PSI vs PM2.5: how to read NEA's haze readings
Updated · By the purifierplan.com team
Singapore reports two numbers during haze. The 24-hour PSI tells you the air quality band and the health advice that goes with it. The 1-hour PM2.5 tells you what the air is like right now.
- 24-hour PSI: an index from 0 to 500, worked out from six pollutants. Official health advice is tied to its bands.
- 1-hour PM2.5: the concentration of fine particles in the last hour, in µg/m³. Use it for decisions about the next hour or two.
- During haze, fine particles usually drive the PSI. A 24-hour PM2.5 of 55 µg/m³ corresponds to a PSI of 100; 150 µg/m³ corresponds to 200.
How NEA builds the PSI
NEA works out a sub-index from 0 to 500 for each of six pollutants: PM2.5, PM10, sulphur dioxide, carbon monoxide, ozone and nitrogen dioxide. The PSI is the highest of the six. Each sub-index is a straight-line conversion between fixed breakpoints (NEA, Computation of the PSI).
| Band | PSI | 24-hour PM2.5 (µg/m³) |
|---|---|---|
| Good | 0–50 | 0–12 |
| Moderate | 51–100 | 13–55 |
| Unhealthy | 101–200 | 56–150 |
| Very unhealthy | 201–300 | 151–250 |
| Hazardous | 301–400 | 251–350 |
| Hazardous | 401–500 | 351–500 |
Worked example from NEA
A 24-hour PM2.5 of 40 µg/m³ lies between the 12 and 55 breakpoints, which map to PSI 50 and 100. So the sub-index is 50 + (100 − 50) × (40 − 12) ÷ (55 − 12) = 83. If every other pollutant's sub-index is lower, the PSI is 83.
Going the other way: PSI to PM2.5
To estimate fine particles from a PSI reading, run the same breakpoints backwards. A PSI of 150 suggests a 24-hour PM2.5 of about 103 µg/m³. Treat that as an upper limit: if another pollutant set the PSI, PM2.5 was lower. Our planner does this only when NEA's PM2.5 reading isn't available, and uses the PM2.5 reading when it is.
Which number to check for what
NEA says its health advisories during transboundary haze refer primarily to the 24-hour PSI. The 1-hour PM2.5 is the better guide to current conditions for immediate activities, such as going for a jog. For planning ahead, use the 24-hour PSI forecast (NEA haze update).
What MOH advises for each band
| 24-hour PSI | Healthy persons | Elderly, pregnant women, children | Chronic lung or heart disease |
|---|---|---|---|
| 0–100 (Good, Moderate) | Normal activities | Normal activities | Normal activities |
| 101–200 (Unhealthy) | Reduce prolonged or strenuous outdoor physical exertion | Minimise prolonged or strenuous outdoor physical exertion | Avoid prolonged or strenuous outdoor physical exertion |
| 201–300 (Very unhealthy) | Avoid prolonged or strenuous outdoor physical exertion | Minimise outdoor activity | Avoid outdoor activity |
| Above 300 (Hazardous) | Minimise outdoor activity | Avoid outdoor activity | Avoid outdoor activity |
MOH also advises N95 masks for prolonged time outdoors when the PSI is forecast above 300, or above 200 for people with chronic heart or lung disease, and says they aren't needed indoors. If you feel unwell, seek medical attention. Check HealthHub for the current advice.
Where to get the readings
- haze.gov.sg and the myENV app, from NEA.
- data.gov.sg publishes the same readings for developers under the Singapore Open Data Licence. Our planner shows them for your region.
Indoors, it's a different number. With windows shut, the air inside is still a large share of what's outside. See getting your home ready for haze, or check how much a purifier would change it in your home.
Plan my homeSources
- NEA, Computation of the Pollutant Standards Index (PSI), last updated March 2014. Breakpoints and worked example.
- NEA, Haze situation update, 19 September 2019. Use of the 24-hour PSI and 1-hour PM2.5.
- HealthHub (MOH), How to protect yourself against haze, reviewed 11 October 2026.
- This page is not affiliated with or endorsed by NEA or MOH.
Questions people ask
Is PSI the same as PM2.5?
No. PSI is an index from 0 to 500 built from six pollutants. PM2.5 is a concentration in micrograms per cubic metre. During haze, PM2.5 usually sets the PSI, but the two numbers are on different scales.
What PM2.5 level matches a PSI of 100?
A 24-hour PM2.5 of 55 µg/m³ gives a PM2.5 sub-index of 100, the top of the Moderate band.
Which reading should I check before going for a run?
NEA says the 1-hour PM2.5 reading is the better guide to current conditions for immediate activities, and the 24-hour PSI forecast is for planning ahead.
Why does the PSI change slowly when the haze clears quickly?
The 24-hour PSI is based on a 24-hour average, so it lags behind sudden changes. The 1-hour PM2.5 responds within the hour.