OREANDA-NEWS. The US Supreme Court today ruled the US Environmental Protection Agency (EPA) must consider costs when deciding to regulate mercury and air toxics emissions from power plants.

The 5-4 ruling in the case, Michigan v. EPA, reversed a 2014 DC Circuit Court of Appeals decision upholding the mercury and air toxics standards. The Supreme Court said EPA acted unlawfully by excluding costs when it made the determination that it was "appropriate and necessary" to regulate mercury from power plants.

"EPA must consider cost — including cost of compliance — before deciding whether regulation is appropriate and necessary. It will be up to the agency to decide (as always, within the limits of reasonable interpretation) how to account for cost," the court said in a decision authored by justice Antonin Scalia.

EPA finalized the mercury standards in February 2012. The rule required coal- and oil-fired power plants to reduce their mercury emissions by 90pc by 16 April. Most utilities have already complied with the rule by installing new pollution controls or retiring older units.

Twenty-one states, the National Mining Association and the Utility Air Regulatory Group appealed the case to the Supreme Court.

The states and industry said that while the EPA estimated the rule would cost $9.6bn/yr to implement, they believed it would generate benefits of only $4-6mn/yr. EPA had said the benefits would be in the range of $37bn-$90bn/yr in part because of co-benefits from reducing emissions of other pollutants.

The court sent the case back to the DC Circuit Court.