Singapore PSI vs AQI: which haze reading to check

The 24-hour PSI

The Pollutant Standards Index is NEA's headline figure. It is reported for five regions of the island, north, south, east, west and central, and built from six pollutants: fine particles (PM2.5), PM10, sulphur dioxide, nitrogen dioxide, ozone and carbon monoxide. Fine particles were folded into it in 2014.

Because each pollutant is averaged over 24 hours, the PSI tells you what the air has been like over the past day. That makes it good for planning tomorrow, and slow to show a change that happened this afternoon. Its bands:

• 0–50: Good
• 51–100: Moderate
• 101–200: Unhealthy
• 201–300: Very unhealthy
• Above 300: Hazardous

The 1-hour PM2.5 reading

For what is happening now, NEA publishes 1-hour PM2.5 concentrations, in micrograms per cubic metre, in four bands: Band I Normal (0–55), Band II Elevated (56–150), Band III High (151–250) and Band IV Very high (251 and above). NEA's own advice is to use this reading for activities in the next few hours, and the 24-hour PSI forecast for the next day.

Where the US AQI fits

The US AQI, which IQAir, Airleaf and most weather apps show, is an hourly index. Its bands start the same way, 0–50 good and 51–100 moderate, but above 100 it adds a band for sensitive groups (101–150) before unhealthy (151–200). Its fine-particle table was tightened in 2024: only 9 µg/m³ of PM2.5 or less now counts as good.

So when haze rolls in, an hourly AQI reacts within the hour while the 24-hour PSI climbs slowly behind it. When the haze clears, the PSI stays high for most of a day after the sky looks fine. The two differ most at exactly the moments people check them.

Where the haze comes from

Singapore's bad-air episodes are almost all transboundary haze from forest and peatland fires in Sumatra and Kalimantan, usually between June and October and worst in dry El Niño years. The worst on record came in June 2013, when the 3-hour PSI reached 401. Drained and cleared peatland burns longest and smokiest, which is why the region's haze and its deforestation are the same story.

What to do

General advice, following the bands of each index:

• Up to 100 on the PSI or the AQI: normal activity for almost everyone
• Unhealthy: cut down long or strenuous outdoor activity and keep windows closed; on the US AQI, sensitive groups should start doing this above 100
• Very unhealthy and above: stay indoors where you can, run an air purifier with a HEPA filter, and wear a well-fitted N95 mask outdoors
• Check the 1-hour PM2.5 band before heading out, and the 24-hour PSI forecast the night before

Checking your own neighbourhood

NEA's readings are regional. For your own estate, Airleaf's Singapore reports show the US AQI, the PM2.5 reading behind it and the past week for neighbourhoods across the island, from Woodlands and Punggol to Ang Mo Kio.