What Is the Air Quality Index?
The Air Quality Index (AQI) is a standardised scale used by governments worldwide to communicate outdoor air pollution levels in a way the public can easily understand. Rather than reporting raw pollutant concentrations, AQI translates them into a 0-500 scale with colour-coded health categories.
AQI Categories
- 0-50 (Green) — Good. Air quality is satisfactory with little or no health risk.
- 51-100 (Yellow) — Moderate. Acceptable, though unusually sensitive individuals may experience mild effects.
- 101-150 (Orange) — Unhealthy for Sensitive Groups. People with respiratory or heart conditions, children, and the elderly should limit prolonged outdoor exposure.
- 151-200 (Red) — Unhealthy. Everyone may begin to experience health effects.
- 201-300 (Purple) — Very Unhealthy. Health alert; significant risk for the entire population.
- 301-500 (Maroon) — Hazardous. Emergency conditions; everyone is affected.
What Pollutants Does AQI Measure?
The AQI in most countries incorporates five or six key pollutants:
- PM2.5 — fine particulate matter (often the dominant driver of high AQI)
- PM10 — coarse particulate matter
- Ozone (O3) — ground-level ozone
- Nitrogen dioxide (NO2) — from vehicle exhaust
- Sulphur dioxide (SO2) — from industrial sources
- Carbon monoxide (CO) — from combustion
Notably, CO2 is not included in standard AQI calculations. CO2 is addressed separately through indoor air quality guidelines because it is primarily an indoor concern driven by occupancy rather than pollution.
AQI vs Indoor Air Quality
AQI is designed for outdoor air. For indoor spaces, CO2 and VOCs are often more relevant metrics. A room can have excellent outdoor AQI but terrible indoor air quality if ventilation is poor. Learn the differences in our IAQ beginner guide.
How to Use AQI Practically
- Check daily — apps like IQAir and AirNow provide real-time local AQI.
- Protect indoors on high-AQI days — keep windows closed and run a HEPA air purifier. See monitors vs purifiers.
- Balance ventilation — on high-PM2.5 days, opening windows to reduce indoor CO2 may raise PM2.5. A multi-sensor monitor helps you find the right balance.
- Use the calculator — the CO2 calculator helps you understand indoor ventilation needs independent of outdoor conditions.