EPA Proposes to Repeal Biden-era Regulations Governing Air Toxic Emissions from Coal- and Oil-Fired Power Plants

MATS Rule Overview

EPA initially promulgated the MATS in 2011 to limit the emissions of mercury and other Hazardous Air Pollutants (“HAP”) from coal- and oil-fired EGUs. In promulgating these standards, EPA projected that power plant closures as a result of the promulgation of this rule could reach 32 or more plants with a total of 4000 megawatts of capacity. While the estimates of actual closures of coal-fired power plants as a result of the MATS rules vary, the number of closures is high with the precise role of these standards in those decisions remaining uncertain. Section 112(d)(6) of the Act obligates EPA to revise these emission standards as “necessary (taking into account developments or practices, processes and control technologies).”

On May 7, 2024, EPA amended the MATS regulations to impose even more stringent limits on these plants.³ Most notably, the amendments:

• tightened by a factor of three the filterable particulate matter (“fPM”) emission standard (from 0.030 lb/MMBtu to 0.010 lb/MMBtu);
• required all plants to use continuous emissions monitoring systems (“PM CEMS”) for determining compliance with these reduced particulate matter limits; and
• established a more stringent mercury limit for lignite coal plants, reducing the allowable emissions from 4.0 lb/TBtu to 1.2 lb/TBtu.

These standards were challenged, and an appeal remains pending before the United States Court of Appeals for the District of Columbia Circuit. That proceeding was abated in light of the change in administration and EPA’s statement that it was reviewing the rulemaking in anticipation of further revisions.⁴ Later, in April 2025, citing national energy security concerns, President Donald Trump issued a proclamation delaying the compliance deadline for these new MATS standards by two years, from July 8, 2027 to July 8, 2029.⁵ This executive action exempted 47 operators of over 60 stationary sources.⁶ That action has been challenged by various environmental non-governmental organizations in federal District Court in Washington, D.C.⁷

The newly proposed MATS rule would repeal the majority of the 2024 MATS amendments, including the three above-listed components. EPA proposes to repeal the rulemaking because the 2024 rulemaking incorrectly determined that the revisions met the statutory test for “necessary” under Section 112(d)(6). EPA asserts that because previous risk assessments show that the 2012 standards “provided an ample margin of safety” to protect public health, further tightening of standards would not result in meaningful additional health benefits.⁸ Indeed, EPA notes that the 2023 proposal accepted the validity of the finding that the pre-existing MATS rule sufficed to create a low residual risk from coal- and oil-fired EGUs. Accordingly, the further reductions in allowable emissions compelled by the 2024 amendments were not “necessary.”

The lack of health benefits from the regulation then affected the remaining analysis, including whether the proposed restrictions were cost-effective. Likewise, the $10.5 million per ton figure for removal of non-Hg HAP metals over the standard was too high, as EPA has determined in the past for other pollutants. EPA also now rejects the technical feasibility determination for the lignite coal industry because EPA relied on test results from two new boilers using a circulating fluidized bed and a quality of lignite that was not characteristic of the industry as a whole. Finally, the continuous emissions monitoring requirement costs could not be justified by the benefits of “annual transparency” and “access to data” where the existing testing requirements had functioned so well.

Key Takeaways and Next Steps

If adopted in its present form, the new MATS rule would repeal the bulk of the 2024 MATS amendments, leaving the original 2012 MATS standards in place. The PM CEMS requirement has been a principal concern for most affected plant operators since its controversial introduction last year. Its elimination, along with the other 2024 amendments, will result in total compliance cost savings of $1 billion over the 2028 to 2037 timeframe, according to EPA estimates included with the proposal.⁹ These costs savings primarily result from avoiding potential operational disruptions or the need for costly equipment upgrades. In addition to the estimated cost savings, EPA asserts that the proposed rule provides greater regulatory certainty, allowing plant operators to rely on established compliance methods (i.e., quarterly stack testing).

Notably, in the rationales provided in the preamble to the proposed repeal of the PM CEMS requirement, EPA did not discuss the prior amendments’ mismatch between the data used to develop the more stringent fPM limit (determined via discrete stack tests) and the accompanying required use of CEMS (not stack tests) to determine compliance. Yet, on this issue, courts have held that a significant difference between techniques used by an agency in arriving at a standard and the requirements presently prescribed for determining compliance raises questions about the validity of the standard.¹⁰ Supportive comments discussing this issue may be appropriate during the upcoming comment period.

Comments on the proposed rule are due August 11, 2025. Interested stakeholders should review the proposal and consider submitting comments on its technical, economic, and policy implications. EPA also announced that a virtual public hearing will be held on the proposed rule on or before July 17, 2025.

¹ 90 Fed. Reg. 25535 (June 17, 2025).

² On June 17, 2025, EPA simultaneously published another proposed rule (90 Fed. Reg. 25752) that aims to repeal greenhouse gas (“GHG”) emissions standards for fossil fuel-fired EGUs, demonstrating EPA’s emphasis on eliminating what it considers to be excessive regulation of the power sector. Our companion article on this GHG proposed rulemaking can be read here.

³ 89 Fed. Reg. 38519 (May 7, 2024).

⁴ North Dakota v. EPA, No. 24-1119 (D.C. Cir. Feb. 20, 2025) (per curiam) (order granting motion to hold case in abeyance and removing cases from oral argument calendar).

⁵ Proclamation No. 10914, 90 Fed. Reg. 16777 (Apr. 21, 2025), https://www.govinfo.gov/content/pkg/FR-2025-04-21/pdf/2025-06936.pdf.

⁶ Annex 1 to Proclamation No. 10914, id. at 16779.

⁷ Air Alliance Houston v. Trump, No. 1:25-cv-01852 (D.D.C. filed June 12, 2025) (complaint for declaratory and injunctive relief).

⁸ 90 Fed. Reg. at 25538.

⁹ Id. (at a 3 percent discount rate).

¹⁰ See Portland Cement Ass’n v. Ruckelshaus, 486 F.2d 375, 396 (D.C. Cir. 1973).