Mercury and Air Toxics Standards (MATS)

The Environmental & Energy Law Program is tracking the environmental regulatory rollbacks of the Trump administration. Click here for the list of rules we are following. If you’re a reporter and would like to speak with an expert on this rule please email us.

Why it Matters

Mercury and Air Toxics Standards (MATS) limit the amount of mercury and other toxic emissions from power plants. Mercury is a powerful neurotoxin that ends up in the water and soil, and concentrates up the food chain, especially in fish. It is particularly dangerous for pregnant women and young children. If the Environmental Protection Agency (EPA) were to reconsider or rescind the rule, power plants could stop operating with installed pollution controls.

Current Status

The Mercury and Air Toxics Standards were signed in December 2011, and coal- and oil-fired power plants were required to make reductions to achieve those standards by the Spring 2016. Industry challenged the rule, but it was upheld by the D.C. Circuit in 2014 and then went up to the Supreme Court.

In 2015, the Supreme Court also left the rule intact — but sent one specific issue, the justification for creating the rule in the first place, back to the D.C. Circuit, who sent back to EPA to ensure they sufficiently considered the costs of compliance with the rule. The Obama EPA re-considered costs and found them to be justified in light of the enormous anticipated health benefits– this “Supplemental Finding” was released in April 2016. Industry challenged the supplemental finding at the D.C. Circuit.

In Spring 2017, the Trump EPA asked the D.C. Circuit to delay that case and litigation has been suspended indefinitely.


On February 16, 2012 EPA issued the final Mercury and Air Toxics Standards. Industry groups and several states challenged the rule. —Michigan v. EPA, No. 14-46 (lead case) (D.C. Cir.)

On June 29, 2015 the Supreme Court remanded MATS to the D.C. Circuit to assess how EPA should proceed with additional cost-benefit analyses. The D.C. Circuit sent the MATS back to EPA to determine if the standards are “appropriate and necessary.”

On April 25, 2016 EPA published its Supplemental Finding  on the costs and benefits of MATS. Within hours, opponents sued. –Murray Energy v. EPA, No. 16-1127 (D.C. Cir.)

Trump Era

On January 31, 2017 petitioners sought a 45-day extension in the briefing schedule. The D.C. Circuit denied the extension on February 9, 2017.

On April 18, 2017 EPA asked the Court to delay oral arguments, scheduled for May 18, 2017.

On April 27, 2017 the D.C. Circuit removed the argument from its calendar, suspending the case indefinitely, and directed EPA to file 90-day status reports.

On July 10, 2018 the Edison Electric Institute and several other industry trade groups and unions sent a letter to EPA air chief Bill Wehrum asking him to leave the MATS in place and finish the residual risk and technology review “as expeditiously as possible.” The letter also emphasized that “…all covered plants have implemented the regulations and that pollution controls — where needed — are installed and operating,”

On August 24, 2018 Senators Tom Carper (D-Del.) and Lamar Alexander (R-Tenn.) sent a letter to acting EPA Administrator Andrew Wheeler urging EPA to retain MATS. The senators explained, “Keeping the current rule in place will provide much-needed certainty for the electric power industry and help protect the health of all Americans.”

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