By a 5-4 ruling the Supreme Court "halted further implementation of the 2011 Mercury and Air Toxics Standards rule, EPA’s landmark regulation that required electric utilities to reduce mercury pollution, linked in multiple studies to respiratory illnesses as well as birth defects and developmental problems in children," Warrick and Barnes write. "The decision’s ultimate impact on pollution controls was uncertain."
MATS was "one of the most ambitious environmental policies of President Obama's first term," ex-Post writer Brad Plumer writes for Vox. "The mercury rule will remain in effect for now, but the EPA will likely need to review and revise it in the months ahead," considering industry costs. But whatever happens, the practical effect could be limited, because utilities have already been shutting or converting coal-fired plants to comply with the rule, Plumer writes. "While a handful of slated-to-retire plants might get a reprieve if the regulation gets nixed, most of the investments in pollution control have already occurred."
The decision could have the most impact on 22 plants that received six-week extensions for complying with the rule because they had contractual obligations to provide power from them. Eric Wolff of SNL Financial has a detailed analysis of that, along with this map showing locations of plants that received various types of extensions.
reports for the Lexington Herald-Leader. West Virginia officials also praised the decision. Bill Raney, president of the West Virginia Coal Association, "said in a statement that the decision 'is an important first step in reigning in a clearly out-of-control, bureaucratic agency,'" Ken Ward reports for the Charleston Gazette.