It is calculated from several sets of air pollution data and was formerly used in mainland China and Hong Kong.
Air Pollution Level If the API exceeds 500, a state of emergency is declared in the reporting area.
Usually, this means that non-essential government services are suspended, and all ports in the affected area are closed.
[4] AQI and Health Implications (Daily Targets)[4] The API was in use in Hong Kong from June 1995 to December 2013.
The API was based on the level of six atmospheric pollutants, namely sulphur dioxide (SO2), nitrogen dioxide (NO2), respirable suspended particulates, carbon monoxide (CO), ozone (O3), and lead (Pb), measured at all the monitoring stations throughout the territory.
The former includes Central / Western, Eastern, Kwai Chung, Kwun Tong, Sha Tin, Sham Shui Po, Tai Po, Tap Mun, Tsuen Wan, Tung Chung, and Yuen Long; the latter Causeway Bay, Central, and Mong Kok.
The index and the air quality objectives were set in 1987; and pollutant levels are measured over varying periods, in μg/m3.
The table below shows the official Health Implications of the respective AQHI levels in Hong Kong.