What Are the Six Criteria Air Pollutants?
Why these six
Under the Clean Air Act, EPA sets National Ambient Air Quality Standards — NAAQS — for six common pollutants, known as the criteria air pollutants because the standards are based on published scientific criteria documents. EPA's criteria air pollutants page is the official home for the current standards. These six are regulated this way because they are common across the country, come from numerous and diverse sources, and are the pollutants for which nationwide ambient standards make sense. Knowing what each one is turns a lot of environmental reporting from jargon into information.
The six, in plain English
- Ground-level ozone (O₃). Not emitted directly. It forms in the air when nitrogen oxides and volatile organic compounds react in sunlight. This is the "smog" pollutant, distinct from the protective ozone layer high in the stratosphere — same molecule, very different location.
- Particulate matter (PM10 and PM2.5). A catch-all term for solid particles and liquid droplets suspended in air: dust, soot, smoke, and particles formed from other pollutants reacting in the atmosphere. The numbers refer to particle diameter in micrometers — PM2.5 particles are roughly thirty times smaller than the width of a human hair.
- Carbon monoxide (CO). A colorless, odorless gas produced by incomplete combustion. Outdoors, vehicle exhaust has historically been a major source.
- Sulfur dioxide (SO₂). A gas produced largely by burning sulfur-containing fuels, historically associated with coal-fired power generation and certain industrial processes. It also contributes to the formation of fine particles.
- Nitrogen dioxide (NO₂). One of a group of reactive gases called nitrogen oxides, formed during high-temperature combustion in engines and power plants. It is both a pollutant in its own right and an ingredient in ozone formation.
- Lead (Pb). A metal that, in air, comes today mainly from certain industrial sources and piston-engine aircraft fuel; ambient levels fell dramatically after lead was removed from automotive gasoline. Lead's significance extends well beyond air — the WHO fact sheet on lead covers its exposure pathways more broadly.
How the standards work, in outline
For each criteria pollutant, EPA sets primary standards and, in some cases, secondary standards. Primary standards are set to protect public health; secondary standards protect public welfare — visibility, crops, buildings, ecosystems. A standard specifies a concentration, an averaging time (an hour, a day, a year), and a statistical form. That is why you cannot compare a single hourly reading directly against an annual standard: the averaging time is part of the standard itself.
When a geographic area's monitored air quality violates a standard, EPA can designate it a "nonattainment area," which triggers state planning obligations to bring the area back into compliance. If you see that phrase in local news coverage, this is what it means: a legal status under the Clean Air Act, tied to one specific pollutant and standard.
Where to go deeper
The research context — why scientists study these pollutants and what kinds of effects are under investigation — is summarized well by the NIEHS air pollution topic page. For the global picture, the World Health Organization's fact sheet on ambient air pollution (linked in the sources below) describes worldwide exposure and burden. And to see how these pollutants translate into the daily number in your weather app, read our companion guide, How to Read the Air Quality Index.