Duke Energy Corporation runs 30 coal-fired electric generating units at eight plants in North and South Carolina. The units were placed in service between 1940 and 1975, and each includes a boiler...

Duke Energy Corporation runs 30 coal-fired electric generating units at eight plants in North and South Carolina. The units were placed in service between 1940 and 1975, and each includes a boiler containing thousands of steel tubes arranged in sets. Between 1988 and 2000, Duke replaced or redesigned 29 tube assemblies in order to extend the life of the units and allow them to run longer each day. The United States sued, claiming, among other things, that Duke violated the Prevention of Significant Deterioration (PSD) provisions by doing this work without permits. Environmental Defense, North Carolina Sierra Club, and North Carolina Public Interest Research Group Citizen Lobby/Education Fund intervened as plaintiffs and filed a complaint charging similar violations. Duke moved for summary judgment, one of its positions being that none of the projects was a “major modification”requiring a PSD permit because none increased hourly rates of emissions. The district court agreed with Duke’s reading of the 1980 PSD regulations. The Court of Appeals for the Fourth Circuit affirmed. How did the Supreme Court rule on appeal? Environmental Defense v. Duke Energy Corp., 127 S. Ct. 1423 (2007).