EPA Proposes Repeal of 2024 Mercury and Air Toxics Standards Amendments
On June 11, 2025, the U.S. Environmental Protection Agency ("EPA") announced a proposed rule to repeal key amendments to the 2024 Mercury and Air Toxics Standards ("MATS") for coal- and oil-fired electric utility steam generating units ("EGUs"), reverting certain emission standards and compliance requirements to those established in the original 2012 MATS rule.
Key Elements of the Proposed Repeal
- Filterable Particulate Matter ("fPM") Emission Standard. The 2024 amendment lowered the fPM standard for existing coal-fired EGUs from 0.030 lb/MMBtu to 0.010 lb/MMBtu. EPA now proposes to revert to the 2012 standard, citing cost-effectiveness values for the revised standard that exceed those previously deemed reasonable in other technology reviews under the Clean Air Act ("CAA") Section 112(d)(6).
- Compliance Demonstration Requirements. The 2024 rule required all coal- and oil-fired EGUs to use particulate matter continuous emission monitoring systems ("PM CEMS") for fPM compliance. The proposed repeal would restore the option for EGU owners and operators to use quarterly stack testing or continuous parametric monitoring systems as alternatives.
- Mercury ("Hg") Emission Standard for Lignite-Fired EGUs. The 2024 amendment reduced the Hg emission limit for lignite-fired EGUs from 4.0 lb/TBtu to 1.2 lb/TBtu. EPA now proposes to revert to the 2012 limit, citing insufficient data to demonstrate that the more stringent standard is achievable across the range of boiler types and variable lignite fuel compositions.
Rationale for the Proposal
- Cost and Feasibility. EPA found that compliance with the fPM, PM CES, and Hg standards for lignite-fired EGUs was either too costly or not feasible. EPA concluded that the cost per ton of pollutant reduced under the 2024 fPM standard is significantly higher than thresholds previously rejected in other CAA Section 112(d)(6) technology reviews. Likewise, EPA stated that the requirement for PM CEMS would impose unnecessary expenses on a majority of affected EGUs, since alternative monitoring methods are sufficient for demonstrating compliance. Finally, EPA concluded that the revised Hg standard for lignite-fired EGUs was based on limited data and may not be achievable for all units, particularly those not employing advanced combustor technologies.
- Lack of Need. EPA noted that the 2020 residual risk review found risks from hazardous air pollutant emissions to be within acceptable limits, and that the 2012 MATS rule provided an ample margin of safety to protect public health and the environment.
EPA Proposes Repeal of Mercury and Air Toxics Standards Amendments
Comments on the proposal are due 45 days after publication in the Federal Register. Affected industries should consider submitting comments and follow developments closely.
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