This section contains the nature of the case and procedural background.

The Clean Air Act regulates air quality through a federal-state collaboration. Periodically, D sets standards for common air pollutants, as necessary to “protect the public health.” Once D 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. States decide how to measure ambient air quality, pick “emission limitations and other control measures,” and provide for the enforcement of their prescribed measures. Under the Act’s “Good Neighbor Provision, States must design these plans to account for pollution across state borders. D has “no authority to question the wisdom of a State’s choices of emission limitations so long as a SIP satisfies the “applicable requirements” of the Act, including the Good Neighbor Provision. If a SIP falls short, D must issue a Federal Implementation Plan, or FIP, for the noncompliant State-that is, “unless” the State corrects the deficiencies in its SIP first. In 2015 D revised its air-quality standards for ozone from 75 to 70 parts per billion. States submitted SIPs. For over two years, D did not act on the SIPs it received. In February 2022, D 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. A few months later, it disapproved four more SIPs. Pursuant to the Act, D issued its proposed SIP disapprovals for public comment before finalizing them. D then proposed a single FIP to bind all 23 States. D identified various emissions-control measures and, using nationwide data, calculated how much each typically costs to reduce a ton of nitrogen-oxide emissions. D then estimated how much, on average, ozone levels would fall in downwind States with the adoption of each measure. D sought measures that would “maximize costeffectiveness” in achieving “downwind ozone air quality improvements.” The flaw in what D proposed was the assumption that the FIP would apply to all covered States. But what happens if some or many of those States are not covered? Two circuits issued stays of EPA’s SIP denials for four States. D ignored the comments and developments and issued its final FIP. D adopted a severability provision stating that, should any jurisdiction drop out, its rule would “continue to be implemented as to any remaining jurisdictions.” D 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. Eventually, D recognized that it could not apply its FIP to 12 of the 23 original States. Those 12 States accounted for over 70 percent of the emissions D had planned to address through its FIP. Some of the remaining States and industry groups challenged the remnants of the FIP in court as “arbitrary” or “capricious.” Ps sought a stay to enforce the FIP against them while their appeal unfolded. The court denied relief, and Ps appealed.

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