But the Supreme Court in 2015 ruled that the EPA improperly failed to consider the costs of regulating power plants' emissions of hazardous air pollutants. Several years later, in 2023, the EPA determined after considering the costs that regulation of power plants is "appropriate and necessary," a standard set by the Clean Air Act.

The U.S. Supreme Court declined on Friday to put on hold new federal air pollution rules from President Joe Biden's administration to tighten limits on mercury and methane, acting in challenges brought by a group of states - most of them Republican-led - and industry groups.

The Supreme Court on Friday let stand Biden administration rules that would cut emissions of the planet-warming gas methane as well as mercury in a rare win for an environmental regulation pending before the conservative high court.