API Calculation Method

API is the conversion of the ambient respirable suspended particulate (RSP), sulphur dioxide (SO₂), carbon monoxide (CO), ozone (O₃) and nitrogen dioxide (NO₂) concentrations measured at our air quality monitoring network to a scale of 0 to 500. An index of 100 corresponds to the short-term Hong Kong Air Quality Objectives (HKAQO) estabished under the Air Pollution Control Ordinance.

Real time air quality data from each monitoring station are transferred via telephone line to the air quality data processing centre of EPD in Revenue Tower for calculation of API. API is calculated by first computing the subindices of scale extending from 0 to 500 for each air pollutant measured at each air quality monitoring station based on the 1-hour, 8-hour and/or 24-hour average concentrations. Subindex levels have been related to ambient pollutant concentrations over a 24 hour period based on the potential health effects of pollutants. A subindex level of 100 corresponds to the short-term Hong Kong Air Quality Objectives (HKAQO) below which there is no adverse acute health effect to human while a level of 500 corresponds to significant harm to human health. API is subsequently calculated for each air quality monitoring station by taking the maximum of the subindices among all the parameters measured at that station to indicate the overall pollution level.