The AQI (Air Quality Index) is a standardised scale converting atmospheric pollutant concentrations into an easy-to-understand number. The European AQI (EAQI) uses a 1–5 scale (good to very poor); the US EPA AQI runs 0–500 with 6 colour-coded categories.
Measured pollutants
Key pollutants are PM2.5, PM10, ground-level ozone (O₃), nitrogen dioxide (NO₂), sulphur dioxide (SO₂), and carbon monoxide (CO). The final AQI is the worst value among all pollutants. Values above 100 (US AQI) are unhealthy for sensitive groups; above 200, hazardous for the entire population.