The name of the law at issue before the Supreme Court on Wednesday is the Clean Air Act. It is not the Coal Industry Protection Act, despite what that industry’s advocates seem to want the justices to believe.

Congress passed the legislation in 1970 and substantially strengthened it in 1990 to safeguard human health from air pollution generated by power plants, vehicles, incinerators and other sources.

One of the most toxic of these pollutants is mercury, a heavy metal that accumulates in waterways and the fish Americans eat. While mercury is particularly dangerous to the vulnerable, developing brains and nervous systems of young children and fetuses, the Environmental Protection Agency estimates that improved air-quality standards prevent the premature deaths of as many as 11,000 Americans each year from exposure to mercury and other toxic air pollutants.

In 2012, the agency issued a rule ordering coal-fired power plants, which are far and away the single biggest source of these emissions, to adopt technology to reduce them. The coal industry sued the government for the same reason it has countless times over the decades: Cleaning up pollution costs money. Business owners and other industry backers argue that the law requires the E.P.A. to weigh those costs against any potential health benefits of a regulation.