Singapore's Pollutant Standards Index (PSI) Explained
Singapore reports air quality with the Pollutant Standards Index — a 0-to-500 scale run by the National Environment Agency. It descends from the older US EPA PSI (the system the US replaced with today's AQI), so the structure looks familiar, but the bands and the way Singapore handles haze season give it its own character. Because Singapore is one of Smog Report's most active markets, this guide goes into the detail that matters there.
What the PSI is
The PSI summarises six pollutants: PM2.5, PM10, sulphur dioxide (SO₂), carbon monoxide (CO), ozone (O₃), and nitrogen dioxide (NO₂). Each pollutant gets a sub-index from its concentration band, and the reported PSI is the highest of the six. NEA publishes a 24-hour PSI for five regions of the island (north, south, east, west, central) plus a national reading, and during haze it also publishes 1-hour PM2.5 concentration readings so the public can see fast-moving changes the 24-hour figure smooths out.
The five bands
| PSI | Band | Colour | In plain English |
|---|---|---|---|
| 0–50 | Good | Green | Normal activities for everyone. |
| 51–100 | Moderate | Yellow | Normal activities; no restrictions. |
| 101–200 | Unhealthy | Orange | Reduce prolonged or strenuous outdoor exertion; the elderly, children, and those with heart or lung conditions should minimise it. |
| 201–300 | Very Unhealthy | Red | Avoid prolonged or strenuous outdoor activity; sensitive groups should stay indoors. |
| 301+ | Hazardous | Maroon | Minimise outdoor activity; everyone should stay indoors where possible. |
The sub-index breakpoints
The PSI sub-indices follow the legacy US EPA PSI breakpoints. PM2.5, PM10, and SO₂ use a 24-hour average; CO and ozone use an 8-hour average (ozone switches to a 1-hour average for higher values); NO₂ uses a 1-hour average and only contributes at "Unhealthy" and above.
| Band | PM2.5 (24-hr, µg/m³) | PM10 (24-hr, µg/m³) | SO₂ (24-hr, µg/m³) | CO (8-hr, mg/m³) |
|---|---|---|---|---|
| Good | 0–12 | 0–50 | 0–80 | 0–5.0 |
| Moderate | 13–55 | 51–150 | 81–365 | 5.1–10 |
| Unhealthy | 56–150 | 151–350 | 366–800 | 10.1–17 |
| Very Unhealthy | 151–250 | 351–420 | 801–1600 | 17.1–34 |
| Hazardous | 251+ | 421+ | 1601+ | 34.1+ |
Haze season: why the 1-hour PM2.5 reading matters
Singapore's worst air comes during the transboundary haze that drifts over from land and peat fires in the region, typically in the dry months from roughly June to October. The headline PSI is a 24-hour figure, which means it lags a fast-moving plume — it can still read "Moderate" while the air outside your window has already turned acrid. That's why NEA publishes the 1-hour PM2.5 concentration alongside the 24-hour PSI during haze: it's the number that tracks the air you're breathing right now.
NEA's own activity guidance during haze is keyed to the 1-hour PM2.5 band as well as the 24-hour PSI forecast, so when haze is active, watch both.
How the PSI compares to the US AQI
Because the PSI uses the legacy US PSI breakpoints rather than today's US AQI breakpoints, the two 0–500 scales are close but not identical — particularly for PM2.5, where the modern US AQI was re-tightened in 2024. As a rough map:
- PSI 0–50 (Good) ≈ US AQI 0–50 (Good)
- PSI 51–100 (Moderate) ≈ US AQI 51–100 (Moderate)
- PSI 101–200 (Unhealthy) ≈ US AQI 101–200 (Unhealthy for Sensitive Groups to Unhealthy)
- PSI 201–300 (Very Unhealthy) ≈ US AQI 201–300 (Very Unhealthy)
- PSI 301+ (Hazardous) ≈ US AQI 301+ (Hazardous)
Air quality on your home screen
Smog Report shows real-time air quality with widgets and Live Activities, and lets you switch between regional scales including Singapore's PSI. Free on iOS.
Download for iOSPrimary sources: Singapore NEA — Air Quality / PSI · Singapore NEA — Haze