How To Interpret The U.S. Supreme Court's MATS Ruling

On Monday, June 29, 2015, by a vote of 5-4 with Justice Kennedy in the majority, the U.S. Supreme Court (Court) invalidated a rule issued by the U.S. Environmental Protection Agency (EPA) limiting power plant emissions of mercury and air toxics (Mercury and Air Toxics Standards or MATS). Writing for the majority, Justice Scalia found unlawful EPA's failure to consider costs at the time EPA determined that regulating these emissions is appropriate and necessary—the first step required under a provision of the Clean Air Act (the Act) unique to hazardous air pollutants (HAPs) emitted by coal- and oil-fired power plants. Michigan v. EPA, No. 14-46, slip op. at 3 (June 29, 2015). While in prior decisions the Court has sometimes found EPA's injection of cost considerations to be unlawful, the decision does not necessarily create any new law. It does suggest, however, a new willingness to scrutinize cost-benefit analyses, which have been required by the last six presidents and