On June 25, 2019, EPA released a pre-publication draft of a proposed rule allowing sources subject to Maximum Achievable Control Technology (MACT) standards under Section 112 of the Clean Air Act to voluntarily limit their emissions and avoid MACT.  The proposed rule, which formalizes and expands on a January 2018 guidance document issued by former EPA Assistant Administrator Bill Wehrum, would allow “major sources” of hazardous air pollutants (HAPs) otherwise subject to MACT standards to take an enforceable limit on HAPs and thus reclassify as “area sources.”  The rulemaking, branded by the Agency as “Major MACT to Area” (MM2A), would eliminate the Agency’s longstanding “once-in-always-in” policy, under which a facility that qualified as a major source of HAPs as of the “first substantive compliance date” of the applicable MACT standard was permanently subject to that standard, even if the source was later able to reduce its emissions below major source applicability thresholds.