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On January 25, 2018, EPA issued a guidance memorandum withdrawing the “Once In Always In” (OIAI) policy for MACT (Maximum Achievable Control Technology) standards applying to major sources of hazardous air pollutants under section 112 of the Clean Air Act. Under the new guidance, sources of hazardous air pollutants previously classified as “major sources” may be reclassified as “area sources” at any time, provided the facility limits its potential to emit below major source thresholds.

This is a reversal of EPA's previous policy from 1995. According to Bill Wehrum, Assistant Administrator of EPA's Office of Air and Radiation, the 1995 policy is contrary to the plain language of the Clean Air Act. From the new guidance: “The OIAI policy… created an artificial time limit that does not exist on the face of the statute by including a temporal limitation on when a major source can become an area source by limiting its PTE.”

EPA plans to publish a notice in the Federal Register to take comment on regulatory language that will reflect the interpretation described in the new memorandum. In the meantime, sources that wish to apply this approach should contact their permit authority to determine what options they may have to change their status to “area source” for purposes of MACT applicability. Note that for some source categories, there is an area source rule (such as for stationary engines), whereas for other categories, there are no requirements for area sources (such as for refineries). EPA's press release, as well as the 2018 withdrawal memorandum and the original 1995 OIAI policy, can be found at

Trinity has extensive experience in evaluating MACT applicability and working with state, local, and federal agencies, and can assist with developing an approach for your particular situation. Contact us at (800) 229-6655.