PSI vs PM2.5: which haze number should I watch?
🦦 Singapore reports two haze numbers, and they answer different questions.
24-hour PSI: the daily picture
The NEA air pollution FAQs explain that the 24-hour PSI is computed from the 24-hour average concentrations of six pollutants, including PM2.5. The highest sub-index becomes the PSI. MOH says a PSI of 101 and above is unhealthy. Because it is a 24-hour average, it changes slowly. NEA says to use the 24-hour PSI forecast and advisory to plan the next day's activities.
1-hour PM2.5: the air right now
MOH says the 1-hour PM2.5 concentration shows the current air quality during haze. You should use the latest reading to plan activities in the next few hours. NEA groups it into four bands:
| 1-hr PM2.5 (µg/m³) | Band | MOH advice, next hour |
|---|---|---|
| 0–55 | 1 Normal | Continue normal activities |
| 56–150 | 2 Elevated | Reduce strenuous outdoor activity. Vulnerable persons: avoid strenuous outdoor activity |
| 151–250 | 3 High | Avoid strenuous outdoor activity. Vulnerable persons: avoid all outdoor activity |
| 251 and above | 4 Very High | Minimise all outdoor activity. Vulnerable persons: avoid all outdoor activity |
Why Haze Buddy shows PM2.5 big
Parents usually decide "playground now, or not?", and the hourly PM2.5 answers that. We show it in large type and the 24-hour PSI underneath. Each region's colour uses whichever is worse.
Why other apps show bigger numbers
Some international apps show a US AQI. That is a different index on a different scale, so its number is not the same as NEA's PM2.5 in µg/m³ or Singapore's PSI. Haze Buddy uses only NEA's official readings so they match the MOH advice. Compare like with like.
Sources: MOH, NEA and haze.gov.sg. Live data: NEA via data.gov.sg.
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Quick answers
What is the difference between PSI and PM2.5?
The 24-hour PSI is a daily index from six pollutants averaged over 24 hours. The 1-hour PM2.5 is the latest hourly concentration of fine particles in µg/m³ and shows current air quality.
Which should I use to decide on outdoor play now?
MOH advises using the latest 1-hour PM2.5 reading for immediate outdoor activities and the 24-hour PSI forecast for the next day.
What PSI is unhealthy in Singapore?
MOH says a PSI reading of 101 and above is unhealthy.
Why is the AQI in my app higher than NEA's numbers?
Many apps show a US AQI, which is a different scale from NEA's PM2.5 in µg/m³ and Singapore's PSI, so the numbers cannot be compared directly.
Last reviewed 16 Oct 2026 against official MOH, MOE and NEA pages.