What is the difference between the China AQI and US AQI? The technical answer is that both systems use the same formula, represented below. The difference, however, is in the AQI thresholds:
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.
You will see that for an AQI above 200 (151 μg/m3 PM2.5 concentration), the US and Chinese AQI systems report the same numbers for the same concentrations. Below a PM2.5 concentration of 151 μg/m3, 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.