A coal fired energy plant in Ohio. (Michael S. Williamson/The Washington Post)

The Supreme Court on Monday left intact a key Obama administration environmental regulation, refusing to take up an appeal from 20 states to block rules that limit the emissions of mercury and other harmful pollutants that are byproducts of burning coal.

The high court’s decision leaves in place a lower-court ruling that found that the regulations, put in place several years ago by the Environmental Protection Agency, could remain in effect while the agency revised the way it had calculated the potential industry compliance costs. The EPA finalized its updated cost analysis in April.

In a statement Monday, the EPA praised the court’s decision not to review the case, saying the mercury standards are an important part of a broader effort to ensure clean air for Americans.

“These practical and achievable standards cut harmful pollution from power plants, saving thousands of lives each year and preventing heart and asthma attacks. Power plants are the largest source of mercury in the United States,” the agency said. “Mercury is a neurotoxin that can damage children’s developing nervous systems, reducing their ability to think and learn. All told, for every dollar spent to make these cuts, the public is receiving up to $9 in health benefits.”

In March, a month after hobbling the Clean Power Plan — the Obama administration’s signature regulation on climate change — Chief Justice John G. Roberts Jr. rejected a separate request to stay the Mercury and Air Toxic Standards rule. More than 20 states had joined a lawsuit opposing the MATS rule, arguing that the controversial pollution controls mandated by the regulation are too expensive relative to the health benefits. The White House and environmental groups argued that the rule was not only economically sound, but also an important public health measure.

Coal-burning power plants are the nation’s largest single source of man-made mercury. Decades of mercury pollution from coal-burning also has contributed to elevated levels of the toxin in fish.

In April, the EPA issued an updated analysis of the estimated costs and benefits of the regulations, arguing that the cost for the industry to comply would amount to a fraction of annual revenue and probably would not lead to steep increases in customer bills. As the fight over the MATS rule has worked its way through the courts in recent years, many utilities have already complied with the new requirements.

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