What are Criteria Air Pollutants? The 6 Pollutants Regulated by EPA (with Standards)
- APSEd
- 22 hours ago
- 3 min read
What are criteria air pollutants? Learn the 6 pollutants designated by the US EPA under NAAQS — CO, Pb, NO₂, O₃, PM, and SO₂ — with their standards, averaging times, and why they're called 'criteria' pollutants.
If you've studied environmental science — or just followed news about air pollution — you've likely encountered the term "criteria air pollutants." But what makes a pollutant a criteria pollutant, which six make the list, and what are the actual legal limits? This article covers everything clearly.
Why "Criteria" Pollutants?
The US Environmental Protection Agency (EPA) is required under the Clean Air Act to set National Ambient Air Quality Standards (NAAQS) for certain common pollutants. These pollutants are called criteria air pollutants because the EPA must establish the standards based on scientific criteria — peer-reviewed evidence about health and environmental effects.
These standards are periodically reviewed and revised as new health science becomes available. There are currently six criteria air pollutants.
For India's equivalent framework — which covers 12 pollutants rather than 6 and underpins the Indian AQI scale — see our guide on India's NAAQS and CPCB limits.
The 6 Criteria Air Pollutants and Their Standards
1. Carbon Monoxide (CO)
An odourless, colourless gas produced by incomplete combustion — vehicles and industrial processes are the main sources.
Standard Type | Averaging Time | Level |
Primary | 8 hours | 9 ppm |
Primary | 1 hour | 35 ppm |
CO only has a primary standard (protecting human health). There is no secondary standard because CO does not cause significant environmental damage at these levels.
2. Lead (Pb)
Lead enters the air from metal smelters, battery manufacturers, and (historically) leaded petrol. Chronic exposure causes neurological damage, especially in children.
Standard Type | Averaging Time | Level |
Primary & Secondary | Rolling 3-month average | 0.15 µg/m³ |
3. Nitrogen Dioxide (NO₂)
Produced when fuels are burned at high temperatures — a key precursor to smog and acid rain. High NO₂ levels are also a major driver of reduced atmospheric visibility, giving polluted skies their distinctive brown tint.
Standard Type | Averaging Time | Level |
Primary | 1 hour | 100 ppb |
Primary & Secondary | 1 year | 53 ppb |
4. Ground-Level Ozone (O₃)
Ozone is not directly emitted — it forms when sunlight reacts with NOₓ and VOCs. It is the key ingredient in photochemical smog.
Standard Type | Averaging Time | Level |
Primary & Secondary | 8 hours | 0.070 ppm |
5. Particulate Matter (PM)
PM is divided into two size categories based on health impact:
PM2.5 (fine particles, ≤ 2.5 µm diameter)
Standard Type | Averaging Time | Level |
Primary | 1 year | 12.0 µg/m³ |
Secondary | 1 year | 15.0 µg/m³ |
Primary & Secondary | 24 hours | 35 µg/m³ |
PM10 (coarse particles, ≤ 10 µm diameter)
Standard Type | Averaging Time | Level |
Primary & Secondary | 24 hours | 150 µg/m³ |
PM2.5 is considered more dangerous than PM10 because finer particles penetrate deeper into the lungs and can enter the bloodstream. PM2.5 and PM10 are also the two pollutants that are mandatory for computing India's Air Quality Index — at least one must be present in the dataset.
6. Sulphur Dioxide (SO₂)
Produced mainly from burning coal and oil. A key cause of acid rain and respiratory irritation.
Standard Type | Averaging Time | Level |
Primary | 1 hour | 75 ppb |
Secondary | 3 hours | 0.5 ppm |
Primary vs. Secondary Standards — What's the Difference?
Type | Protects | Example |
Primary | Human health, including sensitive groups (children, elderly, people with asthma) | CO 8-hr standard of 9 ppm |
Secondary | Public welfare — crops, ecosystems, visibility, property | SO₂ 3-hr standard of 0.5 ppm |
Some pollutants have both primary and secondary standards (e.g., NO₂, PM, Pb), while CO has only a primary standard.
Units Used in Air Quality Standards
Criteria pollutant standards are expressed in three units:
ppm (parts per million by volume) — for gaseous pollutants like CO, SO₂, O₃
ppb (parts per billion) — for lower-concentration gases like NO₂
µg/m³ (micrograms per cubic metre) — for particulate matter and lead
Criteria Pollutants vs. India's NAAQS: A Quick Comparison
India's CPCB regulates 12 pollutants under NAAQS — more than the EPA's 6 criteria pollutants. The six EPA criteria pollutants are all included in India's NAAQS, with India additionally regulating benzene, benzopyrene, arsenic, nickel, and ammonia (NH₃).
India's AQI tracks 8 of these 12 pollutants — it excludes benzene, benzopyrene, arsenic, and nickel from the AQI calculation.
Summary
Remember:
There are 6 criteria pollutants (CO, Pb, NO₂, O₃, PM, SO₂)
Primary = health; Secondary = welfare/environment
Units: ppm/ppb for gases, µg/m³ for particles and lead
O₃ has no primary standard for annual average — only 8-hour
Further Reading
On this blog:
External resources:
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