You may have heard that generally an AQI 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, or 'risk to health', is deemed as the "main pollutant" and that pollutant's AQI becomes the overall AQI.
IAQI 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. More than half of the time, the main pollutant in the air is PM2.5, which is why we put primary importance on measuring this pollutant. In the summer months it's quite common the main pollutant is Ozone, while in particularly sandy or dusty locations it can be PM10.