The Supreme Court ruled Monday that the Environmental Protection Agency acted “unreasonably” when it failed to consider costs before deciding to regulate mercury and other toxic emissions from power plants under the Clean Air Act.

The court’s 5-4 ruling did not void EPA’s authority to regulate the emissions but will require the agency to rewrite the Mercury and Air Toxics Standards (MATS) with a consideration of costs at the beginning of the process. It remanded the case to the D.C. Circuit Court of Appeals for further review.

Muted Impact

The ruling in Michigan v. Environmental Protection Agency is not expected to affect the number of coal-fired plant retirements. Industry analysts say about two-thirds of the nation’s 460 coal plants are already in compliance and investments in emission controls have already been made. (See MATS Challenge Too Late for Targeted Coal Plants.)

“EPA is disappointed that the court did not uphold the rule, but this rule was issued more than three years ago, investments have been made and most plants are already well on their way to compliance,” EPA spokeswoman Melissa Harrison said in a statement.

“Because of the stricter air regulations that have been in place in New England for years, most plants would not have been affected by this rule,” said ISO-NE spokeswoman Marcia Blomberg. “And further, the economics of low-priced natural gas have driven many of the region’s older fossil-fired units to retirement, so we expect there will be limited impact from this ruling.”

NYISO is analyzing the decision, spokesman David Flanagan said.

Coal plants also are under pressure from EPA’s cross-state pollution rule and the carbon emission rule expected later this summer. Even without MATS, EPA Administrator Gina McCarthy told HBO’s “Real Time with Bill Maher” on Friday, “we’re still going to get at the toxic pollution from these facilities.”

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