The Criteria Pollutants

The twelve pollutants for which India sets national air quality limits, what each one is, where it comes from and what it does.

By Saurabh Dhakad, PhD scholar, Atmospheric Chemistry and Climate Change Lab, School of Environmental Sciences, Jawaharlal Nehru University · 30 Oct 2026 · 8 min read

In 1970 the United States passed the Clean Air Act. It required the Environmental Protection Agency to prepare a detailed scientific "criteria document" for each widespread pollutant that harms health or welfare, and then to set a national limit for it. The pollutants covered by these documents came to be called criteria pollutants. The US list has six: carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter and sulphur dioxide.

India follows the same idea. Under the Air (Prevention and Control of Pollution) Act, 1981, the Central Pollution Control Board sets the National Ambient Air Quality Standards (NAAQS). The present NAAQS were notified on 18 November 2009 and cover 12 pollutants. NET questions refer to them as the criteria pollutants under NAAQS.

India’s twelve NAAQS pollutants

Title: Figure 1. The 12 pollutants in India’s NAAQS (2009), grouped by type, with the unit in which each limit is written. - Description: Figure 1. The 12 pollutants in India’s NAAQS (2009), grouped by type, with the unit in which each limit is written. — The 12 pollutants in India’s NAAQS (2009), grouped by type, with the unit in which each limit is written.

The numerical limits (annual and 24-hour values, and the separate limits for ecologically sensitive areas) are covered in Module 4. This lesson looks at each pollutant as a substance.

The six gases

The gaseous NAAQS pollutants
Pollutant	Nature	Main sources	Main harm and exam point
Sulphur dioxide (SO₂)	Colourless, pungent and irritating; very soluble in water	Coal power plants, smelters, refineries, diesel and furnace oil	Irritates the upper airways and causes bronchitis; forms sulphate particles and acid rain; damages plants and marble.
Nitrogen dioxide (NO₂)	Reddish-brown with a sharp smell (NO is colourless)	Vehicles, power plants and industry; mostly formed in the air from emitted NO	Irritates the deep lung; a precursor of ozone and nitrate. NO_x = NO + NO₂. NO₂ gives smog its brown colour.
Carbon monoxide (CO)	Colourless, odourless and tasteless	Incomplete combustion: petrol vehicles, biomass and coal stoves, smoking	Binds haemoglobin about 200–250 times more strongly than oxygen, forming carboxyhaemoglobin and starving tissues of oxygen. Found at the highest mass concentration of all urban pollutants, so its limit is in mg/m³.
Ozone (O₃)	Pale blue gas with a sharp smell; a strong oxidant	Not emitted; formed from NO_x and VOCs in sunlight	Inflames the lungs and reduces lung function; injures leaves and lowers crop yields; also a greenhouse gas.
Ammonia (NH₃)	Colourless and pungent; the only common alkaline gas in air	Agriculture (urea fertiliser, animal waste), sewage, some industry	Neutralises acids to form ammonium sulphate and ammonium nitrate, a large part of secondary PM_2.5. It also reduces the acidity of rain in India.
Benzene (C₆H₆)	An aromatic VOC with a sweet smell	Petrol vapour and exhaust, fuel stations, solvents, tobacco smoke	A Group 1 human carcinogen that causes leukaemia. Petrol in India may contain at most 1% benzene by volume.

Particles and what they carry

Particulate matter and the pollutants measured on particles
Pollutant	What it is	Main sources	Main harm and exam point
PM₁₀	Particles with an aerodynamic diameter of 10 µm or less (inhalable)	Road and construction dust, soil, industry, plus everything in PM_2.5	Reach the airways. The pollutant most often above the standard in Indian cities.
PM_2.5	Particles of 2.5 µm or less (fine particles)	Combustion in vehicles, biomass and coal; secondary sulphate, nitrate and organic aerosol	Reach the alveoli; linked to heart and lung disease and early death. The most harmful air pollutant in terms of health burden.
Lead (Pb)	A toxic metal carried on particles	Formerly leaded petrol (phased out in India by 2000); now smelters and battery recycling	Damages the nervous system, lowers IQ in children and causes anaemia. Measured on PM₁₀ filters.
Arsenic (As)	A toxic metalloid carried on particles	Coal burning, smelting, the glass industry	A Group 1 carcinogen (lung and skin). Its limit is in ng/m³.
Nickel (Ni)	A metal carried on particles	Oil burning, nickel refining, steel making	Nickel compounds are Group 1 carcinogens (lung and nose). Its limit is in ng/m³.
Benzo[a]pyrene (BaP)	A polycyclic aromatic hydrocarbon (PAH), C₂₀H₁₂, with five fused benzene rings; measured on particles	Incomplete combustion: household wood and coal burning, diesel, coke ovens, coal tar, smoking	A Group 1 carcinogen, used as the marker for all PAHs. Its limit is in ng/m³.

Related terms

Several group names appear around the twelve, and NET often tests whether you know exactly what each includes.

Group names used for pollutants
Term	Meaning	Exam point
NO_x	NO + NO₂	It does not include N₂O (a greenhouse gas) or N₂O₅. NO_x is reported as NO₂.
NO_y	All reactive nitrogen: NO_x plus HNO₃, PAN, NO₃, N₂O₅, HONO, organic nitrates, ClONO₂ and HO₂NO₂	NO_y = NO_x + NO_z, where NOz is the set of oxidation products.
SO_x	SO₂ + SO₃	Almost all of it is emitted as SO₂.
VOC	Organic compounds that evaporate easily at room temperature (the WHO uses boiling points of about 50–260 °C)	Includes alkanes, alkenes, aromatics such as BTEX, halocarbons and oxygenated compounds (aldehydes, ketones, alcohols). Larger PAHs are semi-volatile and are usually not counted as VOCs.
NMHC and NMVOC	Non-methane hydrocarbons and non-methane VOCs	Methane is left out because it reacts very slowly and plays little part in urban smog.
BTEX	Benzene, toluene, ethylbenzene and xylenes	Aromatic VOCs from petrol. Benzene is the most volatile and the most toxic of the four.
HAPs (air toxics)	Hazardous air pollutants: chemicals known or suspected to cause cancer or other serious effects	The US lists 187 of them, including benzene, formaldehyde, mercury and dioxins. Criteria pollutants are common and widespread; HAPs are dangerous even in small amounts.
SPM and RSPM	Suspended particulate matter (all sizes) and respirable suspended particulate matter (10 µm or less)	Older Indian terms used before 2009. RSPM is roughly PM₁₀. SPM is no longer in the NAAQS.

Why these twelve?

A pollutant gets a national standard when it is common (many sources, found almost everywhere), harmful at the levels people actually breathe, and measurable by an agreed method. That is why CO and PM are on the list, while very toxic but less widespread chemicals such as dioxins are controlled through emission limits for particular industries instead.

In practice, urban air quality in India is decided mainly by PM₁₀, PM_2.5, NO₂, SO₂, CO and ozone. Of these, particulate matter exceeds the standard most often, and so it decides most AQI values. The AQI is covered in Module 4.

Quick revision

The term criteria pollutant comes from the US Clean Air Act of 1970. The US has six: CO, Pb, NO₂, O₃, PM and SO₂.
India’s NAAQS were notified on 18 November 2009 by the CPCB under the Air Act, 1981, and cover 12 pollutants.
The twelve: SO₂, NO₂, PM₁₀, PM_2.5, O₃, Pb, CO, NH₃, benzene, BaP, As and Ni.
Units: µg/m³ for most; mg/m³ for CO; ng/m³ for BaP, arsenic and nickel.
CO₂, cadmium and mercury are not NAAQS pollutants.
CO binds haemoglobin about 200–250 times more strongly than oxygen.
NO₂ is reddish-brown; NO, CO and SO₂ are colourless.
NO_x = NO + NO₂. NO_y = NO_x + HNO₃ + PAN + NO₃ + N₂O₅ and other oxidised nitrogen.
Ozone is the only one of the twelve that is never emitted directly.
NH₃ is the main alkaline gas in air; it turns acids into ammonium sulphate and nitrate.
Benzene is a Group 1 carcinogen that causes leukaemia, and the most volatile of the BTEX compounds.
BaP is a PAH (C₂₀H₁₂, five rings), a Group 1 carcinogen and the marker for PAHs; household wood and coal burning is its main source.
Larger PAHs are semi-volatile, not VOCs. Methane is excluded from NMHC.
Raoult’s law: pᵢ = xᵢ Pᵢ°. In an equimolar BTEX mixture, benzene has the highest partial pressure.

One more point about wording. NH₃ is a NAAQS pollutant in India but is not one of the original six criteria pollutants. When a question asks for the criteria pollutants and no option includes every gas listed, choose the option with the classic gases (SO₂, NO₂, O₃ and CO).

References

CPCB (2009). National Ambient Air Quality Standards. Gazette of India, notification of 18 November 2009.
US EPA. Criteria Air Pollutants and the NAAQS Table. epa.gov.
WHO (2021). WHO Global Air Quality Guidelines. World Health Organization, Geneva.
IARC Monographs on the Identification of Carcinogenic Hazards to Humans: benzene (vol. 120), benzo[a]pyrene (vol. 100F), arsenic and nickel (vol. 100C).
Seinfeld, J. H. and Pandis, S. N. (2016). Atmospheric Chemistry and Physics, 3rd ed. Wiley. Chapter 2.