Air pollution that travels by wind poses a serious threat both to downwind states’ ability to meet air quality standards and to the health and safety of downwind states’ residents.  Pollution from  factories in states like Illinois, Indiana, and Michigan may wind up causing serious health problems in states like Connecticut, Delaware, and Maryland.

Congress responded to this problem by including in the Clean Air Act a “good neighbor” provision, which requires upwind states to implement policies that prevent them from emitting pollution that “significantly contributes” to air quality problems in downwind states.

To make sure that states follow their statutory obligations under the “good neighbor” provision, the EPA promulgated the Transport Rule.  This rule identified upwind states whose pollution had “significantly contributed” to downwind states’ air quality problems, and which had not done enough about it to comply with the “good neighbor” provision.  Under the Transport Rule, the EPA could then put forward a federal implementation plan for those states to bring them into compliance.   To do so, as explained by SCOTUSblog, the EPA used a cost-based formula that apportioned an “‘upwind’ state’s duty to control emissions from power plants within its own borders according to how feasibly it could reduce the cross-state impact.”  Several upwind states and various industry groups are challenging the rule as invalid under the Clean Air Act; they argue that the statute only permits the EPA to force a state to reduce its “significant contributions” to dirtying downwind states’ air.  But, they say the EPA has gone farther, requiring states to reduce more emissions than those that constitute their “significant contributions.”

During oral argument—which the Court extended to 90 minutes rather than the usual 60—much time was spent discussing how much discretion the EPA is allowed in defining and calculating “significantly contributes” and in devising an enforcement plan.  Several of the justices—with the exception of Justice Scalia—continued to refer back to the actual text of the Clean Air Act and seemed to find that the language invited the EPA to use its judgment in achieving the Act’s goals of advancing downwind states’ ability to meet air quality standards.  Justice Alito recused himself from the case.