Responding to a Supreme Court mandate, the Environmental Protection Agency (“EPA” or “the Agency”) yesterday published a final supplemental finding that it is necessary and appropriate to regulate hazardous air pollutants emitted from coal- and oil-fired power plants under Section 112 of the Clean Air Act. 81 Fed. Reg. 24,420.

The finding was necessitated by a June 29, 2015 Supreme Court ruling that overturned a 2000 determination by the EPA that it was appropriate and necessary to regulate air toxics, including mercury, from these sources. Michigan v. EPA, 135 S. Ct. 2699 (2015). That determination led the EPA to adopt standards regulating air toxics emissions from power plants, known as the mercury and air toxics (“MATS”) rule, in 2012.

The Supreme Court found the EPA was required to consider costs when making its 2000 determination under CAA § 112(n)(1)(A) that regulations were appropriate, a position the EPA initially rejected in making its determination. The Court remanded the rule to the D.C. Circuit, which elected not to vacate the rule while the EPA addressed its flaws. Accordingly, the MATS rule has remained in effect pending the EPA’s new determination. Notwithstanding, the EPA’s latest determination is likely to face further challenges based on the methodology employed by the Agency and its reliance on co-benefits associated with reductions from pollutants not directly regulated by the rule in this analysis.

The EPA’s decision tends to underscore arguments raised by state and industry groups in petitions requesting that the Supreme Court stay to the Agency’s Clean Power Plan, which the Court granted February 9. Petitioners argued that notwithstanding the Supreme Court’s ruling in Michigan v. EPA, the Agency acted as though the MATS rule was a fait accompli requiring no need to revisit the regulation.

Potential petitioners have sixty (60) days to challenge the EPA’s supplemental determination under the Clean Air Act.