On March 18, 2016, a group of 20 states led by Michigan appealed the U.S. Court of Appeals for the D.C. Circuit’s decision to remand the Mercury and Air Toxics Standards (MATS) to EPA without vacatur. The petition for a writ of certiorari filed with the Supreme Court of the United States presented the following question: “When an agency promulgates a rule without any statutory authority, may a reviewing court leave the unlawful rule in place?”

The states argue that because the Supreme Court held in Michigan v. EPA that EPA was required to consider costs before regulating hazardous air pollutant emissions from power plants under §112 of the Clean Air Act, the D.C. Circuit erred by leaving MATS in place while EPA conducts the required “appropriate and necessary” determination. Instead, the states contend, the D.C. Circuit was required to vacate MATS as an unauthorized agency action.

The Supreme Court recently denied the states’ request for an emergency stay of MATS during the D.C. Circuit’s remand of the rule. The Supreme Court is expected to act on the states’ cert petition before the end of the term. Meanwhile, MATS remains in effect, with no change in the rule’s compliance obligations.