Air quality in two Singapore regions enters unhealthy range

SINGAPORE: Air quality in the central and southern parts of Singapore entered the unhealthy range on Monday (Sep 28),

The 24-hour Pollutant Standards Index (PSI) readings for the central and southern regions hit 104 and 103 respectively as of 8pm.

PSI readings have dipped in and out of the unhealthy range over the past weeks.

In its latest media advisory on Monday evening, the National Environment Agency (NEA) said that the 24-hour PSI was forecast to be in the high moderate to mid-unhealthy range for the next 24 hours.

Moderate to dense smoke haze from fires in southern Sumatra and Kalimantan have drifted to Singapore, said NEA.

While some showers are forecast in Singapore on Tuesday, there remains a risk of smoke haze affecting Singapore, with the prevailing winds expected to blow from the east or southeast. 

The 24-hour PSI is Singapore's indicator of daily air quality and forms the basis of NEA's daily health advisories. It is calculated using six air pollutants - PM2.5, PM10, ozone, sulphur dioxide, nitrogen dioxide and carbon monoxide. 

The 1-hour PM2.5 reading, on the other hand, is the hourly average of the PM2.5 pollutant levels. PM2.5 is fine particulate matter with a diameter of 2.5 micrometres or shorter, and is the dominant pollutant during haze that causes harm to health.

While the 24-hour PSI is an average of the past 24 hours, the one-hour PM2.5 reading reflects the air quality at a single point in time.

A 24-hour PSI reading is considered "unhealthy" when it is in the 101 to 200 range, "very unhealthy" when it is within the 201-300 range, and "hazardous" when it goes over 300.