Petition for Writ of Certiorari — Chamber of Commerce, et al., Petitioners v. Environmental Protection Agency, et al.
Supreme Court briefAug 28, 2026
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No. ____
IN THE
Supreme Court of the United States
________________________________
CHAMBER OF COMMERCE OF THE UNITED STATES
OF AMERICA, ET AL.
Petitioners,
v.
UNITED STATES ENVIRONMENTAL PROTECTION
AGENCY; LEE ZELDIN, ADMINISTRATOR, ET AL.
Respondents.
________________________________
ON PETITION FOR WRIT OF CERTIORARI TO THE UNITED
STATES COURT OF APPEALS FOR THE DISTRICT OF
COLUMBIA CIRCUIT
________________________________
APPENDIX VOLUME I OF II
________________________________
LUCINDA MINTON
LANGWORTHY
ERICA N. PETERSON
HUNTON ANDREWS KURTH
LLP
2200 Pennsylvania Avenue,
NW, Suite 900
Washington, D.C. 20037
(202) 955-1500
ELBERT LIN
Counsel of Record
DAVID M. PARKER
HUNTON ANDREWS KURTH
LLP
951 E. Byrd St., East Tower
Richmond, VA 23219
elin@Hunton.com
(804) 788-8200
Counsel for Petitioners Chamber of Commerce of the
United States of America, American Cement Association,
American Chemistry Council, American Forest & Paper
Association, American Petroleum Institute, American
Wood Council, National Association of Manufacturers,
and National Mining Association
(Additional counsel on signature page)
TABLE OF CONTENTS
Page
VOLUME I:
APPENDIX A: Opinion of the U.S. Court of
Appeals for the District of Columbia Circuit
Denying Petitions for Review and EPA’s
Motion for Vacatur, Commonwealth of
Kentucky v. EPA, No. 24-1050 (consolidated
with Nos. 24-1051, 24-1052, 24-1073, 241091) (D.C. Cir. June 26, 2026) ............................... 1a
APPENDIX B:
U.S. Environmental
Protection Agency, Reconsideration of the
National Ambient Air Quality Standards
for Particulate Matter; Final Rule, 89 Fed.
Reg. 16,202 (Mar. 6, 2024) (excerpts) .................... 46a
VOLUME II:
APPENDIX C: Clean Air Act § 109, 42
U.S.C. § 7409 ........................................................ 466a
APPENDIX D:
EPA, Responses to
Significant Comments on the 2023
Proposed Rule for the Reconsideration of
the National Ambient Air Quality
Standards
for
Particulate
Matter
(undated), Docket No. EPA-HQ-OAR2015-0072-6025 (excerpts) ................................... 470a
i
APPENDIX E:
Declaration of Aaron
Szabo (Nov. 24, 2025) ........................................... 500a
ii
1a
APPENDIX A
UNITED STATES COURT OF APPEALS
FOR THE DISTRICT OF COLUMBIA CIRCUIT
_______________
Argued December 16, 2024
Decided June 26, 2026
No. 24-1050
COMMONWEALTH OF KENTUCKY, ET AL.,
PETITIONERS
v.
ENVIRONMENTAL PROTECTION AGENCY AND LEE M.
ZELDIN, IN HIS OFFICIAL CAPACITY AS ADMINISTRATOR
OF THE U.S. ENVIRONMENTAL PROTECTION AGENCY,
RESPONDENTS
ALLIANCE OF NURSES FOR HEALTH ENVIRONMENTS, ET
AL.,
INTERVENORS
_______________
Consolidated with 24-1051, 24-1052, 24-1073, 241091
_______________
On Petitions for Review of a Final Action
of the Environmental Protection Agency
2a
_______________
Elbert Lin argued the cause for Industry
Petitioners. With him on the briefs were Lucinda
Minton Langworthy, Erica N. Peterson, Andrew R.
Varcoe, Stephanie A. Maloney, Nate Curtisi, Michael
A. Tilghman II, Brunn (Beau) W. Roysden III, Kristina
(Tina) R. Van Bockern, Emily C. Schilling, and
Sydney J. Sell.
Jacob M. Abrahamson, Assistant Solicitor
General, Office of the Attorney General for the
Commonwealth of Kentucky, argued the cause for
State Petitioners. With him on the briefs were Russell
Coleman, Attorney General, Matthew F. Kuhn,
Solicitor General, Lindsey R. Keiser, Assistant
Attorney General, Patrick Morrisey, Attorney
General, Office of the Attorney General for the State
of West Virginia, Michael R. Williams, Solicitor
General, Steve Marshall, Attorney General, Office of
the Attorney General for the State of Alabama,
Edmund G. LaCour Jr., Solicitor General, Tim
Griffin, Attorney General, Office of the Attorney
General for the State of Arkansas, Nicholas J. Bronni,
Solicitor General, Dyland L. Jacobs, Deputy Solicitor
General, Treg R. Taylor, Attorney General, Office of
the Attorney General for the State of Alaska, Jennifer
J. Seely, Assistant Attorney General, Ashley Moody,
Attorney General, Office of the Attorney General for
the State of Florida, Henry C. Whitaker, Solicitor
General, James H. Percival, Chief of Staff,
Christopher M. Carr, Attorney General, Office of the
Attorney General for the State of Georgia, Stephen J.
3a
Petrany, Solicitor General, Theodore E. Rokita,
Attorney General, Office of the Attorney General for
the State of Indiana, James A. Barta, Solicitor
General, Raul R. Labrador, Attorney General, Office
of the Attorney General for the State of Idaho, Alan
M. Hurst, Solicitor General, Brenna Bird, Attorney
General, Office of the Attorney General for the State
of Iowa, Eric H. Wessan, Solicitor General, Kris
Kobach, Attorney General, Office of the Attorney
General for the State of Kansas, Anthony J. Powell,
Solicitor General, Lynn Fitch, Attorney General,
Office of the Attorney General for the State of
Mississippi, Justin L. Matheny, Deputy Solicitor
General, Liz Murrill, Attorney General, Office of the
Attorney General for the State of Louisiana, J.
Benjamin Aguinaga, Solicitor General, Andrew
Bailey, Attorney General, Office of the Attorney
General for the State of Missouri, Joshua M. Divine,
Solicitor General, Austin Knudsen, Attorney General,
Office of the Attorney General for the State of
Montana, Christian B. Corrigan, Solicitor General,
Drew Wrigley, Attorney General, Office of the
Attorney General for the State of North Dakota,
Philip Axt, Solicitor General, Michael T. Hilgers,
Attorney General, Office of the Attorney General for
the State of Nebraska, Grant D. Strobl, Assistant
Solicitor General, Dave Yost, Attorney General, Office
of the Attorney General for the State of Ohio, T. Elliot
Gaiser, Solicitor General, Mathura Sridharan,
Deputy Solicitor General, Gentner F. Drummond,
Attorney General, Office of the Attorney General for
the State of Oklahoma, Garry M. Gaskins, II, Solicitor
General, Jennifer L. Lewis, Deputy Attorney General,
4a
Marty J. Jackley, Attorney General, Office of the
Attorney General for the State of South Dakota,
Steven Blair, Deputy Attorney General, Alan Wilson,
Attorney General, Office of the Attorney General for
the State of South Carolina, J. Emory Smith, Jr.,
Deputy Solicitor General, Thomas T. Hydrick,
Assistant Deputy Solicitor General, Joseph D. Spate,
Assistant Deputy Solicitor General, Jonathan
Skrmetti, Attorney General and Reporter, Office of the
Attorney General for the State of Tennessee, Whitney
Hermandorfer, Director of Strategic Litigation,
Harrison Gray Kilgore, Strategic Litigation Counsel
and Assistant Solicitor General, Sean D. Reyes,
Attorney General, Office of the Attorney General for
the State of Utah, Stanford E. Purser, Solicitor
General, Bridget Hill, Attorney General, Office of the
Attorney General for the State of Wyoming, D. David
DeWald, Deputy Attorney General, Ken Paxton,
Attorney General, Office of the Attorney General for
the State of Texas, Kellie E. Billings-Ray, Chief,
Environmental Protection Division, and Clayton
Smith, Assistant Attorney General. Kateland R.
Jackson, Attorney, Office of the Attorney General for
the State of Texas, entered an appearance. Matthew
D. Hardin and Christopher C. Horner were on the
brief for amicus curiae Government Accountability &
Oversight in support of petitioners.
Kathy G. Beckett, David M. Flannery, Keeleigh S.
Huffman, and Edward L. Kropp, were on the brief for
amicus curiae Midwest Ozone Group in support of
Industry and Arizona Coalition petitioners.
5a
Sarah A. Buckley and Alexandra L. St. Romain,
Attorneys, U.S. Department of Justice, argued the
causes for respondents. With them on the brief were
Todd Kim, Assistant Attorney General, and David P.
W. Orlin, Attorney, U.S. Environmental Protection
Agency.
Jonathan A. Wiener, Deputy Attorney General,
Office of the Attorney General for the State of
California, argued the cause for respondentintervenors State and Local Government. With him
on the brief were Rob Bonta, Attorney General, Stacy
Lau and Corey M. Moffat, Deputy Attorneys General,
Kristin K. Mayes, Attorney General, Office of the
Attorney General for the State of Arizona, Kristin M.
Wrobel, Assistant Attorney General, Kwame Raoul,
Attorney General, Office of the Attorney General for
the State of Illinois, Jason E. James, Assistant
Attorney General, Matthew J. Dunn, Chief,
Environmental Enforcement/Asbestos Litigation
Division, William Tong, Attorney General, Office of
the Attorney General for the State of Connecticut,
Matthew I. Levine, Deputy Associate Attorney
General, Jill Lacedonia, Assistant Attorney General,
Anthony G. Brown, Attorney General, Office of the
Attorney General for the State of Maryland, Michael
F. Strande, Assistant Attorney General, Andrea Joy
Campbell, Attorney General, Office of the Attorney
General for the Commonwealth of Massachusetts,
Turner Smith, Assistant Attorney General & Deputy
Chief, Keith Ellison, Attorney General, Office of the
Attorney General for the State of Minnesota, Peter N.
Surdo, Special Assistant Attorney General, Elizabeth
6a
Morrisseau, Assistant Attorney General, Office of the
Attorney General for the State of Michigan, Matthew
J. Platkin, Attorney General, Office of the Attorney
General for the State of New Jersey, Lisa Morelli,
Deputy Attorney General, Letitia James, Attorney
General, Office of the Attorney General for the State
of New York, Barbara D. Underwood, Solicitor
General, Judith N. Vale, Deputy Solicitor General,
Elizabeth A. Brody, Assistant Solicitor General,
Michael J. Myers, Senior Counsel, Nicholas C.
Buttino, Assistant Attorney General, Dave Sunday,
Attorney General, Office of the Attorney General for
the Commonwealth of Pennsylvania, Ann R.
Johnston, Assistant Chief Deputy Attorney General,
Dan Rayfield, Attorney General, Office of the
Attorney General for the State of Oregon, Paul
Garrahan, Attorney-in-Charge, Steve Novick, Special
Assistant Attorney General, Peter F. Neronha,
Attorney General, Office of the Attorney General for
the State of Rhode Island, Alison Hoffman Carney,
Assistant Attorney General, Charity R. Clark,
Attorney General, Office of the Attorney General for
the State of Vermont, Melanie Kehne, Assistant
Attorney General, Joshua L. Kaul, Attorney General,
Office of the Attorney General for the State of
Wisconsin, Bradley J. Motl, Assistant Attorney
General, Nick Brown, Attorney General, Office of the
Attorney General for the State of Washington,
Christopher H. Reitz, Assistant Attorney General,
Brian L. Schwalb, Attorney General, Office of the
Attorney General for the District of Columbia,
Caroline S. Van Zile, Solicitor General, Christopher
Gene King, and Sarah Jane Utley. Jennifer Slocum,
7a
Attorney, Office of the Attorney General for the State
of Washington, entered an appearance.
Seth L. Johnson, Marvin C. Brown IV, Shaun A.
Goho, Hayden W. Hashimoto, John Walke, and Emily
Davis were on the brief for respondent-intervenors
Health, Environmental, and Community Group.
Keri N. Powell was on the brief for amicus curiae
WE ACT for Environmental Justice, et al. in support
of respondents.
Jaclyn Lopez and Rachel Curran were on the brief
for amicus curiae Environmental Protection Network
in support of respondents.
Elizabeth J. Hubertz was on the brief for amicus
curiae National Parks Conservation Association in
support of respondents.
Jessica H. Arnell and Jason A. Schwartz were on
the brief for amicus curiae the Institute for Policy
Integrity at New York University School of Law in
support of respondents.
Before: MILLETT and CHILDS, Circuit Judges,
and GINSBURG, Senior Circuit Judge.
Opinion for the Court filed by Senior Circuit Judge
GINSBURG.
8a
I.
Background .......................................................... 8
A. Statutory Framework .................................... 9
B. The 2020 and 2024 Final Rules ................... 12
C. Procedural History ....................................... 14
II. Analysis .............................................................. 15
A. Statutory Authority...................................... 16
1. Reconsideration or revision? .................... 17
2. Authority to revise NAAQS ..................... 19
B. Arbitrary and Capricious ............................. 25
1. Impermissible considerations .................. 26
2. Considerations other than public health 28
3. Other objections ....................................... 33
III. Conclusion .......................................................... 37
9a
GINSBURG, Senior Circuit Judge: Two groups of
petitioners challenge a 2024 Environmental
Protection Agency rule revising the National Ambient
Air Quality Standards (NAAQS) for particulate
matter. Their claims specifically concern the decision
of the Administrator to revise the primary annual
standard for fine particulate matter (PM2.5) from 12
g/m to 9
g/m . In so doing, he expressly
reconsidered a decision of the previous Administrator
who in 2020 had decided to leave the 12 g/m
standard in place based upon the scientific evidence
then available. 1
A group of industry petitioners argue that the EPA
lacked statutory authority to promulgate the new
rule; several states contend the EPA’s decisionmaking was tainted by impermissible “environmental
justice” considerations; and all claim the agency
action was arbitrary and capricious in violation of the
Clean Air Act, 42 U.S.C. § 7607(d)(9)(A).
After initially defending the new rule, the EPA
now moves to vacate the rule on the grounds that it
exceeded its statutory authority and acted
unreasonably by failing to consider costs. Because
1 In 2025, after this case had been fully briefed and argued,
President Trump appointed a new Administrator of the EPA. For
purposes of this opinion, “the Administrator” refers to the
Administrator appointed by President Biden, who approved the
2024 Final Rule. References to “the previous Administrator” are
to the Administrator during President Trump’s first term, who
approved the 2020 Final Rule.
10a
these arguments lack merit, we deny the petitions for
review and the motion for vacatur.
I. Background
The Clean Air Act (CAA) requires that the
Administrator (1) publish a list of air pollutants, (2)
issue air quality criteria for those pollutants, and (3)
promulgate a primary and a secondary air quality
standard for each pollutant for which criteria have
been issued. See generally 42 U.S.C. §§ 7408-09.
Although this case most directly concerns a revision
to the primary standard for PM2.5, it necessarily
implicates steps antecedent to setting the NAAQS for
fine particulate matter.
A. Statutory Framework
The NAAQS-setting process begins with the
statutory requirement in § 7408 that the
Administrator publish and “from time to time
thereafter revise” a list of air pollutants “[f]or the
purpose of establishing [NAAQS].” § 7408(a)(1). Once
the Administrator lists an air pollutant, he must
publish air quality criteria for that pollutant, which
form the scientific basis upon which the corresponding
standard relies. § 7408(a)(2). Accordingly, the criteria
must “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 such
11a
pollutant in the ambient air, in varying quantities.”
Id. 2
Further,
§ 7408(c)
provides
that
“[t]he
Administrator shall from time to time review, and, as
appropriate, modify, and reissue any criteria . . .
issued pursuant to this section.” The EPA complies
with the requirement by preparing an “Integrated
Science Assessment,” which is a “comprehensive
evaluation and synthesis of the policy-relevant
science” that “serves as the scientific foundation for
the review of” the NAAQS. EPA, INTEGRATED
SCIENCE ASSESSMENT FOR PARTICULATE
MATTER P-9 (2019).
When the Administrator issues criteria for a
pollutant, he must “simultaneously” propose a
corresponding standard. § 7409(a)(2). After a period
for public comment, the proposed standard may be
promulgated and become law. § 7409(a)(1)(B).
As far as “practicable,” the criteria shall include the
following information:
2
(A) those variable factors (including atmospheric conditions)
which of themselves or in combination with other factors
may alter the effects on public health or welfare of such
air pollutant;
(B) the types of air pollutants which, when present in the
atmosphere, may interact with such pollutant to produce
an adverse effect on public health or welfare; and
(C) any known or anticipated adverse effects on welfare.
§ 7408(a)(2).
12a
Substantively, the Administrator must base the
standard upon the scientific criteria and exercise his
“judgment” to set the standard at a level “requisite to
protect the public health” with “an adequate margin
of safety.” § 7409(b)(1). 3 A NAAQS “may be revised in
the same manner as promulgated.” Id.
In short, pursuant to § 7408 the Administrator
maintains a list of air pollutants and publishes
scientific criteria for each one. Then, under § 7409(a)
and (b), the Administrator promulgates a NAAQS for
each listed pollutant and may later revise that
standard in the manner in which it was promulgated.
That brings us to § 7409(d)(1), the interpretation
of which the parties vigorously dispute. In full, the
provision states:
Not later than December 31, 1980, and
at five-year intervals thereafter, the
Administrator shall complete a
thorough review of the criteria published under section 7408 of this title
and the national ambient air quality
3
National primary ambient air quality standards,
prescribed under subsection (a) shall be ambient air quality
standards 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. Such primary standards may be revised in the
same manner as promulgated.
§ 7409(b)(1).
13a
standards promulgated under this
section and shall make such revisions
in such criteria and standards and
promulgate such new standards as
may be appropriate in accordance with
section 7408 [and section 7409(b)]. The
Administrator may review and revise
criteria or promulgate new standards
earlier or more frequently than
required under this paragraph.
As this court has recognized, the first sentence
requires that the Administrator “complete a
‘thorough’ review of the NAAQS every five years” and
“revise the criteria and standards or promulgate new
standards as appropriate.” Murray Energy Corp. v.
EPA, 936 F.3d 597, 605 (2019).
By statutory design, an independent council of
experts — the Clean Air Scientific Advisory
Committee (CASAC) — “assist[s] in this process.” Id.
At five-year intervals, the CASAC is required to
“complete a review” of existing NAAQS and criteria,
and “recommend to the Administrator any new
[NAAQS] and revisions of existing criteria and standards as may be appropriate.” § 7409(d)(2)(B). The
CASAC’s recommendations are timed to arrive
approximately one year prior to the date by which the
Administrator must complete his mandatory
quinquennial review. Compare § 7409(d)(2)(B) (fiveyear intervals for the CASAC beginning on January 1,
1980), with § 7409(d)(1) (five-year intervals for the
Administrator beginning on December 31, 1980).
14a
“[T]he ultimate decision to revise the NAAQS — and
the determination of the new level — rests with the
Administrator.” Murray, 936 F.3d at 605. To the
extent the Administrator’s proposed or promulgated
rule “differs in any important respect from any of [the
CASAC’s] recommendations,” however, it must be
accompanied by a statement that includes “an
explanation of the reasons for such differences.”
§ 7607(d)(3); see § 7607(d)(6)(A); Mississippi v. EPA,
744 F.3d 1334, 1355 (D.C. Cir. 2013).
B. The 2020 and 2024 Final Rules
In December 2020, the previous Administrator
promulgated a final rule that retained the preexisting
primary annual standard for PM2.5 of 12.0 g/m .
Review of the National Ambient Air Quality
Standards for Particulate Matter (2020 Final Rule),
85 Fed. Reg. 82684, 82685/2-3. The Administrator’s
judgment at that time was based upon the EPA’s
science and policy assessments as well as comments
from the CASAC and the public. Id. On the question
whether to retain the standard of 12.0 g/m for PM2.5,
however, the CASAC “did not reach consensus.” Id. at
82706/1-2. Those who supported keeping the standard
at 12.0 g/m “expressed the view that substantial
uncertainty remains in the evidence for associations
between PM2.5 exposures and mortality or serious
morbidity effects.” Id. at 82706/2. Those who
supported lowering the standard “emphasized recent
findings of associations with PM2.5 in areas with
average long-term PM2.5 concentrations below the
level of the annual standard and studies that show
15a
positive associations even when estimated exposures
above 12 g/m are excluded from analyses.” Id. at
82707/1.
The previous Administrator’s approach in 2020
ultimately tracked the views of those CASAC
members who supported keeping the standard at 12.0
g/m . He emphasized “important uncertainties and
limitations” in the epidemiological studies, expressed
concern “about placing too much weight” on those
studies, and noted his “caution in directly comparing
the reported study mean values to the standard level.”
Id. at 82716/3-82717/1. He therefore found it “more
appropriate to focus on the body of studies together”
and took note of “the mean of study-reported means”
— 13.5 g/m — which was “above the level of the
current standard.” Id. at 82717/1. Several groups filed
petitions challenging the 2020 Final Rule, which
petitions were consolidated in this court as California
v. EPA, No. 21-1014.
Meanwhile, President Biden assumed office in
January 2021, and in February the EPA filed an
unopposed motion to hold the consolidated petitions
in abeyance for 90 days. In its motion, the EPA
explained that the President had issued an executive
order directing agency review of certain actions,
including the 2020 Final Rule. See Exec. Order No.
13990, 86 Fed. Reg. 7037 (2021). We granted the
EPA’s motion and its eight subsequent motions to
extend the period of abeyance, as a result of which the
case has remained in abeyance for more than five
years, spanning the Biden and Trump Administra-
16a
tions. During that time, the Administrator initiated
the review that culminated in the 2024 Final Rule
before us in this case. See Reconsideration of the
National Ambient Air Quality Standards for
Particulate Matter (2024 Final Rule), 89 Fed. Reg.
16202, 16210/2 (explaining the EPA announced its
decision to reconsider the 2020 Final Rule in June
2021 “because the available scientific evidence and
technical information indicate that the current
standards may not be adequate to protect public
health”).
As part of its review, the EPA reopened and
revised the underlying air quality criteria to account
for recent scientific literature. Id. at 16211/1-2. The
result was the EPA’s Supplement to the 2019
Integrated Science Assessment for Particulate Matter
and a new policy assessment. See id. at 16212/1-3. The
CASAC also conducted a new review and this time
unanimously concluded that the 12.0 g/m primary
standard should be lowered. See id. at 16204/2,
16256/3. A majority of the CASAC recommended a
standard of 8 to 10 g/m , while a minority deemed a
standard of 10 to 11 g/m adequate to protect public
health. See id. at 16204/2.
The EPA promulgated the 2024 Final Rule, titled
“Reconsideration of the National Ambient Air Quality
Standards for Particulate Matter,” in March of that
year. Id. at 16202/1. The Administrator considered
the various positions of the CASAC members and
concluded that setting the standard at 9 g/m , the
17a
middle of the range recommended by the CASAC
majority, was appropriate. Id. at 16204/3.
C. Procedural History
Industry groups and several states thereafter filed
petitions challenging the 2024 Final Rule, which this
court consolidated. Other states intervened in support
of the rule, as did a group of health, environmental,
and community entities.
We heard oral argument on the petitions for review
on December 16, 2024. Two months later, the new
Administrator appointed by President Trump filed a
motion to hold the case in abeyance to give the agency
time to review the 2024 Final Rule. We granted that
motion and three more extending the period of
abeyance. In two of those motions, the EPA stated its
intention to begin a new rulemaking to replace the
2024 Final Rule.
In November 2025, however, the EPA filed a
motion for vacatur of the 2024 Final Rule and
reversed course in two ways. First, the EPA now
agreed with the Industry Petitioners that the EPA
had exceeded its statutory authority in issuing the
2024 Final Rule and had acted unreasonably by
ignoring costs. Second, the EPA seemingly disavowed
any intention to begin a new rulemaking to replace
the 2024 Final Rule. The EPA explained that it had
simply been considering next steps when it said that
it would begin a new rulemaking and now called any
future rulemaking “irrelevant.”
18a
The intervenors opposed the EPA’s motion for
vacatur. The Health Group Intervenors filed a crossmotion for this court to lift the abeyance and decide
the merits of the petitions for review. The EPA agrees
that we should lift the abeyance and asks us to reach
the merits, hold the 2024 Final Rule unlawful, and
vacate it. We now grant the cross-motion to lift the
abeyance and proceed to the merits of the petitions for
review and the motion for vacatur.
II. Analysis
We have jurisdiction under 42 U.S.C. § 7607(b)(1).
We review the 2024 Final Rule under the same
standard as applies to claims brought under the
Administrative Procedure Act. Heating, Air
Conditioning & Refrigeration Distribs. Int’l v. EPA, 71
F.4th 59, 63 (D.C. Cir. 2023). As relevant here, we will
set aside the rule if it is “arbitrary, capricious, an
abuse of discretion,” or “in excess of statutory
jurisdiction, authority, or limitations, or short of
statutory right.” § 7607(d)(9)(A), (C).
The standard by which we determine whether the
Administrator acted within his statutory authority
differs from the standard applicable when reviewing
the Administrator’s judgment on the merits. With
respect to the former, this court exercises its
“independent judgment in deciding whether an
agency has acted within its statutory authority.”
Loper Bright Enters. v. Raimondo, 603 U.S. 369, 412
(2024). When as here “a particular statute delegates
19a
authority to an agency consistent with constitutional
limits, courts must respect the delegation, while
ensuring that the agency acts within it.” Id. at 413; see
§ 7409(b)(1) (delegating NAAQS-setting authority to
the “judgment of the Administrator” within
statutorily defined limits); Whitman v. Am. Trucking
Ass’ns, 531 U.S. 457, 474 (2001) (upholding the
constitutionality of this delegation).
With respect to this court’s review of the
Administrator’s decision on its merits, we “defer to the
EPA’s scientific judgment while examining the record
to ensure the Agency has considered the relevant
factors and reasonably explained how it reached its
conclusions.” Am. Farm Bureau Fed’n v. EPA, 559
F.3d 512, 519-20 (D.C. Cir. 2009). “We will give an
extreme degree of deference to the agency when it is
evaluating scientific data within its technical
expertise.” Ctr. for Biological Diversity v. EPA, 749
F.3d 1079, 1087-88 (D.C. Cir. 2014) (cleaned up).
A.
Statutory Authority
The Industry Petitioners, the State Petitioners,
and the EPA contend the Administrator lacked
statutory authority to promulgate the 2024 Final
Rule. They argue first that the Administrator acted
unlawfully when he reconsidered the 2020 Final Rule
and issued the Final Rule only four years after the
previous rulemaking; any implicit authority the EPA
may have had to reconsider its rules was displaced,
they say, by § 7409(d)(1), which requires the
Administrator to review and revise (if appropriate)
20a
criteria and NAAQS every five years and allows him
to review and revise them more frequently than
required. They argue second that insofar as the 2024
Final Rule can be characterized as a revision rather
than a reconsideration, § 7409(d)(1) requires the
Administrator to perform a “thorough review” of
criteria and NAAQS as part of any revision —
including any off-cycle revision such as this one.
Because the EPA did not perform such a review, they
ask us to set aside the 2024 Final Rule.
According to the State and Health Group
Intervenors, § 7409(d)(1) does not displace but
supplements the Administrator’s authority to revise a
NAAQS under § 7409(b)(1); § 7409(d)(1) establishes a
mandatory duty — that the Administrator perform a
“thorough review” of NAAQS every five years — that
is “distinct from EPA’s authority to otherwise
reconsider or revise its NAAQS.”
1. Reconsideration or revision?
We need not delineate the precise boundaries of
the Administrator’s reconsideration authority
because the 2024 Final Rule is best understood as a
revision to the criteria and NAAQS for particulate
matter. Although titled a “reconsideration” of the
2020 Final Rule, it is in substance a revision of an
individual NAAQS. Indeed, the first paragraph
unambiguously states that it provides the “rationale
for the Administrator’s final decisions to revise the
primary annual PM2.5 standard.” 89 Fed. Reg. at
16203/1.
21a
The Industry Petitioners argue the 2024 Final
Rule must be a reconsideration because the
Administrator “prepared only an ‘[Integrated Science
Assessment]
Supplement’”
and
“deferred
consideration of a number of available scientific
studies” until the next quinquennial review. They say
those facts are controlling because a true revision
“must build a record from scratch” while a
“reconsideration would take as its starting point the
previous decision not to revise and the associated
record.” Nothing in the statutory scheme, however,
reflects that distinction. We see no reason the
Administrator cannot revise a NAAQS and, in the
process, reconsider a prior rule that left the existing
NAAQS unchanged. 4
The Industry Petitioners call upon SEC v. Chenery
Corp., 318 U.S. 80 (1943), to argue that the EPA
cannot “recast” its reconsideration as a revision.
“Under the Chenery doctrine, a reviewing court must
confine itself to the grounds upon which the record
discloses that the agency’s action was based.” Byers v.
Comm’r, 740 F.3d 668, 680 (D.C. Cir. 2014). The
To the extent the Industry Petitioners argue that the
Administrator lacks any reconsideration authority, they are
mistaken. The CAA explicitly requires the Administrator to
“convene a proceeding for reconsideration” under certain
circumstances. § 7607(d)(7)(B). We need not, however, fully
define the interplay between the Administrator’s reconsideration
authority and his other powers. It is enough that we conclude the
Administrator may revise criteria and NAAQS and, in the course
of so doing, reconsider a previous rule.
4
22a
Chenery doctrine, however, has no application here
because the EPA consistently characterized its rule as
a revision. It did so in its proposed and final rules, see
88 Fed. Reg. 5558, 5560/2 (2023); 89 Fed. Reg. at
16203/1, and it repeatedly referred to its decision to
“revise” the “existing” NAAQS in its responses to
comments, see EPA, Responses to Significant
Comments on the 2023 Proposed Rule for the
Reconsideration of the National Ambient Air Quality
Standards for Particulate Matter (Responses to
Comments) at 27, 34, 46, 56, 118 (2024). This
characterization is unsurprising because the principal
change worked by the 2024 Final Rule was its revision
of the primary annual standard for PM2.5.
2. Authority to revise NAAQS
Our conclusion that the 2024 Final Rule is best
understood as a revision does not answer whether the
Administrator acted within his statutory authority to
revise a NAAQS. The question is particularly
important here because the parties dispute the source
of the Administrator’s revision authority. The
Industry and State Petitioners — joined by the EPA
in its motion for vacatur — argue that § 7409(d)(1)
provides the sole source of the Administrator’s
revision authority and authorizes him to revise
NAAQS off-cycle only if he first performs a “thorough
review” of the criteria and NAAQS, which the EPA
concedes it did not do before promulgating the 2024
Final Rule. The intervenors respond that § 7409(b)
authorizes the Administrator to revise a standard and
23a
does not require him to first perform a “thorough
review.”
“Statutory interpretation, as we always say,
begins with the text.” Ross v. Blake, 578 U.S. 632, 638
(2016). As relevant here, two provisions of § 7409 refer
to the Administrator’s authority to revise a NAAQS.
Section 7409(b) provides the substantive principles
the Administrator is to apply when set-ting a primary
or secondary standard and states that each “may be
revised in the same manner as promulgated.” This
provision was added to the CAA in 1970, Pub. L. No.
91-604, § 4(a), 84 Stat. 1676, 1679-80, and predates
the addition of § 7409(d)(1) by seven years, Clean Air
Act Amendments of 1977, Pub. L. No. 95-95, § 106(a),
91 Stat. 685, 691. Consequently, the Administrator
indisputably had authority to revise primary and
secondary NAAQS at any time. So far, so clear.
The first sentence of § 7409(d)(1) then requires
the Administrator to “complete a thorough review” of
the existing criteria and standards every five years
and to revise them if appropriate. The reason for that
requirement is obvious: By requiring a “thorough
review” the Congress ensured the Administrator
would comprehensively evaluate each and every
existing criterion and standard on a regular basis. In
other words, it serves as an action-forcing device to
ensure NAAQS do not fall too far behind the evolving
scientific evidence.
Together § 7409(b) and the first sentence of
§ 7409(d)(1) provide that the Administrator may
24a
revise NAAQS “in the same manner as promulgated”
and must revise NAAQS as appropriate every five
years. 5 The second sentence of § 7409(d)(1) then adds
that “[t]he Administrator may review and revise
criteria or promulgate new standards earlier or more
frequently than required under this paragraph,” i.e.,
under the first sentence. The plain objective of this
sentence is to clarify that the quinquennial review by
the first sentence of § 7409(d)(1) does not limit the
Administrator’s authority to revise a standard sooner;
nothing in § 7409(d)(1) displaces the Administrator’s
revision authority in § 7409(b).
The petitioners and the EPA raise several
objections to this conclusion. The EPA agrees that it
5 When questioned about § 7409(b)(1) at oral argument, the
Industry Petitioners again invoked Chenery to argue that the
EPA could not rely upon § 7409(b)(1) as the source of its revision
authority. As an initial matter, we have long held the Chenery
doctrine inapplicable “when the question presented [was] one of
statutory construction.” Bldg. & Constr. Trades Dep’t, AFL-CIO
v. U.S. Dep’t of Lab. Wage Appeals Bd., 829 F.2d 1186, 1189
(1987); see also Canonsburg Gen. Hosp. v. Burwell, 807 F.3d 295,
304 (D.C. Cir. 2015) (“We have explained that Chenery only
limits judicial review of factual determinations or policy
judgments that the agency alone is authorized to make” (cleaned
up)). In any event, the Chenery doctrine does not apply here
because the EPA raised § 7409(b) as a basis for the 2024 Final
Rule in response to comments: “The EPA understands [§ 7409(b)]
to authorize the Administrator to revise the NAAQS.” Responses
to Comments at 120. The EPA also cited its “implicit and explicit
authority to revisit earlier decisions on the NAAQS,” id. at 118,
a matter we discuss below at 21, and its “authority under
[§ 7409]” generally, id. at 118, 122, as authorizing its revision.
25a
had revision authority before the Congress added
§ 7409(d)(1), but it disputes the source of that
authority. The EPA claims it had “inherent authority”
to revise a standard before 1977, and § 7409(b) provided only the procedure for doing so. As we have
explained, however, “the term ‘inherent’ is misleading
because it is axiomatic that administrative agencies
may act only pursuant to authority delegated to them
by Congress. Thus, the more accurate label for the
power [the] EPA describes is ‘statutorily implicit.’”
Nat. Res. Def. Council v. Regan, 67 F.4th 397, 401
(2023) (cleaned up). More to the point, § 7409(b)
expressly recognizes that a NAAQS “may be revised.”
The EPA offers no evidence the Congress intended to
limit the Administrator’s pre-existing revision
authority in 1977 when it added the periodic-review
requirement. We therefore do not agree with the EPA
and the petitioners that § 7409(d)(1) provides the
“exclusive source of [the Administrator’s] revision
authority.”
Even if § 7409(b) provides revision authority, says
the EPA, we must read that provision in harmony
with § 7409(d)(1). The petitioners and the EPA reason
that the “thorough review” requirement applies not
only to the quinquennial review mandated by the first
sentence of § 7409(d)(1), but also to the off-cycle
revisions referenced in the second sentence. We do
not, however, understand § 7409(d)(1) to require a
“thorough review” as a precondition to an off-cycle
revision.
26a
We begin again with the statutory text. The
requirement that the Administrator “complete a
thorough review” of the existing criteria and
standards every five years resides in the first sentence
of § 7409(d)(1). The phrase “thorough review,”
however, does not appear in the second sentence of
§ 7409(d)(1): “The Administrator may review and
revise criteria or promulgate new standards earlier or
more frequently than required under this paragraph.”
The Industry Petitioners and the EPA argue this
sentence nonetheless implicitly cross-references the
first sentence through the phrases “review” and
“under this paragraph.” Per this reading of
§ 7409(d)(1), the sole difference between the two
sentences is one of timing; the process for revising a
standard — whether at the required intervals or offcycle — is the same.
We are not persuaded. Starting with “review,” the
absence of the word “thorough” in the second sentence
of § 7409(d)(1) is significant: “When Congress includes
particular language in one section of a statute but
omits it in another — let alone in the very next
provision — this Court presumes that Congress
intended a difference in meaning.” Loughrin v. United
States, 573 U.S. 351, 358 (2014) (cleaned up).
Omitting the word “thorough” in the second sentence
of § 7409(d)(1) suggests the Congress did not intend to
require the Administrator to perform such a review
when making an off-cycle revision.
The phrase “under this paragraph” does refer to
the first sentence of § 7409(d)(1), but we do not read
27a
that reference as importing into the second sentence
all the requirements in the first sentence. As the State
Intervenors explain, that reference simply clarifies
that the Administrator may act sooner than required
by the first sentence of § 7409(d)(1), that is, “earlier or
more frequently” than every five years. Indeed, the
Industry Petitioners themselves characterize that
phrase as “a cross-reference to the five-year intervals
mandated in the first sentence of [§ 7409(d)(1)].” To
read it as also importing the “thorough review”
requirement into the second sentence, as the Industry
Petitioners and the EPA ask us to do, would stretch
its meaning to the point of frustrating it. That seems
certain when one considers that this interpretation
would undermine rather than advance the plain
objective of the first sentence: Requiring a full review
whenever the Administrator seeks to revise a single
criterion or standard off-cycle would prevent the
Administrator from responding promptly to changes
in the relevant science.
When pressed on this point at oral argument, the
Industry Petitioners suggested a “thorough review”
outside the five-year cycle would require only that the
Administrator thoroughly review the scientific
criteria underlying the standard he seeks to revise.
Oral Arg. Tr. 39:24-40:15. Even this more limited
understanding of “thorough review” in the second
sentence would undercut the Administrator’s ability
to keep a NAAQS up-to-date. Requiring a
comprehensive review of all the scientific inputs to the
criteria for a single standard would unnecessarily
delay promulgation of a new standard.
28a
This case illustrates the problem. By 2021 the EPA
became aware of certain scientific studies likely to
require a change in the standard for fine particulate
matter and began updating its science and policy
assessments. That process resulted in the EPA’s
Supplement to the 2019 Integrated Science
Assessment for Particulate Matter and a new policy
assessment, both published in May 2022. See 89 Fed.
Reg. 16212/3-16213/1. Requiring the EPA to expend
limited resources to review other scientific studies
unlikely to affect the final standard could delay but
not improve the result. 6 That the Congress gave the
agency discretion to address the criteria most relevant
to protecting public health is the only sensible reading
of the statute.
The EPA says its reading of § 7409(d)(1) better
consists with other provisions of the Clean Air Act,
particularly the requirement that the Administrator
establish air quality criteria based upon the “latest
scientific knowledge.” § 7408(a)(2). To the contrary, as
6 At oral argument, the Industry Petitioners said that in order
to perform a “thorough review” of a NAAQS off-cycle the EPA
would not need to revisit studies that it had considered during
its prior review. Oral Arg. Tr. 35:21-36:7. As for new studies, the
notice-and-comment process provides an opportunity for
interested parties to argue that the EPA did not consider
relevant studies or that it did not provide a reasoned explanation
for the studies it chose to rely upon. Indeed, the Industry
Petitioners did the former in this rulemaking proceeding, but
they did not raise that argument on appeal. See Oral Arg. Tr.
86:23-88:7.
29a
the Health Group Intervenors argue, importing the
“thorough review” requirement into the second
sentence of § 7409(d)(1) would create tension with
several other parts of the statutory scheme. For
example, the provision requiring the Administrator to
issue criteria for newly designated air pollutants also
requires that the Administrator “from time to time
review, and, as appropriate, modify, and reissue any
criteria . . . issued pursuant to this section.” § 7408(c).
It says nothing of performing a thorough review first
and instead affords the Administrator maximum
flexibility to update criteria so as to reflect the
prevailing science. Under the Industry Petitioners’
and the EPA’s interpretation of the second sentence of
§ 7409(d)(1), however, the Administrator could “revise
criteria . . . more frequently than required” by the
mandatory review only if he performs the “thorough
review” required by the first sentence. That effectively
reads the thoroughness requirement into § 7408(c),
which is inconsistent with the broad and flexible
terms used in that provision. See Loper Bright Enters.,
603 U.S. at 395 (recognizing “appropriate” as a term
that “leaves agencies with flexibility”); cf. Waterkeeper
All. v. EPA, 140 F.4th 1193, 1215 (9th Cir. 2025)
(explaining that the use of the phrase “from time to
time” in the Clean Water Act “suggests that EPA has
discretion about when to revise such standards”).
The Industry Petitioners and the EPA resist this
conclusion by arguing that an off-cycle revision
without a thorough review would undermine the
periodic-review requirement. We do not see how.
Acting sooner than the five-year review cycle required
30a
by § 7409(d)(1) does not excuse the Administrator
from doing a thorough review every fifth year. Nor are
the Industry Petitioners correct that an off-cycle
revision under our interpretation of § 7409 would be
“without any limitations or requirements whatsoever.”
Any revision must satisfy the substantive standard in
§ 7409(b) and comply with the strictures of the CAA,
including its requirement of reasoned decisionmaking. See Heating, Air Conditioning & Refrigeration
Distribs., 71 F.4th at 63. Those requirements are not
toothless and have been applied by this court when
reviewing NAAQS on many occasions.
In sum, the Administrator must “complete a
thorough review” and, if appropriate, revise criteria
and NAAQS every five years pursuant to the first
sentence of § 7409(d)(1), but he may revise them more
frequently without completing a “thorough review.”
The Administrator therefore acted within his
statutory authority by promulgating the 2024 Final
Rule revising the air quality standard for fine
particulate matter.
B. Arbitrary and Capricious
An agency rule will be held arbitrary and
capricious if, among other reasons, in formulating it
the agency (1) relied upon an impermissible
consideration, (2) entirely failed to con-sider an
important aspect of a problem, (3) explained the rule
in a way that runs counter to the evidence before it, or
(4) adopted a rule that “is so implausible that it could
not be ascribed to a difference in view or the product
31a
of agency expertise.” Motor Vehicle Mfrs. Ass’n v. State
Farm Mut. Auto. Ins., 463 U.S. 29, 43 (1983). The
Industry and State Petitioners claim the EPA violated
the CAA in each of these respects, and the EPA itself,
in moving for vacatur, now joins the petitioners in
asserting that the Administrator unreasonably
ignored the costs associated with the mid-cycle review
giving rise to the 2024 Final Rule. Each of these
arguments is either foreclosed by precedent or
unsupported by the record.
1. Impermissible considerations
The
State
Petitioners
argue
that
the
Administrator
relied
upon
impermissible
considerations — namely, promoting environmental
justice and countering climate change — to justify the
2024 Final Rule. They begin by correctly observing
that the Administrator’s remit under the CAA is to set
primary NAAQS “requisite to protect the public
health” with “an adequate margin of safety.”
§ 7409(b)(1). They then point to President Biden’s
Executive Order No. 13990, which served as the
initial catalyst for the EPA’s review, as evidence of the
Administrator’s impermissible environmental goals.
See 89 Fed. Reg. at 16210/2 (explaining the order
“directed review of certain agency actions” including
the 2020 Final Rule).
The EPA, for its part, did not dispute in its
response brief the State Petitioners’ premise that
environmental justice and climate change are
impermissible reasons for setting primary NAAQS
32a
under § 7409(b)(1). Instead, the agency defended the
record it made and characterized the State
Petitioners’ argument as little more than an
“unsubstantiated accusation that climate or
environmental justice considerations improperly
influenced the Agency’s course of action.” We agree
with the latter point.
The EPA here is on solid ground; the
administrative record simply does not support the
State Petitioners’ claim. The 2024 Final Rule
accurately sets out and adheres to the appropriate
legal framework for setting primary NAAQS
consistent with § 7409(b)(1). See 89 Fed. Reg. at
16206/1-16207/1. The 2024 Final Rule explained that
the Administrator announced his decision to
reconsider the 2020 Final Rule in June 2021 “because
the available scientific evidence and technical information indicate[d] that the current standards may not
be adequate to protect public health.” Id. at 16210/2.
Indeed, in the lead-up to its review the EPA
“preliminarily concluded” that certain new studies
“were likely to be relevant” and “would potentially
warrant a reopening of the air quality criteria.” Id. at
16211/1. None of these statements suggests the
Administrator considered environmental justice or
climate change. On the contrary, the EPA made clear
in its response to public comments that it had “no
intention or goal to reduce greenhouse gases through
this rulemaking.” Response to Comments at 135.
Against that, the best the State Petitioners can
muster are repeated references to Executive Order
33a
No. 13990, which they unilaterally christen “the
Climate Order.” The references to cli-mate change in
that order, however, are not specific to NAAQSsetting and cannot overcome the evidence — or lack of
it — in the administrative record.
In any event, the EPA addressed this issue headon when responding to comments regarding the
proposed rule. The agency made clear that the
decision to revisit the NAAQS for particulate matter
“rested with the EPA” and was grounded upon
“additional available information, as well as advice
from the CASAC and public comment.” Id. at 134.
Notably, although the Administrator ultimately
adopted the recommendation of the CASAC, the State
Petitioners do not accuse the CASAC of improper
motives. At bottom, the State Petitioners’ mixture of
conjecture and strident accusations falls far short of
demonstrating that the Administrator relied upon
something other than public health when setting the
NAAQS for particulate matter.
2. Considerations other than public health
The Industry and State Petitioners, now joined by
the EPA, also argue that the 2024 Final Rule is
invalid because the Administrator failed to consider
matters other than public health, principally costs
and attainability. Both the Supreme Court and this
court have repeatedly rejected similar efforts to inject
considerations unrelated to public health into the
NAAQS-setting process. See Am. Trucking, 531 U.S.
at 464 (citing D.C. Circuit cases holding that
34a
“economic considerations may play no part in the
promulgation of ambient air quality standards”
(cleaned up)); Murray, 936 F.3d at 621-22; Am.
Petroleum Inst. v. Costle, 665 F.2d 1176, 1185 (D.C.
Cir. 1981). These cases are controlling.
The petitioners invite us to read these precedents
narrowly in light of the Supreme Court’s opinion in
Michigan v. EPA, 576 U.S. 743 (2015). In that case the
Court considered whether the phrase “appropriate
and necessary” in a different provision of the CAA
required the EPA to consider costs. Id. at 752. Justice
Scalia, writing for the Court, concluded that it did. Id.
He explained that it was not “rational, never mind
‘appropriate,’ to impose billions of dollars in economic
costs in return for a few dollars in health or
environmental benefits.” Id. Yet Justice Scalia
explicitly distinguished that case from American
Trucking, an opinion he had authored a decade earlier. He explained that the provision at issue in the
earlier case — § 7409(b)(1) — “expressly” directed the
EPA to set NAAQS at levels requisite to protect public
health and there-fore should not be read “as implicitly
allowing the [EPA] to consider cost anyway.” Id. at
755-56. Consistent with that understanding of the
statutory scheme, in Murray this court rejected an
invitation based upon Michigan to incorporate cost
considerations into § 7409(d)(1). 936 F.3d at 622
(explaining the word “appropriate” in § 7409(d)(1) did
not require consideration of economic costs and that
the outcome in Michigan turned upon its “statutory
context”). In other words, Michigan reinforces rather
than undermines American Trucking.
35a
Murray similarly forecloses the petitioners’
argument that the Administrator was required to
consider attainability. In that case the petitioners
argued the CAA “requires EPA to set NAAQS that are
attainable.” Id. To the contrary, we explained that the
Congress “recognized the possibility that some states
could not achieve attainment,” but “rather than
watering down the nationally applicable standards,
[it] allowed EPA to relax enforcement on a case-bycase basis.” Id. at 623. The EPA, that is, may consider
attainability “during enforcement, not when setting
standards.” Id.; see also Costle, 665 F.2d at 1190
(“[T]he question of attainability is not relevant to the
setting of ambient air quality standards under the
Clean Air Act”).
The Industry and State Petitioners attempt to
elude these cases by distinguishing between setting a
NAAQS (to which § 7409(b) applies) and the anterior
decision to revise a NAAQS. In their view, the EPA
must consider costs and attain-ability when it initially
decides “whether a revision ‘may be appropriate.’”
They reason that the first sentence of § 7409(d)(1)
provides for revisions “as may be appropriate in
accordance with [§ 7408] and [§ 7409(b)],” and
Michigan says the word “appropriate . . . requires at
least some attention to cost,” 576 U.S. at 752. The
Industry Petitioners attempt to bolster this conclusion
by pointing to § 7409(d)(2)(C), which requires the
CASAC to “advise the Administrator of any adverse
public health, welfare, social, economic, or energy
effects which may result from various strategies for
36a
attainment and maintenance of such national
ambient air quality standards.” It is only when the
EPA formulates a standard that the Industry
Petitioners concede the EPA may disregard costs and
attainability. The EPA, for its part, asserts that
nothing in § 7409(d)(1) prevents it from considering
costs, but it tellingly fails to address American
Trucking or Murray.
The Industry Petitioners’ and the EPA’s
interpretation does not square with that controlling
precedent:
Their
approach
necessarily,
but
impermissibly, injects economic considerations and
attainability into the NAAQS-setting process. See Am.
Trucking, 531 U.S. at 464-65; Murray, 936 F.3d at
622-23. Whether the Administrator considers costs
and attainability when deciding whether to revise a
NAAQS or when setting the NAAQS would have the
same impermissible effect.
Moreover, nothing in § 7409(d)(1) suggests the
Congress intended to distinguish between the
Administrator considering whether to revise a
standard and his setting a standard. That section
requires that the Administrator revise a NAAQS “as
may be appropriate in accordance with [§ 7408] and
[§ 7409(b)].” In this way, the Congress tied the word
“appropriate” in § 7409(d)(1) to § 7409(b), which in
turn pre-scribes NAAQS that “are requisite to protect
the public health.” See Am. Trucking Ass’ns v. EPA,
175 F.3d 1027, 1040 (D.C. Cir. 1999) (explaining that
“the clause immediately following ‘appropriate’ . . .
affirmatively precludes consideration of costs in
37a
revising NAAQS”). 7 Indeed, this court in Murray
explained that we had “already rejected the idea that
‘appropriate’ in [§ 7409(d)(1)] requires consideration
of economic costs.” 936 F.3d at 622 (citing Am.
Trucking Ass’ns, 175 F.3d at 1040-41, aff’d in relevant
part, 531 U.S. at 464-65).
Nor does § 7409(d)(2)(c) support the petitioners’
two-step approach. In Murray we explained why
§ 7409(d)(2)(C) did not imply that economic costs were
permissible considerations in the NAAQS-setting
process: “[T]his provision was intended to ‘enable the
[EPA] to assist the States in carrying out their
statutory role as primary implementers of the
NAAQS,’ but had ‘no bearing upon whether cost
considerations are to be taken into account in
formulating the [NAAQS].’” Id. (quoting Am.
Trucking, 531 U.S. at 470-71). Although Murray
spoke of “formulating the NAAQS,” and not the
anterior decision whether to revise a standard, we do
not see how its reasoning would permit the
Administrator to consider costs at either stage. 8
7 The petitioners briefly argue that this conclusion results in
an interpretation of the statutory scheme that violates the nondelegation doctrine. That argument is foreclosed by American
Trucking, in which the Supreme Court held that the “scope of
discretion [§ 7409(b)(1)] allows is in fact well within the outer
limits of our nondelegation precedents.” 531 U.S. at 474. The
petitioners have preserved the issue whether that case was
correctly decided expressly for further review before the
Supreme Court.
8 The Industry Petitioners also argue, based upon their twostep interpretation of § 7409(d)(1), that the Administrator was
38a
The State Petitioners take a slightly different tack
and, without quite saying so, challenge the holding of
American Trucking directly. They point to the
Supreme Court’s statement that “ozone and
particulate matter are ‘nonthreshold’ pollutants that
inflict a continuum of adverse health effects at any
airborne concentration greater than zero, and hence
require the EPA to make judgments of degree.” 531
U.S. at 475. Consequently, when setting NAAQS for
these pollutants the Administrator must exercise
“discretion” guided by the requirements of
§ 7409(b)(1). Id. at 475-76. Then, quoting Justice
Breyer’s concurrence, the State Petitioners attempt to
read consideration of costs into the exercise of the
Administrator’s discretion. See id. at 494 (Breyer, J.,
concurring in part and concurring in the judgment)
(The CAA “does not require the EPA to eliminate
every health risk, however slight, at any economic
cost, however great, to the point of hurtling industry
required to consider current air quality when deciding whether
to revise the NAAQS. They claim the Administrator “overstated
the benefits of a NAAQS revision by assuming a fictional world
in which no area of the country had better air quality than the
NAAQS standard of 12 g/m ,” when in fact “the current air
quality in most areas significantly out-performs that standard.”
As we have explained, however, it was “not unreasonable for the
EPA to measure expected benefits from the new NAAQS in part
upon the assumption that, if the new NAAQS were not adopted,
then each area would in the future just meet the existing
standard.” Am. Petroleum Inst. v. EPA, 684 F.3d 1342, 1352
(2012).
39a
over the brink of ruin, or
deindustrialization” (cleaned up)).
even
forcing
The opinion of the Court to the contrary, however,
was clear: § 7409(b)(1) “unambiguously bars cost
considerations from the NAAQS-setting process.” 531
U.S. at 471. As we explained in Murray in response to
the petitioners’ reliance on the same passage from
Justice Breyer’s concurrence, “the concurrence does
not govern our decision, and in any event, Justice
Breyer agreed with the majority that economic costs
could not be considered in setting NAAQS.” 936 F.3d
at 622 (citing Am. Trucking, 531 U.S. at 490).
In short, “when Congress directs an agency to
consider only certain factors in reaching an
administrative decision, the agency is not free to
trespass beyond the bounds of its statutory authority
by taking other factors into account.” Lead Indus.
Ass’n v. EPA, 647 F.2d 1130, 1150 (D.C. Cir. 1980).
Here the EPA properly followed the Congress’s
direction and declined to consider non-public health
factors throughout the NAAQS-setting process.
3. Other objections
The petitioners also contend the Administrator’s
decision
was
inadequately
explained
and
insufficiently supported by the evidence. Their
arguments are of three types. First, the Administrator
did not sufficiently justify performing an off-cycle
review. Second, the Administrator failed adequately
to explain his reasons for departing from the
40a
conclusions of the 2020 Final Rule. Third, the
scientific evidence does not justify a standard of 9
g/m .
Off-Cycle Revision. The petitioners’ first
argument requires little additional discussion. Their
charge is that the Administrator’s decision to perform
an off-cycle revision was unprecedented and without
regard for the reliance interests of those affected by
the revised standard. As already explained, however,
the Congress authorized the Administrator to revise
criteria and NAAQS outside the five-year cycle in the
interests of public health.
Changed Course. The petitioners’ second
argument merits more attention but is equally
unavailing. From our review of the 2024 Final Rule,
we hold that the Administrator satisfactorily
explained his basis for revising the standard his
predecessor had decided not to revise in the 2020
Final Rule. Far from ignoring the earlier rule, the
Administrator acknowledged that his analysis
differed from that of “the prior Administrator,” 89
Fed. Reg. at 16276/2, and he explained the bases for
his decision at length, id. at 16273/3-16277/1. His
reasons included newly available scientific evidence,
the unanimous recommendation of CASAC, and a
reweighing of the previously available evidence. Id.
With respect to new evidence, the Administrator
explained that “a number of studies” were published
after the previous literature cutoff date, and he
therefore
had
“additional
information
for
41a
consideration in reaching his final conclusions.” Id. at
16275/2. Several of these studies improved upon those
that were available to the previous Administrator, see
id. at 16276/1 (explaining the studies “employed
statistical approaches that attempted to more
extensively account for confounders and are more
robust to model misspecification”); and at 16276/3
(identifying “several [new] accountability studies”),
including the “study that report[ed] the long-term
mean PM2.5 concentration of 9.3 g/m ,” id. at 16275/3.
The last-referenced study was particularly significant
because the Administrator set the standard
“somewhat below the lowest long-term study-reported
mean PM2.5 concentration reported in key U.S.
epidemiologic studies, which is 9.3 g/m .” Id.
The Administrator also explained that the CASAC
had unanimously agreed that the 2020 standard was
inadequate based upon the newly available scientific
evidence. Id. at 16204/3, 16256/3, 16275/1. Although
most of the CASAC’s membership had changed since
its earlier review, two members remained the same.
Of those, one had previously concluded that the
annual PM2.5 standard should remain at 12 g/m only
later to conclude, as the Administrator did, that this
standard was inadequate to protect public health
based upon the latest evidence. See id. at 16257 n.93.
With respect to his evaluation of the scientific
evidence, the Administrator explained that he
disagreed with the approach of the previous
Administrator in certain respects. Recall that the
previous Administrator took a mean-of-means
42a
approach to the scientific studies — i.e., he “placed
weight on the average of the study-reported means (or
medians) across the U.S. monitor-based studies of
13.5 g/m .” Id. at 16275/3. The new Administrator,
as already explained, set the revised standard “below
the lowest long-term study-reported mean PM2.5
concentration.”
Id.
Additionally,
the
new
Administrator gave weight to “U.S. hybrid modelbased epidemiologic studies,” which he considered “an
advancement in the available science.” Id.
Considering the deference we owe the
Administrator’s scientific judgments, we readily
conclude that he adequately explained his reasons for
not retaining the standard set in the 2020 Final Rule.
See Mississippi, 744 F.3d at 1344 (explaining that the
Administrator is not bound by previous NAAQS).
Appropriate Standard. The petitioners also
argue that the evidence does not support a standard
of 9 g/m . Because § 7409(b)(1) expressly delegates
the setting of NAAQS to the “judgment” of the
Administrator, however, we must respect the
Congress’s delegation and take care not to substitute
our judgment for that of the Administrator, see Loper
Bright Enters., 603 U.S. at 412-13, which “necessarily
requires the exercise of policy judgment,” Mississippi,
744 F.3d at 1358. Put another way, we must “defer to
the EPA’s scientific judgment while examining the
record to ensure the agency has considered the
relevant factors and reasonably explained how it
reached its conclusions.” Am. Farm Bureau Fed’n, 559
F.3d at 520.
43a
Here the petitioners do not claim the
Administrator over-looked an important scientific
study or ignored a key public-health factor. Nor do
they provide meaningful evidence that the
Administrator misapplied the relevant scientific
evidence; rather, the Administrator followed a
methodology that this court has upheld on several
occasions. See Nat’l Ass’n of Mfrs. v. EPA, 750 F.3d
921, 924 (D.C. Cir. 2014) (collecting cases in which
this court has upheld a NAAQS set “somewhat below
the lowest long-term mean concentration shown by
certain key epidemiologic studies to cause adverse
health effects”). Under these circumstances, we
cannot agree that the Administrator’s decision was
unreasoned or contrary to the evidence.
That is not to say the Administrator’s application
of § 7409(b)(1) was the only permissible judgment on
this record. See Mississippi, 744 F.3d at 1348 (“That
the evidence in the record may also support other
conclusions . . . does not prevent us from concluding
that [these] decisions were rational” (cleaned up)).
Setting NAAQS for particulate matter entails the
exercise of judgment in the face of scientific
uncertainty. See 89 Fed. Reg. at 16276/2 (recognizing
“uncertainties and limitations associated with the
epidemiologic studies”); Am. Trucking, 531 U.S. at 475
(explaining that “judgments of degree” are necessary
for “nonthreshold” pollutants). Unsurprisingly, then,
all members of the CASAC recommended revising the
NAAQS downward to levels ranging from 8-11 g/m .
See 89 Fed. Reg. at 16280/1 (explaining that “the
44a
majority and minority of the CASAC . . . weighed the
studies in different ways”). The Administrator
ultimately set the standard near the middle of the
range recommended by the CASAC majority.
The Industry Petitioners fault the Administrator
for not sufficiently explaining why he did not select 10
g/m , which also would have fallen within the range
staked out by the majority and minority members of
the CASAC. The Industry Petitioners contend that
the Administrator’s “own evidence and reasoning
pointed more toward 10.0 g/m than 9.0 g/m .” So
they say because the 2024 Final Rule recognized that
“an annual standard level that is no more than 1518% higher than the study-reported means[, or 10.7 to
11.0 g/m ,] would gen-erally maintain air quality
exposures to be below those . . . for which we have the
strongest support for adverse health effects
occurring.” 89 Fed. Reg. at 16241/3. Yet the
Administrator explained that he was setting the
standard “below the lowest study-reported mean” to
ensure that those living where PM2.5 concentrations
are highest “will be exposed to PM2.5 concentrations
below the PM2.5 concentrations reported in the
epidemiologic studies where there is the highest
confidence of an association” with adverse health
effects. Id. at 16263/2. The Administrator’s regard for
populations in areas with the highest PM2.5
concentration was permissible. See Nat’l Ass’n of
Mfrs., 750 F.3d at 926 (finding the Administrator’s
consideration of populations near heavily trafficked
roads reasonable “[i]n the context of [§ 7409]”).
45a
In short, the Administrator “offered reasoned
explanations for how [he] approached and weighed
the evidence, and why the scientific evidence
supported [his] revision of the NAAQS.” Id. at 924.
That is sufficient for us to uphold his decision on its
merits.
III. Conclusion
For the reasons stated, the petitions for review and
the EPA’s motion for vacatur are
Denied.
46a
APPENDIX B
ENVIRONMENTAL PROTECTION AGENCY
40 CFR Parts 50, 53, and 58
[EPA–HQ–OAR–2015–0072; FRL–8635–02–OAR]
RIN 2060–AV52
Reconsideration of the National Ambient Air
Quality Standards for Particulate Matter
AGENCY: Environmental Protection Agency (EPA).
ACTION: Final rule.
SUMMARY: Based on the Environmental Protection
Agency’s (EPA’s) reconsideration of the air quality
criteria and the national ambient air quality
standards (NAAQS) for particulate matter (PM), the
EPA is revising the primary annual PM2.5 standard by
lowering the level from 12.0 μg/m3 to 9.0 μg/m3. The
Agency is retaining the current primary 24-hour PM2.5
standard and the primary 24-hour PM10 standard.
The Agency also is not changing the secondary 24hour PM2.5 standard, secondary annual PM2.5
standard, and secondary 24-hour PM10 standard at
this time. The EPA is also finalizing revisions to other
key aspects related to the PM NAAQS, including
revisions to the Air Quality Index (AQI) and
monitoring requirements for the PM NAAQS.
DATES: This final rule is effective May 6, 2024.
47a
ADDRESSES: The EPA has established a docket for
this action under Docket ID No. EPA–HQ–OAR–
2015–0072. All documents in the docket are listed on
the https://www.regulations.gov website. Although
listed in the index, some information is not publicly
available, e.g., CBI or other information whose
disclosure is restricted by statute. Certain other
material, such as copyrighted material, is not placed
on the internet and will be publicly available only in
hard copy form. Publicly available docket materials
are
available
electronically
through
https://www.regulations.gov.
FOR FURTHER INFORMATION CONTACT:
Dr. Lars Perlmutt, Health and Environmental
Impacts Division, Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency,
Mail Code C539–04, Research Triangle Park, NC
27711; telephone: (919) 541– 3037; fax: (919) 541–
5315; email: perlmutt.lars@epa.gov.
SUPPLEMENTARY INFORMATION:
The following topics are discussed in this preamble:
Table of Contents
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
48a
II.
3. Review Completed in 2006
4. Review Completed in 2012
5. Review Initiated in 2014
a. 2020 Proposed and Final Decisions
b. Reconsideration of the 2020 PM
NAAQS Final Action
D. Air Quality Information
1. Distribution of Particle Size in Ambient
Air
2. Sources and Emissions Contributing to
PM in the Ambient Air
3. Monitoring of Ambient PM
4. Ambient Concentrations and Trends
a. PM2.5 Mass
b. PM2.5 Components
c. PM10
d. PM10–2.5
e. UFP
5. Characterizing
Ambient
PM2.5
Concentrations for Exposure
a. Predicted
Ambient
PM2.5
and
Exposure Based on Monitored Data
b. Comparison of PM2.5 Fields in
Estimating Exposure and Relative to
Design Values
6. Background PM
Rationale for Decisions on the Primary PM2.5
Standards
A. Introduction
1. Background on the Current Standards
2. Overview of the Health Effects Evidence
a. Nature of Effects
i. Mortality
49a
ii. Cardiovascular Effects
iii. Respiratory Effects
iv. Cancer
v. Nervous System Effects
vi. Other Effects
b. Public Health Implications and AtRisk Populations
c. PM2.5 Concentrations in Key Studies
Reporting Health Effects
i. PM2.5 Exposure Concentrations
Evaluated
in
Experimental
Studies
ii. Ambient PM2.5 Concentrations in
Locations
of
Epidemiologic
Studies
d. Uncertainties in the Health Effects
Evidence
3. Summary of Exposure and Risk
Estimates
a. Key Design Aspects
b. Key Limitations and Uncertainties
c. Summary of Risk Estimates
B. Conclusions on the Primary PM2.5
Standards
1. CASAC Advice
2. Basis for the Proposed Decision
3. Comments on the Proposed Decision
4. Administrator’s Conclusions
C. Decisions on the Primary PM2.5 Standards
[CONTENT OMITTED]
50a
Executive Summary
This document presents the Administrator’s final
decisions for the reconsideration of the 2020 final
decision on the primary (health-based) and secondary
(welfare-based) National Ambient Air Quality
Standards (NAAQS) for Particulate Matter (PM).
More specifically, this document summarizes the
background and rationale for the Administrator’s
final decisions to revise the primary annual PM2.5
standard by lowering the level from 12.0 μg/m3 to 9.0
μg/m3; to retain the current primary 24-hour PM2.5
standard (at a level of 35 μg/m3); to retain the primary
24-hour PM10 standard; and, not to change the
secondary PM standards at this time. In reaching his
final decisions, the Administrator considered the
currently available scientific evidence in the 2019
Integrated Science Assessment (2019 ISA) and the
Supplement to the 2019 ISA (ISA Supplement),
quantitative and policy analyses presented in the
2022 Policy Assessment (2022 PA), advice from the
Clean Air Scientific Advisory Committee (CASAC),
and public comments on the proposal. The EPA has
established primary and secondary standards for
PM2.5, which includes particles with diameters
generally less than or equal to 2.5 μm, and PM10,
which includes particles with diameters generally less
than or equal to 10 μm. The standards include two
primary PM2.5 standards: an annual average
standard, averaged over three years, with a level of
12.0 μg/m3, and a 24-hour standard with a 98th
percentile form, averaged over three years, and a level
of 35 μg/m3. It also includes a primary PM10 standard
51a
with a 24-hour averaging time, and a level of 150
μg/m3, not to be exceeded more than once per year on
average over three years. Secondary PM standards
are set equal to the primary standards, except that the
level of the secondary annual PM2.5 standard is 15.0
μg/m3.
The most recent of the PM NAAQS was completed
in December 2020. In that review, the EPA retained
the primary and secondary NAAQS, without revision
(85 FR 82684, December 18, 2020). Following
publication of the 2020 final action, several parties
filed petitions for review and petitions for
reconsideration of the EPA’s final decision.
In June 2021, the Agency announced its decision to
reconsider the 2020 PM NAAQS final action. 1 The
EPA decided to reconsider the December 2020
decision because the available scientific evidence and
technical information indicated that the current
standards may not be adequate to protect public
health and welfare, as required by the Clean Air Act.
The EPA noted that the 2020 PA concluded that the
scientific evidence and information called into
question the adequacy of the primary PM2.5 standards
and supported consideration of revising the level of
the primary annual PM2.5 standard to below the
current level of 12.0 μg/m3 while retaining the
primary 24-hour PM2.5 standard (U.S. EPA, 2020b).
The press release for this announcement is available at:
https://www.epa.gov/newsreleases/epa-reexamine-healthstandards-harmful-soot-previous-administration-leftunchanged.
1
52a
The EPA also noted that the 2020 PA concluded that
the available scientific evidence and information did
not call into question the adequacy of the primary
PM10 or secondary PM standards and supported
consideration of retaining the primary PM10 standard
and secondary PM standards without revision (U.S.
EPA, 2020b).
The final decisions presented in this document on
the primary PM2.5 standards have been informed by
key aspects of the available health effects evidence
and conclusions contained in the 2019 ISA and ISA
Supplement, quantitative exposure/risk analyses and
policy evaluations presented in the 2022 PA, advice
from the CASAC 2 and public comment received as
part of this reconsideration. 3 The health effects
evidence newly available in this reconsideration, in
conjunction with the full body of evidence critically
evaluated in the 2019 ISA, supports a causal
relationship between long- and short-term exposures
2 In 2021, the Administrator announced his decision to
reestablish the membership of the CASAC. The Administrator
selected seven members to serve on the chartered CASAC, and
appointed a PM CASAC panel to support the chartered CASAC’s
review of the draft ISA Supplement and the draft PA as a part of
this reconsideration (see section I.C.6.b below for more
information).
3 More information regarding the CASAC review of the draft
ISA Supplement and the draft PA, including opportunities for
public comment, can be found in the following Federal Register
notices: 86 FR 54186, September 30, 2021; 86 FR 52673,
September 22, 2021; 86 FR 56263, October 8, 2021; 87 FR 958,
January 7, 2022.
53a
and mortality and cardiovascular effects, and the
evidence supports a likely to be a causal relationship
between long-term exposures and respiratory effects,
nervous system effects, and cancer. The longstanding
evidence base, including animal toxicological studies,
controlled human exposure studies, and epidemiologic
studies, reaffirms, and in some cases strengthens, the
conclusions from past reviews regarding the health
effects of PM2.5 exposures. Epidemiologic studies
available in this reconsideration demonstrate
generally positive, and often statistically significant,
PM2.5 health effect associations. Such studies report
associations between estimated PM2.5 exposures and
non-accidental,
cardiovascular,
or
respiratory
mortality;
cardiovascular
or
respiratory
hospitalizations or emergency room visits; and other
mortality/morbidity outcomes (e.g., lung cancer
mortality or incidence, asthma development). The
scientific evidence available in this reconsideration,
as evaluated in the 2019 ISA and ISA Supplement,
includes a number of epidemiologic studies that use
various methods to characterize exposure to PM2.5
(e.g., ground-based monitors and hybrid modeling
approaches) and to evaluate associations between
health
effects
and
lower
ambient
PM2.5
concentrations. There are a number of recent
epidemiologic studies that use varying study designs
that reduce uncertainties related to confounding and
exposure measurement error. The results of these
analyses provide further support for the robustness of
associations between PM2.5 exposures and mortality
and morbidity. Moreover, the Administrator notes
that recent epidemiologic studies strengthen support
54a
for health effect associations at lower PM2.5
concentrations, with these new studies finding
positive and significant associations when assessing
exposure in locations and time periods with lower
annual mean and 25th percentile concentrations than
those evaluated in epidemiologic studies available at
the time of previous reviews. Additionally, the
experimental evidence (i.e., animal toxicological and
controlled human exposure studies) strengthens the
coherence of effects across scientific disciplines and
provides additional support for potential biological
pathways through which PM2.5 exposures could lead
to the overt population-level outcomes reported in
epidemiologic studies for the health effect categories
for which a causal relationship (i.e., short- and longterm PM2.5 exposure and mortality and cardiovascular
effects) or likely to be causal relationship (i.e., shortand long-term PM2.5 exposure and respiratory effects;
and long-term PM2.5 exposure and nervous system
effects and cancer) was concluded.
The available evidence in the 2019 ISA continues to
provide support for factors that may contribute to
increased risk of PM2.5-related health effects
including lifestage (children and older adults), preexisting diseases (cardiovascular disease and
respiratory
disease),
race/ethnicity,
and
socioeconomic status. For example, the 2019 ISA and
ISA Supplement conclude that there is strong
evidence that Black and Hispanic populations, on
average, experience higher PM2.5 exposures and
PM2.5-related health risks than non-Hispanic White
populations. In addition, studies evaluated in the
55a
2019 ISA and ISA Supplement also provide evidence
indicating that communities with lower socioeconomic
status (SES), as assessed in epidemiologic studies
using indicators of SES including income and
educational attainment are, on average, exposed to
higher concentrations of PM2.5 compared to higher
SES communities.
The quantitative risk assessment, as well as policy
considerations in the 2022 PA, also inform the final
decisions on the primary PM2.5 standards. The risk
assessment in this reconsideration focuses on allcause or nonaccidental mortality associated with longand short-term PM2.5 exposures. The primary
analyses focus on exposure and risk associated with
air quality that might occur in an area under air
quality conditions that just meet the current and
potential alternative standards. The risk assessment
estimates that the current primary PM2.5 standards
could allow a substantial number of PM2.5-associated
premature deaths in the United States, and that
public health improvements would be associated with
just meeting all of the alternative (more stringent)
annual and 24-hour standard levels modeled.
Additionally, the results of the risk assessment
suggest that for most of the U.S., the annual standard
is the controlling standard and that revision to that
standard has the most potential to reduce PM2.5
exposure-related risk. The analyses are summarized
in this document and in the proposal and are
described in detail in the 2022 PA.
In its advice to the Administrator, in its review of
the 2021 draft PA, the CASAC concurred that the
56a
currently available health effects evidence calls into
question the adequacy of the primary annual PM2.5
standard. With regard to the primary annual PM2.5
standard, the majority of the CASAC concluded that
the level of the standard should be revised within the
range of 8.0 to 10.0 μg/m3, while the minority of the
CASAC concluded that the primary annual PM2.5
standard should be revised to a level of 10.0 to 11.0
μg/m3. With regard to the primary 24-hour PM2.5
standard, the CASAC did not reach consensus on the
adequacy of the current standard. The majority of the
CASAC concluded that the primary 24-hour PM2.5 was
not adequate and that the level of the standard should
be revised to within the range of 25 to 30 μg/m3, while
the minority of the CASAC concluded that the
standard was adequate and should be retained,
without revision. Additionally, in their review of the
2019 draft PA, the CASAC did not reach consensus on
the adequacy of the primary annual PM2.5 standard,
with the minority recommending revision and the
majority recommending the standard be retained. In
their review of the 2019 draft PA, the CASAC reached
consensus regarding the adequacy of the primary 24hour PM2.5 standard, concluding that the standard
should be retained.
In considering how to revise the suite of primary
PM2.5 standards to provide the requisite degree of
protection, the Administrator recognizes that the
current annual standard and 24-hour standard,
together, are intended to provide public health
protection against the full distribution of short- and
long-term PM2.5 exposures. Further, he recognizes
57a
that changes in PM2.5 air quality designed to meet
either the annual or the 24-hour standard would
likely result in changes to both long-term average and
short-term peak PM2.5 concentrations.
As in 2012, the Administrator concludes that the
most effective way to reduce total population risk
associated with both long- and short-term PM2.5
exposures is to set a generally controlling annual
standard, and to provide supplemental protection
against the occurrence of peak 24-hour PM2.5
concentrations by means of a 24-hour standard set at
the appropriate level. Based on the current evidence
and quantitative information, as well as consideration
of CASAC advice and public comments, the
Administrator concludes that the current primary
annual PM2.5 standard is not adequate to protect
public health with an adequate margin of safety. The
Administrator notes that the CASAC was unanimous
in its advice on the 2021 draft PA regarding the need
to revise the annual standard. In considering the
appropriate level for a revised annual standard, the
Administrator concludes that a standard set at a level
of 9.0 μg/m3 reflects his judgment about placing the
most weight on the strongest available evidence while
appropriately weighing the uncertainties.
With regard to the primary 24-hour PM2.5 standard,
the Administrator finds the available scientific
evidence and quantitative information to be
insufficient to call into question the adequacy of the
public health protection afforded by the current 24hour standard. He further notes that a more stringent
annual standard set at a level of 9.0 μg/m3 is expected
58a
to reduce both average (annual) concentrations and
peak (daily) concentrations. The Administrator also
notes that, in their review of the 2021 draft PA, the
CASAC did not reach consensus on whether revisions
to the primary 24-hour PM2.5 standard are warranted
at this time. He also notes that, in their review of the
2019 draft PA, the CASAC did reach consensus that
the primary 24-hour PM2.5 standard should be
retained. The Administrator concludes that the 24hour standard should be retained to continue to
provide requisite protection against short-term peak
PM2.5 concentrations, particularly when considered in
conjunction with the protection provided by the suite
of standards and the decision to revise the annual
standard to a level of 9.0 μg/m3.
The primary PM10 standard is intended to provide
public health protection against health effects related
to exposures to PM10–2.5, which are particles with a
diameter between 10 μm and 2.5 μm. The final decision
to retain the current 24-hour PM10 standard has been
informed by key aspects of the available health effects
evidence and conclusions contained in the 2019 ISA,
the policy evaluations presented in the 2022 PA,
advice from the CASAC and public comments.
Specifically, the health effects evidence for PM10–2.5
exposures is somewhat strengthened since past
reviews, although the strongest evidence still only
provides support for a suggestive of, but not sufficient
to infer, causal relationship with long- and short-term
exposures and mortality and cardiovascular effects,
short-term exposures and respiratory effects, and
long-term exposures and cancer, nervous system
59a
effects, and metabolic effects. In reaching his final
decision on the primary PM10 standard, the
Administrator recognizes that, while the available
health effects evidence has expanded, recent studies
are subject to the same types of uncertainties that
were judged to be important in previous reviews. He
also recognizes that, in their review of the 2019 draft
PA and the 2021 draft PA, the CASAC generally
agreed that it was reasonable to retain the primary
24-hour PM10 standard given the available scientific
evidence, including retaining PM10 as the indicator.
He concludes that the newly available evidence does
not call into question the adequacy of the current
primary PM10 standard, and retains that standard,
without revision.
With respect to the secondary PM standards, this
reconsideration focuses on visibility, climate, and
materials effects. 4 The Administrator’s final decision
to not change the current secondary standards at this
4 Consistent with the 2016 Integrated Review Plan (U.S. EPA,
2016), other welfare effects of PM, such as ecological effects, are
being considered in the separate, on-going review of the
secondary NAAQS for oxides of nitrogen, oxides of sulfur and
PM. Accordingly, the public welfare protection provided by the
secondary PM standards against ecological effects such as those
related to deposition of nitrogen- and sulfur-containing
compounds in vulnerable ecosystems is being considered in that
separate review. Thus, the Administrator’s conclusion in this
reconsideration of the 2020 final decision is focused only and
specifically on the adequacy of public welfare protection provided
by the secondary PM standards from effects related to visibility,
climate, and materials and hereafter “welfare effects” refers to
those welfare effects.
60a
time has been informed by key aspects of the currently
available welfare effects evidence as well as the
conclusions contained in the 2019 ISA and ISA
Supplement; quantitative analyses of visibility
impairment; policy evaluations presented in the 2022
PA; advice from the CASAC; and public comments.
Specifically, the welfare effects evidence available in
this reconsideration is consistent with the evidence
available in previous reviews and supports a causal
relationship between PM and visibility, climate, and
materials effects. With regard to visibility effects, the
Administrator notes that he judges that the evidence
supports a target level of protection of 27 dv. He
further notes that the results of quantitative analyses
of visibility impairment suggest that in areas that
meet the current secondary 24-hour PM2.5 standard
that estimated light extinction in terms of a 3-year
visibility metric would be at or well below the target
level of protection. With regard to climate and
materials effects, while the evidence has expanded
since previous reviews, significant limitations and
uncertainties remain in the evidence. While the
evidence has expanded since previous reviews, the
available scientific evidence remains insufficient to
allow the Administrator to make a reasoned judgment
about what specific standard(s) would be requisite to
protect against known or anticipated adverse effects
to public welfare from PM’s effects on materials
damage or climate. In their review of the 2019 draft
PA and the 2021 draft PA, the CASAC did not
recommend revising the secondary PM standards. In
considering the available evidence and quantitative
information, with its inherent uncertainties and
61a
limitations, the Administrator judges that it is
appropriate not to change the secondary PM
standards at this time.
The final revisions to the primary annual PM2.5
NAAQS trigger a process under which States (and
Tribes, if they choose) make recommendations to the
Administrator regarding designations, identifying
areas of the country that either meet or do not meet
the new or revised PM NAAQS. Those areas that do
not meet the revised PM NAAQS will need to develop
plans that demonstrate how they will meet the
standards. As part of these plans, states have the
opportunity to advance environmental justice, in this
case for overburdened communities in areas with high
PM concentrations above the NAAQS, by using the
tools described in the current PM NAAQS
implementation guidance (80 FR 58010, 58136,
August 25, 2016). The EPA is not making changes to
any of the current PM NAAQS implementation
programs in this final rulemaking.
On other topics, the EPA is finalizing two sets of
changes to the PM2.5 subindex of the Air Quality Index
(AQI). First, the EPA is continuing to use the
approach used in the revisions to the AQI in 2012 (77
FR 38890, June 29, 2012) of setting the lower
breakpoints (50, 100 and 150) based on the levels of
the primary annual and 24-hour PM2.5 standards. In
so doing, the EPA is revising the AQI value of 50 to
9.0 μg/m3 and is retaining the AQI values of 100 and
150 at 35.4 μg/m3 and 55.4 μg/m3, respectively.
Second, the EPA is revising the upper AQI
breakpoints (200 and above), and replacing the linear-
62a
relationship approach used in 1999 (64 FR 42530,
August 4, 1999) to set these breakpoints, with an
approach that more fully considers the PM2.5 health
effects evidence from controlled human exposure and
epidemiologic studies that has become available in the
last 20 years. The EPA is also revising the AQI values
of 200, 300 and 500 to 125.4 μg/m3, 225.4 μg/m3, and
325.4 μg/m3, respectively. In addition, this final rule
revises the daily reporting requirement from 5 days
per week to 7 days per week, while also reformatting
appendix G and providing clarifications.
With regard to monitoring-related activities, the
EPA finalizes revisions to data calculations and
ambient air monitoring requirements for PM to
improve the usefulness and appropriateness of data
used in regulatory decision making and to better
characterize air quality in communities that are at
increased risk of PM2.5 exposure and health risk.
These changes are found in 40 CFR part 50
(appendices K, L, and N), part 53, and part 58 with
associated appendices (A, B, C, D, and E). These
changes include addressing updates in data
calculations, approval of reference and equivalent
methods, updates in quality assurance statistical
calculations to account for lower concentration
measurements, updates to support improvements in
PM methods, a revision to the PM2.5 network design
to account for at-risk populations, and updates to the
Probe and Monitoring Path Siting Criteria for NAAQS
pollutants.
In setting the NAAQS, the EPA may not consider
the costs of implementing the standards. This was
63a
confirmed by the Supreme Court in Whitman v.
American Trucking Associations, 531 U.S. 457, 465472, 475–76 (2001), as discussed in section II.A of this
document. As has traditionally been done in NAAQS
rulemaking, the EPA prepared a Regulatory Impact
Analysis (RIA) to provide the public with information
on the potential costs and benefits of attaining several
alternative PM2.5 standard levels. In NAAQS
rulemaking, the RIA is done for informational
purposes only, and the final decisions on the NAAQS
in this rulemaking are not based on consideration of
the information or analyses in the RIA. The RIA
fulfills the requirements of Executive Orders 14094,
13563, and 12866. The RIA estimates the costs and
monetized human health benefits of attaining the
revised and two alternative annual PM2.5 standard
levels and one alternative 24-hour PM2.5 standard
level. Specifically, the RIA examines the revised
annual standard level of 9.0 μg/m3 in combination
with the current 24-hour standard of 35 μg/m3 (i.e.,
9.0/35 μg/m3), as well as the following less and more
stringent alternative standard levels: (1) An
alternative annual standard level of 10.0 μg/m3 in
combination with the current 24-hour standard (i.e.,
10.0/35 μg/m3), (2) an alternative annual standard
level of 8.0 μg/m3 in combination with the current 24hour standard (i.e., 8.0/35 μg/m3), and (3) an
alternative 24-hour standard level of 30 μg/m3 in
combination with an alternative annual standard
level of 10 μg/m3 (i.e., 10.0/30 μg/m3). The RIA
presents estimates of the costs and benefits of
applying illustrative national control strategies in
2032 after implementing existing and expected
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regulations and assessing emissions reductions to
meet the current annual and 24-hour particulate
matter NAAQS (12.0/35 μg/m3).
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
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and allowing an adequate margin of safety, are
requisite to protect the public health.” 5 Under section
109(b)(2), a secondary standard must “specify a level
of air quality the attainment and maintenance of
which, in the judgment of the Administrator, based on
such criteria, is requisite to protect the public welfare
from any known or anticipated adverse effects
associated with the presence of [the] pollutant in the
ambient air.” 6
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
5 The legislative history of section 109 indicates that a
primary standard is to be set at “the maximum permissible
ambient air level . . . which will protect the health of any
[sensitive] group of the population,” and that for this purpose
“reference should be made to a representative sample of persons
comprising the sensitive group rather than to a single person in
such a group.” S. Rep. No. 91–1196, 91st Cong., 2d Sess. 10
(1970).
6 Under CAA section 302(h) (42 U.S.C. 7602(h)), effects on
welfare include, but are not limited to, “effects on soils, water,
crops, vegetation, manmade materials, animals, wildlife,
weather, visibility, and climate, damage to and deterioration of
property, and hazards to transportation, as well as effects on
economic values and on personal comfort and well-being.”
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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
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n.51, Mississippi v. EPA, 744 F.3d at 1351, but rather
at a level that reduces risk sufficiently so as to protect
public health with an adequate margin of safety.
In addressing the requirement for an adequate
margin of safety, the EPA considers such factors as
the nature and severity of the health effects involved,
the size of the sensitive population(s), and the kind
and degree of uncertainties. The selection of any
particular approach to providing an adequate margin
of safety is a policy choice left specifically to the
Administrator’s judgment. See Lead Industries Ass’n
v. EPA, 647 F.2d at 1161–62; Mississippi v. EPA, 744
F.3d at 1353.
Section 109(d)(1) of the Act requires the review
every five years of existing air quality criteria and, if
appropriate, the revision of those criteria to reflect
advances in scientific knowledge on the effects of the
pollutant on public health and welfare. Under the
same provision, the EPA is also to review every five
years and, if appropriate, revise the NAAQS, based on
the revised air quality criteria. Section 109(d)(1) also
provides that the Administrator may review and
revise criteria or promulgate new standards earlier or
more frequently.
Section 109(d)(2) addresses the appointment and
advisory functions of an independent scientific review
committee. Section 109(d)(2)(A) requires the
Administrator to appoint this committee, which is to
be composed of “seven members including at least one
member of the National Academy of Sciences, one
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physician, and one person representing State air
pollution control agencies.” Section 109(d)(2)(B)
provides that the independent scientific review
committee “shall complete a review of the criteria . . .
and the national primary and secondary ambient air
quality standards . . . and shall recommend to the
Administrator any new . . . standards and revisions of
existing criteria and standards as may be appropriate.
. . .” Since the early 1980s, this independent review
function has been performed by the Clean Air
Scientific Advisory Committee (CASAC) of the EPA’s
Science Advisory Board.
As previously noted, the Supreme Court has held
that section 109(b) “unambiguously bars cost
considerations from the NAAQS-setting process.”
Whitman v. Am. Trucking Associations, 531 U.S. 457,
471 (2001). Accordingly, while some of these issues
regarding which Congress has directed the CASAC to
advise the Administrator are ones that are relevant to
the standard setting process, others are not. Issues
that are not relevant to standard setting may be
relevant to implementation of the NAAQS once they
are established.
B.
Related PM Control Programs
States are primarily responsible for ensuring
attainment and maintenance of ambient air quality
standards once the EPA has established them. Under
section 110, Part C, and Part D, Subparts 1 and 4 of
the CAA, and related provisions and regulations,
States are to submit, for the EPA’s approval, State
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implementation plans (SIPs) that provide for the
attainment and maintenance of the NAAQS for PM
through control programs directed to sources of the
pollutants involved. The States, in conjunction with
the EPA, also administer the prevention of significant
deterioration of air quality program that covers these
pollutants (see 42 U.S.C. 7470–7479). In addition,
Federal programs provide for or result in nationwide
reductions in emissions of PM and its precursors
under Title II of the Act, 42 U.S.C. 7521–7574, which
involves controls for motor vehicles and nonroad
engines and equipment; the new source performance
standards under section 111 of the Act, 42 U.S.C.
7411; and the national emissions standards for
hazardous pollutants under section 112 of the Act, 42
U.S.C. 7412.
C.
Review of the Air Quality Criteria and Standards
for Particulate Matter
1.
Reviews Completed in 1971 and 1987
The EPA first established NAAQS for PM in 1971
(36 FR 8186, April 30, 1971), based on the original Air
Quality Criteria Document (AQCD) (DHEW, 1969). 7
The Federal reference method (FRM) specified for
determining attainment of the original standards was
the high-volume sampler, which collects PM up to a
7 Prior to the review initiated in 2007 (see below), the AQCD
provided the scientific foundation (i.e., the air quality criteria) for
the NAAQS. Beginning in that review, the Integrated Science
Assessment (ISA) has replaced the AQCD.
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nominal size of 25 to 45 μm (referred to as total
suspended particulates or TSP). The primary
standards were set at 260 μg/m3, 24-hour average, not
to be exceeded more than once per year, and 75 μg/m3,
annual geometric mean. The secondary standards
were set at 150 μg/m3, 24-hour average, not to be
exceeded more than once per year, and 60 μg/m3,
annual geometric mean.
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. 8 The level of the 24-hour standards
(primary and secondary) was set at 150 μg/m3, 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
μg/m3, and the form was the annual arithmetic mean,
averaged over three years.
8 PM10 refers to particles with a nominal mean aerodynamic
diameter less than or equal to 10 μm. More specifically, 10 μm 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 shortand 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 μm). The new primary standards were as
follows: (1) An annual standard with a level of 15.0
μg/m3, based on the 3-year average of annual
arithmetic mean PM2.5 concentrations from single or
multiple community-oriented monitors; 9 and (2) a 24hour standard with a level of 65 μg/m3, based on the
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).
9
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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 μm and less than or equal to 10 μm),
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.
Following promulgation of the 1997 PM NAAQS,
petitions for review were filed by several parties,
addressing a broad range of issues. In May 1999, the
U.S. Court of Appeals for the District of Columbia
Circuit (D.C. Circuit) upheld the EPA’s decision to
establish fine particle standards and to regulate
coarse particle pollution, but vacated the 1997 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, Inc. v. EPA, 175 F. 3d 1027 (D.C. Cir.
1999). Pursuant to the D.C. Circuit’s decision, the
EPA removed the vacated 1997 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
73a
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
74a
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, 2004a). The EPA’s Office of Air
Quality Planning and Standards (OAQPS) finalized a
Risk Assessment and Staff Paper in December 2005
75a
(Abt Associates, 2005; U.S. EPA, 2005). 10 On
December 20, 2005, the EPA announced its proposed
decision to revise the NAAQS for PM and solicited
public comment on a broad range of options (71 FR
2620, January 17, 2006). On September 21, 2006, the
EPA announced its final decisions to revise the
primary and secondary NAAQS for PM to provide
increased protection of public health and welfare,
respectively (71 FR 61144, October 17, 2006). With
regard to the primary and secondary standards for
fine particles, the EPA revised the level of the 24-hour
PM2.5 standards to 35 μg/m3, retained the level of the
annual PM2.5 standards at 15.0 μg/m3, 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 μg/m3, and revoked the annual
standards. The then-Administrator judged that the
available evidence generally did not suggest a link
between long-term exposure to existing ambient
levels of coarse particles and health or welfare effects.
In addition, a new reference method was added for the
measurement of 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
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.
10
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scientific data to support future reviews of the PM
NAAQS.
Several parties filed petitions for review following
promulgation of the revised PM NAAQS in 2006. On
February 24, 2009, the D.C. Circuit issued its opinion
in the case American Farm Bureau Federation v. EPA,
559 F. 3d 512 (D.C. Cir. 2009). The court remanded
the primary annual 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
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28, 2007). Based on the NAAQS review process, as
revised in 2008 and again in 2009, 11 the EPA held
science/policy issue workshops on the primary and
secondary PM NAAQS (72 FR 34003, June 20, 2007;
72 FR 34005, June 20, 2007), and prepared and
released the planning and assessment documents that
comprise the review process (i.e., Integrated Review
Plan, (IRP; U.S. EPA, 2008), Integrated Science
Assessment (ISA; U.S. EPA, 2009a), Risk and
Exposure Assessment (REA) planning documents for
health and welfare (U.S. EPA, 2009b, U.S. EPA,
2009c), a quantitative health risk assessment (U.S.
EPA, 2010a) and an urban-focused visibility
assessment (U.S. EPA, 2010b), and a Policy
Assessment (PA; U.S. EPA, 2011). In June 2012, the
EPA announced its proposed decision to revise the
NAAQS for PM (77 FR 38890, June 29, 2012).
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 12 to 12.0 μg/m3 and retained
the 24-hour PM2.5 standard, with its level of 35 μg/m3.
For the primary PM10 standard, the EPA retained the
24-hour standard to continue to provide protection
11 The history of the NAAQS review process, including
revisions
to
the
process,
is
discussed
at
https://www.epa.gov/naaqs/historical-information-naaqsreview-process.
12 The EPA also eliminated the option for spatial averaging.
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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 13
and the 24-hour PM10 standard to address visibility
and non-visibility welfare effects.
As with previous reviews, petitioners challenged
the EPA’s final rule. Petitioners argued that the EPA
acted unreasonably in revising the level and form of
the annual standard and in amending the monitoring
network provisions. On judicial review, the revised
standards and monitoring requirements were upheld
in all respects. NAM v. EPA, 750 F.3d 921 (D.C. Cir.
2014).
5.
Review Initiated in 2014
In December 2014, the EPA announced the
initiation of the current periodic review of the air
quality criteria for PM and of the 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 review of the PM
NAAQS (announced in 79 FR 71764, December 3,
2014). Workshop participants, including a wide range
of external experts as well as the EPA staff
representing a variety of areas of expertise (e.g.,
epidemiology, human and animal toxicology, risk/
13 Consistent with the primary standard, the EPA eliminated
the option for spatial averaging with the annual standard.
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exposure analysis, atmospheric science, visibility
impairment, climate effects), were asked to highlight
significant new and emerging PM research, and to
make recommendations to the Agency regarding the
design and scope of the review. This workshop
provided for a public discussion of the key science and
policy-relevant issues around which the EPA
structured the review of the PM NAAQS and of the
most meaningful new scientific information that
would be available in the review to inform
understanding of these issues.
The input received at the workshop guided the EPA
staff in developing a draft IRP, which was reviewed by
the CASAC Particulate Matter Panel and discussed
on public teleconferences held in May 2016 (81 FR
13362, March 14, 2016) and August 2016 (81 FR
39043, June 15, 2016). Advice from the CASAC,
supplemented by the Particulate Matter Panel, and
input from the public were considered in developing
the final IRP (U.S. EPA, 2016). The final IRP
discusses the approaches to be taken in developing
key scientific, technical, and policy documents in the
review and the key policy-relevant issues that frame
the EPA’s consideration of whether the primary
and/or secondary NAAQS for PM should be retained
or revised.
In May 2018, the then-Administrator issued a
memorandum announcing the Agency’s intention to
conduct the review of the PM NAAQS in such a
manner as to ensure that any necessary revisions
were finalized by December 2020 (Pruitt, 2018).
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Following this memo, on October 10, 2018, the thenAdministrator 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 sevenmember chartered CASAC (i.e., rather than the
CASAC Particulate Matter Panel that reviewed the
draft IRP). 14
The EPA released the draft ISA in October 2018 (83
FR 53471, October 23, 2018). The draft ISA was
reviewed by the chartered CASAC at a public meeting
held in Arlington, VA in December 2018 (83 FR 55529,
November 6, 2018) and was discussed on a public
teleconference in March 2019 (84 FR 8523, March 8,
2019). The CASAC provided its advice on the draft
ISA in a letter to the then-Administrator dated April
11, 2019 (Cox, 2019a). The EPA addressed these
comments in the final ISA, which was released in
December 2019 (U.S. EPA, 2019a).
The EPA released the draft PA in September 2019
(84 FR 47944, September 11, 2019). The draft PA was
reviewed by the chartered CASAC and discussed in
October 2019 at a public meeting held in Cary, NC.
Public comments were received via a separate public
teleconference (84 FR 51555, September 30, 2019). A
public meeting to discuss the chartered CASAC letter
and response to charge questions on the draft PA was
Announcement
available
at:
https://www.regulations.gov/document/EPA-HQ-OAR-20150072-0223.
14
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held in Cary, NC, in October 2019 (84 FR 51555,
September 30, 2019), and the CASAC provided its
advice on the draft PA, including its advice on the
current primary and secondary PM standards, in a
letter to the then-Administrator dated December 16,
2019 (Cox, 2019b). With regard to the primary
standards, the CASAC recommended retaining the
current 24-hour PM2.5 and PM10 standards but did not
reach consensus on the adequacy of the current
annual PM2.5 standard. Some CASAC members
expressed support for retaining the current primary
annual PM2.5 standard while other members
expressed support for revising that standard in order
to increase public health protection (Cox, 2019b, p. 1
of letter). These views are described in greater detail
in the letter to the then-Administrator (Cox, 2019b)
and in the notice of final rulemaking (85 FR 82706–
82707, December 18, 2020), as well as below. With
regard to the secondary standards, the CASAC
recommended retaining the current standards. In
response to the CASAC’s comments, the 2020 final PA
incorporated a number of changes (Cox, 2019b, U.S.
EPA, 2020b), as described in detail in section I.C.5 of
the 2020 proposal document (85 FR 24100, April 30,
2020).
a. 2020 Proposed and Final Actions
On April 14, 2020, the EPA proposed to retain all of
the primary and secondary PM standards, without
revision. These proposed decisions were published in
the Federal Register on April 30, 2020 (85 FR
24094, April 30, 2020). The EPA’s final decision on the
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PM NAAQS was published in the Federal Register
on December 18, 2020 (85 FR 82684, December 18,
2020). In the 2020 rulemaking, the EPA retained the
primary and secondary PM2.5 and PM10 standards,
without revision. The then-Administrator’s rationale
for his decisions is described in more detail in section
II, III, and V below, and is briefly summarized here.
In reaching his final decision to retain the primary
annual and 24-hour PM2.5 standards, the thenAdministrator considered the available scientific
evidence, quantitative information, CASAC advice,
and public comments in his supporting rationale in
the 2020 final action (85 FR 82714, December 18,
2020). In so doing, he concluded that the available
controlled human exposure studies did not provide
support for additional public health protection against
exposures to peak PM2.5 concentrations, beyond the
protection provided by the combination of the current
primary annual and 24-hour PM2.5 standards. He also
noted that the available epidemiologic studies did not
indicate that associations in those studies are strongly
influenced by exposures to peak concentrations in the
air quality distribution and thus did not indicate the
need for additional protection against short-term
exposures to peak PM2.5 concentrations. Accordingly,
and taking into account consensus CASAC advice to
retain the current primary 24-hour PM2.5 standard,
the then-Administrator concluded the primary 24hour PM2.5 standard should be retained.
With respect to the annual PM2.5 standard, the
then-Administrator recognized that important
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uncertainties and limitations that were present in
epidemiologic studies in previous reviews remained in
the evidence assessed in the 2019 ISA. In considering
the epidemiologic evidence, the then-Administrator
noted that: (1) The reported mean concentration in the
majority of the key U.S. epidemiologic studies using
ground-based monitoring data are above the level of
the current annual standard; (2) the mean of the
reported study means (or medians) (i.e., 13.5 μg/m3) is
above the level of the current primary annual PM2.5
standard of 12 μg/m3; (3) air quality analyses show the
study means to be lower than their corresponding
design by 10–20%; and (4) that these analyses must
be considered in light of uncertainties inherent in the
epidemiologic evidence. The then-Administrator
further considered other available information,
including the risk assessment, accountability studies,
and controlled human exposure studies, and found
that, in considering all of the evidence together along
with advice from the CASAC, the suite of primary
PM2.5 standards were requisite to protect public
health with an adequate margin of safety, and should
be retained, without revision.
With regard to the primary PM10 standard, the
then-Administrator noted that the expanded body of
evidence has broadened the range of effects that have
been linked with PM10–2.5 exposures. In light of that
information, as well as continued uncertainties in the
evidence and advice from the CASAC to retain the
standard, the then-Administrator judged it
appropriate to retain the primary PM10 standard to
provide the requisite degree of public health
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protection against PM10–2.5 exposures, regardless of
location, source of origin, or particle composition (85
FR 82725, December 18, 2020).
With regard to the secondary PM standards, the
then-Administrator concluded that there was
insufficient information available to establish any
distinct secondary PM standards to address climate
and materials effects of PM. For visibility effects, he
found that in the absence of a monitoring network for
direct measurement of light extinction, a calculated
light extinction indicator that utilizes the IMPROVE
algorithms continued to provide a reasonable basis for
defining a target level of protection against PMrelated visibility impairment. He further found that a
visibility index with a 24-hour averaging time was
reasonable based on its stability and suitability for
representing subdaily periods, and a form based on
the 3-year average of annual 90th percentile values
was reasonable based on its stability and that it
represents the median of the 20 percent worst
visibility days which are targeted under the Regional
Haze program. With regard to the level of a visibility
index, the then-Administrator judged it appropriate
to establish a target level of protection of 30 dv,
reflecting the upper end of the range of visibility
impairment judged to be acceptable by at least 50% of
study participants in the available public preference
studies, taking into consideration the variability,
limitations and uncertainties of the public preference
studies. The then-Administrator judged that the
secondary 24-hour PM2.5 standard with its level of 35
μg/m3 would provide at least the target level of
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protection for visual air quality of 30 dv which he
judged appropriate. Accordingly, taking into
consideration the advice of the CASAC to retain the
current secondary PM standards, the thenAdministrator found the current secondary standards
provide the requisite degree of protection and that
they should be retained (85 FR 82742, December 18,
2020).
Following publication of the 2020 final action,
several parties filed petitions for review and petitions
for reconsideration of the EPA’s final decision. The
petitions for review were filed in the D.C. Circuit and
the Court consolidated the cases. 15 Following EPA’s
decision to reconsider the 2020 final decision, the
Court ordered the consolidated cases to be held in
abeyance.
b. Reconsideration of the 2020 PM NAAQS
Final Action
Executive Order 13990 directed review of certain
agency actions (86 FR 7037, January 25, 2021). 16 An
accompanying fact sheet provided a non-exclusive list
of agency actions that agency heads should review in
15 See California v. EPA, (D.C. Cir., No. 21–2014 consolidated
with Nos. 21–1027, 21–1054).
See
https://www.whitehouse.gov/briefingroom/presidential-actions/2021/01/20/executive-orderprotecting-public-health-and-environment-and-restoring-scienceto-tackle-climate-crisis/.
16
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accordance with that order, including the 2020
Particulate Matter NAAQS Decision. 17
On June 10, 2021, the Agency announced its
decision to reconsider the 2020 PM NAAQS final
action because the available scientific evidence and
technical information indicate that the current
standards may not be adequate to protect public
health and welfare, as required by the Clean Air Act. 18
The Administrator reached this decision in part based
on the fact that the EPA noted that the 2020 PA
concluded that the scientific evidence and information
called into question the adequacy of the primary
annual PM2.5 standard and supported revising the
level to below the current level of 12.0 μg/m3 while
retaining the primary 24-hour PM2.5 standard (U.S.
EPA, 2020b). The EPA also noted that the 2020 PA
concluded that the available scientific evidence and
information supported retaining the primary PM10
standard and secondary PM standards without
revision (U.S. EPA, 2020b).
The EPA staff conclusions detailed in the 2020 PA
in combination with the CASAC advice that informed
the Administrator’s decisions regarding the 2020 final
17
See
https://www.whitehouse.gov/briefingroom/statements-releases/2021/01/20/fact-sheet-list-ofagency-actions-for-review/.
18 The press release for this announcement is available at:
https://www.epa.gov/newsreleases/epa-reexamine-healthstandards-harmful-soot-previous-administration-leftunchanged.
87a
action, studies highlighted by public comments on the
2020 proposal, and the numerous studies published
since the literature cutoff date of the 2019 ISA all
informed the scope of the reconsideration.
In its review of the 2019 draft PA, some members
of the CASAC had recommended that greater
attention should be given to accountability studies
and epidemiologic studies that employ alternative
methods for confounder control (also referred to as
causal inference or causal modeling studies) in order
to “more fully account for effects of confounding,
measurement and estimation errors, model
uncertainty, and heterogeneity” in epidemiologic
studies (Cox, 2019b, p. 8 of consensus responses). In
addition, public commenters submitted a number of
recent studies published after the literature cutoff
date for the 2019 ISA that would have been considered
within the scope of the 2019 ISA. While the EPA
provisionally considered these studies in responding
to public comments, 19 it was determined that, at the
time of the 2020 final action, these studies were
generally consistent with the evidence assessed in the
2019 ISA (85 FR 82690, December 18, 2020; U.S. EPA,
2020a). As such, and consistent with previous NAAQS
reviews, the EPA concluded that the new studies did
not materially change any of the broad scientific
conclusions regarding the health and welfare effects
of PM in ambient air made in the air quality criteria,
19 The list of provisionally considered studies is included in
Appendix A to the 2020 Response to Comments document (U.S.
EPA, 2020a).
88a
and therefore, reopening of the air quality criteria was
not warranted (85 FR 82691, December 18, 2020).
However, at that time, the EPA recognized that its
“provisional consideration of these studies did not and
could not provide the kind of in-depth critical review”
(85 FR 82690, December 18, 2020) that studies
undergo in the development of an ISA.
In preparing to reconsider the 2020 final decision
for the PM NAAQS, the Agency revisited the need to
reopen the air quality criteria, given the amount of
time that had passed since the literature cutoff date
of the 2019 ISA (i.e., approximately January 2018)
and the volume of literature that had become
available, including those studies provisionally
considered in responding to comments in 2020. In so
doing, the EPA preliminarily concluded that at least
some of these studies were likely to be relevant to its
reconsideration of the air quality criteria and the PM
NAAQS and that, in considering public comments on
any proposed decisions for the reconsideration, these
studies were likely to be raised by public commenters
and would potentially warrant a reopening of the air
quality criteria. For example, on February 16, 2021,
the EPA received two petitions to reconsider the PM
NAAQS. One petition objected to the EPA’s
provisional consideration of studies submitted in
public comments on the 2020 proposal and suggested
that the provisional consideration was inadequate
because the studies could be important in determining
whether the existing standards are adequately
protective. See, Petition for Reconsideration of
National Ambient Air Quality Standards for
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Particulate Matter, submitted by American Lung
Association, et al, dated Feb. 16, 2020. The other
petition identified a number of new studies, including
one epidemiologic study that was published after the
provisional consideration was completed that could
further inform the concern expressed by the CASAC
that associations reported in epidemiologic studies do
not adequately account for “uncontrolled confounding
and other potential sources of error and bias.” See
Petition for Reconsideration of “Review of the
National Ambient Air Quality Standards for
Particulate Matter,” submitted by the State of
California, dated Feb. 16, 2020. This was also an
uncertainty noted by the then-Administrator in the
2020
decision,
who
also
recognized
“that
methodological study designs to address confounding,
such as causal inference methods, are an emerging
field of study.” Thus, the Agency concluded it was
appropriate to reconsider not only the standards but
also the air quality criteria, in light of public
comments during the 2020 PM NAAQS proposal and
recent studies published since the cutoff date of the
2019 ISA, as reflected in petitions. In deciding to
reopen the air quality criteria, the Agency concluded
it was reasonable to focus on studies that were most
likely to inform decisions on the appropriate standard,
but not to reassess areas which, based on the
assessment of available science published since the
cutoff date of the 2019 ISA and through 2021, were
judged unlikely to have new information that would
be useful for the Administrator’s decision making. The
Agency accordingly announced that, in support of the
90a
reconsideration, it would develop a supplement to the
2019 ISA and a revised PA.
The EPA also explained that the draft ISA
Supplement and draft PA would be reviewed at a
public meeting by the CASAC, and the public would
have opportunities to comment on these documents
during the CASAC review process, as well as to
provide input during the rulemaking through the
public comment process and public hearings on the
proposed rulemaking.
On March 31, 2021, the Administrator announced
his decision to reestablish the membership of the
CASAC to “ensure the agency received the best
possible scientific insight to support our work to
protect human health and the environment.” 20
Consistent with this memorandum, a call for
nominations of candidates to the EPA’s chartered
CASAC was published in the Federal Register (86
FR 17146, April 1, 2021). On June 17, 2021, the
Administrator announced his selection of the seven
members to serve on the chartered CASAC. 21 22
20 The press release for this announcement is available at:
https://www.epa.gov/newsreleases/administrator-regandirects-epa-reset-criticalscience-focused-federal-advisory.
21 The press release for this announcement is available at:
https://www.epa.gov/newsreleases/epaannounces-selectionscharter-members-clean-airscientific-advisory-committee.
22 The list of members of the chartered CASAC and their
biosketches
are
available
at:
https://casac.epa.gov/ords/sab/r/sab_apex/casac/mems?p14_
91a
Additionally, a call for nominations of candidates to a
PM-specific panel was published in the Federal
Register (86 FR 33703, June 25, 2021). The members
of the PM CASAC panel were announced on August
30, 2021. 23
The draft ISA Supplement was released in
September 2021 (U.S. EPA, 2021a; 86 FR 54186,
September 30, 2021), and included a discussion of the
rationale and scope of the Supplement. As explained
therein, the ISA Supplement focuses on a thorough
evaluation of some studies that became available after
the literature cutoff date of the 2019 ISA that could
either further inform the adequacy of the current PM
NAAQS or address key scientific topics that have
evolved since the literature cutoff date for the 2019
ISA. In selecting the health effects to evaluate within
the ISA Supplement, the EPA focused on health
effects for which the evidence supported a “causal
relationship” because those were the health effects
that were most useful in informing conclusions in the
2020 PA (U.S. EPA, 2022a, section 1.2.1). 24 Consistent
committeeon=2021%20CASAC%20PM%20Panel&session=17433
386035954.
The list of members of the PM CASAC panel and their
biosketches
are
available
at:
https://casac.epa.gov/ords/sab/f?p=105:14:9979229564047:::1
4:P14_COMMITTEEON:2021%20CASAC%20PM%20Panel.
23
24 As described in section 1.2.1 of the ISA Supplement: “In
considering the public health protection provided by the current
primary PM2.5 standards, and the protection that could be
provided by alternatives, [the U.S. EPA, within the 2020 PM PA]
emphasized health outcomes for which the ISA determined that
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with the rationale for the focus on certain health
effects, in selecting the non-ecological welfare effects
to evaluate within the ISA Supplement, the EPA
focused on the non-ecological welfare effects for which
the evidence supported a “causal relationship” and for
which quantitative analyses could be supported by the
evidence because those were the welfare effects that
were most useful in informing conclusions in the 2020
PA. 25 Specifically, for non-ecological welfare effects,
the focus within the ISA Supplement is on visibility
effects. The ISA Supplement also considers recent
health effects evidence that addresses key scientific
topics where the literature has evolved since the 2020
the evidence supports either a ‘causal’ or a ‘likely to be causal’
relationship with PM2.5 exposures” (U.S. EPA, 2020b). Although
the 2020 PA initially focused on this broader set of evidence, the
basis of the discussion on potential alternative standards
primarily focused on health effect categories where the 2019 PM
ISA concluded a ‘causal relationship’ (i.e., short- and long-term
PM2.5 exposure and cardiovascular effects and mortality) as
reflected in Figures 3–7 and 3–8 of the 2020 PA (U.S. EPA,
2020b).”
25 As described in section 1.2.1 of the ISA Supplement: “The
2019 PM ISA concluded a ‘causal relationship’ for each of the
welfare effects categories evaluated (i.e., visibility, climate
effects and materials effects). While the 2020 PA considered the
broader set of evidence for these effects, for climate effects and
material effects, it concluded that there remained ‘substantial
uncertainties with regard to the quantitative relationships with
PM concentrations and concentration patterns that limit[ed]
[the] ability to quantitatively assess the public welfare protection
provided by the standards from these effects’ (U.S. EPA, 2020b).”
93a
review was completed, specifically since the literature
cutoff date for the 2019 ISA. 26
Building on the rationale presented in section 1.2.1,
the ISA Supplement considers peer-reviewed studies
published from approximately January 2018 through
March 2021 that meet the following criteria:
x Health Effects
U.S. and Canadian epidemiologic studies for
health effect categories where the 2019 ISA
concluded a “causal relationship” (i.e., shortand
long-term
PM2.5
exposure
and
cardiovascular effects and mortality).
U.S. and Canadian epidemiologic studies
that employed alternative methods for
confounder
control
or
conducted
accountability analyses (i.e., examined the
effect of a policy on reducing PM2.5
concentrations).
x Welfare Effects
These key scientific topics include experimental studies
conducted at near-ambient concentrations, epidemiologic studies
that employed alternative methods for confounder control or
conducted accountability analyses, studies that assess the
relationship between PM2.5 exposure and severe acute
respiratory syndrome coronavirus 2 (SARS–CoV–2) infection and
coronavirus disease 2019 (COVID–19) death; and in accordance
with recent EPA goals on addressing environmental justice,
studies that examine disparities in PM2.5 exposure and the risk
of health effects by race/ethnicity or socioeconomic status (SES)
(U.S. EPA, 2022a, section 1.2.1).
26
94a
U.S. and Canadian studies that provide new
information on public preferences for visibility
impairment and/or developed methodologies or
conducted quantitative analyses of light
extinction.
x Key Scientific Topics
Experimental studies (i.e., controlled human
exposure and animal toxicological) conducted
concentrations
at
near-ambient
PM2.5
experienced in the U.S.
U.S.- and Canadian-based epidemiologic
studies that examined the relationship between
PM2.5 exposures and severe acute respiratory
syndrome coronavirus 2 (SARS–CoV–2)
infection and coronavirus disease 2019
(COVID– 19) death.
At-Risk Populations.
U.S.- and Canadian-based epidemiologic or
exposure studies examining potential
disparities in either PM2.5 exposures or the
risk of health effects by race/ethnicity or
socioeconomic status (SES).
Given the narrow scope of the ISA Supplement, it
is important to recognize that the evaluation does not
encompass the full multidisciplinary evaluation
presented within the 2019 ISA that would result in
weight-of-evidence conclusions on causality (i.e.,
causality determinations). The ISA Supplement
critically evaluates and provides key study-specific
information for those recent studies deemed to be of
greatest significance for informing preliminary
95a
conclusions on the PM NAAQS in the context of the
body of evidence and scientific conclusions presented
in the 2019 ISA.
In developing a revised PA to support the
reconsideration, the EPA considered the available
scientific evidence, including the evidence presented
in the 2019 ISA and ISA Supplement. The 2022 PA
considered the quantitative and technical information
presented in the 2020 PA, in addition to new and
updated analyses conducted since the 2020 final
decision. For those health and welfare effects for
which the ISA Supplement evaluated recently
available studies (i.e., PM2.5-related health effects and
visibility effects), new updated quantitative analyses
were conducted as a part of the development of the
2022 PA. The newly available scientific and technical
information presented in the 2022 PA were considered
in reaching conclusions regarding the adequacy of the
current standards and any potential alternative
standards. For those health and welfare effects for
which newly available scientific and technical
information were not evaluated (i.e., PM10–2.5-related
health effects and non-visibility welfare effects), the
conclusions presented in the 2022 PA rely heavily on
the information that supported the conclusions in the
2020 PA.
The CASAC PM panel met at a virtual public
meeting in November 2021 to review the draft ISA
Supplement (86 FR 52673, September 22, 2021). A
virtual public meeting was then held in February
2022, and during this meeting the chartered CASAC
96a
considered the CASAC PM panel’s draft letter to the
Administrator on the draft ISA Supplement (87 FR
958, January 7, 2022).
The chartered CASAC provided its advice on the
draft ISA Supplement in a letter to the EPA
Administrator dated March 18, 2022 (Sheppard,
2022b). In its review of the draft ISA Supplement, the
CASAC noted that they found “the Draft ISA
Supplement to be a well-written, comprehensive
evaluation of the new scientific information published
since the 2019 PM ISA” (Sheppard, 2022b, p. 2 of
letter). Furthermore, the CASAC stated that “the
final
Integrated
Science
Assessment
(ISA)
Supplement . . . deserve[s] the Administrator’s full
consideration and [is] adequate for rulemaking”
(Sheppard, 2022b, p. 2 of letter). The CASAC
generally endorsed EPA’s decisions regarding the
limited scope of the draft ISA Supplement, stating
that “this limitation [on scope] is appropriate for the
targeted purpose of the Draft ISA Supplement”
although the CASAC noted it would not be
appropriate for ISAs generally, and recommended
that the EPA provide additional acknowledgment and
explanation for the limited scope (Sheppard, 2022b,
p. 2 of letter; see also pp. 2–3 of consensus responses).
The EPA specifically noted in the final ISA
Supplement, which was released in May 2022 (U.S.
EPA, 2022a; hereafter referred to as the ISA
Supplement throughout this document) that the
“targeted approach to developing the Supplement to
the 2019 PM ISA for the purpose of reconsidering the
2020 PM NAAQS decision does not reflect a change to
97a
EPA’s approach for developing ISAs for NAAQS
reviews.” Thus, the evidence presented within the
2019 ISA, along with the targeted identification and
evaluation of new scientific information in the ISA
Supplement, provides the scientific basis for the
reconsideration of the 2020 PM NAAQS final decision.
The draft PA was released in October 2021 (86 FR
56263, October 8, 2021). The CASAC PM panel met at
a virtual public meeting in December 2021 to review
the draft PA (86 FR 52673, September 22, 2021). A
virtual public meeting was then held in February
2022 and March 2022, and during this meeting the
chartered CASAC considered the CASAC PM panel’s
draft letter to the Administrator on the draft PA (87
FR 958, January 7, 2022). The chartered CASAC
provided its advice on the draft PA in a letter to the
EPA Administrator dated March 18, 2022 (Sheppard,
2022a). The EPA took steps to address these
comments in revising and finalizing the PA. The 2022
PA considers the scientific evidence presented in the
2019 ISA and ISA Supplement and considers the
quantitative and technical information presented in
the 2020 PA, along with updated and newly available
analyses since the completion of the 2020 review. For
those health and welfare effects for which the ISA
Supplement evaluated recently available evidence
and for which updated quantitative analyses were
supported (i.e., PM2.5-related health effects and
visibility effects), the 2022 PA includes consideration
of this newly available scientific and technical
information in reaching preliminary conclusions. For
those health and welfare effects for which newly
98a
available scientific and technical information were not
evaluated (i.e., PM10–2.5-related health effects and
non-visibility effects), the conclusions presented in the
2022 PA rely heavily on the information that
supported the conclusions in the 2020 PA. The final
PA was released in May 2022 (U.S. EPA, 2022b;
hereafter referred to as the 2022 PA throughout this
document).
Drawing from his consideration of the scientific
evidence assessed in the 2019 ISA and ISA
Supplement and the analyses in the 2022 PA,
including the uncertainties in the evidence and
analyses, and from his consideration of advice from
the CASAC, on January 5, 2023, the Administrator
proposed to revise the level of the primary annual
PM2.5 standard and to retain the primary 24-hour
PM2.5 standard, the primary 24-hour PM10 standard,
and the secondary PM standards. These proposed
decisions were published in the Federal Register on
January 27, 2023 (88 FR 5558, January 27, 2023). The
EPA held a multi-day virtual public hearing on
February 21–23, 2023 (88 FR 6215, January 31, 2023).
In total, the EPA received nearly 700,000 comments
on the proposal from members of the public by the
close of the public comment period on March 28, 2023.
Major issues raised in the public comments are
discussed throughout the preamble of this final
action. A more detailed summary of all significant
comments, along with the EPA’s responses
(henceforth “Response to Comments” document), can
be found in the docket for this rulemaking (Docket No.
EPA–HQ–OAR–2015–0072).
99a
As in prior reviews, the EPA is basing its decision
in this reconsideration on studies and related
information in the air quality criteria, which have
undergone CASAC and public review. These studies
assessed in the 2019 ISA 27 and ISA Supplement 28 and
the 2022 PA, and the integration of the scientific
evidence presented in them, have undergone
extensive critical review by the EPA, the CASAC, and
the public. Decisions on the NAAQS should be based
on studies that have been rigorously assessed in an
integrative manner not only by the EPA but also by
the statutorily mandated independent scientific
advisory committee, as well as the public review that
accompanies this process. It is for this reason that the
EPA preliminarily concluded that the scientific
evidence available since the completion of the 2019
ISA, including those raised in public comments on the
proposal in 2020, warranted a partial reopening of the
air quality criteria and prepared an ISA Supplement
27 In addition to the 2020 review’s opening “call for
information” (79 FR 71764, December 3, 2014), the 2019 ISA
identified and evaluated studies and reports that have
undergone scientific peer review and were published or accepted
for publication between January 1, 2009, through approximately
January 2018 (U.S. EPA, 2019a, p. ES–2). References that are
cited in the 2019 ISA, the references that were considered for
inclusion but not cited, and electronic links to bibliographic
information
and
abstracts
can
be
found
at:
https://hero.epa.gov/hero/particulate-matter.
As described above, the ISA Supplement represents an
evaluation of recent studies that are of greatest policy relevance
and utility to the reconsideration of the 2020 final decision on the
PM NAAQS (U.S. EPA, 2022a).
28
100a
to enable the EPA, the CASAC, and the public to
consider them further. Some commenters have
referred to and discussed additional individual
scientific studies on the health effects of PM that were
not included in the 2019 ISA or ISA Supplement (“new
studies”) and that have not gone through this
comprehensive review process. In considering and
responding to comments for which such “new” studies
were cited in support, the EPA has provisionally
considered the cited studies in the context of the
findings of the 2019 ISA and ISA Supplement. The
EPA’s provisional consideration of these studies did
not and could not provide the kind of in-depth critical
review described above, but rather was focused on
determining whether they warranted further
reopening the review of the air quality criteria to
enable the EPA, the CASAC, and the public to
consider them further.
This approach, and the decision to rely on the
studies and related information in the air quality
criteria, which have undergone CASAC and public
review, is consistent with the EPA’s practice in prior
NAAQS reviews and its interpretation of the
requirements of the CAA. Since the 1970
amendments, the EPA has taken the view that
NAAQS decisions are to be based on scientific studies
and related information that have been assessed as a
part of the pertinent air quality criteria, and the EPA
has consistently followed this approach. This
longstanding interpretation was strengthened by new
legislative requirements enacted in 1977, which
added section 109(d)(2) of the Act concerning CASAC
101a
review of air quality criteria. See 71 FR 6114, 61148
(October 17, 2006, final decision on review of NAAQS
for particulate matter) for a detailed discussion of this
issue and the EPA’s past practice.
As discussed in the EPA’s 1993 decision not to
review the O3 NAAQS, “new” studies may sometimes
be of such significance that it is appropriate to delay
a decision in a NAAQS review and to supplement the
pertinent air quality criteria so the studies can be
taken into account (58 FR 13013–13014, March 9,
1993). In the present case, the EPA decided to
partially reopen the air quality criteria and prepared
an ISA Supplement as a part of the reconsideration to
facilitate evaluation of these studies by the EPA, the
CASAC, and the public. The narrow scope of the ISA
Supplement is supported by EPA’s provisional
consideration of “new” studies submitted in response
to public comments on the 2020 proposal which
concluded that, taken in context, the “new”
information and findings do not materially change
any of the broad scientific conclusions regarding the
health and welfare effects of PM in ambient air made
in the air quality criteria. Therefore, a full reopening
of the air quality criteria was not warranted to assess
the health and welfare effects of PM for purposes of
the review.
Accordingly, the EPA is basing the final decisions
in this reconsideration on the studies and related
information included in the PM air quality criteria
(including the 2019 PM ISA and ISA Supplement)
that have undergone rigorous review by the EPA, the
102a
CASAC, and the public. The EPA will consider these
“new” studies for inclusion in the air quality criteria
for the next PM NAAQS review, which the EPA
expects to begin soon after the conclusion of this
reconsideration and which will provide the
opportunity to fully assess these studies through a
more rigorous review process involving the EPA, the
CASAC, and the public.
D.
Air Quality Information
This section provides a summary of basic
information related to PM ambient air quality. It
summarizes information on the distribution of
particle size in ambient air (section I.D.1), sources and
emissions contributing to PM in the ambient air
(section I.D.2), monitoring ambient PM in the U.S.
(section I.D.3), ambient PM concentrations and trends
in the U.S. (I.D.4), characterizing ambient PM2.5
concentrations for exposure (section I.D.5), and
background PM (section I.D.6). Additional detail on
PM air quality can be found in Chapter 2 of the 2022
PA (U.S. EPA, 2022b).
1.
Distribution of Particle Size in Ambient Air
In ambient air, PM is a mixture of substances
suspended as small liquid and/or solid particles (U.S.
EPA, 2019a, section 2.2) and distinct health and
welfare effects have been linked with exposures to
particles of different sizes. Particles in the
atmosphere range in size from less than 0.01 to more
than 10 mm in diameter (U.S. EPA, 2019a, section
103a
2.2). The EPA defines PM2.5, also referred to as fine
particles, as particles with aerodynamic diameters
generally less than or equal to 2.5 μm. 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 μm and less than
or equal to 10 μm. PM10, which is comprised of both
fine and coarse fractions, includes those particles with
aerodynamic diameters generally less than or equal to
10 μm. In addition, ultrafine particles (UFP) are often
defined as particles with a diameter of less than 0.1
μm based on physical size, thermal diffusivity or
electrical mobility (U.S. EPA, 2019a, section 2.2).
Atmospheric lifetimes are generally longest for PM2.5,
which often remains in the atmosphere for days to
weeks (U.S. EPA, 2019a, Table 2–1) before being
removed by wet or dry deposition, while atmospheric
lifetimes for UFP and PM10–2.5 are shorter and are
generally removed from the atmosphere within hours,
through wet or dry deposition (U.S. EPA, 2019a, Table
2–1; U.S. EPA, 2022b, section 2.1).
2.
Sources and Emissions Contributing to PM in
the Ambient Air
PM is composed of both primary (directly emitted
particles) and secondary particles. Primary PM is
derived from direct particle emissions from specific
PM sources while secondary PM originates from gasphase precursor chemical compounds present in the
atmosphere that have participated in new particle
formation or condensed onto existing particles (U.S.
EPA, 2019a, section 2.3). As discussed further in the
104a
2019 ISA (U.S. EPA, 2019a, section 2.3.2.1),
secondary PM is formed in the atmosphere by
photochemical oxidation reactions of both inorganic
and organic gas-phase precursors. Precursor gases
include sulfur dioxide (SO2), nitrogen oxides (NOX),
and volatile organic compounds (VOC) (U.S. EPA,
2019a, section 2.3.2.1). Ammonia also plays an
important role in the formation of nitrate PM by
neutralizing sulfuric acid and nitric acid. Sources and
emissions of PM are discussed in more detail the 2022
PA (U.S. EPA, 2022b, section 2.1.1). Briefly,
anthropogenic sources of PM include both stationary
(e.g., fuel combustion for electricity production and
other purposes, industrial processes, agricultural
activities) and mobile (e.g., diesel- and gasolinepowered highway vehicles and other engine-driven
sources) sources. Natural sources of PM include dust
from the wind erosion of natural surfaces, sea salt,
wildfires, primary biological aerosol particles (PBAP)
such as bacteria and pollen, oxidation of biogenic
hydrocarbons, such as isoprene and terpenes to
produce secondary organic aerosol (SOA), and
geogenic sources, such as sulfate formed from volcanic
production of SO2. Wildland fire, which encompass
both wildfire and prescribed fire, accounts for 44% of
emissions of primary PM2.5 emissions (U.S. EPA,
2021b). Emissions from wildfire comprises 29% of
primary PM2.5 emissions.
In recent years, the frequency and magnitude of
wildfires have increased (U.S. EPA, 2019a). The
magnitude of the public health impact of wildfires is
substantial both because of the increase in PM2.5
105a
concentrations as well as the duration of the wildfire
smoke season, which is considered to range from May
to November. Wildfire can make a large contribution
to air pollution (including PM2.5), and wildfire events
can threaten public safety and life. The impacts of
wildfire events can be mitigated through management
of wildland vegetation, including through prescribed
fire. Prescribed fire (and some wildfires) can mimic
the natural processes necessary to maintain firedependent ecosystems, minimizing catastrophic
wildfires and the risks they pose to safety, property
and air quality (see, e.g., 81 FR 58010, 58038,
August 24, 2016). The EPA views the strategic use of
prescribed fire as an important tool for reducing
wildfire risk and the severity of wildfires and wildfire
smoke (88 FR, 54118, 54126, August 9, 2023). 29 As
noted in the PM NAAQS proposal, agencies have
efforts in place to reduce the frequency and severity of
human-caused wildfires (88 FR 5570, January 27,
2023).
Wildfire events produce high PM emissions that
may impact the PM concentrations in ambient air to
the extent that the concentrations result in an
exceedance or violation which may affect the design
value in a given area. The EPA’s Exceptional Events
Rule (81 FR 68216, October 3, 2016) describes the
process by which air agencies may request to exclude
‘event-influenced’ data caused by exceptional events,
See
also:
https://www.usda.gov/sites/default/files/documents/usdaepa-doi-cdc-mou.pdffiles/documents/usda-epa-doi-cdc-mou.pdf.
29
106a
which can include wildfires and prescribed fires on
wildland. The EPA has issued guidance specifically
addressing exceptional events demonstrations for
both wildfires and prescribed fires on wildland. These
documents are available on EPA’s Exceptional Events
Program website. 30 The EPA will develop fire-related
exceptional events implementation tools, including
updates as needed to existing guidance to facilitate
more efficient processing of PM2.5-related exceptional
events demonstrations for both the 24-hour and
annual standards.
3.
Monitoring of Ambient PM
To promote uniform application of the air quality
standards set forth under the CAA and to achieve the
degree of public health and welfare protection
intended for the NAAQS, the EPA establishes PM
Federal Reference Methods (FRMs) for both PM10 and
PM2.5 in appendices J and L to 40 CFR part 50, both
of which were amended following the 2006 and 2012
PM NAAQS reviews. The current PM monitoring
network relies on FRMs and automated continuous
Federal Equivalent Methods (FEMs) approved
pursuant to 40 CFR part 53, in part to support
changes necessary for implementation of the revised
PM standards. Additionally, 40 CFR part 58,
appendices A through E, detail the requirements to
measure ambient air quality and report ambient air
See: https://www.epa.gov/air-quality-analysis/final2016-exceptional-events-rule-supporting-guidance-documentsupdated-faqs.
30
107a
quality data and related information. More
information on PM ambient monitoring networks is
available in section 2.2 of the 2022 PA (U.S. EPA,
2022b).
The PM2.5 monitoring program is one of the major
ambient air monitoring programs with a robust,
nationally consistent network of ambient air
monitoring sites providing mass and/or chemical
speciation measurements. 40 CFR part 58, appendix
D, section 4.7 provides the
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