Rule2024-02637
Reconsideration of the National Ambient Air Quality Standards for Particulate Matter
Primary source
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Published
March 6, 2024
Effective
May 6, 2024
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 is revising the primary annual PM<INF>2.5</INF> standard by lowering the level from 12.0 [micro]g/m\3\ to 9.0 [micro]g/m\3\. The Agency is retaining the current primary 24-hour PM<INF>2.5</INF> standard and the primary 24-hour PM<INF>10</INF> standard. The Agency also is not changing 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 is also finalizing 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 89, Number 45 (Wednesday, March 6, 2024)]
[Rules and Regulations]
[Pages 16202-16406]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2024-02637]
[[Page 16201]]
Vol. 89
Wednesday,
No. 45
March 6, 2024
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; Final Rule
Federal Register / Vol. 89 , No. 45 / Wednesday, March 6, 2024 /
Rules and Regulations
[[Page 16202]]
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ENVIRONMENTAL PROTECTION AGENCY
40 CFR Parts 50, 53, and 58
[EPA-HQ-OAR-2015-0072; FRL-8635-02-OAR]
RIN 2060-AV52
Reconsideration of the National Ambient Air Quality Standards for
Particulate Matter
AGENCY: Environmental Protection Agency (EPA).
ACTION: Final 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 is
revising the primary annual PM<INF>2.5</INF> standard by lowering the
level from 12.0 [micro]g/m\3\ to 9.0 [micro]g/m\3\. The Agency is
retaining the current primary 24-hour PM<INF>2.5</INF> standard and the
primary 24-hour PM<INF>10</INF> standard. The Agency also is not
changing 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 is also finalizing 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: This final rule is effective May 6, 2024.
ADDRESSES: The EPA has established a docket for this action under
Docket ID No. EPA-HQ-OAR-2015-0072. All documents in the docket are
listed on the <a href="https://www.regulations.gov">https://www.regulations.gov</a> website. 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 will be publicly available only in hard copy form.
Publicly available docket materials are available electronically
through <a href="https://www.regulations.gov">https://www.regulations.gov</a>.
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#d9a9bcabb5b4acadadf7b5b8abaa99bca9b8f7beb6af"><span class="__cf_email__" data-cfemail="bdcdd8cfd1d0c8c9c993d1dccfcefdd8cddc93dad2cb">[email protected]</span></a>.
SUPPLEMENTARY INFORMATION:
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
5. Review Initiated in 2014
a. 2020 Proposed and Final Decisions
b. Reconsideration of the 2020 PM NAAQS Final Action
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 Decisions on the Primary PM<INF>2.5</INF>
Standards
A. Introduction
1. Background on the Current Standards
2. Overview of the Health Effects Evidence
a. Nature of Effects
i. Mortality
ii. Cardiovascular Effects
iii. Respiratory Effects
iv. Cancer
v. Nervous System Effects
vi. Other Effects
b. Public Health Implications and At-Risk Populations
c. PM<INF>2.5</INF> Concentrations in Key Studies Reporting
Health Effects
i. PM<INF>2.5</INF> Exposure Concentrations Evaluated in
Experimental Studies
ii. Ambient PM<INF>2.5</INF> Concentrations in Locations of
Epidemiologic Studies
d. Uncertainties in the Health Effects Evidence
3. Summary of Exposure and Risk Estimates
a. Key Design Aspects
b. Key Limitations and Uncertainties
c. Summary of Risk Estimates
B. Conclusions on the Primary PM<INF>2.5</INF> Standards
1. CASAC Advice
2. Basis for the Proposed Decision
3. Comments on the Proposed Decision
4. Administrator's Conclusions
C. Decisions on the Primary PM<INF>2.5</INF> Standards
III. Rationale for Decisions on the Primary PM<INF>10</INF> Standard
A. Introduction
1. Background on the Current Standard
2. Overview of Health Effects Evidence
a. Nature of Effects
i. Mortality
ii. Cardiovascular Effects
iii. Respiratory Effects
iv. Cancer
v. Metabolic Effects
vi. Nervous System Effects
B. Conclusions on the Primary PM<INF>10</INF> Standard
1. CASAC Advice
2. Basis for the Proposed Decision
3. Comments on the Proposed Decision
4. Administrator's Conclusions
C. Decisions on the 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. Summary of Proposed Revisions
a. Air Quality Index Values of 50, 100, and 150
b. Air Quality Index Values of 200 and Above
2. Summary of Significant Comments on Proposed Revisions
a. Air Quality Index Values of 50, 100, and 150
b. Air Quality Index Values of 200 and Above
c. Other Comments
3. Summary of Final Revisions
C. Air Quality Index Category Breakpoints for PM<INF>10</INF>
D. Air Quality Index Reporting
1. Summary of Proposed Revisions
2. Summary of Significant Comments on Proposed Revisions
3. Summary of Final Revisions
V. Rationale for Decisions on the Secondary PM Standards
A. Introduction
1. Background on the Current Standards
a. Non-Visibility Effects
b. Visibility Effects
2. Overview of Welfare Effects Evidence
a. Nature of Effects
i. Visibility
ii. Climate
iii. Materials
3. Summary of Air Quality and Quantitative Information
a. Visibility Effects
i. Target Level of Protection in Terms of a PM<INF>2.5</INF>
Visibility Index
ii. Relationship Between the PM<INF>2.5</INF> Visibility Index
and the Current Secondary 24-Hour PM<INF>2.5</INF> Standard
b. Non-Visibility Effects
B. Conclusions on the Secondary PM Standards
1. CASAC Advice
2. Basis for the Proposed Decision
3. Comments on the Proposed Decision
4. Administrator's Conclusions
C. Decisions on the Secondary PM Standards
VI. Interpretation of the NAAQS for PM
A. Amendments to Appendix K: Interpretation of the NAAQS for
Particulate Matter
B. Amendments to Appendix N: Interpretation of the NAAQS for
PM<INF>2.5</INF>
VII. Amendments to Ambient Monitoring and Quality Assurance
Requirements
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A. Amendment to 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)
C. Changes to 40 CFR Part 58 (Ambient Air Quality Surveillance)
D. Incorporating Data From Next-Generation Technologies
VIII. Clean Air Act Implementation Requirements for the Revised
Primary Annual PM<INF>2.5</INF> NAAQS
A. Designation of Areas
B. Section 110(a)(1) and (2) Infrastructure SIP Requirements
C. Implementing the Revised Primary Annual 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 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 14094: Modernizing 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 and Executive Order 14096: Revitalizing Our Nation's
Commitment to Environmental Justice for All
K. Congressional Review Act (CRA)
References
Executive Summary
This document presents the Administrator's final 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 final decisions to revise the primary annual
PM<INF>2.5</INF> standard by lowering the level from 12.0 [micro]g/m\3\
to 9.0 [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\); to retain
the primary 24-hour PM<INF>10</INF> standard; and, not to change the
secondary PM standards at this time. In reaching his final decisions,
the Administrator 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 2022 Policy Assessment (2022 PA), advice from
the Clean Air Scientific Advisory Committee (CASAC), and public
comments on the proposal. 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 most recent 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 decided to reconsider the
December 2020 decision because the available scientific evidence and
technical information indicated 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, 2020b). 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, 2020b).
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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 final 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 2022 PA, advice from the CASAC \2\ and
public comment received as part of this reconsideration.\3\ The health
effects evidence newly 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 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
[[Page 16204]]
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 annual 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 risks 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 2022 PA, also inform the final decisions on the primary
PM<INF>2.5</INF> standards. The risk assessment in this reconsideration
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. The
analyses are summarized in this document and in the proposal and are
described in detail in the 2022 PA.
In its advice to the Administrator, in its review of the 2021 draft
PA, the CASAC concurred 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 CASAC did not reach consensus on the
adequacy of the current 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
standard was adequate and should be retained, without revision.
Additionally, in their review of the 2019 draft PA, the CASAC did not
reach consensus on the adequacy of the primary annual PM<INF>2.5</INF>
standard, with the minority recommending revision and the majority
recommending the standard be retained. In their review of the 2019
draft PA, the CASAC reached consensus regarding the adequacy of the
primary 24-hour PM<INF>2.5</INF> standard, concluding that the standard
should be retained.
In considering how to revise the suite of primary PM<INF>2.5</INF>
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.
As in 2012, the Administrator concludes that the most effective way
to reduce total population risk associated with both long- and short-
term PM<INF>2.5</INF> exposures is to set a generally controlling
annual standard, and to provide supplemental protection against the
occurrence of peak 24-hour PM<INF>2.5</INF> concentrations by means of
a 24-hour standard set at the appropriate level. Based on the current
evidence and quantitative information, as well as consideration of
CASAC advice and public comments, the Administrator concludes that the
current primary annual PM<INF>2.5</INF> standard is not adequate to
protect public health with an adequate margin of safety. The
Administrator notes that the CASAC was unanimous in its advice on the
2021 draft PA regarding the need to revise the annual standard. In
considering the appropriate level for a revised annual standard, the
Administrator concludes that a standard set at a level of 9.0 [micro]g/
m\3\ reflects his judgment about placing the most weight on the
strongest available evidence while appropriately weighing the
uncertainties.
With regard to the primary 24-hour PM<INF>2.5</INF> standard, the
Administrator finds the available scientific evidence and quantitative
information to be insufficient to call into question the adequacy of
the public health protection afforded by the current 24-hour standard.
He further notes that a more stringent annual standard set at a level
of 9.0 [micro]g/m\3\ is expected to reduce both average (annual)
concentrations and peak (daily) concentrations. The Administrator also
notes that, in their review of the 2021 draft PA, the CASAC did not
reach consensus on whether revisions to the primary 24-hour
PM<INF>2.5</INF> standard are warranted at this time. He also notes
that, in their review of the 2019 draft PA, the CASAC did reach
consensus that the primary 24-hour PM<INF>2.5</INF> standard should be
retained. The Administrator concludes that the 24-hour standard should
be retained to
[[Page 16205]]
continue to provide requisite protection against short-term peak
PM<INF>2.5</INF> concentrations, particularly when considered in
conjunction with the protection provided by the suite of standards and
the decision to revise the annual standard to a level of 9.0 [micro]g/
m\3\.
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 final 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 2022 PA, advice from the
CASAC and public comments. 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 final decision on the primary PM<INF>10</INF> standard,
the Administrator recognizes that, while the available health effects
evidence has expanded, recent studies are subject to the same types of
uncertainties that were judged to be important in previous reviews. He
also recognizes that, in their review of the 2019 draft PA and the 2021
draft PA, the CASAC generally agreed that it was reasonable to retain
the primary 24-hour PM<INF>10</INF> standard given the available
scientific evidence, including retaining PM<INF>10</INF> as the
indicator. He concludes that the newly available evidence does not call
into question the adequacy of the current primary PM<INF>10</INF>
standard, and retains that standard, without revision.
With respect to the secondary PM standards, this reconsideration
focuses on visibility, climate, and materials effects.\4\ The
Administrator's final 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 2022 PA;
advice from the CASAC; and public comments. 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 visibility effects, the Administrator notes that he
judges that the evidence supports a target level of protection of 27
dv. He further notes that the results of quantitative analyses of
visibility impairment 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 target level of protection. With regard to climate and
materials effects, while the evidence has expanded since previous
reviews, significant limitations and uncertainties remain in the
evidence. While the evidence has expanded since previous reviews, the
available scientific evidence remains insufficient to allow the
Administrator to make a reasoned judgment about what specific
standard(s) would be requisite to protect against known or anticipated
adverse effects to public welfare from PM's effects on materials damage
or climate.-In their review of the 2019 draft PA and the 2021 draft PA,
the CASAC did not recommend revising the secondary PM standards. In
considering the available evidence and quantitative information, with
its inherent uncertainties and limitations, the Administrator judges
that it is appropriate not to change the secondary PM standards at this
time.
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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 is 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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The final revisions to the primary annual PM<INF>2.5</INF> NAAQS
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 revised 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
advance environmental justice, in this case for overburdened
communities in areas with high PM concentrations above the NAAQS, by
using the tools described in the current PM NAAQS implementation
guidance (80 FR 58010, 58136, August 25, 2016). The EPA is not making
changes to any of the current PM NAAQS implementation programs in this
final rulemaking.
On other topics, the EPA is finalizing two sets of changes to the
PM<INF>2.5</INF> sub-index of the Air Quality Index (AQI). First, the
EPA is continuing 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) based on the levels of the primary annual and 24-hour
PM<INF>2.5</INF> standards. In so doing, the EPA is revising the AQI
value of 50 to 9.0 [micro]g/m\3\ and is retaining 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 is revising the upper AQI breakpoints (200 and above),
and replacing 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 is also revising 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. In addition, this final rule revises
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 finalizes
revisions to data calculations and ambient air monitoring requirements
for PM to improve the usefulness 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 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 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
[[Page 16206]]
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 final 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 14094, 13563, and
12866. The RIA estimates the costs and monetized human health benefits
of attaining the revised and two 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 revised annual standard level
of 9.0 [micro]g/m\3\ in combination with the current 24-hour standard
of 35 [micro]g/m\3\ (i.e., 9.0/35 [micro]g/m\3\), as well as the
following less and more stringent alternative standard levels: (1) An
alternative annual standard level of 10.0 [micro]g/m\3\ in combination
with the current 24-hour standard (i.e., 10.0/35 [micro]g/m\3\), (2) an
alternative annual standard level of 8.0 [micro]g/m\3\ in combination
with the current 24-hour standard (i.e., 8.0/35 [micro]g/m\3\), and (3)
an alternative 24-hour standard level of 30 [micro]g/m\3\ in
combination with an alternative annual standard level of 10 [micro]g/
m\3\ (i.e., 10.0/30 [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 (12.0/35 [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 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)(1) also provides that the Administrator may
review and revise criteria or promulgate new standards earlier or more
frequently.
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
[[Page 16207]]
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.
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, Part C, and Part D, Subparts 1 and
4 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 the NAAQS for PM 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 Criteria Document (AQCD)
(DHEW, 1969).\7\ 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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\7\ 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.\8\ 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 the annual arithmetic mean, averaged over three years.
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\8\ 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; \9\ 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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\9\ 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 and 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,
Inc. v. EPA, 175 F. 3d 1027 (D.C. Cir. 1999). 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.
[[Page 16208]]
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 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).\10\ 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. The
then-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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\10\ 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.
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Several parties filed petitions for review following promulgation
of the revised PM NAAQS in 2006. 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,\11\ 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), Integrated
Science Assessment (ISA; U.S. EPA, 2009a), Risk and Exposure Assessment
(REA) planning documents for health and welfare (U.S. EPA, 2009b, U.S.
EPA, 2009c), a quantitative health risk assessment (U.S. EPA, 2010a)
and an urban-focused visibility assessment (U.S. EPA, 2010b), and a
Policy Assessment (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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\11\ The history of the NAAQS review process, including
revisions to the process, is discussed at <a href="https://www.epa.gov/naaqs/historical-information-naaqs-review-process">https://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 \12\ 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
[[Page 16209]]
and annual PM<INF>2.5</INF> standards \13\ and the 24-hour
PM<INF>10</INF> standard to address visibility and non-visibility
welfare effects.
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\12\ The EPA also eliminated the option for spatial averaging.
\13\ Consistent with the primary standard, the EPA eliminated
the option for spatial averaging with the annual standard.
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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 Initiated in 2014
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/
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 then-Administrator issued a memorandum announcing
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 (Pruitt, 2018). Following this memo, on October 10, 2018,
the then-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).\14\
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\14\ 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 then-Administrator dated April 11, 2019
(Cox, 2019a). The EPA addressed 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 October 2019 (84 FR 51555, September 30,
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 then-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. Some CASAC members expressed support for
retaining the current 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, 2019b, p. 1 of letter). These
views are described in greater detail in the letter to the then-
Administrator (Cox, 2019b) and in the notice of final rulemaking (85 FR
82706-82707, December 18, 2020), as well as below. 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 (Cox, 2019b, U.S. EPA, 2020b), as
described in detail in section I.C.5 of the 2020 proposal document (85
FR 24100, April 30, 2020).
a. 2020 Proposed and Final Actions
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. The then-Administrator's rationale for his decisions is
described in more detail in section II, III, and V below, and is
briefly summarized here.
In reaching his final decision to retain the primary annual and 24-
hour PM<INF>2.5</INF> standards, the then-Administrator considered the
available scientific evidence, quantitative information, CASAC advice,
and public comments in his supporting rationale in the 2020 final
action (85 FR 82714, December 18, 2020). In so doing, he concluded that
the available controlled human exposure studies did not provide support
for additional public health protection against exposures to peak
PM<INF>2.5</INF> concentrations, beyond the protection provided by the
combination of the current primary annual and 24-hour PM<INF>2.5</INF>
standards. He also noted that the available epidemiologic studies did
not indicate that associations in those studies are 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. Accordingly,
and taking into account consensus CASAC advice to retain the current
primary 24-hour PM<INF>2.5</INF> standard, the then-Administrator
concluded the primary 24-hour PM<INF>2.5</INF> standard should be
retained.
With respect to the annual PM<INF>2.5</INF> standard, the then-
Administrator recognized that important uncertainties and limitations
that were present in epidemiologic studies in previous
[[Page 16210]]
reviews remained in the evidence assessed in the 2019 ISA. In
considering the epidemiologic evidence, the then-Administrator noted
that: (1) The reported mean concentration in the majority of the key
U.S. epidemiologic studies using ground-based monitoring data are above
the level of the current annual standard; (2) the mean of the reported
study means (or medians) (i.e., 13.5 [micro]g/m\3\) is above the level
of the current primary annual PM<INF>2.5</INF> standard of 12 [micro]g/
m\3\; (3) air quality analyses show the study means to be lower than
their corresponding design by 10-20%; and (4) that these analyses must
be considered in light of uncertainties inherent in the epidemiologic
evidence. The then-Administrator further considered other available
information, including the risk assessment, accountability studies, and
controlled human exposure studies, and found that, in considering all
of the evidence together along with advice from the CASAC, the suite of
primary PM<INF>2.5</INF> standards were requisite to protect public
health with an adequate margin of safety, and should be retained,
without revision.
With regard to the primary PM<INF>10</INF> standard, the then-
Administrator noted that the expanded body of evidence has broadened
the range of effects that have been linked with PM<INF>10-2.5</INF>
exposures. In light of that information, as well as continued
uncertainties in the evidence and advice from the CASAC to retain the
standard, the then-Administrator judged it appropriate to retain the
primary PM<INF>10</INF> standard to provide the requisite degree of
public health protection against PM<INF>10-2.5</INF> exposures,
regardless of location, source of origin, or particle composition (85
FR 82725, December 18, 2020).
With regard to the secondary PM standards, the then-Administrator
concluded that there was insufficient information available to
establish any distinct secondary PM standards to address climate and
materials effects of PM. For visibility effects, he found that in the
absence of a monitoring network for direct measurement of light
extinction, a calculated light extinction indicator that utilizes the
IMPROVE algorithms continued to provide a reasonable basis for defining
a target level of protection against PM-related visibility impairment.
He further found that a visibility index with a 24-hour averaging time
was reasonable based on its stability and suitability for representing
subdaily periods, and a form based on the 3-year average of annual 90th
percentile values was reasonable based on its stability and that it
represents the median of the 20 percent worst visibility days which are
targeted under the Regional Haze program. With regard to the level of a
visibility index, the then-Administrator judged it appropriate to
establish a target level of protection of 30 dv, reflecting the upper
end of the range of visibility impairment judged to be acceptable by at
least 50% of study participants in the available public preference
studies, taking into consideration the variability, limitations and
uncertainties of the public preference studies. The then-Administrator
judged that the secondary 24-hour PM<INF>2.5</INF> standard with its
level of 35 [micro]g/m\3\ would provide at least the target level of
protection for visual air quality of 30 dv which he judged appropriate.
Accordingly, taking into consideration the advice of the CASAC to
retain the current secondary PM standards, the then-Administrator found
the current secondary standards provide the requisite degree of
protection and that they should be retained (85 FR 82742, 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. The petitions for review were filed in the D.C.
Circuit and the Court consolidated the cases.\15\ Following EPA's
decision to reconsider the 2020 final decision, the Court ordered the
consolidated cases to be held in abeyance.
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\15\ See California v. EPA, (D.C. Cir., No. 21-2014 consolidated
with Nos. 21-1027, 21-1054).
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b. Reconsideration of the 2020 PM NAAQS Final Action
Executive Order 13990 directed review of certain agency actions (86
FR 7037, January 25, 2021).\16\ 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.\17\
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\16\ 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>.
\17\ 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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On June 10, 2021, the Agency announced its decision to reconsider
the 2020 PM NAAQS final action 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.\18\ The Administrator reached this decision in
part based on the fact that the EPA noted that the 2020 PA concluded
that the scientific evidence and information called into question the
adequacy of the primary annual PM<INF>2.5</INF> standard and supported
revising the level 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, 2020b). 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, 2020b).
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\18\ 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 EPA staff conclusions detailed in the 2020 PA in combination
with the CASAC advice that informed the Administrator's decisions
regarding the 2020 final action, studies highlighted by public comments
on the 2020 proposal, and the numerous studies published since the
literature cutoff date of the 2019 ISA all informed the scope of the
reconsideration.
In its review of the 2019 draft PA, some members of the CASAC had
recommended that greater attention should be given to accountability
studies and epidemiologic studies that employ alternative methods for
confounder control (also referred to as causal inference or causal
modeling studies) in order to ``more fully account for effects of
confounding, measurement and estimation errors, model uncertainty, and
heterogeneity'' in epidemiologic studies (Cox, 2019b, p. 8 of consensus
responses). In addition, public commenters submitted a number of recent
studies published after the literature cutoff date for the 2019 ISA
that would have been considered within the scope of the 2019 ISA. While
the EPA provisionally considered these studies in responding to public
comments,\19\ it was determined that, at the time of the 2020 final
action, these studies were generally consistent with the evidence
assessed in the 2019 ISA (85 FR 82690, December 18, 2020; U.S. EPA,
2020a). As such, and consistent with previous NAAQS reviews, the EPA
concluded that the new studies did not materially change any of the
broad scientific conclusions regarding the health and welfare effects
of PM in ambient air made in the air quality criteria, and therefore,
reopening of the air quality criteria was not warranted (85 FR 82691,
December 18, 2020). However, at that time, the EPA
[[Page 16211]]
recognized that its ``provisional consideration of these studies did
not and could not provide the kind of in-depth critical review'' (85 FR
82690, December 18, 2020) that studies undergo in the development of an
ISA.
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\19\ The list of provisionally considered studies is included in
Appendix A to the 2020 Response to Comments document (U.S. EPA,
2020a).
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In preparing to reconsider the 2020 final decision for the PM
NAAQS, the Agency revisited the need to reopen the air quality
criteria, given the amount of time that had passed since the literature
cutoff date of the 2019 ISA (i.e., approximately January 2018) and the
volume of literature that had become available, including those studies
provisionally considered in responding to comments in 2020. In so
doing, the EPA preliminarily concluded that at least some of these
studies were likely to be relevant to its reconsideration of the air
quality criteria and the PM NAAQS and that, in considering public
comments on any proposed decisions for the reconsideration, these
studies were likely to be raised by public commenters and would
potentially warrant a reopening of the air quality criteria. For
example, on February 16, 2021, the EPA received two petitions to
reconsider the PM NAAQS. One petition objected to the EPA's provisional
consideration of studies submitted in public comments on the 2020
proposal and suggested that the provisional consideration was
inadequate because the studies could be important in determining
whether the existing standards are adequately protective. See, Petition
for Reconsideration of National Ambient Air Quality Standards for
Particulate Matter, submitted by American Lung Association, et al,
dated Feb. 16, 2020. The other petition identified a number of new
studies, including one epidemiologic study that was published after the
provisional consideration was completed that could further inform the
concern expressed by the CASAC that associations reported in
epidemiologic studies do not adequately account for ``uncontrolled
confounding and other potential sources of error and bias.'' See
Petition for Reconsideration of ``Review of the National Ambient Air
Quality Standards for Particulate Matter,'' submitted by the State of
California, dated Feb. 16, 2020. This was also an uncertainty noted by
the then-Administrator in the 2020 decision, who also recognized ``that
methodological study designs to address confounding, such as causal
inference methods, are an emerging field of study.'' Thus, the Agency
concluded it was appropriate to reconsider not only the standards but
also the air quality criteria, in light of public comments during the
2020 PM NAAQS proposal and recent studies published since the cutoff
date of the 2019 ISA, as reflected in petitions. In deciding to reopen
the air quality criteria, the Agency concluded it was reasonable to
focus on studies that were most likely to inform decisions on the
appropriate standard, but not to reassess areas which, based on the
assessment of available science published since the cutoff date of the
2019 ISA and through 2021, were judged unlikely to have new information
that would be useful for the Administrator's decision making. The
Agency accordingly announced 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 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/r/sab_apex/casac/mems?p14_committeeon=2021%20CASAC%20PM%20Panel&session=17433386035954">https://casac.epa.gov/ords/sab/r/sab_apex/casac/mems?p14_committeeon=2021%20CASAC%20PM%20Panel&session=17433386035954</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), and included a discussion of
the rationale and scope of the Supplement. As explained therein, 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,
[[Page 16212]]
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, 2020b).
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, 2020b).''
\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, 2020b).''
\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:
<bullet> 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 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 developing a revised PA to support the reconsideration, the EPA
considered the available scientific evidence, including the evidence
presented in the 2019 ISA and ISA Supplement. The 2022 PA considered
the quantitative and technical information presented in the 2020 PA, in
addition to new and updated analyses conducted since the 2020 final
decision. For those health and welfare effects for which the ISA
Supplement evaluated recently available studies (i.e.,
PM<INF>2.5</INF>-related health effects and visibility effects), new
updated quantitative analyses were conducted as a part of the
development of the 2022 PA. The newly available scientific and
technical information presented in the 2022 PA were considered in
reaching conclusions regarding the adequacy of the current standards
and any potential alternative standards. 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 welfare effects), the conclusions presented in the
2022 PA rely heavily on the information that supported the conclusions
in the 2020 PA.
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). 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). The
CASAC generally endorsed EPA's decisions regarding the limited scope of
the draft ISA Supplement, stating that ``this limitation [on scope] is
appropriate for the targeted purpose of the Draft ISA Supplement''
although the CASAC noted it would not be appropriate for ISAs
generally, and recommended that the EPA provide additional
acknowledgment and explanation for the limited scope (Sheppard, 2022b,
p. 2 of letter; see also pp. 2-3 of consensus responses). The EPA
specifically noted 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) that the ``targeted approach to developing
the Supplement to the 2019 PM ISA for the purpose of reconsidering the
2020 PM NAAQS decision does not reflect a change to EPA's approach for
developing ISAs for NAAQS reviews.'' Thus, 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 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 2022 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 2022 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
[[Page 16213]]
information were not evaluated (i.e., PM<INF>10-2.5</INF>-related
health effects and non-visibility effects), the conclusions presented
in the 2022 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 2022 PA throughout this
document).
Drawing from his consideration of the scientific evidence assessed
in the 2019 ISA and ISA Supplement and the analyses in the 2022 PA,
including the uncertainties in the evidence and analyses, and from his
consideration of advice from the CASAC, on January 5, 2023, the
Administrator proposed to revise the level of the primary annual
PM<INF>2.5</INF> standard and to retain the primary 24-hour
PM<INF>2.5</INF> standard, the primary 24-hour PM<INF>10</INF>
standard, and the secondary PM standards. These proposed decisions were
published in the Federal Register on January 27, 2023 (88 FR 5558,
January 27, 2023). The EPA held a multi-day virtual public hearing on
February 21-23, 2023 (88 FR 6215, January 31, 2023). In total, the EPA
received nearly 700,000 comments on the proposal from members of the
public by the close of the public comment period on March 28, 2023.
Major issues raised in the public comments are discussed throughout the
preamble of this final action. A more detailed summary of all
significant comments, along with the EPA's responses (henceforth
``Response to Comments'' document), can be found in the docket for this
rulemaking (Docket No. EPA-HQ-OAR-2015-0072).
As in prior reviews, the EPA is basing its decision in this
reconsideration on studies and related information in the air quality
criteria, which have undergone CASAC and public review. These studies
assessed in the 2019 ISA \27\ and ISA Supplement \28\ and the 2022 PA,
and the integration of the scientific evidence presented in them, have
undergone extensive critical review by the EPA, the CASAC, and the
public. Decisions on the NAAQS should be based on studies that have
been rigorously assessed in an integrative manner not only by the EPA
but also by the statutorily mandated independent scientific advisory
committee, as well as the public review that accompanies this process.
It is for this reason that the EPA preliminarily concluded that the
scientific evidence available since the completion of the 2019 ISA,
including those raised in public comments on the proposal in 2020,
warranted a partial reopening of the air quality criteria and prepared
an ISA Supplement to enable the EPA, the CASAC, and the public to
consider them further. Some commenters have referred to and discussed
additional individual scientific studies on the health effects of PM
that were not included in the 2019 ISA or ISA Supplement (``new
studies'') and that have not gone through this comprehensive review
process. In considering and responding to comments for which such
``new'' studies were cited in support, the EPA has provisionally
considered the cited studies in the context of the findings of the 2019
ISA and ISA Supplement. The EPA's provisional consideration of these
studies did not and could not provide the kind of in-depth critical
review described above, but rather was focused on determining whether
they warranted further reopening the review of the air quality criteria
to enable the EPA, the CASAC, and the public to consider them further.
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\27\ 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>.
\28\ As described above, the ISA Supplement represents an
evaluation of recent studies that are of greatest policy relevance
and utility to the reconsideration of the 2020 final decision on the
PM NAAQS (U.S. EPA, 2022a).
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This approach, and the decision to rely on the studies and related
information in the air quality criteria, which have undergone CASAC and
public review, is consistent with the EPA's practice in prior NAAQS
reviews and its interpretation of the requirements of the CAA. Since
the 1970 amendments, the EPA has taken the view that NAAQS decisions
are to be based on scientific studies and related information that have
been assessed as a part of the pertinent air quality criteria, and the
EPA has consistently followed this approach. This longstanding
interpretation was strengthened by new legislative requirements enacted
in 1977, which added section 109(d)(2) of the Act concerning CASAC
review of air quality criteria. See 71 FR 6114, 61148 (October 17,
2006, final decision on review of NAAQS for particulate matter) for a
detailed discussion of this issue and the EPA's past practice.
As discussed in the EPA's 1993 decision not to review the
O<INF>3</INF> NAAQS, ``new'' studies may sometimes be of such
significance that it is appropriate to delay a decision in a NAAQS
review and to supplement the pertinent air quality criteria so the
studies can be taken into account (58 FR 13013-13014, March 9, 1993).
In the present case, the EPA decided to partially reopen the air
quality criteria and prepared an ISA Supplement as a part of the
reconsideration to facilitate evaluation of these studies by the EPA,
the CASAC, and the public. The narrow scope of the ISA Supplement is
supported by EPA's provisional consideration of ``new'' studies
submitted in response to public comments on the 2020 proposal which
concluded that, taken in context, the ``new'' information and findings
do not materially change any of the broad scientific conclusions
regarding the health and welfare effects of PM in ambient air made in
the air quality criteria. Therefore, a full reopening of the air
quality criteria was not warranted to assess the health and welfare
effects of PM for purposes of the review.
Accordingly, the EPA is basing the final decisions in this
reconsideration on the studies and related information included in the
PM air quality criteria (including the 2019 PM ISA and ISA Supplement)
that have undergone rigorous review by the EPA, the CASAC, and the
public. The EPA will consider these ``new'' studies for inclusion in
the air quality criteria for the next PM NAAQS review, which the EPA
expects to begin soon after the conclusion of this reconsideration and
which will provide the opportunity to fully assess these studies
through a more rigorous review process involving the EPA, the CASAC,
and the public.
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 2022 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
[[Page 16214]]
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 2022 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 44% of
emissions of primary PM<INF>2.5</INF> emissions (U.S. EPA, 2021b).
Emissions from wildfire comprises 29% of primary PM<INF>2.5</INF>
emissions.
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
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). The EPA
views the strategic use of prescribed fire as an important tool for
reducing wildfire risk and the severity of wildfires and wildfire smoke
(88 FR, 54118, 54126, August 9, 2023).\29\ As noted in the PM NAAQS
proposal, agencies have efforts in place to reduce the frequency and
severity of human-caused wildfires (88 FR 5570, January 27, 2023).
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\29\ See also: <a href="https://www.usda.gov/sites/default/files/documents/usda-epa-doi-cdc-mou.pdf">https://www.usda.gov/sites/default/files/documents/usda-epa-doi-cdc-mou.pdf</a>.
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Wildfire events produce high PM emissions that may impact the PM
concentrations in ambient air to the extent that the concentrations
result in an exceedance or violation which may affect the design value
in a given area. The EPA's Exceptional Events Rule (81 FR 68216,
October 3, 2016) describes the process by which air agencies may
request to exclude `event-influenced' data caused by exceptional
events, which can include wildfires and prescribed fires on wildland.
The EPA has issued guidance specifically addressing exceptional events
demonstrations for both wildfires and prescribed fires on wildland.
These documents are available on EPA's Exceptional Events Program
website.\30\ The EPA will develop fire-related exceptional events
implementation tools, including updates as needed to existing guidance
to facilitate more efficient processing of PM<INF>2.5</INF>-related
exceptional events demonstrations for both the 24-hour and annual
standards.
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\30\ See: <a href="https://www.epa.gov/air-quality-analysis/final-2016-exceptional-events-rule-supporting-guidance-documents-updated-faqs">https://www.epa.gov/air-quality-analysis/final-2016-exceptional-events-rule-supporting-guidance-documents-updated-faqs</a>.
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3. Monitoring of Ambient PM
To promote uniform application 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 establishes PM
Federal Reference Methods (FRMs) for both PM<INF>10</INF> and
PM<INF>2.5</INF> in appendices J and L to 40 CFR part 50, both of which
were amended following the 2006 and 2012 PM NAAQS reviews. The current
PM monitoring network relies on FRMs and automated continuous Federal
Equivalent Methods (FEMs) approved pursuant to 40 CFR part 53, in part
to support changes necessary for implementation of the revised PM
standards. Additionally, 40 CFR part 58, appendices A through E, detail
the requirements to measure ambient air quality and report ambient air
quality data and related information. More information on PM ambient
monitoring networks is available in section 2.2 of the 2022 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. 40 CFR part 58, appendix D, section 4.7 provides the
applicable PM<INF>2.5</INF> network design criteria. For most urban
locations, PM<INF>2.5</INF> monitors are sited at the neighborhood
scale,\31\ 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
[[Page 16215]]
area-wide air quality is sited in an area of expected maximum
concentration.\32\ 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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\31\ For PM<INF>2.5</INF>, neighborhood scale is defined at 40
CFR part 58, appendix D, 4.7.1(c)(3) 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.
\32\ 40 CFR part 58, app. D, 4.7.1(b)(2).
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Under 40 CFR part 50, appendix L, and 40 CFR part 53, and 40 CFR
part 58 appendix D 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
2022 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 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 2022
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\<INF></INF>,
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 [micro]g/m\3\ higher than at the
highest measuring non-near-road sites (range is 0.1 to 2.1 [micro]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 subdaily
patterns, and because some health studies examine PM exposure durations
shorter than 24-hours, it is useful to understand the broader
distribution of subdaily PM<INF>2.5</INF> concentrations across the
U.S. The 2022 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
[[Page 16216]]
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. 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 2022 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 2022 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
[[Page 16217]]
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). Ground-based PM<INF>2.5</INF> monitors
are generally sited in areas of expected maximum concentration. 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 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 2022 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 design 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. 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. Analyses in the 2022 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
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
2022 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> Hybrid Modeling Approaches 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 2022 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 2022 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 2022 PA evaluates the
DI2019 \40\ and HA2020 \41\ hybrid surfaces, surfaces that are used in
several of the key epidemiologic studies in the 2022 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
[[Page 16218]]
influenced by the approach used to compute that mean and how that value
might compare to monitor reported concentrations. The PM<INF>2.5</INF>
standards are expected to achieve a pattern of air quality through the
attainment of a specific design value at each monitor in the monitoring
network. As a result, it is important to be able to assess the
relationship between monitor concentrations and patterns of air quality
evaluated in the epidemiologic studies.
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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 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 2022 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>
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 [micro]g/m\3\ to 8.1 [micro]g/m\3\, compared to the CBSA scale,
which ranges from 5.7 [micro]g/m\3\ to 8.7 [micro]g/m\3\. (U.S. EPA,
2022b, section 2.3.3.2.4, Table 2-6).
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The 2022 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 2022 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
[micro]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 [micro]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 [micro]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 2022 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 2022 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 2022 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
[micro]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 [micro]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 Decisions on the Primary PM2.5 Standards
This section presents the rationale for the Administrator's
decision to revise the primary annual PM<INF>2.5</INF> standard down to
a level of 9 [micro]g/m\3\ 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 evaluated 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
[[Page 16219]]
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,
2022a).\47\ The Administrator's rationale also takes into account: (1)
The 2022 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; 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, 2022a).
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In presenting the rationale for the Administrator's decisions and
its foundations, section II.A provides background on the general
approach for this reconsideration and the basis for the existing
standard, and also presents brief summaries of key aspects of the
currently available health effects and risk information. Section II.B
summarizes the CASAC advice and the basis for the proposed conclusions,
addresses public comments received on the proposal and presents the
Administrator's conclusions on the adequacy of the current standards,
drawing on consideration of the scientific evidence and quantitative
risk information, advice from the CASAC, and comments from the public.
Section II.C summarizes the Administrator's decision on the primary
PM<INF>2.5</INF> standards.
A. Introduction
The general approach for this reconsideration of the 2020 final
decision on the primary PM<INF>2.5</INF> standards is fundamentally
based 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 2022 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 2022 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 2022 PA. In evaluating the
public health protection afforded by the current standards, the four
basic elements of the NAAQS (i.e., 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 (also referred to as ``at-risk'') 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]); see also Am. Lung Ass'n v.
EPA, 134 F.3d 388, 389 (D.C. Cir. 1998).
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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 information
available at that time, as well as the then-Administrator's judgments
regarding the available health effects evidence and the appropriate
degree of public health protection afforded by the existing standards
(85 FR 82718, December 18, 2020). With the 2020 decision, the then-
Administrator retained the primary annual PM<INF>2.5</INF> standard
with its level of 12.0 [mu]g/m\3\ and retained the primary 24-hour
PM<INF>2.5</INF> standard with its level of 35 [mu]g/m\3\. The key
considerations and the then-Administrator's conclusions regarding the
primary PM<INF>2.5</INF> standards in the 2020 review are summarized
below.
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, 2019a). 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-related 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.
[[Page 16220]]
Based on this evidence, the 2019 ISA concluded 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> exposure
and respiratory effects, and between long-term PM<INF>2.5</INF>
exposure 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), including children and older adults, people
with pre-existing respiratory or cardiovascular disease, minority
populations, and low socioeconomic status (SES) populations.
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, 2020b, 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.
As an initial matter, the then-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 then-Administrator recognized that,
consistent with the evidence available in prior reviews, the scientific
evidence 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 then-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 then-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 subdaily estimates are less consistent and, in some
cases, smaller in magnitude (U.S. EPA, 2019a, section 1.5.2.1; U.S.
EPA, 2020b, section 3.5.2.2). In addition, controlled human exposure
and panel-based studies of subdaily 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
concluded that epidemiologic studies did not indicate that subdaily
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, 2020b, section 3.2.3.1). Thus, these
studies also did not suggest the need for additional protection against
subdaily PM<INF>2.5</INF> exposures (U.S. EPA, 2020b, section 3.5.2.2).
Therefore, the then-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 then-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, 2020b, 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 supported
a 98th percentile form for a 24-hour standard. The then-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, 2020b, section 3.5.2.3). As such, the then-
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 then-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, 2020b), 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).
[[Page 16221]]
As an initial matter, the then-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 then-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 then-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
then-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 then-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 then-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
then-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 then-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, 2020b, 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, 2020b, 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 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, 2020b,
section 3.2.3.1; 85 FR 82716, December 18, 2020).
With respect to the epidemiologic evidence, the then-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, 2020b, section 3.5.1). The
then-Administrator noted that this was consistent with CASAC consensus
support for retaining the current 24-hour standard. Thus, the then-
Administrator concluded that the 24-hour standard with its level of 35
[micro]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 then-
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 effects observed in epidemiologic
studies. However, the then-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 then-
Administrator's judgment. Thus, consistent with the approaches in
previous NAAQS reviews, the then-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 then-Administrator emphasized uncertainties and limitations
that were present in epidemiologic studies in previous reviews and
persisted in the 2020 review. These uncertainties
[[Page 16222]]
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 then-
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, 2019b, 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, 2019b, p. 8 consensus responses).
Taking into consideration the views expressed by the CASAC members
who supported retaining the annual standard, the then-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, 2020b, 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 then-Administrator viewed that when
assessing the mean concentrations of the key short-term and long-term
epidemiologic studies in the U.S. that used 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
[micro]g/m\3\, a concentration level above the existing level of the
primary annual standard of 12 [micro]g/m\3\. The then-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, 2020b, 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 then-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
[micro]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 then-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 82716-17, 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 [micro]g/m\3\, which was
also above the level of the existing standard.
---------------------------------------------------------------------------
In addition to the evidence, the then-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 then-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, 2020b, section 3.3.2.4). Additionally,
the then-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 then-
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 then-Administrator additionally considered an emerging body of
evidence from accountability studies that examined past reductions in
ambient PM<INF>2.5</INF> and the degree to which those reductions
resulted in public health improvements. While the then-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, 2020b, 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, 2020b, 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[micro]g/m\3\ (i.e., U.S. EPA, 2020b, Table 3-3). Given the lack
of available accountability studies
[[Page 16223]]
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 then-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 then-
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 then-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 in the 2020 review, the then-
Administrator judged that the existing standards provided an adequate
margin of safety. With respect to the annual standard, the level of 12
[micro]g/m\3\ was below the lowest ``starting'' concentration (i.e.,
13.2 [micro]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 then-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 [micro]g/m\3\). In addition, the then-
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 then-Administrator's review of the
science in the 2020 review, 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 then- 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 in the 2020 review, the then-
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 then-
Administrator's view that there were important limitations and
uncertainties that remained in the evidence. The then-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 [micro]g/m\3\, 35 [micro]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. Overview of the Health Effects Evidence
The information summarized here and further detailed in section
II.B of the proposal (88 FR 5580, January 27, 2023), 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 (U.S. EPA, 2019a) and ISA Supplement (U.S. EPA, 2022a) and
its policy implications are further discussed in the 2022 PA (U.S. EPA,
2022b). 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, which 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, in
selecting the health effects to evaluate within the ISA Supplement
(i.e., newly available evidence related to short- and long-term
PM<INF>2.5</INF> exposure and mortality and cardiovascular effects),
the primary rationale is based on the causality determinations for
health effect categories presented in the 2019 PM ISA, and the
subsequent use of the health effects evidence in the 2020 PM PA.
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. Additionally, studies examining
associations outside the U.S. or Canada reflect air quality and
exposure patterns that may be less typical of the U.S., and thus less
likely to be informative for purposes of reviewing the NAAQS (U.S. EPA,
2022b, p.1-3). While the ISA Supplement does not include information
for health effects other than mortality and cardiovascular effects, the
[[Page 16224]]
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.
---------------------------------------------------------------------------
\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.
---------------------------------------------------------------------------
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 (II.A.2.a), with a focus on those health effects
for which the 2019 ISA concluded a ``causal'' or ``likely to be
causal'' relationship, the potential public health implications and
populations at risk (II.A.2.b), and PM<INF>2.5</INF> concentrations in
key studies reporting health effects (II.A.2.c).
a. 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, 2009a; 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, and in more detail in section II.B.1 of the proposal (88 FR
5580, January 27, 2023), summarize the relationships between long-and
short-term PM<INF>2.5</INF> exposures and mortality (II.A.2.a.i),
cardiovascular effects (II.A.2.a.ii), respiratory effects
(II.A.2.a.iii), cancer (II.A.2.a.iv), nervous system effects
(II.A.2.a.v) and other effects (II.A.2.a.vi). 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.
---------------------------------------------------------------------------
i. Mortality
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
[[Page 16225]]
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 and report that reductions in ambient
PM<INF>2.5</INF> are associated with improvements in longevity (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 [micro]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 ambient
PM<INF>2.5</INF> concentrations and found a decrease in long-term
PM<INF>2.5</INF> concentration continued to be associated with an
increase in life expectancy, though the magnitude of the increase was
smaller than during the earlier time period (i.e., a 10 [micro]g/m\3\
decrease in long-term PM<INF>2.5</INF> concentration was associated
with a 0.35-year increase in life expectancy) (Correia et al., 2013).
Additional studies conducted in the U.S. or Europe similarly report
that reductions in ambient PM<INF>2.5</INF> are associated with
improvements in longevity (U.S. EPA, 2022a, section 11.2.2.5).
Since the literature cutoff date for the 2019 ISA, a few
epidemiologic studies were published that examined the relationship
between long-term PM<INF>2.5</INF> exposure and life-expectancy (U.S.
EPA, 2022a, section 3.2.1.3) and report results that are consistent
with and expand upon the body of evidence from the 2019 ISA. For
example, Bennett et al. (2019) reported that PM<INF>2.5</INF>
concentrations above the lowest observed concentration (2.8 [micro]g/
m\3\) were associated with a 0.15 year decrease in national life
expectancy for women and 0.13 year decrease in national life expectancy
for men (U.S. EPA, 2022a, section 3.2.2.2.4, Figure 3-25). Another
study compared participants living in areas with PM<INF>2.5</INF>
concentrations >12 [micro]g/m\3\ to participants living in areas with
PM<INF>2.5</INF> concentrations <12 [micro]g/m\3\ and reported that the
number of years of life lost due to living in areas with higher
PM<INF>2.5</INF> concentrations was 0.84 years over a 5-year period
(Ward-Caviness et al., 2020; U.S. EPA, 2022a, section 3.2.2.2.4).
Additionally, a number of accountability studies, which are
epidemiologic studies that evaluate whether an environmental policy or
air quality intervention resulted in reductions in ambient air
pollution concentrations and subsequent reductions in mortality or
morbidity, have emerged and were evaluated in the ISA Supplement (U.S.
EPA, 2022a, section 3.2.2.3). For example, Sanders et al. (2020a)
examined whether policy actions (i.e., the first annual
PM<INF>2.5</INF> NAAQS implementation rule in 2005 for the 1997 annual
PM<INF>2.5</INF> standard with a 3-year annual average of 15.0 [mu]g/
m\3\) reduced PM<INF>2.5</INF> concentrations and mortality rates in
Medicare beneficiaries between 2000-2013, and found that following
implementation of the annual PM<INF>2.5</INF> NAAQS, annual
PM<INF>2.5</INF> concentrations decreased by 1.59 [mu]g/m\3\ (95% CI:
1.39, 1.80) which corresponded to a 0.93% reduction in mortality rates
among individuals 65 years and older ([95% CI: 0.10%, 1.77%) in non-
attainment counties relative to attainment counties.
The 2019 ISA also evaluated a small number of studies that used
alternative methods for confounder control to further assess
relationship between long-term PM<INF>2.5</INF> exposure and mortality
(U.S. EPA, 2019a, section 11.2.2.4). In addition, multiple
epidemiologic studies that implemented alternative metho
[…truncated; see source link]Indexed from Federal Register on March 6, 2024.
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