Every haze season, Singapore turns into a nation of people refreshing the same two numbers. One is the 24-hour PSI. The other is the 1-hour PM2.5 reading. They are not the same thing, and checking the wrong one can send you jogging at the wrong time or cancel a perfectly fine dinner.
This guide explains what each number measures, which bands NEA uses, and how to turn them into decisions at home. Bands come from NEA's published scales. For official health advice, always check NEA's latest advisory.
- What is the 24-hour PSI?
- What is the 1-hour PM2.5 reading?
- Which number should I check first?
- What the numbers mean for the air inside your home
- A simple haze routine
- FAQ
What is the 24-hour PSI?
The Pollutant Standards Index (PSI) is calculated from six pollutants: PM2.5, PM10, ozone, sulphur dioxide, nitrogen dioxide and carbon monoxide. The 24-hour PSI reflects the average air quality over the past 24 hours, which is why it moves slowly and forms the basis of NEA's daily health advisories.
The bands: 0 to 50 is Good, 51 to 100 is Moderate, 101 to 200 is Unhealthy, 201 to 300 is Very Unhealthy, and anything above 300 is Hazardous.
What is the 1-hour PM2.5 reading?
PM2.5 is fine particulate matter 2.5 micrometres across or smaller, and it is the dominant pollutant during haze. The 1-hour reading is the hourly average at a point in time, so it reacts much faster than the 24-hour PSI.
NEA groups it into four bands: Normal at 55 µg/m³ and below, Elevated at 56 to 150, High at 151 to 250, and Very High above 250. NEA publishes activity advice for each band, so read the current advisory rather than guessing.
Which number should I check first?
Heading out in the next few hours? Look at the 1-hour PM2.5. Planning your day or your week? Look at the 24-hour PSI and NEA's forecast. The two can disagree, and that is normal: PSI lags because it averages a full day, while hourly PM2.5 swings with the wind.
What the numbers mean for the air inside your home
Both are outdoor readings. What you breathe indoors depends on how leaky your home is, whether windows were open an hour ago, and what you are frying for dinner. A purifier sized for the room helps, and a PM2.5 monitor lets you see what is actually happening in your own space.
Not sure how much airflow a room needs? Use the CADR calculator, then browse bedroom purifiers or living room purifiers.
A simple haze routine
Check the 24-hour PSI and forecast in the morning. Check the 1-hour PM2.5 before you head out. When either starts climbing, close windows and switch the purifier on early, rather than after the room already smells like a campfire. NEA's haze.gov.sg and the myENV app are the official places to look.
Quick answers
Where do I find the official PSI and PM2.5 numbers?
NEA publishes them on haze.gov.sg and the myENV app, and the data is also available through data.gov.sg. The live reading on this site's homepage comes from that public feed.
Is a PSI of 100 dangerous?
A 24-hour PSI up to 100 falls in the Good or Moderate bands, where NEA advises the general population to carry on with normal activities. People who are sensitive to air pollution should still follow NEA's current advisory.
Why does PSI differ from PM2.5?
PSI blends six pollutants over 24 hours. PM2.5 is a single pollutant averaged over one hour. During haze, PM2.5 is the main driver, but the two move at different speeds.
