On June 29, 2015, the United States Supreme Court held that the U.S. Environmental Protection Agency (EPA) acted unreasonably when it deemed cost irrelevant to the decision to regulate emissions of mercury and other hazardous air pollutants (HAPs) from power plants under the Mercury and Air Toxics Standards (MATS).  Michigan v. E.P.A., 135 S. Ct. 2699 (2015). The MATS, which is now on remand to EPA, is one of the Obama administration’s signature environmental rulemakings. Section 112 of the Clean Air Act (CAA) requires EPA to regulate emissions of HAPs from “major sources”—a stationary source or group of stationary sources that emit or have the potential to emit 10 tons per year or more of a HAP or 25 tons per year or more…