(Slip Opinion) OCTOBER TERM, 2023 1 

Syllabus

NOTE: Where it is feasible, a syllabus (headnote) will be released, as is 

being done in connection with this case, at the time the opinion is issued.

The syllabus constitutes no part of the opinion of the Court but has been 

prepared by the Reporter of Decisions for the convenience of the reader.

See United States v. Detroit Timber & Lumber Co., 200 U. S. 321, 337.

SUPREME COURT OF THE UNITED STATES

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 Neighbor Provision,” state plans must prohibit emissions “in amounts which 

will . . . contribute significantly 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 requirements” of the Act may EPA issue a Federal Implementation Plan, 

or FIP, for the noncompliant State that fails to correct the deficiencies 

in its SIP. §§7410(k)(3), (c)(1).

 In 2015, EPA revised its air-quality standards for ozone, thus triggering 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 

—————— 

*Together with No. 23A350, Kinder Morgan, Inc., et al. v. Environmental Protection Agency et al.; No. 23A351, American Forest & Paper 

Assn. et al. v. Environmental Protection Agency, No. 23A384, United 

States Steel Corp. v. Environmental Protection Agency et al., also on applications for stay.
2 OHIO v. EPA

Syllabus

all those States. EPA designed its proposed FIP based on which emissions-control measures would maximize cost-effectiveness in improving ozone levels downwind and on the assumption the FIP would apply 

to all covered States. Commenters warned that the proposed SIP disapprovals were flawed 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 

improvements in light of a different set of States. EPA proceeded to 

issue its final FIP without addressing this concern. Instead, EPA announced that its plan was severable: Should any jurisdiction drop out, 

the plan would continue to apply unchanged to the remaining jurisdictions. 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. 9–20.

 (a) When deciding an application for a stay, the Court 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. ___, ___ 

(KAVANAUGH, J., concurring in grant of stay). Because that is true 

here, resolution of applicants’ stay request ultimately turns on the first 

question: Who is likely to prevail at the end. See Nken, 556 U. S., at 

434. Pp. 9–11.

 (b) Applicants are likely to prevail on their arbitrary-or-capricious 

claim. An agency action qualifies 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 important aspect of the problem.” Motor Vehicle Mfrs. Assn. of United 

States, Inc. v. State Farm Mut. Automobile Ins. Co., 463 U. S. 29, 43. 
Cite as: 603 U. S. ____ (2024) 3 

Syllabus

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 airquality improvements might shift. To this question, EPA offered no 

reasoned response. As a result, the applicants are likely to prevail on 

their argument that EPA’s final rule was not “reasonably explained,” 

Prometheus Radio Project, 592 U. S., at 423, and that it instead ignored “an important aspect of the problem” before it, State Farm Mut. 

Automobile Ins. Co., 463 U. S., at 43. Pp. 11–13.

(c) EPA’s alternative arguments are unavailing. First, EPA argues 

that adding a “severability” provision to its final rule—i.e., providing 

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 sidestepped them. Nothing in the final rule’s severability provision actually 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 insists that no one raised that concern during the public comment period. 

The Act’s “reasonable specificity” 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 actions confirm the agency appreciated the concern. Third, EPA 

argues that applicants must return to EPA and file a motion asking it 

to reconsider its final rule before presenting their objection in court 

because the “grounds for [their] objection arose after the period for 

public comment.” §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. 13–17.

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., filed 

a dissenting opinion, in which SOTOMAYOR, KAGAN, and JACKSON, JJ., 

joined.
Cite as: 603 U. S. ____ (2024) 1 

Opinion of the Court

NOTICE: This opinion is subject to formal revision before publication in the 

United States Reports. Readers 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.

SUPREME COURT OF THE UNITED STATES _________________ 

Nos. 23A349, 23A350, 23A351 and 23A384

_________________ 

OHIO, ET AL. 

23A349 v.

ENVIRONMENTAL PROTECTION AGENCY, ET AL. 

KINDER MORGAN, INC., ET AL. 

23A350 v.

ENVIRONMENTAL PROTECTION AGENCY, ET AL. 

AMERICAN FOREST & PAPER ASSOCIATION, ET AL. 

23A351 v.

ENVIRONMENTAL PROTECTION AGENCY, ET AL. 

UNITED STATES STEEL CORPORATION

23A384 v.

ENVIRONMENTAL PROTECTION AGENCY, ET AL. 

ON APPLICATIONS FOR STAY

[June 27, 2024] 

 JUSTICE GORSUCH delivered the opinion of the Court.

The Clean Air Act envisions States and the federal government 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 compliant plan, however, the federal government may sometimes 

step in and assume that authority for itself. §7410(c)(1). 
2 OHIO v. EPA

Opinion of the Court

Here, the federal government announced its intention to reject over 20 States’ plans for controlling ozone pollution. In 

their place, the government sought to impose a single, 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 Environmental 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 

standard, the clock starts ticking: States have three years 

to design and submit a plan—called a State Implementation Plan, or SIP—providing for the “implementation, 

maintenance, and enforcement” of that standard in their jurisdictions. §7410(a)(1); see EPA v. EME Homer City Generation, 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 measures.” §7410(a)(2)(A). And States provide for 

the enforcement 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 

pollution across state borders, emissions in upwind States 

sometimes affect air quality in downwind States. See EME 

Homer, 572 U. S., at 496. To address that externality problem, under the Act’s “Good Neighbor Provision,” state plans 

must prohibit emissions “in amounts which will . . . contribute significantly to nonattainment in, or interfere with 

maintenance by, any other State” of the relevant air-quality 

standard. §7410(a)(2)(D)(i)(I).
Cite as: 603 U. S. ____ (2024) 3 

Opinion of the Court

Because the States bear “primary responsibility” for developing compliance plans, §7401(a)(3), EPA has “no authority 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 satisfies the “applicable requirements” of the Act, including 

the Good Neighbor Provision, EPA “shall approve” it within 

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 

deficiencies in its SIP first. §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 National Resources Defense Council 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, groundlevel 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 

requirement for States to submit new SIPs. Id., at 65437. 

Along the way, EPA issued a guidance document advising 

States that they had “flexibility” in choosing how to address 

their Good Neighbor obligations. See EPA, Memorandum, 

Information on the Interstate Transport State Implementation 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 

submitted SIPs. See 84 Fed. Reg. 66612 (2019). And many 
4 OHIO v. EPA

Opinion of the Court

of the States that did submit SIPs said that they need not 

adopt emissions-control measures to comply with the Good 

Neighbor Provision because, among other things, they were 

not linked to downwind air-quality problems or they could 

identify 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 received. 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 submitting them had failed to address adequately their obligations 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 disapprovals for public comment before finalizing 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, 

20038 (2022). Rather than continue to encourage “‘flexi-

—————— 1See 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). 2See 87 Fed. Reg. 31443 (California); 87 Fed. Reg. 31470 (2022) (Utah); 

87 Fed. Reg. 31485 (2022) (Nevada); 87 Fed. Reg. 31495 (2022) (Wyoming). 3EPA also added three more States: Pennsylvania and Virginia, which 

had not submitted SIPs, and Delaware, whose SIP, EPA said, it had approved in “error.” 87 Fed. Reg. 20036, 20038 (2022).
Cite as: 603 U. S. ____ (2024) 5 

Opinion of the Court

bilit[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 “uniform framework” and “[n]ationwide consistency.” 87 

Fed. Reg. 9841; see 87 Fed. Reg. 20073. The FIP the agency 

proposed set as its target the reduction of the emissions of 

one ozone precursor in particular: nitrous oxide. See id., at 

20038. And it sought to impose nitrous oxide emissionscontrol measures that “maximized cost-effectiveness” in 

achieving “downwind ozone air quality improvements.” Id., 

at 20055; see also id., at 20043.

In broad strokes, here is how EPA’s proposed rule worked 

to eliminate a State’s “significant contribution” to downwind ozone problems. First, the agency identified various 

emissions-control measures and, using nationwide data, 

calculated how much each typically costs to reduce a ton of 

nitrous-oxide emissions. Id., at 20076; see, e.g., id., at

20077–20081. Next, the agency sought to predict how much 

each upwind State’s nitrous-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 Analysis). In making those predictions, EPA often considered 

data specific to the emissions-producing facilities in the 

State, and fed “unit-level and state-level” values into its calculations. 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, 

“determin[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.

To pick which measures would “maximiz[e] cost- 
6 OHIO v. EPA

Opinion of the Court

effectiveness” in achieving “downwind ozone air quality improvements,” 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 (hyphenation omitted). EPA used this point to select a “uniform level” of cost, and so a uniform package of emissionsreduction 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. Pursuant 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 determined which 

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 commenters portrayed the SIPs, this was not an entirely speculative possibility. Many believed EPA’s disapprovals of 

the SIPs were legally flawed. See, e.g., Comments of Missouri Dept. of Natural Resources 3 (June 17, 2022) (referencing “all the technical, legal, and procedural issues” with 

the proposed SIP disapproval); see also, e.g., Comments of 

Louisiana 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 Proposed Rule 9–11 (EPA–HQ–OAR–2021–0668). 

They added that EPA’s FIP was “inextricably linked” to the 

SIP disapprovals. E.g., Comments of Missouri Dept. of Natural Resources, 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 3.

Commenters added that failing to include a State could 
Cite as: 603 U. S. ____ (2024) 7 

Opinion of the Court

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 

findings 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 “significant contribution” by determining what measures in upwind States would maximize 

cost-effective ozone-level improvements in the States downwind of them. Supra, at 5–6. 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 preexisting 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 disap- 

—————— 4Commenters pointed out the variance among emissions-producing facilities too. See, e.g., Comments of Indiana Municipal Power Agency 9 

(June 20, 2022) (the “cost effectiveness” of one tool “will be highly variable” 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 significant costs 

to achieve [nitrous 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 

benefits from controlling one industry—pulp and paper mills—had a 

“maximum estimated improvement” in ozone levels in downwind States 

of just 0.0117 parts per billion).
8 OHIO v. EPA

Opinion of the Court

provals soon seemed to vindicate at least some of the commenters’ concerns. Two circuits issued stays of EPA’s SIP 

denials 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 final 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 final FIP, litigation over the agency’s 

SIP disapprovals continued. One court after another issued 

one stay after another.6 Each new stay meant another

 

—————— 5The final 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 

intention to disapprove those States’ SIPs. Ibid.; see also supra, at 4, 

and nn. 1–2. EPA has since proposed a plan for Tennessee and several 

other States. 89 Fed. Reg. 12666 (2024). 6See, 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).
Cite as: 603 U. S. ____ (2024) 9 

Opinion of the Court

State to which EPA could not apply its FIP. See §7410(c)(1). 

Ultimately, EPA recognized that it could not apply its FIP 

to 12 of the 23 original States.7 Together, these 12 States 

accounted for over 70 percent of the emissions EPA had 

planned to address through its FIP. See Application for 

Ohio et al. in No. 23A349, p. 1 (States’ Application); 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 standard. 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 applicants 

renewed their request here. The Court has received and 

reviewed over 400 pages of briefing and a voluminous record, held over an hour of oral argument on the applications, 

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 

—————— 7See 88 Fed. Reg. 49295 (2023) (Arkansas, Kentucky, Louisiana, Mississippi, Missouri, and Texas); 88 Fed. Reg. 67102 (2023) (Alabama, Minnesota, Nevada, Oklahoma, Utah, and West Virginia). EPA has since 

proposed settling the litigation over the Nevada SIP disapproval. 89 Fed. 

Reg. 35091 (2024). 8Of course, this could change again as litigation over the SIP denials 

progresses past preliminary stay litigation and toward final decisions on 

the merits.
10 OHIO v. EPA

Opinion of the Court

request, we must provide the applicants with an answer—

“grant or deny.” Labrador v. Poe, 601 U. S. ___, ___ (2024) 

(KAVANAUGH, J., concurring in grant of stay) (slip op., at 2).

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). Specifically, 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 Respondent EPA in No. 23A349 etc., p. 16 (EPA Response).9

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, 601 U. S., at ___ (opinion of KAVANAUGH, J.) (slip op., 

at 3). That is certainly the case here, for both sides have 

strong arguments with respect to the latter three Nken factors. On one side of the ledger, the federal government 

points to the air-quality benefits its FIP offers downwind 

States. EPA Response 48–50. On the other side, the States 

observe that a FIP issued unlawfully (as they contend this 

one was) necessarily impairs their sovereign interests in 

regulating their own industries and citizens—interests the 

Act expressly recognizes. 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 litigation risks placing them at a “competitive 

—————— 9Approaching 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 field, 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).
Cite as: 603 U. S. ____ (2024) 11

Opinion of the Court

disadvantage” to their exempt peers. States’ Application

21. The States and the private applicants also stress that 

complying with the FIP during the pendency of this litigation would require them to incur “hundreds of millions[,] if 

not billions of dollars.” 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.) (slip op., at 4).

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 final FIP “arbitrary” or “capricious” 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 qualifies as “arbitrary” or “capricious” if it is not “reasonable and reasonably explained.” 

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 explanation for 

its action[,] including a rational connection between the 
12 OHIO v. EPA

Opinion of the Court

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 omitted). Accordingly, an agency cannot simply ignore “an important aspect of the problem.” Ibid. 

We agree with the applicants that EPA’s final FIP likely 

runs afoul of these long-settled standards. The problem 

stems from the way EPA chose to determine which emissions “contribute[d] significantly” to downwind States’ difficulty meeting national ozone standards. 42 U. S. C. 

§7410(a)(2)(D)(i)(I). Recall that EPA’s plan rested on an assumption that all 23 upwind States would adopt emissionsreduction 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 emissions 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 “maximiz[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 government acknowledged that it could not represent with certainty whether the cost-effectiveness analysis it performed 

collectively for 23 States would yield the same results and 
Cite as: 603 U. S. ____ (2024) 13

Opinion of the Court

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 there is an explanation, it does not appear in 

the final rule. As a result, the applicants are likely to prevail on their argument that EPA’s final 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[al]” of the 

FIP’s mandates on them. §7607(d)(9).10

III

A 

Resisting this conclusion, EPA advances three alternative arguments.

First, the government insists, the agency did offer a reasoned response to the applicants’ concern, just not the one 

they hoped. When finalizing its rule in response to public 

comments, the government represents, “the agency did consider whether the [FIP] could cogently be applied to a subset of the 23 covered States.” EPA Response 27; see also 

post, at 17–18 (BARRETT, J., dissenting). And that consideration, the government stresses, led EPA to add a “severability” provision to its final 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 

—————— 10Various 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.
14 OHIO v. EPA

Opinion of the Court

27 (quoting 88 Fed. Reg. 36693). In support of its severability provision, EPA cited, among other things, its intent 

to address “‘important public health and environmental 

benefits” and encourage reliance by others “on th[e] final 

rule in their planning.’” Ibid.

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. Ibid. But none of that, nor anything else EPA 

said in support of its severability provision, addresses 

whether and how measures found to maximize cost effectiveness 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 originally targeted emissions, might be subject to the 

agency’s plan. Put simply, EPA’s response did not address 

the applicants’ concern so much as sidestep it.11

—————— 11As 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 intended to cover. To be sure, after this Court heard argument, 

EPA issued a document in which it sought to provide further explanations for the course it pursued. See 89 Fed. Reg. 23526 (2024). But the 

government has not suggested that we should consult this analysis in 

assessing the validity of the final rule. See Letter from E. Prelogar, Solicitor General, 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 promulgated 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 
Cite as: 603 U. S. ____ (2024) 15

Opinion of the Court

Second, the government pivots in nearly the opposite direction. Now, it says, if its final rule lacks a reasoned response 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 first “‘raised with reasonable specificity’” before the agency during the public comment period. 

Id., at 19–20 (quoting §7607(d)(7)(B)); see also post, at 8–

11.

We cannot agree. The Act’s “reasonable specificity” requirement does not call for “a hair-splitting approach.” Appalachian 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 confirm that the government 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 “conduct a new assessment and modeling of contribution” to determine what emissions-control measures maximized cost 

effectiveness in securing downwind ozone air-quality improvements. Comments of Air Stewardship Coalition, at 

13–14; see also Part I–C, supra (noting examples of other 

—————— 

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” promulgated the FIP. Michigan 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). 
16 OHIO v. EPA

Opinion of the Court

comments). And, as we have just seen, EPA’s own statements and actions confirm the agency appreciated that concern. In preparing the final rule in response to public comments, 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 final rule. Ibid. By its own 

words and actions, then, the agency demonstrated that it 

was 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 return to EPA and file a motion asking it to reconsider its final 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, 

—————— 12The dissent resorts to a “hair-splitting approach” to the public comments. Post, at 8–11. It stresses, for example, that some comments highlighted variances among specific emissions-producing facilities and industries, “not States.” Post, at 8 (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 

Analysis). 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 10. But why would the 

sequencing matter? Because the FIP cannot apply to a State if its SIP is 

not disapproved. 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 concern, EPA needed to, and by its own admission sought to, “consider” whether its FIP could apply to a subset of States. EPA Response 

27.
Cite as: 603 U. S. ____ (2024) 17

Opinion of the Court

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. Nothing 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 

dissent posits that every “objection that [a] final rule was 

not reasonably explained” must be raised in a motion for 

reconsideration. Post, at 7 (internal quotation marks omitted; emphasis deleted). But there is a reason why the government does not go so far. The Clean Air Act opens the 

courthouse doors to those with objections the agency already ignored. If an “objection [is] raised with reasonable 

specificity during the period for public comment” but not 

reasonably addressed in the final 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. Tellingly, the case on which the dissent relies involves an entirely different situation: a “‘logical outgrowth’ 

challeng[e].” Post, at 7. There, the objection was that EPA 

had supposedly “‘significantly amend[ed] the [r]ule between the proposed and final 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 important problem the public could and did raise 

during the comment period.

B 

With the government’s theories unavailing, the dissent 

advances 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 depend on the number of covered States.” Post, at 12–17, 18–
18 OHIO v. EPA

Opinion of the Court

19. Then, coming at the same point from another direction, 

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 20.

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 briefing. See supra, at 9. 

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.

If anything, we see one reason for caution after another. 

Start with the fact the dissent itself expresses little confidence in its own theories, contending no more than it “appear[s]” EPA’s methodology did not depend on the number 

of covered States. Post, at 14 (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 included 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 described 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 significant role in EPA’s work.13 Finally, observe that, 

while the Act seems to anticipate, as the dissent suggests, 

—————— 13The agency, we said, “first calculated, for each upwind State, the 

quantity of emissions the State could eliminate at each of several cost 

[levels]”; next, it “conducted complex modeling to establish the combined 

effect of 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] significant cost [levels]” to use in setting its emissions 

budgets. EME Homer City, 572 U. S., at 501–502.
Cite as: 603 U. S. ____ (2024) 19

Opinion of the Court

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) 

(emphasis added). With so many reasons for caution, we 

think sticking to our normal course of declining to consider 

forfeited arguments the right course here.14

* 

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 disposition 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 

terminate automatically. If the petition is granted, this order shall terminate upon the sending down of the judgment 

of this Court.

It is so ordered. 

—————— 14Admittedly, the dissent points to some statements in the FIP suggesting EPA considered nationwide data in parts of its analysis. See, 

e.g., post, at 14; see also, e.g., 88 Fed. Reg. 36721, 36727. But other statements in that rule and supporting documents also seem to suggest EPA 

considered state-specific information. See Part I–C, supra. If, as the 

dissent posits, only nationwide data informed EPA’s analysis, why would 

EPA say that, “for purposes of identifying the appropriate level of control,” it focused on “the 23 upwind states that were linked” to the downwind States, rather than, say, “all states in the contiguous U.S.”? Final 

Ozone Analysis 3 (footnote omitted). Why would EPA explain that its 

“findings regarding air quality improvement” downwind were a “central 

component” of picking the appropriate cost levels and so defining a 

State’s significant 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 

“calibrat[e]” that relationship “based on state-specific source apportionment”? Final Ozone Analysis 43. In asking these questions, we do not 

profess answers; we simply highlight further reasons for caution.
Cite as: 603 U. S. ____ (2024) 1 

BARRETT, J., dissenting

SUPREME COURT OF THE UNITED STATES _________________ 

Nos. 23A349, 23A350, 23A351 and 23A384

_________________ 

OHIO, ET AL. 

23A349 v.

ENVIRONMENTAL PROTECTION AGENCY, ET AL. 

KINDER MORGAN, INC., ET AL. 

23A350 v.

ENVIRONMENTAL PROTECTION AGENCY, ET AL. 

AMERICAN FOREST & PAPER ASSOCIATION, ET AL. 

23A351 v.

ENVIRONMENTAL PROTECTION AGENCY, ET AL. 

UNITED STATES STEEL CORPORATION

23A384 v.

ENVIRONMENTAL PROTECTION AGENCY, ET AL. 

ON APPLICATIONS FOR STAY

[June 27, 2024] 

 JUSTICE BARRETT, with whom JUSTICE SOTOMAYOR, 

JUSTICE KAGAN, and JUSTICE JACKSON join, dissenting.

The Court today enjoins the enforcement of a major Environmental Protection Agency rule based on an underdeveloped theory that is unlikely to succeed on the merits. In 

so doing, the Court grants emergency relief in a factintensive 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 sufficiently in response to comments, this theory must 

surmount sizable procedural obstacles and contrary record 
2 OHIO v. EPA

BARRETT, J., dissenting

evidence. 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 ensuring that States meet air-quality standards. Ante, at 2–3. 

The Clean Air Act directs EPA to “establish national ambient air quality standards (NAAQS) for pollutants at levels 

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 Implementation 

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,” relieving 

upwind States “of the associated costs” and making it difficult for downwind States to “maintain satisfactory air quality.” EME, 572 U. S., at 496. So the Act’s Good Neighbor 

Provision requires SIPs to “prohibi[t]” the State’s emissions 

sources from “emitting any air pollutant in amounts which 

will . . . contribute significantly to nonattainment in, or interfere with maintenance by, any other State with respect 

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 significantly 

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 significant contributions by the statutory deadline for the affected downwind States to achieve compliance. See Wisconsin v. EPA, 938 F. 3d 303, 313–314 (CADC 2019)
Cite as: 603 U. S. ____ (2024) 3 

BARRETT, J., dissenting

(per curiam); §7511.

Second, the Court fails to recognize that EPA’s SIP disapprovals may, in fact, be valid. EPA justified its findings 

that 23 States had failed to submit adequate SIPs. It found 

that these States all significantly contributed to ozone pollution in downwind States. See 88 Fed. Reg. 36656 (2023). 

But 21 of these States, including applicants, proposed to 

do nothing to reduce their ozone-precursor (i.e., NOx) 

emissions—arguing that they did not actually contribute to 

downwind nonattainment or that there were no other costeffective 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 original 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 covered fewer States, but EPA failed to respond. Ante, at 6–8. 

Not exactly. As I will elaborate below, commenters merely 

criticized EPA’s decision to propose a FIP before its SIP disapprovals were final. EPA responded that this sequencing 

was “consistent with [its] past practice in [its] efforts 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 yearslong process of promulgating a FIP so that one could be effective 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 relying on an “assumption that [it] would apply to all covered 
4 OHIO v. EPA

BARRETT, J., dissenting

States.” Ante, at 6; see ante, at 12. But try as it might, the 

Court identifies no evidence that the FIP’s emissions limits 

would have been different for a different set of States or 

that EPA’s consideration of state-specific inputs was anything but confirmatory of the limits it calculated based on 

nationwide data. See ante, at 5–6, 19, n. 14. The Court 

leans on the fact that EPA “considered data specific to the 

emissions-producing facilities in [each] State” to calculate 

“how much each upwind State’s [NOx] emissions would fall” 

if the State’s emitters “adopted each [emissions-control] 

measure.” Ante, at 5 (citing EPA, Ozone Transport Policy 

Analysis Proposed Rule TSD 9–10, 13, 22–23, (EPA–HQ–

OAR–2021–0668–0133, Feb. 2022) (Proposed Ozone Analysis)). But the Proposed Ozone Analysis makes clear that 

EPA did these state-specific calculations to determine each 

State’s “emissions budget.” Proposed Ozone Analysis 7–13. 

A State’s budget consists of the “emissions that would remain” after the State’s power plants meet the emissions 

limits that EPA independently calculated. 88 Fed. Reg. 

36762; see Proposed Ozone Analysis 13 (“adjust[ed]” “unitlevel 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 

identify any NOx limitfor any industry that relied on statespecific data.

On the contrary, as I will explain in Part II–B, the final 

rule and its supporting documents suggest that EPA’s 

methodology 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 confirmed this interpretation. During this litigation, EPA received petitions 

seeking reconsideration of the FIP on the ground that it 
Cite as: 603 U. S. ____ (2024) 5 

BARRETT, J., dissenting

should not be implemented in just a subset of the original

States. EPA denied these petitions on April 4, 2024. 89 

Fed. Reg. 23526. It thoroughly explained how its “methodology for defining” 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 costeffectively achieve.” EPA, Basis for Partial Denial of Petitions for Reconsideration on Scope 1, (EPA–HQ–OAR–

2021–0668–1255, Apr. 2024) (Denial). The “control technologies and cost-effectiveness figures 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 obligations” regardless of 

the number of covered States. Id., at 3–4.

II

To obtain emergency relief, applicants must, at a minimum, 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 

applicants’ 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 significantly, 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 “capricious.” 42 U. S. C. §7607(d)(9). The “arbitrary-andcapricious standard requires that agency action” be both 
6 OHIO v. EPA

BARRETT, J., dissenting

“[1] reasonable and [2] reasonably explained.” FCC v. Prometheus Radio Project, 592 U. S. 414, 423 (2021). The 

Court’s theory is that EPA did not “‘reasonably explai[n]’” 

“why the number and identity of participating States does 

not affect what measures maximize cost-effective downwind air-quality improvement.” Ante, at 13 (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 significant evidence in the record (not to mention 

EPA’s denial 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 decision is the argument that EPA failed to provide 

“‘a satisfactory explanation for its action’” and a “reasoned 

response” to comments. Ante, at 12–13 (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 challenges that a plaintiff can bring in court: Only objections 

that were “raised with reasonable specificity during the period for public comment . . . may be raised during judicial 

review.” §7607(d)(7)(B). If it was “impracticable to raise 

such objection 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 
Cite as: 603 U. S. ____ (2024) 7 

BARRETT, J., dissenting

challenging the plan itself, not the denial of reconsideration. So the procedural bar on objections not raised in the 

comments presents a significant obstacle—in two ways.

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 

covered. Ante, at 12. That EPA failed to adequately explain 

its final rule in response to comments is “an objection to the 

notice and comment process itself,” which applicants “obviously 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 “final rule was not ‘reasonably explained.’” Ante, at 13 (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 “significantly amend[ed] the Rule between the proposed and final 

versions without providing additional opportunity for notice and comment.” 795 F. 3d, at 137. But because this procedural objection could not have been raised during the 

comment period, “the only appropriate path for petitioners” 

under §7607(d)(7)(B) was to raise it “through an initial petition for reconsideration to EPA.” Ibid. So the D. C. Circuit 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 reconsideration proceedings. This Court’s failure-to-explain objection may face the same problem: It is not judicially reviewable in its current posture.1

—————— 1The Court offers a feeble response to this application of 
8 OHIO v. EPA

BARRETT, J., dissenting

Second, even putting aside this aspect of §7607(d)(7)(B), 

it is not clear that any commenter raised with “reasonable 

specificity” the underlying substantive issue: that the exclusion of some States from the FIP would undermine 

EPA’s cost-effectiveness analyses and resulting emissions 

controls. §7607(d)(7)(B); see ante, at 13. The Court concludes otherwise only by putting in the commenters’ 

mouths words they did not say. It first cites a bevy of comments arguing that EPA’s “disapprovals of the SIPs were 

legally flawed” and noting the obvious point that EPA cannot “include a State in its FIP” unless it validly disapproves 

the State’s SIP. Ante, at 6. These comments do not address 

the continued efficacy of a FIP that applies to a subset of 

the originally covered States.

Another collection of the Court’s inapposite comments relates to the inclusion of specific sources, emissions controls, 

and industries in the proposed plan—not States. See ante, 

at 7, 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 expecta-

—————— 

§7607(d)(7)(B)’s procedural bar. Ante, at 17. It simply quotes 

§7607(d)(7)(B) and asserts without support that it means that a plaintiff 

“need not go back to the agency and insist on an explanation a second 

time.” Ibid. The Court fails to engage with the logic of this argument: 

The objection that the final rule did not contain sufficient explanation 

was not and could not have 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 logicaloutgrowth challenge the D. C. Circuit considered in EME Homer. Ante, 

at 17. But the Court ignores the fact that its failure-to-explain challenge 

and logical-outgrowth challenges are both “objection[s] to the notice and 

comment process itself” that depend on the content of the final rule. 

EME Homer, 795 F. 3d, at 137. Even if the public raised an “important 

problem . . . during the comment period,” ante, at 17, the Court’s basis 

for enjoining the FIP’s enforcement is not that the alleged problem is 

real, but that the final rule did not address it.
Cite as: 603 U. S. ____ (2024) 9 

BARRETT, J., dissenting

tions in even the most optimistic case.” Comments of Indiana Municipal Power Agency 9 (June 20, 2022). That is a 

challenge to EPA’s endorsement of a particular emissionscontrol 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). Similarly, another commenter argued that pulp and 

paper mills should not be included because the “maximum 

estimated improvement” in ozone levels from controlling 

their emissions would be “too small to even measure.” Comments of Wisconsin Paper Council 2 (June 21, 2022).2 An 

argument that the maximum benefits from regulating an 

industry are too small is not an argument that those benefits would become too small if fewer States were covered.3

The closest comment that the Court can find—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 

—————— 2The Court claims that in distinguishing comments about particular 

industries from comments that question whether the plan depends on a 

number of States, I “fai[l] to acknowledge” that the FIP treats States as 

the “sum of their emissions-producing facilities.” Ante, at 16, n. 12. But 

in reality, it is the Court that ignores how the FIP works. The FIP determines emissions limits for particular sources based on their industries; the total NOx 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, infra. So comments critiquing a particular industry-specific emissions limit or technology assumption say 

nothing about the FIP’s dependence on a certain number of States. 3Nor did the Air Stewardship Coalition’s comment about the “knee in 

the curve” raise concerns about which States are included. See ante, at

7, n. 4. Rather, this comment questioned EPA’s proposed average costeffectiveness 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 Stewardship Comments). It did not link its concern about cost thresholds to the 

States covered by the plan.
10 OHIO v. EPA

BARRETT, J., dissenting

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’ might 

shift” and change the cost-effective “emissions-control 

measures.” Ante, at 7. 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 finalized 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 Stewardship 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 assessment and modeling of contribution and subject 

those findings to public comment.” Id., at 14.

This sentence says nothing about what would be required if 

after EPA finalizes its SIP disapprovals and issues a final 

FIP, some States drop out of the plan. Nor does it suggest 

that the plan’s cost-effectiveness thresholds or emissions 

controls 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 upwind 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 difficult to see how this comment raised 

with “reasonable specificity” the objection that the removal 

of some States from the final plan would invalidate EPA’s 
Cite as: 603 U. S. ____ (2024) 11

BARRETT, J., dissenting

cost-effectiveness thresholds and chosen emissions-control 

measures.4 That is not how EPA understood it. EPA characterized this comment as arguing that “by taking action 

before considering comments on the proposed disapprovals, 

the EPA is presupposing the outcome of its proposed rulemakings 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 August 2024 NAAQS attainment deadline. Id., at 150. If a 

commenter had said with reasonable specificity what the 

Court says today—that “a different set of States might 

mean that the ‘knee in the curve’ might shift,” ante, at 7—

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 

penalized if it did not have reasonable notice of this objection.5

—————— 4So too with Portland Cement’s comment. See ante, at 7. 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 finalizing its SIP disapprovals. Comments 

of Portland Cement Association 7 (June 21, 2022). 5The Court concludes to the contrary only by building out the comment’s bare reference to a “new assessment and modeling” with its own 

inferences about the possible effect of different numbers of States on “the 

math.” Ante, at 16, n. 12 (internal quotation marks omitted). But as 

explained above, the comment itself said nothing about States dropping 

out of the final plan or the possible impact of different numbers of States 

on the FIP’s cost thresholds or emissions limits. It is hard to believe that 

a single sentence with no elaboration or explanation of the potential issue—in a sea of thousands of pages of comments—gave EPA reasonable 

notice that it should have included in its final rule a detailed explanation 

of why the FIP’s emissions limits did not depend on the number 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’”).
12 OHIO v. EPA

BARRETT, J., dissenting

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.

B 

Even 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 judgment 

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. ArkansasBest Freight System, Inc., 419 U. S. 281, 286 (1974)). Given 

the explanations and state-agnostic methodology apparent 

in the final rule and its supporting documentation—and the 

paucity of comments specifically 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 costeffectiveness thresholds and imposing emissions controls 

did not depend on the number of covered States. The rule 

applied EPA’s longstanding “4-step interstate transport 

framework” to create emissions limits that will prevent NOx

sources in upwind States from significantly 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) identifies “downwind receptors that are expected to have 

problems attaining or maintaining the NAAQS”; (2) identifies which upwind States are “‘link[ed]’” to those downwind 

receptors because they contribute at least 1% of a receptor’s 
Cite as: 603 U. S. ____ (2024) 13

BARRETT, J., dissenting

ozone; (3) determines which NOx sources in the linked upwind States “significantly contribute” to downwind nonattainment or interference; and (4) implements emissions 

limits to stop those sources’ significant contributions. 88 

Fed. Reg. 36659; see EME, 572 U. S., at 500–501 (describing similar approach used in earlier FIP). The first two 

steps determine which States the FIP must cover. The rubber meets the road at steps 3 and 4: How much do sources 

in those States “significantly contribute” to downwind pollution, and what must they do about it?

Here is how EPA explains that methodology. A source 

“significantly 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 eliminated at a cost “under the cost threshold set by the Agency” 

for sources in that industry. EME, 572 U. S., at 518 (upholding 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 effective.” 88 Fed. Reg. 36676. EPA calculates for each type of 

source a “uniform level of NOx 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 become excessively costly on a per-ton basis while also delivering 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 

emissions accordingly.6

—————— 6For power plants, EPA implements these requirements by allocating 

each State an “‘emissions budget’ . . . representing the EPA’s quantification 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 
14 OHIO v. EPA

BARRETT, J., dissenting

Crucially, the final rule suggests that EPA calculated 

cost-effectiveness 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 12, these thresholds and the FIP’s resulting emissions limits appear not to depend on the number of 

covered States. Consider the plan’s approach to power 

plants (“electric generating units,” or EGUs). EPA assessed 

the cost and impact of different NOx mitigation strategies 

that EGUs could implement. One strategy was to fully operate “selective catalytic reduction” (SCR) technology. 88 

Fed. Reg. 36655; see id., at 36720. EPA estimated that a 

“representative 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 “uniform control stringency to EGUs within 

the covered upwind states.” Id., at 36680.7

—————— 

marketplace for emissions. Ibid. With respect to other sources, EPA 

determined that nine industries in the covered States produced the most 

significant emissions. Id., at 36817. The rule requires sources in each of 

those industries to meet specific emissions limits that were calculated 

based on the reductions that they can cost-effectively achieve. Ibid. 7EPA ultimately selected (based on nationwide data) a cost threshold 

of $1,800 per ton of NOx 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 “retrofi[t] state-of-the-art combustion controls” and “[o]ptimiz[e] idled SCRs” 

for less than $1,800 per ton, and they can “instal[l new] SCRs” for less 
Cite as: 603 U. S. ____ (2024) 15

BARRETT, J., dissenting

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 

states”—an approach that would have made the costeffectiveness thresholds depend on which States were covered. Id., at 36723. But EPA expressly defended its approach based on its “intention to identify a technology-specific 

representative emissions rate” and its interest in “the performance potential of a technology”—which were best 

served by the “largest dataset possible (i.e., nationwide).” 

Id., at 36723–36724 (emphasis added). EPA explained 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

—————— 

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 assumption 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 

EPA selected cost thresholds based on nationwide data, each State’s 

budget would be the same even if the FIP covered different States. 8While EPA’s methodology with respect to other industrial sources 

(non-EGUs) was more complicated, it also seems to have relied on nationwide 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 “ozone season NOx reduction 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–2021–0668–150, Feb. 2022) (boldface omitted). This 

figure thus appears to have been determined based on industry-wide cost 

and emissions data rather than state-specific calculations. So too with 

the specific emissions limits EPA decided could be implemented for less 

than that cost. See, e.g., 88 Fed. Reg. 36825 (“EPA based the proposed 
16 OHIO v. EPA

BARRETT, J., dissenting

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 statements mentioning state data. See ante, at 5–6, 

19, n. 14. None stick. The fundamental problem with the 

Court’s citations is that they discuss analyses that EPA performed after it chose cost thresholds and emissions limits 

based on nationwide industry data. EPA did assess the impact 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 5, 19, n. 14. 

But EPA said that these “‘findings regarding air quality improvement,’” ante, at 19, n. 14 (quoting 88 Fed. Reg. 36741), 

served only to “cement EPA’s identification 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 “eliminate 

significant contribution” depends on the implementation of 

cost-effective emissions controls at individual “industrial 

sources,” not some overall impact on “downwind areas’ nonattainment and maintenance problems.” 88 Fed. Reg. 

36741. EPA assessed the FIP’s impact assuming the participation of particular States primarily to ensure that its 

emissions limits did not result in “overcontrol”—i.e., more 

reductions 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 

5, makes this point clear: “The downwind air quality impacts are used to inform EPA’s assessment of potential 

overcontrol.” Proposed Ozone Analysis 31.

EPA’s analysis confirmed 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. 

—————— 

emissions limits for cement kilns on the types of limits being met across 

the nation”).
Cite as: 603 U. S. ____ (2024) 17

BARRETT, J., dissenting

36741. Importantly, implementing the FIP “in fewer upwind states does not (and cannot possibly) result in overcontrol” 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-specific data in 

its overcontrol analysis does not mean that the FIP’s emissions 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 Confirming this interpretation, the final 

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 justified the FIP’s severability: EPA 

“must address good neighbor obligations as expeditiously as 

practicable and by no later than the next applicable attainment date”; severability serves “important public health 

and environmental benefits” and ensures that stakeholders 

can “rely on this final rule in their planning.” Ibid. These 

—————— 9The Court argues that EPA equated the framework it used here with 

the one that we described in EME Homer City. Ante, at 18, and n. 13. 

But even if EME described an approach that selected cost thresholds 

“only after” conducting downwind air-quality assessments, ibid., 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 final 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 final FIP identifies 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. 
18 OHIO v. EPA

BARRETT, J., dissenting

rationales align with EPA’s response to critics of its decision 

to propose a FIP before finalizing 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 of assuring that [EPA’s] statutory obligation to reduce air pollution affecting the health and welfare of people 

in downwind states is implemented without delay.” Response to Comments 151.

Given these justifications and the state-agnostic methodology apparent in the final 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 

significant way on the number of States included, so the 

drawbacks of severability were minimal. On the other 

hand, severability was necessary so that EPA could fulfill, 

to the greatest extent possible, its statutory obligation to 

eliminate the significant ozone contributions 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 attainment 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). 

—————— 10The 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 justifications for it did not address (alleged) concerns about how 

the cost-effectiveness thresholds would change with fewer States. Ante, 

at 15–16. But as explained above, commenters did not raise that issue 

with specificity; 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.
Cite as: 603 U. S. ____ (2024) 19

BARRETT, J., dissenting

An agency must respond to “‘relevant’ and ‘significant’ public comments,” and that requirement is not “particularly demanding”; 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. 

FAA, 988 F. 2d 186, 197 (CADC 1993) (quoting Home Box 

Office, Inc. v. FCC, 567 F. 2d 9, 35, and n. 58 (CADC 1977); 

emphasis added); see §7607(d)(6)(B) (EPA must respond to 

“significant” 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 comments purportedly raising that possibility might not be “relevant” and 

“significant.” Moreover, the one comment that vaguely referred to a need for a “new assessment and modeling,” Air 

Stewardship Comments 14, was “purely speculative” and 

“disclose[d]” no “factual or policy basis”; it likely merited no 

response, Home Box Office, 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 

—————— 11Despite the Court’s suggestion of forfeiture, ante, at 17, 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 (raising the Air Stewardship Coalition “modeling” comment for the first time).
20 OHIO v. EPA

BARRETT, J., dissenting

the errors were so serious and related to matters of such 

central relevance to the rule that there is a substantial likelihood that the rule would have been significantly 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 

contexts. 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] . . . procedure 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 

unlikely to prevail. Given the apparent lack of connection 

between the number of States covered and the FIP’s methodology for determining cost thresholds and emissions limits, it is difficult to imagine a “substantial” likelihood that 

the rule would have been “significantly” different had EPA 

just responded more thoroughly. In fact, applicants seem 

to have conceded as much. See Tr. of Oral Arg. 6 (“[W]ith 

full candor 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 15. Yet EPA still chose to 

make the FIP severable because of its statutory obligation 

to reduce downwind pollution—an obligation it repeatedly 
Cite as: 603 U. S. ____ (2024) 21

BARRETT, J., dissenting

referenced. See, e.g., 88 Fed. Reg. 36693; Response to Comments 149–151. Would that same EPA have “significantly

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, EPA has in fact refused to reconsider the 

FIP now that it applies to fewer States, explaining in detail 

why its methodology was unaffected by the States it covered.13

* * * 

With little to say in response to the FIP’s apparent stateagnostic methodology for setting emissions limits and the 

Clean Air Act’s stringent harmless-error rule, the Court resorts to raising forfeiture. Ante, at 17–19. But it is the 

Court that goes out of its way to develop a failure-to-explain 

—————— 12The 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 18. But §7607(d)(8) does not 

require the Government to show that the rule would be the same; it is 

most naturally read to require the challenger to demonstrate a “substantial likelihood that the rule would have been significantly 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 19, §7607(d)(8)’s more specific harmless-error rule and 

§7607(d)(9)(D)’s more specific requirements for reversal based on arbitrary or capricious procedural errors would seem to control. 13The Court claims that the Clean Air Act prevents us from considering EPA’s denial of reconsideration. Ante, at 14–15, n. 11. But it is not 

obvious that the relevant provision of the Act—§7607(d)(7)(A)’s definition 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 itself 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 arguments 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 Independently by State”); id., at 15, and nn. 16–18 

(citing the final rule and technical support documents on the rulemaking 

docket).
22 OHIO v. EPA

BARRETT, J., dissenting

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 final 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 crystallized 

until oral argument, during which counsel for the state applicants 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 6, 7, 

and n. 4, 12, one might expect to find 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 brief. That is particularly true given the compressed 

briefing 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-specific 

technical challenges, filed 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 coherently.” Id., at 24. EPA could also have demonstrated 

how the FIP’s state-agnostic methodology for selecting cost 

thresholds was apparent in the final rule. But EPA cannot 

have forfeited that more specific 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 approach to forfeited arguments is justified. See ante, at 18. 
Cite as: 603 U. S. ____ (2024) 23

BARRETT, J., dissenting

It remains applicants’ burden to show that the FIP’s alleged 

dependence on the covered States likely was an “important” 

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 “significant,” specific comments raising this issue. §7607(d)(6)(B). 

Finally, I would exercise our discretion to consider 

§7607(d)(8)’s harmless-error rule. Even putting aside the 

expedited briefing schedule and the limited discussion of 

the Court’s theory in applicants’ briefs, applicants bear the 

burden 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 briefing and argument 

in the lower courts and, potentially, again in this Court. 

Section 7607(d)(8)’s harmless-error rule is one such important 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 interdependent” 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

application of the Clean Air Act’s procedural bar and

harmless-error rule are tentative. But even a tentative adverse conclusion can undermine applicants’ likelihood of 

success. And applicants, to prevail, must run the table; 

they face the daunting task of surmounting all of these significant 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-
24 OHIO v. EPA

BARRETT, J., dissenting

tiorari. Ante, at 19. Given the number of companies included and the timelines for review, the Court’s injunction 

leaves large swaths of upwind States free to keep contributing significantly to their downwind neighbors’ ozone 

problems for the next several years—even though the temporarily stayed SIP disapprovals may all be upheld and the 

FIP may yet cover all the original States. The Court justifies this decision based on an alleged procedural error that 

likely had no impact on the plan. So its theory would require EPA only to confirm what we already know: EPA 

would have promulgated 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 benefit of full briefing 

and reasoned lower court opinions. See Does 1–3, 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.