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 HEALTHY ENVIRONMENTS, ET AL.,
INTERVENORS
Consolidated with 24-1051, 24-1052, 24-1073, 24-1091
On Petitions for Review of a Final Action
of the Environmental Protection Agency
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.
2
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. 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
3
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, 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.
4
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.
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 respondent-intervenors 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 Morrisseau, Assistant Attorney General,
Office of the Attorney General for the State of Michigan,
5
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, 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.
6
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.
7
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
8
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 µg/m³ to 9 µg/m³. In so doing, he expressly reconsidered a
decision of the previous Administrator who in 2020 had
decided to leave the 12 µg/m³ 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 decision-making 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 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
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.
9
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 pollutant in the ambient air,
in varying quantities.” Id. 2
2
As far as “practicable,” the criteria shall include the following information:
(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
10
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). 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.
(C) any known or anticipated adverse effects on
welfare.
§ 7408(a)(2).
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).
11
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 fiveyear 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 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
12
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)
(five-year intervals for the CASAC beginning on January 1,
1980), with § 7409(d)(1) (five-year intervals for the
Administrator beginning on December 31, 1980). “[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 µg/m³. 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 µg/m³ for PM2.5, however, the
CASAC “did not reach consensus.” Id. at 82706/1-2. Those
who supported keeping the standard at 12.0 µg/m³ “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
13
studies that show positive associations even when estimated
exposures above 12 µg/m³ 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 µg/m³. 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 µg/m³ — 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 Administrations. 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
14
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 µg/m³ primary
standard should be lowered. See id. at 16204/2, 16256/3. A
majority of the CASAC recommended a standard of 8 to
10 µg/m³, while a minority deemed a standard of 10 to
11 µg/m³ 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 µg/m³, the 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
15
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.”
The intervenors opposed the EPA’s motion for vacatur.
The Health Group Intervenors filed a cross-motion 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).
16
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 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
17
§ 7409(d)(1), which requires the Administrator to review and
revise (if appropriate) 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.
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
18
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 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.
4
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.
19
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 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 setting 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
20
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 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 required by the first sentence of
§ 7409(d)(1) does not limit the Administrator’s authority to
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.
21
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 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.
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,”
22
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 crossreferences 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 off-cycle — 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 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 crossreference 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
23
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.
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
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-andcomment 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.
24
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 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 “sugIndeed, 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.
25
gests 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 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 consider 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 of agency exper26
tise.” 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 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
27
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 No. 13990, which they
unilaterally christen “the Climate Order.” The references to climate change in that order, however, are not specific to
NAAQS-setting and cannot overcome the evidence — or lack
of it — in the administrative record.
In any event, the EPA addressed this issue head-on 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
28
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 “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 therefore should not be read “as implicitly allowing the [EPA] to
consider cost anyway.” Id. at 755-56. Consistent with that
29
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.
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-by-case 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 attainability 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 attainment
30
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 prescribes 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 revising NAAQS”). 7 Indeed, this court in Murray explained
7
The petitioners briefly argue that this conclusion results in an
interpretation of the statutory scheme that violates the non-delegation 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
31
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
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 two-step
interpretation of § 7409(d)(1), that the Administrator was 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 µg/m³,” 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).
32
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 over the brink
of ruin, or even forcing deindustrialization” (cleaned up)).
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
33
to consider non-public health factors throughout the NAAQSsetting 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 offcycle review. Second, the Administrator failed adequately to
explain his reasons for departing from the conclusions of the
2020 Final Rule. Third, the scientific evidence does not justify
a standard of 9 µg/m³.
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.
34
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 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 µg/m³,” 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 µg/m³.” 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 µg/m³ 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
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 µg/m³.” Id. at 16275/3.
35
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 model-based 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 µg/m³. 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.
Here the petitioners do not claim the Administrator overlooked an important scientific study or ignored a key publichealth 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
36
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 µg/m³. See 89 Fed. Reg. at 16280/1 (explaining that “the
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 µg/m³, 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 µg/m³ than 9.0 µg/m³.” So
they say because the 2024 Final Rule recognized that “an
annual standard level that is no more than 15-18% higher than
the study-reported means[, or 10.7 to 11.0 µg/m³,] would generally 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
37
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]”).
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.