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 Neigh- bor 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 re- quirements” 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 trig- gering 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. Environ- mental 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 ap- plications for stay. 2 OHIO v. EPA

Syllabus

all those States. EPA designed its proposed FIP based on which emis- sions-control measures would maximize cost-effectiveness in improv- ing ozone levels downwind and on the assumption the FIP would apply to all covered States. Commenters warned that the proposed SIP dis- approvals were flawed and that a failure to achieve all the SIP disap- provals as EPA envisioned would mean that EPA would need to reas- sess 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 an- nounced that its plan was severable: Should any jurisdiction drop out, the plan would continue to apply unchanged to the remaining jurisdic- tions. Ongoing litigation over the SIP disapprovals soon vindicated at least some of the commenters’ concerns. Courts stayed 12 of the SIP disapprovals, which meant EPA could not apply its FIP to those States. A number of the remaining States and industry groups challenged the FIP in the D. C. Circuit. They argued that EPA’s decision to apply the FIP after so many other States had dropped out was “arbitrary” or “capricious,” and they asked the court to stay any effort to enforce the FIP against them while their appeal unfolded. The D. C. Circuit de- nie