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.The image below helps to explain this difference.
*Please note that the concentration values in the image above have been rounded to the nearest whole number.
You will see that for an AQI above 200 (151ug/3 PM2.5 concentration), the US and Chinese AQI systems report the same numbers for the same concentrations. Below a PM2.5 concentration of 151ug/3, the US AQI formula reports higher AQI numbers than the Chinese AQI formula. It is for this reason that generally the US AQI system is thought to be more stringent - and perhaps a better guideline for minimizing exposure.