2026 MATS Rule Repeal: What Changed Under 40 CFR Part 63 Subpart UUUUU
- Mar 5
- 3 min read
The United States Environmental Protection Agency finalized the repeal of specific amendments to the National Emission Standards for Hazardous Air Pollutants (NESHAP) for Coal- and Oil-Fired Electric Utility Steam Generating Units (EGUs), commonly referred to as the Mercury and Air Toxics Standards (MATS), that were promulgated on May 7, 2024.
The rule takes effect April 27, 2026, and restores specific compliance provisions that facilities have historically relied on under 40 CFR Part 63 Subpart UUUUU.
This article summarizes what changed, what didn’t, and what it means for coal and oil-fired electric generating units (EGUs) going forward.
What Did the EPA Repeal in 2026?
The EPA removed three major elements of the 2024 amendments, each of which would have tightened emissions or monitoring requirements. These repeals are documented in the rule and are listed below.
1. Stricter Filterable PM (fPM) Limit
Repealed: 0.010 lb/MMBtu for existing coal units
Restored: 0.030 lb/MMBtu (2012 standard)
Also reverses the associated tighter metal HAP limits.
2. Mandatory PM CEMS Requirement
Repealed: PM CEMS as the only compliance method
Restored: Flexibility to use:
Quarterly stack testing
PM CPMS
PM CEMS
3. Tightened Mercury Limit for Lignite Units
Repealed: 1.2 lb/TBtu
Restored: 4.0 lb/TBtu
What Was Not Repealed?
The EPA removed only three elements of the 2024 amendments. All other provisions of the 2012 MATS rule and the 2020 Residual Risk and Technology Review (RTR) remain fully enforceable.
Key Requirements That Remain in Effect
All original 2012 emission limits (PM, metals, acid gases, Hg, SO₂ surrogate)
All 2012/2020 work practice standards
All performance testing requirements (Methods 5, 29, 26A, 320, 30B, etc.)
Continuous monitoring requirements (Hg CEMS, sorbent traps, HCl/HF CEMS, SO₂ CEMS, PM CPMS)
Electronic reporting through ECMPS
2020 residual risk findings
All Subpart UUUUU definitions, test methods, appendices, and rule text (except where directly tied to the repealed 2024 amendments)
As a result, facilities must continue to maintain existing monitoring systems, meet emissions limits, and comply with performance testing and reporting requirements under 40 CFR Part 63 Subpart UUUUU.
Additional Updates to Electronic Reporting
The EPA also clarified electronic reporting requirements under the MATS rule. These revisions do not alter the underlying reporting requirements but update references and file format specifications used for electronic submissions.
Removed references to “ECMPS” and replacing with “ECMPS Reporting Tool”
Revised the XML file format to “any file format specified by the Administrator”
What This Means for Power Plant Operators
The 2026 repeal simplifies MATS compliance by returning to a familiar regulatory framework. Operators regain flexibility in monitoring methods, face less stringent fPM and Hg limits, and may benefit from reduced compliance costs.
At the same time, the core structure of MATS—emission limits, monitoring, reporting, and work practices—remains unchanged. Facilities must continue to meet all 2012 MATS and the 2020 Residual Risk and Technology Review (RTR) requirements.
How Alliance Technical Group Can Help
Alliance Technical Group supports power plants and industrial facilities with MATS compliance programs, including stack testing, monitoring strategy support, CEMS, QA/QC procedures, and ECMPS reporting.
If you are evaluating how the 2026 MATS repeal may impact your compliance approach, Alliance Technical Group can help review your current program and identify any adjustments needed to remain aligned with the rule.
Susan Butler-Kennedy, QAP
Regulatory Manager, ACS/DAS
Susan has 25+ years of experience in the power industry, helping power plants and related facilities comply with various environmental laws and regulations. Her expertise lies in the communication, analysis, and documentation of Federal and State air emission compliance. She holds a Bachelor of Science ('92) and a Master of Science ('96) from Auburn University.
