Reading AQI numbers without getting confused
India's AQI, the US AQI and PM2.5 in µg/m³ are three different numbers for the same air. How to read each, and which to trust.
You might see “AQI 380” on the news, “AQI 270” on a phone weather app and “PM2.5 190” on a monitor, all on the same morning. They can all be right. They’re different scales.
The three numbers
- India’s National Air Quality Index (NAQI), published by the Central Pollution Control Board (CPCB). Scale 0–500, six categories from Good to Severe. This is the official number for Delhi and the one GRAP decisions are based on.
- The US EPA AQI, used by many international apps and some consumer monitors. Also 0–500, but its categories are much stricter: the same air gets a higher number and a worse label.
- PM2.5 concentration in µg/m³ (micrograms per cubic metre): the actual measurement, with no scale applied. Indoor monitors and the WHO guidelines use this.
Try it
AQI translator
Apps and news reports quote different scales. Enter whichever number you have to see it on the other scales and next to the WHO guideline.
How rough?
A rule of thumb from Berkeley Earth (22 µg/m³ ≈ 1 cigarette a day) that compares population death rates, not the dose you inhale. It is a communication aid, not a personal risk estimate.Both scales use 24-hour average PM2.5. The AQI shown by CPCB is the highest sub-index across several pollutants, so in dusty weather it may be driven by PM10 and the PM2.5 implied here will be an overestimate. Hourly “live” numbers are not the same as 24-hour averages.
The breakpoints
| Category | Index | PM2.5, 24-h average (µg/m³) | CPCB health statement |
|---|---|---|---|
| Good | 0–50 | 0–30 | Minimal impact |
| Satisfactory | 51–100 | 31–60 | May cause minor breathing discomfort to sensitive people |
| Moderate | 101–200 | 61–90 | May cause breathing discomfort to people with lung disease such as asthma, and discomfort to people with heart disease, children and older adults |
| Poor | 201–300 | 91–120 | May cause breathing discomfort to people on prolonged exposure, and discomfort to people with heart disease with short exposure |
| Very Poor | 301–400 | 121–250 | May cause respiratory illness on prolonged exposure; effect may be more pronounced in people with lung and heart diseases |
| Severe | 401–500 | 251+ | May cause respiratory effects even in healthy people and serious health impacts in people with lung or heart disease; may be felt even during light physical activity |
CPCB does not define a concentration ceiling for Severe. For interpolation this site maps AQI 500 to 380 µg/m³ and shows 500+ above it.
| Category | Index | PM2.5, 24-h average (µg/m³) |
|---|---|---|
| Good | 0–50 | 0–9 |
| Moderate | 51–100 | 9.1–35.4 |
| Unhealthy for Sensitive Groups | 101–150 | 35.5–55.4 |
| Unhealthy | 151–200 | 55.5–125.4 |
| Very Unhealthy | 201–300 | 125.5–225.4 |
| Hazardous | 301–500 | 225.5+ |
Reading AQI well
- It’s the worst pollutant that sets the AQI. India’s AQI is the highest of several pollutant sub-indices (PM2.5, PM10, NO₂, ozone and others). On dusty days, PM10 may drive the number.
- AQI is based on averages. The official AQI uses 24-hour averages for particles, so it lags behind sudden changes. For “should I go out now?”, look at the latest hourly PM2.5 at the nearest station.
- Use the nearest station, with its timestamp. Pollution varies a lot across Delhi NCR. A city average can hide a much worse reading near you.
- Apps may convert numbers. If an app’s number looks very different from CPCB, check whether it uses the US scale.
Where to check
- The SAMEER app (official, from CPCB)
- CPCB National AQI dashboard (if it doesn’t load, try the mirror)
- IITM’s Air Quality Early Warning System for Delhi (forecasts) and the Delhi government’s AQI portal
More in resources.