The Clean Air Act doesn’t target just one gas. It regulates dozens of specific pollutants across several categories, from common exhaust gases like carbon monoxide to potent greenhouse gases like methane. The law’s broadest reach covers six “criteria” air pollutants that the EPA monitors nationwide, but it also addresses greenhouse gases, nearly 200 toxic air pollutants, and chemicals that damage the ozone layer.
The Six Criteria Air Pollutants
The core of the Clean Air Act is built around six widely found pollutants that the EPA sets concentration limits for under the National Ambient Air Quality Standards (NAAQS). These are the pollutants most people breathe in everyday life, and each has a legally enforceable ceiling that states must meet:
- Carbon monoxide (CO), a colorless, odorless gas produced by burning fuel in vehicles and industrial processes.
- Lead (Pb), now capped at 0.15 micrograms per cubic meter, measured as a three-month average. Lead levels have dropped dramatically since leaded gasoline was phased out, but smelters and some aviation fuels still contribute.
- Nitrogen dioxide (NO₂), released primarily by cars and power plants, which irritates airways and contributes to smog.
- Ground-level ozone (O₃), the main ingredient in smog. Unlike the other five, ozone isn’t emitted directly. It forms when volatile organic compounds (VOCs) and nitrogen oxides react in sunlight, so the Act controls ozone by regulating those precursor gases.
- Particulate matter (PM), tiny particles and droplets that penetrate deep into the lungs. In 2024, the EPA tightened the annual fine particulate (PM2.5) standard to 9.0 micrograms per cubic meter, down from the previous limit, to better reflect current health science.
- Sulfur dioxide (SO₂), a sharp-smelling gas from coal-fired power plants and industrial facilities that causes respiratory problems and contributes to acid rain.
Sulfur Dioxide and the Acid Rain Program
Sulfur dioxide gets special attention under Title IV of the 1990 amendments, which created the Acid Rain Program. The law mandated a permanent 10-million-ton reduction in SO₂ emissions from 1980 levels, and it did so with a novel tool: a cap-and-trade system. Power plants receive allowances, each permitting one ton of SO₂ emissions per year. Plants that cut emissions below their limit can sell unused allowances to other facilities. Any source that exceeds its allowances faces a $2,000-per-ton penalty and must offset the excess the following year. This market-based approach is widely considered one of the most cost-effective environmental regulations ever enacted.
Greenhouse Gases
The Clean Air Act also covers six greenhouse gases as a group: carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride. For regulatory purposes, their emissions are converted into “CO₂ equivalent” by multiplying each gas’s output by its global warming potential, then summing the totals. A stationary source, like a power plant or refinery, generally falls under greenhouse gas permitting requirements only if it emits 75,000 tons or more of CO₂ equivalent per year.
The EPA’s authority over greenhouse gases has been contested in court. In a major ruling, the Supreme Court found that the EPA cannot impose state-level caps on carbon emissions from power plants under the Clean Air Act, saying that kind of sweeping energy policy must come from Congress. The decision limited the EPA’s ability to use one specific section of the law but did not eliminate all federal, state, or local authority to regulate climate pollution.
Methane, in particular, has become a growing regulatory focus. Under the Inflation Reduction Act, the EPA collects a “waste emissions charge” on methane from facilities in the oil and gas sector that report their greenhouse gas output. The charge started at $900 per metric ton of methane and rises to $1,500 after two years, creating a direct financial incentive to reduce leaks and venting.
Hazardous Air Pollutants
Beyond the six criteria pollutants and greenhouse gases, the Clean Air Act targets 188 hazardous air pollutants (sometimes called “air toxics”) under Section 112. These are substances that cause cancer, neurological damage, or other serious health effects even at low concentrations. The list includes benzene (found in gasoline), mercury compounds released by coal combustion, lead compounds from industrial processes, and dozens of solvents and chemical byproducts used in manufacturing. Rather than setting ambient air standards the way it does for criteria pollutants, the EPA regulates these toxics by requiring specific industries to install maximum achievable control technology at their facilities.
Ozone-Depleting Substances
Title VI of the Clean Air Act addresses a completely different kind of gas: chemicals that destroy the protective ozone layer in the upper atmosphere. These ozone-depleting substances include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), halons, methyl bromide, carbon tetrachloride, and methyl chloroform. CFCs were once common in refrigerators, air conditioners, and aerosol sprays. The Act phased out their production in alignment with international treaties, and the stratospheric ozone layer has been slowly recovering as a result. HCFCs, used as transitional replacements, are also being phased down on a set schedule.
How These Categories Work Together
The Clean Air Act’s reach is layered. A single coal-fired power plant, for example, might be regulated for sulfur dioxide (as a criteria pollutant and under the Acid Rain Program), for nitrogen oxides and particulate matter (criteria pollutants), for mercury (a hazardous air pollutant), and for carbon dioxide (a greenhouse gas). Each category has its own legal framework, standards, and enforcement mechanisms. The criteria pollutants use air quality thresholds that states must meet. Greenhouse gases use permitting requirements tied to emission volume. Hazardous pollutants use technology-based standards applied to specific industries. And ozone-depleting substances follow production and import phase-out schedules.
In total, the Clean Air Act covers well over 200 individual substances across these categories, making it one of the most comprehensive air quality laws in the world.