EPA v. EME Homer City Generation, L.P.

The Clean Air Act's Good Neighbor Provision requires state implementation plans (SIPs) to prevent emissions that significantly contribute to another state's noncompliance with national air quality standards; the EPA's 2011 Transport Rule found a state violated this provision if it produced at least 1 percent of another state's pollution and that pollution could be cost-effectively eliminated, and simultaneously found the violating states' existing SIPs inadequate, prompting the EPA to craft federal implementation plans (FIPs) for them. EME Homer City (plaintiff) challenged the rule, and the D.C. Circuit invalidated it for requiring states to solve interstate pollution without first receiving EPA guidance on specific reduction targets.