[Federal Register Volume 85, Number 84 (Thursday, April 30, 2020)]
[Proposed Rules]
[Pages 24094-24144]
[FR Doc No: 2020-08143]

Vol. 85

Thursday,

No. 84

April 30, 2020

Part II

Environmental Protection Agency

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40 CFR Part 50

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

Federal Register / Vol. 85 , No. 84 / Thursday, April 30, 2020 /
Proposed Rules

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[EPA-HQ-OAR-2015-0072; FRL-10008-31-OAR]
RIN 2060-AS50

Review of the National Ambient Air Quality Standards for
Particulate Matter

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed 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 proposed 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 proposes to
retain the current standards, without revision.

DATES: Comments must be received on or before June 29, 2020.
    Public Hearings: The EPA will hold one or more virtual public
hearings on this proposed rule. These will be announced in a separate
Federal Register notice that provides details, including specific
dates, times, and contact information for these hearings.

ADDRESSES: You may submit comments, identified by Docket ID No. EPA-HQ-
OAR-2015-0072, by any of the following means:
     Federal eRulemaking Portal: https://www.regulations.gov
(our preferred method). Follow the online instructions for submitting
comments.
     Email<EMAIL_ADDRESS>Include the Docket ID No.
EPA-HQ-OAR-2015-0072 in the subject line of the message.
    Instructions: All submissions received must include the Docket ID
No. for this document. Comments received may be posted without change
to https://www.regulations.gov, including any personal information
provided. For detailed instructions on sending comments, see the
SUPPLEMENTARY INFORMATION section of this document. Out of an abundance
of caution for members of the public and our staff, the EPA Docket
Center and Reading Room was closed to public visitors on March 31,
2020, to reduce the risk of transmitting COVID-19. Our Docket Center
staff will continue to provide remote customer service via email,
phone, and webform. We encourage the public to submit comments via
https://www.regulations.gov or email, as there is a temporary
suspension of mail delivery to EPA, and no hand deliveries are
currently accepted. For further information of EPA Docket Center
services and the current status, please visit us online at https://www.epa.gov/dockets.

FOR FURTHER INFORMATION CONTACT: Dr. Scott Jenkins, Health and
Environmental Impacts Division, Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency, Mail Code C504-06,
Research Triangle Park, NC 27711; telephone:<PHONE_NUMBER>; fax: (919)
541-5315; email<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:

General Information

    Written Comments: Submit your comments, identified by Docket ID No.
EPA-HQ-OAR-2015-0072, at https://www.regulations.gov (our preferred
method), or the other methods identified in the ADDRESSES section. Once
submitted, comments cannot be edited or removed from the docket. The
EPA may publish any comment received to its public docket. Do not
submit electronically any information you consider to be Confidential
Business Information (CBI) or other information whose disclosure is
restricted by statute. Multimedia submissions (audio, video, etc.) must
be accompanied by a written submission. The written submission is
considered the official submission and should include discussion of all
points you wish to make. The EPA will generally not consider
submissions or submission content located outside of the primary
submission (i.e., on the web, cloud, or other file sharing system). For
additional submission methods, the full EPA public comment policy,
information about CBI or multimedia submissions, and general guidance
on making effective comments, please visit https://www.epa.gov/dockets/commenting-epa-dockets.
    The EPA is temporarily suspending its Docket Center and Reading
Room for public visitors to reduce the risk of transmitting COVID-19.
Written comments submitted by mail are temporarily suspended and no
hand deliveries will be accepted. Our Docket Center staff will continue
to provide remote customer service via email, phone, and webform. We
encourage the public to submit comments via https://www.regulations.gov. For further information and updates on EPA Docket
Center services, please visit us online at https://www.epa.gov/dockets.
    The EPA continues to carefully and continuously monitor information
from the Centers for Disease Control and Prevention (CDC), local area
health departments, and our Federal partners so that we can respond
rapidly as conditions change regarding COVID-19.

Availability of Information Related to This Action

    A number of the documents that are relevant to this proposed
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, and 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. 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. Review of the Air Quality Criteria and Standards for
Particulate Matter
    1. Reviews Completed in 1971 and 1987
    2. Review Completed in 1997
    3. Review Completed in 2006
    4. Review Completed in 2012
    5. Current Review
    D. Air Quality Information
    1. Distribution of Particle Size in Ambient Air
    2. Sources and Emissions Contributing to PM in the Ambient Air
    3. Monitoring of Ambient PM
    4. Ambient Concentrations and Trends
    a. PM2.5 mass
    b. PM2.5 components
    c. PM10
    d. PM10-2.5
    a. UFP
    5. Background PM
II. Rationale for Proposed Decisions on the Primary PM2.5
Standards
    A. General Approach

    1. Approach Used in the Last Review
    a. Indicator
    b. Averaging Time
    c. Form
    d. Level
    2. Approach in the Current Review
    B. Health Effects Related to Fine Particle Exposures
    1. Nature of Effects
    a. Mortality
    b. Cardiovascular Effects
    c. Respiratory Effects
    d. Cancer
    e. Nervous System Effects
    2. Populations at Risk of PM2.5-Related Health
Effects
    3. CASAC Advice
    C. Proposed Conclusions on the Current Primary PM2.5
Standards
    1. Evidence- and Risk-Based Considerations in the Policy
Assessment
    a. Evidence-Based Considerations
    b. Risk-Based Considerations
    2. CASAC Advice
    3. Administrator's Proposed Decision on the Current Primary
PM2.5 Standards
III. Rationale for Proposed Decisions on the Primary PM10
Standard
    A. General Approach
    1. Approach Used in the Last Review
    2. Approach in the Current Review
    B. Health Effects Related to Thoracic Coarse Particle Exposures
    1. Mortality
    a. Long-Term Exposures
    b. Short-Term Exposures
    2. Cardiovascular Effects
    a. Long-Term Exposures
    b. Short-Term Exposures
    3. Respiratory Effects--Short-Term Exposures
    4. Cancer--Long-Term Exposures
    5. Metabolic Effects--Long-Term Exposures
    6. Nervous System Effects--Long-Term Exposures
    C. Proposed Conclusions on the Current Primary PM10
Standard
    1. Evidence-Based Considerations in the Policy Assessment
    2. CASAC Advice
    3. Administrator's Proposed Decision on the Current Primary
PM10 Standard
IV. Rationale for Proposed Decisions on the Secondary PM Standards
    A. General Approach
    1. Approach Used in the Last Review
    a. Non-Visibility Effects
    b. Visibility Effects
    2. Approach for the Current Review
    B. PM-Related Visibility Impairment
    1. Nature of PM-Related Visibility Impairment
    2. Relationship between Ambient PM and Visibility
    3. Public Perception of Visibility Impairment
    C. Other PM-Related Welfare Effects
    1. Climate
    2. Materials
    D. Proposed Conclusions on the Current Secondary PM Standards
    1. Evidence- and Quantitative Information-Based Considerations
in the Policy Assessment
    2. CASAC Advice
    3. Administrator's Proposed Decision on the Current 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 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)
References

Executive Summary

    This document presents the Administrator's proposed decisions on
the primary (health-based) and secondary (welfare-based) National
Ambient Air Quality Standards (NAAQS) for particulate matter (PM). 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 proposed 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), and advice from the Clean Air Scientific
Advisory Committee (CASAC).
    For the primary PM2.5 standards, the Administrator
proposes to conclude that there are important uncertainties in the
evidence for adverse health effects below the current standards and in
the potential public health impacts of reducing ambient
PM2.5 concentrations below those standards. As a result, he
proposes to conclude that the available evidence and information do not
call into question the adequacy of the current primary PM2.5
standards, and he proposes to retain those standards (i.e., both the
annual and 24-hour standards) without revision in this review.
    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 proposes to conclude
that the newly available evidence does not call into question the
adequacy of the current primary PM10 standard, and he
proposes to retain that standard without revision in this review.
    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. Therefore, he proposes to conclude
that the newly available evidence and updated analyses do not call into
question the adequacy of the current secondary PM standards, and he
proposes to retain those standards without revision in this review.
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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. 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 proposed 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 did
not reach consensus on the primary annual PM2.5 standard,
with some committee members

recommending that EPA retain the current standard and other members
recommending revision of that standard.

I. Background

A. Legislative Requirements

    Two sections of the Clean Air Act (CAA) govern the establishment
and revision of the NAAQS. Section 108 (42 U.S.C. 7408) directs the
Administrator to identify and list certain air pollutants and then to
issue air quality criteria for those pollutants. The Administrator is
to list those pollutants ``emissions of which, in his judgment, cause
or contribute to air pollution which may reasonably be anticipated to
endanger public health or welfare''; ``the presence of which in the
ambient air results from numerous or diverse mobile or stationary
sources''; and for which he ``plans to issue air quality criteria . . .
. '' (42 U.S.C. 7408(a)(1)). Air quality criteria are intended to
``accurately reflect the latest scientific knowledge useful in
indicating the kind and extent of all identifiable effects on public
health or welfare which may be expected from the presence of [a]
pollutant in the ambient air . . . . '' (42 U.S.C. 7408(a)(2)).
    Section 109 [42 U.S.C. 7409] directs the Administrator to propose
and promulgate ``primary'' and ``secondary'' NAAQS for pollutants for
which air quality criteria are issued [42 U.S.C. 7409(a)]. Section
109(b)(1) defines primary standards as ones ``the attainment and
maintenance of which in the judgment of the Administrator, based on
such criteria and allowing an adequate margin of safety, are requisite
to protect the public health.'' \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
specifically to the Administrator's judgment. See Lead Industries Ass'n
v. EPA, 647 F.2d at 1161-62; Mississippi v. EPA, 744 F.3d at 1353.
    Section 109(d)(1) of the Act requires the review every five years
of existing air quality criteria and, if appropriate, the revision of
those criteria to reflect advances in scientific knowledge on the
effects of the pollutant on public health and welfare. Under the same
provision, the EPA is also to review every five years and, if
appropriate, revise the NAAQS, based on the revised air quality
criteria.
    Section 109(d)(2) addresses the appointment and advisory functions
of an independent scientific review committee. Section 109(d)(2)(A)
requires the Administrator to appoint this committee, which is to be
composed of ``seven members including at least one member of the
National Academy of Sciences, one physician, and one person
representing State air pollution control agencies.'' Section
109(d)(2)(B) provides that the independent scientific review committee
``shall complete a review of the criteria . . . and the national
primary and secondary ambient air quality standards . . . and shall
recommend to the Administrator any new . . . standards and revisions of
existing criteria and standards as may be appropriate. . . .'' Since
the early 1980s, this independent review function has been performed by
the Clean Air Scientific Advisory Committee (CASAC) of the EPA's
Science Advisory Board. 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 EPA
to consider costs of implementation when reviewing and revising the
standards ``it would be grounds for vacating the NAAQS.'' Whitman,
531 U.S. at 471 n.4. At the same time, the CAA directs the CASAC to
provide advice on ``any adverse public health, welfare, social,
economic, or energy effects which may result from various strategies
for attainment and maintenance'' of the NAAQS to the Administrator
under section 109(d)(2)(C)(iv). In Whitman, the Court clarified that
most of that advice would be relevant to implementation but not
standard setting, as it ``enable[s] the Administrator to assist the
States in carrying out their statutory role as primary implementers
of the NAAQS.'' Id. at 470 (emphasis in original). However, the
Court also noted that the CASAC's ``advice concerning certain
aspects of `adverse public health . . . effects' from various
attainment strategies is unquestionably pertinent'' to the NAAQS
rulemaking record and relevant to the standard setting process. Id.
at 470 n.2.
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B. Related PM Control Programs

    States are primarily responsible for ensuring attainment and
maintenance of ambient air quality standards once the EPA has
established them. Under section 110 and 171-190 of the CAA, and related
provisions and regulations, states are to submit, for 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. Review of the Air Quality Criteria and Standards for Particulate
Matter

1. Reviews Completed in 1971 and 1987
    The EPA first established NAAQS for PM in 1971 (36 FR 8186, April
30, 1971), based on the original Air Quality Criteria Document (AQCD)
(DHEW, 1969).\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 below), the AQCD
provided the scientific foundation (i.e., the air quality criteria)
for the NAAQS. Beginning in that review, the Integrated Science
Assessment (ISA) has replaced the AQCD.
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    In October 1979 (44 FR 56730, October 2, 1979), the EPA announced
the first periodic review of the air quality criteria and NAAQS for PM.
Revised primary and secondary standards were promulgated in 1987 (52 FR
24634, July 1, 1987). In the 1987 decision, the EPA changed the
indicator for particles from TSP to PM10, 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.\6\ 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 PM NAAQS, petitions for review
were filed by several parties, addressing a broad range of issues. In
May 1999, the U.S. Court of Appeals for the District of Columbia
Circuit (D.C. Circuit) upheld the EPA's decision to establish fine
particle standards, holding that ``the growing empirical evidence
demonstrating a relationship between fine particle pollution and
adverse health effects amply justifies establishment of new fine
particle standards.'' American Trucking Associations, Inc. v. EPA, 175
F. 3d 1027, 1055-56 (D.C. Cir. 1999). The D.C. Circuit also found
``ample support'' for the EPA's decision to regulate coarse particle
pollution, but vacated the 1997 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 holding on the cost and constitutional issues
were appealed to the United States Supreme Court. In February 2001, the
Supreme Court issued a unanimous decision upholding the EPA's position
on both the cost and constitutional issues. Whitman v. American
Trucking Associations, 531 U.S. 457, 464, 475-76. On the constitutional
issue, the Court held that the statutory requirement that NAAQS be
``requisite'' to protect public health with an adequate margin of
safety sufficiently guided the EPA's discretion, affirming the EPA's
approach of setting standards that are neither more nor less stringent
than necessary.
    The Supreme Court remanded the case to the D.C. Circuit for
resolution of any remaining issues that had not been addressed in that
court's earlier rulings. Id. at 475-76. In a March 2002 decision, the
D.C. Circuit rejected all remaining challenges to the standards,
holding that the EPA's 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.
---------------------------------------------------------------------------

    \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.
    \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).
---------------------------------------------------------------------------

    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
(discussed below).
4. Review Completed in 2012
    In June 2007, the EPA initiated the fourth periodic review of the
air quality criteria and the PM NAAQS by issuing a call for information
(72 FR 35462, June 28, 2007). Based on the NAAQS review process, as
revised in 2008 and again in 2009,\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, 2010a), and PA (U.S. EPA, 2011)). In June 2012, the EPA
announced its proposed decision to revise the NAAQS for PM (77 FR
38890, June 29, 2012).
---------------------------------------------------------------------------

    \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).
5. Current Review
    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). On February 9 to 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 the EPA staff in
developing a draft IRP, which was reviewed by the CASAC Particulate
Matter Panel and discussed on public teleconferences held in May 2016
(81 FR 13362, March 14, 2016) and August 2016 (81 FR 39043, June 15,
2016). Advice from the CASAC, supplemented by the Particulate Matter
Panel, and input from the public were considered in developing the
final IRP (U.S. EPA, 2016). The final IRP discusses the approaches to
be taken in developing key scientific, technical, and policy documents
in 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. Rather, the
EPA has taken steps to address the CASAC's comments in the Final PM 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 PM 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 discussed below
in sections II.B.1.d and II.B.1.e.
---------------------------------------------------------------------------

    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 this document.
    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, including the following (U.S. EPA, 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).
---------------------------------------------------------------------------

     Text was added to Chapter 1 to clarify the process
followed for this review of the PM NAAQS, including how the process has
evolved since the initiation of the review.
     Text and figures were added to Chapter 2 on emissions of
PM and PM precursors, and a section discussing uncertainty in emissions
estimates was added. A discussion of measurement uncertainty for FRM,
FEM, CSN, and IMPROVE monitors was also added.
     Chapter 3 and Appendices B and C include a number of
changes, including:
    [cir] An expanded characterization and discussion of the evidence
related to exposure measurement error, the potential confounders
examined by key studies, the shapes of concentration-response
functions, and the results of causal inference and quasi-experimental
studies.
    [cir] An expanded and clarified discussion of uncertainties in the
risk assessment, and additional air quality model performance
evaluations for each of the urban study areas included in the risk
assessment.
    [cir] Additional detail on the procedure used to derive
concentration-response functions used in the risk assessment.
    [cir] Changes in the text to reflect the change in the final ISA's
causality determination from ``likely to be causal'' to ``suggestive
of, but not sufficient to infer, a causal relationship.''
     Throughout the document (Chapters 3, 4 and 5), summaries
of the CASAC advice on the PM standards are included, and expanded
discussions of data gaps and areas for future research in the health
and welfare effects evidence are presented.

D. Air Quality Information

    This section provides a summary of basic information related to PM
ambient air quality. It summarizes information on the distribution of
particle size in ambient air (I.D.1), sources and emissions
contributing to PM in the ambient air (I.D.2), monitoring of ambient PM
in the U.S. (I.D.3), ambient PM concentrations and trends in the U.S.
(I.D.4), and background PM (I.D.5). Additional detail on PM air quality
can be found in Chapter 2 of the Policy Assessment (U.S. EPA, 2020;
PA).
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). Particle
size is an important consideration for PM, as 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). When describing PM, subscripts are used to denote the aerodynamic
diameter \17\ of the particle size range, in [micro]m, of 50% cut
points of sampling devices. 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).
---------------------------------------------------------------------------

    \17\ Aerodynamic diameter is the size of a sphere of unit
density (i.e., 1 g/cm\3\) that has the same terminal settling
velocity as the particle of interest (U.S. EPA, 2019, section
4.1.1).
---------------------------------------------------------------------------

    Atmospheric distributions of particle size generally exhibit
distinct modes that roughly align with the PM size fractions defined
above. The nucleation mode is made up of freshly generated particles,
formed either during combustion or by atmospheric reactions of
precursor gases. The nucleation mode is especially prominent near
sources like heavy traffic, industrial emissions, biomass burning, or
cooking (Vu et al., 2015). While nucleation mode particles are only a
minor contributor to overall ambient PM mass and surface area, they are
the main contributors to ambient particle number (U.S. EPA, 2019,
section 2.2). By number, most nucleation mode particles fall into the
UFP size range, though some fraction of the nucleation mode number
distribution can extend above 0.1 [mu]m in diameter. Nucleation mode
particles can grow rapidly through coagulation or uptake of gases by
particle surfaces, giving rise to the accumulation mode. The
accumulation mode is typically the predominant contributor to
PM2.5 mass, though only a minor contributor to particle
number (U.S. EPA, 2019, section 2.2). PM2.5 sampling methods
measure most of the accumulation mode mass, although a small fraction
of particles that make up the accumulation mode are greater than 2.5
[mu]m in diameter. Coarse mode particles are formed by mechanical
generation, and through processes like dust resuspension and sea spray
formation

(Whitby et al., 1972). Most coarse mode mass is captured by
PM10-2.5 sampling, but small fractions of coarse mode mass
can be smaller than 2.5 [mu]m or greater than 10 [mu]m in diameter
(U.S. EPA, 2019, section 2.2).
    Most particles are found in the lower troposphere, where they can
have residence times ranging from a few hours to weeks. Particles are
removed from the atmosphere by wet deposition, such as when they are
carried by rain or snow, or by dry deposition, when particles settle
out of suspension due to gravity. 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. In contrast, atmospheric lifetimes for UFP and
PM10-2.5 are shorter. Within hours, UFP can undergo
coagulation and condensation that lead to formation of larger particles
in the accumulation mode, or can be removed from the atmosphere by
evaporation, deposition, or reactions with other atmospheric
components. PM10-2.5 are also generally removed from the
atmosphere within hours, through wet or dry deposition (U.S. EPA, 2019,
Table 2-1).
2. Sources and Emissions Contributing to PM in the Ambient Air
    PM is composed of both primary (directly emitted particles) and
secondary particles. Primary PM is derived from direct particle
emissions from specific PM sources while secondary PM originates from
gas-phase 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. Precursor gases include sulfur dioxide
(SO2), nitrogen oxides (NOX), and volatile
organic compounds (VOC) (U.S. EPA, 2019, section <IP_ADDRESS>). Ammonia also
plays an important role in the formation of nitrate PM by neutralizing
sulfuric acid and nitric acid. Sources and emissions of PM are
discussed in more detail in section 2.1.1 of the PA (U.S. EPA, 2020).
    Direct emissions of PM have remained relatively unchanged in recent
years, while emissions of some precursor gases have declined
substantially.\18\ From 1990 to 2014, SO2 emissions have
undergone the largest declines while NH3 emissions have
undergone the smallest change. Declining SO2 emissions
during this time period are primarily a result of reductions at
stationary sources such as EGUs, with substantial reductions also from
mobile sources (U.S. EPA, 2019, section <IP_ADDRESS>).\19\
---------------------------------------------------------------------------

    \18\ More information on these trends, including details on
methods and explanations on the noted changes over time is available
at https://gispub.epa.gov/neireport/2014/.
    \19\ State-specific emission trends data for 1990 to 2014 can be
found at: https://www.epa.gov/air-emissions-inventories/air-pollutant-emissions-trends-data.
---------------------------------------------------------------------------

3. Monitoring of Ambient PM
    To promote uniform enforcement of the air quality standards set
forth under the CAA and to achieve the degree of public health and
welfare protection intended for the NAAQS, the EPA established PM
Federal Reference Methods (FRMs) \20\ for both PM10 and
PM2.5 (40 CFR appendix J and L to Part 50) and performance
requirements for approval of Federal Equivalent Methods (FEMs) (40 CFR
part 53). Amended following the 2006 and 2012 p.m. NAAQS reviews, the
current PM monitoring network relies on FRMs and automated continuous
FEMs, in part to support changes necessary for implementation of the
revised PM standards. The requirements for measuring ambient air
quality and reporting ambient air quality data and related information
are the basis for 40 CFR appendices A through E to Part 58. More
information on PM ambient monitoring networks is available in section
2.2 of the PA (U.S. EPA, 2020).
---------------------------------------------------------------------------

    \20\ FRMs provide the methodological basis for comparison to the
NAAQS and also serve as the ``gold-standard'' for the comparison of
other methods being reviewed for potential approval as equivalent
methods. The EPA keeps a complete list of designated reference and
equivalent methods available on its Ambient Monitoring Technology
Information Center (AMTIC) website (https://www.epa.gov/amtic/air-monitoring-methods-criteria-pollutants).
---------------------------------------------------------------------------

4. Ambient Concentrations and Trends
    This section summarizes available information on recent ambient PM
concentrations in the U.S. and on trends in PM air quality. Sections
I.D.4.a and I.D.4.b summarize information on PM2.5 mass and
components, respectively. Section I.D.4.c summarizes information on
PM10. Sections I.D.4.d and I.D.4.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 section
2.3 of the PA (U.S. EPA, 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>).\21\
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>).
---------------------------------------------------------------------------

    \21\ 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,\22\ 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,\23\
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>).
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    \22\ A design value is considered valid if it meets the data
handling requirements given in 40 CFR appendix N to Part 50. Several
large CBSAs such as Chicago-Naperville-Elgin, IL-IN-WI and Houston-
The Woodlands-Sugar Land, TX had near-road sites that did not have
valid PM2.5 design values for the 2015-2017 period.
    \23\ The Elizabeth Lab site in Elizabeth, NJ is situated
approximately 30 meters from travel lanes of the Interchange 13 toll
plaza of the New Jersey Turnpike and within 200 meters of travel
lanes for Interstate 278 and the New Jersey Turnpike.
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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>).\24\ 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.
---------------------------------------------------------------------------

    \24\ 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>).\25\ These PM10 concentrations have
generally declined in the eastern U.S., while concentrations in the
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.
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    \25\ For more information, see https://www.epa.gov/air-trends/particulate-matter-pm10-trends#pmnat.
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d. PM10-2.5
    Since the last review, the availability of PM10-2.5
ambient concentration data has greatly increased. 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>).
5. 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 section 2.4 of the PA (U.S. EPA, 2020).
    At annual and national scales, estimated background PM
concentrations in the U.S. are small compared to contributions from
domestic anthropogenic emissions. 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 PM sources, including sources of
background PM, may have changed over time. As discussed further in the
PA (U.S. EPA, 2020, section 2.4), such data suggests that estimates of
background concentrations at IMPROVE monitors are around 1-3 [micro]g/
m\3\, and have not changed significantly since the last PM NAAQS
Review.
    As discussed further in the PA (U.S. EPA, 2020, section 2.4),
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. 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).
    The magnitude and sources of background PM can vary widely by
region and time of year. Coastal sites may experience a consistent
contribution of PM from sea spray aerosol, while other areas covered
with dense vegetation may be impacted by biogenic aerosol production
during the summertime. Sources of background PM also operate across a
range of time scales. While some sources like biogenic aerosol vary at
monthly to seasonal scales, many sources of background PM are episodic
in nature. These episodic sources (e.g., large wildfires) can be
characterized by infrequent contributions to high-concentration events
occurring over shorter periods of time (e.g., hours to several days).
Such episodic events are sporadic and do not necessarily occur in all
years. While these exceptional episodes can lead to exceedances of the
24-hour PM2.5 standard (35 [micro]g/m\3\) in some cases
(Schweizer et al., 2017), such events are routinely screened for and
usually identifiable in the monitoring data. As described further in
the PA (U.S. EPA, 2020, section 2.4), 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.

II. Rationale for Proposed Decisions on the Primary PM2.5 Standards

    This section provides the rationale supporting the Administrator's
proposed decisions on the primary PM2.5 standards. Section
II.A describes the Agency's approach to reaching decisions on the
primary PM2.5 standards in the last review and summarizes
the general approach to reaching proposed decisions in this review.
Section II.B summarizes the scientific evidence for PM2.5-
related health effects. Section II.C presents the Administrator's
proposed conclusions regarding the adequacy of the current primary
PM2.5 standards and his proposed decision to retain those
standards in this review.\26\
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    \26\ Sections III and IV provide the rationales supporting the
Administrator's proposed decisions on the primary PM10
standard and secondary standards, respectively.
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A. General Approach

1. Approach Used in the Last Review
    The last review of the primary PM NAAQS was completed in 2012 (78
FR 3086, January 15, 2013). As noted above (section 1.3), 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\,\27\ 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, 2012a). She particularly noted the ``strong and generally
robust body of evidence of serious health effects associated with both
long- and short-term exposures to PM2.5'' (78 FR 3120,
January 15, 2013). This included epidemiologic studies reporting health
effect associations based on long-term average PM2.5
concentrations ranging from about 15.0 [mu]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.
---------------------------------------------------------------------------

    \27\ 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). These considerations, and the prior
Administrator's conclusions, are summarized in sections II.A.1.a to
II.A.1.d below.
a. Indicator
    In the last review, the EPA considered issues related to the
appropriate indicator for fine particles, with a focus on evaluating
support for the existing PM2.5 mass-based indicator and for
potential alternative indicators based on the UFP fraction or on fine
particle composition (78 FR 3121, January 15, 2013).\28\ With regard to
PM2.5 mass, as in the 1997 and 2006 reviews, 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
fine particles indexed largely by PM2.5 mass (78 FR 3121,
January 15, 2013). With regard to the ultrafine fraction of ambient PM,
the 2011 PA noted the limited body of health 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 characterize
ambient concentrations of UFP (U.S. EPA, 2011, section 1.3.2). With
regard to PM composition, 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). 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).
---------------------------------------------------------------------------

    \28\ In the last review, the ISA defined 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).

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b. Averaging Time
    In 1997, the EPA set an annual PM2.5 standard to provide
protection from health effects associated with long- and short-term
exposures to PM2.5, and a 24-hour standard to supplement the
protection afforded by the annual standard (62 FR 38667 to 38668, July
18, 1997). In the 2006 review, the EPA retained both annual and 24-hour
averaging times (71 FR 61164, October 17, 2006). In the last review,
the EPA again considered issues related to the appropriate averaging
times 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 ISA, the 2011 PA noted that
the overwhelming majority of studies that had been conducted since the
2006 review continued to utilize annual (or multi-year) or 24-hour PM
averaging periods (U.S. EPA, 2011, section 2.3.2). Given this, 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 current annual and 24-hour
averaging times (U.S. EPA, 2011, p. 2-58). The CASAC agreed that these
conclusions were reasonable (Samet, 2010a, p. 13). The prior
Administrator concurred with the PA conclusions and 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).
c. Form
    In 1997, the EPA established the form of the annual
PM2.5 standard as an annual arithmetic mean, averaged over 3
years, from single or multiple community-oriented monitors.\29\ That
is, the level of the annual standard was to be compared to measurements
made at each community-oriented monitoring site or, if specific
criteria were met, measurements from multiple community-oriented
monitoring sites could be averaged together (i.e., spatial averaging)
\30\ (62 FR 38671 to 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 at
38671 to 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).\31\
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    \29\ 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.
    \30\ 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 to 38672, July
18, 1997).
    \31\ 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).
---------------------------------------------------------------------------

    In the last review, the EPA's consideration of the form of the
annual PM2.5 standard again included 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, including minority
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).
    The prior Administrator concluded that public health would not be
protected with an adequate margin of safety in all locations, as
required by law, if disproportionately higher PM2.5
concentrations in low income and minority communities were averaged
together with lower concentrations measured at other sites in a large
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).
    In the last review, the EPA also considered the form of the 24-hour
PM2.5 standard. The Agency recognized that the existing 98th
percentile form for the 24-hour standard 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.\32\ 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).
---------------------------------------------------------------------------

    \32\ 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.
---------------------------------------------------------------------------

d. Level
    The EPA's approach to considering alternative levels of the
PM2.5 standards in the last review was based on evaluating
the public health protection afforded by the annual and 24-hour
standards, taken together, against mortality and morbidity effects
associated with long-term 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
standard, 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 prior Administrator considered the degree to which
epidemiologic studies indicate confidence in the reported health effect
associations over distributions of ambient PM2.5
concentrations. She noted that a level of 12.0 [micro]g/m\3\ was below
the long-term mean PM2.5 concentrations reported in key
epidemiologic 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 epidemiologic 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).\33\
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    \33\ With respect to cancer, mutagenic, and genotoxic effects,
the Administrator observed that the PM2.5 concentrations
reported in studies evaluating these effects generally included
ambient concentrations that are equal to or greater than ambient
concentrations observed in studies that reported mortality and
cardiovascular and respiratory effects (U.S. EPA, 2009c, section
7.5). Therefore, the Administrator concluded that, in selecting a
standard level that provides protection from mortality and
cardiovascular and respiratory effects, it is reasonable to
anticipate that protection will also be provided for carcinogenic
effects (78 FR 3161-3162, January 15, 2013).
---------------------------------------------------------------------------

    Given the uncertainties remaining in the scientific information,
the prior 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 errors in
epidemiologic studies, and the nature and magnitude of estimated risks
at relatively low ambient PM2.5 concentrations. Furthermore,
she noted that epidemiologic studies had reported heterogeneity in
responses both within and between cities and in geographic regions
across the U.S. She recognized that this heterogeneity may be
attributed, in part, to differences in fine particle 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 following: The limited number of studies
evaluating such effects; uncertainties related to identifying the
relevant exposure time periods of concern; and limited toxicological
evidence providing little 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, interpreted in light of remaining uncertainties, 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. Approach in the Current Review
    The EPA's approach to reaching proposed decisions on the primary
PM2.5 standards in the current review builds on the
decisions made in the last review. Consistent with that review, the
approach focuses on evaluating the public health protection afforded by
the annual and 24-hour standards, taken together, against mortality and
morbidity associated with long-term or short-term PM2.5
exposures. As discussed in the PA (U.S. EPA, 2020, section 3.1.2), in
adopting this approach the EPA recognizes that changes in
PM2.5 air quality designed to meet an annual standard would
likely result not only in lower annual average PM2.5
concentrations, but also in fewer and lower short-term peak
PM2.5 concentrations. Additionally, changes designed to meet
a 24-hour standard, with a 98th percentile form, would result not only
in fewer and lower peak 24-hour PM2.5 concentrations, but
also in lower annual average PM2.5 concentrations. Thus, the
EPA's approach recognizes that it is appropriate to consider the
protection provided by attaining the air quality needed to meet the
suite of standards.
    This approach to reviewing the primary PM2.5 standards
is based most fundamentally on considering the available scientific
evidence and technical information as assessed and discussed in the ISA
(U.S. EPA, 2019) and PA (U.S. EPA, 2020), including the uncertainties
inherent in that evidence and information, and on consideration of
advice received from the CASAC in this review (Cox, 2019a). The EPA
emphasizes the health outcomes for which the ISA determines that the
evidence supports either a ``causal'' or a ``likely to be causal''
relationship with PM2.5 exposures (U.S. EPA, 2019). This
approach focuses proposed decisions on the health outcomes for which
the evidence is strongest. Such a focus, which is supported by the
CASAC (Cox, 2019a, p. 12 of consensus responses), recognizes that
standards set based on evidence supporting ``causal'' and ``likely to
be causal'' health outcomes will also provide some measure of
protection against the broader range of PM2.5-associated
outcomes, including those for which the evidence is less certain.
    As in past reviews, the EPA's approach recognizes that there is no
bright line clearly directing the choice of standards. Rather, the
choice of what is appropriate is a public health policy judgment
entrusted to the Administrator. Specifically, the CAA requires primary
standards that, in the judgment of the Administrator, are requisite to
protect public health with an adequate margin of safety. In setting
primary standards that are ``requisite'' to protect public health, the
EPA's task is to establish standards that are neither more nor less
stringent than necessary for this purpose. Thus, as discussed above
(I.A), the CAA does not require that primary standards be set at a
zero-risk level, but rather at a level that, in the judgment of the
Administrator, limits risk sufficiently so as to protect public health
with an adequate margin of safety. As in previous reviews, this
judgment includes consideration of the strengths and limitations of the
scientific and technical information, and the appropriate inferences to
be drawn from that information.

B. Health Effects Related to Fine Particle Exposures

    This section draws from the EPA's synthesis and assessment of the
scientific evidence presented in the ISA (U.S. EPA, 2019) and the
summary of that evidence in the PA (U.S. EPA, 2020, section 3.2.1). The
ISA uses a weight-of-evidence framework for characterizing the strength
of the available scientific evidence for health effects attributable to
PM exposures (U.S. EPA, 2015, Preamble, Section 5). As in the last
review (U.S. EPA, 2009c), the ISA for this review has adopted a five-
level hierarchy to classify the overall weight-of-evidence into one of
the following categories: Causal relationship; a likely to be causal
relationship; suggestive of, but not sufficient to infer, a causal
relationship; \34\ inadequate to infer the presence or absence of a
causal relationship; and not likely to be a causal relationship (U.S.
EPA, 2015, Preamble Table II). In using the weight-of-evidence approach
to inform judgments about the likelihood that various health effects
are caused by PM exposures, evidence is evaluated for major outcome
categories or groups of related outcomes (e.g., respiratory effects),
integrating evidence from across disciplines, including epidemiologic,
controlled human exposure, and animal toxicological studies and
evaluating the coherence of evidence across a spectrum of related
endpoints as well as biological plausibility of the effects observed
(U.S. EPA, 2015, Preamble, Section 5.c.). Based on application of this
approach, the EPA believes that the final ISA ``accurately reflects 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 [PM] in the ambient air, in varying
quantities'' as required by the CAA (42 U.S.C. 7408(a)(2)).
---------------------------------------------------------------------------

    \34\ As noted in the 2019 p.m. ISA (U.S. EPA, 2019, p. ES-15),
this causality determination language has been updated since the
last review.
---------------------------------------------------------------------------

    In this review of the 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.
The ISA defines these causality determinations as follows (U.S. EPA,
2019, p. p-20):
     Causal relationship: The pollutant has been shown to
result in health effects at relevant exposures based on studies
encompassing multiple lines of evidence and chance, confounding, and
other biases can be ruled out with reasonable confidence.
     Likely to be a causal relationship: There are studies in
which results are not explained by chance, confounding, or other
biases, but uncertainties remain in the health effects evidence
overall. For example, the influence of co-occurring pollutants is
difficult to address, or evidence across scientific disciplines may be
limited or inconsistent.
    The sections below briefly summarize the health effects evidence
determined in the ISA to support either a ``causal'' or a ``likely to
be causal'' relationship with fine particle exposures (II.B.1), the
populations potentially at increased risk for PM-related effects
(II.B.2), and the CASAC's advice on the draft ISA (II.B.3). Additional
detail on these topics can be found in the ISA (U.S. EPA, 2019) and in
the PA (U.S. EPA, 2020, section 3.2).
1. 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.B.1.a), cardiovascular effects (II.B.1.b), respiratory effects
(II.B.1.c), cancer (II.B.1.d), and nervous system effects (II.B.1.e).
For these outcomes, the ISA concludes that the evidence supports either
a ``causal'' or a ``likely to be causal'' relationship with
PM2.5 exposures.
a. Mortality
i. Long-term PM2.5 exposures
    In the last review, the 2009 PM 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 epidemiologic studies, particularly those examining two
seminal cohorts, the American Cancer Society (ACS) cohort and the
Harvard Six Cities cohort. Analyses of the Harvard Six Cities cohort
included 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 also 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>, 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.
---------------------------------------------------------------------------

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

    A recent series of retrospective 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 concentration continued to be associated with an
increase in life expectancy, though the magnitude of the increase was
smaller than during the earlier time period (i.e., a 10 [micro]g/m\3\
decrease in long-term 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 2019 ISA specifically evaluates the degree to which recent
studies that examine the relationship between long-term
PM2.5 exposure and mortality have addressed key policy-
relevant issues and/or previously identified data gaps in the
scientific evidence. For example, based on its assessment of the
evidence, 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).
    An additional important consideration in characterizing the public
health impacts associated with 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 (U.S. EPA, 2015, Preamble section 6.c).
    The biological plausibility of PM2.5-attributable
mortality is supported by the coherence of effects across scientific
disciplines (i.e., animal toxicological, controlled human exposure
studies, and epidemiologic), including in recent studies evaluating the
morbidity effects that are the largest contributors to total
(nonaccidental) mortality. 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 epidemiologic
studies consistently report positive associations with mortality across
different geographic locations, populations, and analytic approaches.
Such studies reduce key uncertainties identified in the last review,
including those related to potential copollutant confounding, and
provide additional information on the shape of the concentration-
response curve. Recent experimental and epidemiologic 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).
ii. Short-term PM2.5 exposures
    The 2009 PM 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 epidemiologic studies that consistently reported
positive associations between short-term PM2.5 exposure and
non-accidental mortality. These associations were strongest, in terms
of magnitude and precision, primarily at lags of 0 to 1 days.
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).
---------------------------------------------------------------------------

    \36\ As detailed in the Preface to the ISA, risk estimates are
for a 10 [micro]g/m\3\ increase in 24-hour avg PM2.5
concentrations, unless otherwise noted (U.S. EPA, 2019).
---------------------------------------------------------------------------

    Some recent studies have expanded the examination of potential
confounders (e.g., U.S. EPA, 2019, section <IP_ADDRESS>), including
copollutants. 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 examines 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.
    The positive associations for total mortality reported across the
majority of studies evaluated are further supported by analyses
reporting generally consistent, positive associations with both
cardiovascular and respiratory mortality (U.S. EPA, 2019, section
11.1.3). For both cardiovascular and respiratory mortality, there has
been only 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. This evidence further supports the copollutant
analyses conducted for total mortality. 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, particularly from
controlled human exposure studies of respiratory effects, 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>). Analyses focusing on effect modification of the
PM2.5-mortality relationship by PM2.5 components,
regional patterns in PM2.5 components and city-specific
differences in composition and sources indicate some differences in the
PM2.5 composition and sources across cities and regions, but
these differences do not fully explain the observed heterogeneity.
Additional studies find that factors related to potential exposure
differences, such as housing stock and commuting, as well as city-
specific factors (e.g., land-use, port volume, and traffic
information), may explain some of the observed heterogeneity (U.S. EPA,
2019, section <IP_ADDRESS>). Collectively, recent studies indicate that the
heterogeneity in PM2.5-mortality risk estimates cannot be
attributed to one factor, but instead 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 number of recent studies conducted systematic evaluations of the
lag structure of associations for the PM2.5-mortality
relationship by examining either a series of single-day or multiday
lags and these studies continue to support an immediate effect (i.e.,
lag 0 to 1 days) of short-term PM2.5 exposures on mortality
(U.S. EPA, 2019, section <IP_ADDRESS>). Recent studies also 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.
    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; Figure 11-12 in
U.S. EPA, 2019). 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.
---------------------------------------------------------------------------

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

    Overall, recent epidemiologic 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
co-pollutant 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,
pp. 11-58).
b. Cardiovascular Effects
i. Long-Term PM2.5 Exposures
    The scientific evidence reviewed in the 2009 PM 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 epidemiologic 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 epidemiologic
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\
---------------------------------------------------------------------------

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

    The body of literature examining the relationship between long-term
PM2.5 exposure and cardiovascular morbidity has greatly
expanded since the 2009 PM ISA, with positive associations reported in
several cohorts (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. Positive associations with cardiovascular morbidity (e.g.,
coronary heart disease, stroke) and atherosclerosis progression (e.g.,
coronary artery calcification) are observed in several epidemiologic
studies (U.S. EPA, 2019, sections 6.2.2. to 6.2.9). Associations in
such studies are supported by toxicological 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 epidemiologic
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 toxicological
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 toxicological studies demonstrating a
relationship between long-term exposure to PM2.5 and
consistent increases in blood pressure in rats and mice are coherent
with epidemiologic studies reporting positive associations between
long-term exposure to PM2.5 and hypertension.
    Longitudinal epidemiologic 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 toxicological 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 epidemiologic 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
epidemiologic 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
epidemiologic 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).
ii. Short-Term PM2.5 Exposures
    The 2009 PM 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 PM ISA
was from epidemiologic studies of emergency department visits and
hospital admissions for ischemic heart disease (IHD) and heart failure
(HF), with supporting evidence from epidemiologic studies of
cardiovascular mortality (U.S. EPA, 2009c). Animal toxicological
studies provided coherence and biological plausibility for the positive
associations reported with myocardial ischemia, emergency department
visits, and hospital admissions. These included studies reporting
reduced myocardial blood flow during ischemia and studies indicating
altered vascular reactivity. In addition, effects of PM2.5
exposure on a potential indicator of ischemia (i.e., ST segment
depression on an electrocardiogram) were reported in both animal
toxicological and epidemiologic panel studies.\39\ Key uncertainties
from the last review resulted from 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 PM ISA identified a growing body of evidence from
controlled human exposure and animal toxicological studies,
uncertainties remained with respect to biological plausibility.
---------------------------------------------------------------------------

    \39\ Some animal studies included in the 2009 PM 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.
---------------------------------------------------------------------------

    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 epidemiologic
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) using Medicare
records report positive associations between PM2.5 exposures
and hospital admissions for HF (U.S. EPA, 2019, section <IP_ADDRESS>).
Additional multicity studies conducted in the northeast U.S. report
positive associations between short-term PM2.5 exposures and