[9][fn 1] At the same time, Congress developed a procedure to determine the applicability of the Hazardous Air Pollutants Program to power plants that generated electricity from fossil fuels.

[20] In 2014, the Supreme Court of the United States granted certiorari to resolve the question of whether the EPA must consider costs when regulating power plants under the Clean Air Act.

[21] Writing for a 5–4 majority, Justice Antonin Scalia held that the EPA interpreted the Clean Air Act unreasonably when it decided that it should not consider costs when regulating power plants.

[24] Looking at the language of the Clean Air Act, Justice Scalia concluded that when "[r]ead naturally in the present context, the phrase 'appropriate and necessary' requires at least some attention to cost.

"[27] Justice Clarence Thomas wrote a separate concurring opinion urging the Court to re-evaluate the extent to which it defers to agency interpretations of statutes.

[4] However, some commentators have criticized Justice Scalia's decision to not consider health benefits in his opinion; one analyst wrote, "hundreds of thousands of people might have lived longer if regulations on mercury and other coal pollutants had not been tied up in court battles.

[33][35] Some commentators have suggested that Michigan v. EPA may foreshadow a retreat from the Court's prior administrative law jurisprudence, which generally gave deference to an agency's reasonable interpretation of an ambiguous statute.