Whether challenges to the Environmental Protection Agency's (EPA) disapprovals of State Implementation Plans (SIPs) must be brought in the U.S. Court of Appeals for the D. C. Circuit or a regional Circuit under 42 U. S. C. § 7607(b)(1). Specifically, the Court must determine if EPA's aggregation of these disapprovals into one omnibus rule constitutes a "nationally applicable" action. Additionally, the Court must decide if such actions fall under the "nationwide scope or effect" exception that would mandate D. C. Circuit review.

• Each individual SIP disapproval constitutes its own discrete "action" rather than a single omnibus rule.
• These disapprovals are prototypical "locally or regionally applicable" actions because they apply only to the specific state that proposed the plan.
• The EPA's decisions were driven by state-specific factual analyses, not by determinations of nationwide scope or effect.
• Consequently, venue is proper in a regional Circuit under 42 U. S. C. § 7607(b)(1).

• The omnibus rule is a single "nationally applicable" action because it covers 21 states and applies a uniform statutory interpretation and common analytical methods.
• If the actions are considered locally or regionally applicable, they still belong in the D. C. Circuit under the "nationwide scope or effect" exception of 42 U. S. C. § 7607(b)(1).
• The use of a "nationally consistent 4-step . . . framework" and the goal of avoiding inconsistent results constitute determinations of nationwide scope or effect.
• Respondent sought to have all challenges transferred to the D. C. Circuit.

• The Court applied a two-step inquiry from *EPA v. Calumet Shreveport Refining, L.L.C.*: first identifying the relevant "action" and its applicability, then determining if the "nationwide scope or effect" exception applies.
• An "action" is defined as a "particular exercis[e] of EPA authority undertaken pursuant to [a] particular CAA provisio[n]," determined by the statute rather than how EPA packages the decision (*Calumet*).
• Because 42 U. S. C. § 7607(b)(1) explicitly lists SIP approvals as locally or regionally applicable, individual SIP disapprovals—which are "opposite sides of the same coin" under 42 U. S. C. § 7410—are also discrete, locally applicable actions.
• The Court rejected the Tenth Circuit's view that an action is whatever EPA has "chosen to issue," noting that aggregation in the Federal Register has no significance for venue (*Calumet*).
• An action only "applies" nationally if it has binding effect throughout the country on its face; EPA's use of uniform analytical methods goes to reasoning, not formal geographical scope (*Calumet*).
• For the "nationwide scope or effect" exception to apply, a justification of nationwide breadth must be the "primary explanation for and driver of EPA's action," and cannot apply if EPA relied significantly on "intensely factual" considerations (*Calumet*).
• The Court found that EPA's disapprovals were based on state-specific facts—such as Oklahoma's emissions in Texas and Utah's in Colorado—resulting in a "unique mixture of reasons" for each state.
• While EPA's 2016-based modeling and 1% contribution threshold are determinations of nationwide scope, they served only as "heuristics" or "screening" tools rather than the primary drivers of the disapprovals.