In 2012, the Environmental Protection Agency (EPA) promulgated the Mercury and Air Toxics Standards (MATS) to regulate the emissions of mercury and other air toxics at electricity generating units (EPA 2012). The agency argued that this rule-making is “appropriate and necessary” because: (1) electricity generating units are the largest domestic source of mercury emissions, and they emit other hazardous air pollutants; (2) these emissions pose a hazard to public health; and (3) effective emission controls are available. In 2015, the Supreme Court ruled that EPA must consider costs in making an appropriate and necessary finding and remanded the regulatory finding to EPA, but permitted implementation of the regulation to proceed (Michigan v. EPA). Coal- and oil-fired power plants began demonstrating compliance with the MATS in April 2016. In the same month, the agency responded to the Supreme Court decision by issuing a Supplemental Cost Finding (EPA 2016) based on several cost metrics, including the rule’s original benefit-cost analysis showing the benefits far exceeding the costs. The 2016 Supplemental Cost Finding reached the same conclusion as in 2012, namely that regulating mercury and air toxics at electricity generating units is appropriate and necessary.
Aldy, Joseph, Matthew Kotchen, Mary Evans, Meredith Fowlie, Arik Levinson, and Karen Palmer. "Report on the Proposed Changes to the Federal Mercury and Air Toxics Standards." External Environmental Economics Advisory Committee, December 2019.