Why Singapore Smells Smoky Before Haze Numbers Rise

Last week Singapore’s air quality hit a new high, with the Pollutant Standards Index (PSI) climbing to 154 in the central region. The spike prompted schools and workplaces to limit outdoor activities.

The haze does not affect the whole island equally; wind patterns, local terrain and the city’s heat‑island effect all shape where the smoke settles.

Assistant Professor Wang Jingyu, a climate and environment researcher at Nanyang Technological University’s National Institute of Education, explains that wind direction decides if smoke reaches Singapore, while wind speed determines how fast it arrives.

With southeasterly winds of about 5 metres per second, smoke from Central Kalimantan—roughly 1,000 km away—can take up to three days to travel to Singapore, Wang said.

Professor Rajasekhar Balasubramanian of the National University of Singapore’s College of Design and Engineering adds that smoke from Sumatra can reach the island in about one to two days when wind speeds are between 3 and 4 metres per second.

When the haze arrives, it does not spread uniformly. Local breezes steer the pollutants, and tall buildings can slow their movement. “Certain parts of Singapore could be more exposed to the polluted air coming from the affected areas,” Balasubramanian noted, “but at the same time, we also have tall buildings … that tend to slow down the movement of air.”

Wang also points out that the central region’s higher haze readings are linked to its elevated terrain, such as Bukit Timah Hill, and to the urban heat‑island effect. The hill can block ground‑level airflow, while heat‑absorbing buildings warm the air and help keep smoke particles aloft longer.

Haze is a mix of fine particles and gases. PM2.5 particles, which are 2.5 micrometres or smaller, can stay suspended for days, but gases travel faster. Balasubramanian said, “The odour … tends to diffuse or travel relatively faster … so you smell the air first before you see a spike in PM2.5 concentrations.”

The PSI is derived from six pollutants—PM2.5, PM10, sulphur dioxide, carbon monoxide, ozone and nitrogen dioxide—averaged over a rolling 24‑hour period. Because of this lag, the one‑hour PM2.5 reading provides a more immediate picture, and the NEA advises the public to check this figure for short‑term outdoor plans.

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