Pollutant Standards Index (PSI)

The Pollutant Standards Index (PSI) it is an index that the National Environment Agency in Singapore uses to provide information about daily levels of air quality in Singapore. The PSI is computed based on the concentration levels of particulate matter (PM10), fine particulate matter (PM2.5), sulphur dioxide (SO2), nitrogen dioxide (NO2), ozone (O3), and carbon monoxide (CO). These six pollutant parameters determine the PSI, hence, it provides an overall index about the outdoor air quality. The NEA monitors the outdoor air pollution using a network of air monitoring stations located in different parts of Singapore. It is important to remember that the PSI value is based on historical data. Consequently during haze episodes it is recommended to refer to 1-hour PM_2.5 concentration readings which are a better indicator of the outdoor air pollution.

The NEA calculates the 24-h PSI value firstly by linearly mapping each individual sub-pollutant (i.e., 24-h PM10, 24-h PM2.5, 8-h CO, 24-h SO2, 1-h NO2, 8-h O3) to a sub-index ranging from 0 to 500. Then the overall 24-h PSI value is assumed to be equal to the maximum value of any of the pollutants sub-indices. The table below shows the air quality descriptor relative to the specific PSI range.

Difference between the PSI and the Air Quality Index (AQI)

The U.S. Environmental Protection Agency (EPA) and other air quality websites like aqicn.org use the Air Quality Index (AQI) to provide information about the outdoor air quality. The AQI is calculated based on four major air pollutants: particulate matter (PM), sulphur dioxide (SO2), ozone (O3), and carbon monoxide (CO). Hence, currently it does not includes nitrogen dioxide (NO2). As the PSI the AQI ranges on a scale from 0 to 500. Where higher values refer to higher concentrations of outdoor air pollutants. Values of AQI below 100 are considered by the EPA to be satisfactory, while values above 100 are considered to be unhealthy. You can find out more about the AQI in this brochure published by EPA. The table below shows how the EPA divides the AQI in six levels of health concern.