PSI (Pollutant Standards Index) is the number NEA publishes to describe Singapore's air quality — the same figure behind headlines about haze. It's a composite reading built from several pollutants, most often driven by PM2.5 (fine particulate matter) during haze episodes.
| PSI range | Classification | What to do |
|---|---|---|
| 0–50 | Good | No precautions needed. |
| 51–100 | Moderate | Generally fine for most people; those with existing respiratory or heart conditions may want to reduce prolonged outdoor exertion. |
| 101+ | Unhealthy and above | Reduce prolonged or heavy outdoor exertion, especially for children, the elderly, and those with existing health conditions. Consider an N95 mask outdoors and keeping windows closed. |
NEA reports PSI for five regions (north, south, east, west, central) as well as a national figure, updated hourly. Haze conditions can shift quickly depending on wind direction, so the number where you are can differ meaningfully from the headline national figure.
PSI is a composite index; PM2.5 is one of the individual pollutant readings that feeds into it. During haze episodes, PM2.5 is usually the pollutant driving the PSI number up. If you have asthma or another respiratory condition, the PM2.5 sub-reading specifically is often the more clinically relevant number to watch.