In a major setback for the Obama administration, the U.S. Supreme Court ruled on June 29 that the U.S. Environmental Protection Agency did not consider the costs firms would face to comply with a 2013 federal rule aimed at reducing emissions of mercury and other air pollutants.

The high court is sending the Mercury and Air Toxics Standards rule (MATS) back to the U.S. Court of Appeals for the District of Columbia and to EPA to evaluate compliance costs.

Although EPA critics such as Senate Majority Leader Mitch McConnell (R-Ky.) were quick to praise the 5-4 opinion written by Associate Justice Antonin Scalia, environmental advocates say the decision has a limited scope and does not undermine the agency's authority to regulate pollutants.

They add that the rule remains in place, pending an EPA analysis of the compliance costs.

Construction contractors typically install equipment in powerplants to help reduce mercury, sulfur dioxide and other air pollutants. EPA officials have said that several other air quality regulations put in place over the past decade have significantly reduced pollution and improved public health.

Lisa Garcia, Earthjustice’s vice president of litigation for healthy communities, said in a statement: “The Supreme Court’s decision does not change the importance of EPA’s role in protecting our families and communities from toxic air pollution. The Court gave EPA the ability to finalize these critical public health protections once and for all. Now, EPA must act quickly. Thousands of lives are at stake. Further delay is not an option.”

William Becker,  National Association of Clean Air Agencies executive director, said that more than two-thirds of powerplants already had complied with the MATS rule by the April 2015 deadline.

He added, “It is disappointing that the Supreme Court has reversed the agency’s actions. The successful implementation of the rule underscores the technological feasibility and affordability of the MATS rule.”

Rule opponents say that the Supreme Court's decision will help curb what they view as EPA overreach.

The ruling “represents a cutting rebuke to the administration’s callous attitude," McConnell said in a statement. "While much of the damage of this regulation has already been done, the ruling serves as a critical reminder to every governor contemplating the administration’s demands to impose more regressive—and likely illegal—regulations that promise even more middle-class pain.”

Scott Segal, director of the Electric Reliability Coordinating Council, a group of powerplants working on Clean Air Act implementation, said the ruling could be an indication of how the court might view upcoming legal challenges to the administration’s rules to limit greenhouse gas emissions from powerplants.

“The court clearly had cautionary words for an agency that would proceed without due consideration of cost.”  Because powerplants already are heavily regulated and play a significant role in the economy, EPA should consider “cost and reliability in a thorough manner before it finalizes its carbon rules,” he said.

In his opinion, Scalia wrote that EPA "must consider cost—including, most importantly, cost of compliance—before deciding whether regulation is appropriate and necessary.”

However, he said that the agency does not need to conduct a “formal cost-benefit analysis in which each advantage and disadvantage is assigned a monetary value. It will be up to the agency…how to account for cost.”

While some opponents of the administration’s upcoming Clean Power Plan could try to use this ruling as ammunition to challenge that regulation when it is released this summer, Elizabeth Ouzts, a spokeperson for Environment America, says that “legal experts are confident that the ruling will not affect the Clean Power Plan.”

That is because the EPA’s authority for the carbon emissions rule arises out of a different section of the Clean Air Act than the program involving the standards for hazardous air pollutants, she says.

Moreover, Ouzts adds, “the authority the EPA is relying on for the Clean Power Plan expressly provides for the consideration of costs.”