You open a weather app before your morning run and see “Unhealthy”, but Singapore’s official NEA reading says “Moderate”. Same sky, same moment, two different verdicts. This isn’t an app error: it’s two distinct systems delivering different numbers, and knowing which one to follow matters.
Our companion piece on haze covers the deeper health risks of what’s actually in that air. This one is narrower and more practical: why the numbers disagree, and which reading should decide whether you run today.
Key Takeaways
- Singapore’s official PSI and the “AQI” shown on international apps like IQAir or AQICN are calculated using different methods, and can genuinely disagree for the exact same air, particularly during haze.
- PSI takes the highest reading among five pollutants (PM10, sulphur dioxide, nitrogen dioxide, ozone, and carbon monoxide) averaged over 3 hours, while most third-party AQI apps report a PM2.5-specific index using US EPA conversion formulas on roughly hourly data.
- When fine particulate matter (PM2.5) is the dominant pollutant, as is typical during haze, the app-based AQI number often runs higher and is more alarming than Singapore’s official PSI at that same moment.
- For decisions in Singapore specifically, NEA guidance points to the 1-hour PM2.5 reading for immediate activity decisions and the 24-hour PSI for planning ahead, both sourced from NEA’s own monitoring, not a third-party app.
- NEA publishes exact numeric bands for both: PSI 101-200 is “Unhealthy” (24-hour), and PM2.5 151-250 µg/m³ is “High” (1-hour), each with specific exercise guidance for healthy adults versus more vulnerable groups, this article includes both full tables.
- If any official reading, PSI or PM2.5, is in the unhealthy range, that’s the one to act on, regardless of what a differently calculated app number says.
Why Your Phone and Government Readings Differ
Singapore’s PSI uses a 3-hour average of five pollutants: PM10, sulphur dioxide, nitrogen dioxide, ozone, and carbon monoxide. The highest sub-index determines the overall score on a 0–500 scale.
In contrast, third-party apps like IQAir and AQICN display a PM2.5-focused AQI using US EPA breakpoints updated roughly hourly. During haze, PM2.5 spikes rapidly, making app-based AQIs higher and faster-changing than official PSI figures.
PSI vs. App-Based AQI, at a Glance
| Feature | Singapore PSI | Third-Party App AQI |
|---|---|---|
| What it measures | Highest of 5 pollutants (PM10, SO2, NO2, O3, CO) | Typically PM2.5 specifically |
| Averaging window | 3-hour average | Roughly 1-hour |
| Standard used | Singapore’s own PSI methodology (NEA) | US EPA breakpoint conversion |
| Tends to run higher during haze? | Comparatively lower/slower to move | Often higher and faster-moving |
| Official for Singapore health advisories? | Yes | No |
So Which One Should You Actually Check Before Running?
For activity decisions in Singapore, follow NEA readings. Use 1-hour PM2.5 for immediate workout decisions and 24-hour PSI for advance planning. Third-party AQIs offer helpful early warnings, but official health advisories rely on NEA methods.
Deciding to Run: Official NEA Bands Explained
The NEA links specific activity advice to defined air quality bands for healthy adults and vulnerable groups. Identifying your reading’s band removes the guesswork from outdoor exercise decisions.
24-Hour PSI: What Each Band Means for Exercise
| PSI Range | Band | Healthy Adults | Elderly, Pregnant Women, Children | Chronic Heart/Lung Conditions |
|---|---|---|---|---|
| 0-50 | Good | Normal activity | Normal activity | Normal activity |
| 51-100 | Moderate | Normal activity | Normal activity | Normal activity |
| 101-200 | Unhealthy | Reduce prolonged or strenuous outdoor exertion | Minimise prolonged or strenuous outdoor exertion | Avoid prolonged or strenuous outdoor exertion |
| 201-300 | Very Unhealthy | Avoid prolonged or strenuous outdoor exertion | Minimise outdoor activity | Avoid outdoor activity |
| >300 | Hazardous | Minimise outdoor activity | Avoid outdoor activity | Avoid outdoor activity |
1-Hour PM2.5: What Each Band Means
| PM2.5 (µg/m³) | Band |
|---|---|
| 0-55 | Normal |
| 56-150 | Elevated |
| 151-250 | High |
| >250 | Very High |
Before running, check the 1-hour PM2.5 reading. Normal or Elevated levels are generally fine for healthy adults. High or Very High levels signal that you should shorten, relocate, or skip your workout, especially if you belong to a vulnerable group. For sessions planned later, use the 24-hour PSI band guidelines instead.
Handling Conflicting Numbers
If your phone displays “Unhealthy” while the PSI reads “Moderate”, check official NEA readings directly. Discrepancies usually stem from calculation differences rather than errors. When any official reading reaches an unhealthy range, shorten, relocate indoors, or postpone outdoor exercise, regardless of third-party app figures.
Frequently Asked Questions
Why does my weather app differ from official NEA readings?
Most apps show an AQI based strictly on PM2.5 via US EPA formulas, whereas Singapore’s PSI averages five pollutants over three hours. They can genuinely diverge during haze.
Which number should I rely on for outdoor exercise?
Use NEA’s 1-hour PM2.5 reading for immediate decisions and the 24-hour PSI for advance planning.
Is a high app AQI as serious as a high PSI?
They are not directly comparable due to different metrics, but any unhealthy rating warrants reducing outdoor exertion.
What threshold should stop my run?
For healthy adults, a 1-hour PM2.5 over 150 (“High”) or a 24-hour PSI over 100 (“Unhealthy”) means you should shorten, move indoors, or skip the run. Vulnerable groups should exercise caution starting at “Elevated” or “Unhealthy” thresholds.
Where can I check official Singapore readings?
NEA publishes official PSI and PM2.5 data directly on their portal.
The Bottom Line
Your phone and Singapore’s official reading do not contradict each other; they measure differently on changing air. For running decisions, rely on NEA’s official PSI and PM2.5 readings. Use app figures as a quick check, but let official numbers guide your workout.
Editorial Note
This article is intended for general educational purposes only and does not constitute medical advice. Air quality readings and their associated health guidance can change; always check the National Environment Agency’s current advisory for the most up-to-date information. People with asthma, heart or lung conditions, or other risk factors should follow more cautious thresholds and consult a healthcare professional about appropriate precautions during haze or poor air quality.
Sources
- AQICN (World Air Quality Index Project). Singapore PSI and PM2.5 AQI: Why Is There a Difference Between the Two Readings? https://aqicn.org/faq/2013-06-25/singapore-psi-and-pm25-aqi-why-is-there-a-difference-between-the-two-readings/
- Odysseus Aquatics. The Difference Between PSI, AQI and PM2.5. https://www.swimlesson.com.sg/blog/the-difference-between-psi-aqi-and-pm25
- National Environment Agency (Singapore). Managing Haze. https://www.nea.gov.sg/our-services/pollution-control/air-pollution/managing-haze
- National Environment Agency (Singapore). Air Pollution FAQs: Air Quality, Health Effects and Haze Precautions. https://www.nea.gov.sg/our-services/pollution-control/air-pollution/faqs
- Haze.gov.sg (Singapore Government). PSI Readings over the last 24 Hours. https://www.haze.gov.sg/resources/readings-over-the-last-24-hours
- Haze.gov.sg (Singapore Government). 1-hr PM2.5 Readings. https://www.haze.gov.sg/resources/1-hr-pm2.5-readings