meeting held in Cary, NC.
Public comments were received via a separate public teleconference (84
FR 51555, September 30, 2019). A public meeting to discuss the
chartered CASAC letter and response to charge questions on the draft PA
was held in Cary, NC, in December 2019 (84 FR 58713, November 1, 2019),
and the CASAC provided its advice on the draft PA, including its advice
on the current primary and secondary PM standards, in a letter to the
EPA Administrator dated December 16, 2019 (Cox, 2019b). With regard to
the primary standards, the CASAC recommended retaining the current 24-
hour PM<INF>2.5</INF> and PM<INF>10</INF> standards but did not reach
consensus on the adequacy of the current annual PM<INF>2.5</INF>
standard. With regard to the secondary standards, the CASAC recommended
retaining the current standards. In response to the CASAC's comments,
the 2020 final PA incorporated a number of changes (U.S. EPA, 2020a),
as described in detail in section I.C.5 of the 2020 proposal document
(85 FR 24100, April 30, 2020).
On April 14, 2020, the EPA proposed to retain all of the primary
and secondary PM standards, without revision. These proposed decisions
were published in the Federal Register on April 30, 2020 (85 FR 24094,
April 30, 2020). The EPA's final decision on the PM NAAQS was published
in the Federal Register on December 18, 2020 (85 FR 82684, December 18,
2020). In the 2020 rulemaking, the EPA retained the primary and
secondary PM<INF>2.5</INF> and PM<INF>10</INF> standards, without
revision.
Following publication of the 2020 final action, several parties
filed petitions for review and petitions for reconsideration of the
EPA's final decision. The petitions for review were filed in the D.C.
Circuit and the Court consolidated the cases. In order to consider
whether reconsideration of the 2020 final action was warranted, the EPA
moved for two 90-day abeyances in these consolidated cases, which the
Court granted. After the EPA announced that it is reconsidering the
2020 final decision, the EPA filed a motion with the Court to hold the
consolidated cases in abeyance until March 1, 2023, which the court
granted on October 1, 2021.
6. Reconsideration of the 2020 PM NAAQS Final Action
On January 20, 2021, President Biden issued an ``Executive Order on
Protecting Public Health and the Environment and Restoring Science to
Tackle the Climate Crisis'' (Executive Order 13990; 86 FR 7037, January
25, 2021),\17\ which directed review of certain agency actions. An
accompanying fact sheet provided a non-exclusive list of agency actions
that agency heads should review in accordance with that order,
including the 2020 Particulate Matter NAAQS Decision.\18\
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\17\ See <a href="https://www.whitehouse.gov/briefing-room/presidential-actions/2021/01/20/executive-order-protecting-public-health-and-environment-and-restoring-science-to-tackle-climate-crisis/">https://www.whitehouse.gov/briefing-room/presidential-actions/2021/01/20/executive-order-protecting-public-health-and-environment-and-restoring-science-to-tackle-climate-crisis/</a>.
\18\ See <a href="https://www.whitehouse.gov/briefing-room/statements-releases/2021/01/20/fact-sheet-list-of-agency-actions-for-review/">https://www.whitehouse.gov/briefing-room/statements-releases/2021/01/20/fact-sheet-list-of-agency-actions-for-review/</a>.
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a. Decision To Initiate a Reconsideration
On June 10, 2021, the Agency announced its decision to reconsider
the 2020 PM NAAQS final action.\19\ The EPA is reconsidering the
December 2020 decision because the available scientific evidence and
technical information indicate that the current standards may not be
adequate to protect public health and welfare, as required by the Clean
Air Act. The EPA noted that the 2020 PA concluded that the scientific
evidence and information supported revising the level of the primary
annual PM<INF>2.5</INF> standard to below the current level of 12.0
[micro]g/m\3\ while retaining the primary 24-hour PM<INF>2.5</INF>
standard (U.S. EPA, 2020a). The EPA also noted that the 2020 PA
concluded that the available scientific evidence and information
supported retaining the primary PM<INF>10</INF> standard and secondary
PM standards without revision (U.S. EPA, 2020a).
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\19\ The press release for this announcement is available at:
<a href="https://www.epa.gov/newsreleases/epa-reexamine-health-standards-harmful-soot-previous-administration-left-unchanged">https://www.epa.gov/newsreleases/epa-reexamine-health-standards-harmful-soot-previous-administration-left-unchanged</a>.
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b. Process for Reconsideration of the 2020 PM NAAQS Decision
In its announcement of the reconsideration of the PM NAAQS, the
Agency explained that, in support of the reconsideration, it would
develop a supplement to the 2019 ISA and a revised PA. The EPA also
explained that the draft ISA Supplement and draft PA would be reviewed
at a public meeting by the CASAC, and the public would have
opportunities to comment on these documents during the CASAC review
process, as well as to provide input during the rulemaking through the
[[Page 5568]]
public comment process and public hearings on the proposed rulemaking.
On March 31, 2021, the Administrator announced his decision to
reestablish the membership of the CASAC to ``ensure the agency received
the best possible scientific insight to support our work to protect
human health and the environment.'' \20\ Consistent with this
memorandum, a call for nominations of candidates to the EPA's chartered
CASAC was published in the Federal Register (86 FR 17146, April 1,
2021). On June 17, 2021, the Administrator announced his selection of
the seven members to serve on the chartered CASAC.<SUP>21 22</SUP>
Additionally, a call for nominations of candidates to a PM-specific
panel was published in the Federal Register (86 FR 33703, June 25,
2021). The members of the PM CASAC panel were announced on August 30,
2021.\23\
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\20\ The press release for this announcement is available at:
<a href="https://www.epa.gov/newsreleases/administrator-regan-directs-epa-reset-critical-science-focused-federal-advisory">https://www.epa.gov/newsreleases/administrator-regan-directs-epa-reset-critical-science-focused-federal-advisory</a>.
\21\ The press release for this announcement is available at:
<a href="https://www.epa.gov/newsreleases/epa-announces-selections-charter-members-clean-air-scientific-advisory-committee">https://www.epa.gov/newsreleases/epa-announces-selections-charter-members-clean-air-scientific-advisory-committee</a>.
\22\ The list of members of the chartered CASAC and their
biosketches are available at: <a href="https://casac.epa.gov/ords/sab/f?p=113:29:1706195567016:::RP,29:P29_COMMITTEEON:CASAC">https://casac.epa.gov/ords/sab/f?p=113:29:1706195567016:::RP,29:P29_COMMITTEEON:CASAC</a>.
\23\ The list of members of the PM CASAC panel and their
biosketches are available at: <a href="https://casac.epa.gov/ords/sab/f?p=105:14:9979229564047:::14:P14_COMMITTEEON:2021%20CASAC%20PM%20Panel">https://casac.epa.gov/ords/sab/f?p=105:14:9979229564047:::14:P14_COMMITTEEON:2021%20CASAC%20PM%20Panel</a>.
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The draft ISA Supplement was released in September 2021 (U.S. EPA,
2021a; 86 FR 54186, September 30, 2021). The CASAC PM panel met at a
virtual public meeting in November 2021 to review the draft ISA
Supplement (86 FR 52673, September 22, 2021). A virtual public meeting
was then held in February 2022, and during this meeting the chartered
CASAC considered the CASAC PM panel's draft letter to the Administrator
on the draft ISA Supplement (87 FR 958, January 7, 2022). The chartered
CASAC provided its advice on the draft ISA Supplement in a letter to
the EPA Administrator dated March 18, 2022 (Sheppard, 2022b). The EPA
took steps to address these comments in the final ISA Supplement, which
was released in May 2022 (U.S. EPA, 2022a; hereafter referred to as the
ISA Supplement throughout this document).
The evidence presented within the 2019 ISA, along with the targeted
identification and evaluation of new scientific information in the ISA
Supplement, provides the scientific basis for the reconsideration of
the 2020 PM NAAQS final decision. The ISA Supplement focuses on a
thorough evaluation of some studies that became available after the
literature cutoff date of the 2019 ISA that could either further inform
the adequacy of the current PM NAAQS or address key scientific topics
that have evolved since the literature cutoff date for the 2019 ISA. In
selecting the health effects to evaluate within the ISA Supplement, the
EPA focused on health effects for which the evidence supported a
``causal relationship'' because those were the health effects that were
most useful in informing conclusions in the 2020 PA (U.S. EPA, 2022a,
section 1.2.1).\24\ Consistent with the rationale for the focus on
certain health effects, in selecting the non-ecological welfare effects
to evaluate within the ISA supplement, the EPA focused on the non-
ecological welfare effects for which the evidence supported a ``causal
relationship'' and for which quantitative analyses could be supported
by the evidence because those were the welfare effects that were most
useful in informing conclusions in the 2020 PA.\25\ Specifically, for
non-ecological welfare effects, the focus within the ISA Supplement is
on visibility effects. The ISA Supplement also considers recent health
effects evidence that addresses key scientific topics where the
literature has evolved since the 2020 review was completed,
specifically since the literature cutoff date for the 2019 ISA.\26\
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\24\ As described in section 1.2.1 of the ISA Supplement: ``In
considering the public health protection provided by the current
primary PM<INF>2.5</INF> standards, and the protection that could be
provided by alternatives, [the U.S. EPA, within the 2020 PM PA]
emphasized health outcomes for which the ISA determined that the
evidence supports either a `causal' or a `likely to be causal'
relationship with PM<INF>2.5</INF> exposures'' (U.S. EPA, 2020a).
Although the 2020 PA initially focused on this broader set of
evidence, the basis of the discussion on potential alternative
standards primarily focused on health effect categories where the
2019 PM ISA concluded a `causal relationship' (i.e., short- and
long-term PM<INF>2.5</INF> exposure and cardiovascular effects and
mortality) as reflected in Figures 3-7 and 3-8 of the 2020 PA (U.S.
EPA, 2020a).'' As described in section 1.2.1 of the ISA Supplement:
``In considering the public health protection provided by the
current primary PM<INF>2.5</INF> standards, and the protection that
could be provided by alternatives, [the U.S. EPA, within the 2020 PM
PA] emphasized health outcomes for which the ISA determined that the
evidence supports either a `causal' or a `likely to be causal'
relationship with PM<INF>2.5</INF> exposures'' (U.S. EPA, 2020a).
Although the 2020 PA initially focused on this broader set of
evidence, the basis of the discussion on potential alternative
standards primarily focused on health effect categories where the
2019 PM ISA concluded a `causal relationship' (i.e., short- and
long-term PM<INF>2.5</INF> exposure and cardiovascular effects and
mortality) as reflected in Figures 3-7 and 3-8 of the 2020 PA (U.S.
EPA, 2020a).''
\25\ As described in section 1.2.1 of the ISA Supplement: ``The
2019 PM ISA concluded a `causal relationship' for each of the
welfare effects categories evaluated (i.e., visibility, climate
effects and materials effects). While the 2020 PA considered the
broader set of evidence for these effects, for climate effects and
material effects, it concluded that there remained `substantial
uncertainties with regard to the quantitative relationships with PM
concentrations and concentration patterns that limit[ed] [the]
ability to quantitatively assess the public welfare protection
provided by the standards from these effects' (U.S. EPA, 2020a).''
\26\ These key scientific topics include experimental studies
conducted at near-ambient concentrations, epidemiologic studies that
employed alternative methods for confounder control or conducted
accountability analyses, studies that assess the relationship
between PM<INF>2.5</INF> exposure and severe acute respiratory
syndrome coronavirus 2 (SARS-CoV-2) infection and coronavirus
disease 2019 (COVID-19) death; and in accordance with recent EPA
goals on addressing environmental justice, studies that examine
disparities in PM<INF>2.5</INF> exposure and the risk of health
effects by race/ethnicity or socioeconomic status (SES) (U.S. EPA,
2022a, section 1.2.1).
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Building on the rationale presented in section 1.2.1, the ISA
Supplement considers peer-reviewed studies published from approximately
January 2018 through March 2021 that meet the following criteria:
Health Effects
[cir] U.S. and Canadian epidemiologic studies for health effect
categories where the 2019 ISA concluded a ``causal relationship''
(i.e., short- and long-term PM<INF>2.5</INF> exposure and
cardiovascular effects and mortality).
[ssquf] U.S. and Canadian epidemiologic studies that employed
alternative methods for confounder control or conducted accountability
analyses (i.e., examined the effect of a policy on reducing
PM<INF>2.5</INF> concentrations).
<bullet> Welfare Effects
[cir] U.S. and Canadian studies that provide new information on
public preferences for visibility impairment and/or developed
methodologies or conducted quantitative analyses of light extinction.
<bullet> Key Scientific Topics
[cir] Experimental studies (i.e., controlled human exposure and
animal toxicological) conducted at near-ambient PM<INF>2.5</INF>
concentrations experienced in the U.S.
[cir] U.S.- and Canadian-based epidemiologic studies that examined
the relationship between PM<INF>2.5</INF> exposures and severe acute
respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and
coronavirus disease 2019 (COVID-19) death.
[cir] At-Risk Populations:
[ssquf] U.S.- and Canadian-based epidemiologic or exposure studies
examining potential disparities in either PM<INF>2.5</INF> exposures or
the risk of health
[[Page 5569]]
effects by race/ethnicity or socioeconomic status (SES).
Given the narrow scope of the ISA Supplement, it is important to
recognize that the evaluation does not encompass the full
multidisciplinary evaluation presented within the 2019 ISA that would
result in weight-of-evidence conclusions on causality (i.e., causality
determinations). The ISA Supplement critically evaluates and provides
key study specific information for those recent studies deemed to be of
greatest significance for informing preliminary conclusions on the PM
NAAQS in the context of the body of evidence and scientific conclusions
presented in the 2019 ISA. In its review of the draft ISA Supplement,
the CASAC noted that they found ``the Draft ISA Supplement to be a
well-written, comprehensive evaluation of the new scientific
information published since the 2019 PM ISA'' (Sheppard, 2022b, p. 2 of
letter). Furthermore, the CASAC stated that ``the final Integrated
Science Assessment (ISA) Supplement . . . deserve[s] the
Administrator's full consideration and [is] adequate for rulemaking''
(Sheppard, 2022b, p. 2 of letter). However, recognizing the limited
scope of the draft ISA Supplement, the CASAC stated that ``[a]lthough
this limitation is appropriate for the targeted purpose of the Draft
ISA Supplement . . . this limiting of scope applies only to this
document and is not intended to establish a precedent for future ISAs''
(Sheppard, 2022b, p. 2 of letter).
The draft PA was released in October 2021 (86 FR 56263, October 8,
2021). The CASAC PM panel met at a virtual public meeting in December
2021 to review the draft PA (86 FR 52673, September 22, 2021). A
virtual public meeting was then held in February 2022 and March 2022,
and during this meeting the chartered CASAC considered the CASAC PM
panel's draft letter to the Administrator on the draft PA (87 FR 958,
January 7, 2022). The chartered CASAC provided its advice on the draft
PA in a letter to the EPA Administrator dated March 18, 2022 (Sheppard,
2022a). The EPA took steps to address these comments in revising and
finalizing the PA. The PA considers the scientific evidence presented
in the 2019 ISA and ISA Supplement and considers the quantitative and
technical information presented in the 2020 PA, along with updated and
newly available analyses since the completion of the 2020 review. For
those health and welfare effects for which the ISA Supplement evaluated
recently available evidence and for which updated quantitative analyses
were supported (i.e., PM<INF>2.5</INF>-related health effects and
visibility effects), the PA includes consideration of this newly
available scientific and technical information in reaching preliminary
conclusions. For those health and welfare effects for which newly
available scientific and technical information were not evaluated
(i.e., PM<INF>10-2.5</INF>-related health effects and non-visibility
effects), the conclusions presented in the PA rely heavily on the
information that supported the conclusions in the 2020 PA. The final PA
was released in May 2022 (U.S. EPA, 2022b; hereafter referred to as the
PA throughout this document).
D. Air Quality Information
This section provides a summary of basic information related to PM
ambient air quality. It summarizes information on the distribution of
particle size in ambient air (section I.D.1), sources and emissions
contributing to PM in the ambient air (section I.D.2), monitoring
ambient PM in the U.S. (section I.D.3), ambient PM concentrations and
trends in the U.S. (I.D.4), characterizing ambient PM<INF>2.5</INF>
concentrations for exposure (section I.D.5), and background PM (section
I.D.6). Additional detail on PM air quality can be found in Chapter 2
of the PA (U.S. EPA, 2022b).
1. Distribution of Particle Size in Ambient Air
In ambient air, PM is a mixture of substances suspended as small
liquid and/or solid particles (U.S. EPA, 2019a, section 2.2) and
distinct health and welfare effects have been linked with exposures to
particles of different sizes. Particles in the atmosphere range in size
from less than 0.01 to more than 10 [mu]m in diameter (U.S. EPA, 2019a,
section 2.2). The EPA defines PM<INF>2.5</INF>, also referred to as
fine particles, as particles with aerodynamic diameters generally less
than or equal to 2.5 [mu]m. The size range for PM<INF>10-2.5</INF>,
also called coarse or thoracic coarse particles, includes those
particles with aerodynamic diameters generally greater than 2.5 [mu]m
and less than or equal to 10 [mu]m. PM<INF>10</INF>, which is comprised
of both fine and coarse fractions, includes those particles with
aerodynamic diameters generally less than or equal to 10 [mu]m. In
addition, ultrafine particles (UFP) are often defined as particles with
a diameter of less than 0.1 [mu]m based on physical size, thermal
diffusivity or electrical mobility (U.S. EPA, 2019a, section 2.2).
Atmospheric lifetimes are generally longest for PM<INF>2.5</INF>, which
often remains in the atmosphere for days to weeks (U.S. EPA, 2019a,
Table 2-1) before being removed by wet or dry deposition, while
atmospheric lifetimes for UFP and PM<INF>10-2.5</INF> are shorter and
are generally removed from the atmosphere within hours, through wet or
dry deposition (U.S. EPA, 2019a, Table 2-1; U.S. EPA, 2022b, section
2.1).
2. Sources and Emissions Contributing to PM in the Ambient Air
PM is composed of both primary (directly emitted particles) and
secondary particles. Primary PM is derived from direct particle
emissions from specific PM sources while secondary PM originates from
gas-phase precursor chemical compounds present in the atmosphere that
have participated in new particle formation or condensed onto existing
particles (U.S. EPA, 2019a, section 2.3). As discussed further in the
2019 ISA (U.S. EPA, 2019a, section 2.3.2.1), secondary PM is formed in
the atmosphere by photochemical oxidation reactions of both inorganic
and organic gas-phase precursors. Precursor gases include sulfur
dioxide (SO<INF>2</INF>), nitrogen oxides (NO<INF>X</INF>), and
volatile organic compounds (VOC) (U.S. EPA, 2019a, section 2.3.2.1).
Ammonia also plays an important role in the formation of nitrate PM by
neutralizing sulfuric acid and nitric acid. Sources and emissions of PM
are discussed in more detail the PA (U.S. EPA, 2022b, section 2.1.1).
Briefly, anthropogenic sources of PM include both stationary (e.g.,
fuel combustion for electricity production and other purposes,
industrial processes, agricultural activities) and mobile (e.g.,
diesel- and gasoline-powered highway vehicles and other engine-driven
sources) sources. Natural sources of PM include dust from the wind
erosion of natural surfaces, sea salt, wildfires, primary biological
aerosol particles (PBAP) such as bacteria and pollen, oxidation of
biogenic hydrocarbons, such as isoprene and terpenes to produce
secondary organic aerosol (SOA), and geogenic sources, such as sulfate
formed from volcanic production of SO<INF>2</INF>. Wildland fire, which
encompass both wildfire and prescribed fire, accounts for over 30% of
emissions of primary PM<INF>2.5</INF> emissions (U.S. EPA, 2021).
In recent years, the frequency and magnitude of wildfires have
increased (U.S. EPA, 2019a). The magnitude of the public health impact
of wildfires is substantial both because of the increase in
PM<INF>2.5</INF> concentrations as well as the duration of the wildfire
smoke season, which is considered to range from May to November.
Wildfire can make a large contribution to air pollution (including
PM<INF>2.5</INF>), and wildfire events can threaten public safety and
life. The impacts of wildfire events can be mitigated through
[[Page 5570]]
management of wildland vegetation, including through prescribed fire.
Prescribed fire (and some wildfires) can mimic the natural processes
necessary to maintain fire dependent ecosystems, minimizing
catastrophic wildfires and the risks they pose to safety, property and
air quality (see, e.g., 81 FR 58010, 58038, August 24, 2016).
Landowners, land managers and government public safety agencies are
strongly motivated to reduce the frequency and severity of human caused
wildfires. Additionally, land managers, landowners, air agencies and
communities may be able to lessen the impacts of wildfires by working
collaboratively to take steps to minimize fuel loading in areas
vulnerable to fire. Fuel load minimization steps can consist of both
prescribed fire and mechanical treatments, such as using mechanical
equipment to reduce accumulated understory (81 FR 68249, October 3,
2016). There are specific Federal plans of the Department of the
Interior \27\ and United States Forest Service \28\ to increase fuel
load minimization efforts in areas at high risk of wildfire. The
recently passed Bipartisan Infrastructure Law \29\ and Inflation
Reduction Act \30\ further direct agencies and provide funding for such
efforts at the Federal level as well as at state, Tribal, local, and
private landowner levels.\31\
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\27\ See U.S. Department of the Interior, ``Infrastructure
Investment and Jobs Act Wildfire Risk Five-Year Monitoring,
Maintenance, and Treatment Plan'' (April 2022), available at:
<a href="https://www.doi.gov/sites/doi.gov/files/bil-5-year-wildfire-risk-mmt-plan.04.2022.owf_.final_.pdf">https://www.doi.gov/sites/doi.gov/files/bil-5-year-wildfire-risk-mmt-plan.04.2022.owf_.final_.pdf</a>.
\28\ See U.S. Department of Agriculture, Forest Service,
``Confronting the Wildfire Crisis: A Strategy for Protecting
Communities and Improving Resilience in America's Forests'', FS-
1187d (April 2022) available at: <a href="https://www.fs.usda.gov/sites/default/files/Confronting-Wildfire-Crisis.pdf">https://www.fs.usda.gov/sites/default/files/Confronting-Wildfire-Crisis.pdf</a>.
\29\ Inflation Reduction Act, Public Law 117-169 available at
<a href="https://www.congress.gov/117/plaws/publ169/PLAW-117publ169.pdf">https://www.congress.gov/117/plaws/publ169/PLAW-117publ169.pdf</a>.
\30\ Infrastructure Investment and Jobs Act, Public Law 117-58,
available at <a href="https://www.congress.gov/117/plaws/publ58/PLAW-117publ58.pdf">https://www.congress.gov/117/plaws/publ58/PLAW-117publ58.pdf</a>.
\31\ Inflation Reduction Act, Public Law 117-169 available at
<a href="https://www.congress.gov/117/plaws/publ169/PLAW-117publ169.pdf">https://www.congress.gov/117/plaws/publ169/PLAW-117publ169.pdf</a>.
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Wildfire events produce high PM emissions that impact the PM
concentrations in ambient air to the extent that such days with high PM
concentrations from wildfire smoke events may affect the design values
in a given area. The annual and daily design values affected by
potential exceptional events associated with wildfire smoke may qualify
to be excluded from design value calculations used for comparison to
the NAAQS. The EPA's Exceptional Events Rule (81 FR 68216, October 3,
2016) describes the process by which exceedances caused by fire events,
including certain prescribed fires, can be excluded from the design
values. It should be noted that potential exceptional events associated
with prescribed fires on wildland may also qualify to be excluded from
design value calculations used for comparison to the NAAQS under the
Exceptional Events Rule (as described in more detail in section VIII
below).
While the EPA is not proposing changes to implementation as a part
of this proposal (as described in more detail in section VIII below),
the EPA acknowledges that increases in PM<INF>2.5</INF> emissions due
to increases in wildfire and prescribed fire on wildland present a
number of challenges relevant to the implementation of the PM NAAQS,
particularly if one or more standards are strengthened. Stakeholders
have expressed concern about the growing health challenges associated
with such emissions, the importance of prescribed fire for managing
fire-dependent ecosystems and reducing fuel loads, and the potential
for further increases in the frequency and magnitude of wildfires due
to climate change. Though such issues are outside the scope of this
proposal, the EPA acknowledges that these topics may arise in the
context of implementation of any revised PM<INF>2.5</INF> NAAQS and
intends to work with stakeholders to address these issues.
3. Monitoring of Ambient PM
To promote uniform enforcement of the air quality standards set
forth under the CAA and to achieve the degree of public health and
welfare protection intended for the NAAQS, the EPA established PM
Federal Reference Methods (FRMs) for both PM<INF>10</INF> and
PM<INF>2.5</INF> (appendices J and L to 40 CFR part 50). Amended
following the 2006 and 2012 PM NAAQS reviews, the current PM monitoring
network relies on FRMs and automated continuous Federal Equivalent
Methods (FEMs), in part to support changes necessary for implementation
of the revised PM standards. The requirement for measuring ambient air
quality and reporting ambient air quality data and related information
are the basis for appendices A through E to 40 CFR part 58. More
information on PM ambient monitoring networks is available in section
2.2 of the PA (U.S. EPA, 2022b).
The PM<INF>2.5</INF> monitoring program is one of the major ambient
air monitoring programs with a robust, nationally consistent network of
ambient air monitoring sites providing mass and/or chemical speciation
measurements. For most urban locations, PM<INF>2.5</INF> monitors are
sited at the neighborhood scale,\32\ where PM<INF>2.5</INF>
concentrations are reasonably homogeneous throughout an entire urban
sub-region. In each CBSA with a monitoring requirement, at least one
PM<INF>2.5</INF> monitoring station representing area-wide air quality
is sited in an area of expected maximum concentration. By ensuring the
area of expected maximum concentration in a CBSA has a site compared to
both the annual and 24-hour NAAQS, all other similar locations are thus
protected. Sites that represent relatively unique microscale, localized
hot-spot, or unique middle scale impact sites are only eligible for
comparison to the 24-hour PM<INF>2.5</INF> NAAQS.
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\32\ For PM<INF>2.5</INF>, neighborhood scale is defined as
follows: Measurements in this category would represent conditions
throughout some reasonably homogeneous urban sub-region with
dimensions of a few kilometers and of generally more regular shape
than the middle scale. Homogeneity refers to the particulate matter
concentrations, as well as the land use and land surface
characteristics. Much of the PM<INF>2.5</INF> exposures are expected
to be associated with this scale of measurement. In some cases, a
location carefully chosen to provide neighborhood scale data would
represent the immediate neighborhood as well as neighborhoods of the
same type in other parts of the city. PM<INF>2.5</INF> sites of this
kind provide good information about trends and compliance with
standards because they often represent conditions in areas where
people commonly live and work for periods comparable to those
specified in the NAAQS. In general, most PM<INF>2.5</INF> monitoring
in urban areas should have this scale.
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There are three main methods components of the PM<INF>2.5</INF>
monitoring program: filter-based FRMs measuring PM<INF>2.5</INF> mass,
FEMs measuring PM<INF>2.5</INF> mass, and other samplers used to
collect the aerosol used in subsequent laboratory analysis for
measuring PM<INF>2.5</INF> chemical speciation. The FRMs are primarily
used for comparison to the NAAQS, but also serve other important
purposes, such as developing trends and evaluating the performance of
FEMs. PM<INF>2.5</INF> FEMs are typically continuous methods used to
support forecasting and reporting of the Air Quality Index (AQI) but
are also used for comparison to the NAAQS. Samplers that are part of
the Chemical Speciation Network (CSN) and Interagency Monitoring of
Protected Visual Environments (IMPROVE) network are used to provide
chemical composition of the aerosol and serve a variety of objectives.
More detail on of each of these components of the PM<INF>2.5</INF>
monitoring program and of recent changes to PM<INF>2.5</INF> monitoring
requirements are described in detail in the PA (U.S. EPA, 2022b,
section 2.2.3).
4. Ambient Concentrations and Trends
This section summarizes available information on recent ambient PM
concentrations in the U.S. and on trends
[[Page 5571]]
in PM air quality. Sections I.D.4.a and I.D.4.b summarize information
on PM<INF>2.5</INF> mass and components, respectively. Section I.D.4.c
summarizes information on PM<INF>10</INF>. Sections I.D.4.d and I.D.4.e
summarize the more limited information on PM<INF>10-2.5</INF> and UFP,
respectively. Additional detail on PM air quality and trends can be
found in the PA (U.S. EPA, 2022b, section 2.3).
a. PM<INF>2.5</INF> Mass
At monitoring sites in the U.S., annual PM<INF>2.5</INF>
concentrations from 2017 to 2019 averaged 8.0 [mu]g/m\3\ (with the 10th
and 90th percentiles at 5.9 and 10.0 [mu]g/m\3\, respectively) and the
98th percentiles of 24-hour concentrations averaged 21.3 [mu]g/m\3\
(with the 10th and 90th percentiles at 14.0 and 29.7 [mu]g/m\3\,
respectively) (U.S. EPA, 2022b, section 2.3.2.1). The highest ambient
PM<INF>2.5</INF> concentrations occur in the western U.S., particularly
in California and the Pacific Northwest (U.S. EPA, 2022b, Figure 2-15).
Much of the eastern U.S. has lower ambient concentrations, with annual
average concentrations generally at or below 12.0 [mu]g/m\3\ and 98th
percentiles of 24-hour concentrations generally at or below 30 [mu]g/
m\3\ (U.S. EPA, 2022b, section 2.3.2.1).
Recent ambient PM<INF>2.5</INF> concentrations reflect the
substantial reductions that have occurred across much of the U.S. (U.S.
EPA, 2022b, section 2.3.2.1). From 2000 to 2019, national annual
average PM<INF>2.5</INF> concentrations declined from 13.5 [mu]g/m\3\
to 7.6 [mu]g/m\3\, a 43% decrease (U.S. EPA, 2022b, section
2.3.2.1).\33\ These declines have occurred at urban and rural
monitoring sites, although urban PM<INF>2.5</INF> concentrations remain
consistently higher than those in rural areas (Chan et al., 2018) due
to the impact of local sources in urban areas. Analyses at individual
monitoring sites indicate that declines in ambient PM<INF>2.5</INF>
concentrations have been most consistent across the eastern U.S. and in
parts of coastal California, where both annual average and 98th
percentiles of 24-hour concentrations declined significantly (U.S. EPA,
2022b, section 2.3.2.1). In contrast, trends in ambient
PM<INF>2.5</INF> concentrations have been less consistent over much of
the western U.S., with no significant changes since 2000 observed at
some sites in the Pacific Northwest, the northern Rockies and plains,
and the southwest, particularly for 98th percentiles of 24-hour
concentrations (U.S. EPA, 2022b, section 2.3.2.1). As noted below, some
sites in the northwestern U.S. and California, where wildfire have been
relatively common in recent years, have experienced high concentrations
over shorter periods (i.e., 2-hour averages).
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\33\ See <a href="https://www.epa.gov/air-trends/particulate-matter-pm25-trends">https://www.epa.gov/air-trends/particulate-matter-pm25-trends</a> for up-to-date PM<INF>2.5</INF> trends information.
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The recent deployment of PM<INF>2.5</INF> monitors near major roads
in large urban areas provides information on PM<INF>2.5</INF>
concentrations near an important emissions source. For 2016-2018, Gantt
et al. (2021) reported that 52% and 24% of the time near-road sites
reported the highest annual and 24-hour PM<INF>2.5</INF> design value
\34\ in the CBSA, respectively. Of the CBSAs with the highest annual
design values at near-road sites reported by Gantt et al. (2021), those
design values were, on average, 0.8 [mu]g/m\3\ higher than at the
highest measuring non-near-road sites (range is 0.1 to 2.1 [mu]g/m\3\
higher at near-road sites). Although most near-road monitoring sites do
not have sufficient data to evaluate long-term trends in near-road
PM<INF>2.5</INF> concentrations, analyses of the data at one near-road-
like site in Elizabeth, NJ,\35\ show that the annual average near-road
increment has generally decreased between 1999 and 2017 from about 2.0
[mu]g/m\3\ to about 1.3 [mu]g/m\3\ (U.S. EPA, 2022b, section 2.3.2.1).
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\34\ A design value is considered valid if it meets the data
handling requirements given in appendix N to 40 CFR part 50.
\35\ The Elizabeth Lab site in Elizabeth, NJ, is situated
approximately 30 meters from travel lanes of the Interchange 13 toll
plaza of the New Jersey Turnpike and within 200 meters of travel
lanes for Interstate 278 and the New Jersey Turnpike.
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Ambient PM<INF>2.5</INF> concentrations can exhibit a diurnal cycle
that varies due to impacts from intermittent emission sources,
meteorology, and atmospheric chemistry. The PM<INF>2.5</INF> monitoring
network in the U.S. has an increasing number of continuous FEM monitors
reporting hourly PM<INF>2.5</INF> mass concentrations that reflect this
diurnal variation. The 2019 ISA describes a two-peaked diurnal pattern
in urban areas, with morning peaks attributed to rush-hour traffic and
afternoon peaks attributed to a combination of rush hour traffic,
decreasing atmospheric dilution, and nucleation (U.S. EPA, 2019a,
section 2.5.2.3, Figure 2-32). Because a focus on annual average and
24-hour average PM<INF>2.5</INF> concentrations could mask sub-daily
patterns, and because some health studies examine PM exposure durations
shorter than 24-hours, it is useful to understand the broader
distribution of sub-daily PM<INF>2.5</INF> concentrations across the
U.S. The PA presents information on the frequency distribution of 2-
hour average PM<INF>2.5</INF> mass concentrations from all FEM
PM<INF>2.5</INF> monitors in the U.S. for 2017-2019. At sites meeting
the current primary PM<INF>2.5</INF> standards, these 2-hour
concentrations generally remain below 10 [mu]g/m\3\, and rarely exceed
30 [mu]g/m\3\. Two-hour concentrations are higher at sites violating
the current standards, generally remaining below 16 [mu]g/m\3\ and
rarely exceeding 80 [mu]g/m\3\ (U.S. EPA, 2022b, section 2.3.2.2.3).
The extreme upper end of the distribution of 2-hour PM<INF>2.5</INF>
concentrations is shifted higher during the warmer months, generally
corresponding to the period of peak wildfire frequency (April to
September) in the U.S. At sites meeting the current primary standards,
the highest 2-hour concentrations measured rarely occur outside of the
period of peak wildfire frequency. Most of the sites measuring these
very high concentrations are in the northwestern U.S. and California,
where wildfires have been relatively common in recent years (see U.S.
EPA, 2022b, Appendix A, Figure A-1). When the period of peak wildfire
frequency is excluded from the analysis, the extreme upper end of the
distribution is reduced (U.S. EPA, 2022b, section 2.3.2.2.3).
b. PM<INF>2.5</INF> Components
Based on recent air quality data, the major chemical components of
PM<INF>2.5</INF> have distinct spatial distributions. Sulfate
concentrations tend to be highest in the eastern U.S., while in the
Ohio Valley, Salt Lake Valley, and California nitrate concentrations
are highest, and relatively high concentrations of organic carbon are
widespread across most of the continental U.S. (U.S. EPA, 2022b,
section 2.3.2.3). Elemental carbon, crustal material, and sea salt are
found to have the highest concentrations in the northeast U.S.,
southwest U.S., and coastal areas, respectively.
An examination of PM<INF>2.5</INF> composition trends can provide
insight into the factors contributing to overall reductions in ambient
PM<INF>2.5</INF> concentrations. The biggest change in PM<INF>2.5</INF>
composition that has occurred in recent years is the reduction in
sulfate concentrations due to reductions in SO<INF>2</INF> emissions.
Between 2000 and 2015, the nationwide annual average sulfate
concentration decreased by 17% at urban sites and 20% at rural sites.
This change in sulfate concentrations is most evident in the eastern
U.S. and has resulted in organic matter or nitrate now being the
greatest contributor to PM<INF>2.5</INF> mass in many locations (U.S.
EPA, 2019a, Figure 2-19). The overall reduction in sulfate
concentrations has contributed substantially to the decrease in
national average PM<INF>2.5</INF> concentrations as well as the decline
in the fraction of PM<INF>10</INF> mass accounted for by
PM<INF>2.5</INF> (U.S.
[[Page 5572]]
EPA, 2019a, section 2.5.1.1.6; U.S. EPA, 2022b, section 2.3.1).
c. PM<INF>10</INF>
At long-term monitoring sites in the U.S., the 2017-2019 average of
2nd highest 24-hour PM<INF>10</INF> concentration was 68 [mu]g/m\3\
(with 10th and 90th percentiles at 28 and 124 [mu]g/m\3\, respectively)
(U.S. EPA, 2022b, section 2.3.2.4).\36\ The highest PM<INF>10</INF>
concentrations tend to occur in the western U.S. Seasonal analyses
indicate that ambient PM<INF>10</INF> concentrations are generally
higher in the summer months than at other times of year, though the
most extreme high concentration events are more likely in the spring
(U.S. EPA, 2019a, Table 2-5). This is due to fact that the major
PM<INF>10</INF> emission sources, dust and agriculture, are more active
during the warmer and drier periods of the year.
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\36\ The form of the current 24-hour PM<INF>10</INF> standard is
one-expected-exceedance, averaged over three years.
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Recent ambient PM<INF>10</INF> concentrations reflect reductions
that have occurred across much of the U.S. (U.S. EPA, 2022b, section
2.3.2.4). From 2000 to 2019, 2nd highest 24-hour PM<INF>10</INF>
concentrations have declined by about 46% (U.S. EPA, 2022b, section
2.3.2.4).\37\ Analyses at individual monitoring sites indicate that
annual average PM<INF>10</INF> concentrations have generally declined
at most sites across the U.S., with much of the decrease in the eastern
U.S. associated with reductions in PM<INF>2.5</INF> concentrations
(U.S. EPA, 2022b, section 2.3.2.4). Annual 2nd highest 24-hour
PM<INF>10</INF> concentrations have generally declined in the eastern
U.S., while concentrations in much of the midwest and western U.S. have
remained unchanged or increased since 2000 (U.S. EPA, 2022b, section
2.3.2.4).
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\37\ For more information, see <a href="https://www.epa.gov/air-trends/particulate-matter-pm10-trends#pmnat">https://www.epa.gov/air-trends/particulate-matter-pm10-trends#pmnat</a>.
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Compared to previous reviews, data available from the NCore
monitoring network in the current reconsideration allows a more
comprehensive analysis of the relative contributions of
PM<INF>2.5</INF> and PM<INF>10-2.5</INF> to PM<INF>10</INF> mass.
PM<INF>2.5</INF> generally contributes more to annual average
PM<INF>10</INF> mass in the eastern U.S. than the western U.S. (U.S.
EPA, 2022b, Figure 2-23). At most sites in the eastern U.S., the
majority of PM<INF>10</INF> mass is comprised of PM<INF>2.5</INF>. As
ambient PM<INF>2.5</INF> concentrations have declined in the eastern
U.S. (U.S. EPA, 2022b, section 2.3.2.2), the ratios of PM<INF>2.5</INF>
to PM<INF>10</INF> have also declined. For sites with days having
concurrently very high PM<INF>2.5</INF> and PM<INF>10</INF>
concentrations (U.S. EPA, 2022b, Figure 2-24), the PM<INF>2.5</INF>/
PM<INF>10</INF> ratios are typically higher than the annual average
ratios. This is particularly true in the northwestern U.S. where the
high PM<INF>10</INF> concentrations can occur during wildfires with
high PM<INF>2.5</INF> (U.S. EPA, 2022b, section 2.3.2.4).
d. PM<INF>10-2.5</INF>
Since the 2012 review, the availability of PM<INF>10-2.5</INF>
ambient concentration data has greatly increased because of additions
to the PM<INF>10-2.5</INF> monitoring capabilities to the national
monitoring network. As illustrated in the PA (U.S. EPA, 2022b, section
2.3.2.5), annual average and 98th percentile PM<INF>10-2.5</INF>
concentrations exhibit less distinct differences between the eastern
and western U.S. than for either PM<INF>2.5</INF> or PM<INF>10</INF>.
Due to the short atmospheric lifetime of PM<INF>10-2.5</INF>
relative to PM<INF>2.5</INF>, many of the high concentration sites are
isolated and likely near emission sources associated with wind-blown
and fugitive dust. The spatial distributions of annual average and 98th
percentile concentrations of PM<INF>10-2.5</INF> are more similar than
that of PM<INF>2.5</INF>, suggesting that the same dust-related
emission sources are affecting both long-term and episodic
concentrations (U.S. EPA, 2022b, Figure 2-25). The highest
concentrations of PM<INF>10-2.5</INF> are in the southwest U.S. where
widespread dry and windy conditions contribute to wind-blown dust
emissions. Additionally, compared to PM<INF>2.5</INF> and
PM<INF>10</INF>, changes in PM<INF>10-2.5</INF> concentrations have
been small in magnitude and inconsistent in direction (U.S. EPA, 2022b,
Figure 2-25). The majority of PM<INF>10-2.5</INF> sites in the U.S. do
not have a concentration trend from 2000-2019, reflecting the
relatively consistent level of dust emissions across the U.S. during
the same time period (U.S. EPA, 2022b, section 2.3.2.5).\38\
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\38\ PM from dust emissions in the National Emissions Inventory
(NEI) remain fairly consistent from year-to-year, except when there
are severe weather incursions or there is a dust event that
transports or causes major local dust storms to occur (particularly
in the western U.S.). These dust events and weather incursions
needed to effect dust emissions on a national level are not common
and only seldomly occur. In the emissions trends analysis presented
in the PA (U.S. EPA, 2022b, section 2.1.1), dust is included in the
NEI sector labeled ``miscellaneous.''
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e. UFP
Compared to PM<INF>2.5</INF> mass, there is relatively little data
on U.S. particle number concentrations, which are dominated by UFP. In
the published literature, annual average particle number concentrations
reaching about 20,000 to 30,000 cm\3\ have been reported in U.S. cities
(U.S. EPA, 2019a). In addition, based on UFP measurements in two urban
areas (New York City, Buffalo) and at a background site (Steuben
County) in New York, there is a pronounced difference in particle
number concentration between different types of locations (U.S. EPA,
2022b, Figure 2-26; U.S. EPA, 2019a, Figure 2-18). Urban particle
number counts were several times higher than at the background site,
and the highest particle number counts in an urban area with multiple
sites (Buffalo) were observed at a near-road location (U.S. EPA, 2022b,
section 2.3.2.6).
Long-term trends in UFP are not routinely available at U.S.
monitoring sites. At one background site in Illinois with long-term
data available, the annual average particle number concentration
declined between 2000 and 2019, closely matching the reductions in
annual PM<INF>2.5</INF> mass over that same period (U.S. EPA, 2022b,
section 2.3.2.6). In addition, a small number of published studies have
examined UFP trends over time. While limited, these studies also
suggest that UFP number concentrations have declined over time along
with decreases in PM<INF>2.5</INF> (U.S. EPA, 2022b, section 2.3.2.6).
However, the relationship between changes in ambient PM<INF>2.5</INF>
and UFPs cannot be comprehensively characterized due to the high
variability and limited monitoring of UFPs (U.S. EPA, 2022b, section
2.3.2.6).
5. Characterizing Ambient PM<INF>2.5</INF> Concentrations for Exposure
Epidemiologic studies use various methods to characterize exposure
to ambient PM<INF>2.5</INF>. The methods used to estimate
PM<INF>2.5</INF> concentrations can vary from traditional methods using
monitoring data from ground-based monitors to newer methods using more
complex hybrid modeling approaches. Studies using hybrid modeling
approaches aim to broaden the spatial coverage, as well as estimate
more spatially-resolved ambient PM<INF>2.5</INF> concentrations, by
expanding beyond just those areas with monitors and providing estimates
in areas that do not have ground-based monitors (i.e., areas that are
generally less densely populated and tend to have lower
PM<INF>2.5</INF> concentrations) and at finer spatial resolutions
(e.g., 1 km x 1 km grid cells). As such, the hybrid modeling approaches
tend to broaden the areas captured in the exposure assessment, and in
doing so, the studies that utilize these methods tend to report lower
mean PM<INF>2.5</INF> concentrations than monitor-based approaches.
Further, other aspects of the approaches applied in the various
epidemiologic studies to
[[Page 5573]]
estimate PM<INF>2.5</INF> exposure and/or to calculate the related
study-reported mean concentration (i.e., population weighting, trim
mean approaches) can affect those data values. More detail related to
hybrid modeling methods, performance of the methods, and how the
reported mean concentrations compare across approaches is provided in
section 2.3.3.2 of the PA (U.S. EPA, 2022b). The subsections below
discuss the characterization of PM<INF>2.5</INF> concentrations based
on monitoring data (I.D.5.a) and using hybrid modeling approaches
(I.D.5.b).
a. Predicted Ambient PM<INF>2.5</INF> and Exposure Based on Monitored
Data
Ambient concentrations of PM<INF>2.5</INF> are often characterized
using measurements from national monitoring networks due to the
accuracy and precision of the measurements and the public availability
of data. For applications requiring PM<INF>2.5</INF> characterizations
across large areas or provide complete coverage from the site
measurements, data interpolation and averaging techniques (such as
Average Nearest Neighbor tools, and area-wide or population-weighted
averaging of monitors) are sometimes used (U.S. EPA, 2019a, chapter 3).
For an area to meet the NAAQS, all valid design values \39\ in that
area, including the highest annual and 24-hour values, must be at or
below the levels of the standards. Because the monitoring network
siting requirements are specified to capture the high PM<INF>2.5</INF>
concentrations (U.S. EPA, 2022b, section 2.2.3), areas meeting an
annual PM<INF>2.5</INF> standard with a particular level would be
expected to have long-term average monitored PM<INF>2.5</INF>
concentrations (i.e., averaged across space and over time in the area)
somewhat below that standard level. Analyses in the PA indicate that,
based on recent air quality in U.S. CBSAs, maximum annual
PM<INF>2.5</INF> design values are often 10% to 20% higher than annual
average concentrations (i.e., averaged across multiple monitors in the
same CBSA) (U.S. EPA, 2022b, section 2.3.3.1, Figures 2-28 and 2-29).
This means that the PM<INF>2.5</INF> design value in an area is
associated with a distribution of PM<INF>2.5</INF> concentrations in
that area, and based on monitoring siting requirements, should
represent the highest concentration location applicable to be monitored
under the PM<INF>2.5</INF> NAAQS. This difference between the maximum
annual design value and the average concentration in an area can vary,
depending on factors such as the number of monitors, monitor siting
characteristics, and the distribution of ambient PM<INF>2.5</INF>
concentrations. Given that higher PM<INF>2.5</INF> concentrations have
been reported at some near-road monitoring sites relative to the
surrounding area (U.S. EPA, 2022b, section 2.3.2.2.2), recent
requirements for PM<INF>2.5</INF> monitoring at near-road locations in
large urban areas (U.S. EPA, 2022b, section 2.2.3.3) may increase the
ratios of maximum design values to average annual design values in some
areas. Such ratios may also depend on how the averages are calculated
(i.e., averaged across monitors versus across modeled grid cells, as
described below in section I.5.b). Compared to annual design values,
the analysis in the PA indicates a more variable relationship between
maximum 24-hour PM<INF>2.5</INF> design values and annual average
concentrations (U.S. EPA, 2022b, section 2.3.3.1, Figure 2-29).
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\39\ For the annual PM<INF>2.5</INF> standard, design values are
calculated as the annual arithmetic mean PM<INF>2.5</INF>
concentration, averaged over 3 years. For the 24-hour standard,
design values are calculated as the 98th percentile of the annual
distribution of 24-hour PM<INF>2.5</INF> concentrations, averaged
over three years (appendix N of 40 CFR part 50).
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b. Comparison of PM<INF>2.5</INF> Fields in Estimating Exposure and
Relative to Design Values
Two types of hybrid approaches that have been utilized in several
key PM<INF>2.5</INF> epidemiologic studies in the 2019 ISA and ISA
Supplement include neural network approaches and a satellite-based
method with regression of residual PM<INF>2.5</INF> with land-use and
other variables to improve estimates of PM<INF>2.5</INF> concentration
in the U.S. As such, the PA further compares these two types of
approaches across various scales (e.g., CBSA versus nationwide), taking
into account population weighting approaches utilized in epidemiologic
studies when estimating PM<INF>2.5</INF> exposure (U.S. EPA, 2022b,
section 2.3.3.2.4). Additionally, the PA assesses how average
PM<INF>2.5</INF> concentrations computed in epidemiologic studies using
these hybrid surfaces compare to the maximum design values measured at
ground-based monitors. For this assessment, the PA evaluates the DI2019
\40\ and HA2020 \41\ hybrid surfaces, surfaces that are used in several
of the key epidemiologic studies in the PA. This analysis is intended
to help inform how the magnitude of the overall study reported mean
PM<INF>2.5</INF> concentrations in epidemiologic studies may be
influenced by the approach used to compute that mean and how that value
might compare to monitor reported concentrations.
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\40\ This analysis includes an updated version of the surface
used in Di et al. (2016). Predictions in Di et al. (2016) were for
2000 to 2012 using a neural network model. The Di et al. (2019)
study improved on that effort in several ways. First, a generalized
additive model was used that accounted for geographic variations in
performance to combine predictions from three models (neural
network, random forest, and gradient boosting) to make the final
optimal PM<INF>2.5</INF> predictions. Second, the datasets were
updated that were used in model training and included additional
variables such as 12-km community multiscale air quality (CMAQ)
modeling as predictors. Finally, more recent years were included in
the Di et al. (2019) study.
\41\ The HA2020 field is based on the V4.NA.03 product available
at: <a href="https://sites.wustl.edu/acag/datasets/surface-pm2-5/">https://sites.wustl.edu/acag/datasets/surface-pm2-5/</a>. The name
``HA2020'' comes from the references for this product (Hammer et
al., 2020; van Donkelaar et al., 2019).
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In estimating exposure, some studies focus on estimating
concentrations in urban areas, while others examine the entire U.S. or
large portions of the country. In general, the areas that are not
included in the CBSA-only analysis tend to be more rural or less
densely populated areas, tend to have lower PM<INF>2.5</INF>
concentrations, and likely correspond to those locations where
monitoring data availability is limited or nonexistent (U.S. EPA,
2022b, section 2.3.3.2.4, Figure 2-37). To evaluate the differences in
mean PM<INF>2.5</INF> concentrations across different spatial scales,
the PA analysis compares the DI2019 and HA2020 surfaces. At the
national scale, the two surfaces generally produce similar average
annual PM<INF>2.5</INF> concentrations, with the DI2019 surface being
slightly higher compared to the HA2020 surface. The average annual
PM<INF>2.5</INF> concentrations are also slightly higher using the
DI2019 surface compared to the HA2020 surface when the analyses are
conducted for CBSAs. Also, regardless of which surface is used, the
average annual and 3-year average of the average annual
PM<INF>2.5</INF> concentrations for the CBSA-only analyses are somewhat
higher than for the nationwide analyses (4-8% higher) (U.S. EPA, 2022b,
section 2.3.3.2.4, Table 2-5).\42\ Overall, these analyses suggest that
there are only slight differences in the average PM<INF>2.5</INF>
[[Page 5574]]
concentrations depending on the hybrid modeling method employed, though
including other hybrid modeling methods in this comparison could result
in larger differences.
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\42\ For the national scale, 3-year averages of the average
annual PM<INF>2.5</INF> concentrations generally range from about
5.3 [mu]g/m\3\ to 8.1 [mu]g/m\3\, compared to the CBSA scale, which
ranges from 5.7 [mu]g/m\3\ to 8.7 [mu]g/m\3\. (U.S. EPA, 2022b,
section 2.3.3.2.4, Table 2-6).
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The PA next evaluates how the averages of the hybrid model surfaces
compare to regulatory design values using both the DI2019 and HA2020
surfaces and how population weighting influences the mean
PM<INF>2.5</INF> concentration.\43\ As presented in the PA, the results
using the DI2019 and HA2020 surfaces are similar for the average annual
PM<INF>2.5</INF> concentrations, for each 3-year period. When
population weighting is not applied, the average annual
PM<INF>2.5</INF> concentrations generally range from 7.0 to 8.6 [mu]g/
m\3\. When population weighting is applied, the average annual
PM<INF>2.5</INF> concentrations are slightly higher, ranging from 8.2
to 10.2 [mu]g/m\3\. As with CBSAs versus the national comparison above,
population weighting results in a higher average PM<INF>2.5</INF>
concentration than when population weighting is not applied (U.S. EPA,
2022b, section 2.3.3.2.4, Table 2-7). For the CBSAs included in the
population weighted analyses, the average maximum annual design values
generally range from 9.5 to 11.7 [mu]g/m\3\. The results are similar
for both the DI2019 and HA2020 surfaces and the maximum annual
PM<INF>2.5</INF> design values measured at the monitors are often 40%
to 50% higher than average annual PM<INF>2.5</INF> concentrations
predicted by hybrid modeling methods when population weighting is not
applied. However, when population weighting is applied, the ratio of
the maximum annual PM<INF>2.5</INF> design values to the predicted
average annual PM<INF>2.5</INF> concentrations are lower than when
population weighting is not applied, with monitored design values
generally 15% to 18% higher than population-weighted hybrid modeling
average annual PM<INF>2.5</INF> concentrations (U.S. EPA, 2022b,
section 2.3.3.2.4, Table 2-7).
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\43\ For this analysis, the PA includes CBSAs with three or more
valid design values for the 3-year period. The regulatory design
values for the CBSAs were calculated for each 3-year period for the
CBSAs with 3 or more design values in each of the 3-year periods.
Using the maximum design value for each CBSA and by each 3-year
period, the ratio of maximum design values to modeled average annual
PM<INF>2.5</INF> concentrations were calculated, for each 3-year
period. More details about the analytical methods used for this
analysis are described in section A.6 of Appendix A in the PA (U.S.
EPA, 2022b).
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6. Background PM
In this reconsideration, background PM is defined as all particles
that are formed by sources or processes that cannot be influenced by
actions within the jurisdiction of concern. U.S. background PM is
defined as any PM formed from emissions other than U.S. anthropogenic
(i.e., manmade) emissions. Potential sources of U.S. background PM
include both natural sources (i.e., PM that would exist in the absence
of any anthropogenic emissions of PM or PM precursors) and
transboundary sources originating outside U.S. borders. Background PM
is discussed in more detail in the PA (U.S. EPA, 2022b, section 2.4).
At annual and national scales, estimated background PM concentrations
in the U.S. are small compared to contributions from domestic
anthropogenic sources.\44\ For example, based on zero-out modeling in
the last review of the PM NAAQS, annual background PM<INF>2.5</INF>
concentrations were estimated to range from 0.5-3 [mu]g/m\3\ across the
sites examined. In addition, speciated monitoring data from IMPROVE
sites can provide some insights into how contributions from different
sources, including sources of background PM, may have changed over
time. Such data suggests the estimates of background concentrations
using speciated monitoring data from IMPROVE monitors are around 1-3
[mu]g/m\3\ and have not changed significantly since the 2012 review.
Contributions to background PM in the U.S. result mainly from sources
within North America. Contributions from intercontinental events have
also been documented (e.g., transport from dust storms occurring in
deserts in North Africa and Asia), but these events are less frequent
and represent a relatively small fraction of background PM in most of
the U.S. (U.S. EPA, 2022b, section 2.4).
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\44\ Sources that contribute to natural background PM include
dust from the wind erosion of natural surfaces, sea salt, wildland
fires, primary biological aerosol particles such as bacteria and
pollen, oxidation of biogenic hydrocarbons such as isoprene and
terpenes to produce secondary organic aerosols (SOA), and geogenic
sources such as sulfate formed from volcanic production of
SO<INF>2</INF> and oceanic production of dimethyl-sulfide (U.S. EPA,
2022b, section 2.4). While most of these sources release or
contribute predominantly to fine aerosol, some sources including
windblown dust, and sea salt also produce particles in the coarse
size range (U.S. EPA, 2019a, section 2.3.3).
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II. Rationale for Proposed Decisions on the Primary PM2.5 Standards
This section presents the rationale for the Administrator's
proposed decision to revise the primary annual PM<INF>2.5</INF>
standard and retain the primary 24-hour PM<INF>2.5</INF> standard. This
rationale is based on a thorough review of the scientific evidence
generally published through January 2018,\45\ as presented in the 2019
ISA (U.S. EPA, 2019a), on the human health effects of PM<INF>2.5</INF>
associated with long- and short-term exposures \46\ to PM<INF>2.5</INF>
in the ambient air. Additionally, this rationale is based on a thorough
evaluation of some studies that became available after the literature
cutoff date of the 2019 ISA, as evaluated in the ISA Supplement, that
could either further inform the adequacy of the current PM NAAQS or
address key scientific topics that have evolved since the literature
cutoff date for the 2019 ISA, generally through March 2021 (U.S. EPA,
2022b).\47\ The Administrator's rationale also takes into account: (1)
the PA evaluation of the policy-relevant information in the 2019 ISA
and ISA Supplement and presentation of quantitative analyses of air
quality and health risks; (2) CASAC advice and recommendations, as
reflected in discussions of the drafts of the ISA Supplement and PA at
public meetings and in the CASAC's letters to the Administrator; and
(3) public comments received during the development of these documents.
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\45\ In addition to the 2020 review's opening ``call for
information'' (79 FR 71764, December 3, 2014), the 2019 ISA
identified and evaluated studies and reports that have undergone
scientific peer review and were published or accepted for
publication between January 1, 2009, through approximately January
2018 (U.S. EPA, 2019a, p. ES-2). References that are cited in the
2019 ISA, the references that were considered for inclusion but not
cited, and electronic links to bibliographic information and
abstracts can be found at: <a href="https://hero.epa.gov/hero/particulate-matter">https://hero.epa.gov/hero/particulate-matter</a>.
\46\ Short-term exposures are defined as those exposures
occurring over hours up to 1 month, whereas long-term exposures are
defined as those exposures occurring over 1 month to years (U.S.
EPA, 2019a, section P.3.1).
\47\ The ISA Supplement represents an evaluation of recent
studies that are of greatest policy relevance to the reconsideration
of the 2020 final decision on the PM NAAQS. Specifically, the ISA
Supplement focuses on studies of health effects for which the
evidence in the 2019 ISA supported a ``causal relationship'' (i.e.,
short- and long-term PM<INF>2.5</INF> exposure and mortality and
cardiovascular effects) because those were the health effects that
were most useful in informing conclusions in the 2020 PA. The ISA
Supplement does not include an evaluation of studies for other
PM<INF>2.5</INF>-related health effects (U.S. EPA, 2022b).
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In presenting the rationale for the Administrator's proposed
decisions and its foundations, section II.A provides background and
introductory information for this reconsideration of the primary
PM<INF>2.5</INF> standards. It includes background on the 2020 final
decision to retain the primary PM<INF>2.5</INF> standards (section
II.A.1) and also describes the general approach for this
reconsideration (section II.A.2). Section II.B summarizes the key
aspects of the currently available health effects evidence, focusing on
consideration of
[[Page 5575]]
the key policy-relevant aspects. Section II.C summarizes the risk
information for this reconsideration, drawing on the quantitative
analyses for PM<INF>2.5</INF>, presented in the PA. Section II.D
presents the Administrator's proposed conclusions on the current
primary annual and 24-hour PM<INF>2.5</INF> standards (section II.D.3),
drawing on both the evidence-based and risk-based considerations
(section II.D.2) and advice from the CASAC (section II.D.1).
A. General Approach
This reconsideration of the 2020 final decision on the primary
PM<INF>2.5</INF> standards relies on using the EPA's assessment of the
current scientific evidence and associated quantitative analyses to
inform the Administrator's judgment regarding primary PM<INF>2.5</INF>
standards that protect public health with an adequate margin of safety.
The EPA's assessments are primarily documented in the 2019 ISA, ISA
Supplement, and PA, all of which have received CASAC review and public
comment (83 FR 53471, October 23, 2018; 83 FR 55529, November 6, 2018;
85 FR 4655, January 27, 2020; 86 FR 52673, September 22, 2021; 86 FR
54186, September 30, 2021; 86 FR 56263, October 8, 2021; 87 FR 958,
January 7, 2022; 87 FR 22207, April 14, 2022; 87 FR 31965, May 26,
2022). In bridging the gap between the scientific assessments of the
2019 ISA and ISA Supplement and the judgments required of the
Administrator in determining whether the current standards provide the
requisite public health protection, the PA evaluates policy
implications of the evaluation of the current evidence in the 2019 ISA
and ISA Supplement, and the risk information documented in the PA. In
evaluating the public health protection afforded by the current
standards, the four basic elements of the NAAQS (indicator, averaging
time, level, and form) are considered collectively.
The final decision on the adequacy of the current primary
PM<INF>2.5</INF> standards is a public health policy judgment to be
made by the Administrator. In reaching conclusions with regard to the
standards, the decision will draw on the scientific information and
analyses about health effects and population risks, as well as
judgments about how to consider the range and magnitude of
uncertainties that are inherent in the scientific evidence and
analyses. This approach is based on the recognition that the available
health effects evidence generally reflects a continuum, consisting of
levels at which scientists generally agree that health effects are
likely to occur, through lower levels at which the likelihood and
magnitude of the response become increasingly uncertain. This approach
is consistent with the requirements of the NAAQS provisions of the
Clean Air Act and with how the EPA and the courts have historically
interpreted the Act (summarized in section I.A above). These provisions
require the Administrator to establish primary standards that, in the
judgment of the Administrator, are requisite to protect public health
with an adequate margin of safety. In so doing, the Administrator seeks
to establish standards that are neither more nor less stringent than
necessary for this purpose. The Act does not require that primary
standards be set at a zero-risk level, but rather at a level that
avoids unacceptable risks to public health, including the health of
sensitive groups.\48\
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\48\ As noted in section I.A above, the legislative history
describes such protection for the sensitive group of individuals and
not for a single person in the sensitive group (see S. Rep. No. 91-
1196, 91st Cong, 2d Sess. 10 [1970]).
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The subsections below provide background and introductory
information. Background on the 2020 decision to retain the current
standards, including the rationale for that decision, is summarized in
section II.A.1. This is followed, in section II.A.2, by an overview of
the general approach for the reconsideration of the 2020 final
decision. Following this introductory section and subsections, the
subsequent sections summarize current information and analyses,
including that newly available in this reconsideration. The
Administrator's proposed conclusions on the primary PM<INF>2.5</INF>
standards, based on the current information, are provided in section
II.D.3.
1. Background on the Current Standards
The current primary PM<INF>2.5</INF> standards were retained in
2020 based on the scientific evidence and quantitative risk analyses
available at that time, as well as the Administrator's judgments
regarding the available scientific information, the appropriate degree
of public health protection for the standards, and the available risk
information regarding the exposures and risk that may be allowed by the
current standards (85 FR 82718, December 18, 2020). With the 2020 final
decision, the EPA retained the primary 24-hour PM<INF>2.5</INF>
standard, with its level of 35 [mu]g/m\3\, and the primary annual
PM<INF>2.5</INF> standard, with its level of 12.0 [mu]g/m\3\, this
decision was informed by the scientific evidence evaluated in the 2019
ISA, the evidence and quantitative risk information in the 2020 PA, the
advice and recommendations of the CASAC, and public comments on the
proposed decision (85 FR 24094, April 30, 2020).
The health effects evidence base available in the 2020 review
included extensive evidence from previous reviews as well as the
evidence that had emerged since the prior review had been completed in
2012. This evidence base, spanning several decades, documents the
relationship between short- and long-term PM<INF>2.5</INF> exposure and
mortality or serious morbidity effects. The evidence available in the
2019 ISA reaffirmed, and in some cases strengthened, the conclusions
from the 2009 ISA regarding the health effects of PM<INF>2.5</INF>
exposures (U.S. EPA, 2009a). Much of the evidence came from
epidemiologic studies conducted in North America, Europe, or Asia
examining short-term and long-term exposures that demonstrated
generally positive, and often statistically significant,
PM<INF>2.5</INF> health effect associations with a range of outcomes
including non-accidental, cardiovascular, or respiratory mortality;
cardiovascular or respiratory hospitalizations or emergency department
visits; and other mortality/morbidity outcomes (e.g., lung cancer
mortality or incidence, asthma development). Experimental evidence, as
well as evidence from panel studies, strengthened support for potential
biological pathways through which PM<INF>2.5</INF> exposures could lead
to health effects reported in many population-based epidemiologic
studies, including support for pathways that could lead to
cardiovascular, respiratory, nervous system, and cancer-related
effects. Based on this evidence, the 2019 ISA concludes there to be a
causal relationship between long- and short-term PM<INF>2.5</INF>
exposure and mortality and cardiovascular effects, as well as likely to
be causal relationships between long- and short-term PM<INF>2.5</INF>
exposures and respiratory effects, and between long-term
PM<INF>2.5</INF> exposures and cancer and nervous system effects (U.S.
EPA, 2019a, section 1.7).
Epidemiologic studies reported PM<INF>2.5</INF> health effect
associations with mortality and/or morbidity across multiple U.S.
cities and in diverse populations, including in studies examining
populations and lifestages that may be at increased risk of
experiencing a PM<INF>2.5</INF>-related health effect (e.g., older
adults, children). The 2019 ISA cited extensive evidence indicating
that ``both the general population as well as specific populations and
lifestages are at risk for PM<INF>2.5</INF>-related health effects''
(U.S. EPA, 2019a, p. 12-1). Some of the evidence that supported
conclusions on at-risk populations and lifestages also
[[Page 5576]]
contributed to the conclusions of causal and likely to be causal
relationships within the 2019 ISA, including:
<bullet> PM<INF>2.5</INF>-related mortality and cardiovascular
effects in older adults (U.S. EPA, 2019a, sections 11.1, 11.2, 6.1, and
6.2);
<bullet> PM<INF>2.5</INF>-related cardiovascular effects in people
with pre-existing cardiovascular disease (U.S. EPA, 2019a, section
6.1);
<bullet> PM<INF>2.5</INF>-related respiratory effects in people
with pre-existing respiratory disease, particularly asthma (U.S. EPA,
2019a, section 5.1);
<bullet> PM<INF>2.5</INF>-related impairments in lung function
growth and asthma development in children (U.S. EPA, 2019a, sections
5.1, 5.2, and 12.5.1.1).
The 2019 ISA also noted that stratified analyses (i.e., analyses
that allow for the comparison of PM-related health effects across
different populations) provided strong evidence for racial and ethnic
differences in PM<INF>2.5</INF> exposures and PM<INF>2.5</INF>-related
health risk. Such analyses indicated that certain racial and ethnic
groups, specifically Hispanic and non-Hispanic Black populations have
higher PM<INF>2.5</INF> exposures than non-Hispanic White populations,
thus contributing to risk of adverse PM<INF>2.5</INF>-related health
effects in minority populations (U.S. EPA, 2019a, section 12.5.4).
Stratified analyses focusing on other groups also suggested that
populations with pre-existing cardiovascular or respiratory disease,
populations that are overweight or obese, populations that have
particular genetic variants, and populations that are of low
socioeconomic status (SES) could be at increased risk for
PM<INF>2.5</INF>-related adverse health effects (U.S. EPA, 2019a,
chapter 12).
The risk information available in the 2020 review included risk
estimates for air quality conditions just meeting the existing primary
PM<INF>2.5</INF> standards, and also for air quality conditions just
meeting potential alternative standards. The general approach to
estimating PM<INF>2.5</INF>-associated health risks combined
concentration-response (C-R) functions from epidemiologic studies with
model-based PM<INF>2.5</INF> air quality surfaces, baseline health
incidence data, and population demographics for 47 urban areas (U.S.
EPA, 2022b, section 3.3, Figure 3-10, Appendix C). The risk assessment
estimated that the existing primary PM<INF>2.5</INF> standards could
allow a substantial number of PM<INF>2.5</INF>-associated deaths in the
U.S. Uncertainty in risk estimates (e.g., in the size of risk
estimates) can result from a number of factors, including assumptions
about the shape of the C-R relationship with mortality at low ambient
PM<INF>2.5</INF> concentrations, the potential for confounding and/or
exposure measurement error, and the methods used to adjust
PM<INF>2.5</INF> air quality.
Consistent with the general approach routinely employed in NAAQS
reviews, the initial consideration in the 2020 review of the primary
PM<INF>2.5</INF> standards was with regard to the adequacy of the
protection provided by the existing standards. Key aspects of the
consideration are summarized in section II.A.1.a below.
a. Considerations Regarding the Adequacy of the Existing Standards in
the 2020 Review
With the 2020 final decision, the EPA retained the primary 24-hour
PM<INF>2.5</INF> standard, with its level of 35 [micro]g/m\3\, and the
primary annual PM<INF>2.5</INF> standard, with its level of 12.0
[micro]g/m\3\. The Administrator's conclusions regarding the adequacy
of the primary PM<INF>2.5</INF> standards at the time of the 2020
review was based on consideration of the evidence, analyses and
conclusions contained in the 2019 ISA; the quantitative risk assessment
in the 2020 PA; advice from the CASAC; and public comments. Key
considerations informing the Administrator's decision to retain the
standards that were promulgated in the 2012 review are summarized
below.
As an initial matter, the Administrator considered the range of
scientific evidence evaluating these effects, including studies of at-
risk populations, to inform his review of the primary PM<INF>2.5</INF>
standards, placing the greatest weight on evidence of effects for which
the 2019 ISA determined there to be a causal or likely to be causal
relationship with long- and short-term PM<INF>2.5</INF> exposures (85
FR 82714-82715, December 18, 2020).
With regard to indicator, the Administrator recognized that,
consistent with the evidence available in prior reviews, the scientific
evidence in the 2020 review continued to provide strong support for
health effects following short- and long-term PM<INF>2.5</INF>
exposures. He noted the 2020 PA conclusions that the information
continued to support the PM<INF>2.5</INF> mass-based indicator and
remained too limited to support a distinct standard for any specific
PM<INF>2.5</INF> component or group of components, and too limited to
support a distinct standard for the ultrafine fraction. Thus, the
Administrator concluded that it was appropriate to retain
PM<INF>2.5</INF> as the indicator for the primary standards for fine
particles (85 FR 82715, December 18, 2020).
With respect to averaging time and form, the Administrator noted
that the scientific evidence continued to provide strong support for
health effects associations with both long-term (e.g., annual or multi-
year) and short-term (e.g., mostly 24-hour) exposures to
PM<INF>2.5</INF>, consistent with the conclusions in the 2020 PA. In
the 2019 ISA, epidemiologic and controlled human exposure studies
examined a variety of PM<INF>2.5</INF> exposure durations.
Epidemiologic studies continued to provide strong support for health
effects associated with short-term PM<INF>2.5</INF> exposures based on
24-hour PM<INF>2.5</INF> averaging periods, and the EPA noted that
associations with sub-daily estimates are less consistent and, in some
cases, smaller in magnitude (U.S. EPA, 2019a, section 1.5.2.1; U.S.
EPA, 2020a, section 3.5.2.2). In addition, controlled human exposure
and panel-based studies of sub-daily exposures typically examined
subclinical effects, rather than the more serious population-level
effects that have been reported to be associated with 24-hour exposures
(e.g., mortality, hospitalizations). Taken together, the 2019 ISA
concludes that epidemiologic studies did not indicate that sub-daily
averaging periods were more closely associated with health effects than
the 24-hour average exposure metric (U.S. EPA, 2019a, section 1.5.2.1).
Additionally, while controlled human exposure studies provided
consistent evidence for cardiovascular effects following
PM<INF>2.5</INF> exposures for less than 24 hours (i.e., < 30 minutes
to 5 hours), exposure concentrations in the studies were well-above the
ambient concentrations typically measured in locations meeting the
existing standards (U.S. EPA, 2020a, section 3.2.3.1). Thus, these
studies also did not suggest the need for additional protection against
sub-daily PM<INF>2.5</INF> exposures (U.S. EPA, 2020a, section
3.5.2.2). Therefore, the Administrator judged that the 24-hour
averaging time remained appropriate (85 FR 82715, December 18, 2020).
With regard to the form of the 24-hour standard (98th percentile,
averaged over three years), the Administrator noted that epidemiologic
studies continued to provide strong support for health effect
associations with short-term (e.g., mostly 24-hour) PM<INF>2.5</INF>
exposures (U.S. EPA, 2020a, section 3.5.2.3) and that controlled human
exposure studies provided evidence for health effects following single
short-term ``peak'' PM<INF>2.5</INF> exposures. Thus, the evidence
supported retaining a standard focused on providing supplemental
protection against short-term peak exposures and
[[Page 5577]]
supported a 98th percentile form for a 24-hour standard. The
Administrator further noted that this form also provided an appropriate
balance between limiting the occurrence of peak 24-hour
PM<INF>2.5</INF> concentrations and identifying a stable target for
risk management programs (U.S. EPA, 2020a, section 3.5.2.3). As such,
the Administrator concluded that the available information supported
retaining the form and averaging time of the current 24-hour standard
(98th percentile, averaged over three years) and annual standard
(annual average, averaged over three years) (85 FR 82715, December 18,
2020).
With regard to the level of the standards, in reaching his final
decision, the Administrator considered the large body of evidence
presented and assessed in the 2019 ISA (U.S. EPA, 2019a), the policy-
relevant and risk-based conclusions and rationales as presented in the
2020 PA (U.S. EPA, 2020a), advice from the CASAC, and public comments.
In particular, in considering the 2019 ISA and 2020 PA, he considered
key epidemiologic studies that evaluated associations between
PM<INF>2.5</INF> air quality distributions and mortality and morbidity,
including key accountability studies; the availability of experimental
studies to support biological plausibility; controlled human exposure
studies examining effects following short-term PM<INF>2.5</INF>
exposures; air quality analyses; and the important uncertainties and
limitations associated with the information (85 FR 82715, December 18,
2020).
As an initial matter, the Administrator considered the protection
afforded by both the annual and 24-hour standards together against
long- and short-term PM<INF>2.5</INF> exposures and health effects. The
Administrator recognized that the annual standard was most effective in
controlling ``typical'' PM<INF>2.5</INF> concentrations near the middle
of the air quality distribution (i.e., around the mean of the
distribution), but also provided some control over short-term peak
PM<INF>2.5</INF> concentrations. On the other hand, the 24-hour
standard, with its 98th percentile form, was most effective at limiting
peak 24-hour PM<INF>2.5</INF> concentrations, but in doing so also had
an effect on annual average PM<INF>2.5</INF> concentrations. Thus,
while either standard could be viewed as providing some measure of
protection against both average exposures and peak exposures, the 24-
hour and annual standards were not expected to be equally effective at
limiting both types of exposures. Thus, consistent with previous
reviews, the Administrator's consideration of the public health
protection provided by the existing primary PM<INF>2.5</INF> standards
was based on his consideration of the combination of the annual and 24-
hour standards. Specifically, he recognized that the annual standard
was more likely to appropriately limit the ``typical'' daily and annual
exposures that are most strongly associated with the health effects
observed in epidemiologic studies. The Administrator concluded that an
annual standard (as the arithmetic mean, averaged over three years)
remained appropriate for targeting protection against the annual and
daily PM<INF>2.5</INF> exposures around the middle portion of the
PM<INF>2.5</INF> air quality distribution. Further, recognizing that
the 24-hour standard (with its 98th percentile form) was more directly
tied to short-term peak PM<INF>2.5</INF> concentrations, and more
likely to appropriately limit exposures to such concentrations, the
Administrator concluded that the current 24-hour standard (with its
98th percentile form, averaged over three years) remained appropriate
to provide a balance between limiting the occurrence of peak 24-hour
PM<INF>2.5</INF> concentrations and identifying a stable target for
risk management programs. However, the Administrator recognized that
changes in PM<INF>2.5</INF> air quality to meet an annual standard
would likely result not only in lower short- and long-term
PM<INF>2.5</INF> concentrations near the middle of the air quality
distribution, but also in fewer and lower short-term peak
PM<INF>2.5</INF> concentrations. The Administrator further recognized
that changes in air quality to meet a 24-hour standard, with a 98th
percentile form, would result not only in fewer and lower peak 24-hour
PM<INF>2.5</INF> concentrations, but also in lower annual average
PM<INF>2.5</INF> concentrations (85 FR 82715-82716, December 18, 2020).
Thus, in considering the adequacy of the 24-hour standard, the
Administrator noted the importance of considering whether additional
protection was needed against short-term exposures to peak
PM<INF>2.5</INF> concentrations. In examining the scientific evidence,
he noted the limited utility of the animal toxicological studies in
directly informing conclusions on the appropriate level of the standard
given the uncertainty in extrapolating from effects in animals to those
in human populations. The Administrator noted that controlled human
exposure studies provided evidence for health effects following single,
short-term PM<INF>2.5</INF> exposures that corresponded best to
exposures that might be experienced in the upper end of the
PM<INF>2.5</INF> air quality distribution in the U.S. (i.e., ``peak''
concentrations). However, most of these studies examined exposure
concentrations considerably higher than are typically measured in areas
meeting the standards (U.S. EPA, 2020a, section 3.2.3.1). In
particular, controlled human exposure studies often reported
statistically significant effects on one or more indicators of
cardiovascular function following 2-hour exposures to PM<INF>2.5</INF>
concentrations at and above 120 [mu]g/m\3\ (at and above 149 [mu]g/m\3\
for vascular impairment, the effect shown to be most consistent across
studies). To provide insight into what these studies may indicate
regarding the primary PM<INF>2.5</INF> standards, the 2020 PA (U.S.
EPA, 2020a, p. 3-49) noted that 2-hour ambient concentrations of
PM<INF>2.5</INF> at monitoring sites meeting the current standards
almost never exceeded 32 [mu]g/m\3\. In fact, even the extreme upper
end of the distribution of 2-hour PM<INF>2.5</INF> concentrations at
sites meeting the primary PM<INF>2.5</INF> standards remained well-
below the PM<INF>2.5</INF> exposure concentrations consistently shown
in controlled human exposure studies to elicit effects (i.e., 99.9th
percentile of 2-hour concentrations at these sites is 68 [mu]g/m\3\
during the warm season). Thus, the available experimental evidence did
not indicate the need for additional protection against exposures to
peak PM<INF>2.5</INF> concentrations, beyond the protection provided by
the combination of the 24-hour and the annual standards (U.S. EPA,
2020a, section 3.2.3.1; 85 FR 82716, December 18, 2020).
With respect to the epidemiologic evidence, the Administrator noted
that the studies did not indicate that associations in those studies
were strongly influenced by exposures to peak concentrations in the air
quality distribution and thus did not indicate the need for additional
protection against short-term exposures to peak PM<INF>2.5</INF>
concentrations (U.S. EPA, 2020a, section 3.5.1 The Administrator noted
that this was consistent with CASAC consensus support for retaining the
current 24-hour standard. Thus, the Administrator concluded that the
24-hour standard with its level of 35 [mu]g/m\3\ was adequate to
provide supplemental protection (i.e., beyond that provided by the
annual standard alone) against short-term exposures to peak
PM<INF>2.5</INF> concentrations (85 FR 82716, December 18, 2020).
With regard to the level of the annual standard, the Administrator
recognized that the annual standard, with its form based on the
arithmetic mean concentration, was most appropriately meant to limit
the ``typical'' daily and annual exposures that were most strongly
associated with the health
[[Page 5578]]
effects observed in epidemiologic studies. However, the Administrator
also noted that while epidemiologic studies examined associations
between distributions of PM<INF>2.5</INF> air quality and health
outcomes, they did not identify particular PM<INF>2.5</INF> exposures
that cause effects and thus, they could not alone identify a specific
level at which the standard should be set, as such a determination
necessarily required the Administrator's judgment. Thus, consistent
with the approaches in previous NAAQS reviews, the Administrator
recognized that any approach that used epidemiologic information in
reaching decisions on what standards are appropriate necessarily
required judgments about how to translate the information from the
epidemiologic studies into a basis for appropriate standards. This
approach included consideration of the uncertainties in the reported
associations between daily or annual average PM<INF>2.5</INF> exposures
and mortality or morbidity in the epidemiologic studies. Such an
approach is consistent with setting standards that are neither more nor
less stringent than necessary, recognizing that a zero-risk standard is
not required by the Clean Air Act (CAA) (85 FR 82716, December 18,
2020).
The Administrator emphasized uncertainties and limitations that
were present in epidemiologic studies in previous reviews and persisted
in the 2020 review. These uncertainties included exposure measurement
error, potential confounding by copollutants, increasing uncertainty of
associations at lower PM<INF>2.5</INF> concentrations, and
heterogeneity of effects across different cities or regions (85 FR
82716, December 18, 2020). The Administrator also noted the advice
given by the CASAC on this matter. As described in section I.C.5 above,
the CASAC did not reach consensus on the adequacy of the primary annual
PM<INF>2.5</INF> standard. ``Some CASAC members'' expressed support for
retaining the primary annual PM<INF>2.5</INF> standard while ``other
members'' expressed support for revising that standard in order to
increase public health protection (Cox, 2019a, p. 1 of consensus
letter). The CASAC members who supported retaining the annual standard
expressed their concerns with the epidemiologic studies, asserting that
these studies did not provide a sufficient basis for revising the
existing standards. They also identified several key concerns regarding
the associations reported in epidemiologic studies and concluded that
``while the data on associations should certainly be carefully
considered, this data should not be interpreted more strongly than
warranted based on its methodological limitations'' (Cox, 2019a, p. 8
consensus responses).
Taking into consideration the views expressed by the CASAC members
who supported retaining the annual standard, the Administrator
recognized that epidemiologic studies examined associations between
distributions of PM<INF>2.5</INF> air quality and health outcomes, and
they did not identify particular PM<INF>2.5</INF> exposures that cause
effects (U.S. EPA, 2020a, section 3.1.2). While the Administrator
remained concerned about placing too much weight on epidemiologic
studies to inform conclusions on the adequacy of the primary standards,
he noted the approach to considering such studies in the 2012 review.
In the 2012 review, it was noted that the evidence of an association in
any epidemiologic study was ``strongest at and around the long-term
average where the data in the study are most concentrated'' (78 FR
3140, January 15, 2013). In considering the characterization of
epidemiologic studies, the Administrator viewed that when assessing the
mean concentrations of the key short-term and long-term epidemiologic
studies in the U.S. that use ground-based monitoring (i.e., those
studies where the mean is most directly comparable to the current
annual standard), the majority of studies had mean concentrations at or
above the level of the existing annual standard, with the mean of the
study-reported means or medians equal to 13.5 [mu]g/m\3\, a
concentration level above the existing level of the primary annual
standard of 12 [mu]g/m\3\. The Administrator further noted his caution
in directly comparing the reported study mean values to the standard
level given that study-reported mean concentrations, by design, are
generally lower than the design value of the highest monitor in an
area, which determines compliance. In the 2020 PA, analyses of recent
air quality in U.S. CBSAs indicated that maximum annual
PM<INF>2.5</INF> design values for a given three-year period were often
10% to 20% higher than average monitored concentrations (i.e., averaged
across multiple monitors in the same CBSA) (U.S. EPA, 2020a, Appendix
B, section B.7). He further noted his concern in placing too much
weight on any one epidemiologic study but instead judged that it was
more appropriate to focus on the body of studies together and therefore
noted the calculation of the mean of study-reported means (or medians).
Thus, while the Administrator was cautious in placing too much weight
on the epidemiologic evidence alone, he noted that: (1) the reported
mean concentration in the majority of the key U.S. epidemiologic
studies using ground-based monitoring data were above the level of the
existing annual standard; (2) the mean of the reported study means (or
medians) (i.e., 13.5 [mu]g/m\3\) was above the level of the current
standard; \49\ (3) air quality analyses showed the study means to be
lower than their corresponding design values by 10-20%; and (4) these
analyses must be considered in light of uncertainties inherent in the
epidemiologic evidence. When taken together, the Administrator judged
that, even if it were appropriate to place more weight on the
epidemiologic evidence, this information did not call into question the
adequacy of the current standards (85 FR 82716-82717, December 18,
2020).
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\49\ The median of the study-reported mean (or median)
PM<INF>2.5</INF> concentrations is 13.3 [mu]g/m\3\, which was also
above the level of the existing standard.
---------------------------------------------------------------------------
In addition to the evidence, the Administrator also considered the
potential implications of the risk assessment. He noted that all risk
assessments have limitations and that he remained concerned about the
uncertainties in the underlying epidemiologic data used in the risk
assessment. The Administrator also noted that in previous reviews,
these uncertainties and limitations have often resulted in less weight
being placed on quantitative estimates of risk than on the underlying
scientific evidence itself (e.g., 78 FR 3086, 3098-99, January 15,
2013). These uncertainties and limitations included uncertainty in the
shapes of C-R functions, particularly at low concentrations;
uncertainties in the methods used to adjust air quality; and
uncertainty in estimating risks for populations, locations and air
quality distributions different from those examined in the underlying
epidemiologic study (U.S. EPA, 2020a, section 3.3.2.4). Additionally,
the Administrator noted similar concern expressed by some members of
the CASAC who support retaining the existing standards; they
highlighted similar uncertainties and limitations in the risk
assessment (Cox, 2019b). In light of all of this, the Administrator
judged it appropriate to place little weight on quantitative estimates
of PM<INF>2.5</INF>-associated mortality risk in reaching conclusions
about the level of the primary PM<INF>2.5</INF> standards (85 FR 82717,
December 18, 2020).
The Administrator additionally considered an emerging body of
evidence from accountability studies that examined past reductions in
[[Page 5579]]
ambient PM<INF>2.5</INF> and the degree to which those reductions
resulted in public health improvements. While the Administrator agreed
with public commenters that well-designed and conducted accountability
studies can be informative, he viewed the interpretation of such
studies in the context of the primary PM<INF>2.5</INF> standards as
complicated by the fact that some of the available studies had not
evaluated PM<INF>2.5</INF> specifically (e.g., as opposed to
PM<INF>10</INF> or total suspended particulates), did not show changes
in PM<INF>2.5</INF> air quality, or had not been able to disentangle
health impacts of the interventions from background trends in health
(U.S. EPA, 2020a, section 3.5.1). He further recognized that the small
number of available studies that did report public health improvements
following past declines in ambient PM<INF>2.5</INF> had not examined
air quality meeting the existing standards (U.S. EPA, 2020a, Table 3-
3). This included U.S. studies that reported increased life expectancy,
decreased mortality, and decreased respiratory effects following past
declines in ambient PM<INF>2.5</INF> concentrations. Such studies
examined ``starting'' annual average PM<INF>2.5</INF> concentrations
(i.e., prior to the reductions being evaluated) ranging from about 13.2
to >20 [mu]g/m\3\ (i.e., U.S. EPA, 2020a, Table 3-3). Given the lack of
available accountability studies reporting public health improvements
attributable to reductions in ambient PM<INF>2.5</INF> in locations
meeting the existing standards, together with his broader concerns
regarding the lack of experimental studies examining PM<INF>2.5</INF>
exposures typical of areas meeting the existing standards, the
Administrator judged that there was considerable uncertainty in the
potential for increased public health protection from further
reductions in ambient PM<INF>2.5</INF> concentrations beyond those
achieved under the existing primary PM<INF>2.5</INF> standards (85 FR
82717, December 18, 2020).
When the above considerations were taken together, the
Administrator concluded that the scientific evidence assessed in the
2019 ISA, together with the analyses in the 2020 PA based on that
evidence and consideration of CASAC advice and public comments, did not
call into question the adequacy of the public health protection
provided by the existing annual and 24-hour PM<INF>2.5</INF> standards.
In particular, the Administrator judged that there was considerable
uncertainty in the potential for additional public health improvements
from reducing ambient PM<INF>2.5</INF> concentrations below the
concentrations achieved under the existing primary standards and that,
therefore, standards more stringent than the existing standards (e.g.,
with lower levels) were not supported. That is, he judged that more
stringent standards would be more than requisite to protect the public
health with an adequate margin of safety. This judgment reflected the
Administrator's consideration of the uncertainties in the potential
implications of the lower end of the air quality distributions from the
epidemiologic studies due in part to the lack of supporting evidence
from experimental studies and retrospective accountability studies
conducted at PM<INF>2.5</INF> concentrations meeting the existing
standards (85 FR 82717, December 18, 2020).
In reaching this conclusion, the Administrator judged that the
existing standards provided an adequate margin of safety. With respect
to the annual standard, the level of 12 [mu]g/m\3\ was below the lowest
``starting'' concentration (i.e., 13.2 [mu]g/m\3\) in the available
accountability studies that showed public health improvements
attributable to reductions in ambient PM<INF>2.5</INF>. In addition,
while the Administrator placed less weight on the epidemiologic
evidence for selecting a standard, he noted that the level of the
annual standard was below the reported mean (and median) concentrations
in the majority of the key U.S. epidemiologic studies using ground-
based monitoring data (noting that these means tend to be 10-20% lower
than their corresponding area design values which is the more relevant
metric when considering the level of the standard) and below the mean
of the reported means (or medians) of these studies (i.e., 13.5 [mu]g/
m\3\). In addition, the Administrator recognized that concentrations in
areas meeting the existing 24-hour and annual standards remained well-
below the PM<INF>2.5</INF> exposure concentrations consistently shown
to elicit effects in human exposure studies (85 FR 82717-82718,
December 18, 2020).
In addition, based on the Administrator's review of the science,
including controlled human exposure studies examining effects following
short-term PM<INF>2.5</INF> exposures, the epidemiologic studies, and
accountability studies conducted at levels just above the existing
annual standard, he judged that the degree of public health protection
provided by the existing annual standard is not greater than warranted.
This judgment, together with the fact that no CASAC member expressed
support for a less stringent standard, led the Administrator to
conclude that standards less stringent than the existing standards
(e.g., with higher levels) were also not supported (85 FR 82718,
December 18, 2020).
In reaching his final decision, the Administrator concluded that
the scientific evidence and technical information continued to support
the existing annual and 24-hour PM<INF>2.5</INF> standards. This
conclusion reflected the Administrator's view that there were important
limitations and uncertainties that remained in the evidence. The
Administrator concluded that these limitations contributed to
considerable uncertainty regarding the potential public health
implications of revising the existing primary PM<INF>2.5</INF>
standards. Given this uncertainty, and noting the advice from some
CASAC members, he concluded that the primary PM<INF>2.5</INF>
standards, including the indicators (PM<INF>2.5</INF>), averaging times
(annual and 24-hour), forms (arithmetic mean and 98th percentile,
averaged over three years) and levels (12.0 [mu]g/m\3\, 35 [mu]g/m\3\),
when taken together, remained requisite to protect the public health.
Therefore, in the 2020 review, the Administrator reached the conclusion
that the primary 24-hour and annual PM<INF>2.5</INF> standards,
together, were requisite to protect public health from fine particles
with an adequate margin of safety, including the health of at-risk
populations, and retained the standards, without revision (85 FR 82718,
December 18, 2020).
2. General Approach and Key Issues in This Reconsideration of the 2020
Final Decision
To evaluate whether it is appropriate to consider retaining the
current primary PM<INF>2.5</INF> standards, or whether consideration of
revision is appropriate, the EPA has adopted an approach in this
reconsideration that builds upon the general approach used in past
reviews. This includes the substantial assessments and evaluations
performed in those reviews, and also takes into account the more recent
scientific evidence and risk information now available to inform
understanding of the key policy-relevant issues in the reconsideration.
As summarized above, the Administrator's decisions in the 2020 review
were based on an integration of PM health effects information with the
judgments on the adversity and public health significance of key health
effects, policy judgments as to when the standard is requisite to
protect public health with an adequate margin of safety, and
consideration of CASAC advice and public comments.
Similarly, in this reconsideration, we draw on the current evidence
and quantitative assessments of exposure
[[Page 5580]]
pertaining to the public health risk of PM in ambient air. In
considering the scientific and technical information here, we consider
both the information available at the time of the 2020 review and
information more recently available, including that which has been
critically analyzed and characterized in the 2019 ISA and ISA
Supplement. The quantitative risk analyses, including a newly conducted
at-risk analysis, provide a context for interpreting the evidence of
mortality and the potential public health significance of risks
associated with air quality conditions that just meet the current and
potential alternative standards. The overarching purpose of these
analyses is to inform the Administrator's conclusions on the public
health protection afforded by the current primary standards, with an
important focus on evaluating the potential for exposures and risks
beyond those indicated by the information available at the time the
current standards were established.
B. Overview of the Health Effects Evidence
The information summarized here is an overview of the policy-
relevant aspects of the health effects evidence available in this
reconsideration; the assessment of this evidence is documented in the
2019 ISA and ISA Supplement and its policy implications are further
discussed in the PA. While the 2019 ISA provides the broad scientific
foundation for this reconsideration, additional literature has become
available since the cutoff date of the 2019 ISA that expands the body
of evidence related to mortality and cardiovascular effects for both
short- and long-term PM<INF>2.5</INF> exposure that can inform the
Administrator's judgment on the adequacy of the current primary
PM<INF>2.5</INF> standards. As such, the ISA Supplement builds on the
information presented within the 2019 ISA with a targeted
identification and evaluation of new scientific information (U.S. EPA,
2022a, section 1.2). The ISA Supplement focuses on PM<INF>2.5</INF>
health effects evidence where the 2019 ISA concludes a ``causal
relationship,'' because such health effects are given the most weight
in an Administrator's decisions in a NAAQS review. As such, the ISA
Supplement evaluates newly available evidence related to short- and
long-term PM<INF>2.5</INF> exposure and mortality and cardiovascular
effects given the strength of the evidence available in the 2019 ISA
and past ISAs and AQCDs, as well as the clear adversity of these
endpoints. Specifically, U.S. and Canadian epidemiologic studies for
mortality and cardiovascular effects along with controlled human
exposure studies associated with cardiovascular effects at near ambient
concentrations, were considered to be of greatest utility in informing
the Administrator's conclusions on the adequacy of the current primary
PM<INF>2.5</INF> standards. While the ISA Supplement does not include
information for health effects other than mortality and cardiovascular
effects, the scientific evidence for other health effect categories is
evaluated in the 2019 ISA, which in combination with the ISA Supplement
represents the complete scientific record for the reconsideration of
the 2020 final decision.
The ISA Supplement also assessed accountability studies because
these types of epidemiologic studies were part of the body of evidence
that was a focus of the 2020 review. Accountability studies inform our
understanding of the potential for public health improvements as
ambient PM<INF>2.5</INF> concentrations have declined over time.
Further, the ISA Supplement considered studies that employed
statistical approaches that attempt to more extensively account for
confounders and are more robust to model misspecification (i.e., used
alternative methods for confounder control),\50\ given that such
studies were highlighted by the CASAC and identified in public comments
in the 2020 review. Since the literature cutoff date for the 2019 ISA,
multiple accountability studies and studies that employ alternative
methods for confounder control have become available for consideration
in the ISA Supplement and, subsequently, in this reconsideration.
---------------------------------------------------------------------------
\50\ As noted in the ISA Supplement (U.S. EPA, 2022a, p. 1-3):
``In the peer-reviewed literature, these epidemiologic studies are
often referred to as causal inference studies or studies that used
causal modeling methods. For the purposes of this Supplement, this
terminology is not used to prevent confusion with the main
scientific conclusions (i.e., the causality determinations)
presented within an ISA. In addition, as is consistent with the
weight-of-evidence framework used within ISAs and discussed in the
Preamble to the Integrated Science Assessments, an individual study
on its own cannot inform causality, but instead represents a piece
of the overall body of evidence.''
---------------------------------------------------------------------------
The ISA Supplement also considered recent health effects evidence
that addresses key scientific issues where the literature has expanded
since the completion of the 2019 ISA.\51\ The 2019 ISA evaluated a
couple of controlled human exposure studies that investigated the
effect of exposure to near-ambient concentrations of PM<INF>2.5</INF>
(U.S. EPA, 2019a, section 6.1.10 and 6.1.13). The ISA Supplement adds
to this limited evidence, including a recent study conducted in young
healthy individuals exposed to near-ambient PM<INF>2.5</INF>
concentrations (U.S. EPA, 2022a, section 3.3.1). Given the importance
of identifying populations at increased risk of PM<INF>2.5</INF>-
related effects, the ISA Supplement also included epidemiologic or
exposure studies that examined whether there is evidence of exposure or
risk disparities by race/ethnicity or SES. These types of studies
provide additional information related to factors that may increase
risk of PM<INF>2.5</INF>-related health effects and provide additional
evidence for consideration by the Administrator in reaching conclusions
regarding the adequacy of the current standards. In addition, the ISA
Supplement evaluated studies that examined the relationship between
short- and long-term PM<INF>2.5</INF> exposures and SARS-CoV-2
infection and/or COVID-19 death, as these studies are a new area of
research and were raised by a number of public commenters in the 2020
review.
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\51\ As with the epidemiologic studies for long- and short-term
PM<INF>2.5</INF> exposure and mortality and cardiovascular effects,
epidemiologic studies of exposure or risk disparities and SARS-CoV-2
infection and/or COVID-19 death were limited to those conducted in
the U.S. and Canada.
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The evidence presented within the 2019 ISA, along with the targeted
identification and evaluation of new scientific information in the ISA
Supplement, provides the scientific basis for the reconsideration of
the 2020 final decision on the primary PM<INF>2.5</INF> standards. The
subsections below briefly summarize the nature of PM<INF>2.5</INF>-
related health effects, with a focus on those health effects for which
the 2019 ISA concluded a ``causal'' or ``likely to be causal''
relationship.
1. Nature of Effects
The evidence base available in the reconsideration includes decades
of research on PM<INF>2.5</INF>-related health effects (U.S. EPA,
2004b; U.S. EPA, 2009b; U.S. EPA, 2019a), including the full body of
evidence evaluated in the 2019 ISA (U.S. EPA, 2019a), along with the
targeted evaluation of recent evidence in the ISA Supplement (U.S. EPA,
2022a). In considering the available scientific evidence, the sections
below summarize the relationships between long- and short-term
PM<INF>2.5</INF> exposures and mortality (II.B.1.a), cardiovascular
effects (II.B.1.b), respiratory effects (II.B.1.c), cancer (II.B.1.d),
and nervous system effects (II.B.1.e). For these outcomes, the 2019 ISA
concluded that the evidence supports either a ``causal'' or a ``likely
to be causal'' relationship.\52\
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\52\ In this reconsideration of the PM NAAQS, the EPA considers
the full body of health evidence, placing the greatest emphasis on
the health effects for which the evidence has been judged in the
2019 ISA to demonstrate a ``causal'' or ``likely to be causal''
relationship with PM<INF>2.5</INF> exposures.
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[[Page 5581]]
a. Mortality
i. Long-Term PM<INF>2.5</INF> Exposures
In the 2012 review, the 2009 ISA reported that the evidence was
``sufficient to conclude that the relationship between long-term
PM<INF>2.5</INF> exposures and mortality is causal'' (U.S. EPA, 2009a,
p. 7-96). The strongest evidence supporting this conclusion was
provided by epidemiologic studies, particularly those examining two
seminal cohorts, the American Cancer Society (ACS) cohort and the
Harvard Six Cities cohort. Analyses of the Harvard Six Cities cohort
included evidence indicating that reductions in ambient
PM<INF>2.5</INF> concentrations are associated with reduced mortality
risk (Laden et al., 2006) and increases in life expectancy (Pope et
al., 2009). Further support was provided by other cohort studies
conducted in North America and Europe that reported positive
associations between long-term PM<INF>2.5</INF> exposure and mortality
(U.S. EPA, 2019a).
Cohort studies, which have become available since the completion of
the 2009 ISA and evaluated in the 2019 ISA, continue to provide
consistent evidence of positive associations between long-term
PM<INF>2.5</INF> exposures and mortality. These studies add support for
associations with all-cause and total (non-accidental) mortality,\53\
as well as with specific causes of mortality, including cardiovascular
disease and respiratory disease (U.S. EPA, 2019a, section 11.2.2).
Several of these studies conducted analyses over longer study durations
and periods of follow-up than examined in the original ACS and Harvard
Six Cities cohort studies and continue to report positive associations
between long-term exposure to PM<INF>2.5</INF> and mortality (U.S. EPA,
2019a, section 11.2.2.1; Figures 11-18 and 11-19). In addition to
studies focusing on the ACS and Harvard Six Cities cohorts, additional
studies examining other cohorts also provide evidence of consistent,
positive associations between long-term PM<INF>2.5</INF> exposure and
mortality across a wide range of demographic groups (e.g., age, sex,
occupation), spatial and temporal extents, exposure assessment metrics,
and statistical techniques (U.S. EPA, 2019a, sections 11.2.2.1, 11.2.5;
U.S. EPA, 2022a, Table 11-8). This includes some of the largest cohort
studies conducted to date, such as analyses of the U.S. Medicare cohort
that includes nearly 61 million enrollees and studies that control for
a range of individual and ecological covariates, including race, age,
SES, smoking status, body mass index, and annual weather variables
(e.g., temperature, humidity) (U.S. EPA, 2019a).
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\53\ The majority of these studies examined non-accidental
mortality outcomes, though some Medicare studies lack cause-specific
death information and, therefore, examine total mortality.
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In addition to those cohort studies evaluated in the 2019 ISA,
recent North American cohort studies evaluated in the ISA Supplement
continue to examine the relationship between long-term PM<INF>2.5</INF>
exposure and mortality and report consistent, positive and
statistically significant associations. These recent studies also
utilize large and demographically diverse cohorts that are generally
representative of the national populations in both the U.S. and Canada.
These ``studies published since the 2019 ISA support and extend the
evidence base that contributed to the conclusion of a causal
relationship between long-term PM<INF>2.5</INF> exposure and
mortality'' (U.S. EPA, 2022a, section 3.2.2.2.1, Figure 3-19, Figure 3-
20).
Furthermore, studies evaluated in the 2019 ISA and the ISA
Supplement that examined cause-specific mortality expand upon previous
research that found consistent, positive associations between
PM<INF>2.5</INF> exposure and specific mortality outcomes, which
include cardiovascular and respiratory mortality, as well as other
mortality outcomes. For cardiovascular-related mortality, the evidence
evaluated in the ISA Supplement is consistent with the evidence
evaluated in the 2019 ISA with recent studies reporting positive
associations with long-term PM<INF>2.5</INF> exposure. When evaluating
cause-specific cardiovascular mortality, recent studies reported
positive associations for a number of outcomes, such as ischemic heart
disease (IHD) and stroke mortality (U.S. EPA, 2022a, Figure 3-23).
Moreover, recent studies also provide some initial evidence that
individuals with pre-existing health conditions, such as heart failure
and diabetes, are at an increased risk of PM<INF>2.5</INF>-related
health effects (U.S. EPA, 2022a, section 3.2.2.4) and that these
individuals have a higher risk of mortality overall, which was
previously only examined in studies that used stratified analyses
rather than a cohort of people with an underlying health condition
(U.S. EPA, 2022a, section 3.2.2.4). With regard to respiratory
mortality, epidemiologic studies evaluated in the 2019 ISA and ISA
Supplement continue to provide support for associations between long-
term PM<INF>2.5</INF> exposure and respiratory mortality (U.S. EPA,
2019a, section 5.2.10; U.S. EPA, 2022a, Table 3-2).
A series of epidemiologic studies evaluated in the 2019 ISA tested
the hypothesis that past reductions in ambient PM<INF>2.5</INF>
concentrations are associated with increased life expectancy or a
decreased mortality rate (U.S. EPA, 2022a, section 11.2.2.5). Pope et
al. (2009) conducted a cross-sectional analysis using air quality data
from 51 metropolitan areas across the U.S., beginning in the 1970s
through the early 2000s, and found that a 10 [mu]g/m\3\ decrease in
long-term PM<INF>2.5</INF> concentration was associated with a 0.61-
year increase in life expectancy. In a subsequent analysis, the authors
extended the period of analysis to include 2000 to 2007, a time period
with lower a
[…truncated; see source link]Indexed from Federal Register on January 27, 2023.
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