How to Read the Air Quality Index (AQI)

The Air Quality Index converts raw pollutant concentrations into a simple 0–500 scale, with six color-coded categories.Green (0–50) means air quality is satisfactory.Yellow (51–100) is acceptable but may concern unusually sensitive individuals.Orange (101–150) is unhealthy for sensitive groups—children, elderly, and those with heart or lung disease.

Red (151–200) affects everyone; outdoor exercise should be limited.Purple (201–300) triggers health alerts.Maroon (301–500) is hazardous; everyone should avoid outdoor activity.Different countries use different AQI formulas.

The US EPA method converts PM2.5, PM10, O3, NO2, SO2, and CO into sub-indices and reports the highest.China uses its own scale with slightly different breakpoints.India’s NAQI adds lead and ammonia.When reading AQI, always note which pollutant drives the index—it’s usually PM2.5 in urban areas.

Raw concentration data (in µg/m³) is more precise for health decisions than the simplified index..