Singapore's air turns unhealthy in all five regions for the first time since 2019

Every region of Singapore recorded unhealthy air on the morning of 15 September, the first time the whole island has sat in that band since September 2019. At 6 a.m. the 24-hour Pollutant Standards Index was 154 in the centre, 128 in the east and west, 111 in the south and 104 in the north.

Unhealthy begins at 101 and runs to 200. The National Environment Agency tells people in that range to cut outdoor activity. Moderate is 51 to 100. Good is 50 and below.

Four of the five regions had already crossed 101 on Monday evening. The north joined them at about 1 a.m., moving from 99 at midnight to 102. By 7 a.m. the island range was 104 to 153, with the centre still the worst reading. One-hour PM2.5, a shorter gauge, was improving overnight. At 7 a.m. only the centre was still marginally elevated, at 57. The other four regions sat between 24 and 38, inside the normal band.

Where the smoke is coming from

NEA's 14 September advisory said moderate to dense plumes from fires in Indonesian Kalimantan had drifted toward Singapore after the prevailing wind shifted to east-southeast. Some haze was also moving from southern Sumatra. Dry weather is forecast for Singapore and the surrounding region in the coming days. Brief showers fell over parts of the island on Monday and did not clear the plume.

IQAir ranked Singapore the world's sixth most polluted city on the same morning. Kuala Lumpur sat first. Jakarta sat fourth. Malaysia had already spent early September managing a haze emergency in Sarawak's Serian division, where the air pollutant index peaked at 519 on 4 September before falling into the 200s. Sultan Ibrahim lifted that emergency on 7 September. Serian and Samarahan remained in the very unhealthy band after the order ended.

Singapore last saw islandwide unhealthy PSI readings in September 2019. The last time haze alone pushed any Singapore PSI into the unhealthy band was October 2023. Authorities had warned of a June-to-October haze risk this year because of warmer, drier weather tied to El Nino and the Indian Ocean Dipole.

What "unhealthy" asks of a city-state

Singapore does not declare school closures on a single 154 reading. NEA's public advice is to reduce prolonged outdoor exercise, especially for children, older people and anyone with heart or lung disease. Construction and the port keep running. The practical constraint is sport, military training and the outdoor lunch hour in the central business district, where the index was highest.

Haze policy in the region is older than this plume. ASEAN's haze agreement has never produced a reliable halt to peat and forest fires in Kalimantan and Sumatra during a dry southwest or southeast monsoon. Each year the same two variables decide the outcome in Singapore: how many fires are burning, and whether the wind points at the island. This week the wind pointed.

The 24-hour PSI is a lagging average. That is why PM2.5 can fall while the PSI is still rising. If the east-southeast flow holds and the Kalimantan fires stay lit, NEA's own forecast (high moderate to mid-unhealthy) will remain the working assumption for several days. If the wind backs or rain builds over the fire ground, the centre's 154 will be the peak of this pulse rather than the start of a longer event.

The number that matters for the next advisory is not the overnight high. It is whether the north stays above 101 after sunrise, and whether one-hour PM2.5 in the centre drops out of the elevated band and stays there.