Audit, Compliance and Risk Blog

Regulating routine emissions of air toxics

Posted by Jon Elliott on Fri, Sep 11, 2020


The federal Clean Air Act (CAA) requires the U.S. Environmental Protection Agency (EPA) to establish and maintain a list of air toxics, named as Hazardous Air Pollutants (HAPs), and to set emissions standards for many sources of such pollutants. HAPs include heavy metals, organics, and other airborne pollutants that are not otherwise regulated as “criteria” air pollutants (such as carbon monoxide, particulate matter, and ground level ozone). This note summarizes requirements applicable to stationary sources.

What are hazardous air pollutants?

The CAA Amendments of 1990 included a list of specific materials that EPA is required to regulate as HAPs, by limiting emissions. After limited regulatory revisions in the intervening 30 years, EPA oversees a list of 188 HAPs. Most are industrial chemicals, such as benzene, methylene chloride, and perchloroethylene.

CAA assigns most day-to-day regulation of HAPs to states, which are also allowed to define additional air toxics. Several states have done so. For example, California defines a list of toxic air contaminants (TACs) that includes all the federal HAPs, plus more than 20 additional materials; most additional TACs are pesticides, although California also identifies “environmental tobacco smoke.”

How are stationary source emissions regulated?

CAA directs EPA to define National Emissions Standards for Hazardous Air Pollutants (NESHAPs), which are to protect public health with an “ample margin of safety.” Standard development is organized around dozens of source “categories” and sub-categories, based on pollutants and/or the activities that generate them – including halogenated solvent cleaning, petroleum refining, and benzene waste operations. EPA also distinguishes between two types of emissions sources:

  • Major Sources — stationary sources that emit, or have the potential to emit, 10 tons per year (tpy) of any individual HAP or 25 tpy of any combination of HAPs.

  • Area Sources — all stationary sources of HAPs that are not defined as major sources.

Different standards apply to each source category, based on EPA’s assessment of available control methods. Major sources generally are subject to stricter standards, which EPA refers to as maximum achievable control technology (MACT) standards for major sources. Area sources are subject to less-stringent generally available control technology (GACT) standard. 

When developing MACT standards, EPA must meet the following criteria:

  • For new sources, the MACT standard must be as stringent as the “best controlled similar source” anywhere in the world.

  • For existing sources, the MACT standard can be no less stringent than the average emission limitation achieved by the best-performing 12% of similar sources in the U.S. (or the top 5 sources when fewer than 30 sources exist for the source category in question).

Because even “maximum” controls may not eliminate all risks, EPA also reviews the “residual risk” left after application of a promulgated standard. In addition, EPA is to review the technical basis for each NESHAP standard at least every eight years taking into account any developments in practices, processes, and control technologies. EPA maintains an extensive compilation of NESHAP determinations in an Applicability Determination Index (ADI).

How do HAP source permits work?

HAP sources follow compliance requirements similar to those for sources of conventional pollutants. For major sources, these include:

  • Preconstruction review—for compliance with NESHAP/MACT requirements in addition to any associated with conventional pollutants

  • Operating permit -- with detailed permit information and conditions including:

       - description of the source, including operations and technologies employed

       - emission standards and limitations - controls to be employed, which  may include schedules for implementation

       - monitoring and related record keeping

       - permit duration

       - provisions for entry and inspection by air quality regulators

Now what?

The HAP regulatory program has now been in place for decades, and continues to update and revise requirements. For example, on August 13, 2020 EPA published the final results of its latest residual risk and technology review for the Plywood and Composite Wood Products source category.

Self-evaluation checklist

Does the organization conduct any activities that emit pollutants regulated as HAPs under EPA’s CAA provisions?

If so, do any of these activities include stationary sources that qualify as major sources of HAPs?

If so, has each such activity undergone permitting by EPA or a delegated state or local air agency?

If so, is each such activity conducted in compliance with permit requirements?

If any such activity is not subject to permit requirements, does the organization  manage each to ensure that emissions meet requirements necessary to remain exempt?

Where can I go for more information?

  • EPA 

- hazardous air pollutants webpage 

- applicability determination index 

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About the Author

Jon Elliott is President of Touchstone Environmental and has been a major contributor to STP’s product range for over 30 years. 

Mr. Elliott has a diverse educational background. In addition to his Juris Doctor (University of California, Boalt Hall School of Law, 1981), he holds a Master of Public Policy (Goldman School of Public Policy [GSPP], UC Berkeley, 1980), and a Bachelor of Science in Mechanical Engineering (Princeton University, 1977).

Mr. Elliott is active in professional and community organizations. In addition, he is a past chairman of the Board of Directors of the GSPP Alumni Association, and past member of the Executive Committee of the State Bar of California's Environmental Law Section (including past chair of its Legislative Committee).

You may contact Mr. Elliott directly at:

Image by Ralf Vetterle from Pixabay

Tags: OSHA, CAA, mact, Air Toxics, NESHAPs, GACT, HAPs, ADI