October 13, 2023

Proposal to Keep MM2A With Additional Safeguards

Starting in 1995, EPA’s CAA “once in, always in” policy (OIAI) meant a facility that was a major source under a Part 63 NESHAP would be required to permanently comply with that major source standard even if the source limited its potential-to-emit (PTE) to area source levels. On January 25, 2018, EPA initially withdrew OIAI when it issued the “Wehrum Memorandum” and formalized the withdrawal in its 2020 major MACT to area source (MM2A) rule. [85 FR 73854] The withdrawal meant a source could reduce its PTE and possibly increase its actual emissions while avoiding compliance requirements, sometimes called “backsliding.” In response to Executive Order 13990 “Protecting Public Health and the Environment and Restoring Science to Tackle the Climate Crisis”, the agency proposed a reconsideration of MM2A on September 27, 2023. [88 FR 66340] The proposal would retain MM2A while instituting federally enforceable safeguards to ensure reclassified sources cannot increase their emissions as a result of reclassification.


A source that reclassifies from major source to area source may find it is no longer subject to a source category standard and could backslide on its emissions. To prevent backsliding, EPA proposes three safeguard emission control options to apply to sources that reclassify after January 25, 2018. A source could:

  1. Continue to employ the emission control methods required under the major source NESHAP requirements, including previously approved alternatives under the applicable NESHAP and associated monitoring, recordkeeping, and reporting;
  2. Employ control methods prescribed for reclassification under a specific NESHAP; or
  3. Use emission controls the permitting authority has reviewed and approved, ensuring HAP emissions from units or activities previously covered will not exceed the emission standard or level acceptable under the major source NESHAP requirements at the time of reclassification.

The chosen safeguard would be selected by the permitting authority and determined on a case-by-case basis. EPA is also considering additional, more stringent safeguards for sources that emit persistent and bioaccumulative HAPs listed in CAA Section 112(c)(6), such as hexachlorobenzene and mercury.

Enforceability and applicability

The agency believes compliance is best achieved when the federal government and citizens may enforce permit limits, not just state and local permitting authorities. As such, EPA proposes any limits taken by a source to reclassify must be federally enforceable, and citizens would be provided the ability to enforce such permits directly. These provisions are believed to enhance a chosen safeguard’s effectiveness. EPA also seeks comment on the prevalence and effectiveness of citizen suit provisions in state and local enforceable HAP PTE limiting programs.

EPA states the proposal would only apply to sources that are or were subject to a major source NESHAP and take a restriction to limit the PTE to below the major source threshold to become a synthetic minor. The proposal would not apply to sources that became a synthetic minor before the first compliance date of the applicable major source standard. Nor would the proposal apply to reclassified true area sources that modify operations such that they can no longer emit HAPs above the major source threshold.

Comments may be submitted through November 13, 2023 via Docket ID No. EPA-HQ-OAR-2023-0330.


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