What Is 40 CFR Part 63? The Air Toxics Rules, and Why They're Not the Criteria Pollutant Standards
40 CFR Part 63 is the section of the Code of Federal Regulations that contains the National Emission Standards for Hazardous Air Pollutants, usually abbreviated NESHAP. These are the federal rules limiting emissions of air toxics — chemicals like benzene and mercury — from specific categories of industrial facilities. They are a different program from the criteria air pollutant standards that govern the six common pollutants, and confusing the two is the most common reason people land on this citation and get lost.
Decoding the citation
Federal regulations are organized like an outline, and the citation reads from broad to narrow:
- 40 is the title. Title 40 of the Code of Federal Regulations is "Protection of Environment" — it holds the EPA's regulations.
- CFR is the Code of Federal Regulations, the compiled body of federal agency rules.
- Part 63 is one part within Title 40, and it houses the hazardous air pollutant emission standards written after the 1990 Clean Air Act Amendments.
Within Part 63, the rules are divided into subparts, each covering a category of industrial sources — one subpart for one kind of facility, another for a different kind. So a permit document citing "40 CFR 63, Subpart X" is pointing to the emission rules for one particular industry category.
What Part 63 regulates — and what it doesn't
Part 63 covers hazardous air pollutants (HAPs), the long list of air toxics Congress directed the EPA to regulate at the facility level. These rules are often called MACT standards, for "maximum achievable control technology," because they generally require sources to match the emission performance of the best-controlled facilities in their category.
What Part 63 does not cover are the six criteria pollutants — ozone, particulate matter, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. Those are handled through an entirely different mechanism: ambient concentration limits listed in the NAAQS table, which cap how much of each pollutant can be in the outdoor air of a region, rather than how much a particular smokestack can emit. If you want the full contrast between the two systems, see criteria pollutants vs. hazardous air pollutants.
A quick way to keep them straight:
- NAAQS (criteria pollutants): limits on the air itself, enforced region by region. A region that exceeds them becomes a nonattainment area — explained in attainment vs. nonattainment.
- 40 CFR Part 63 (hazardous air pollutants): limits on emissions from specific facilities, enforced source category by source category. There is no ambient standard and no AQI category for these chemicals.
Why you might encounter the citation
Most non-specialists meet "40 CFR 63" in one of a few places: an environmental permit or compliance notice for a local facility, a public-comment announcement about a proposed rule for an industry, or an environmental report that lists which regulations apply to a site. In each case, the citation tells you the document is dealing with air toxics from an industrial source category — not with everyday outdoor air quality readings or the AQI.
If what you actually want to know is what a specific hazardous chemical is and how people can be exposed to it, the CFR is the wrong place to read — it is written for facility operators and regulators. The ATSDR ToxFAQs collection provides plain-language fact sheets on individual toxic substances, and we have a walkthrough on how to look up any chemical in ToxFAQs.
The short version
40 CFR Part 63 is where the federal air toxics emission rules live: Title 40 for environment, Part 63 for the post-1990 hazardous air pollutant standards, subparts for individual industries. It sits alongside — not inside — the criteria pollutant system. If your question is about the six common pollutants and their legal limits, you want the NAAQS, not Part 63; start with the six criteria air pollutants.