EHS Fundamentals

Criteria air pollutants: 6 emissions you need to know

The list of criteria air pollutants has been nearly the same since 1978 yet legislation around them keeps evolving.

by Nathaniel Gajasa

Criteria air pollutants are a set of six emissions recognized as those posing a significant risk to health, the environment, and property.

In the US, the Environmental Protection Agency (EPA) determined the criteria air pollutants for evaluating the contaminants that diminish air quality in 1978.

Since then, the list has remained essentially unchanged in the US, but the same pollutants appear as part of similar lists of standards in other regions — namely the EU.


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The 6 criteria air pollutants

A graphic of the six common pollutants

Here’s an overview of the 6 emissions commonly known as criteria air pollutants, all of which should be on your facilities’ to-watch list:

Carbon Monoxide (CO)

Carbon Monoxide is primarily emitted by vehicles and gasoline-burning equipment such as generators. CO can also be released as part of various industrial processes, including food, chemical, and metal manufacturing.

When inhaled at elevated levels, CO can reduce the amount of oxygen in the blood, which can lead to dizziness and even death. More commonly, CO causes complications for people with heart conditions, including reducing oxygen to the heart and chest pain.



Common sources of this criteria air pollutant include ore and metal processing, waste incinerators, and lead-acid battery manufacturing.

Historically, one of the largest sources of lead in the air was the burning of leaded gasoline by vehicles. Lead is a probable human carcinogen and exposure can also lead to developmental effects in children, including lowered IQ, as outlined in research by the International Agency for Research on Cancer (IARC).


Nitrogen Dioxide (NO2)

Like CO, Nitrogen Dioxide is emitted by vehicles as well as electrical generating equipment and non-road engines that burn gasoline. NO2 exposure can cause a host of health effects, especially for people with respiratory diseases, such as asthma. Inhaling even small quantities can lead to wheezing, coughing, or difficulty breathing which is severe enough to require hospitalization. Extended exposure can even lead to the development of asthma in healthy individuals.


Ground-level Ozone (O3)

Ozone is not directly emitted by human processes. However, it’s produced by the interaction of nitrogen oxides and volatile organic compounds, which are common industrial emissions. Exposure to ozone can cause difficulty breathing and damage to the airways. As with other pollutants, O3 can aggravate asthma and other lung diseases when inhaled.


Particulate Matter

Particulate Matter (PM) refers to a broad set of pollutants generally grouped together based on the size of the emitted particles. These groups include particles smaller than 2.5 micrometers (PM2.5) and those between 2.5 and 10 micrometers (PM10).

Sources of these particles vary but often include farming operations, combustion sources, construction, and various industrial processes.

According to the EPA, PM exposure can lead to a host of dangerous health effects including heart attacks and an irregular heartbeat, increased respiratory symptoms and aggravated asthma, as well as premature death in people with heart or lung disease.


Sulfur Dioxide (SO2)

This criteria air pollutant is primarily emitted by electrical generation sources that burn sulfur-containing fuels as well as metal processing and smelting facilities. Exposure to SO2 can lead to difficulty breathing, wheezing, and shortness of breath. Even short-term exposures can be associated with hospitalizations for respiratory-related symptoms.

While the list of criteria air pollutants has remained nearly the same since 1978, legislation around them keeps evolving to keep up with various global regulations regarding EHS, ESG, and sustainability.


Regulations around emissions are continually evolving and changing.

Find out what environmental requirements are on the horizon with our 5 changes set to shake up environmental requirements whitepaper.

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A regulatory foundation for better air quality

The classification of 6 criteria air pollutants is meant to help companies evaluate operations — and eventually, act on their findings — to improve results.

 The term refers to specific contaminants that serve as key indicators for air quality, which are therefore regulated by agencies. Although these specific contaminants are regulated as part of clean air standards across the globe, the term “criteria air pollutants” refers specifically to US programs under the Clean Air Act.

In the US, the identified pollutants serve as a baseline for one of the country’s primary regulatory programs under the US Clean Air Act — namely the National Ambient Air Quality Standards (NAAQS). The EPA organizes NAAQS into two types of standards: primary, referring to human health, and secondary, referring to welfare (e.g. environment, livestock, and property). Both standard types are established for each of the criteria air pollutants.

Why is the classification of criteria air pollutants important?

Criteria air pollutants help set the standard for good air quality. They’re essentially the foundation upon which air pollution and emissions regulations are built.

In a broader sense, they guide companies to build a better future for our communities, providing a way to evaluate the risk that these pollutants can pose to our health, property, and environment — and provide measurable targets for their reduction. 

Countries across the globe now use the EPA’s list of the most crucial 6 air pollutants to control as a gauge for businesses to upgrade their efforts for cleaner, safer air.

Criteria air pollutants: Important regulatory activity

While the set of criteria air pollutants stays relatively the same, regulatory activity around them is ever-evolving.

Much of the regulatory activity revolves around the designation of areas as either “attainment” or “nonattainment” in relation to the NAAQS for each of the pollutants described above. This is due to their potentially wide-ranging effects on communities, the environment, and regulated industries.

Agencies tasked with issuing air permits in nonattainment areas are required under the CAA to impose tougher controls on sources than those in attainment. This covers areas where the ambient air quality is worse than the health-based primary standards, the welfare-based secondary standards, or both.

The Biden administration’s EPA has proposed expanding the areas throughout the nation deemed in nonattainment for the federal ozone standards. It’s expected that they’ll also propose the expansion of nonattainment zones for other criteria pollutants.

Why is prioritizing criteria air pollutants important?

While reducing emissions from criteria air pollutants is only one-way companies are charged with helping to improve air quality, it is a big one. Whether you’re starting out at a new facility or making changes to your operations, use this list as a guide of problematic pollutants to check for in your company.

Keeping track of the emissions at your facilities — as well as the attainment status at your sites — can go a long way in meeting your obligations to the surrounding communities and keep you well-prepared for when it comes time to renew your air permit.

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