How is the air index in the weather forecast derived?

The air index in the weather forecast is the air quality index, which is compared with the graded concentration limits of various pollutants (the concentration limits of AQI refer to (GB3095-2012), the concentration limits of API refer to (GB3095) -1996)), based on the measured concentrations of various pollutants such as fine particulate matter (PM2.5), respirable particulate matter (PM10), sulfur dioxide (SO2), nitrogen dioxide (NO2), ozone (O3), carbon monoxide (CO), etc. The air quality sub-index (Individual Air Quality Index, referred to as IAQI) is calculated from the values ​​(PM2.5 and PM10 are the 24-hour average concentrations) respectively.