The U.S. Court of Appeals for the 8th Circuit has ruled that the EPA may pursue injunctive relief against emission sources for past violations of new source review (NSR) permit requirements under the Clean Air Act. In the suit, the EPA was pursuing Ameren, Missouri, a coal-fired power plant, for the company’s failure to obtain a prevention of significant deterioration (PSD) permit for “major” modifications to its Rush Island power plant. The EPA argued that Ameren should have conducted a PSD permit review, and based on projected emission increases, that the facility should have applied the “best available control technology (BACT)” which is the most stringent level of emissions control available. The case was a test of the EPA’s ability to obtain injunctive relief for past violations of NSR rules. In a unanimous opinion, the Appeals Court agreed that the plain text of the Clean Air Act authorized the injunctive relief and that the EPA could require retroactive application of PSD permitting and BACT technology.