EPA Finalizes Changes to “Once In, Always In” Rule

On November 19, 2020, the United States Environmental Protection Agency (EPA) published a final rule to revise its “once in, always in” policy under the 40 CFR Part 63 of the Maximum Achievable Control Technology (MACT), or National Emission Standards for Hazardous Air Pollutants (NESHAP), rules. The revisions clarify “major source” and “minor source” definitions and allow sources that were previously MACT-major to reclassify to an “area source” if their hazardous air pollutants (HAP) potential to emit (PTE) falls below 10 tons per year (tpy) for an individual HAP or 25 tpy combined for HAPs. EPA has also included specific notification requirements whenever reclassification occurs. Further, the revisions specify that if a source goes from major to minor and back to major, the applicable MACT requirements become immediately effective.

Some background

Until 2018, the EPA had maintained a “once in, always in” policy for 25 years regarding major HAP sources. In 2018, EPA issued a memorandum rescinding the “once in, always in” policy. The latest rulemaking, which becomes effective January 19, 2021, codifies the tenets of the 2018 memorandum, meaning a major source can become an area source at any time by limiting its PTE. Without a “once in, always in” policy, facilities could "backslide" from MACT control levels by obtaining PTE limits that remove MACT standard applicability and then can increase emissions to the major-source threshold.

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Jay Christopher
Senior Scientist

Jeremy Sell, P.E.
Vice President, Air & Process Services

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