“Once In Always In” Guidance for Major Sources under the Clean Air Act – More Information

Click here to return to our primary page on the “once in always in” guidance for major sources under the Clean Air Act, with the current status and discussion of why this rule matters. 


In May 1995, The Environmental Protection Agency (EPA) issued “once in, always in” guidance for major sources under the Clean Air Act (CAA). This guidance declared that, prior to the first Maximum Available Control Technology (MACT) standard compliance date, a major source could be reclassified and not required to operate MACT if its potential to emit fell below the threshold amounts. After the first compliance date, a major source would no longer have this option and would be required to limit its Hazardous Air Pollutants (HAPs) emissions through MACT.

In 2007, EPA proposed a rule to replace the “once in, always in” guidance. EPA did not finalize this rule.

Trump Era

January 25, 2018 — EPA issued guidance withdrawing the “once in, always in” policy. This new guidance establishes that, at any time, “a major source which takes an enforceable limit on its potential to emit and takes measures to bring its HAP emissions below the applicable threshold” may be reclassified and no longer be required to use MACT.

February 8, 2018 — The guidance was published in the Federal Register.

On March 26, 2018 — A coalition of environmental groups sued EPA over the weakening of the rule, and California sued EPA as well on April 10, 2018.


Thank you to Harvard student Laura Bloomer, JD/MPP 2019 and William Neibling, Research Counsel and JD 2013, for their assistance with this rule.

HLS Student Orgs

Search Our Site

HLS Evironmental Law on Twitter