EPA NESHAP — National Emission Standards for Hazardous Air Pollutants

The National Emission Standards for Hazardous Air Pollutants (NESHAP), codified at 40 CFR Part 63, are EPA's technology-based emission standards for 187 toxic air pollutants — chemicals like benzene, mercury, vinyl chloride, and dioxins that cause cancer, neurological damage, or reproductive harm even at concentrations below criteria pollutant thresholds. Congress listed these Hazardous Air Pollutants (HAPs) in the 1990 Clean Air Act Amendments and directed EPA to set Maximum Achievable Control Technology (MACT) standards for each industrial source category that emits them. Part 63 now contains hundreds of industry-specific MACT subparts covering petroleum refineries, chemical plants, paint strippers, dry cleaners, hospital sterilizers, and dozens more.

Current Rule (2026)

What This Rule Does

Not all air pollution is treated equally. The six "criteria pollutants" — particulate matter, ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide, and lead — are regulated through the National Ambient Air Quality Standards (NAAQS) framework, targeting safe concentration levels in outdoor air. Hazardous Air Pollutants (HAPs) are a different category: 187 toxic chemicals that cause cancer, neurological damage, reproductive harm, or other serious health effects even at levels below criteria pollutant thresholds. Congress listed these HAPs in 1990 and directed EPA to set Maximum Achievable Control Technology (MACT) standards for industrial categories that emit them — resulting in 40 CFR Part 63, one of the most extensive regulatory programs in U.S. environmental law.

Part 63 currently contains hundreds of industry-specific subparts — each a stand-alone MACT rule for a particular source category — covering facilities from petroleum refineries and chemical plants to paint stripping operations, chromium electroplating shops, dry cleaners, hospital sterilizers, and rubber tire manufacturing. When EPA promulgates a MACT standard for a source category, every major source in that category must meet the standard's emission limits, work practice requirements, equipment specifications, monitoring requirements, recordkeeping obligations, and reporting deadlines. MACT standards are technology-based, not health-based: they require sources to match the best-performing 12% of existing sources in the category (the "MACT floor"), then allow EPA to set more stringent standards if technically feasible.

After a MACT standard has been in place for at least eight years, EPA must conduct a Risk and Technology Review (RTR) — an assessment of residual cancer and non-cancer risks to the public from sources complying with the MACT floor. If residual risks remain above acceptable levels (generally, more than 1-in-10,000 excess cancer risk in the most-exposed population), EPA must strengthen the standards. This RTR process drives ongoing revisions to existing Part 63 subparts as EPA works through its backlog of reviews.

How It Works

The NESHAP framework divides emission sources into two tiers. A major source — any facility that emits 10 tons or more per year of any single HAP, or 25 tons or more of all HAPs combined — must meet Maximum Achievable Control Technology (MACT) standards. EPA sets MACT at no less than the MACT floor: the average emission level achieved by the best-performing 12% of existing sources in each category. EPA may then set standards "beyond the floor" if further reductions are economically and technically feasible. An area source (any source below the major thresholds) must meet Generally Achievable Control Technology (GACT) standards, which are typically less stringent. When EPA promulgates a new standard, existing sources generally get three years to comply from the rule's effective date; new sources must comply immediately on startup.

Each regulated source category gets its own subpart within 40 CFR Part 63, identified by a letter or letter combination. Subpart A contains the general provisions that all other subparts incorporate by reference: the applicability determination process (§ 63.1), recordkeeping and reporting (§ 63.10), control device requirements (§ 63.11), and definitions (§ 63.2). A facility covered by NESHAP must therefore understand both its source-category subpart — which sets the emission limits, work practice standards, and monitoring requirements specific to its industry — and the general provisions, which govern how those requirements are demonstrated and documented. States that have received delegation from EPA may administer NESHAP programs directly, but must apply standards at least as stringent as the federal requirements.

How It Affects You

If you operate an industrial facility: Your first task is determining which Part 63 subparts apply to your operations. Applicability depends on whether your facility qualifies as a major or area source, what processes you operate, and which source categories EPA has listed. EPA's website provides applicability tools, but the threshold question — major or area source — depends on your actual potential to emit each HAP, which requires a careful analysis of your equipment and operational limits. Operating below the major source threshold (often done through a "synthetic minor" permit limiting your allowable emissions) keeps you in the less stringent area source tier.

If you are in a regulated industry category: Once your applicable subpart is identified, the compliance path is clear — you must meet the emission limits or work practice standards in your subpart. Many MACT standards specify control technology (continuous emission monitors, thermal oxidizers, carbon adsorbers, enclosures) that you must install and maintain. The monitoring and recordkeeping obligations are extensive: Part 63 requires documenting equipment performance, maintaining deviation reports, and submitting semi-annual compliance reports. Continuous Emission Monitoring Systems (CEMS) are required for many major source categories.

If you are near a facility regulated under Part 63: MACT standards reduce HAP emissions from industrial sources by requiring the best available technology — but the "residual risk" that remains after MACT controls may still present health risks in communities adjacent to facilities. EPA's Residual Risk and Technology Review process is intended to address this, but many reviews have been delayed. EPA's AirToxScreen tool provides community-level cancer risk estimates from air toxics, allowing residents to understand the cumulative HAP exposure in their area.

If you operate a small or area source (dry cleaner, auto body shop, chromium plater, commercial sterilizer): EPA has promulgated area source MACT and GACT standards for many categories affecting small businesses. Work practice standards — how you manage chemicals, store solvents, conduct equipment inspections — rather than continuous monitoring often define compliance for area sources. Check whether your industry is a listed source category and what the applicable standard requires.

Statutory Authority

This rule implements:

• 42 U.S.C. § 7412 (Clean Air Act § 112) — Establishes the 187-HAP list; directs EPA to list all major source categories emitting HAPs; requires promulgation of MACT standards for listed categories; requires Risk and Technology Review no less than every 8 years; authorizes area source GACT standards; provides for state delegation of NESHAP implementation

Recent Developments

• Trump deregulatory rollbacks (2025-2026): EPA rescinded or proposed to rescind MACT standards for integrated iron and steel manufacturing, proposed to reconsider the 2024 ethylene oxide sterilization NESHAP that required significant emission reductions at commercial hospital sterilizers, and extended compliance deadlines for oil and gas sector NESHAPs. These rollbacks represent a significant departure from the Obama and Biden-era approach of tightening MACT floors through RTR reviews.
• RTR backlog: EPA maintains a large backlog of required Risk and Technology Reviews. Courts have held that EPA must complete RTRs for overdue source categories; litigation over RTR delays has produced court-ordered schedules for several source categories.
• Part 61 legacy standards: The earlier 40 CFR Part 61 contains NESHAP promulgated before 1990, covering categories such as asbestos (Subpart A), benzene (Subpart F), and vinyl chloride (Subpart F). These pre-1990 standards are health-based (not MACT-based) and apply to specific source types independently of the Part 63 framework.
• Electronic reporting: EPA has transitioned Part 63 compliance reports to electronic submission through the Compliance and Emissions Data Reporting Interface (CEDRI) in EPA's WebFIRE system; most Part 63 subparts now require electronic submission of compliance certifications and deviation reports.

Pending Action

The Trump EPA's deregulatory agenda has targeted multiple Part 63 MACT standards for reconsideration, including the 2024 ethylene oxide sterilization rule (which required significant emission reductions at commercial hospital sterilizer facilities), integrated iron and steel MACT standards, and several oil and gas sector NESHAPs. Litigation from both industry (challenging tighter MACT floors from Biden-era RTR reviews) and environmental groups (challenging delayed RTR schedules and deregulatory rollbacks) is proceeding in the D.C. Circuit. Watch for EPA action on court-ordered RTR completion schedules for source categories where EPA has missed statutory deadlines.