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

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Why it Matters

As part of the Clean Air Act (CAA), EPA regulates Hazardous Air Pollutants (HAPs). HAPs include benzene, metals, and other pollutants that are known to cause cancer and other serious health effects. A facility is considered a “major source” if it has the potential to emit 10 tons per year of any one HAP or 25 tons per year of any combination of HAPs. All other facilities are considered “area sources.” Major sources, such as power plants and petroleum refineries, are subject to Maximum Available Control Technology (MACT) standards for regulated pollutants. MACT standards are stringent pollution control requirements based on the technology used in the best-controlled sources in the industry.

Because MACT often reduces emissions to below the major source thresholds of 10 and 25 tons per year, the standards significantly limit the amount of hazardous pollutants in our environment. Under current law, a major source remains a major source even after the application of MACT and the resulting achievement of emissions reductions. That means the source must continue to operate under the more stringent requirements that are applied to major sources and maintain MACT-level low emissions.

A different approach – allowing the source to be treated as an area source after it reduces its emissions below the threshold – would have the effect of replacing the source’s initial MACT requirements, and the greater level of emissions reductions achieved, with the 10 and 25 tons per year limit. Meaning, for all practical purposes, the newly re-classified area source would be constrained by the thresholds not by the more stringent MACT requirements.

The result could be a large increase in pollution. For example, after first applying the MACT, the source could switch to less effective pollution controls, or operate its controls less frequently or at lower removal efficiencies, and release more HAPs up to the major source threshold amounts. This increase could have significant health effects on local communities, especially those that are located near major stationary sources of toxic air pollutants.

Allowing a major source to stop operating these stringent controls would be counter to the primary goal of the CAA and, especially its MACT provisions, of protecting public health and the environment by minimizing emissions consistent with standards such as MACT-based ones. 

Current Status

On January 25, 2018 EPA issued a memorandum (published in the Federal Register on February 8, 2018) that weakens the pollution control technology requirements for major sources of HAPs, if those sources simply commit to limiting their emissions to the less constraining threshold levels of 10 tons per year for any one HAP, and 25 tons per year for any combination of HAPs. By amending its operating permit to incorporate those threshold levels for its HAP emissions, a major source can be reclassified as an area source subject to less stringent emissions control and compliance requirements.



In May 1995, EPA issued “once in, always in” guidance for major sources under the CAA. This guidance declared that, prior to the first MACT 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 HAP 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

On 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.

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

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

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