Haze in S’pore: Air quality back to moderate after brief dip into unhealthy range

SINGAPORE – Air quality in the central region of the island slipped back to a moderate range late on Oct 4, with the Pollutant Standards Index (PSI) value falling from an unhealthy 101 at 9pm to 99 at 11pm.

The 24-hour PSI reading in that area had mostly been in the unhealthy range since 11am on Oct 3, but dipped temporarily into the moderate range at 11pm that night, before gradually dropping from 100 at 9am on Oct 4, to 94 at 4pm.

It then started to climb back late in the afternoon and in the early evening.

The readings for the other parts of Singapore at 11pm remained in the moderate range, ranging from 75 to 86.

The PSI reading is a 24-hour rolling average of six key air pollutants and is used in planning for outdoor activities the next day.

Air quality is considered unhealthy when the 24-hour PSI is in the range of 101 to 200, during which the National Environment Agency (NEA) advises the public to at least reduce outdoor activities. PSI readings of 51 to 100 are considered moderate, while values of zero to 50 are in the “good” range.

The air quality started worsening at 11am on Oct 3 when the 24-hour PSI hit 102, the first time the reading in any region on the island reached unhealthy levels since 10pm on Sept 30.

Meanwhile, the one-hour PM 2.5 readings, which are used as a gauge for immediate activities, reached the elevated range for the eastern and western regions at 11pm on Oct 4, registering values of 51 and 89, respectively.

The readings for the other parts of Singapore stood in the normal range, between 30 and 51 at 11pm.

PM2.5 readings, which indicate the current concentrations of fine particulate matter, are considered elevated if they fall between 56 and 150, and high if they are between 151 and 250.

In its daily haze advisory on Oct 4, NEA said the 24-hour PSI is forecast to be in the “high moderate to low unhealthy” range.

Moderate to dense smoke plumes from southern Sumatra were observed drifting towards Singapore and the surrounding region with smoke haze also observed from fires in Kalimantan.

Dry conditions are forecast to continue over Singapore and the surrounding region on Oct 4. The risk of smoke haze affecting the Republic remains as the prevailing winds are expected to blow from the south-east or south-west.

The PSI crossed into the unhealthy range a month ago, hitting 101 in central Singapore on the morning of Sept 4. In the days that followed, readings in the central, eastern and western parts of the island occasionally crossed into the unhealthy range.

The developments in September and October mark the first time since October 2023 that the PSI in Singapore hit unhealthy levels because of haze, with the authorities earlier warning of the risk of haze between June and October as a result of warmer and drier conditions brought on by both El Nino and the Indian Ocean Dipole.

Meanwhile, fair and warm conditions are expected on several days, particularly during the first week of the month, said the Meteorological Service Singapore in its fortnightly weather advisory on Oct 1.

The total rainfall for the first half of the month is forecast to be below average across most of the island, making it drier than the previous two weeks.

However, localised thundery showers may fall over parts of Singapore in the late morning and afternoon on a few days.

Daily temperatures are likely to reach a high of between 34 deg C and 35 deg C on most days, and could exceed 35 deg C on a few days.

Some nights may be warm and humid, with temperatures above 27 deg C.