What Are Criteria Pollutants?

Criteria pollutants are common air pollutants used to measure and compare outdoor air quality.

The most widely monitored criteria pollutants are:

• PM₂.₅
• PM₁₀
• Nitrogen dioxide (NO₂)
• Sulfur dioxide (SO₂)
• Ground-level ozone (O₃)

These pollutants are commonly used in Air Quality Index (AQI) systems because they can be measured reliably using established monitoring methods. To understand how these pollutants are converted into AQI values, read our detailed guide on how AQI is calculated in India.

In India, the Central Pollution Control Board (CPCB) monitors these pollutants through national air-quality monitoring networks.

This article explains:

• what these pollutants are
• how particulate and gaseous pollutants are classified
• how air-quality monitoring systems measure them
• why they are important in AQI reporting and pollution monitoring

Why Are These Pollutants Monitored?

Criteria pollutants are monitored because they provide a practical and reliable way to study outdoor air quality.

These pollutants help scientists and environmental agencies:

• track pollution levels over time
• compare air quality between cities
• calculate Air Quality Index (AQI) values
• study long-term pollution trends

Particulate pollutants such as PM₂.₅ and PM₁₀ are classified mainly by particle size, while gaseous pollutants such as NO₂, SO₂, and O₃ are identified by their chemical properties.

Many other pollutants also exist in the atmosphere, including volatile organic compounds (VOCs) and air toxics. However, criteria pollutants are widely used in routine monitoring because reliable measurement systems and long-term monitoring data are available for these pollutants.

Although monitoring systems may differ slightly between countries, the basic goal remains the same: to measure air pollution using pollutants that can be tracked using similar monitoring methods across different cities and regions.

Criteria pollutants are not grouped because they are chemically similar. Instead, they are treated as standard reference pollutants that help scientists and environmental agencies track air pollution levels across different locations and time periods.

Main Criteria Pollutants

The table below shows the most commonly monitored criteria pollutants and their typical pollution sources.

Particulate Matter (PM₂.₅ and PM₁₀)

Particulate matter refers to tiny solid particles and liquid droplets suspended in the air. These particles may include dust, smoke, soot, ash, and other microscopic materials.

Because airborne particles vary greatly in composition, shape, and origin, air-quality systems classify particulate matter mainly by particle size. This allows monitoring systems to measure and compare particles using similar measurement standards.

PM₂.₅ vs PM₁₀

PM₂.₅ includes particles with a diameter of 2.5 micrometers (µm) or smaller. These particles are extremely small and can remain suspended in the air for long periods. A human hair is usually much wider than a PM₂.₅ particle. Because PM₂.₅ particles are so small, they are often invisible to the naked eye even when pollution levels are high.

PM₁₀ includes particles up to 10 micrometers in diameter and contains both fine and larger coarse particles such as road dust and construction dust.

Air-quality monitoring systems use size-selective instruments to separate these particle categories during measurement.

Why Particle Size Matters

Particle size affects how particles move through the atmosphere and how they are measured by monitoring systems.

Smaller particles usually remain suspended in the air longer, while larger particles settle more quickly. For this reason, particle size is one of the main classification methods used in modern air-quality monitoring systems and AQI frameworks worldwide.

Gaseous Criteria Pollutants (NO₂, SO₂, and O₃)

Besides particulate matter, air-quality systems also monitor several important gaseous pollutants.

The most commonly monitored gaseous criteria pollutants are:

• Nitrogen dioxide (NO₂)
• Sulfur dioxide (SO₂)
• Ground-level ozone (O₃)

These gases are monitored because they are commonly found in polluted urban air and can be tracked using established air-quality monitoring systems.

Nitrogen Dioxide (NO₂)

NO₂ is mainly associated with vehicle emissions, fuel combustion, and urban traffic pollution. It is commonly found in cities with heavy traffic and is widely used as an indicator of urban air pollution.

Because NO₂ can be measured continuously using air-quality monitoring instruments, it is included in AQI reporting systems in many countries, including India.

Sulfur Dioxide (SO₂)

SO₂ is mainly linked to industrial activities and the burning of sulfur-containing fuels such as coal and oil.

Air-quality monitoring systems track SO₂ because it is commonly associated with industrial emissions and thermal power generation in many regions.

Ground-Level Ozone (O₃)

Ground-level ozone is a gaseous pollutant found in the lower atmosphere. Unlike pollutants that are released directly into the air, ozone forms through chemical reactions involving other pollutants in sunlight.

Ground-level ozone is considered a secondary pollutant because it forms through atmospheric chemical reactions rather than being released directly from a source. This makes ozone different from pollutants that are emitted directly into the air from vehicles or industrial activities.

High ground-level ozone concentrations are often associated with urban smog conditions during hot and sunny weather.

Because ozone levels can vary depending on weather conditions and sunlight intensity, monitoring systems track ozone as an important component of air-quality assessment.

Why These Gases Are Important

NO₂, SO₂, and O₃ help scientists and environmental agencies study changes in urban air pollution over time.

These pollutants are also used in AQI calculations and long-term air-quality monitoring programs.

How Air-Quality Monitoring Systems Measure These Pollutants

Air-quality monitoring systems use specialized instruments to measure pollutant concentrations in the atmosphere.

Particulate matter such as PM₂.₅ and PM₁₀ is measured using instruments that separate airborne particles by size before calculating particle concentration.

Gaseous pollutants such as NO₂, SO₂, and O₃ are measured using gas analyzers designed to detect specific pollutants in ambient air.

Pollutant concentrations are usually reported using standard units such as:

• micrograms per cubic meter (µg/m³)
• parts per billion (ppb)

Using common measurement units allows pollution data to be compared across locations and time periods.

Different monitoring instruments are used for particulate matter and gaseous pollutants because these pollutants behave differently in the atmosphere and require different measurement methods.

In India, organizations such as the Central Pollution Control Board (CPCB) use national air-quality monitoring stations to track these pollutants and support AQI reporting systems.

During winter in many Indian cities, including Delhi, AQI values may increase because cooler weather and weaker air movement can trap pollutants closer to the ground for longer periods.

Learn how air-quality monitoring stations measure pollutants such as PM₂.₅, ozone, and nitrogen dioxide using specialized monitoring instruments.

Limitations of Criteria Pollutant Classification

Criteria pollutants are important reference pollutants in air-quality monitoring systems, but they do not represent every pollutant present in the atmosphere.

Many other pollutants, including volatile organic compounds (VOCs), toxic metals, and region-specific industrial pollutants, may also affect air quality.

Particulate matter categories such as PM₂.₅ and PM₁₀ are based mainly on particle size rather than exact chemical composition. As a result, particles within the same size category may still differ in origin and chemical properties.

In addition, monitoring systems and pollutant lists may vary slightly between countries depending on monitoring infrastructure, environmental conditions, and national air-quality frameworks.

Despite these limitations, criteria pollutants remain widely used because they provide a practical way to measure and compare outdoor air quality across different locations and time periods.

Frequently Asked Questions

Conclusion

Criteria pollutants such as PM₂.₅, PM₁₀, NO₂, SO₂, and O₃ are important components of modern air-quality monitoring systems.

In India, organizations such as the CPCB use these pollutants to track air-quality conditions across cities and regions.

Understanding these pollutants helps explain how AQI systems and air-quality monitoring networks interpret pollution conditions in different environments.

References

1. World Health Organization (WHO). (2021). WHO Global Air Quality Guidelines: Particulate Matter (PM₂.₅ and PM₁₀), Ozone, Nitrogen Dioxide, Sulfur Dioxide and Carbon Monoxide. Geneva: WHO.
2. Ministry of Environment, Forest and Climate Change (MoEFCC), Government of India. (2009). National Ambient Air Quality Standards (NAAQS).
3. Central Pollution Control Board (CPCB), Government of India. National Air Quality Monitoring Programme (NAMP): Guidelines and Methodology.
4. Central Pollution Control Board (CPCB), Government of India. National Air Quality Index (AQI): Technical Framework.
5. Seinfeld, J. H., & Pandis, S. N. (2016). Atmospheric Chemistry and Physics: From Air Pollution to Climate Change (3rd ed.). Wiley.

Last Update: June 2026