Smoke and steam billow from cooling towers and smokestacks at the Bruce Mansfield power plant near Shippingport, Pa. (Joby Warrick/The Washington Post)

The Environmental Protection Agency on Tuesday asked a federal court to delay an oral argument in a challenge involving a 2012 regulation limiting the amount of mercury, lead and other airborne toxins emitted from power plants.

While the power sector has largely already complied with the rule, several companies and 15 states — including Oklahoma, which was represented by current EPA head Scott Pruitt when he was the state’s attorney general — are seeking to overturn it. The U.S. Court of Appeals for the District of Columbia Circuit was set to hear the case on May 18.

Under President Trump, the EPA has asked judges to stay multiple rules adopted under the Obama administration that target power plants, including ones regulating carbon emissions and the release of toxic metals in plants’ wastewater, as well as one limiting ozone emissions generated by fossil-fuel burning.

In all three cases, the EPA is considering rewriting the regulations.

The rule on mercury and other air toxins, known as the MATS rule, has been the subject of litigation for years. While the Supreme Court initially required EPA to do a more thorough cost-benefit analysis of the measure, it allowed the new standards to take effect in 2012. Under the rule, coal and oil-fired utilities had to install pollution controls that put them on par with the 12 percent cleanest facilities in their sector.

Power plants are the single biggest emitter of mercury, a neurotoxin that can cause brain damage, especially in young children. Over time, these emissions have built up in fish, whose elevated levels of mercury are absorbed by people who eat it.

Congress gave the EPA the authority to regulate the toxic metals that are the byproduct of burning coal — a list that also includes arsenic, nickel, selenium and cyanide — in the 1990s, but it took the agency years to settle on a standard. At the time the regulation came out, the EPA estimated it would prevent as many as 11,000 premature deaths and 4,700 heart attacks a year when fully implemented and would cost the industry $9.6 billion in compliance that year. It projects that reducing these emissions would save between $37 billion and $90 billion in 2016 in annual health costs and lost workdays.

In correspondence with other litigants in the case this week, the Trump administration said it would seek the delay “in order to give the time to fully review” the case “after the change in administration.” In its court filing Tuesday, officials argued that the delay was warranted, in part, because “agencies have inherent authority to reconsider past decisions and to revise, replace or repeal a decision to the extent permitted by law and supported by a reasoned explanation.”

Industry groups such as the National Mining Association have long opposed the rule, saying that it has been responsible for shutting down numerous coal-fired power plants and eliminating jobs.

“The Mercury and Air Toxics Standards has already had far-reaching and costly impacts not only on our industry but on many states and their citizens whose assurance of reliable electricity supply has been cast in doubt by this rule,” the group has argued. “EPA’s rule reflects a stunningly unbalanced approach to regulation. The agency decided to impose expensive standards for certain emissions that it never found posed a threat to public health.”

Earthjustice staff attorney James Pew, who has filed a brief supporting the rule, said in an interview that the effort to overturn the rule made no sense given that an agency can only deregulate when it can prove human health will not be harmed in the process.

“There’s no way it could ever make that showing when it comes to power plants,” Pew said. “It’s really hard to understand why the administration would want to do this.”

The Clean Air Task Force, whose attorneys represent litigants in the case, said it would join with other environmental groups to make sure the rule remains in place.

“The Trump Administration and EPA Administrator Pruitt are again playing fast and loose with Americans’ health,” the organization’s legal director, Ann Weeks, said in a statement. “Seeking to delay the oral argument scheduled for May 18th serves no public purpose whatsoever.”

She said the regulation’s limits on toxic air emissions from coal- and oil-fired power plants had saved thousands of lives and reduced childhood asthma attacks, among other public health benefits, and that its benefits far outweigh its costs.