U.S. EPA Proposes to Repeal Certain Provisions of the Mercury and Air Toxics Standards for Power Plants

On June 11, 2025, the United States Environmental Protection Agency (U.S. EPA) proposed to repeal certain provisions of the 2024 amendments to the National Emission Standards for Hazardous Air Pollutants (NESHAP) for Coal- and Oil-Fired Electric Utility Steam Generating Units (EGUs), also known as the Mercury and Air Toxics Standards (MATS). MATS includes standards to limit emissions of mercury, acid gas hazardous air pollutants (HAPs), non-mercury HAP metals, and organic HAPs from coal- and oil-fired EGUs. In the rule, an EGU is a fossil fuel-fired combustion unit of more than 25 megawatts (MW) that serves a generator that produces electricity for sale. A unit that cogenerates steam and electricity and supplies more than one-third of its potential electric output capacity and more than 25 MW electric output to any utility power distribution system for sale is also considered an EGU.  

What’s New? 

Broadly, the new Administration has announced a number of deregulation activities in response to several Presidential Executive Orders and U.S. EPA Administrator Lee Zeldin’s “Powering the Great American Comeback” initiative. Reconsideration of the 2024 MATS rulemaking was among the list of deregulation activities affecting fossil fuel-fired power plants.  

The recently proposed amendments to the MATS rule were published in the Federal Register on June 17, 2025. The rule proposes to repeal certain provisions of the 2024 MATS amendments. The 2024 MATS amendments were promulgated as part of U.S. EPA’s technology review for the source category and revised certain requirements promulgated in the original 2012 MATS rule. Technology reviews are conducted by U.S. EPA every eight years to assess whether there are developments in practices, process, and control technologies since the promulgation of the original standard. When deciding whether to revise the existing standards, U.S. EPA considers a number of factors including cost. The 2024 MATS amendments lowered the filterable particulate matter (fPM) emissions standard from 0.030 to 0.010 pounds per million British thermal units (lb/MMBtu) for existing coal-fired EGU, required the use of continuous emissions monitoring systems (CEMS) for demonstrating compliance with the fPM standard for coal- and oil-fired EFU, lowered the mercury emissions limit from 4.0 to 1.2 pounds per trillion British thermal units (lb/TBtu) for existing lignite-fired EGUs, and revised startup requirements. 

Specifically for fPM, which serves as a surrogate for non-mercury HAP, U.S. EPA proposes to revert to the 0.03 lb/MMBtu standard for existing coal-fired EGUs. This proposed revision also includes reverting the total and individual non-mercury HAP emissions standards to the levels in the 2012 rule because these standards had been proportionally lowered in the 2024 rule. U.S. EPA proposes these revisions citing both high total cost and high cost per mass emissions reduced (i.e., cost effectiveness). U.S. EPA noted that the cost effectiveness accepted in the 2024 MATS rule was higher than cost effectiveness values rejected in other technology reviews (e.g., Taconite Ore Processing, Petroleum Refinery Sector, Integrated Iron and Steel Manufacturing). 

The proposed rule would also reinstate the options for demonstrating compliance included in the original 2012 rule. The compliance options will once again include quarterly stack testing, and PM continuous parameter monitoring systems (CPMS) in addition to PM CEMS. U.S. EPA explained that in the 2024 rule, it was estimated that the compliance costs for stack testing at the lower limit would increase to the extent that it would be comparable to the cost of a CEMS when combined with the reduced costs of CEMS as compared to 2012. The 2024 rulemaking also noted that data transparency and public access to such data was an advantage of requiring CEMS. In its repeal, U.S. EPA noted that because the standard is proposed to be reverted to its original value, the cost of stack testing is not expected to increase and that stack testing data is publicly accessible on WebFIRE. As part of reinstating the various compliance demonstration options, U.S. EPA also proposes to reinstate the low emitting EGU (LEE) program for fPM and non-Hg HAP metals that provides a pathway for facilities electing to conduct stack testing to reduce their testing frequency.  

Finally, U.S. EPA proposes to revert to the 4.0 pounds mercury per trillion British thermal units (lb/TBtu) for existing lignite-fired EGUs, citing insufficient data to inform a lower standard and technical feasibility concerns regarding boiler, fuel, and control technology variability. 

Now what? 

This is just one of many rules that is expected to be reconsidered that will affect fossil fuel-fired power plants, and ALL4 is tracking them all. U.S. EPA will conduct a public hearing for the proposed MATS rule on July 10, 2025 and has opened a public comment period which will end on August 11, 2025. ALL4 can assist you with preparing comments on the proposed revisions, with stakeholder engagement meetings, and with evaluating the impact of the proposed rule on your facility. If you have questions about how the proposed MATS revisions could affect your facility’s program, or what your next steps should be, please reach out to your ALL4 project manager or to me at lpearce@all4inc.com. ALL4 is monitoring all updates published by U.S. EPA on this topic, and we are here to answer your questions and assist your facility with any aspects of MATS compliance.