Air quality in central Singapore deteriorated into the unhealthy range on Saturday, October 3, as smoke haze from fires in parts of Indonesia drifted towards the island.

The 24-hour Pollutant Standards Index (PSI) in the central region crossed the unhealthy threshold at 11am, while air quality readings in Singapore’s other regions remained within the moderate range.

By 7pm, the 24-hour PSI reading in central Singapore had reached 103. The eastern region recorded 94, followed by the west at 92, the south at 84 and the north at 77.

The 1-hour PM2.5 reading, which provides an indication of air quality at a particular point in time, was in the elevated band in both the central and eastern parts of Singapore.

The National Environment Agency (NEA) said moderate to dense smoke plumes originating from southern Sumatra had been observed moving towards Singapore and neighbouring areas. Smoke haze from fires in Kalimantan was also detected.

The deteriorating air quality comes as dry conditions continue across Singapore and the surrounding region. The NEA forecast that dry weather would persist on Sunday, accompanied by strengthening southeasterly or southwesterly winds.

The agency warned that there remained a possibility of smoke haze affecting Singapore, with the 24-hour PSI forecast to remain within the high-moderate to low-unhealthy range.

The NEA advised healthy people to reduce prolonged or strenuous outdoor physical activity if the 24-hour PSI reaches the unhealthy range.

It also issued stronger guidance for more vulnerable groups. Elderly people, pregnant women and children were advised to minimise prolonged or strenuous outdoor activity, while people living with chronic heart or lung conditions were advised to avoid such activities.

The agency said the potential health effects of haze vary according to an individual’s health status, the level of pollution and the duration and intensity of outdoor exposure.

For people making decisions about immediate outdoor activities, the NEA recommended referring to the 1-hour PM2.5 readings and the corresponding health guidance. The 24-hour PSI forecast and health advisories are intended to help residents plan outdoor activities for the following day.

Singapore’s 24-hour PSI is the country’s main indicator of daily air quality and is used as the basis for the NEA’s daily health advisories. It incorporates six pollutants: PM2.5, PM10, ozone, sulphur dioxide, nitrogen dioxide and carbon monoxide.

The index measures average air quality over a 24-hour period, while the 1-hour PM2.5 reading measures the hourly average concentration of fine particulate matter.

PM2.5 particles are 2.5 micrometres or smaller in diameter and are considered the dominant pollutant during haze episodes.

Under Singapore’s PSI classification, readings between 101 and 200 are considered unhealthy. Levels from 201 to 300 are classified as very unhealthy, while readings above 300 are considered hazardous.

For hourly PM2.5 readings, the elevated band covers levels from 56 to 150. Readings between 151 and 250 are classified as high, while levels above 250 are considered very high.

The latest deterioration in Singapore’s air quality highlights the continuing regional impact of smoke from fires in Indonesia, particularly during periods of dry weather and favourable winds that can carry smoke across national borders.