Why Are They Called "Criteria" Pollutants?
If you've read about the six criteria air pollutants, you may have wondered about the name. "Criteria" sounds like a strange label for a group of substances. It isn't describing the pollutants themselves — it's describing the process the law requires before the government can limit them.
The name comes from the law, not the chemistry
Under the Clean Air Act, the EPA cannot simply declare a limit for a common air pollutant. It first has to assemble the scientific evidence about what that pollutant does — to human health, and to the broader public welfare, meaning things like visibility, crops, vegetation, and buildings. That body of evidence is referred to as the air quality criteria for the pollutant.
The national standards are then set based on those criteria. So the pollutants that go through this process picked up the shorthand name criteria pollutants: pollutants for which air quality criteria have been developed and national ambient standards issued. The EPA's criteria air pollutants page identifies the six: carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide.
"Criteria" is about evidence, and the standards can change
Because the standards rest on scientific criteria rather than a fixed number written into the statute, they are not frozen. The EPA reviews the science and the standards periodically, and the levels and averaging times can be revised over time. The current legal limits — with their exact concentrations and averaging periods — are published in the official NAAQS table. If you compare an old textbook to the current table, you may find the numbers have moved; that's the criteria process working as designed.
What the label tells you in practice
When you see "criteria pollutant" in an air quality report, it signals three practical things:
- There is a national ambient standard for it. Each criteria pollutant has one or more legal concentration limits for outdoor air, listed in the NAAQS table.
- It is routinely monitored. Networks of official monitors track these pollutants across the country, which is why daily reports and forecasts can cover them.
- It feeds into the AQI. The Air Quality Index that appears in weather apps is built on a subset of these same pollutants. AirNow's AQI basics page explains how the index translates measured concentrations into the familiar 0–500 scale and color categories.
What the label does not mean
"Criteria pollutant" does not mean "the six most dangerous chemicals in the air." Plenty of hazardous substances — benzene, asbestos, many solvents and metals — are regulated through entirely different programs and never appear in an AQI number. The criteria pollutants are the common, widespread pollutants for which nationwide outdoor-air standards make sense: they come from many sources, they occur essentially everywhere, and everyone is exposed to some degree.
It's also worth keeping "criteria pollutants" separate from two look-alike distinctions. Primary vs. secondary pollutants is about chemistry — whether a pollutant is emitted directly or forms in the atmosphere. And the standards themselves come in primary and secondary versions, which is about who or what is being protected, not about the pollutant. All of those terms coexist, and mixing them up is one of the most common sources of confusion when reading about air quality.
So the short answer: they're called criteria pollutants because the law requires scientific criteria to be established for them, and national air quality standards are then set on the basis of those criteria. The name is a fossil of the legal process — but a useful one, because it flags exactly the pollutants that official standards, monitors, and the AQI are built around.