Let us explain!
The Air Quality Index (AQI) is calculated by weighting 6 key pollutants in a formula. These pollutants are: PM2.5, PM10, Ozone, NO2, CO and SO2. The formula is intended to translate sometimes confusing concentration measurements of pollutants in the air, into one easy-to-understand scale, which shows a comparable level of health risk across pollutants (which are clearly colour-coded). Whichever pollutant is at the most 'risky' concentration (highest AQI number), will dictate overall AQI.
On the app we display the main pollutant in text below the AQI number.
The main pollutant is often PM2.5. Sometimes in hot summer months, Ozone will be the main pollutant, meanwhile - in particularly sandy or dusty places, PM10 will often be the main pollutant. The main pollutant meaning that that pollutant will dictate the AQI curve.