The A.E.P. coal burning plant in Conesville, Ohio, (Michael S. Williamson/Washington Post)

The Supreme Court said Tuesday that it would decide whether the Environmental Protection Agency improperly adopted regulations requiring power plants to reduce mercury and other toxins from their emissions without first considering how much it would cost.

The regulations have been in the works for years — begun under the Clinton administration, derailed when George W. Bush became president and renewed after President Obama was elected.

The new rules — regulating the emissions of mercury and other pollutants, such as arsenic, acid gas, nickel, selenium and cyanide — are aimed at coal- and oil-fired power plants and were finalized at the end of 2011.

But about two dozen states and the industry have been fighting them in court. A panel of the U.S. Court of Appeals for the D.C. Circuit upheld the rules early this year, but Judge Brett Kavanaugh, an influential conservative jurist, dissented in part.

“In my view, it is unreasonable for EPA to exclude consideration of costs in determining that it is ‘appropriate’ to impose significant new regulations on electric utilities,” Kavanaugh wrote, emphasizing the law’s language authorizing EPA intervention when “appropriate and necessary.”

This Oct. 3, 2014 file photo shows the Supreme Court in Washington. (Susan Walsh/AP)

“To be sure, EPA could conclude the benefits outweigh the costs. But the problem here is that EPA did not even consider the costs. And the costs are huge . . . $9.6 billion a year, by EPA’s own calculation,” Kavanaugh wrote.

In accepting the case, the Supreme Court said it would consider “whether the Environmental Protection Agency unreasonably refused to consider costs in determining whether it is appropriate to regulate hazardous air pollutants emitted by electric utilities.”

Solicitor General Donald B. Verrilli Jr., representing the Obama administration, had asked the justices not to take the case but rather to leave in place the appeals court decision upholding the regulations.

Verrilli said the appeals court found that the EPA had followed Congress’s intent in the Clean Air Act “to consider costs when establishing the appropriate level of any regulation of hazardous air pollutant emissions from power plants, but not when deciding whether to regulate those plants in the first place.”

In fact, the EPA says the final regulations will create huge benefits for the public.

The government’s brief said the agency believes that once the standards are fully implemented in 2016, they will “yield total annual monetized benefits of between $37 billion and $90 billion (measured in 2007 dollars), as compared to annual costs of $9.6 billion.”

The reduction of mercury is particularly important, the government said. Studies link it to cancer in adults and neurological disorders in young children. Emissions of mercury from utilities fall on land and water, contaminating humans and fish.

The government’s brief said the new regulations would prevent “up to 11,000 premature deaths each year” and “IQ loss to children whose mothers consume noncommercial freshwater fish caught by recreational anglers in modeled watersheds during pregnancy.”

The states and industry groups dispute the value of the new regulations and said the agency’s decision that it did not initially have to consider the costs of regulations set a dangerous precedent.

“In this case, EPA’s decision to ignore entirely the costs of its decision has led to one of the most far-reaching and costly rules — if not the most costly rule — ever imposed” under the Clean Air Act, according to the industry’s petition.

States and industry on one side and the federal government on the other have increasingly battled over the powers of the Obama administration’s EPA, with the agency emerging victorious in the most recent cases.

The court consolidated three cases to hear in the spring and decide by next June: Michigan v. EPA; Utility Air Regulatory Group v. EPA and National Mining Association v. EPA.