What is the Air Quality Index (AQI)?
The Air Quality Index turns the levels of several pollutants into a single number from 0 to 500, so you can tell at a glance how clean the air is and what to do about it. The higher the number, the greater the health risk.
Each pollutant gets its own sub-index from its concentration, and the AQI is the highest of them. That is why every AQI reading has a "main pollutant", usually PM2.5 in cities. Different countries use different breakpoints, so the same air can carry different numbers. Good Air shows the official national index first and the US AQI alongside it for comparison.
US AQI scale (US EPA)
| AQI | Category | What it means |
|---|---|---|
| 0–50 | Good | Air quality is satisfactory and poses little or no risk. |
| 51–100 | Moderate | Air quality is acceptable. People who are unusually sensitive to air pollution may feel effects with long or heavy exertion outdoors. |
| 101–150 | Unhealthy for Sensitive Groups | Children, older adults, pregnant people and anyone with heart or lung disease may feel effects. Most other people are unlikely to be affected. |
| 151–200 | Unhealthy | Some members of the general public may feel health effects; sensitive groups may feel more serious effects. |
| 201–300 | Very Unhealthy | Health alert: the risk of health effects is increased for everyone. |
| 301–500 | Hazardous | Health warning of emergency conditions: everyone is more likely to be affected. |
Choose a range for activity advice: 0–50 · 51–100 · 101–150 · 151–200 · 201–300 · 301–500
India's National AQI (CPCB)
| AQI | Category | Health impact (CPCB) |
|---|---|---|
| 0–50 | Good | Minimal impact on health. |
| 51–100 | Satisfactory | May cause minor breathing discomfort to sensitive people. |
| 101–200 | Moderate | May cause breathing discomfort to people with lung disease such as asthma, and discomfort to people with heart disease, children and older adults. |
| 201–300 | Poor | May cause breathing discomfort to most people on prolonged exposure, and discomfort to people with heart disease. |
| 301–400 | Very Poor | May cause respiratory illness on prolonged exposure. The effect may be more pronounced in people with lung and heart diseases. |
| 401–500 | Severe | May cause respiratory effects even in healthy people, and serious health impacts on people with lung or heart diseases. Health effects may be felt even during light physical activity. |
India's index treats moderate pollution more gently than the US scale: PM2.5 up to 30 µg/m³ counts as Good, against 9 µg/m³ on the US scale since 2024.
China's AQI (MEE)
| AQI | Category |
|---|---|
| 0–50 | Excellent |
| 51–100 | Good |
| 101–150 | Lightly Polluted |
| 151–200 | Moderately Polluted |
| 201–300 | Heavily Polluted |
| 301+ | Severely Polluted |
The pollutants behind the number
| Pollutant | What it is | WHO guideline |
|---|---|---|
| PM2.5 | Particles smaller than 2.5 micrometres. They reach deep into the lungs and the bloodstream, and drive most of the health harm from polluted air. | 15 µg/m³ (24 h) |
| PM10 | Dust, smoke and road particles up to 10 micrometres. They irritate the nose, throat and airways. | 45 µg/m³ (24 h) |
| NO₂ | A gas mostly from vehicle exhausts and power plants. It inflames the airways and worsens asthma. | 25 µg/m³ (24 h) |
| O₃ | Formed when sunlight reacts with traffic and industrial emissions. It peaks on hot, sunny afternoons. | 100 µg/m³ (8 h) |
| SO₂ | Released by burning coal and oil. It narrows the airways, especially in people with asthma. | 40 µg/m³ (24 h) |
| CO | A colourless gas from incomplete burning. High levels reduce the oxygen the blood can carry. | 4000 µg/m³ (24 h) |
WHO Global Air Quality Guidelines, 2021.
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Frequently asked questions
What is a good AQI?
On the US scale, 0 to 50 is Good and 51 to 100 is Moderate, which is acceptable for almost everyone. On India's NAQI, 0 to 50 is Good and 51 to 100 Satisfactory.
At what AQI should I stop exercising outside?
People in sensitive groups should cut back from US AQI 101. Everyone should move hard exercise indoors from 151 and avoid outdoor exercise from 201. See AQI 151 to 200.
Why does my app show a different AQI from the government?
Different countries use different scales. The same air in Delhi might read NAQI 180 (Moderate) and US AQI 160 (Unhealthy), because the US scale treats PM2.5 more strictly. Apps also differ in which stations they use and how they average them. Read the full explanation.
Is the AQI an hourly or daily number?
Officially, AQI categories are defined on 24-hour averages for particles (8 hours for ozone and CO). Live AQI numbers, including those on Good Air, use the latest hourly readings, so they react faster to a sudden smoke plume or a clean wind.
What is the highest AQI possible?
Most scales stop at 500. Readings above that, which do happen in severe smoke or dust, are usually shown as 500 or "500+".