Can You Give Me an Example of a Criteria Pollutant? Carbon Monoxide, Explained

Carbon monoxide (CO) is a textbook example of a criteria pollutant. It is one of six common air pollutants — along with ground-level ozone, particulate matter, sulfur dioxide, nitrogen dioxide, and lead — for which the EPA sets National Ambient Air Quality Standards under the Clean Air Act. Any one of those six qualifies as an example, but carbon monoxide is a useful one to walk through because it shows every feature that makes a pollutant a "criteria" pollutant.

What makes carbon monoxide a criteria pollutant

A criteria pollutant is not simply any harmful substance in the air. It is a pollutant that meets three conditions, and CO checks all of them:

• It is common and widespread. According to the EPA's carbon monoxide page, the greatest sources of CO in outdoor air are cars, trucks, and other vehicles or machinery that burn fossil fuels. Because combustion happens everywhere people live, CO shows up everywhere people live.
• It comes from many and diverse sources. No single factory or industry accounts for it, so it cannot be handled with a single facility-level rule. It has to be managed with an ambient standard — a limit on how much can be in the outdoor air itself.
• The EPA has published "air quality criteria" for it. These are the science documents summarizing what is known about the pollutant's effects, and they are what the word "criteria" actually refers to. We cover that naming history in why they're called criteria pollutants.

The standard that comes with the example

Being a criteria pollutant means having a legal ceiling. The official NAAQS table lists carbon monoxide's standards as concentrations in parts per million measured over defined averaging times — one standard for an 8-hour average and another for a 1-hour average. Every criteria pollutant has an entry in that same table, each with its own level, averaging time, and form.

That is the practical meaning of the label: a criteria pollutant is a pollutant you can look up in the NAAQS table and find a nationwide outdoor-air limit for.

Would any of the other five work as an example?

Yes — each of the six fits the same pattern, just with different sources and forms:

• Ground-level ozone is a secondary pollutant formed in the air from other emissions rather than released directly, per the EPA's ground-level ozone page.
• Particulate matter is a mixture of solid and liquid particles rather than a single chemical, described on the EPA's particulate matter page. It is regulated in two size classes, which we compare in PM2.5 vs. PM10.
• Sulfur dioxide, nitrogen dioxide, and lead round out the list, each with its own row in the NAAQS table.

If you were asked for an example on a quiz or in a report, naming any one of the six is correct. If you want the full list with what each pollutant is and where it comes from, see our guide to the six criteria air pollutants.

What would not be an example

A common trip-up: not every regulated air pollutant is a criteria pollutant. Benzene, mercury, and asbestos are regulated under a completely separate Clean Air Act program for hazardous air pollutants, with different rules and no ambient standard in the NAAQS table. Citing one of those as a "criteria pollutant" is the most frequent mistake on this topic — the distinction is laid out in criteria pollutants vs. hazardous air pollutants.

So: carbon monoxide is your example. Common, emitted from many sources, backed by published criteria, and capped by a nationwide outdoor-air standard — that combination is exactly what the term means.