By The Diary desk · Last verified 28 September 2026

A hazy day comes down to two numbers. The 1-hour PM2.5 reading tells you what the air is like right now; the 24-hour PSI forecast tells you how to plan tomorrow. A 24-hour PSI of 0 to 100 means normal activities. Between 101 and 200 it is Unhealthy, and healthy people should cut back on prolonged or strenuous exertion outdoors. On 4 September 2026, the 24-hour PSI entered the Unhealthy range for the first time since October 2023.

Quick facts

• Check the 1-hour PM2.5 for what to do now, and the 24-hour PSI forecast to plan ahead.
• PSI: 0–50 Good, 51–100 Moderate, 101–200 Unhealthy, then Very Unhealthy and Hazardous.
• PM2.5: Normal up to 55, Elevated 56–150, High 151–250, Very High from 251.
• NEA issues daily haze advisories during an episode.
• The smoke comes from fires in Sumatra and Kalimantan, carried by the wind.
• The elderly, pregnant women, children and people with heart or lung conditions need extra care.

Which number should I check?

Both, for different things. PM2.5 is the main pollutant during haze, and the 1-hour reading is the best guide to the air right now, so check it before you head out. NEA bases its health advice on the 24-hour PSI, so use the PSI forecast to plan the next day’s outdoor activities.

Going out now: the PM2.5 bands

Band	1-hour PM2.5 (µg/m³)	What to do
Normal	0–55	Carry on as usual
Elevated	56–150	Reduce strenuous outdoor activity; vulnerable people should avoid it
High	151–250	Avoid strenuous outdoor activity; vulnerable people should avoid outdoor activity
Very High	251 and above	Minimise all outdoor activity; vulnerable people should avoid it

Planning tomorrow: the PSI bands

24-hour PSI	Healthy people	Elderly, pregnant women, children	Chronic heart or lung disease
0–100 (Good, Moderate)	Normal activities	Normal activities	Normal activities
101–200 (Unhealthy)	Reduce prolonged or strenuous outdoor exertion	Minimise it	Avoid it
Above 200	Stricter advice applies; follow NEA’s advisory for the day

Anyone in the more vulnerable groups who feels unwell should see a doctor.

Where does the haze come from?

From land and forest fires in the region. In September 2026, smoke from fires in southern and central Sumatra and parts of Kalimantan drifted towards Singapore on southeasterly and southerly winds. When the wind shifts or the rain comes, the air usually clears.

What changes on a hazy day?

When the PSI enters the Unhealthy range, the government’s Haze Task Force activates its plans and NEA starts issuing daily advisories, with a 24-hour PSI forecast for the next day. LTA advises drivers to switch on their headlights when visibility drops. Schools, the armed forces and event organisers scale back outdoor activity according to the readings.

How to move your plans

• Check the 1-hour PM2.5 before a run, a hike or a day at the beach.
• Swap long outdoor plans for museums, malls and indoor sport when the forecast is Unhealthy.
• Keep windows shut at home when readings are high.
• Check with organisers before outdoor events; many follow NEA’s advisories.

The WahLiao Verdict

Check PM2.5 before you leave the house, and the PSI forecast before you plan tomorrow. Up to 100, carry on; above it, the hike can wait a week, and the museum is right there.

Questions people ask

Where do I check the readings?

On NEA’s website and its myENV app, which carry the latest PM2.5 readings and the PSI forecast.

When is haze most likely?

In the drier southwest monsoon months, roughly June to October, when fires in the region are most common and the winds blow the smoke this way.

Why do the numbers differ across the island?

Readings are taken in five regions, and smoke doesn’t spread evenly, so check the region you’re heading to.

Is a PSI of 90 safe for a run?

For healthy people, NEA’s advice at Moderate levels is normal activities. Check the 1-hour PM2.5 just before you go.

Sources: NEA, haze situation updates (4 September 2026, 9 September 2026); NEA, government agencies’ measures to mitigate the haze (4 September 2026); NEA, haze situation update (7 October 2023); Human Resources Online on NEA’s PSI and PM2.5 bands (September 2026).

Read next: Singapore Weather by Month · Back to The Diary

For what’s worth it this week, with the bill shown, read The WahLiao Week.