Air quality guide
What the AQI measures: PM2.5, PM10 and the other pollutants
India’s AQI is the worst of up to eight pollutant scores, not a single measurement. This page says what each pollutant is, where it comes from, and the three sets of numbers it is judged against: CPCB’s index breakpoints, India’s legal standards, and the WHO’s 2021 guidelines.
Sources: Central Pollution Control Board; Gazette of India, 18 November 2009; World Health Organization, 2021.
How the AQI is put together
CPCB's National Air Quality Index turns the concentration of each pollutant into a sub-index on a 0–500 scale, using that pollutant's own breakpoints. The sub-index is a straight-line function of concentration between breakpoints: for PM2.5 it is 51 at 31 µg/m³, 75 at 45 µg/m³ and 100 at 60 µg/m³.
The worst sub-index is the AQI for that monitoring location. That is why every reading names a prominent pollutant: it is the one whose sub-index was highest.
Sub-indices use 24-hour average concentrations, except carbon monoxide and ozone, which use 8-hour values. An AQI is published only when at least three pollutants were measured and one of them is PM2.5 or PM10, and a sub-index needs at least 16 hours of data.
The index is a running average: the AQI shown at 6 am covers 6 am the previous day to 6 am today. A single smoky hour moves it slowly, which is why an hourly particulate reading can run well ahead of the index on a morning when smoke is building.
Source: CPCB, National Air Quality Index.
The six AQI bands
Every sub-index and every AQI falls in one of six bands. The health descriptions are CPCB's.
| Band | AQI | What CPCB says it means |
|---|---|---|
| Good | 0–50 | Minimal impact. |
| Satisfactory | 51–100 | Minor breathing discomfort to sensitive people. |
| Moderate | 101–200 | Breathing discomfort to people with lung or heart disease, children and older adults. |
| Poor | 201–300 | Breathing discomfort to most people on prolonged exposure. |
| Very Poor | 301–400 | Respiratory illness on prolonged exposure. |
| Severe | 401–500 | Affects healthy people; serious impact on those with existing disease. |
PM2.5
Fine particulate matter (PM2.5)
Airborne particles 2.5 micrometres across or smaller. It is measured as the mass of those particles in a cubic metre of air.
Where it comes from
Combustion: vehicle exhaust, coal and biomass burning, crop-residue fires, industry and diesel generators, plus particles that form in the air from gases such as sulphur dioxide, nitrogen oxides and ammonia.
The limits it is judged against
- India NAAQS, annual
- 40 µg/m³
- India NAAQS, 24-hour
- 60 µg/m³
- WHO 2021, annual
- 5 µg/m³
- WHO 2021, 24-hour
- 15 µg/m³
NAAQS values are for industrial, residential, rural and other areas.
| AQI band | Sub-index | PM2.5, 24-hour (µg/m³) |
|---|---|---|
| Good | 0–50 | 0–30 |
| Satisfactory | 51–100 | 31–60 |
| Moderate | 101–200 | 61–90 |
| Poor | 201–300 | 91–120 |
| Very Poor | 301–400 | 121–250 |
| Severe | 401–500 | 250+ |
CPCB breakpoints for the PM2.5 sub-index. City pages show this pollutant's live sub-index when CPCB's feed has it.
PM10
Coarse particulate matter (PM10)
Airborne particles 10 micrometres across or smaller. The figure includes PM2.5, so a PM10 reading is always at least as high as the PM2.5 reading taken beside it.
Where it comes from
Road and construction dust, soil and sand lifted by wind, unpaved surfaces, mining and quarrying, along with the combustion sources that produce PM2.5.
The limits it is judged against
- India NAAQS, annual
- 60 µg/m³
- India NAAQS, 24-hour
- 100 µg/m³
- WHO 2021, annual
- 15 µg/m³
- WHO 2021, 24-hour
- 45 µg/m³
NAAQS values are for industrial, residential, rural and other areas.
| AQI band | Sub-index | PM10, 24-hour (µg/m³) |
|---|---|---|
| Good | 0–50 | 0–50 |
| Satisfactory | 51–100 | 51–100 |
| Moderate | 101–200 | 101–250 |
| Poor | 201–300 | 251–350 |
| Very Poor | 301–400 | 351–430 |
| Severe | 401–500 | 430+ |
CPCB breakpoints for the PM10 sub-index. City pages show this pollutant's live sub-index when CPCB's feed has it.
NO₂
Nitrogen dioxide (NO₂)
A reddish-brown gas formed when fuel burns at high temperature. It is one of the gases that later turns into fine particles and ozone.
Where it comes from
Road traffic above all, then power plants, industry and diesel generators.
The limits it is judged against
- India NAAQS, annual
- 40 µg/m³
- India NAAQS, 24-hour
- 80 µg/m³
- WHO 2021, annual
- 10 µg/m³
- WHO 2021, 24-hour
- 25 µg/m³
NAAQS values are for industrial, residential, rural and other areas.
| AQI band | Sub-index | NO₂, 24-hour (µg/m³) |
|---|---|---|
| Good | 0–50 | 0–40 |
| Satisfactory | 51–100 | 41–80 |
| Moderate | 101–200 | 81–180 |
| Poor | 201–300 | 181–280 |
| Very Poor | 301–400 | 281–400 |
| Severe | 401–500 | 400+ |
CPCB breakpoints for the NO₂ sub-index. City pages show this pollutant's live sub-index when CPCB's feed has it.
SO₂
Sulphur dioxide (SO₂)
A sharp-smelling gas released when fuel containing sulphur is burned. In the air it converts to sulphate, a component of fine particulate matter.
Where it comes from
Coal-fired power plants, refineries, smelters and industries burning coal or heavy fuel oil.
The limits it is judged against
- India NAAQS, annual
- 50 µg/m³
- India NAAQS, 24-hour
- 80 µg/m³
- WHO 2021, 24-hour
- 40 µg/m³
NAAQS values are for industrial, residential, rural and other areas.
| AQI band | Sub-index | SO₂, 24-hour (µg/m³) |
|---|---|---|
| Good | 0–50 | 0–40 |
| Satisfactory | 51–100 | 41–80 |
| Moderate | 101–200 | 81–380 |
| Poor | 201–300 | 381–800 |
| Very Poor | 301–400 | 801–1600 |
| Severe | 401–500 | 1600+ |
CPCB breakpoints for the SO₂ sub-index. City pages show this pollutant's live sub-index when CPCB's feed has it.
CO
Carbon monoxide (CO)
A colourless, odourless gas produced when fuel burns without enough oxygen. It is the one pollutant in the index measured in milligrams, not micrograms, per cubic metre.
Where it comes from
Petrol vehicles in slow traffic, biomass and waste burning, and poorly ventilated combustion.
The limits it is judged against
- India NAAQS, 8-hour
- 2 mg/m³
- India NAAQS, 1-hour
- 4 mg/m³
- WHO 2021, 24-hour
- 4 mg/m³
NAAQS values are for industrial, residential, rural and other areas.
| AQI band | Sub-index | CO, 8-hour (mg/m³) |
|---|---|---|
| Good | 0–50 | 0–1.0 |
| Satisfactory | 51–100 | 1.1–2.0 |
| Moderate | 101–200 | 2.1–10 |
| Poor | 201–300 | 10–17 |
| Very Poor | 301–400 | 17–34 |
| Severe | 401–500 | 34+ |
CPCB breakpoints for the CO sub-index. City pages show this pollutant's live sub-index when CPCB's feed has it.
O₃
Ground-level ozone (O₃)
A gas that is not emitted directly: it forms near the ground when sunlight acts on nitrogen oxides and volatile organic compounds. It peaks on hot, sunny afternoons, the opposite season to winter smog.
Where it comes from
Formed in the air from traffic and industrial emissions and solvent vapours, in sunlight.
The limits it is judged against
- India NAAQS, 8-hour
- 100 µg/m³
- India NAAQS, 1-hour
- 180 µg/m³
- WHO 2021, peak season
- 60 µg/m³
- WHO 2021, 8-hour
- 100 µg/m³
NAAQS values are for industrial, residential, rural and other areas.
| AQI band | Sub-index | O₃, 8-hour (µg/m³) |
|---|---|---|
| Good | 0–50 | 0–50 |
| Satisfactory | 51–100 | 51–100 |
| Moderate | 101–200 | 101–168 |
| Poor | 201–300 | 169–208 |
| Very Poor | 301–400 | 209–748 |
| Severe | 401–500 | 748+ |
CPCB breakpoints for the O₃ sub-index. City pages show this pollutant's live sub-index when CPCB's feed has it.
NH₃
Ammonia (NH₃)
A pungent gas. It rarely sets a city's AQI on its own, but it reacts with sulphur and nitrogen compounds in the air to form fine particles.
Where it comes from
Fertiliser use and livestock, sewage and waste, and some industrial processes.
The limits it is judged against
- India NAAQS, annual
- 100 µg/m³
- India NAAQS, 24-hour
- 400 µg/m³
- WHO 2021
- No level in the 2021 guidelines
NAAQS values are for industrial, residential, rural and other areas.
| AQI band | Sub-index | NH₃, 24-hour (µg/m³) |
|---|---|---|
| Good | 0–50 | 0–200 |
| Satisfactory | 51–100 | 201–400 |
| Moderate | 101–200 | 401–800 |
| Poor | 201–300 | 801–1200 |
| Very Poor | 301–400 | 1200–1800 |
| Severe | 401–500 | 1800+ |
CPCB breakpoints for the NH₃ sub-index. City pages show this pollutant's live sub-index when CPCB's feed has it.
Pb
Lead (Pb)
A toxic metal carried on airborne particles. It is the eighth pollutant in the index definition, but it is not part of the real-time station feed, so it does not appear in the live readings on city pages.
Where it comes from
Lead smelting and battery recycling, some industrial processes and waste burning.
The limits it is judged against
- India NAAQS, annual
- 0.50 µg/m³
- India NAAQS, 24-hour
- 1.0 µg/m³
- WHO 2021
- No level in the 2021 guidelines
NAAQS values are for industrial, residential, rural and other areas.
| AQI band | Sub-index | Pb, 24-hour (µg/m³) |
|---|---|---|
| Good | 0–50 | 0–0.5 |
| Satisfactory | 51–100 | 0.5–1.0 |
| Moderate | 101–200 | 1.1–2.0 |
| Poor | 201–300 | 2.1–3.0 |
| Very Poor | 301–400 | 3.1–3.5 |
| Severe | 401–500 | 3.5+ |
CPCB breakpoints for the Pb sub-index. Not part of the real-time station feed, so it has no live reading on city pages.
Which pollutant leads where
From CPCB's daily bulletins for 2025.
Across the 62 cities mapped here, the pollutant most often named as prominent over the year was PM10 in 49, PM2.5 in 12, SO2 in 1 cities. See it for a city:
Or open the India Air Quality Index by city. For what the two particle sizes mean inside a home, read PM2.5 vs PM10: what is the difference and which matters more indoors?
Pollutants and the AQI: questions
What is PM2.5?
PM2.5 is airborne particulate matter 2.5 micrometres across or smaller, measured in micrograms per cubic metre of air (µg/m³). India's National Ambient Air Quality Standards set a limit of 60 µg/m³ over 24 hours and 40 µg/m³ as an annual average; the WHO's 2021 guidelines recommend 15 µg/m³ over 24 hours and 5 µg/m³ annually.
What is the difference between PM2.5 and PM10?
Size. PM10 counts every particle up to 10 micrometres across, and PM2.5 counts only those up to 2.5 micrometres, so PM2.5 is a subset of PM10. PM10 is dominated by dust from roads, construction and soil; PM2.5 is dominated by combustion. India's 24-hour standard is 100 µg/m³ for PM10 and 60 µg/m³ for PM2.5.
What PM2.5 level is safe?
On CPCB's scale a 24-hour PM2.5 average up to 30 µg/m³ is Good and 31–60 µg/m³ is Satisfactory; 60 µg/m³ is also India's 24-hour legal standard. The WHO's 2021 guideline is stricter, at 15 µg/m³ over 24 hours.
How is AQI calculated from PM2.5?
CPCB converts the 24-hour average PM2.5 concentration to a sub-index using fixed breakpoints: 0–30 µg/m³ maps to 0–50, 31–60 to 51–100, 61–90 to 101–200, 91–120 to 201–300, 121–250 to 301–400, and above 250 to 401–500, interpolating in a straight line inside each band. The city's AQI is then the highest sub-index among the pollutants measured.
Which pollutants make up India's AQI?
Eight: PM10, PM2.5, nitrogen dioxide (NO₂), sulphur dioxide (SO₂), carbon monoxide (CO), ozone (O₃), ammonia (NH₃) and lead (Pb). An AQI is calculated only when at least three were measured, one of which must be PM2.5 or PM10.
Why does the prominent pollutant change between cities?
Because the AQI is the highest sub-index, the prominent pollutant is whichever one is furthest up its own scale that day. Dusty, dry cities are usually led by PM10; cities with heavy combustion and winter inversions by PM2.5; ozone can lead on hot, sunny afternoons.
Sources
- CPCB, National Air Quality Index
- National Ambient Air Quality Standards, CPCB notification of 18 November 2009
- WHO global air quality guidelines, 2021
Index breakpoints are from CPCB’s “About National Air Quality Index”; legal limits from the 2009 notification, for industrial, residential, rural and other areas; WHO values from Table 0.1 of the 2021 guidelines. This page is general information, not medical advice.