The EPA completed its review of the Mercury and Air Toxics Standards (MATS) Supplemental Cost Finding (81 FR 24420, April 25, 2016). On February 7, 2019 (84 FR 2670), EPA proposed a revised response to the U.S. Supreme Court decision in Michigan v. EPA, which held that the EPA erred by not considering cost in its determination that regulation of hazardous air pollutant (HAP) emissions from coal- and oil-fired electric utility steam generating units (EGUs) is appropriate and necessary under CAA section 112. After considering the cost of compliance relative to the HAP benefits of regulation, the EPA proposed to find that it is not appropriate and necessary to regulate HAP emissions from coal- and oil-fired EGUs, thereby reversing the Agency’s prior conclusion under CAA section 112(n)(1)(A) and correcting flaws in the Agency’s prior response to Michigan v. EPA. The EPA also proposed that finalizing the new response to Michigan v. EPA will not remove the Coal- and Oil-Fired EGU source category from the CAA section 112(c) list of sources that must be regulated under CAA section 112(d) and will not affect the existing CAA section 112(d) emissions standards that regulate HAP emissions from coal- and oil-fired EGUs. The EPA also proposed the results of the residual risk and technology review (RTR) of MATS that the Agency is required to conduct in accordance with CAA section 112. Based on the results of the reviews, the EPA proposed that no revisions to MATS are warranted. After consideration of public comments, the EPA will issue an action that finalizes decisions and revisions for the rule.