On June 29, 2015, the U.S. Supreme Court reversed the U.S. Court of Appeals for the D.C. Circuit and invalidated U.S. EPA's 2012 regulation of mercury and other hazardous air pollutant ("HAP") emissions from coal and oil-fired power plants. Michigan v. Environmental Protection Agency, No. 14-46.
On February 16, 2012, EPA issued national emission standards for mercury and other HAPs from fossil fuel-fired powered electric utility steam generating units (a/k/a "power plants"). 77 Fed. Reg. 9304. The regulations are also known as the Mercury and Air Toxics Standards ("MATS"). The MATS were issued pursuant to the Clean Air Act Amendments of 1990, which required EPA to study "the hazards to public health reasonably anticipated to occur as a result of emissions by [power plants] of [HAPs] after imposition of [other] requirements [of the CAA]." 42 U.S.C. § 7412(n)(1)(A). Congress then required EPA to regulate HAPs from power plants if the EPA "finds such regulation is appropriate and necessary after considering the results of the study…." Id.
EPA studied the hazards from power plant emissions in 2000 and again in 2011, and determined that further regulation of power plants was "appropriate and necessary," in large part due to mercury emissions. When EPA issued the proposed emission standards for power plants, it expressly stated that it did not consider the costs of regulating HAPs when it determined that such regulation was "necessary and appropriate." 77 Fed. Reg. 9326-27. Michigan, other states, and members of industry contested the final MATS,