You may have heard that generally an AQI (Air Quality Index) number is calculated from 6 key pollutants. While this is true, the AQI formula itself does not use all 6 pollutants in one equation. Rather, each of the 6 pollutants has both a concentration and AQI value. The pollutant with the highest AQI level, or 'risk to health', is deemed the "main pollutant" and that pollutant's AQI determines the overall AQI number across all the included pollutants.
AQI is calculated by using the following formula:
Ip = [(Ihi-Ilow)/(BPhi-BPlow)] (Cp-BPlow)+Ilow,
Where Ip is the index of the pollutant; Cp is the rounded concentration of pollutant p; BPhi is the breakpoint greater or equal to Cp; BPlow is the breakpoint less than or equal to Cp; Ihi is the AQI corresponding to BPhi; Ilow is the AQI corresponding to BPlow.
While overall AQI is the highest AQI of the 6 main pollutants, for a majority of locations, the main pollutant in the air is PM2.5 most of the time, which is why we put primary importance on measuring this pollutant. Less frequently, during summer months in many locations the main pollutant may be Ozone, while in particularly sandy or dusty locations it may be PM10.