Haze in S’pore: Air quality in all regions except central back in moderate range

SINGAPORE – Air quality improved across most of Singapore on the night of Sept 15, with all areas except the central region recording PSI readings in the moderate range at midnight on Sept 16.

The 24-hour Pollutant Standards Index (PSI) level at midnight in the central region was 112, in the unhealthy range.

It was 83 in the north, 80 in the south, 93 in the east and 96 in the west, all in the moderate range.

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 reduce outdoor activities.

PSI readings of 51 to 100 are considered moderate, while values of zero to 50 are in the “good” range.

Air quality on Sept 15 started improving in the south and north first.

The PSI readings in those regions first fell back below the 101 threshold in the afternoon, after all areas across Singapore earlier recorded levels in the unhealthy range at 6am. The central region had recorded the highest reading at 154.

It was the first time since September 2019 that unhealthy levels of pollutants were recorded all across the country.

Air quality falling back into the moderate range happened at 11pm on Sept 15 for the east, and midnight on Sept 16 for the west.

The air quality in Singapore had worsened on Sept 14 with the PSI across the island, except for the north, hitting unhealthy levels, following several days in the moderate range.

The PSI reading for the north crossed the 101 threshold at about 1am on Sept 15, rising to 102 from 99 at midnight. 

The one-hour PM2.5 readings for all regions were within the normal range as at midnight on Sept 16, ranging from 29 mcg per cubic m in the north to 47 mcg per cubic m in the central region.

Earlier on Sept 15 at 5pm, one-hour PM2.5 readings were elevated in all regions, with the south recording 56 mcg per cubic m and the central region hitting 96 mcg per cubic m.

Readings of 56 mcg per cubic m to 150 mcg per cubic m are considered elevated, while levels of 151 mcg to 250mcg per cubic m are in the “high” range. Anything 55 and below is considered normal.

The one-hour PM2.5, an indicator of fine micro-pollutants in the air, points to current air quality and should be checked immediately before outdoor activities.

The 24-hour PSI is computed based on the daily average concentration of six pollutants, including PM2.5, sulphur dioxide, carbon monoxide and ozone. The calculations are done based on readings from the past 24 hours.

The index provides easily understandable information about daily levels of air quality.

In an advisory at about 6.30pm on Sept 15, NEA said air quality in Singapore for the next 24 hours was forecast to be in the high moderate to mid-unhealthy range.

The PSI crossed into the unhealthy range two weeks 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 Singapore occasionally crossed into the unhealthy range.

September’s developments 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.

In its Sept 15 advisory, NEA said that there were some improvements in the haze situation on Sept 15 compared with Sept 14 due to overnight showers over some areas in the surrounding region.

However, prevailing winds blowing from the east-south-east or south-east over Singapore and the surrounding region continued to bring haze from Kalimantan and Sumatra to Singapore.

Dry conditions are expected over central and southern Sumatra and southern Kalimantan in the coming days, NEA added.

“With the prevailing winds forecast to continue blowing from the south-east to east-south-east, there is a risk of smoke haze affecting Singapore,” the agency said.