The Difference Between Air Toxics and Criteria Pollutants
The Clean Air Act establishes two fundamentally different regulatory tracks for air pollutants: National Ambient Air Quality Standards (NAAQS) for "criteria pollutants," and hazardous air pollutant (HAP) standards for "air toxics." Understanding the difference between these tracks is essential for interpreting industrial facility emissions data and for understanding what regulatory requirements apply to facilities in your area.
Criteria Pollutants: Area-Based Standards
The six criteria pollutants — ozone, particulate matter (PM₂.₅ and PM₁₀), carbon monoxide, nitrogen dioxide, sulfur dioxide, and lead — are regulated through ambient air quality standards. EPA sets a maximum allowable concentration in outdoor air that applies everywhere in the country, then requires states to develop plans to achieve those standards in areas that are out of compliance ("nonattainment areas"). The regulatory focus is on the ambient concentration in the air that people breathe, not the emissions from individual facilities per se.
Hazardous Air Pollutants: Source-Based Standards
Hazardous air pollutants (HAPs) — 187 compounds listed in the Clean Air Act, including benzene, mercury, lead compounds, and formaldehyde — are regulated through emission standards applied directly to source categories. Under Section 112 of the CAA, EPA establishes National Emission Standards for Hazardous Air Pollutants (NESHAP) for specific industry categories (refineries, chemical plants, plating shops, etc.) that set technology-based limits on HAP emissions from those sources. These standards apply regardless of where the facility is located or what ambient concentrations exist in the area.
TRI and HAP Data
The TRI chemical list overlaps significantly with CAA HAPs — many TRI chemicals are also HAPs regulated under NESHAP standards. When a facility reports TRI releases of a HAP like benzene, those releases are occurring within a regulatory framework set by applicable NESHAP standards. But TRI reporting thresholds and NESHAP applicability are separate — a facility can be required to report benzene to TRI without necessarily being subject to a specific benzene NESHAP.
Practical Implications for Facility Research
When researching a specific facility\'s air emissions, it helps to look at both TRI data (for toxic chemical quantities) and EPA ECHO compliance data (for Clean Air Act permit compliance). A facility can be releasing substantial quantities of TRI chemicals while technically in full compliance with its CAA permit — because the permitted limits are based on available control technology, not on ambient health benchmarks. Browse facilities by their air program compliance at the facility list and explore specific HAP chemicals at the chemical index.