Ohio v. Environmental Protection Agency, 603 U.S. 279

The Clean Air Act requires a collaborative federal-state process to regulate air quality, where states propose State Implementation Plans (SIPs) that must also consider cross-state air pollution (the "Good Neighbor Provision").
In 2015, the EPA updated ozone standards, triggering states to submit new SIPs. Many states did, frequently claiming they either did not significantly contribute to downwind problems or had no additional cost-effective control measures.
Years later, EPA proposed to disapprove over 20 SIPs, believing they failed Good Neighbor obligations, and concurrently published a uniform Federal Implementation Plan (FIP) designed to apply to all covered states.
During the comment period and litigation, some courts stayed EPA’s disapproval of several SIPs, excusing some states from the FIP. EPA responded by stating the FIP would continue for remaining states, without reevaluation.
States and industry groups challenged the FIP in the D.C. Circuit, claiming EPA’s unchanged application of the FIP, despite multiple states falling out, was arbitrary and capricious. The D.C. Circuit denied a stay; applicants sought relief from the Supreme Court.
The Supreme Court stayed enforcement of EPA’s FIP against applicants pending appeal, finding applicants likely to succeed on their arbitrary or capricious claim.

Issue	Plaintiff's Argument	Defendant's Argument	Held
Whether EPA's unchanged FIP is arbitrary/capricious after several states were excused due to stayed SIP disapprovals	EPA failed to re-justify the FIP for a subset of states, ignoring key concerns raised in comments	FIP is severable; methodology does not depend on number of states; no party specifically raised this issue in comments	EPA’s unchanged FIP likely arbitrary/capricious for failing to address this important aspect; stay granted
Whether FIP enforcement causes irreparable harm	Compliance costs are substantial and unrecoverable, creating competitive disadvantages	FIP serves critical public health and environmental needs	Both sides have compelling equities; decision turns on likelihood of success on the merits
Sufficiency of public comments on EPA’s methodology	Comments gave EPA notice FIP’s logic might change with fewer states; EPA failed to respond adequately	No comment specifically addressed this issue; any objection must be raised during notice period	Comments sufficed to trigger EPA’s duty to explain; agency failed to provide a reasoned response
Need for further administrative exhaustion (reconsideration petition)	Not required; EPA had an opportunity to respond during comment period and failed	Must first petition EPA for reconsideration before court challenge	No need for another petition; issue properly before the court

United States v. Detroit Timber & Lumber Co., 200 U.S. 321 (U.S. 1906) (syllabus not part of opinion)
FCC v. Prometheus Radio Project, 592 U.S. 414 (2021) (arbitrary/capricious standard requires reasonable explanation)
Motor Vehicle Mfrs. Assn. of United States, Inc. v. State Farm Mut. Automobile Ins. Co., 463 U.S. 29 (1983) (agency must explain rational connection between facts and choices; must not ignore important aspect)
Nken v. Holder, 556 U.S. 418 (2009) (stay factors and standards)
Train v. Natural Res. Def. Council, Inc., 421 U.S. 60 (1975) (EPA must approve SIPs if requirements met; limited oversight)