[Federal Register Volume 85, Number 244 (Friday, December 18, 2020)]
[Rules and Regulations]
[Pages 82684-82748]
[FR Doc No: 2020-27125]

Vol. 85

Friday,

No. 244

December 18, 2020

Part III

 Environmental Protection Agency

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40 CFR Part 50

 Review of the National Ambient Air Quality Standards for Particulate
Matter; Final Rule

Federal Register / Vol. 85 , No. 244 / Friday, December 18, 2020 /
Rules and Regulations

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[EPA-HQ-OAR-2015-0072; FRL-10018-11-OAR]
RIN 2060-AS50

Review of the National Ambient Air Quality Standards for
Particulate Matter

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final action.

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SUMMARY: Based on the Environmental Protection Agency's (EPA's) review
of the air quality criteria and the national ambient air quality
standards (NAAQS) for particulate matter (PM), the Administrator has
reached final decisions on the primary and secondary PM NAAQS. With
regard to the primary standards meant to protect against fine particle
exposures (i.e., annual and 24-hour PM2.5 standards), the
primary standard meant to protect against coarse particle exposures
(i.e., 24-hour PM10 standard), and the secondary
PM2.5 and PM10 standards, the EPA is retaining
the current standards, without revision.

DATES: This final action is effective December 18, 2020.

ADDRESSES: The EPA has established a docket for this action under
Docket ID No. EPA-HQ-OAR-2015-0072. Incorporated into this docket is a
separate docket established for the Integrated Science Assessment
(Docket ID No. EPA-HQ-ORD-2014-0859). All documents in the docket are
listed in https://www.regulations.gov/. Although listed in the index,
some information is not publicly available, e.g., Confidential Business
Information (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. With the exception of such material, publicly available
docket materials are available electronically through https://www.regulations.gov/. Out of an abundance of caution for members of the
public and our staff, the EPA Docket Center and Reading Room are closed
to the public, with limited exceptions, to reduce the risk of
transmitting COVID-19. Our Docket Center staff will continue to provide
remote customer service via email, phone, and webform. For further
information on EPA Docket Center services and the current status,
please visit us online at https://www.epa.gov/dockets.

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:<PHONE_NUMBER>; fax: (919)
541-5315; email<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:

Basis for Immediate Effective Date

    In accordance with section 307(d)(1)(V), the Administrator has
designated this action as being subject to the rulemaking procedures in
section 307(d) of the Clean Air Act (CAA). Section 307(d)(1) of the CAA
states that: ``The provisions of section 553 through 557 * * * of Title
5 shall not, except as expressly provided in this subsection, apply to
actions to which this subsection applies.'' Thus, section 553(d) of the
Administrative Procedure Act (APA), which requires publication of a
substantive rule to be made ``not less than 30 days before its
effective date'' subject to limited exceptions, does not apply to this
action. In the alternative, the EPA concludes that it is s consistent
with APA section 553(d) to make this action effective December 18,
2020.
    Section 553(d)(3) of the APA, 5 U.S.C. 553(d)(3), provides that
final rules shall not become effective until 30 days after publication
in the Federal Register ``except . . . as otherwise provided by the
agency for good cause found and published with the rule.'' ``In
determining whether good cause exists, an agency should `balance the
necessity for immediate implementation against principles of
fundamental fairness which require that all affected persons be
afforded a reasonable amount of time to prepare for the effective date
of its ruling.'' Omnipoint Corp. v. Fed. Commc'n Comm'n, 78 F.3d 620,
630 (D.C. Cir. 1996) (quoting United States v. Gavrilovic, 551 F.2d
1099, 1105 (8th Cir. 1977)). The purpose of this provision is to ``give
affected parties a reasonable time to adjust their behavior before the
final rule takes effect.'' Id.; see also Gavrilovic, 551 F.2d at 1104
(quoting legislative history).
    The EPA is determining that in light of the nature of this action,
good cause exists to make this final action effective immediately
because the Agency seeks to provide regulatory certainty as soon as
possible and the Administrator's decision to retain the current NAAQS
does not change the status quo or impose new obligations on any person
or entity. As a result, there is no need to provide parties additional
time to adjust their behavior, and no person will be harmed by making
the action immediately effective as opposed to delaying the effective
date by 30 days. Accordingly, the EPA is making this action effective
immediately upon publication.

General Information

Availability of Information Related to This Action

    A number of the documents that are relevant to this final decision
are available through the EPA's website at https://www.epa.gov/naaqs/particulate-matter-pm-air-quality-standards. These documents include
the Integrated Review Plan for the National Ambient Air Quality
Standards for Particulate Matter (U.S. EPA, 2016), available at https://www3.epa.gov/ttn/naaqs/standards/pm/data/201612-final-integrated-review-plan.pdf, the Integrated Science Assessment for Particulate
Matter (U.S. EPA, 2019), available at https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=347534, the Policy Assessment for the Review of
the National Ambient Air Quality Standards for Particulate Matter (U.S.
EPA, 2020), available at https://www.epa.gov/naaqs/particulate-matter-pm-standards-policy-assessments-current-review-0, and the notice of
proposed rulemaking, available at https://www.epa.gov/naaqs/particulate-matter-pm-standards-federal-register-notices-current-review. These and other related documents are also available for
inspection and copying in the EPA docket identified above.

Table of Contents

    The following topics are discussed in this preamble:

Executive Summary
I. Background
    A. Legislative Requirements
    B. Related PM Control Programs
    C. History of the PM Air Quality Criteria and Standards
    1. Reviews Completed in 1971 and 1987
    2. Review Completed in 1997
    3. Review Completed in 2006
    4. Review Completed in 2012
    D. Current Review of the Air Quality Criteria and Standards
    E. Air Quality Information
    1. Distribution of Particle Size in Ambient Air
    2. Sources and Emissions Contributing to PM in the Ambient Air
    3. Ambient Concentrations and Trends
    a. PM2.5 Mass
    b. PM2.5 Components
    c. PM10
    d. PM10-2.5
    e. UFP
    4. Background PM
II. Rationale for Decisions on the Primary PM2.5
Standards

    A. Introduction
    1. Background on the Current Standards
    2. Overview of 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. At-Risk Populations
    c. Evidence-Based Considerations
    i. PM2.5 Concentrations Evaluated in Experimental
Studies
    ii. Ambient Concentrations in Locations of Epidemiological
Studies
    3. Overview of Risk and Exposure Assessment Information
    B. Conclusions on the Primary PM2.5 Standards
    1. CASAC Advice in This Review
    2. Basis for Proposed Decision
    3. Comments on the Proposed Decision
    4. Administrator's Conclusions
    C. Decision on the Primary PM2.5 Standards
III. Rationale for Decisions on the Primary PM10 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 PM10 Standard
    1. CASAC Advice in This Review
    2. Basis for the Proposed Decision
    3. Comments on the Proposed Decision
    4. Administrator's Conclusions
    C. Decision on the Primary PM10 Standard
IV. Rationale for Decision on the Secondary PM Standards
    A. Introduction
    1. Background on the Current Standards
    2. Overview of Welfare Effects Evidence
    a. Nature of Effects
    i. Visibility
    ii. Climate
    iii. Materials
    3. Overview of Air Quality and Quantitative Information
    a. Visibility Effects
    b. Non-Visibility Effects
    B. Conclusions on the Secondary Standards
    1. CASAC Advice in This Review
    2. Basis for the Proposed Decision
    3. Comments on the Proposed Decision
    4. Administrator's Conclusions
    C. Decision on the Secondary PM Standards
V. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
    B. Executive Order 13771: Reducing Regulations and Controlling
Regulatory Costs
    C. Paperwork Reduction Act (PRA)
    D. Regulatory Flexibility Act (RFA)
    E. Unfunded Mandates Reform Act (UMRA)
    F. Executive Order 13132: Federalism
    G. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
    H. Executive Order 13045: Protection of Children From
Environmental Health Risks and Safety Risks
    I. Executive Order 13211: Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution or Use
    J. National Technology Transfer and Advancement Act (NTTAA)
    K. Executive Order 12898: Federal Actions To Address
Environmental Justice in Minority Populations and Low-Income
Populations
    L. Determination Under Section 307(d)
    M. Congressional Review Act (CRA)
References

Executive Summary

    This notice presents the Administrator's final decisions to retain
the current primary (health-based) and secondary (welfare-based)
National Ambient Air Quality Standards (NAAQS) for particulate matter
(PM), without revision.
    In ambient air, PM is a mixture of substances suspended as small
liquid and/or solid particles. Particles in the atmosphere range in
size from less than 0.01 to more than 10 micrometers ([mu]m) in
diameter. Particulate matter and its precursors are emitted from both
anthropogenic sources (e.g., electricity generating units, cars and
trucks, agricultural operations) and natural sources (e.g., sea salt,
wildland fires, biological aerosols). When describing PM, subscripts
are used to denote particle size. For example, PM2.5
includes particles with diameters generally less than or equal to 2.5
[mu]m and PM10 includes particles with diameters generally
less than or equal to 10 [mu]m.
    The EPA has established primary (health-based) and secondary
(welfare-based) NAAQS for PM2.5 and PM10. This
includes two primary PM2.5 standards, an annual average
standard with a level of 12.0 [mu]g/m\3\ and a 24-hour standard with a
98th percentile form and a level of 35 [mu]g/m\3\. It also includes a
primary PM10 standard with a 24-hour averaging time, a 1-
expected exceedance form, and a level of 150 [mu]g/m\3\. Secondary PM
standards are set equal to the primary standards, except that the level
of the secondary annual PM2.5 standard is 15.0 [mu]g/m\3\.
In reaching decisions on these PM standards in the current review, the
Administrator has considered the available scientific evidence assessed
in the Integrated Science Assessment (ISA), analyses in the Policy
Assessment (PA), advice from the Clean Air Scientific Advisory
Committee (CASAC), and public comments on the proposal.
    For the primary PM2.5 standards, the Administrator
concludes that there are important uncertainties in the evidence for
adverse health effects below the current standards and in the potential
for additional public health improvements from reducing ambient
PM2.5 concentrations below those standards. Based on the
available evidence, the Administrator has concluded that the current
primary PM2.5 standards are requisite to protect public
health, with an adequate margin of safety, from effects of
PM2.5 in ambient air and should be retained, without
revision. Therefore, the EPA is retaining those standards (i.e., both
the annual and 24-hour standards), without revision.
    For the primary PM10 standard, the Administrator
observes that, while the available health effects evidence has
expanded, recent studies are subject to the same types of uncertainties
that were judged important in the last review. He concludes that, based
on the newly available evidence with its inherent uncertainties, the
current primary PM10 standard is requisite to protect public
health, with an adequate margin of safety, from effects of
PM10 in ambient air, and should be retained, without
revision. Therefore, the EPA is retaining that standard, without
revision.
    For the secondary standards, the Administrator observes that the
expanded evidence for non-ecological welfare effects is consistent with
the last review \1\ and that updated quantitative analyses show results
similar to those in the last review. Based on his consideration of the
available evidence and quantitative information, he concludes that the
current secondary PM standards are requisite to protect public welfare,
against visibility effects and that there is insufficient information
to establish distinct

secondary PM standards to address materials and climate effects.
Therefore, the EPA is retaining those standards, without revision.
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    \1\ The welfare effects considered in this review include
visibility impairment, climate effects, and materials effects.
Ecological effects associated with PM, and the adequacy of
protection provided by the secondary PM standards for those effects,
are being addressed in the separate review of the secondary NAAQS
for oxides of nitrogen, oxides of sulfur and PM (U.S. EPA, 2016,
section 5.2; U.S. EPA, 2020, section 5.1.1) in recognition of the
linkages between oxides of nitrogen, oxides of sulfur, and PM with
respect to atmospheric deposition and ecological effects. Addressing
the pollutants together enables the EPA to take a comprehensive
approach to considering the nature and interactions of the
pollutants, which is important for ensuring that all scientific
information relevant to ecological effects is thoroughly evaluated.
Information on the current review of these secondary NAAQS can be
found at https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-secondary-air-quality-standards.
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    These decisions are consistent with the CASAC's consensus advice on
the primary 24-hour PM2.5 standard, the primary
PM10 standard, and the secondary standards. The CASAC
provided differing views on the primary annual PM2.5
standard, with some committee members recommending that the EPA retain
the current standard and other members recommending revision of that
standard.

I. Background

A. Legislative Requirements

    Two sections of the 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.'' \2\ 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.'' \3\
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    \2\ 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).
    \3\ 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 to
the Administrator's judgment. See Lead Industries Ass'n v. EPA, 647
F.2d at 1161-62; Mississippi v. EPA, 744 F.3d at 1353.
    Section 109(d)(1) of the Act requires the review every five years
of existing air quality criteria and, if appropriate, the revision of
those criteria to reflect advances in scientific knowledge on the
effects of the pollutant on public health and welfare. Under the same
provision, the EPA is also to review every five years and, if
appropriate, revise the NAAQS, based on the revised air quality
criteria.
    Section 109(d)(2) addresses the appointment and advisory functions
of an independent scientific review committee. Section 109(d)(2)(A)
requires the Administrator to appoint this committee, which is to be
composed of ``seven members including at least one member of the
National Academy of Sciences, one physician, and one person
representing State air pollution control agencies.'' Section
109(d)(2)(B) provides that the independent scientific review committee
``shall complete a review of the criteria . . . and the national
primary and secondary ambient air quality standards . . . and shall
recommend to the Administrator any new . . . standards and revisions of
existing criteria and standards as may be appropriate. . . .'' Since
the early 1980s, this independent review function has been performed by
the Clean Air Scientific Advisory Committee (CASAC) of the EPA's
Science Advisory Board. A number of other advisory functions are also
identified for the committee by section 109(d)(2)(C), which reads:

    Such committee shall also (i) advise the Administrator of areas
in which additional knowledge is required to appraise the adequacy
and basis of existing, new, or revised national ambient air quality
standards, (ii) describe the research efforts necessary to provide
the required information, (iii) advise the Administrator on the
relative contribution to air pollution concentrations of natural as
well as anthropogenic activity, and (iv) advise the Administrator of
any adverse public health, welfare, social, economic, or energy
effects which may result from various strategies for attainment and
maintenance of such national ambient air quality standards.

    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.\4\
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    \4\ Some aspects of the CASAC's advice may not be relevant to
the EPA's process of setting primary and secondary standards that
are requisite to protect public health and welfare. Indeed, were the
EPA to consider costs of implementation when reviewing and revising
the standards ``it would be grounds for vacating the NAAQS.''
Whitman, 531 U.S. at 471 n.4. At the same time, the CAA directs the
CASAC to provide advice on ``any adverse public health, welfare,
social, economic, or energy effects which may result from various
strategies for attainment and maintenance'' of the NAAQS to the
Administrator under section 109(d)(2)(C)(iv). In Whitman, the Court
clarified that most of that advice would be relevant to
implementation but not standard setting, as it ``enable[s] the
Administrator to assist the States in carrying out their statutory
role as primary implementers of the NAAQS.'' Id. at 470 (emphasis in
original). However, the Court also noted that the CASAC's ``advice
concerning certain aspects of `adverse public health . . . effects'
from various attainment strategies is unquestionably pertinent'' to
the NAAQS rulemaking record and relevant to the standard setting
process. Id. at 470 n.2.
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B. Related PM Control Programs

    States are primarily responsible for ensuring attainment and
maintenance of ambient air quality standards once the EPA has
established them. Under sections 110 and 171-190 of the CAA, and
related provisions and regulations, states are to submit, for the EPA's
approval, state implementation plans (SIPs) that provide for the
attainment and maintenance of such standards through control programs
directed to sources of the pollutants involved. The states, in
conjunction with the EPA, also administer the Prevention of Significant
Deterioration (PSD) program (CAA sections 160 to 169). In addition,
Federal programs provide for nationwide reductions in emissions of PM
and other air pollutants through the Federal motor vehicle and motor
vehicle fuel control program under title II of the Act (CAA sections
202 to 250), which involves controls for emissions from mobile sources
and controls for the fuels used by these sources, and new source
performance standards for stationary sources under section 111 of the
CAA.

C. History of the PM Air Quality Criteria and Standards

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).\5\ 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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    \5\ Prior to the review initiated in 2007 (see section I.C.4),
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 PM10,\6\ 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. The level of the 24-hour standards (primary and secondary)
was set at 150 [micro]g/m\3\, and the form was one expected exceedance
per year, on average over three years. The level of the annual
standards (primary and secondary) was set at 50 [micro]g/m\3\, and the
form was annual arithmetic mean, averaged over three years.
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    \6\ PM10 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 PM10 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 PM10 standards. The EPA added
new standards, using PM2.5 as the indicator for fine
particles (with PM2.5 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 PM2.5 concentrations from single or multiple community-
oriented monitors; \7\ 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 PM2.5 concentrations at each monitor within an area.
Also, the EPA established a new reference method for the measurement of
PM2.5 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 PM10 (referred to as
thoracic coarse particles or PM10-2.5; 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 PM10 standard and revised the form of the
primary 24-hour PM10 standard to be based on the 99th
percentile of 24-hour PM10 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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    \7\ The 1997 annual PM2.5 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 p.m. NAAQS, petitions for review
were filed by several parties, addressing a broad range of issues. In
May 1999, the U.S. Court of Appeals for the District of Columbia
Circuit (D.C. Circuit) upheld the EPA's decision to establish fine
particle standards, holding that ``the growing empirical evidence
demonstrating a relationship between fine particle pollution and
adverse health effects amply justifies establishment of new fine
particle standards.'' American Trucking Associations, Inc. v. EPA, 175
F. 3d 1027, 1055-56 (D.C. Cir. 1999). The D.C. Circuit also found
``ample support'' for the EPA's decision to regulate coarse particle
pollution, but vacated the 1997 PM10 standards, concluding
that the

EPA had not provided a reasonable explanation justifying use of
PM10 as an indicator for coarse particles. American Trucking
Associations v. EPA, 175 F. 3d at 1054-55. Pursuant to the D.C.
Circuit's decision, the EPA removed the vacated 1997 PM10
standards, and the pre-existing 1987 PM10 standards remained
in place (65 FR 80776, December 22, 2000). The D.C. Circuit also upheld
the EPA's determination not to establish more stringent secondary
standards for fine particles to address effects on visibility. American
Trucking Associations v. EPA, 175 F. 3d at 1027.
    The D.C. Circuit also addressed more general issues related to the
NAAQS, including issues related to the consideration of costs in
setting NAAQS and the EPA's approach to establishing the levels of
NAAQS. Regarding the cost issue, the court reaffirmed prior rulings
holding that in setting NAAQS the EPA is ``not permitted to consider
the cost of implementing those standards.'' American Trucking
Associations v. EPA, 175 F. 3d at 1040-41. Regarding the levels of
NAAQS, the court held that the EPA's approach to establishing the level
of the standards in 1997 (i.e., both for PM and for the ozone NAAQS
promulgated on the same day) effected ``an unconstitutional delegation
of legislative authority.'' American Trucking Associations v. EPA, 175
F. 3d at 1034-40. Although the court stated that ``the factors EPA uses
in determining the degree of public health concern associated with
different levels of ozone and PM are reasonable,'' it remanded the rule
to the EPA, stating that when the EPA considers these factors for
potential non-threshold pollutants ``what EPA lacks is any determinate
criterion for drawing lines'' to determine where the standards should
be set.
    The D.C. Circuit's holdings on the cost and constitutional issues
were appealed to the U.S. 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 PM2.5 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, 2004). 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).\8\
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 PM2.5 standards to 35 [micro]g/m\3\,
retained the level of the annual PM2.5 standards at 15.0
[micro]g/m\3\, and revised the form of the annual PM2.5
standards by narrowing the constraints on the optional use of spatial
averaging. With regard to the primary and secondary standards for
PM10, the EPA retained the 24-hour standards, with levels at
150 [micro]g/m\3\, and revoked the annual standards.\9\ The
Administrator judged that the available evidence generally did not
suggest a link between long-term exposure to existing ambient levels of
coarse particles and health or welfare effects. In addition, a new
reference method was added for the measurement of PM10-2.5
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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    \8\ 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 (PA).
    \9\ In the 2006 proposal, the EPA proposed to revise the 24-hour
PM10 standard in part by establishing a new
PM10-2.5 indicator for thoracic coarse particles (i.e.,
particles generally between 2.5 and 10 [micro]m in diameter). The
EPA proposed to include any ambient mix of PM10-2.5 that
was dominated by resuspended dust from high density traffic on paved
roads and by PM from industrial sources and construction sources.
The EPA proposed to exclude any ambient mix of PM10-2.5
that was dominated by rural windblown dust and soils and by PM
generated from agricultural and mining sources. In the final
decision, the existing PM10 standard was retained, in
part due to an ``inability . . . to effectively and precisely
identify which ambient mixes are included in the
[PM10-2.5] indicator and which are not'' (71 FR 61197,
October 17, 2006).
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    Several parties filed petitions for review following promulgation
of the revised PM NAAQS in 2006. These petitions addressed the
following issues: (1) Selecting the level of the primary annual
PM2.5 standard; (2) retaining PM10 as the
indicator of a standard for thoracic coarse particles, retaining the
level and form of the 24-hour PM10 standard, and revoking
the PM10 annual standard; and (3) setting the secondary
PM2.5 standards identical to the primary standards. On
February 24, 2009, the D.C. Circuit issued its opinion in the case
American Farm Bureau Federation v. EPA, 559 F. 3d 512 (D.C. Cir. 2009).
The court remanded the primary annual PM2.5 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
PM10, the court upheld the EPA's decisions to retain the 24-
hour PM10 standard to provide protection from thoracic
coarse particle exposures and to revoke the annual PM10
standard. Id. at 533-38. With regard to the secondary PM2.5
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.
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,\10\ 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., IRP (U.S. EPA, 2008), ISA (U.S. EPA, 2009c), REA planning
documents for health and welfare (U.S. EPA, 2009b, U.S. EPA, 2009a), a
quantitative health risk assessment (U.S. EPA, 2010a) and an urban-
focused visibility assessment (U.S. EPA, 2010b), and PA (U.S. EPA,
2011)). In June 2012, the EPA announced its proposed decision to revise
the NAAQS for PM (77 FR 38890, June 29, 2012).
---------------------------------------------------------------------------

    \10\ The history of the NAAQS review process, including
revisions to the process, is discussed at https://www.epa.gov/naaqs/historical-information-naaqs-review-process.
---------------------------------------------------------------------------

    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 PM2.5, the EPA revised the level of the annual
PM2.5 standard \11\ to 12.0 [micro]g/m\3\ and retained the
24-hour PM2.5 standard, with its level of 35 [micro]g/m\3\.
For the primary PM10 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.,
PM10-2.5). With regard to the secondary PM standards, the
EPA generally retained the 24-hour and annual PM2.5
standards \12\ and the 24-hour PM10 standard to address
visibility and non-visibility welfare effects.
---------------------------------------------------------------------------

    \11\ The EPA also eliminated the option for spatial averaging.
    \12\ Consistent with the primary standard, the EPA eliminated
the option for spatial averaging with the annual standard.
---------------------------------------------------------------------------

    As with previous reviews, petitioners challenged the EPA's final
rule. Petitioners argued that the EPA acted unreasonably in revising
the level and form of the annual standard and in amending the
monitoring network provisions. On judicial review, the revised
standards and monitoring requirements were upheld in all respects. NAM
v. EPA, 750 F.3d 921 (D.C. Cir. 2014).

D. Current Review of the Air Quality Criteria and Standards

    In December 2014, the EPA announced the initiation of the current
periodic review of the air quality criteria for PM and of the
PM2.5 and PM10 NAAQS and issued a call for
information (79 FR 71764, December 3, 2014). From February 9 to
February 11, 2015, the EPA's NCEA and OAQPS held a public workshop to
inform the planning for the current 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 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 this review. This workshop provided
for a public discussion of the key science and policy-relevant issues
around which the EPA has structured the current review of the PM NAAQS
and of the most meaningful new scientific information that would be
available in this review to inform understanding of these issues.
    The input received at the workshop guided EPA staff in developing a
draft IRP, which was reviewed by the CASAC Particulate Matter Review
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 chartered CASAC, supplemented by the Particulate Matter
Review 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 this review and the key policy-relevant issues.
    In May 2018, the Administrator issued a memorandum describing a
``back-to-basics'' process for reviewing the NAAQS (Pruitt, 2018). This
memo announced the Agency's intention to conduct the current review of
the PM NAAQS in such a manner as to ensure that any necessary revisions
are finalized by December 2020. Following this memo, on October 10,
2018 the Administrator additionally announced that the role of
reviewing the key assessments developed as part of the ongoing review
of the PM NAAQS (i.e., drafts of the ISA and PA) would be performed by
the seven-member chartered CASAC (i.e., rather than the CASAC
Particulate Matter Panel that reviewed the draft IRP).\13\
---------------------------------------------------------------------------

    \13\ The CASAC charter is available at: https://
yosemite.epa.gov/sab/sabproduct.nsf/WebCASAC/2019casaccharter/$File/
CASAC%202019%20Renewal%20Charter%203.21.19%20-%20final.pdf. The
Administrator's announcement is available at: https://archive.epa.gov/epa/newsreleases/acting-administrator-wheeler-announces-science-advisors-key-clean-air-act-committee.html.
---------------------------------------------------------------------------

    The EPA released the draft ISA in October 2018 (83 FR 53471,
October 23, 2018). The draft ISA was reviewed by the chartered CASAC at
a public meeting held in Arlington, VA in December 2018 (83 FR 55529,
November 6, 2018) and was discussed on a public teleconference in March
2019 (84 FR 8523, March 8, 2019). The CASAC provided its advice on the
draft ISA in a letter to the EPA Administrator dated April 11, 2019
(Cox, 2019b). In that letter, the CASAC's recommendations address both
the draft ISA's assessment of the science for PM-related effects and
the process under which this review of the PM NAAQS is being conducted.
    Regarding the assessment of the evidence, the CASAC letter states
that ``the Draft ISA does not provide a sufficiently comprehensive,
systematic assessment of the available science relevant to
understanding the health impacts of exposure to particulate matter
(PM)'' (Cox, 2019b, p. 1 of letter). The CASAC recommended that this
and other limitations (i.e., ``[i]nadequate evidence for altered causal
determinations'' and the need for a ``[c]learer discussion of causality
and causal biological mechanisms and pathways'') be remedied in a
revised ISA (Cox, 2019b, p. 1 of letter).
    Given the Administrator's timeline for this review, as noted above
(Pruitt, 2018), the EPA did not prepare a second draft ISA (Wheeler,
2019). Rather, the EPA has taken steps to address the CASAC's comments
in the final ISA (U.S. EPA, 2019). In particular, the final ISA
includes additional text and a new appendix to clarify the
comprehensive and systematic process employed by the EPA to develop the
ISA. In addition, several causality determinations were re-examined
and, consistent with the CASAC advice, the final ISA reflects a revised
causality determination for long-term ultrafine particle (UFP)
exposures and nervous system effects (i.e., from ``likely to be
causal'' to ``suggestive of, but not sufficient to infer, a causal
relationship'').\14\ The final ISA also contains additional text to
clarify the evidence for biological pathways of particular PM-related
effects and the role of that evidence in causality determinations.
---------------------------------------------------------------------------

    \14\ Based on the CASAC's comments, the EPA also re-examined the
causality determinations for cancer and for nervous system effects
following long-term PM2.5 exposures. The EPA's
consideration of these comments in the final ISA is described in
detail in the proposal in sections II.B.1.d (85 FR 24111, April 30,
2020) and II.B.1.e (85 FR 24113, April 30, 2020).
---------------------------------------------------------------------------

    Among its comments on the process, the chartered CASAC recommended
``that the EPA reappoint the previous CASAC PM panel (or appoint a
panel with similar expertise)'' (Cox, 2019b). The Agency's response to
this advice was provided in a letter from the Administrator to the
CASAC chair dated

July 25, 2019.\15\ In that letter, the Administrator announced his
intention to identify a pool of non-member subject matter expert
consultants to support the CASAC's review activities for the PM and
ozone NAAQS. A Federal Register notice requesting the nomination of
scientists from a broad range of disciplines ``with demonstrated
expertise and research in the field of air pollution related to PM and
ozone'' was published in August 2019 (84 FR 38625, August 7, 2019). The
Administrator selected consultants from among those nominated, and
input from members of this pool of consultants informed the CASAC's
review of the draft PA.
---------------------------------------------------------------------------

    \15\ Available at: https://yosemite.epa.gov/sab/sabproduct.nsf/
0/6CBCBBC3025E13B4852583D90047B352/$File/EPA-CASAC-19-
002_Response.pdf.
---------------------------------------------------------------------------

    The EPA released the draft PA in September 2019 (84 FR 47944,
September 11, 2019). The draft PA drew from the assessment of the
evidence in the draft ISA. It was reviewed by the chartered CASAC and
discussed in October 2019 at a public meeting held in Cary, NC. Public
comments were received via a separate public teleconference (84 FR
51555, September 30, 2019). A public meeting to discuss the chartered
CASAC letter and response to charge questions on the draft PA was held
in Cary, NC in December 2019 (84 FR 58713, November 1, 2019), and the
CASAC provided its advice on the draft PA, including its advice on the
current primary and secondary PM standards, in a letter to the EPA
Administrator dated December 16, 2019 (Cox, 2019a).
    With regard to the primary standards, the CASAC recommended
retaining the current 24-hour PM2.5 and PM10
standards but did not reach consensus on the adequacy of the current
annual PM2.5 standard. With regard to the secondary
standards, the CASAC recommended retaining the current standards. The
CASAC's advice on the primary and secondary PM standards, and the
Administrator's consideration of that advice in reaching proposed
decisions, is discussed in detail in sections II.C.2 and II.C.3
(primary PM2.5 standards), III.C.2 and III.C.3 (primary
PM10 standards), and IV.D.2 and IV.D.3 (secondary standards)
of the proposal notice (85 FR 24094, April 30, 2020).
    The CASAC additionally made a number of recommendations regarding
the information and analyses presented in the draft PA. Specifically,
the CASAC recommended that a revised PA include: (1) Additional
discussion of the current CASAC and NAAQS review process; (2)
additional characterization of PM-related emissions, monitoring and air
quality information, including uncertainties in that information; (3)
additional discussion and examination of uncertainties in the
PM2.5 health evidence and the risk assessment; (4) updates
to reflect changes in the ISA's causality determinations; and (5)
additional discussion of the evidence for PM-related welfare effects,
including uncertainties (Cox, 2019a, pp. 2-3 in letter). In response to
the CASAC's comments, the final PA \16\ incorporated a number of
changes, as described in detail in section I.C.5 of the proposal (85 FR
24100, April 2020).
---------------------------------------------------------------------------

    \16\ Given the Administrator's timeline for this review, as
noted above (Pruitt, 2018), the EPA did not prepare a second draft
PA. Rather, the CASAC's advice was considered in developing the
final PA (U.S. EPA, 2020).
---------------------------------------------------------------------------

    Drawing from his consideration of the scientific evidence assessed
in the ISA and the analyses in the PA, including uncertainties in the
evidence and analyses, and from his consideration of advice from the
CASAC, on April 14, 2020 the Administrator 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 held virtual public
hearings on the proposal on May 20-22, 2020 and May 27, 2020 (85 FR
26634, May 5, 2020). In total, the EPA received more than 66,000
comments on the proposal from members of the public and various
stakeholder groups by the close of the public comment period on June
29, 2020. 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''), can be found in the docket for this
rulemaking (Docket No. EPA-HQ-OAR-2015-0072).
    As in prior NAAQS reviews, the EPA is basing its decision in this
review on studies and related information included in the air quality
criteria, which have undergone CASAC and public review. The studies
assessed in the ISA \17\ and PA, and the integration of the scientific
evidence presented in them, have undergone extensive critical review by
the EPA, the CASAC, and the public. The rigor of that review makes
these studies, and their integrative assessment, the most reliable
source of scientific information on which to base decisions on the
NAAQS, decisions that all parties recognize as of great import.
Decisions on the NAAQS can have profound impacts on public health and
welfare, and NAAQS decisions 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.
Some commenters have referred to and discussed individual scientific
studies on the health effects of PM that were not included in the ISA
(``'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 ISA.
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 reopening the
review of the air quality criteria to enable the EPA, the CASAC, and
the public to consider them further.
---------------------------------------------------------------------------

    \17\ Studies identified for the ISA were based on the review's
opening ``call for information'' (79 FR 71764, December 3, 2014), as
well as literature searches conducted routinely to identify and
evaluate ``studies and reports that have undergone scientific peer
review and were published or accepted for publication between
January 1, 2009 and March 31, 2017. A limited literature update
identified some additional studies that were published before
December 31, 2017'' (U.S. EPA, 2019, Appendix, p. A-3). References
that are cited in the ISA, the references that were considered for
inclusion but not cited, and electronic links to bibliographic
information and abstracts can be found at: https://hero.epa.gov/hero/particulate-matter.
---------------------------------------------------------------------------

    This approach, and the decision to rely on studies and related
information included 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 61144, 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 revise the
O3 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 at 13013-13014, March 9,
1993). In the present case, the EPA's provisional consideration of
``new'' studies concludes 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. For this reason,
reopening the air quality criteria review would not be warranted.
    Accordingly, the EPA is basing the final decisions in this review
on the studies and related information included in the PM air quality
criteria that have undergone rigorous review by the EPA, 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 review and which
will provide the opportunity to fully assess these studies through a
more rigorous review process involving the EPA, CASAC, and the public.

E. 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 (I.E.1), sources and emissions
contributing to PM in the ambient air (I.E.2), ambient PM
concentrations and trends in the U.S. (I.E.3), and background PM
(I.E.4). Additional detail on PM air quality can be found in Chapter 2
of the Policy Assessment (U.S. EPA, 2020; PA) and section I.D of the
proposal (85 FR 24100, April 30, 2020).
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, 2019, section 2.2) and
distinct health and welfare effects have been linked with exposures to
particles of different sizes. Particles in the atmosphere range in size
from less than 0.01 to more than 10 [mu]m in diameter (U.S. EPA, 2019,
section 2.2).The EPA defines PM2.5, 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 PM10-2.5, 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. PM10, 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, 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, 2019, section 2.2). Atmospheric lifetimes are generally
longest for PM2.5, which often remains in the atmosphere for
days to weeks (U.S. EPA, 2019, Table 2-1) before being removed by wet
or dry deposition, while atmospheric lifetimes for UFP and
PM10-2.5 are shorter and are generally removed from the
atmosphere within hours, through wet or dry deposition (U.S. EPA, 2019,
Table 2-1; 85 FR 24100, April 30, 2020).
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 chemical compounds present in the atmosphere that have
participated in new particle formation or condensed onto existing
particles (U.S. EPA, 2019, section 2.3). As discussed further in the
ISA (U.S. EPA, 2019, section <IP_ADDRESS>), secondary PM is formed in the
atmosphere by photochemical oxidation reactions of both inorganic and
organic gas-phase precursors. Sources and emissions of PM are discussed
in more detail the PA (U.S. EPA, 2020, section 2.1.1) and in the
proposal (85 FR 24101, April 30, 2020).
3. 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.E.3.a and I.E.3.b summarize information on PM2.5 mass and
components, respectively. Section I.E.3.c summarizes information on
PM10. Sections I.E.3.d and I.E.3.e summarize the more
limited information on PM10-2.5 and UFP, respectively.
Additional detail on PM air quality and trends can be found in the PA
(U.S. EPA, 2020, section 2.3) and in the proposal (85 FR 24100, April
30, 2020).
a. PM2.5 Mass
    At monitoring sites in the U.S., annual PM2.5
concentrations from 2015 to 2017 averaged 8.0 [mu]g/m\3\ (and ranged
from 3.0 to 18.2 [mu]g/m\3\) and the 98th percentiles of 24-hour
concentrations averaged 20.9 [mu]g/m\3\ (and ranged from 9.2 to 111
[mu]g/m\3\) (U.S. EPA, 2020, section <IP_ADDRESS>). The highest ambient
PM2.5 concentrations occur in the west, particularly in
California and the Pacific northwest (U.S. EPA, 2020, Figure 2-8). 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, 2020, section 2.3.2).
    Recent ambient PM2.5 concentrations reflect the
substantial reductions that have occurred across much of the U.S. (U.S.
EPA, 2020, section <IP_ADDRESS>). From 2000 to 2017, national annual average
PM2.5 concentrations have declined from 13.5 [mu]g/m\3\ to
8.0 [mu]g/m\3\, a 41% decrease (U.S. EPA, 2020, section <IP_ADDRESS>).\18\
These declines have occurred at urban and rural monitoring sites,
although urban PM2.5 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 PM2.5 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 have declined significantly (U.S. EPA, 2020,
section <IP_ADDRESS>). In contrast, trends in ambient PM2.5
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,
2020, section <IP_ADDRESS>).
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    \18\ See https://www.epa.gov/air-trends/particulate-matter-pm25-trends and https://www.epa.gov/air-trends/particulate-matter-pm25-trends#pmnat for more information.
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    The recent deployment of PM2.5 monitors near major roads
in large urban areas provides information on PM2.5
concentrations near an important emissions source. Of the 25 CBSAs with
valid design values at near-road monitoring sites,\19\ 52% measured the
highest annual design value at the near-road site while 24% measured
the highest 24-hour design value at the near-road site (U.S. EPA, 2020,
section <IP_ADDRESS>). Of the CBSAs with highest annual design values at
near-road sites, those design values were, on average, 0.7 [mu]g/m\3\
higher than at the highest measuring non-near-road sites (range is 0.1
to 2.0 [mu]g/m\3\ higher at near-road sites). Although most near-road
monitoring sites do not have sufficient data to evaluate long-term
trends in

near-road PM2.5 concentrations, analyses of the data at one
near-road-like site in Elizabeth, NJ,\20\ 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, 2020, section
<IP_ADDRESS>).
---------------------------------------------------------------------------

    \19\ A design value is considered valid if it meets the data
handling requirements given in 40 CFR Appendix N to part 50.
    \20\ 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.
---------------------------------------------------------------------------

b. PM2.5 Components
    Based on recent air quality data, the major chemical components of
PM2.5 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, 2020, section
<IP_ADDRESS>). 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 PM2.5 composition trends can provide
insight into the factors contributing to overall reductions in ambient
PM2.5 concentrations. The biggest change in PM2.5
composition that has occurred in recent years is the reduction in
sulfate concentrations due to reductions in SO2 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 PM2.5 mass in many locations (U.S.
EPA, 2019, Figure 2-19). The overall reduction in sulfate
concentrations has contributed substantially to the decrease in
national average PM2.5 concentrations as well as the decline
in the fraction of PM10 mass accounted for by
PM2.5 (U.S. EPA, 2019, section <IP_ADDRESS>.6; U.S. EPA, 2020,
section 2.3.1).
c. PM10
    At monitoring sites in the U.S., the 2015-2017 average of 2nd
highest 24-hour PM10 concentration was 56 [mu]g/m\3\
(ranging from 18 to 173 [mu]g/m\3\) (U.S. EPA, 2020, section
<IP_ADDRESS>).\21\ The highest PM10 concentrations tend to occur
in the western U.S. Seasonal analyses indicate that ambient
PM10 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, 2019,
Table 2-5). This is due to fact that the major PM10 emission
sources, dust and agriculture, are more active during the warmer and
drier periods of the year.
---------------------------------------------------------------------------

    \21\ The form of the current 24-hour PM10 standard is
one-expected-exceedance, averaged over three years.
---------------------------------------------------------------------------

    Recent ambient PM10 concentrations reflect reductions
that have occurred across much of the U.S. (U.S. EPA, 2020, section
<IP_ADDRESS>). From 2000 to 2017, annual second highest 24-hour
PM10 concentrations have declined by about 30% (U.S. EPA,
2020, section <IP_ADDRESS>).\22\ These PM10 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, 2020, section <IP_ADDRESS>). Analyses at individual
monitoring sites indicate that annual average PM10
concentrations have also declined at most sites across the U.S., with
much of the decrease in the eastern U.S. associated with reductions in
PM2.5 concentrations.
---------------------------------------------------------------------------

    \22\ For more information, see https://www.epa.gov/air-trends/particulate-matter-pm10-trends#pmnat.
---------------------------------------------------------------------------

d. PM10-2.5
    Since the last review, the availability of PM10-2.5
ambient concentration data has greatly increased because of additions
to the PM10-2.5 monitoring capabilities to the national
monitoring network. As illustrated in the PA (U.S. EPA, 2020, section
<IP_ADDRESS>), annual average and 98th percentile PM10-2.5
concentrations exhibit less distinct differences between the eastern
and western U.S. than for either PM2.5 or PM10.
Additionally, compared to PM2.5 and PM10, changes
in PM10-2.5 concentrations have been small in magnitude and
inconsistent in direction (U.S. EPA, 2020, section <IP_ADDRESS>).
e. UFP
    Compared to PM2.5 mass, there is relatively little data
on U.S. particle number concentrations, which are dominated by UFP.
Based on measurements in two urban areas (New York City, Buffalo) and
at a background site (Steuben County) in New York, urban particle
number counts were several times higher than at the background site
(U.S. EPA, 2020, section <IP_ADDRESS>; U.S. EPA, 2019, Figure 2-18). The
highest particle number counts in an urban area with multiple sites
(Buffalo) were observed at a near-road location.
    Long-term trends in UFP are not routinely available at U.S.
monitoring sites. At one site in Illinois with long-term data
available, the annual average particle number concentration declined
between 2000 and 2017, closely matching the reductions in annual
PM2.5 mass over that same period (U.S. EPA, 2020, section
<IP_ADDRESS>). 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 PM2.5 (U.S. EPA, 2020, section <IP_ADDRESS>).
4. Background PM
    In this review, background PM is defined as all particles that are
formed by sources or processes that cannot be influenced by actions
within the jurisdiction of concern. U.S. background PM is defined as
any PM formed from emissions other than U.S. anthropogenic (i.e.,
manmade) emissions. Potential sources of U.S. background PM include
both natural sources (i.e., PM that would exist in the absence of any
anthropogenic emissions of PM or PM precursors) and transboundary
sources originating outside U.S. borders. Background PM is discussed in
more detail in the PA (U.S. EPA, 2020, section 2.4) and in the proposal
(85 FR 24102, April 30, 2020). At annual and national scales, estimated
background PM concentrations in the U.S. are small compared to
contributions from domestic anthropogenic emissions.\23\ For example,
based on zero-out modeling in the last review of the PM NAAQS, annual
background PM2.5 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 last 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 places.
---------------------------------------------------------------------------

    \23\ 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
SO2 and oceanic production of dimethyl-sulfide (U.S. EPA,
2020, 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, 2019, 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 retain the current primary PM2.5 standards. This
decision is based on a thorough review in the ISA of the latest
scientific information, published through December 2017,\24\ on human
health effects associated with long-and short-term exposures to
PM2.5 in the ambient air. This decision also takes into
account analyses in the PA of policy-relevant information from the ISA,
as well as information on air quality; the analyses of human health
risks in the PA; CASAC advice; and consideration of public comments
received on the proposal.
---------------------------------------------------------------------------

    \24\ In addition to the review's opening ``call for
information'' (79 FR 71764, December 3, 2014), ``the current ISA
identified and evaluated studies and reports that have undergone
scientific peer review and were published or accepted for
publication between January 1, 2009 and March 31, 2017. A limited
literature update identified some additional studies that were
published before December 31, 2017'' (U.S. EPA, 2019, Appendix, p.
A-3). References that are cited in the ISA, the references that were
considered for inclusion but not cited, and electronic links to
bibliographic information and abstracts can be found at: https://hero.epa.gov/hero/particulate-matter.
---------------------------------------------------------------------------

    Section II.A provides background on the general approach for this
review 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 proposed conclusions and
CASAC advice, addresses public comments received on the proposal and
presents the Administrator's conclusions on the adequacy of the current
standard, 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 PM2.5 standards.

A. Introduction

    As in prior reviews, the general approach to reviewing the current
primary PM2.5 standards is based, most fundamentally, on
using the EPA's assessment of current scientific evidence and
associated quantitative analyses to inform the Administrator's judgment
regarding primary PM2.5 standards that protects public
health with an adequate margin of safety. In drawing conclusions with
regard to the primary PM2.5 standards, the final decision on
the adequacy of the standard is largely a public health policy judgment
to be made by the Administrator. The Administrator's final decision
draws upon scientific information and analyses about health effects,
population risks, as well as judgments about how to consider the range
and magnitude of uncertainties that are inherent in the scientific
evidence and risk analyses. The approach to informing these judgments,
discussed more fully below, 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 CAA
and with how the EPA and the courts have historically interpreted the
Act. These provisions require the Administrator to establish primary
standards that, in his judgment, are requisite to protect public health
with an adequate margin of safety. In so doing, the Administrator seeks
to establish standards that are neither more nor less stringent than
necessary for this purpose. The Act does not require that primary
standards be set at a zero-risk level, but rather at a level that
avoids unacceptable risks to public health including the health of
sensitive groups.\25\ The four basic elements of the NAAQS (indicator,
averaging time, form, and level) are considered collectively in
evaluating the health protection afforded by a standard.
---------------------------------------------------------------------------

    \25\ As noted in section I.A above, such protection is specified
for the sensitive group of individuals and not to a single person in
the sensitive group (see S. Rep. No. 91-1196, 91st Cong., 2d Sess.
10 [1970]).
---------------------------------------------------------------------------

    In evaluating the appropriateness of retaining or revising the
current primary PM2.5 standards, the EPA has adopted an
approach that builds upon the general approach used in the last review
and reflects the body of evidence of information now available. As
summarized in section II.A.1 below, the Administrator's decisions in
the prior review were based on an integration of information on health
effects associated with exposure to PM2.5 with information
on the public health significance of key health effects, as well as on
policy judgments as to when the standard is requisite to protect public
health with an adequate margin of safety and on consideration of advice
from the CASAC and public comments. These decisions were also informed
by air quality and related analyses and quantitative risk information.
    Similarly, in this review, as described in the PA, the proposal,
and elsewhere in this document, we draw on the current evidence and
quantitative assessments of public health risk of PM2.5 in
ambient air. The past and current approaches are both based, most
fundamentally, on the EPA's assessments of the current scientific
information and associated quantitative analyses. The EPA's assessments
are primarily documented in the ISA and PA, which have received CASAC
review and public comment (83 FR 53471, October 23, 2018; 83 FR 55529,
November 6, 2018; 84 FR 8523, March 8, 2019; 84 FR 47944, September 11,
2019; 84 FR 51555, September 30, 2019; 84 FR 58713, September 30,
2019). To bridge the gap between the scientific assessments of the ISA
and quantitative assessments of the PA and the judgments required of
the Administrator in determining whether the current standard remains
requisite to protect public health with an adequate margin of safety,
the PA evaluates the policy implications of the current evidence in the
ISA and of the quantitative analyses in the PA.
    In considering the scientific and technical information, we
consider both the information available at the time of the last review
and information newly available since the last review, including most
particularly that which has been critically analyzed and characterized
in the current ISA. We additionally consider the quantitative risk
information described in the PA that estimated population-level health
risks associated with ambient PM2.5 concentrations that have
been adjusted to simulate air quality scenarios of policy interest
(e.g., ``just meeting'' the current standards) in multiple study areas.
The evidence-based discussions presented below (and summarized more
fully in the proposal) draw upon evidence from studies evaluating
health effects related to exposures to PM2.5, as discussed
in the ISA. The risk-based discussions also presented below (and
summarized more fully in the proposal) have been drawn from the
quantitative analyses for PM2.5, as discussed in the PA.
Sections II.A.2 and II.A.3 below provide an overview for the current
health effects evidence related to short- and long-term exposures to
PM2.5 and quantitative risk information with a focus on
specific policy-relevant questions identified for these categories of
information in the PA.

1. Background on the Current Standards
    The last review of the primary PM NAAQS was completed in 2012 (78
FR 3086, January 15, 2013). As noted above (section I.C.4), in the last
review the EPA lowered the level of the primary annual PM2.5
standard from 15.0 to 12.0 [mu]g/m\3\,\26\ and retained the existing
24-hour PM2.5 standard with its level of 35 [mu]g/m\3\. The
2012 decision to strengthen the suite of primary PM2.5
standards was based on the prior Administrator's consideration of the
extensive body of scientific evidence assessed in the 2009 ISA (U.S.
EPA, 2009c); the quantitative risk analyses presented in the 2010
health risk assessment (U.S. EPA, 2010a); the advice and
recommendations of the CASAC (Samet, 2009; Samet, 2010c; Samet, 2010b);
and public comments on the proposed rule (78 FR 3086, January 15, 2013;
U.S. EPA, 2012). In particular, she noted the ``strong and generally
robust body of evidence of serious health effect associated with both
long- and short-term exposures to PM2.5'' (78 FR 3120,
January 15, 2013). This included epidemiological studies reporting
health effect associations based on long-term average PM2.5
concentrations ranging from about 15.0 [micro]g/m\3\ or above (i.e., at
or above the level of the then-existing annual standard) to
concentrations ``significantly below the level of the annual standard''
(78 FR 3120, January 15, 2013). Based on her ``confidence in the
association between exposure to PM2.5 and serious public
health effects, combined with evidence of such an association in areas
that would meet the current standards'' (78 FR 3120, January 15, 2013),
the prior Administrator concluded that revision of the suite of primary
PM2.5 standards was necessary in order to provide increased
public health protection.
---------------------------------------------------------------------------

    \26\ The Agency also eliminated spatial averaging provisions as
part of the form of the annual standard.
---------------------------------------------------------------------------

    The prior Administrator next considered what specific revisions to
the existing primary PM2.5 standards were appropriate, given
the available evidence and quantitative risk information. She
considered both the annual and 24-hour PM2.5 standards,
focusing on the basic elements of those standards (i.e., indicator,
averaging time, form, and level). With regard to the indicator, the EPA
recognized that the health studies available during the last review
continued to link adverse health outcomes (e.g., premature mortality,
hospital admissions, emergency department visits) with long- and short-
term exposures to PM2.5 (78 FR 3121, January 15, 2013). In
assessing the appropriateness of PM2.5 mass as the
indicator, the EPA also considered the available scientific evidence
and information available related to ultrafine particles
27 28 and PM components,\29\ noting the significant
uncertainties and limitations associated with the evidence, as well as
the availability of monitoring data. Consistent with the considerations
and conclusions in the 2011 PA, the CASAC advised that it was
appropriate to consider retaining PM2.5 as the indicator for
fine particles. In light of the evidence and the CASAC's advice, the
prior Administrator concluded that it was ``appropriate to retain
PM2.5 as the indicator for fine particles'' (78 FR 3123,
January 15, 2013).
---------------------------------------------------------------------------

    \27\ In the last review, the ISA defined ultrafine particles
(UFP) as generally including particles with a mobility diameter less
than or equal to 0.1 [micro]m. Mobility diameter is defined as the
diameter of a particle having the same diffusivity or electrical
mobility in air as the particle of interest and is often used to
characterize particles of 0.5 [micro]m or smaller (U.S. EPA, 2009c,
pp. 3-2 to 3-3).
    \28\ The 2011 PA noted the limited body of evidence assessed in
the 2009 ISA (summarized in U.S. EPA, 2009c, section 2.3.5 and Table
2-6) and the limited monitoring information available to
characterized ambient concentrations of UFP (U.S. EPA, 2011, section
1.3.2).
    \29\ The 2009 ISA concluded that ``the evidence is not yet
sufficient to allow differentiation of those constituents or sources
that are more closely related to specific health outcomes'' (U.S.
EPA, 2009c, pp. 2-26 and 6-212; 78 FR 3123, January 15, 2013). The
2011 PA further noted that ``many different constituents of the fine
particle mixture as well as groups of components associated with
specific source categories of fine particles are linked to adverse
health effects'' (U.S. EPA, 2011, p. 2-55; 78 FR 3123, January 15,
2013).
---------------------------------------------------------------------------

    With regard to averaging time, in the last review, the EPA
considered issues related to the appropriate averaging time for
PM2.5 standards, with a focus on evaluating support for the
existing annual and 24-hour averaging times and for potential
alternative averaging times based on sub-daily or seasonal metrics.
Based on the evidence assessed in the 2009 ISA, the 2011 PA noted that
the overwhelming majority of studies utilized annual (or multi-year) or
24-hour PM averaging periods (U.S. EPA, 2011, section 2.3.2). Given
this evidence-base, and limitations in the data for alternatives, the
2011 PA reached the overall conclusions that the available information
provided strong support for considering retaining the existing annual
and 24-hour averaging times (U.S. EPA, 2011, p. 2-58). The CASAC agreed
that these conclusions were reasonable (Samet, 2010a, p. 2-58). The
prior Administrator concurred with the CASAC's advice. Specifically,
she judged that it was ``appropriate to retain the current annual and
24-hour averaging times for the primary PM2.5 standards to
protect against health effects associated with long- and short-term
exposure periods'' (78 FR 3124, January 15, 2013).
    With regard to form, the EPA first noted that the form of the
annual PM2.5 standard was established in 1997 as an annual
arithmetic mean, averaged over 3 years, from single or multiple
community-oriented monitors.\30\ That is, the level of the annual
standard was to be compared to measurements made at each community-
oriented monitoring site, or if criteria were met, measurements from
multiple community-oriented monitoring sites could be averaged together
(i.e., spatial averaging) \31\ (62 FR 38671-38672, July 18, 1997). In
the 1997 review, the EPA also established the form of the 24-hour
PM2.5 standard as the 98th percentile of 24-hour
concentrations at each monitor within an area (i.e., no spatial
averaging), averaged over three years (62 FR 38671-38674, July 18,
1997). In the 2006 review, the EPA retained these standard forms but
tightened the criteria for using spatial averaging with the annual
standard (71 FR 61167, October 17, 2006).\32\
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    \30\ In the last review, the EPA replaced the term ``community-
oriented'' monitor with the term ``area-wide'' monitor (U.S. EPA,
2020, section 1.3). 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). CBSAs are required to have at least one area-wide
monitor sited in the area of expected maximum PM2.5
concentration.
    \31\ The original criteria for spatial averaging included: (1)
The annual mean concentration at each site shall be within 20% of
the spatially averaged annual mean, and (2) the daily values for
each monitoring site pair shall yield a correlation coefficient of
at least 0.6 for each calendar quarter (62 FR 38671-38672, July 18,
1997).
    \32\ Specifically, the Administrator revised spatial averaging
criteria such that ``(1) [t]he annual mean concentration at each
site shall be within 10 percent of the spatially averaged annual
mean, and (2) the daily values for each monitoring site pair shall
yield a correlation coefficient of at least 0.9 for each calendar
quarter'' (71 FR 61167, October 17, 2006).
---------------------------------------------------------------------------

    At the time of the last review, the EPA again considered the form
of the standard with a focus on the issue of spatial averaging. An
analysis of air quality and population demographic information
indicated that the highest PM2.5 concentrations in a given
area tended to be measured at monitors in locations where the
surrounding populations were more likely to live below the poverty line
and to include larger percentages of racial and ethnic minorities (U.S.
EPA, 2011, p. 2-60). Based on this analysis, the 2011 PA concluded that
spatial averaging could result in disproportionate impacts in at-risk
populations and populations with

lower socioeconomic status (SES). Therefore, the PA concluded that it
was appropriate to consider revising the form of the annual
PM2.5 standard such that it did not allow for the use of
spatial averaging across monitors (U.S. EPA, 2011, p. 2-60). The CASAC
agreed with the PA conclusions that it was ``reasonable'' for the EPA
to eliminate the spatial averaging provisions (Samet, 2010c, p. 2).
    With regard to the form of the annual PM2.5 standard,
the prior Administrator concluded that public health would not be
protected with an adequate margin of safety in all locations if
disproportionately higher PM2.5 concentrations in low income
and minority communities were averaged together with lower
concentrations measured at other sites in a larger urban area.
Therefore, she concluded that the form of the annual PM2.5
standard should be revised to eliminate spatial averaging provisions
(78 FR 3124, January 15, 2013).
    With regard to the form of the 24-hour PM2.5 standard,
the EPA recognized that the existing 98th percentile form was
originally selected to provide a balance between limiting the
occurrence of peak 24-hour PM2.5 concentrations and
identifying a stable target for risk management programs.\33\ Updated
air quality analyses in the last review provided additional support for
the increased stability of the 98th percentile PM2.5
concentration, compared to the 99th percentile (U.S. EPA, 2011, Figure
2-2, p. 2-62). Consistent with the PA conclusions based on this
analysis, the prior Administrator concluded that it was appropriate to
retain the 98th percentile form for the 24-hour PM2.5
standard (78 FR 3127, January 15, 2013).
---------------------------------------------------------------------------

    \33\ See ATA III, 283 F.3d at 374-76 which concludes that it is
legitimate for the EPA to consider overall stability of the standard
and its resulting promotion of overall effectiveness of NAAQS
control programs in setting a standard that is requisite to protect
the public health.
---------------------------------------------------------------------------

    With regard to alternative levels of the annual and 24-hour
PM2.5 standards, in the last review, the EPA considered the
public health protection provided by the standards, taken together,
against mortality and morbidity effects associated with long- or short-
term PM2.5 exposures. This approach recognized that it is
appropriate to consider the protection provided by attaining the air
quality needed to meet the suite of standards, and that there is no
bright line clearly directing the choice of levels. Rather, the choice
of what is appropriate is a public health policy judgment entrusted to
the Administrator. See Mississippi, 744 F.3d at 1358, Lead Industries
Ass'n, 647 F.2d at 1147.
    In selecting the levels of the annual and 24-hour PM2.5
standards, the prior Administrator placed the greatest emphasis on
health endpoints for which the evidence was strongest, based on the
assessment of the evidence in the ISA and on the ISA's causality
determinations (U.S. EPA, 2009c, section 2.3.1). She particularly noted
that the evidence was sufficient to conclude a causal relationship
exists between PM2.5 exposures and mortality and
cardiovascular effects (i.e., for both long- and short-term exposures)
and that the evidence was sufficient to conclude a causal relationship
is ``likely'' to exist between PM2.5 exposures and
respiratory effects (i.e., for both long- and short-term exposures).
She also noted additional, but more limited, evidence for a broader
range of health endpoints, including evidence ``suggestive of a causal
relationship'' between long-term exposures and developmental and
reproductive effects as well as carcinogenic effects (78 FR 3158,
January 15, 2013).
    To inform her decisions on an appropriate level for the annual
standard, the Administrator considered the degree to which
epidemiological studies indicate confidence in the reported health
effect associations over distributions of PM2.5
concentrations in ambient air. She noted that a level of 12.0 [micro]g/
m\3\ was below the long-term mean PM2.5 concentrations
reported in key epidemiological studies that provided evidence of an
array of serious health effects (78 FR 3161, January 15, 2013). She
further noted that 12.0 [micro]g/m\3\ generally corresponded to the
lower portions (i.e., about the 25th percentile) of distributions of
health events in the limited number of epidemiological studies for
which population-level information was available. A level of 12.0
[micro]g/m\3\ also reflected placing some weight on studies of
reproductive and developmental effects, for which the evidence was more
uncertain (78 FR 3161-3162, January 15, 2013).
    Given the uncertainties remaining in the scientific evidence, the
Administrator judged that an annual standard level below 12.0 [micro]g/
m\3\ was not supported. She specifically noted uncertainties related to
understanding the relative toxicity of the different components in the
fine particle mixture, the role of PM2.5 in the complex
ambient mixture, exposure measurement error in epidemiological studies,
and the nature and magnitude of estimated risks at relatively low
ambient PM2.5 concentrations. Furthermore, she noted that
epidemiological studies had reported heterogeneity in effect estimates
both within and between cities and in geographic regions of the U.S.
She recognized that this heterogeneity may be attributed, in part, to
difference in PM2.5 composition in different regions and
cities. With regard to evidence for reproductive and developmental
effects, the prior Administrator recognized that there were a number of
limitations associated with this body of evidence, including the
limited number of studies evaluating such effects; uncertainties
related to identifying the relevant exposure time periods of concern,
and limited toxicologic evidence providing information on the mode of
action(s) or biological plausibility for an association between long-
term PM2.5 exposures and adverse birth outcomes. On balance,
she found that the available evidence, interpreted in light of these
remaining uncertainties, did not justify an annual standard level set
below 12.0 [micro]g/m\3\ as being requisite to protect public health
with an adequate margin of safety (i.e., a standard with a lower level
would have been more stringent than necessary).
    In conjunction with a revised annual standard with a level of 12.0
[micro]g/m\3\, the prior Administrator concluded that the evidence
supported retaining the 35 [micro]g/m\3\ level of the 24-hour
PM2.5 standard. She noted that the existing 24-hour
standard, with its 35 [micro]g/m\3\ level and 98th percentile form,
would provide supplemental protection, particularly for areas with high
peak-to-mean ratios possibly associated with strong seasonal sources
and for areas with PM2.5-related effects that may be
associated with shorter than daily exposure periods (78 FR 3163,
January 15, 2013). Thus, she concluded that the available evidence and
information, considered together with its inherent uncertainties and
limitations, supported an annual standard with a level of 12.0
[micro]g/m\3\ combined with a 24-hour standard with a level of 35
[micro]g/m\3\.
2. Overview of Health Effects Evidence
    In this section, we provide an overview of the policy-relevant
aspects of the health effects evidence available for consideration in
this review. Section II.B of the proposal provides a detailed summary
of key information contained in the ISA (U.S. EPA, 2019) and in the PA
(U.S. EPA, 2020) on the health effects associated with PM2.5
exposures, and the related public health implications, focusing
particularly on the information most relevant to consideration of
effects associated with the presence of PM2.5 in ambient
air. The subsections below briefly

summarize the information discussed in more detail in section II.B of
the proposal (85 FR 24106 to 24114, April 30, 2020).
a. Nature of Effects
    Drawing from the assessment of the evidence in the ISA (U.S. EPA,
2019), and the summaries of that assessment in the PA (U.S. EPA, 2020),
the sections below summarize the evidence for relationships between
long- or short-term PM2.5 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
ISA concludes that the evidence supports either a ``causal'' or a
``likely to be causal'' relationship with PM2.5
exposures.\34\
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    \34\ In this review 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 ISA to
demonstrate a ``causal'' or a ``likely to be causal'' relationship
with PM exposures.
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i. Mortality
Long-Term PM2.5 Exposures
    In the last review, the 2009 ISA reported that the evidence was
``sufficient to conclude that the relationship between long-term
PM2.5 exposures and mortality is causal'' (U.S. EPA, 2009c,
p. 7-96). The strongest evidence supporting this conclusion was
provided by epidemiological 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 demonstrations that reductions in ambient PM2.5
concentrations are associated with reduced mortality risk (Laden et
al., 2006) and with 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 PM2.5 exposures and risk of mortality (U.S. EPA,
2009c).
    Recent cohort studies, which have become available since the 2009
ISA, continue to provide consistent evidence of positive associations
between long-term PM2.5 exposures and mortality. These
studies add support for associations with total and non-accidental
mortality,\35\ as well as with specific causes of death, including
cardiovascular disease and respiratory disease (U.S. EPA, 2019, section
11.2.2). Many of these recent studies have extended the follow-up
periods originally evaluated in the ACS and Harvard Six Cities cohort
studies and continue to observe positive associations between long-term
PM2.5 exposures and mortality (U.S. EPA, 2019, section
<IP_ADDRESS>, Figures 11-18 and 11-19). Adding to recent evaluations of the
ACS and Six Cities cohorts, studies conducted with other cohorts also
show consistent, positive associations between long-term
PM2.5 exposure and mortality across various demographic
groups (e.g., age, sex, occupation), spatial and temporal extents,
exposure assessment metrics, and statistical techniques (U.S. EPA,
2019, sections <IP_ADDRESS> and 11.2.5). This includes some of the largest
cohort studies conducted to date, with analyses of the U.S. Medicare
cohort that include nearly 61 million enrollees (Di et al., 2017b) and
studies that control for a range of individual and ecological
covariates.
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    \35\ 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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    A recent series of accountability studies has additionally tested
the hypothesis that past reductions in ambient PM2.5
concentrations have been associated with increased life expectancy or a
decreased mortality rate (U.S. EPA, 2019, section <IP_ADDRESS>). 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 PM2.5 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 (Correia et
al., 2013), a time period with lower ambient PM2.5
concentrations. In this follow-up study, a decrease in long-term
PM2.5 concentrations 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 PM2.5 concentration was associated
with a 0.35-year increase in life expectancy). Additional studies
conducted in the U.S. or Europe similarly report that reductions in
ambient PM2.5 are associated with improvements in longevity
(U.S. EPA, 2019, section <IP_ADDRESS>).
    The ISA concludes that positive associations between long-term
PM2.5 exposures and mortality are robust across analyses
examining a variety of study designs (e.g., U.S. EPA, 2019, section
<IP_ADDRESS>), approaches to estimating PM2.5 exposures (U.S.
EPA, 2019, section <IP_ADDRESS>), approaches to controlling for confounders
(U.S. EPA, 2019, sections 11.2.3 and 11.2.5), geographic regions and
populations, and temporal periods (U.S. EPA, 2019, sections <IP_ADDRESS>
and <IP_ADDRESS>). Recent evidence further demonstrates that associations
with mortality remain robust in copollutant analyses (U.S. EPA, 2019,
section 11.2.3), and that associations persist in analyses restricted
to long-term exposures below 12 [mu]g/m\3\ (Di et al., 2017b) or 10
[mu]g/m\3\ (Shi et al., 2016).
    Another important consideration in characterizing the potential for
additional public health improvements associated with changes in
PM2.5 exposure is whether concentration-response
relationships are linear across the range of concentrations or if
nonlinear relationships exist along any part of this range. Several
recent studies examine this issue, and continue to provide evidence of
linear, no-threshold relationships between long-term PM2.5
exposures and all-cause and cause-specific mortality (U.S. EPA, 2019,
section 11.2.4). However, interpreting the shapes of these
relationships, particularly at PM2.5 concentrations near the
lower end of the air quality distribution, can be complicated by
relatively low data density in the lower concentration range, the
possible influence of exposure measurement error, and variability among
individuals with respect to air pollution health effects. These sources
of variability and uncertainty tend to smooth and ``linearize''
population-level concentration-response functions, and thus could
obscure the existence of a threshold or nonlinear relationship (85 FR
24107, April 30, 2020).
    The biological plausibility of PM2.5-attributable
mortality is supported by the coherence of effects across scientific
disciplines (i.e., animal toxicologic, controlled human exposure
studies, and epidemiologic). The ISA outlines the available evidence
for plausible pathways by which inhalation exposure to PM2.5
could progress from initial events (e.g., respiratory tract
inflammation, autonomic nervous system modulation) to endpoints
relevant to population outcomes, particularly those related to
cardiovascular diseases such as ischemic heart disease, stroke and
atherosclerosis (U.S. EPA, 2019, section 6.2.1), and to metabolic
disease and diabetes (U.S. EPA, 2019, section 7.2.1). The ISA notes
``more limited evidence from respiratory morbidity'' (U.S. EPA, 2019,
p. 11-101) to support the biological plausibility of mortality due

to long-term PM2.5 exposures (U.S. EPA, 2019, section
11.2.1).
    Taken together, recent studies reaffirm and further strengthen the
body of evidence from the 2009 ISA for the relationship between long-
term PM2.5 exposure and mortality. Recent epidemiological
studies consistently report positive associations with mortality across
different geographic locations, populations, and analytic approaches.
Recent experimental and epidemiological evidence for cardiovascular
effects, and respiratory effects to a more limited degree, supports the
plausibility of mortality due to long-term PM2.5 exposures.
The 2019 ISA concludes that, ``collectively, this body of evidence is
sufficient to conclude that a causal relationship exists between long-
term PM2.5 exposure and total mortality'' (U.S. EPA, 2019,
section 11.2.7; p. 11-102).
Short-Term PM2.5 Exposures
    The 2009 ISA concluded that ``a causal relationship exists between
short-term exposure to PM2.5 and mortality'' (U.S. EPA,
2009c). This conclusion was based on the evaluation of both multi- and
single-city epidemiological studies that consistently reported positive
associations between short-term PM2.5 exposure and non-
accidental mortality. Examination of the potential confounding effects
of gaseous copollutants was limited, though evidence from single-city
studies indicated that gaseous copollutants have minimal effect on the
PM2.5-mortality relationship (i.e., associations remain
robust to inclusion of other pollutants in copollutant models). The
evaluation of cause-specific mortality found that effect estimates were
larger in magnitude, but also had larger confidence intervals, for
respiratory mortality compared to cardiovascular mortality. Although
the largest mortality risk estimates were for respiratory mortality,
the interpretation of the results was complicated by the limited
coherence from studies of respiratory morbidity. However, the evidence
from studies of cardiovascular morbidity provided both coherence and
biological plausibility for the relationship between short-term
PM2.5 exposure and cardiovascular mortality.
    Recent multicity studies evaluated since the 2009 ISA continue to
provide evidence of primarily positive associations between daily
PM2.5 exposures and mortality, with percent increases in
total mortality ranging from 0.19% (Lippmann et al., 2013) to 2.80%
(Kloog et al., 2013) \36\ at lags of 0 to 1 days in single-pollutant
models. Whereas most studies rely on assigning exposures using data
from ambient monitors, associations are also reported in recent studies
that employ hybrid modeling approaches using additional
PM2.5 data (i.e., from satellites, land use information, and
modeling, in addition to monitors), allowing for the inclusion of more
rural locations in analyses (Kloog et al., 2013, Shi et al., 2016, Lee
et al., 2015).
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    \36\ As detailed in the ISA, risk estimates are for a 10
[micro]g/m\3\ increase in 24-hour average PM2.5
concentrations, unless otherwise noted (U.S. EPA, 2019, Preface).
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    Some recent studies have expanded the examination of potential
confounders (e.g., U.S. EPA, 2019, section <IP_ADDRESS>) to include not
only copollutants, but also systematic evaluations of the potential
impact of inadequate control from long-term temporal trends and
weather. Associations between short-term PM2.5 exposures and
mortality remain positive and relatively unchanged in copollutant
models with both gaseous pollutants and PM10-2.5 (U.S. EPA,
2019, Section 11.1.4). Additionally, the low (r <0.4) to moderate
correlations (r = 0.4-0.7) between PM2.5 and gaseous
pollutants and PM10-2.5 increase the confidence in
PM2.5 having an independent effect on mortality (U.S. EPA,
2019, section 11.1.4).
    The generally positive associations reported with mortality are
supported by a small group of studies employing causal inference or
quasi-experimental statistical approaches (U.S. EPA, 2019, section
<IP_ADDRESS>). For example, a recent study examined whether a specific
regulatory action in Tokyo, Japan (i.e., a diesel emission control
ordinance) resulted in a subsequent reduction in daily mortality
(Yorifuji et al., 2016). The authors report a reduction in mortality in
Tokyo due to the ordinance, compared to Osaka, which did not have a
similar diesel emission control ordinance in place.
    Positive associations with total mortality are further supported by
analyses reporting positive associations with cause-specific mortality,
including cardiovascular and respiratory mortality (U.S. EPA, 2019,
section 11.1.3). For cause-specific mortality, there has been only a
limited assessment of potential copollutant confounding, though initial
evidence indicates that associations remain positive and relatively
unchanged in models with gaseous pollutants and PM10-2.5.
The evidence for ischemic events and heart failure, as detailed in the
assessment of cardiovascular morbidity (U.S. EPA, 2019, chapter 6),
provides biological plausibility for PM2.5-related
cardiovascular mortality, which comprises the largest percentage of
total mortality (i.e., ~33%) (U.S. National Institutes of Health,
2013). Although there is evidence for exacerbations of chronic
obstructive pulmonary disease (COPD) and asthma, the collective body of
evidence for respiratory effects, particularly from controlled human
exposure studies, provides only limited support for the biological
plausibility of PM2.5-related respiratory mortality (U.S.
EPA, 2019, chapter 5).
    In the 2009 ISA, one of the main uncertainties identified was the
regional and city-to-city heterogeneity in PM2.5-mortality
associations. Recent studies examine both city-specific as well as
regional characteristics to identify the underlying contextual factors
that could contribute to this heterogeneity (U.S. EPA, 2019, section
<IP_ADDRESS>). Collectively, these studies indicate that the heterogeneity
in PM2.5-mortality risk estimates cannot be attributed to
one factor, but instead to a combination of factors including, but not
limited to, PM composition and sources as well as community
characteristics that could influence exposures (U.S. EPA, 2019, section
11.1.12).
    A few recent studies have conducted analyses comparing the
traditional 24-hour average exposure metric with a sub-daily metric
(i.e., 1-hour max). These initial studies provide evidence of a similar
pattern of associations for both the 24-hour average and 1-hour max
metric, with the association larger in magnitude for the 24-hour
average metric (U.S. EPA, 2019, section <IP_ADDRESS>).
    Recent multicity studies indicate that positive and statistically
significant associations with mortality persist in analyses restricted
to short-term PM2.5 exposures below 35 [mu]g/m\3\ (Lee et
al., 2015),\37\ below 30 [mu]g/m\3\ (Shi et al., 2016), and below 25
[mu]g/m\3\ (Di et al., 2017a). Additional studies examine the shape of
the concentration-response relationship and whether a threshold exists
specifically for PM2.5 (U.S. EPA, 2019, section 11.1.10).
These studies have used various statistical approaches and consistently
found linear relationships with no evidence of a threshold. Recent
analyses provide initial evidence indicating that PM2.5-
mortality associations persist and may be stronger (i.e., a steeper
slope) at lower

concentrations (e.g., Di et al., 2017a; U.S. EPA, 2019, Figure 11-12).
However, given the limited data available at the lower end of the
distribution of ambient PM2.5 concentrations, the shape of
the concentration-response curve remains uncertain at these low
concentrations and, to date, studies have not conducted extensive
analyses exploring alternatives to linearity when examining the shape
of the PM2.5-mortality concentration-response relationship.
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    \37\ Lee et al. (2015) also report that positive and
statistically significant associations between short-term
PM2.5 exposures and mortality persist in analyses
restricted to areas with long-term concentrations below 12 [mu]g/
m\3\.
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    Overall, recent epidemiological studies build upon and extend the
conclusions of the 2009 ISA for the relationship between short-term
PM2.5 exposures and total mortality. Supporting evidence for
PM2.5-related cardiovascular morbidity, and more limited
evidence from respiratory morbidity, provides biological plausibility
for mortality due to short-term PM2.5 exposures. The
primarily positive associations observed across studies conducted in
diverse geographic locations is further supported by the results from
copollutant analyses indicating robust associations, along with
evidence from analyses of the concentration-response relationship. The
2019 ISA states that, collectively, ``this body of evidence is
sufficient to conclude that a causal relationship exists between short-
term PM2.5 exposure and total mortality'' (U.S. EPA, 2019,
p. 11-58).
ii. Cardiovascular Effects
Long-Term PM2.5 Exposures
    The scientific evidence reviewed in the 2009 ISA was ``sufficient
to infer a causal relationship between long-term PM2.5
exposure and cardiovascular effects'' (U.S. EPA, 2009c). The strongest
line of evidence comprised findings from several large epidemiological
studies of U.S. cohorts that consistently showed positive associations
between long-term PM2.5 exposure and cardiovascular
mortality (Pope et al., 2004, Krewski et al., 2009, Miller et al.,
2007, Laden et al., 2006). Studies of long-term PM2.5
exposure and cardiovascular morbidity were limited in number.
Biological plausibility and coherence with the epidemiological findings
were provided by studies using genetic mouse models of atherosclerosis
demonstrating enhanced atherosclerotic plaque development and
inflammation, as well as changes in measures of impaired heart
function, following 4- to 6-month exposures to PM2.5
concentrated ambient particles (CAPs), and by a limited number of
studies reporting CAPs-induced effects on coagulation factors, vascular
reactivity, and worsening of experimentally induced hypertension in
mice (U.S. EPA, 2009c).
    Studies conducted since the last review continue to support the
relationship between long-term exposure to PM2.5 and
cardiovascular effects. As discussed above, results from recent U.S.
and Canadian cohort studies consistently report positive associations
between long-term PM2.5 exposure and cardiovascular
mortality (U.S. EPA, 2019, Figure 6-19) in evaluations conducted at
varying spatial scales and employing a variety of exposure assessment
and statistical methods (U.S. EPA, 2019, section 6.2.10). Positive
associations between long-term PM2.5 exposures and
cardiovascular mortality are generally robust in copollutant models
adjusted for ozone, NO2, PM10-2.5, or
SO2. In addition, most of the results from analyses
examining the shape of the concentration-response relationship for
cardiovascular mortality support a linear relationship with long-term
PM2.5 exposures and do not identify a threshold below which
effects do not occur (U.S. EPA, 2019, section 6.2.16, Table 6-52).\38\
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    \38\ As noted above for mortality, uncertainty in the shape of
the concentration-response relationship increases near the upper and
lower ends of the concentration distribution where the data are
limited.
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    The available evidence examining the relationship between long-term
PM2.5 exposure and cardiovascular morbidity has greatly
expanded since the 2009 ISA, with positive associations reported in
several cohorts examining a range of cardiovascular outcomes (U.S. EPA,
2019, section 6.2). Though results for cardiovascular morbidity are
less consistent than those for cardiovascular mortality (U.S. EPA,
2019, section 6.2), recent studies provide some evidence for
associations between long-term PM2.5 exposures and the
progression of cardiovascular disease, including cardiovascular
morbidity (e.g., coronary heart disease, stroke) and atherosclerosis
progression (e.g., coronary artery calcification) (U.S. EPA, 2019,
sections 6.2.2. to 6.2.9). Associations reported in such studies are
supported by toxicologic evidence for increased plaque progression in
mice following long-term exposure to PM2.5 collected from
multiple locations across the U.S. (U.S. EPA, 2019, section <IP_ADDRESS>). A
small number of epidemiological studies also report positive
associations between long-term PM2.5 exposure and heart
failure, changes in blood pressure, and hypertension (U.S. EPA, 2019,
sections 6.2.5 and 6.2.7). Associations with heart failure are
supported by animal toxicologic studies demonstrating decreased cardiac
contractility and function, and increased coronary artery wall
thickness following long-term PM2.5 exposure (U.S. EPA,
2019, section <IP_ADDRESS>). Similarly, a limited number of animal
toxicologic studies demonstrating a relationship between long-term
exposure to PM2.5 and consistent increases in blood pressure
in rats and mice are coherent with epidemiological studies reporting
positive associations between long-term exposure to PM2.5
and hypertension. Further, a recent animal toxicologic study also
demonstrates increased plaque progression in mice following long-term
exposure to PM2.5 and provides coherent results with
epidemiological evidence reporting positive associations between long-
term exposure to PM2.5 and indicators of atherosclerosis
(U.S. EPA, 2019, section <IP_ADDRESS>).
    Longitudinal epidemiological analyses also report positive
associations with markers of systemic inflammation (U.S. EPA, 2019,
section 6.2.11), coagulation (U.S. EPA, 2019, section 6.2.12), and
endothelial dysfunction (U.S. EPA, 2019, section 6.2.13). These results
are coherent with animal toxicologic studies generally reporting
increased markers of systemic inflammation, oxidative stress, and
endothelial dysfunction (U.S. EPA, 2019, section <IP_ADDRESS> and 6.2.14).
    In summary, the 2019 ISA concludes that there is consistent
evidence from multiple epidemiological studies illustrating that long-
term exposure to PM2.5 is associated with mortality from
cardiovascular causes. Associations with CHD, stroke and
atherosclerosis progression were observed in several additional
epidemiological studies providing coherence with the mortality
findings. Results from copollutant models generally support an
independent effect of PM2.5 exposure on mortality.
Additional evidence of the independent effect of PM2.5 on
the cardiovascular system is provided by experimental studies in
animals, which support the biological plausibility of pathways by which
long-term exposure to PM2.5 could potentially result in
outcomes such as CHD, stroke, CHF and cardiovascular mortality. The
combination of epidemiological and experimental evidence results in the
ISA conclusion that ``a causal relationship exists between long-term
exposure to PM2.5 and cardiovascular effects'' (U.S. EPA,
2019, p. 6-222).
Short-Term PM2.5 Exposures
    The 2009 ISA concluded that ``a causal relationship exists between
short-

term exposure to PM2.5 and cardiovascular effects'' (U.S.
EPA, 2009c). The strongest evidence in the 2009 ISA was from
epidemiological studies of emergency department visits and hospital
admissions for ischemic heart disease (IHD) and heart failure (HF),
with supporting evidence from epidemiological studies of cardiovascular
mortality (U.S. EPA, 2009c). Animal toxicologic studies reported
evidence of reduced myocardial blood flow during ischemia and studies
indicating altered vascular reactivity (i.e., vascular function), which
provided coherence and biological plausibility for the effects observed
in epidemiological studies. In addition, both animal toxicologic and
epidemiological panel studies reported effects of PM2.5
exposure on ST segment depression, an electrocardiogram change that
potentially indicates ischemia.\39\ Key uncertainties from the last
review included inconsistent results across disciplines with respect to
the relationship between short-term exposure to PM2.5 and
changes in blood pressure, blood coagulation markers, and markers of
systemic inflammation. In addition, while the 2009 ISA identified a
growing body of evidence from controlled human exposure and animal
toxicologic studies, uncertainties remained with respect to biological
plausibility.
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    \39\ Some animal studies included in the 2009 ISA examined
exposures to mixtures, such as motor vehicle exhaust or woodsmoke.
In these studies, it was unclear if the resulting cardiovascular
effects could be attributed specifically to the particulate
components of the mixture.
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    A large body of recent evidence confirms and extends the evidence
from the 2009 ISA supporting the relationship between short-term
PM2.5 exposure and cardiovascular effects. This includes
generally positive associations observed in multicity epidemiological
studies of emergency department visits and hospital admissions for IHD,
HF, and combined cardiovascular-related endpoints. In particular,
nationwide studies of older adults (65 years and older) report positive
associations between PM2.5 exposures and hospital admissions
for HF (U.S. EPA, 2019, section <IP_ADDRESS>). Single-city epidemiological
studies contribute some support, though associations reported are less
consistently positive than in multicity studies, and include a number
of studies reporting null associations (U.S. EPA, 2019, sections 6.1.2
and 6.1.3).
    In addition, a number of more recent controlled human exposure,
animal toxicologic, and epidemiological panel studies provide evidence
that PM2.5 exposure could plausibly result in IHD or HF
through pathways that include endothelial dysfunction, arterial
thrombosis, and arrhythmia (U.S. EPA, 2019, section 6.1.1). The most
consistent evidence from recent controlled human exposure studies is
for endothelial dysfunction, as measured by changes in brachial artery
diameter or flow mediated dilation (U.S. EPA, 2019, section <IP_ADDRESS>).
These studies report variable results regarding the timing of the
effect and the mechanism by which reduced blood flow occurs (i.e.,
availability of vs. sensitivity to nitric oxide). Some controlled human
exposure studies using PM2.5 CAPs report evidence for small
increases in blood pressure (U.S. EPA, 2019, section <IP_ADDRESS>). In
addition, although not entirely consistent, there is also some evidence
across controlled human exposure studies for conduction abnormalities/
arrhythmia (U.S. EPA, 2019, section <IP_ADDRESS>), changes in heart rate
variability (HRV) (U.S. EPA, 2019, section <IP_ADDRESS>), changes in
hemostasis that could promote clot formation (U.S. EPA, 2019, section
<IP_ADDRESS>), and increases in inflammatory cells and markers (U.S. EPA,
2019, section <IP_ADDRESS>). Thus, when taken as a whole, controlled human
exposure studies are coherent with epidemiological studies in that they
provide evidence that short-term exposures to PM2.5 may
result in the types of cardiovascular endpoints that could lead to
emergency department visits and hospital admissions for IHD or HF.
    Animal toxicologic studies published since the 2009 ISA also
support a relationship between short-term PM2.5 exposure and
cardiovascular effects. A recent study demonstrating decreased cardiac
contractility and left ventricular pressure in mice is coherent with
the results of epidemiological studies that report associations between
short-term PM2.5 exposure and heart failure (U.S. EPA, 2019,
section <IP_ADDRESS>). In addition, similar to results of controlled human
exposure studies, there is generally consistent evidence in animal
toxicologic studies for indicators of endothelial dysfunction (U.S.
EPA, 2019, section <IP_ADDRESS>). Studies in animals also provide evidence
for changes in a number of other cardiovascular endpoints following
short-term PM2.5 exposure. Although not entirely consistent,
these studies provide some evidence of conduction abnormalities and
arrhythmia (U.S. EPA, 2019, section <IP_ADDRESS>), changes in HRV (U.S. EPA,
2019, section <IP_ADDRESS>), changes in blood pressure (U.S. EPA, 2019,
section <IP_ADDRESS>), and evidence for systemic inflammation and oxidative
stress (U.S. EPA, 2019, section <IP_ADDRESS>).
    In summary, recent evidence supports the conclusions reported in
the 2009 ISA indicating relationships between short-term
PM2.5 exposures and hospital admissions and ED visits for