Ohio v. Environmental Protection Agency

P R E L I M I N A R Y P R I N T
Volume 603 U. S. Part 1
Pages 279–323
OFFICIAL REPORTS
OF
T H E S U P R E M E C O U R T
June 27, 2024
REBECCA A. WOMELDORF
reporter of decisions
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N O T I C E : This preliminary print is subject to formal revision before
the bound volume is published. Users are requested to notify the Reporter
of Decisions, Supreme Court of the United States, Washington, D.C. 20543,
pio@supremecourt.gov, of any typographical or other formal errors.

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TERM, 2023
279
Syllabus
OHIO et al v. ENVIRONMENTAL PROTECTION
AGENCY
et al
on applications for stay
No. 23A349. Argued February 21, 2024—Decided June 27, 2024*
The Clean Air Act envisions a collaborative effort between States and the
federal government to regulate air quality. When the Environmental
Protection Agency sets standards for common air pollutants, States
must submit a State Implementation Plan, or SIP, providing for the
“implementation, maintenance, and enforcement” of those standards in
their jurisdictions. See 42 U. S. C. § 7410(a)(1). Because air currents
can carry pollution across state borders, States must also design their
plans with neighboring States in mind. Under the Act's “Good Neigh-
bor Provision,” state plans must prohibit emissions “in amounts which
will . . . contribute signifcantly to nonattainment in, or interfere with
maintenance by, any other State” of the relevant air-quality standard.
§ 7410(a)(2)(D)(i)(I). Only if a SIP fails to satisfy the “applicable re-
quirements” of the Act may EPA issue a Federal Implementation Plan,
or FIP, for the noncompliant State that fails to correct the defciencies
in its SIP. §§ 7410(k)(3), (c)(1).
In 2015, EPA revised its air-quality standards for ozone, thus trigger-
ing a requirement for States to submit new SIPs. Years later, EPA
announced its intention to disapprove over 20 SIPs because the agency
believed they had failed to address adequately obligations under the
Good Neighbor Provision. During the public comment period for the
proposed SIP disapprovals, EPA issued a single proposed FIP to bind
all those States. EPA desig ned its proposed FIP based on which
emissions-control measures would maximize cost effectiveness in im-
proving ozone levels downwind and on the assumption the FIP would
apply to all covered States. Commenters warned that the proposed
SIP disapprovals were fawed and that a failure to achieve all the SIP
disapprovals as EPA envisioned would mean that EPA would need to
reassess the measures necessary to maximize cost-effective ozone-level
*Together with No. 23A350, Kinder Morgan, Inc., et al. v. Environmen-
tal Protection Agency et al., No. 23A351, American Forest & Paper Assn.
et al. v. Environmental Protection Agency, et al., and No. 23A384, United
States Steel Corp. v. Environmental Protection Agency et al., also on ap-
plications for stay.

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280 OHIO
v. EPA
Syllabus
improvements in light of a different set of States. EPA proceeded to
issue
its fnal FIP without addressing this concern. Instead, EPA an-
nounced that its plan was severable: Should any jurisdiction drop out,
the plan would continue to apply unchanged to the remaining jurisdic-
tions. Ongoing litigation over the SIP disapprovals soon vindicated at
least some of the commenters' concerns. Courts stayed 12 of the SIP
disapprovals, which meant EPA could not apply its FIP to those States.
A number of the remaining States and industry groups challenged the
FIP in the D. C. Circuit. They argued that EPA's decision to apply the
FIP after so many other States had dropped out was “arbitrary” or
“capricious,” and they asked the court to stay any effort to enforce the
FIP against them while their appeal unfolded. The D. C. Circuit denied
relief, and the parties renewed their request in this Court.
Held: The applications for a stay are granted; enforcement of EPA's rule
against the applicants shall be stayed pending the disposition of the
applicants' petition for review in the D. C. Circuit and any petition for
writ of certiorari, timely sought. Pp. 290–300.
(a) W hen decidi ng an applicati on for a st ay, the Cour t asks (1)
whether the applicant is likely to succeed on the merits, (2) whether it
will suffer irreparable injury without a stay, (3) whether the stay will
substantially injure the other parties interested in the proceedings, and
(4) where the public interest lies. Nken v. Holder, 556 U. S. 418, 434.
When States and other parties seek to stay the enforcement of a federal
regulation against them, often “the harms and equities [will be] very
weighty on both sides.” Labrador v. Poe, 601 U. S. –––, ––– (Kava-
naugh, J., concurring in grant of stay). Because that is true here, reso-
lution of applicants' stay requests ultimately turns on the frst question:
W ho is likely to prevai l at the end. See Nken, 556 U. S., at 434.
Pp. 290–292.
(b) Applicants are likely to prevail on their arbitrary-or-capricious
claim. An agency action qualifes as “arbitrary” or “capricious” if it is
not “reasonable and reasonably explained.” FCC v. Prometheus Radio
Project, 592 U. S. 414, 423. Thus, the agency must offer “a satisfactory
explanation for its action[,] including a rational connection between the
facts found and the choice made,” and cannot simply ignore “an impor-
tant aspect of the problem.” Motor Vehicle Mfrs. Assn. of United
States, Inc. v. State Farm Mut. Automobile Ins. Co., 463 U. S. 29, 43.
EPA's plan rested on an assumption that all the upwind States would
adopt emissions-reduction measures up to a uniform level of costs to the
point of diminishing returns. Commenters posed their concerns that if
upwind States fell out of the planned FIP, the point at which emissions-
control measures maximize cost-effective downwind air-quality im-

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Cite
as: 603 U. S. 279 (2024)
281
Syllabus
provements might shift. To this question, EPA offered no reasoned
response
. As a result, the applicants are likely to prevail on their argu-
ment that EPA's fnal rule was not “reasonably explained,” Prometheus
Radio Project, 592 U. S., at 423, and that it instead ignored “an impor-
tant aspect of the problem” before it, State Farm Mut. Automobile Ins.
Co., 463 U. S., at 43. Pp. 292–294.
(c) EPA's alternative arguments are unavailing. First, EPA argues
that adding a “severability” provision to its fnal rule—i. e., provid-
ing the FIP would “continue to be implemented” without regard to the
number of States remaining—responded to commenters' concerns. But
EPA's response did not address those concerns so much as it side-
stepped them. Nothing in the fnal rule's severability provision actu-
ally addressed whether and how measures found to maximize cost
effectiveness in achieving downwind ozone air-quality improvements
with the participation of all the upwind States remain so when many
fewer States might be subject to the agency's plan. Second, EPA in-
sists that no one raised that concern during the public comment period.
The Act's “reasonable specifcity” requirement, however, does not mean
a party must rehearse the identical argument made before the agency.
Here, EPA had notice of the objection, and its own statements and ac-
tions confrm the agency appreciated the concern. Third, EPA argues
that applicants must return to EPA and fle a motion asking it to recon-
sider its fnal rule before presenting their objection in court because the
“grounds for [their] objection arose after the period for public com-
ment.” § 7607(d)(7)(B). Nothing requires the applicants to return to
EPA to raise (again) a concern EPA already had a chance to address.
Pp. 294–298.
Applications for stay granted.
Gorsuch, J., delivered the opinion of the Court, in which Roberts,
C. J., and Thomas, Alito, and Kavanaugh, JJ., joined. Barrett, J., fled
a dissenting opinion, in which Sotomayor, Kagan, and Jackson, JJ.,
joined, post, p. 300.
Mathura J. Sridharan, Deputy Solicitor General of Ohio,
argued the cause for state applicants in No. 23A349. With
her on the application for stay and reply were Dave Yost,
Attorney General, Zachery P. Keller, Deputy Solicitor Gen-
era l, Theodo re E. Roki ta, Attor ney General of Indiana,
James A. Barta, Deputy Solicitor General, Patrick Morri-
sey, Attorney General of West Virginia, Lindsay S. See, So-

282 OHIO
v. EPA
Counsel
licitor General, and Michael Williams, Principal Deputy
Sol
icitor General.
Catherine E. Stetson argued the cause for industry appli-
cants in all cases. With her on the application for stay and
reply in No. 23A350 were Ana M. Gutiérrez, Michael D.
Miller, Brittany M. Pemberton, and Eric D. McArthur.
Jonathan Y. Ellis, Allison D. Wood, Makram B. Jaber, Mi-
chael B. Schon, Mithun Mansinghani, David M. Flannery,
and Kathy G. Beckett fled an application for stay and reply
in No. 23A351. John D. Lazzaretti fled an application for
stay and reply in No. 23A384.
Deputy Solicitor General Stewart argued the cause for
federal respondents in all cases. Solicitor General Prelogar
fled a response in opposition to the applications for stay in
all cases.
Judith N. Vale, Deputy Solicitor General of New York, ar-
gued the cause for state respondents in all cases. With her
on the briefs were Letitia James, Attorney General, Barbara
D. Underwood, Solicitor General, Elizabeth A. Brody and
Stephen J. Yanni, Assistant Solicitors General, and Morgan
A. Costello and Claiborne E. Walthall, Assistant Attorneys
General, and Sylvia Hinds-Radix, Corporation Counsel for
New York City, and by the Attorneys General for their re-
spective jurisdictions as follows: William Tong of Connecti-
cut, Kathleen Jennings of Delaware, Brian L. Schwalb of
the District of Columbia, Kwame Raoul of Illinois, Anthony
G. Brown of Maryland, Andrea Joy Campbell of Massachu-
setts, Matthew J. Platkin of New Jersey, Michelle A. Henry
of Pennsylvania, and Joshua L. Kaul of Wisconsin. Megan
M. Herzog, Sean H. Donahue, David T. Goldberg, Vickie L.
Patton, Shaun A. Goho, Neil Gormley, and Kathleen L.
Riley fled a response in opposition to the applications for
stay for public interest respondents in all cases.†
†Jeremy C. Marwell and Eric Groten fled a brief for the Energy Infra-
structure Council as amicus curiae urging grant of application for stay in
No. 23A350.
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as: 603 U. S. 279 (2024)
283
Opinion of the Court
Justice Gorsuch delivered the opinion of the Court.
The
Clean Air Act envisions States and the federal gov-
ernment working together to improve air quality. Under
that law's terms, States bear “primary responsibility” for
developing plans to achieve air-quality goals. 42 U. S. C.
§ 7401(a)(3). Should a State fail to prepare a legally compli-
ant plan, however, the federal government may sometimes
step in and assume that authority for itself. § 7410(c)(1).
Here, the federal government announced its intention to re-
ject over 20 States' plans for controlling ozone pollution. In
their place, the gover nment sought to impose a sing le,
uniform federal plan. This litigation concerns whether, in
adopting that plan, the federal government complied with
the terms of the Act.
I
A
“The Clean Air Act regulates air quality through a federal-
state collaboration.” EME Homer City Generation, L.P. v.
EPA, 795 F. 3d 118, 124 (CADC 2015). Periodically, the En-
vironmental Protection Agency (EPA) sets standards for
common air pollutants, as necessary to “protect the public
health.” §§ 7409(a)(1), (b)(1). Once EPA sets a new stand-
ard, the clock starts ticking: States have three years to de-
sign and submit a plan—called a State Implementation
Plan, or SIP—providing for the “implementation, mainte-
nance, and enforcement” of that standard in their jurisdic-
tions. § 7410(a)(1); see EPA v. EME Homer City Genera-
tion, L. P., 572 U. S. 489, 498 (2014). Under the Act, States
decide how to measure ambient air quality. § 7410(a)(2)(B).
States pick “emission limitations and other control meas-
ures.” § 7410(a)(2)(A). And States provide for the enforce-
ment of their prescribed measures. § 7410(a)(2)(C).
At the same time, States must design these plans with
their neighbors in mind. Because air currents can carry pol-
lution across state borders, emissions in upwind States
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284 OHIO
v. EPA
Opinion of the Court
sometimes affect air quality in downwind States. See EME
H
omer, 572 U. S., at 496. To address that externality prob-
lem, under the Act's “Good Neighbor Provision,” state plans
must prohibit emissions “in amounts which will . . . contrib-
ute signifcantly to nonattainment in, or interfere with main-
tenance by, any other State” of the relevant air-qua lity
standard. § 7410(a)(2)(D)(i)(I).
Because the States bear “primary responsibility” for de-
veloping compliance plans, § 7401(a)(3), EPA has “no author-
ity to question the wisdom of a State's choices of emission
limitations.” Train v. Natural Resources Defense Council,
Inc., 421 U. S. 60, 79 (1975). So long as a SIP satisfes the
“applicable requirements” of the Act, including the Good
Neighbor Prov isi on, EPA “sha l l approve” it w ith i n 18
months of its submission. § 7410(k)(3); see §§ 7410(k)(1)(B),
(k)(2). If, however, a SIP falls short, EPA “shall” issue a
Federal Implementation Plan, or FIP, for the noncompliant
State—that is, “unless” the State corrects the defciencies in
its SIP frst. § 7410(c)(1); EME Homer, 572 U. S., at 498.
EPA must also ensure States meet the new air-quality
standard by a statutory deadline. See § 7511.
B
A layer of ozone in the atmosphere shields the world from
the sun's radiation. See Natural Resources Defense Coun-
cil v. EPA, 464 F. 3d 1, 3 (CADC 2006). But closer to earth,
ozone can hurt more than it helps. Forming when sunlight
interacts with a wide range of precursor pollutants, ground-
level ozone can trigger and exacerbate health problems and
damage vegetation. 80 Fed. Reg. 65299, 65302, 65370 (2015).
To mitigate those and other problems, in 2015 EPA revised
its air-quality standards for ozone from 75 to 70 parts per
billion. Id., at 65293–65294. That change triggered a re-
quirement for States to submit new SIPs. Id., at 65437.
Along the way, EPA issued a guidance document advising
States that they had “fexibility” in choosing how to address
their Good Neighbor obligations. See EPA, Memorandum,

Cite
as: 603 U. S. 279 (2024)
285
Opinion of the Court
Information on the Interstate Transport State Implementa-
ti
on Plan Submissions for the 2015 Ozone National Ambient
Air Quality Standards 3 (Mar. 27, 2018). With that and
other guidance in hand, many (though not all) States submit-
ted SIPs. See 84 Fed. Reg. 66612 (2019). And many of the
States that did submit SIPs said that they need not adopt
emissions-control measures to comply with the Good Neigh-
bor Provision because, among other things, they were not
linked to downwind air-quality problems or they could iden-
tify no additional cost-effective methods of controlling the
emissions beyond those they were currently employing.
See, e. g., 87 Fed. Reg. 9798, 9810 (2022); 87 Fed. Reg. 9545,
9552 (2022); see generally 88 Fed. Reg. 9336, 9354–9361
(2023).
For over two years, EPA did not act on the SIPs it re-
ceived. See, e. g., 87 Fed. Reg. 9838, 9845 –9851 (2022).
Then, in February 2022, the agency announced its intention
to disapprove 19 of them on the ground that the States sub-
mitting them had failed to address adequately their obliga-
tions under the Good Neighbor Provision.
1
A few months
later, the agency proposed disapproving four more SIPs.
2
Pursuant to the Act, the agency issued its proposed SIP dis-
approvals for public comment before fnalizing them. See
§ 7607(d)(3).
C
During that public comment period, the agency proposed
a single FIP to bind all 23 States.
3
87 Fed. Reg. 20036,
1
See 87 Fed. Reg. 9463 (2022) (Maryland); 87 Fed. Reg. 9484 (2022) (New
York, New Jersey); 87 Fed. Reg. 9498 (2022) (Kentucky); 87 Fed. Reg. 9516
(2022) (West Virginia); 87 Fed. Reg. 9533 (2022) (Missouri); 87 Fed. Reg.
9545 (2022) (Alabama, Mississippi, Tennessee); 87 Fed. Reg. 9838 (2022)
(Illinois, Indiana, Michigan, Minnesota, Ohio, Wisconsin); 87 Fed. Reg.
9878 (2022) (Arkansas, Louisiana, Oklahoma, Texas).
2
See 87 Fed. Reg. 31443 (California); 87 Fed. Reg. 31470 (2022) (Utah);
87 Fed. Reg. 31485 (2022) (Nevada); 87 Fed. Reg. 31495 (2022) (Wyoming).
3
EPA also added three more States: Pennsylvania and Virginia, which
had not submitted SIPs, and Delaware, whose SIP, EPA said, it had ap-
proved in “error.” 87 Fed. Reg. 20036, 20038 (2022).
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286 OHIO
v. EPA
Opinion of the Court
20038 (2022). Rather than continue to encourage “ `fexibil-
it[
y]' ” and different state approaches, EPA now apparently
took the view that “[e]ffective policy solutions to the problem
of interstate ozone transport” demanded that kind of “uni-
form framework” and “[n]ationwide consistency.” 87 Fed.
Reg. 9841; see 87 Fed. Reg. 20073. The FIP the agency pro-
posed set as its target the reduction of the emissions of one
family of ozone precursors in particular: nitrogen oxides.
See id., at 20038. And it sought to impose nitrogen-
oxide emissions-control measures that “maximized cost-
effectiveness” in achieving “downwind ozone air quality im-
provements.” Id., at 20055; see also id., at 20043.
In broad strokes, here is how EPA's proposed rule worked
to eliminate a State's “signifcant contribution” to down-
wind ozone problems. First, the agency identifed various
emissions-control measures and, using nationwide data, cal-
culated how much each typically costs to reduce a ton of
nitrogen-oxide emissions. Id., at 20076; see, e. g., id., at
20077–20081. Next, the agency sought to predict how much
each upwind State's nitrogen-oxide emissions would fall if
emissions-producing facilities in the State adopted each
measure. Id., at 20076; see, e. g., id., at 20088–20089; EPA,
Ozone Transport Policy Analysis Proposed Rule TSD 22–23
(EPA–HQ–OAR–2021–0668, 2022) (Proposed Ozone Analy-
sis). In making those predictions, EPA often considered
data specifc to the emissions-producing facilities in the
State, and fed “unit-level and state-level” values into its cal-
culations. See id., at 9–10, 13. Then, the agency estimated
how much, on average, ozone levels would fall in downwind
States with the adoption of each measure. 87 Fed. Reg.
20076; see, e. g., id., at 20092–20093, 20096–20097; Proposed
Ozone Analysis 51–52. In making those estimations, too,
EPA calibrated its modeling to each State's features, “deter-
min[ing] the relationship between changes in emissions and
changes in ozone contributions on a state-by-state . . . basis.”
Id., at 33; see also id., at 40, 42.
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as: 603 U. S. 279 (2024)
287
Opinion of the Court
To pick wh ich measures wou ld “maxi mi z[e] cost-
effec
tiveness” in achieving “downwind ozone air quality im-
provements,” 87 Fed. Reg. 20055, EPA focused on what it
called the “ `knee in the curve,' ” or the point at which more
expenditures in the upwind States were likely to produce
“very little” in the way of “additional emissions reductions
and air quality improvement” downwind, id., at 20095 (hy-
phenation omitted). EPA used this point to select a “uni-
form level” of cost, and so a uniform package of emissions-
reduction tools, for upwind States to adopt. Id., at 20076.
And EPA performed this analysis on two “parallel tracks”—
one for power plants, one for other industries. Ibid. Pur-
suant to the Clean Air Act, §§ 7607(d)(1)(B), (d)(3)–(6), the
agency published its proposed FIP for notice and comment
in April 2022, 87 Fed. Reg. 20036.
Immediately, commenters warned of a potential pitfall
in the agency's approach. EPA had deter mi ned wh ich
emissions-control measures were cost effective at addressing
downwind ozone levels based on an assumption that the FIP
would apply to all covered States. But what happens if
some or many of those States are not covered? As the com-
menters portrayed the SIPs, this was not an entirely specu-
lative possibility. Many believed EPA's disapprovals of the
SIPs were legally fawed. See, e. g., Comments of Missouri
Dept. of Natural Resources 3 (June 17, 2022) (referencing
“all the technical, legal, and procedural issues” with the pro-
posed SIP disapproval); see also, e. g., Comments of Louisi-
ana Dept. of Environmental Quality 1–3 (June 21, 2022);
Comments of Texas Comm'n on Environmental Quality 2–4
(June 21, 2022); EPA, Response to Public Comments on Pro-
posed Rule 9–11 (EPA–HQ–OAR–2021–0668). They added
that EPA's FIP was “inextricably linked” to the SIP disap-
provals. E. g., Comments of Missouri Dept. of Natural Re-
sources, at 4. Without a SIP disapproval or missing SIP,
after all, EPA could not include a State in its FIP. See, e. g.,
id., at 3; supra, at 284.
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288 OHIO
v. EPA
Opinion of the Court
Commenters added that failing to include a State could
have
consequences for the proposed FIP. If the FIP did
not wind up applying to all 23 States as EPA envisioned,
commenters argued, the agency would need “to conduct a
new assessment and modeling of contribution and subject
those fndings to public comment.” E. g., Comments of Air
Stewardship Coalition 13–14 (June 21, 2022); Comments of
Portland Cement Association 7 (June 21, 2022). Why? As
noted above, EPA assessed “signifcant contribution” by de-
termining what measures in upwind States would maximize
cost-effective ozone-level improvements in the States down-
wind of them. Supra, at 286–287. And a different set of
States might mean that the “knee in the curve” would shift.
After all, each State differs in its mix of industries, in its
pre-existing emissions-control measures, and in the impact
those measures may have on emissions and downwind air
quality. See 87 Fed. Reg. 20052, 20060, 20071–20073; EPA,
Technical Memorandum, Screening Assessment of Potential
Emissions Reductions, Air Quality Impacts, and Costs from
Non-EGU Emissions Units for 2026, pp. 12–13 (2022).
4
As it happened, ongoing litigation over the SIP disapprov-
als soon seemed to vindicate at least some of the comment-
4
Commenters pointed out the variance among emissions-producing facil-
ities too. See, e. g., Comments of Indiana Municipal Power Agency 9
(June 20, 2022) (the “cost effectiveness” of one tool “will be highly vari-
able” across different power plants); Comments of Lower Colorado River
Authority 21 (June 21, 2022) (power plants that “have already invested”
in one emissions-control tool “have already undertaken signifcant costs to
achieve [nitrogen-oxide] reductions and have less to gain from additional
control installation”); Comments of Air Stewardship Coalition 27 (June 21,
2022) (noting that the “knee in the curve” appeared to be at a different
cost depending on which mix of industries were considered); Comments of
Wisconsin Paper Council 2 (June 21, 2022) (the air-quality benefts from
controlling one industry—pulp and paper mills—had a “maximum esti-
mated improvement” in ozone levels in downwind States of just 0.0117
parts per billion).
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289
Opinion of the Court
ers' concerns. Two circuits issued stays of EPA's SIP de-
nia
ls for four States. See Order in No. 23–60069 (CA5,
May 1, 2023) (Texas and Louisiana); Order in No. 23–1320
(CA8, May 25, 2023) (Arkansas); Order in No. 23–1719 (CA8,
May 26, 2023) (Missouri).
Despite those comments and developments, the agency
proceeded to issue its fnal FIP. 88 Fed. Reg. 36654 (2023).
5
In response to the problem commenters raised, EPA adopted
a severability provision stating that, should any jurisdiction
drop out, its rule would “continue to be implemented as to
any remaining jurisdictions.” Id., at 36693. But in doing
so, EPA did not address whether or why the same emissions-
control measures it mandated would continue to further the
FIP's stated purpose of maximizing cost-effective air-quality
improvement if fewer States remained in the plan.
D
After EPA issued its fnal FIP, litigation over the agency's
SIP disapprovals continued. One court after another is-
sued one stay after another.
6
Each new stay meant another
S t a t e t o w h i c h E PA c o u l d n o t a p p l y i t s F I P. S e e
§ 7410(c)(1). Ultimately, EPA recognized that it could not
5
The fnal FIP covered 23 States. 88 Fed. Reg. 36654, 36656 (2023).
That plan included Pennsylvania and Virginia, but EPA declined to cover
Tennessee or Wyoming at the time, even though it had announced its in-
tention to disapprove those States' SIPs. Ibid.; see also supra, at 285,
and nn. 1–2. EPA has since proposed a plan for Tennessee and several
other States. 89 Fed. Reg. 12666 (2024).
6
See, e. g., Order in No. 23–60069 (CA5, June 8, 2023) (Mississippi);
Order in No. 23–682 (CA9, July 3, 2023) (Nevada); Order in No. 23–1776
(CA8, July 5, 2023) (Minnesota); Order in No. 23–3216 (CA6, July 25, 2023)
(Kentucky); Order in No. 23–9520 etc. (CA10, July 27, 2023) (Utah and
Oklahoma); Order in No. 23–11173 (CA11, Aug. 17, 2023) (Alabama); see
also Order in No. 23–1418 (CA4, Aug. 10, 2023) (West Virginia, pending
oral argument on preliminary motions to stay and to transfer); Order in
No. 23–1418 (CA4, Jan. 10, 2024) (West Virginia, after oral argument and
pending merits review of petition).

290 OHIO
v. EPA
Opinion of the Court
apply its FIP to 12 of the 23 original States.
7
T
ogether,
these 12 States accounted for over 70 percent of the emis-
sions EPA had planned to address through its FIP. See
Application for Ohio et al. in No. 23A349, p. 1 (States' Appli-
cation); see also 88 Fed. Reg. 36738–36739.
8
A number of the remaining States and industry groups
challenged the remnants of the FIP in the D. C. Circuit.
They pointed to the Act's provisions authorizing a court to
“reverse any . . . action” taken in connection with a FIP that
is “arbitrary” or “capricious.” § 7607(d)(9)(A). And they
argued that EPA's decision to apply the FIP to them even
after so many other States had dropped out met that stand-
ard. As part of their challenge, they asked that court to
stay any effort to enforce the FIP against them while their
appeal unfolded. After that court denied relief, the appli-
cants renewed their request here. The Court has received
and reviewed over 400 pages of briefng and a voluminous
record, held over an hour of oral argument on the applica-
tions, and engaged in months of postargument deliberations
as we often do for the cases we hear.
II
A
Stay applications are nothing new. They seek a form of
interim relief perhaps “as old as the judicial system of the
nation.” Scripps-Howard Radio, Inc. v. FCC, 316 U. S. 4,
17 (1942). Like any other federal court faced with a stay
request, we must provide the applicants with an answer—
7
See 88 Fed. Reg. 49295 (2023) (Arkansas, Kentucky, Louisiana, Mis-
sissippi, Missouri, and Texas); 88 Fed. Reg. 67102 (2023) (Alabama, Min-
nesota, Nevada, Oklahoma, Utah, and West Virginia). EPA has since
proposed settling the litigation over the Nevada SIP disapproval. 89 Fed.
Reg. 35091 (2024).
8
Of course, this could change again as litigation over the SIP denials
progresses past preliminary stay litigation and toward fnal decisions on
the merits.
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Opinion of the Court
“grant or deny.” Labrador v. Poe, 601 U. S. –––, ––– (2024)
(
Kavanaugh, J., concurring in grant of stay).
In deciding whether to issue a stay, we apply the same
“sound . . . principles” as other federal courts. Nken v.
Holder, 556 U. S. 418, 434 (2009) (internal quotation marks
omitted). Specifcally, in this litigation, we ask (1) whether
the applicant is likely to succeed on the merits, (2) whether
it will suffer irreparable injury without a stay, (3) whether
the stay will substantially injure the other parties interested
in the proceedings, and (4) where the public interest lies.
Ibid.; States' Application 13; Response in Opposition for Re-
spondent EPA in No. 23A349 etc., p. 16 (EPA Response).
9
When States and other parties seek to stay the enforce-
ment of a federal regulation against them, often “the harms
and equities [will be] very weighty on both sides.” Labra-
dor, 601 U. S., at ––– (opinion of Kavanaugh, J.). That
is certainly the case here, for both sides have strong argu-
ments with respect to the latter three Nken factors. On one
side of the ledger, the federal government points to the air-
quality benefts its FIP offers downwind States. EPA Re-
sponse 48–50. On the other side, the States observe that a
FIP issued unlawfully (as they contend this one was) neces-
sarily impairs their sovereign interests in regulating their
own industries and citizens—interests the Act expressly rec-
ognizes. See Part I–A, supra; States' Application 24–26;
Maryland v. King, 567 U. S. 1301, 1303 (2012) (Roberts,
C. J., in chambers). The States observe, too, that having
to comply with the FIP during the pendency of this litiga-
tion risks placing them at a “competitive disadvantage” to
their exempt peers. States' Application 21. The States
and the private applicants also stress that complying with
9
Approaching the applications before us like any other stay request
both accords with the Clean Air Act's text, see 42 U. S. C. § 7607(d), and
usual practice in this feld, see, e. g., Texas v. EPA, 829 F. 3d 405, 424 (CA5
2016); West Virginia v. EPA, 90 F. 4th 323, 331 (CA4 2024); In re Murray
Energy Corp., 788 F. 3d 330, 335 (CADC 2015).

292 OHIO
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the FIP during the pendency of this litigation would require
them
to incur “hundreds of millions[,] if not billions of dol-
lars.” Tr. of Oral Arg. 96. Those costs, the applicants note,
are “nonrecoverable.” Thunder Basin Coal Co. v. Reich,
510 U. S. 200, 220–221 (1994) (Scalia, J., concurring in part
and concurring in judgment); see, e. g., States' Application
24; Application for American Forest & Paper Association et
al. 25; see also Alabama Assn. of Realtors v. Department
of Health and Human Servs., 594 U. S. 758, 765 (2021)
(per curiam).
Because each side has strong arguments about the harms
they face and equities involved, our resolution of these stay
requests ultimately turns on the merits and the question who
is likely to prevail at the end of this litigation. See Nken,
556 U. S., at 434; Labrador, 601 U. S., at ––– (opinion of
Kavanaugh, J.).
B
When it comes to that question, the parties agree on the
rules that guide our analysis. The applicants argue that a
court is likely to hold EPA's fnal FIP “arbitrary” or “capri-
cious” within the meaning of the Act and thus enjoin its
enforcement against them. 42 U. S. C. § 7607(d)(9)(A); see,
e. g., States' Application 15–16; Application for American
Forest & Paper Association et al. 14; see also 5 U. S. C.
§ 706(2)(A). An agency action qualifes as “arbitrary” or
“capricious” if it is not “reasonable and reasonably ex-
plained.” FCC v. Prometheus Radio Project, 592 U. S. 414,
423 (2021). In reviewing an agency's action under that
standard, a court may not “ `substitute its judgment for that
of the agency.' ” FCC v. Fox Television Stations, Inc., 556
U. S. 502, 513 (2009). But it must ensure, among other
things, that the agency has offered “a satisfactory explana-
tion for its action[,] including a rational connection between
the facts found and the choice made.” Motor Vehicle Mfrs.
Assn. of United States, Inc. v. State Farm Mut. Automobile
Ins. Co., 463 U. S. 29, 43 (1983) (internal quotation marks
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omitted). Accordingly, an agency cannot simply ignore “an
i
mportant aspect of the problem.” Ibid.
We agree with the applicants that EPA's fnal FIP likely
runs afoul of these long-settled standards. The problem
stems from the way EPA chose to deter mi ne wh ich
emissions “contribute[d] signifcantly” to downwind States'
diffculty meeting national ozone standards. 42 U. S. C.
§ 7410(a)(2)(D)(i)(I). Recall that EPA's plan rested on an as-
sumption that all 23 upwind States would adopt emissions-
reduction tools up to a “uniform” level of “costs” to the point
of diminishing returns. 87 Fed. Reg. 20076, 20095; 88 Fed.
Reg. 36661, 36683–36684, 36719; see Part I–C, supra. But
as the applicants ask: What happens—as in fact did happen—
when many of the upwind States fall out of the planned FIP
and it may now cover only a fraction of the States and emis-
sions EPA anticipated? See, e. g., States' Application 16–21;
Application for American Forest & Paper Association et al.
14–15, 19–20. Does that affect the “knee in the curve,” or
the point at which the remaining States might still “maxi-
miz[e] cost-effectiv[e]” downwind ozone-level improvements?
87 Fed. Reg. 20055. As “the mix of states changes, . . . and
their particular technologies and industries drop out with
them,” might the point at which emissions-control measures
maximize cost-effective downwind air-quality improvements
also shift? Tr. of Oral Arg. 6.
Although commenters posed this concern to EPA during
the notice and comment period, see Part I–C, supra, EPA
offered no reasoned response. Indeed, at argument the gov-
ernment acknowledged that it could not represent with cer-
tainty whether the cost-effectiveness analysis it performed
collectively for 23 States would yield the same results and
command the same emissions-control measures if conducted
for, say, just one State. Tr. of Oral Arg. 58–59. Perhaps
there is some explanation why the number and identity of
participating States does not affect what measures maximize
cost-effective downwind air-quality improvements. But if

294 OHIO
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there is an explanation, it does not appear in the fnal rule.
As
a result, the applicants are likely to prevail on their argu-
ment that EPA's fnal rule was not “reasonably explained,”
Prometheus Radio Project, 592 U. S., at 423, that the agency
failed to supply “a satisfactory explanation for its action[,]”
State Farm Mut. Automobile Ins. Co., 463 U. S., at 43, and
that it instead ignored “an important aspect of the problem”
before it, ibid. The applicants are therefore likely to be
entitled to “revers[a l] ” of the FIP's mandates on them.
§ 7607(d)(9).
10
III
A
Resisting this conclusion, EPA advances three alterna-
tive arguments.
First, the government insists, the agency did offer a rea-
soned response to the applicants' concern, just not the one
they hoped. When fnalizing its rule in response to public
comments, the government represents, “the agency did con-
sider whether the [FIP] could cogently be applied to a subset
of the 23 covered States.” EPA Response 27; see also post,
at 316–317 (Barrett, J., dissenting). And that consider-
ation, the government stresses, led EPA to add a “severabil-
ity” provision to its fnal rule in which the agency announced
that the FIP would “ `continue to be implemented' ” without
regard to the number of States remaining, even if just one
State remained subject to its terms. EPA Response 27
(quoting 88 Fed. Reg. 36693). In support of its severability
provision, EPA cited, among other things, its intent to ad-
dress “ `important public health and environmental benefts' ”
and encourage reliance by others “ `on th[e] fnal rule in their
planning.' ” Ibid.
10
Various applicants offer various other reasons why they believe they
are likely to succeed in challenging EPA's FIP. Having found that they
are likely to succeed on the basis discussed above, however, we have no
occasion to address those other arguments.
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None of this, however, solves the agency's problem. True,
the
severability provision highlights that EPA was aware
of the applicants' concern. But awareness is not itself an
explanation. The severability provision highlights, too, the
agency's desire to apply its rule expeditiously and “ `to the
greatest extent possible,' ” no matter how many States it
could cover. Ib id. But none of that, nor anything else
EPA said in support of its severability provision, addresses
whether and how measures found to maximize cost effective-
ness in achieving downwind ozone air-quality improvements
with the participation of 23 States remain so when many
fewer States, responsible for a much smaller amount of the
or ig i na l ly t argeted emissi ons, might be subjec t to the
agency's plan. Put simply, EPA's response did not address
the applicants' concern so much as sidestep it.
11
11
As the applicants conceded at oral argument, see Tr. of Oral Arg. 25–
26, EPA did not need to address every possible permutation when it
sought to adopt a multi-State FIP. Our conclusion is narrower: When
faced with comments like the ones it received, EPA needed to explain why
it believed its rule would continue to offer cost-effective improvements in
downwind air quality with only a subset of the States it originally in-
tended to cover. To be sure, after this Court heard argument, EPA is-
sued a document in which it sought to provide further explanations for
the course it pursued. See 89 Fed. Reg. 23526 (2024). But the govern-
ment has not suggested that we should consult this analysis in assessing
the validity of the fnal rule. See Letter from E. Prelogar, Solicitor Gen-
eral, to S. Harris, Clerk of Court 1 (Mar. 28, 2024). Nor could it, since
the Clean Air Act prevents us (and courts that may in the future assess
the FIP's merits) from consulting explanations and information offered
after the rule's promulgation. See 42 U. S. C. §§ 7607(d)(6)(C) (“The pro-
mulgated rule may not be based (in part or whole) on any information
or data which has not been placed in the docket as of the date of such
promulgation”), 7607(d)(7)(A) (restricting the “record for judicial review”).
We therefore look to only “the grounds that the agency invoked when it”
promu lgated the FIP. M i ch i gan v. EPA, 576 U. S. 743, 758 (2015).
Should the applicants show the FIP was arbitrary or capricious on the
existing record, as we have concluded is likely, the Clean Air Act entitles
them to “revers[al]” of that rule's mandates on them. § 7607(d)(9)(A).

296 OHIO
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Second, the government pivots in nearly the opposite di-
rec
tion. Now, it says, if its fnal rule lacks a reasoned re-
sponse to the applicants' concern, it is because no one raised
that concern during the public comment period. And, the
agency stresses, a litigant may pursue in court only claims
premised on objections frst “ `raised with reasonable speci-
fcity' ” before the agency during the public comment period.
Id., at 19–20 (quoting § 7607(d)(7)(B)); see also post, at 307–310.
We cannot agree. The Act's “reasonable specifcity” re-
quirement does not call for “a hair-splitting approach.” Ap-
palachian Power Co. v. EPA, 135 F. 3d 791, 817 (CADC
1998). A party need not “rehears[e]” the identical argument
made before the agency; it need only confrm that the gov-
ernment had “notice of [the] challenge” during the public
comment period and a chance to consider “in substance, if
not in form, the same objection now raised” in court. Id.,
at 818; see also, e. g., Bahr v. Regan, 6 F. 4th 1059, 1070
(CA9 2021).
Here, EPA had notice of the objection the applicants seek
to press in court. Commenters alerted the agency that,
should some States no longer participate in the plan, the
agency would need to return to the drawing board and “con-
duct a new assessment and modeling of contribution” to
determine what emissions-control measures maximized cost
effectiveness in securing downwind ozone air-quality im-
provements. Comments of Air Stewardship Coalition, at
13–14; see also Part I–C, supra (noting examples of other
comments). And, as we have just seen, EPA's own state-
ments and actions confrm the agency appreciated that con-
cern. In preparing the fnal rule in response to public com-
ments, the agency emphatically insists, it “did consider
whether the [r]ule could cogently be applied to a subset of
the 23 covered States.” EPA Response 27. And as a result
of that consideration, the agency observes, it opted to add a
severability provision to its fnal rule. Ibid. By its own
words and actions, then, the agency demonstrated that it was
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on notice of the applicants' concern. Yet, as we have seen,
it
failed to address the concern adequately.
12
Third, the government pursues one more argument in the
alternative. As the agency sees it, the applicants must re-
turn to EPA and fle a motion asking it to reconsider its fnal
rule before presenting their objection in court. They must,
the agency says, because the “grounds for [their] objection
arose after the period for public comment.” § 7607(d)(7)(B);
see EPA Response 20–21. As just discussed, however, EPA
had the basis of the applicants' objection before it during
the comment period. It chose to respond with a severability
provision that in no way grappled with their concern. Noth-
ing requires the applicants to return to EPA to raise (again)
a concern EPA already had a chance to address.
Taking the government's argument (much) further, the dis-
sent posits that every “objection that [a] fnal rule was not
reasonably explained” must be raised in a motion for recon-
sideration. Post, at 306 (internal quotation marks omitted;
emphasis deleted). But there is a reason why the govern-
ment does not go so far. The Clean Air Act opens the court-
12
The dissent resorts to a “hair-splitting approach” to the public com-
ments. Post, at 307–310. It stresses, for example, that some comments
highlighted variances among specifc emissions-producing facilities and in-
dustries, “not States.” Post, at 307 (emphasis deleted). But the dissent
fails to acknowledge that, for purposes of the FIP, States are a sum of their
emissions-producing facilities. See, e. g., Ozone Transport Policy Analysis
Final Rule TSD 12 (EPA–HQ–OAR–2021–0668, 2023) (Final Ozone Analy-
sis). Similarly, the dissent characterizes the comment indicating EPA
would need to “conduct a new assessment and modeling ” if States dropped
out of the FIP as a complaint about the “sequencing ” of the proposed SIP
disapprovals and the FIP. Post, at 309. But why would the sequencing
matter? Because the FIP cannot apply to a State if its SIP is not disap-
proved. See Part I–A, supra. And why would EPA need to perform a
“new assessment and modeling of contribution”? Because it may be that
“the math . . . wouldn't necessarily turn out the same” if some States were
not covered by the FIP. Tr. of Oral Arg. 59. Fairly on notice of the con-
cern, EPA needed to, and by its own admission sought to, “consider”
whether its FIP could apply to a subset of States. EPA Response 27.
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house doors to those with objections the agency already ig-
nored.
If an “objection [is] raised with reasonable specifcity
during the period for public comment” but not reasonably
addressed in the fnal rule, the Act permits an immediate
challenge. § 7607(d)(7)(B). A person need not go back to
the agency and insist on an explanation a second time. Tell-
ingly, the case on which the dissent relies involves an en-
tirely different situation: a “ `logical outgrowth' challeng[e].”
Post, at 306. There, the objection was that EPA had sup-
posedly “ `signifcantly amend[ed] the [r]ule between the pro-
posed and fnal versions,' ” making it impossible for people
to comment on the rule during the comment period. Ibid.
(quoting EME Homer, 795 F. 3d, at 137). That is nothing
like the challenge here, where EPA failed to address an im-
portant problem the public could and did raise during the
comment period.
B
With the government's theories unavailing, the dissent ad-
vances others of its own. It begins by suggesting that the
problem the applicants raise was not “ `important' ” enough
to warrant a reasoned reply from the agency because the
methodology EPA employed in its FIP “appear[s] not to de-
pend on the number of covered States.” Post, at 311–316,
317–318. Then, coming at the same point from another di-
rection, the dissent seeks to excuse the agency's lack of a
reasoned reply as “harmless” given, again, “the apparent
lack of connection between the number of States covered and
the FIP's methodology.” Post, at 319.
The trouble is, if the government had arguments along
these lines, it did not make them. It did not despite its
ample resources and voluminous briefng. See supra, at 290.
This Court “normally decline[s] to entertain” arguments
“forfeited” by the parties. Kingdomware Technologies, Inc.
v. United States, 579 U. S. 162, 173 (2016). And we see no
persuasive reason to depart from that rule here.
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If anything, we see one reason for caution after another.
St
art with the fact the dissent itself expresses little conf-
dence in its own theories, contending no more than it “ap-
pear[s]” EPA's methodology did not depend on the number
of covered States. Post, at 313 (emphasis added). Add to
that the fact that, at oral argument, even the government
refused to say with certainty that EPA would have reached
the same conclusions regardless of which States were in-
cluded in the FIP. See Tr. of Oral Arg. 59. Combine all
that with the further fact that, in developing the FIP, EPA
said it used the “same regulatory framework” this Court de-
scribed in EME Homer City Generation, L. P. v. EPA, 572
U. S. 489. E. g., EPA Response 7–8. And, at least as the
Court described that framework, state-level analyses play a
signifcant role in EPA's work.
13
Finally, observe that, while
the Act seems to anticipate, as the dissent suggests, that
the agency's “procedural determinations” may be subject to
harmless-error review, § 7607(d)(8), the Act also seems to
treat separately challenges to agency “actions” like the FIP
before us, authorizing courts to “reverse any . . . action”
found to be “arbitrary” or “capricious,” § 7607(d)(9)(A).
With so many reasons for caution, we think sticking to our
normal course of declining to consider forfeited arguments
the right course here.
14
13
The agency, we said, “frst calculated, for each upwind State, the quan-
tity of emissions the State could eliminate at each of several cost [ levels]”;
next, it “conducted complex modeling to establish the combined effect the
upwind reductions projected at each cost [ level] would have on air quality
in downwind States”; and only after all that did the agency “then identif[y]
`signifcant cost [ levels]' ” to use in setting its emissions budgets. EME
Homer City, 572 U. S., at 501–502.
14
Admittedly, the dissent points to some statements in the FIP suggest-
ing EPA considered nationwide data in parts of its analysis. See, e. g.,
post, at 313; see also, e. g., 88 Fed. Reg. 36721, 36727. But other state-
ments in that rule and supporting documents also seem to suggest EPA
considered state-specifc information. See Part I–C, supra. If, as the
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*
The
applications for a stay in Nos. 23A349, 23A350,
23A351, and 23A384 are granted. Enforcement of EPA's
rule against the applicants shall be stayed pending the dispo-
sition of the applicants' petitions for review in the United
States Court of Appeals for the D. C. Circuit and any petition
for writ of certiorari, if such writ is timely sought. Should
the petition for certiorari be denied, this order will termi-
nate automatically. If the petition is granted, this order
shall terminate upon the sending down of the judgment of
this Court.
It is so ordered.
Justice Barrett, with whom Justice Sotomayor, Jus-
tice Kagan, and Justice Jackson join, dissenting.
The Court today enjoins the enforcement of a major Envi-
ronmental Protection Agency rule based on an underdevel-
oped theory that is unlikely to succeed on the merits. In so
doing, the Court grants emergency relief in a fact-intensive
and highly technical case without fully engaging with both
the relevant law and the voluminous record. While the
Court suggests that the EPA failed to explain itself suff-
ciently in response to comments, this theory must surmount
sizable procedural obstacles and contrary record evidence.
dissent posits, only nationwide data informed EPA's analysis, why would
EPA say that, “for purposes of identifying the appropriate level of con-
trol,” it focused on “the 23 upwind states that were linked” to the down-
wind States, rather than, say, “all states in the contiguous U. S.”? Final
Ozone Analysis 3 (footnote omitted). Why would EPA explain that its
“fndings regarding air quality improvement” downwind were a “central
component” of picking the appropriate cost levels and so defning a State's
signifcant contribution? 88 Fed. Reg. 36741. And why would EPA
bother to “determine the relationship between changes in emissions and
changes in ozone contributions on a state-by-state . . . basis” and “cali-
brat[e]” that relationship “based on state-specifc source apportionment”?
Final Ozone Analysis 43. In asking these questions, we do not profess
answers; we simply highlight further reasons for caution.
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Barrett, J., dissenting
Applicants therefore cannot satisfy the stringent conditions
for
relief in this posture.
I
I will start by setting the record straight with respect to
some important background.
First, the Court downplays EPA's statutory role in ensur-
ing that States meet air-quality standards. Ante, at 283–
284. The Clean Air Act directs EPA to “establish national
ambient air quality standards (NAAQS) for pollutants at lev-
els that will protect public health.” EPA v. EME Homer
City Generation, L. P., 572 U. S. 489, 498 (2014); see 42
U. S. C. §§ 7408, 7409. States must create State Implemen-
tation Plans (SIPs) to ensure that their air meets these
standards. § 7410(a)(1). But States also face an externality
problem: “Pollutants generated by upwind sources are often
transported by air currents . . . to downwind States,” reliev-
ing upwind States “of the associated costs” and making it
diffcult for downwind States to “maintain satisfactory air
quality.” EME, 572 U. S., at 496. So the Act's Good Neigh-
bor Provision requires SIPs to “prohibi[t]” the State's emis-
sions sources from “emitting any air pollutant in amounts
which will . . . contribute signifcantly to nonattainment in,
or interfere with maintenance by, any other State with re-
spect to any [NAAQS].” § 7410(a)(2)(D)(i)(I).
Given the incentives of upwind States to underregulate
the pollution they send downwind, the Act requires EPA to
determine whether a State “has failed to submit an adequate
SIP.” EME, 572 U. S., at 498; see § 7410(c)(1). If a SIP
does not prevent the State's polluters from signifcantly
contributing to nonattainment in downwind States, EPA
“shall” promulgate a Federal Implementation Plan (FIP)
that does. § 7410(c)(1). And EPA must stop the State's
signifcant contributions by the statutory deadline for the
affected downwind States to achieve compliance. See Wis-
consin v. EPA, 938 F. 3d 303, 313–314 (CADC 2019) (per
curiam); § 7511.
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Second, the Court fails to recognize that EPA's SIP disap-
prova
ls may, in fact, be valid. EPA justifed its fndings that
23 States had failed to submit adequate SIPs. It found that
these States all signifcantly contributed to ozone pollution
in downwind States. See 88 Fed. Reg. 36656 (2023). But
21 of these States, including applicants, proposed to do noth-
ing to reduce their ozone-precursor (i. e., NO
x
) emissions—
arguing that they did not actually contribute to downwind
nonattainment or that there were no other cost-effective
emissions-reduction measures they could impose. See 88
Fed. Reg. 9354–9361 (2023). The other two States failed to
submit a SIP at all. See 84 Fed. Reg. 66614 (2019). While
12 of EPA's SIP disapprovals have been temporarily stayed,
no court yet has invalidated one. So EPA's replacement
FIP—the Good Neighbor Plan—may yet apply to all 23 orig-
inal States. Indeed, EPA and the plaintiffs who challenged
Nevada's SIP disapproval have proposed a settlement that
would lift that stay. 89 Fed. Reg. 35091 (2024).
Third, the Court claims that commenters on the proposed
FIP warned that its emissions limits might change if it cov-
ered fewer States, but EPA failed to respond. Ante, at 287–
289. Not exactly. As I will elaborate below, commenters
merely criticized EPA's decision to propose a FIP before its
SIP disapprovals were fnal. EPA responded that this se-
quencing was “consistent with [its] past practice in [its] ef-
forts to timely address good neighbor obligations”: Given the
August 2024 deadline for certain States to comply with the
2015 ozone NAAQS, EPA was “obligated” to start the years-
long process of promulgating a FIP so that one could be ef-
fective in time. EPA, Response to Public Comments on
Proposed Rule 149–150 (EPA–HQ–OAR–2021–0668–1127,
June 2023) (Response to Comments); see Wisconsin, 938
F. 3d, at 313–314.
Finally, the Court repeatedly characterizes the FIP as re-
lying on an “assumption that [it] would apply to all covered
States.” Ante, at 287; see ante, at 293. But try as it might,
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the Court identifes no evidence that the FIP's emissions lim-
its
would have been different for a different set of States or
that EPA's consideration of state-specifc inputs was any-
thing but confrmatory of the limits it calculated based on
nationwide data. See ante, at 286–287, 299–300, n. 14. The
Court leans on the fact that EPA “considered data specifc to
the emissions-producing facilities in [each] State” to calculate
“how much each upwind State's [NO
x
] emissions would fall”
if the State's emitters “adopted each [emissions-control]
measure.” Ante, at 286 (citing EPA, Ozone Transport Pol-
icy Analysis Proposed Rule TSD 9–10, 13, 22–23 (EPA–HQ–
OAR–2021–0668–0133, Feb. 2022) (Proposed Ozone Analy-
sis)). But the Proposed Ozone Analysis makes clear that
EPA did these state-specifc calculations to determine each
State's “emissions budget.” Proposed Ozone Analysis 7–13.
A State's budget consists of the “emissions that would re-
main” after the State's power plants meet the emissions lim-
its that EPA independently calculated. 88 Fed. Reg. 36762;
see Proposed Ozone Analysis 13 (“adjust[ed]” “unit-level
emissions are summed up to the state level”); n. 6, infra.
Of course each State's emissions budget will depend on the
emitters in that State. What matters is whether the limits
the FIP imposes on each emitter depend on the number of
States the FIP covers. Tellingly, the Court does not iden-
tify any NO
x
limit for any industry that relied on state-
specifc data.
On the contrary, as I will explain in Part II–B, the fnal
rule and its supporting documents suggest that EPA's meth-
odology for setting emissions limits did not depend on the
number of States in the plan, but on nationwide data for the
relevant industries—and the FIP contains many examples of
emissions limits that EPA created using nationwide inputs.
Moreover, EPA has now confrmed this interpretation.
During this litigation, EPA received petitions seeking recon-
sideration of the FIP on the ground that it should not be
implemented in just a subset of the original States. EPA
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Barrett, J., dissenting
denied these petitions on April 4, 2024. 89 Fed. Reg. 23526.
It
thoroughly explained how its “methodology for defning ”
each State's emissions obligations is “independent of the
number of states included in the Plan” because it “relies
on a determination regarding what emissions reductions
each type of regulated source can cost-effectively achieve.”
EPA, Basis for Partial Denial of Petitions for Reconsidera-
tion on Scope 1 (EPA–HQ–OAR–2021–0668–1255, Apr. 2024)
(Denial). The “control technologies and cost-effectiveness
fgures the EPA consider[ed] . . . do not depend in any way
on the number of states included.” Id., at 2. So “[s]ources
in the remaining upwind states currently regulated by the
Plan . . . would bear the same actual emission reduction obli-
gations” regardless of the number of covered States. Id.,
at 3–4.
II
To obtain emergency relief, applicants must, at a mini-
mum, show that they are likely to succeed on the merits, that
they will be irreparably injured absent a stay, and that the
balance of the equities favors them. Nken v. Holder, 556
U. S. 418, 425–426 (2009). Moreover, we should grant relief
only if we would be likely to grant certiorari were the appli-
cants' case to come to us in the usual course. See Does 1–3
v. Mills, 595 U. S. –––, ––– (2021) (Barrett, J., concurring
in denial of application for injunctive relief ); Hollingsworth
v. Perry, 558 U. S. 183, 190 (2010) (per curiam). In my view,
the applicants cannot satisfy the stay factors. Most signif-
cantly, they have not shown a likelihood of success on the
merits.
The Court holds that applicants are likely to succeed on a
claim that the Good Neighbor Plan is “arbitrary” or “capri-
ci ous. ” 42 U. S. C. § 7607(d)(9). The “arbitrary-and-
capricious standard requires that agency action” be both “[1]
reasonable and [2] reasonably explained.” FCC v. Prometh-
eus Radio Project, 592 U. S. 414, 423 (2021). The Court's
theory is that EPA did not “ `reasonably explai[n]' ” “why
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the number and identity of participating States does not af-
fec
t what measures maximize cost-effective downwind air-
quality improvements.” Ante, at 293–294 (quoting only the
second part of Prometheus Radio's formulation (emphasis
added)). So to be clear, the Court does not conclude that
EPA's actions were substantively unreasonable—e. g., that
the FIP cannot rationally be applied to fewer States because
a change in the number of participants would undermine its
rationale or render it ineffective. Nor could it, given the
signifcant evidence in the record (not to mention EPA's de-
nial of reconsideration) that the covered States did not, in
fact, affect the plan's emissions-reduction obligations. See
Part II–B, infra. Thus, the only basis for the Court's deci-
sion is the argument that EPA failed to provide “ `a satisfac-
tory explanation for its action' ” and a “reasoned response” to
comments. Ante, at 293–294 (quoting Motor Vehicle Mfrs.
Assn. of United States, Inc. v. State Farm Mut. Automobile
Ins. Co., 463 U. S. 29, 43 (1983)). There are at least three
major barriers to success on such a claim.
A
The Clean Air Act imposes a procedural bar on the chal-
lenges that a plaintiff can bring in court: Only objections that
were “raised with reasonable specifcity during the period
for public comment . . . may be raised during judicial review.”
§ 7607(d)(7)(B). If it was “impracticable to raise such objec-
tion within such time or if the grounds for such objection
arose after the period for public comment,” the challenger
may petition for reconsideration of the rule and can obtain
judicial review only if EPA refuses. Ibid. While EPA has
now separately denied petitions for reconsideration of the
Good Neighbor Plan, this case came to us directly; we are
assessing applicants' likelihood of success in challenging the
plan itself, not the denial of reconsideration. So the proce-
dural bar on objections not raised in the comments presents
a signifcant obstacle—in two ways.
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v. EPA
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First, consider the Court's basic theory: that EPA offered
“
no reasoned response” to comments allegedly questioning
whether the plan's emissions limits depend on the States cov-
ered. Ante, at 293. That EPA failed to adequately explain
its fnal rule in response to comments is “an objection to the
notice and comment process itself,” which applicants “obvi-
ously did not and could not have raised . . . during the period
for public comment.” EME Homer City Generation, L. P.
v. EPA, 795 F. 3d 118, 137 (CADC 2015) (Kavanaugh, J.). No
one could have raised during the proposal's comment period
the objection that the “fnal rule was not `reasonably ex-
plained.' ” Ante, at 294 (emphasis added).
The D. C. Circuit, on remand in EME Homer, considered
a similar objection that EPA had “violated the Clean Air
Act's notice and comment requirements”: EPA had “signif-
cantly amend[ed] the Rule between the proposed and fnal
versions without providing additional opportunity for notice
and comment.” 795 F. 3d, at 137. But because this proce-
dural objection could not have been raised during the com-
ment period, “the only appropriate path for petitioners”
under § 7607(d)(7)(B) was to raise it “through an initial peti-
tion for reconsideration to EPA.” Ibid. So the D. C. Cir-
cuit lacked “authority at th[at] time to reach this question.”
Ibid. While such “logical outgrowth” challenges typically
are cognizable under the Administrative Procedure Act, see
Shell Oil Co. v. EPA, 950 F. 2d 741, 747 (CADC 1991), the
Clean Air Act channels these challenges through reconsider-
ation proceedings. This Court's failure-to-explain objection
may face the same problem: It is not judicially reviewable in
its current posture.
1
1
The Court offers a feeble response to this application of § 7607(d)(7)(B)'s
procedural bar. Ante, at 297–298. It simply quotes § 7607(d)(7)(B) and as-
serts without support that it means that a plaintiff “need not go back to the
agency and insist on an explanation a second time.” Ante, at 298. The
Court fails to engage with the logic of this argument: The objection that the
fnal rule did not contain suffcient explanation was not and could not have

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Second, even putting aside this aspect of § 7607(d)(7)(B),
it
is not clear that any commenter raised with “reasonable
specifcity” the underlying substantive issue: that the exclu-
sion of some States from the FIP would undermine EPA's
cost-effectiveness analyses and resulting emissions controls.
§ 7607(d)(7)(B); see ante, at 293–294. The Court concludes
otherwise only by putting in the commenters' mouths words
they did not say. It frst cites a bevy of comments arguing
that EPA's “disapprovals of the SIPs were legally fawed”
and noting the obvious point that EPA cannot “include a
State in its FIP” unless it validly disapproves the State's
SIP. Ante, at 287. These comments do not address the
continued effcacy of a FIP that applies to a subset of the
originally covered States.
Another collection of the Court's inapposite comments re-
lates to the inclusion of specifc sources, emissions controls,
and industries in the proposed plan—not States. See ante,
at 288, n. 4. For example, one commenter argued that the
“cost effectiveness of the requirement to employ SNCR will
be highly variable, and is unlikely to meet EPA expectations
in even the most optimistic case.” Comments of Indiana
Municipal Power Agency 9 (June 20, 2022). That is a chal-
lenge to EPA's endorsement of a particular emissions-control
technology; it says nothing about the FIP's dependence on
a particular number of States. See also, e. g., Comments
of Lower Colorado River Authority 21–22 (June 21, 2022).
been raised during the comment period, so it must be raised in a petition
for reconsideration. EME Homer, 795 F. 3d, at 137. The Court claims
that its theory is different from the logical-outgrowth challenge the D. C.
Circuit considered in EME Homer. Ante, at 298. But the Court ignores
the fact that its failure-to-explain challenge and logical-outgrowth chal-
lenges are both “objection[s] to the notice and comment process itself ”
that depend on the content of the fnal rule. EME Homer, 795 F. 3d, at
137. Even if the public raised an “important problem . . . during the
comment period,” ante, at 298, the Court's basis for enjoining the FIP's
enforcement is not that the alleged problem is real, but that the fnal rule
did not address it.
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Similarly, another commenter argued that pulp and paper
mi
lls should not be included because the “maximum esti-
mated improvement” in ozone levels from controlling their
emissions would be “too small to even measure.” Com-
ments of Wisconsin Paper Council 2 (June 21, 2022).
2
An
arg ument that the m ax imum benefits from reg u lat-
ing an industry are too small is not an argument that those
benefts would become too small if fewer States were
covered.
3
The closest comment that the Court can fnd—which it
quotes repeatedly—is one sentence that obliquely refers to
some “new assessment and modeling of contribution” that
EPA might need to perform. Comments of Air Stewardship
Coalition 13–14 (June 21, 2022). The Court dresses up this
comment by characterizing it as a warning about what might
happen “[i]f the FIP did not wind up applying to all 23
States” and responding to the concern that a “different set
of States might mean that the `knee in the curve' would
shift” and change the cost-effective “emissions-control meas-
2
The Court claims that in distinguishing comments about particular in-
dustries from comments that question whether the plan depends on a num-
ber of States, I “fai[ l] to acknowledge” that the FIP treats States as the
“sum of their emissions-producing facilities.” Ante, at 297, n. 12. But in
reality, it is the Court that ignores how the FIP works. The FIP deter-
mines emissions limits for particular sources based on their industries;
the total NO
x
emissions limit for each State is simply the sum of the limits
the plan imposes on each of the State's sources. See, e. g., 88 Fed. Reg.
36678, 36762; Part II–B, in fra. So comments critiquing a particular
industry-specifc emissions limit or technology assumption say nothing
about the FIP's dependence on a certain number of States.
3
Nor did the Air Stewardship Coalition's comment about the “knee in
the curve” raise concerns about which States are included. See ante, at
288, n. 4. Rather, this comment questioned EPA's proposed average cost-
effectiveness threshold of $7,500 per ton for non-power-plant sources; it
argued that EPA should use different thresholds for different industries.
Comments of Air Stewardship Coalition 27 (June 21, 2022) (Air Steward-
ship Comments). It did not link its concern about cost thresholds to the
States covered by the plan.
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ures.” Ante, at 288. But those words are the Court's, not
the
commenter's.
The commenter's actual objection was to EPA's sequencing
of its actions—proposing a FIP before it fnalized its SIP
disapprovals. The commenter titled this section “EPA Step
Two Screening is Premised on the Premature Disapproval of
19 Upwind States['] Good Neighbor SIPs.” Air Steward-
ship Comments 13 (boldface omitted). And the relevant
sentence reads in full:
“The proposed FIP essentially prejudges the outcome of
those pending SIP actions and, in the event EPA takes
a different action on those SIPs than contemplated in
this proposal, it would be required to conduct a new as-
sessment and modeling of contribution and subject those
fndings to public comment.” Id., at 14.
This sentence says nothing about what would be required if
after EPA fnalizes its SIP disapprovals and issues a fnal
FIP, some States drop out of the plan. Nor does it suggest
that the plan's cost-effectiveness thresholds or emissions con-
trols would change with a different number of States. Nor
is it clear what the comment means by its bare reference to
a “new assessment and modeling of contribution”: Would
EPA be required to perform a new evaluation of which up-
wind States cause pollution in downwind States? A new
analysis of how much pollution each source must eliminate?
A new assessment of the plan's impact on downwind States?
It is therefore diffcult to see how this comment raised
with “reasonable specifcity” the objection that the removal
of some States from the fnal plan would invalidate EPA's
cost-effectiveness thresholds and chosen emissions-control
measures.
4
That is not how EPA understood it. EPA char-
4
So too with Portland Cement's comment. See ante, at 288. That
comment simply echoes the quoted sentence from the Air Stewardship
Coalition almost word for word, also in the context of objecting to EPA's
decision to propose a FIP before fnalizing its SIP disapprovals. Com-
ments of Portland Cement Association 7 (June 21, 2022).
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acterized this comment as arguing that “by taking action
before
considering comments on the proposed disapprovals,
the EPA is presupposing the outcome of its proposed rule-
makings on the SIPs.” Response to Comments 147 (noting
this comment's ID number, 0518). And EPA explained that
it “disagree[d]” with the argument that the “sequence” of its
actions was “improper, unreasonable, or bad policy”; EPA
had a statutory obligation to promulgate a FIP by the Au-
gust 2024 NAAQS attainment deadline. Id., at 150. If a
commenter had said with reasonable specifcity what the
Court says today—that “a different set of States might mean
that the `knee in the curve' would shift,” ante, at 288—EPA
could have responded with more explanation of why its
methodology did not depend on the number of covered
States—as it has recently explained. But EPA cannot be
pena l i zed i f it did not have reasonable notice of th is
objection.
5
In sum, § 7607(d)(7)(B)'s procedural bar likely forecloses
both the failure-to-explain objection that the Court credits
and any substantive challenge to the reasonableness of
applying the FIP to a subset of the originally covered States.
5
The Court concludes to the contrary only by building out the comment's
bare reference to a “new assessment and modeling ” with its own infer-
ences about the possible effect of different numbers of States on “the
math.” Ante, at 297, n. 12 (internal quotation marks omitted). But as
explained above, the comment itself said nothing about States drop-
ping out of the fnal plan or the possible impact of different numbers of
States on the FIP's cost thresholds or emissions limits. It is hard to be-
lieve that a single sentence with no elaboration or explanation of the po-
tential issue—in a sea of thousands of pages of comments—gave EPA rea-
sonable notice that it should have included in its fnal rule a detailed
explanation of why the FIP's emissions limits did not depend on the num-
ber of States. Cf. Public Citizen, Inc. v. FAA, 988 F. 2d 186, 197 (CADC
1993) (An “agency need not respond at all to comments that are `purely
speculative and do not disclose the factual or policy basis on which they
rest' ”).
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B
E
ven if applicants clear § 7607(d)(7)(B)'s procedural bar,
they face an uphill battle on the merits. To prevail on the
Court's theory, applicants must show that EPA's actions
were “arbitrary” or “capricious. ” §§ 7607(d)(9)(A), (D).
“The scope of review under the `arbitrary and capricious'
standard is narrow and a court is not to substitute its judg-
ment for that of the agency.” State Farm, 463 U. S., at 43.
A rule is arbitrary and capricious if the agency “entirely
failed to consider an important aspect of the problem.”
Ibid. (emphasis added). But we will “ `uphold a decision of
less than ideal clarity if the agency's path may reasonably
be discerned. ' ” Ibid. (quoting Bowman Transp., Inc. v.
Arkansas-Best Freight System, Inc., 419 U. S. 281, 286
(1974)). Given the explanations and state-agnostic method-
ology apparent in the fnal rule and its supporting documen-
tation—and the paucity of comments specifcally raising the
issue—EPA may well have done enough to justify its plan's
severability.
To begin, the rule and its supporting documents arguably
make clear that EPA's methodology for calculating cost-
effectiveness thresholds and imposing emissions controls did
not depend on the number of covered States. The rule ap-
plied EPA's longstanding “4-step interstate transport frame-
work” to create emissions limits that will prevent NO
x
sources in upwind States from signifcantly contributing to
ozone pollution in downwind States. 88 Fed. Reg. 36659; see
42 U. S. C. § 7410(a)(2)(D). Under that framework, EPA (1)
identifes “downwind receptors that are expected to have
problems attaining or maintaining the NAAQS”; (2) identi-
fes which upwind States are “ `link[ed]' ” to those downwind
receptors because they contribute at least 1% of a receptor's
ozone; (3) determines which NO
x
sources in the linked up-
wind States “signifcantly contribute” to downwind nonat-
tainment or interference; and (4) implements emissions lim-
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its to stop those sources' signifcant contributions. 88 Fed.
Reg
. 36659; see EME, 572 U. S., at 500–501 (describing simi-
lar approach used in earlier FIP). The frst two steps deter-
mine which States the FIP must cover. The rubber meets
the road at steps 3 and 4: How much do sources in those
States “signifcantly contribute” to downwind pollution, and
what must they do about it?
Here is how EPA explains that methodology. A source
“signifcantly contributes” to downwind pollution if there are
cost-effective measures it could implement to reduce its
emissions: It must halt those emissions that can be elimi-
nated at a cost “under the cost threshold set by the Agency”
for sources in that industry. EME, 572 U. S., at 518 (uphold-
ing this approach). So the “ `amount' of pollution” that
sources must eliminate is “that amount . . . in excess of the
emissions control strategies the EPA has deemed cost effec-
tive.” 88 Fed. Reg. 36676. EPA calculates for each type of
source a “uniform level of NO
x
emissions control stringency”
expressed as a “cost per ton of emissions reduction.” Id., at
36719. This cost-effectiveness threshold is based on the
point “at which further emissions mitigation strategies be-
come excessively costly on a per-ton basis while also deliver-
ing far fewer additional emissions reductions.” Id., at 36683
(describing this “ `knee in the curve' ” analysis). The plan
requires sources in each covered State to reduce their emis-
sions accordingly.
6
6
For power plants, EPA implements these requirements by allocating
each State an “ `emissions budget' . . . representing the EPA's quantifca-
tion of the emissions that would remain” if plants in that State eliminated
all the emissions that EPA determines can be eliminated for less than
the cost threshold. 88 Fed. Reg. 36762. EPA then allocates tradeable
“ `allowances' ” proportionally among the State's sources, creating a mar-
ketplace for emissions. Ibid. With respect to other sources, EPA deter-
mined that nine industries in the covered States produced the most sig-
nifcant emissions. Id., at 36817. The rule requires sources in each of
those industries to meet specifc emissions limits that were calculated
based on the reductions that they can cost-effectively achieve. Ibid.
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Crucially, the fnal rule suggests that EPA calculated cost-
effec
tiveness thresholds based on the likely cost and impact
of available emissions-reduction technology given national,
industry-wide data. Contrary to the Court's speculations,
ante, at 293, these thresholds and the FIP's resulting emis-
sions limits appear not to depend on the number of covered
States. Consider the plan's approach to power plants (“elec-
tric generating units,” or EGUs). EPA assessed the cost
and impact of different NO
x
mitigation strategies that EGUs
could implement. One strategy was to fully operate “selec-
tive catalytic reduction” (SCR) technology. 88 Fed. Reg.
36655; see id., at 36720. EPA estimated that a “representa-
tive marginal cost” for this strategy would be $1,600 per ton,
and a “reasonable level of performance” would be 0.08 lb/
mmBtu—based on “nationwide” power plant “emissions
data.” Id., at 36720–36721. EPA thus determined that
SCR optimization was a “viable mitigation strategy for the
2023 ozone season” and built this assumption into the plan's
emissions limits. Id., at 36720. In other words, EPA relied
on nationwide industry data to select cost thresholds that
corresponded to how much it would cost to use particular
emissions-reduction technologies, and it applied that “uni-
form control stringency to EGUs within the covered upwind
states.” Id., at 36680.
7
7
EPA ultimately selected (based on nationwide data) a cost threshold
of $1,800 per ton of NO
x
reduction that would apply in earlier years, and
a cost threshold of $11,000 per ton that would apply in later years. 88
Fed. Reg. 36749, 36846. These cost thresholds corresponded to the cost
of different emissions-control measures: For example, EGUs can “[r]etro-
f[t] state-of-the-art combustion controls” and “[o]ptimiz[e] idled SCRs” for
less than $1,800 per ton, and they can “[i]nstal[ l new] SCR[s]” for less than
$11,000 per ton. Ozone Transport Policy Analysis Final Rule TSD 5
(EPA–HQ– OAR– 2021– 0668 –1080, Mar. 2023) (Final Ozone Analysis).
EPA then calculated each State's emissions budget based on the assump-
tion that the State's EGUs would implement the emissions-reductions
strategies that cost less than the chosen thresholds. See n. 6, supra;
88 Fed. Reg. 36762; Final Ozone Analysis 6, 9. Given the likelihood that

314 OHIO
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In fact, some commenters criticized EPA's reliance on a
“
nationwide data set” to calculate emissions limits, arguing
that EPA should “limit the dataset to . . . just the covered
st ates”—an approach that wo ul d have made the cost-
effectiveness thresholds depend on which States were cov-
ered. Id., at 36723. But EPA expressly defended its
approach based on its “intention to identify a technology-
specifc representative emissions rate” and its interest in
“the performance potential of a technology”—which were
best served by the “largest dataset possible (i. e., nation-
wide).” Id., at 36723–36724 (emphasis added). EPA ex-
plained that it used the same approach it had successfully
applied in previous rulemakings: It “derive[d] technology
performance averages” based on nationwide data. Id., at
36724. Then it applied the relevant industry standard “on a
uniform basis” to each emitter across the covered States.
Id., at 36817.
8
The Court, perhaps recognizing the problem that the FIP's
seemingly state-agnostic methodology poses for its theory,
throws at the wall a cherry-picked assortment of EPA state-
EPA selected cost thresholds based on nationwide data, each State's
budget would be the same even if the FIP covered different States.
8
While EPA's methodology with respect to other industrial sources
(non-EGUs) was more complicated, it also seems to have relied on nation-
wide data. EPA chose a “$7,500 marginal cost-per-ton threshold,” 88 Fed.
Reg. 36740, which corresponded to the point of diminishing returns (the
“knee in the curve”) when EPA assessed the impact of emissions controls
in the highest impact industries and in “all industries” on the total “[o]zone
[s]eason NO
x
[r]eductio[n]” potential, Technical Memorandum, Screening
Assessment of Potential Emissions Reductions, Air Quality Impacts, and
Costs From Non-EGU Emissions Units for 2026, p. 4 (EPA–HQ–OAR–
2021–0668–0150, Feb. 2022) (boldface omitted). This fgure thus appears
to have been determined based on industry-wide cost and emissions data
rather than state-specifc calculations. So too with the specifc emissions
limits EPA decided could be implemented for less than that cost. See,
e. g., 88 Fed. Reg. 36825 (“EPA based the proposed emissions limits for
cement kilns on the types of limits being met across the nation”).
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ments mentioning state data. See ante, at 286–287, 299–
300,
n. 14. None stick. The fundamental problem with the
Court's citations is that they discuss analyses that EPA per-
formed after it chose cost thresholds and emissions limits
based on nationwide industry data. EPA did assess the im-
pact on downwind States if particular upwind States met the
proposed emissions limits, and that impact depended on the
States included in the modeling. Ante, at 286, 299–300, n.
14. But EPA said that these “ `fndings regarding air qual-
ity improvement,' ” ante, at 300, n. 14 (quoting 88 Fed. Reg.
36741), served only to “cement EPA's identifcation of the
selected . . . mitigation measures as the appropriate control
stringency,” 88 Fed. Reg. 36741 (emphasis added); see Denial
18. EPA explained that the statutory requirement to “elim-
inate signifcant contribution” depends on the implementa-
tion of cost-effective emissions controls at individual “indus-
trial sources,” not some overall impact on “downwind areas'
nonattainment and maintenance problems.” 88 Fed. Reg.
36741. EPA assessed the FIP's impact assuming the partic-
ipation of particular States primarily to ensure that its emis-
sions limits did not result in “overcontrol”—i. e., more reduc-
tions than necessary to help downwind States comply with
the NAAQS. Ibid.; see EME Homer City, 572 U. S., at 521.
The technical document that the Court cites, ante, at 286,
makes this point clear: “The downwind air quality impacts
are used to inform EPA's assessment of potential overcon-
trol.” Proposed Ozone Analysis 31.
EPA's analysis confrmed that its chosen emissions limits
would not result in overcontrol if they were implemented in
the States originally covered by the FIP. 88 Fed. Reg.
36741. Importantly, implementing the FIP “in fewer up-
wind states does not (and cannot possibly) result in overcon-
trol” given that “there was no overcontrol even when more
states, making more emission reductions, were included.”
Denial 22. So the fact that EPA used state-specifc data in
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its overcontrol analysis does not mean that the FIP's emis-
si
ons limits depended on the number of States it covered.
And the inclusion of fewer States in that analysis logically
could not have affected the results.
Thus, EPA generally characterized the FIP's emissions
limits as dependent on nationwide data, not on any particular
set of States.
9
Confrming this interpretation, the fnal rule
contemplates its application to a different number of States.
It recognizes that “states may replace FIPs with SIPs if
EPA approves them,” and several sections explain how
States may exit this FIP. 88 Fed. Reg. 36753, 36838–36843.
And the rule's severability provision explains that EPA
views the plan as “severable along . . . state and/or tribal
jurisdictional lines.” Id., at 36693.
Moreover, EPA justifed the FIP's severability: EPA
“must address good neighbor obligations as expeditiously as
practicable and by no later than the next applicable attain-
ment date”; severability serves “important public health and
environmental benefts” and ensures that stakeholders can
“rely on this fnal rule in their planning.” Ibid. These ra-
tionales align with EPA's response to critics of its decision to
propose a FIP before fnalizing its SIP disapprovals: Quickly
proposing a FIP—just like keeping the FIP in place even if
some States drop out—“is a reasonable and prudent means
9
The Court argues that EPA equated the framework it used here with
the one that we described in EME Homer City. Ante, at 299, and n. 13.
But even if EME described an approach that selected cost thresholds
“only after” conducting downwind air-quality assessments, ante, at 299, n.
13, it is not clear that the Good Neighbor Plan adopted this aspect of the
EME framework. In fact, there are other key similarities between this
FIP and EME's approach. For example, the fnal FIP refers to EME in
order to note that EPA's “uniform framework of policy judgments”—i. e.,
applying the same cost thresholds with “[n]ationwide consistency”—was
upheld in that case. 88 Fed. Reg. 36673. And the fnal FIP identifes
cost thresholds that EPA chose based on nationwide data—like $1,800
based on the cost of EGUs “optimiz[ing] . . . existing SCRs and SNCRs.”
Id., at 36749.
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of assuring that [EPA's] statutory obligation to reduce air
pol
lution affecting the health and welfare of people in down-
wind states is implemented without delay.” Response to
Comments 151.
Given these justifcations and the state-agnostic methodol-
ogy apparent in the fnal rule, EPA's “ `path may reasonably
be discerned.' ” State Farm, 463 U. S., at 43. The FIP's
cost thresholds and emissions limits did not depend in any
signifcant way on the number of States included, so the
drawbacks of severability were minima l. On the other
hand, severability was necessary so that EPA could fulfll,
to the greatest extent possible, its statutory obligation to
eliminate the sig nificant ozone contr ibutions of upwind
States and reduce harmful pollution in downwind States in
time to meet the attainment deadlines. See Response to
Comments 150 (noting the August 2024 ozone-NAAQS at-
tainment deadline). If the FIP were not severable, EPA
would have to go back to the drawing board for all States
whenever a single State is removed—thwarting its mission
for little reason.
10
Finally, it is unlikely that EPA's response to comments
evinces a “fail[ure] to consider an important aspect of the
problem.” State Farm, 463 U. S., at 43 (emphasis added).
An agency must respond to “ `relevant' and `signifcant' pub-
lic comments,” and that requirement is not “particularly de-
manding ”; the “agency need not respond at all to comments
that are `purely speculative and do not disclose the factual
or policy basis on which they rest.' ” Public Citizen, Inc. v.
10
The Court claims that the severability clause is evidence that EPA
“had notice of the objection the applicants seek to press in court,” yet
EPA's justifcations for it did not address (alleged) concerns about how the
cost-effectiveness thresholds would change with fewer States. Ante, at
296–297. But as explained above, commenters did not raise that issue
with specifcity; they simply pointed out that some SIP disapprovals might
be invalid. The severability clause is evidence that EPA was aware of
that possibility, and the clause was EPA's response to it.
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318 OHIO
v. EPA
Barrett, J., dissenting
FAA, 988 F. 2d 186, 197 (CADC 1993) (quoting Home Box
O
ffce, Inc. v. FCC, 567 F. 2d 9, 35, and n. 58 (CADC 1977)
(per curiam); emphasis added); see § 7607(d)(6)(B) (EPA
must respond to “signifcant” comments). EPA received
hundreds of comments, and its response numbered nearly
1,100 pages. Given the likelihood that the FIP's emissions
limits did not depend on the covered States, the risk of it
applying to fewer States may not be “important,” and com-
ments purportedly raising that possibility might not be “rele-
vant” and “signifcant.” Moreover, the one comment that
vaguely referred to a need for a “new assessment and model-
ing,” Air Stewardship Comments 14, was “purely specula-
tive” and “disclose[d]” no “factual or policy basis”; it likely
merited no response, Home Box Offce, 567 F. 2d, at 35, n. 58.
Requiring more from EPA risks the “sort of unwarranted
judicial examination of perceived procedural shortcomings”
that might “seriously interfere with that process prescribed
by Congress.” Vermont Yankee Nuclear Power Corp. v.
Natural Resources Defense Council, Inc., 435 U. S. 519, 548
(1978).
11
C
Applicants face one more impediment: the Clean Air Act's
stringent harmless-error rule. A court “reviewing alleged
procedural errors . . . may invalidate [an EPA] rule only if
the errors were so serious and related to matters of such
central relevance to the rule that there is a substantial like-
lihood that the rule would have been signifcantly changed
if such errors had not been made.” § 7607(d)(8) (emphasis
added). This provision appears “tailor-made to undo” any
“rigid presumption of vacatur” that might apply in other con-
11
Despite the Court's suggestion of forfeiture, ante, at 298, EPA could
not have forfeited the argument that the comments the Court cites were
too insubstantial to merit a response. The Court relies on comments that
were not raised until the applicants' reply briefs or that were uncovered
later by the Court itself. See, e. g., Reply Brief in No. 23A351, p. 11 (rais-
ing the Air Stewardship Coalition “modeling ” comment for the frst time).
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319
Barrett, J., dissenting
texts. N. Bagley, Remedial Restraint in Administrative
Law
, 117 Colum. L. Rev. 253, 291 (2017).
The alleged error here plausibly is subject to § 7607(d)(8)'s
harmless-error rule. As explained above, the Court does
not suggest that it is substantively “[un]reasonable” to apply
the FIP to fewer States, only that EPA did not “reasonably
explai[n]” the FIP's severability in response to comments.
Prometheus, 592 U. S., at 423. That is arguably an “alleged
procedural error” within the meaning of § 7607(d)(8). In
fact, the Act contemplates that at least some “arbitrary or
capricious” challenges allege failures to “observ[e] . . . proce-
dure required by law,” and such challenges may only succeed
if § 7607(d)(8)'s “condition is . . . met.” § 7607(d)(9)(D).
If the Act's harmless-error rule applies, applicants are un-
likely to prevail. Given the apparent lack of connection be-
tween the number of States covered and the FIP's methodol-
ogy for determining cost thresholds and emissions limits, it
is diffcult to imagine a “substantial” likelihood that the rule
would have been “signifcantly” different had EPA just re-
sponded more thoroughly. In fact, applicants seem to have
conceded as much. See Tr. of Oral Arg. 6 (“[W]ith full can-
dor to the Court, [the cost threshold] could be the same or
even be more expensive”); id., at 9 (“I can't tell you what that
looks like, whether there is a difference in the obligations or
not”). And EPA, the Court says, had “notice” of the alleged
concern that the cost thresholds might change with different
States. Ante, at 296. Yet EPA still chose to make the FIP
severable because of its statutory obligation to reduce down-
wind pollution—an obligation it repeatedly referenced. See,
e. g., 88 Fed. Reg. 36693; Response to Comments 149–151.
Would that same EPA have “signifcantly changed” the FIP
had it just explained more thoroughly why the plan did not
depend on the States covered?
12
And on top of all this,
12
The Court faults EPA for “refus[ing] to say with certainty” at oral
argument that it would have reached the same conclusions if different
States were included in the FIP. Ante, at 299. But § 7607(d)(8) does not
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320 OHIO
v. EPA
Barrett, J., dissenting
EPA has in fact refused to reconsider the FIP now that it
appl
ies to fewer States, explaining in detail why its method-
ology was unaffected by the States it covered.
13
***
With little to say in response to the FIP's apparent state-
agnostic methodology for setting emissions limits and the
Clean Air Act's stringent harmless-error rule, the Court re-
sorts to raising forfeiture. Ante, at 298–299. But it is the
Court that goes out of its way to develop a failure-to-explain
theory largely absent from applicants' briefs. One can
search diligently in the hundreds of pages of applicants'
opening briefs for the Court's theory—that EPA failed to
explain in its fnal rule why the FIP's cost-effectiveness
thresholds for imposing emissions limits do not shift with a
different mix of States—and be left wondering where the
Court found it. That theory appears not to have crystal-
require the Government to show that the rule would be the same; it is
most naturally read to require the challenger to demonstrate a “sub-
stantial likelihood that the rule would have been signifcantly changed
if such errors had not been made. ” (Emphasis added.) And though
§ 7607(d)(9)(A) appears to allow reversal of “ `any' ” arbitrary or capricious
“ `action,' ” ante, at 299, § 7607(d)(8)'s more specifc harmless-error rule and
§ 7607(d)(9)(D)'s more specifc requirements for reversal based on arbitrary
or capricious procedural errors would seem to control.
13
The Court claims that the Clean Air Act prevents us from considering
EPA's denial of reconsideration. Ante, at 295, n. 11. But it is not obvious
that the relevant provision of the Act—§ 7607(d)(7)(A)'s defnition of the
“record for judicial review”—bars consideration of later developments for
purposes of the Act's stringent harmless-error rule, § 7607(d)(8). Even
assuming that the denial of reconsideration itsel f cannot count as evidence
of harmlessness, we are judging applicants' likelihood of success on the
merits. On the merits, we can expect EPA to make just the sort of argu-
ments it made in its denial: EPA likely will explain why the covered States
did not matter by citing and interpreting material in the record. See,
e. g., Denial 11 (“Record Basis Establishing Why the Plan Functions Inde-
pendently by State”); id., at 15, and nn. 16–18 (citing the fnal rule and
technical support documents on the rulemaking docket).
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321
Barrett, J., dissenting
lized until oral argument, during which counsel for the state
appl
icants struggled to locate it in the States' brief. Tr. of
Oral Arg. 11–12. Consider just one illustrative example.
Given the importance to the Court's theory of how the “knee
in the curve” might change with different States, see ante,
at 287, 288, and n. 4, 293, one might expect to fnd some mention
of that idea in applicants' briefs. One would be wrong.
Given that applicants' theory has evolved throughout the
course of this litigation, we can hardly fault EPA for failing
to raise every potentially meritorious defense in its response
br ief. That is par ticu lar ly tr ue g iven the compressed
briefng schedule in this litigation's emergency posture: The
Court gave EPA less than two weeks to respond to multiple
applications raising a host of general and industry-specifc
technical challenges, fled less than a week earlier. Even
still, EPA raised § 7607(d)(7)(B)'s procedural bar. Brief for
Respondents 19. And on the merits, EPA expressly argued
that the FIP's “viability and validity do not depend on the
number of jurisdictions it covers”; the “Rule need not apply
to any minimum number of States in order to operate coher-
ently.” Id., at 24. EPA could also have demonstrated how
the FIP's state-agnostic methodology for selecting cost
thresholds was apparent in the fnal rule. But EPA cannot
have forfeited that more specifc point because applicants did
not raise it to begin with.
Because EPA did not forfeit these responses to the merits
of applicants' arbitrary-or-capricious challenge, there is no
need to consider whether a departure from our typical ap-
proach to forfeited arguments is justifed. See ante, at 298–
299. It remains applicants' burden to show that the FIP's
alleged dependence on the covered States likely was an “im-
portant” problem that EPA “entirely failed to consider.”
State Farm, 463 U. S., at 43. And that is on top of their
burden to overcome § 7607(d)(7)(B)'s procedural bar and the
lack of “signifcant,” specifc comments raising this issue.
§ 7607(d)(6)(B).

322 OHIO
v. EPA
Barrett, J., dissenting
Fi na l ly, I wou ld exercise our discreti on to consider
§
7607(d)(8)'s harmless-error rule. Even putting aside the
expedited briefng schedule and the limited discussion of the
Court's theory in applicants' briefs, applicants bear the bur-
den in seeking emergency relief to show a likelihood of
success on the merits. In other words, we must predict
whether applicants will overcome every barrier to relief at
the end of the day, after full merits briefng and argument
in the lower courts and, potentially, again in this Court.
Section 7607(d)(8)'s harmless-error rule is one such impor-
tant obstacle, and EPA has already signaled that it will raise
it as litigation progresses. See Denial 35, n. 38 (arguing that
any failure to more fully explain “how the Rule is not inter-
dependent” is harmless error under § 7607(d)(8)). I see no
reason not to consider it now.
III
Given the emergency posture of this litigation, my views
on the merits of the failure-to-explain objection and the ap-
plication of the Clean Air Act's procedural bar and harmless-
error rule are tentative. But even a tentative adverse con-
clusion can undermine applicants' likelihood of success. And
applicants, to prevail, must run the table; they face the daun-
ting task of surmounting all of these signifcant obstacles.
They are unlikely to succeed.
The Court, seizing on a barely briefed failure-to-explain
theory, grants relief anyway. It enjoins the Good Neighbor
Plan's enforcement against any state or industry applicant
pending review in the D. C. Circuit and any petition for cer-
tiorari. Ante, at 300. Given the number of companies in-
cluded and the timelines for review, the Court's injunction
leaves large swaths of upwind States free to keep contribut-
ing signifcantly to their downwind neighbors' ozone prob-
lems for the next several years—even though the temporar-
ily stayed SIP disapprovals may all be upheld and the FIP
may yet cover all the original States. The Court justifes
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323
Barrett, J., dissenting
this decision based on an alleged procedural error that likely
had
no impact on the plan. So its theory would require EPA
only to confrm what we already know: EPA would have pro-
mulgated the same plan even if fewer States were covered.
Rather than require this years-long exercise in futility, the
equities counsel restraint.
Our emergency docket requires us to evaluate quickly the
merits of applications without the beneft of full briefng and
reasoned lower court opinions. See Does, 595 U. S., at –––
(opinion of Barrett, J.). Given those limitations, we should
proceed all the more cautiously in cases like this one with
voluminous, technical records and thorny legal questions. I
respectfully dissent.
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Reporter’s Note
The attached opinion has been revised to refect the usual publication
and citation style of the United States Reports. The revised pagination
makes available the offcial United States Reports citation in advance of
publication. The syllabus has been prepared by the Reporter of Decisions
for the convenience of the reader and constitutes no part of the opinion of
the Court. A list of counsel who argued or fled briefs in this case, and
who were members of the bar of this Court at the time this case was
argued, has been inserted following the syllabus. Other revisions may
include adjustments to formatting, captions, citation form, and any errant
punctuation. The following additional edits were made:
p. 284, line 13 from bottom: “National” is replaced with “Natural”
p. 286, line 8: “family of ” is inserted before “ozone”; “nitrous oxide” is
replaced with “nitrogen oxides”
p. 286, line 9: “nitrous oxide” is replaced with “nitrogen-oxide“
p. 286, line 18: “nitrous” is replaced with “nitrogen”
p. 286, line 20: “nitrous” is replaced with “nitrogen”
p. 288, n. 4, line 7: “nitrous oxide” is replaced with “nitrogen-oxide”
p. 308, line 18: “might” is replaced with “would”
p. 310, line 12: “might” is replaced with “would”