Air Pollution Index (API)
The Air Pollution Index (API) serves as a way to identify days when air quality is good or poor and it is primarily associated with human health. The index values range from 0 to 500, with lower values associated with better air quality and lower health risks. Within this range, there are 5 API categories which are defined based on a given subrange of index values and these categories are associated with potential health concerns. For example, the index range 0-50 is defined as Good while 101 – 200 is defined as Unhealthy. The Malaysian API follows closely the United States Air Quality Index (AQI) categories, with the exception that currently, the Unhealthy for Sensitive Group and Unhealthy AQI categories are grouped together under Unhealthy API in Malaysia.
The API values currently broadcasted through APIMS (Air Pollutant Index of Malaysia) is based on the following categories:
API index value is derived hourly from ambient measurement or forecast values of pollutants. In Malaysia, the Environmental Quality Monitoring Programme (EQMP) under the
Department of Environment provides the input needed for obtaining API values. The EQMP network includes 65 continuous air quality monitoring sites throughout Malaysia. The criteria pollutants that are monitored and included in API calculations are Particulate Matter (PM10 and PM2.5), Ozone (O3) , Nitrogen Dioxide (NO2) , Sulfur Dioxide (SO2) and Carbon Monoxide (CO).
To obtain hourly API value, an API sub-index value is first calculated for each pollutant based on its concentration and the sub-index with the highest value is then selected as the hourly API value:
The pollutant concentration used as input in API sub-index calculations do not have the same averaging hour. For example, 1-hour averages are used for deriving hourly NO2 sub-index while 24-hour running averages are used for obtaining hourly PM2.5 sub-index. Hence, in some cases, although the air appears polluted such as during a haze episode with high PM2.5 concentrations, the API value may only show Unhealthy or Very Unhealthy values several hours after the incident. One reason for the difference in the averaging hour used as input value for API is that health impacts related to the specific pollutant is linked to duration of exposure.
Typically, the shift from Moderate to Unhealthy API category is closely linked to ambient air quality standards. Hence, as the Malaysian Ambient Air Quality Standards become more stringent, air quality may appear to deteriorate when new standards are implemented although the actual pollutant concentration has not necessarily increased.