PSI stands for ‘Pollutant Standards Index’. It is an index to provide accurate and easily understandable information about daily levels of air quality.
PSI is computed based on 24-hour average of PM2.5 concentration levels, among other pollutants. PM2.5 is the dominant pollutant during haze episodes.
The ambient air in Singapore is monitored through a network of air monitoring stations located in different parts of Singapore.
The monitoring stations measure 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.
The PSI value gives an indication of the air quality as shown:
| PSI Value | PSI Descriptor |
| 0 – 50 | Good |
| 51 – 100 | Moderate |
| 101 – 200 | Unhealthy |
| 201 – 300 | Very unhealthy |
| Above 300 | Hazardous |
Technical details on how the PSI is calculated can be found here: Computation of PSI