On Tuesday morning at 9 am, the 24‑hour Pollutant Standards Index (PSI) showed unhealthy levels in four of the five regions: central 128, west 114, east 113, north 107. The south recorded a moderate 96.

The 1‑hour PM2.5 reading in central Singapore was in the ‘elevated’ band at the same time.

The 24‑hour PSI is Singapore’s daily air‑quality indicator. It is calculated from six pollutants – PM2.5, PM10, ozone, sulphur dioxide, nitrogen dioxide and carbon monoxide – and reflects the average conditions over the previous 24 hours.

According to the National Environment Agency (NEA), a 24‑hour PSI of 101‑200 is labelled ‘unhealthy’, 201‑300 ‘very unhealthy’ and any value above 300 ‘hazardous’. The hourly PM2.5 reading is ‘elevated’ at 56‑150, ‘high’ at 151‑250 and ‘very high’ above 250.

NEA advises people to look at the 1‑hour PM2.5 number for immediate outdoor plans. When the reading is elevated, healthy adults should cut back on strenuous outdoor activity for the next hour, while the elderly, pregnant women, children and those with chronic heart or lung conditions should avoid it altogether.

For activities scheduled later in the day, the 24‑hour PSI forecast and its health advisory guide should be consulted, as they give an overview of air quality for the coming 24 hours.

The readings are driven by smoke drifting from Sumatra and Kalimantan, where dry conditions and prevailing southeast winds continue to push haze toward Singapore, according to NEA.