Air quality across Singapore is back to moderate levels on the evening of Sept 20 as the 24-hour Pollutant Standards Index (PSI) reading in the island’s central region dipped out of the unhealthy spectrum from 7pm.
The PSI value for that area was 95 at 11pm, after dropping to 100 at 7pm from 120 at 10am. Earlier, the PSI readings for the eastern and western regions had dropped into the moderate zone, below the 101 threshold for the unhealthy range.
The PSI reading in the west dropped to 91 as at 11pm, after hitting a peak of 106 at 10am on Sept 20.
In the east, the level dipped to 75 as at 11pm, after it entered the unhealthy range earlier in the day at 3am with a reading of 102.
The PSI readings in the north (68) and south (71) remained in the moderate range.
The PSI reading in the central area had been in the unhealthy range since 1pm on Sept 19, with air quality in the rest of the country remaining in the moderate range throughout the 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 hourly PM2.5 readings for all regions returned to the normal band as at 9pm on Sept 20. At points earlier in the day, the readings for the eastern, western and central regions had ticked over to the elevated range.
As at 11pm on Sept 20, the PM2.5 readings for all regions are between 19 and 32.
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.
The NEA has advised the public to refer to the hourly PM2.5 reading if they are considering immediate activities and the 24-hour PSI readings to plan for the day ahead.
In its daily haze advisory on Sept 20, the NEA said the 24-hour PSI is forecast to be in the “moderate to unhealthy” range, even following afternoon showers over the northern and eastern parts of Singapore.
Dry conditions are forecast to continue over parts of southern Sumatra and south-western Kalimantan and smoke haze might affect Singapore as the prevailing winds are expected to blow from the south-east.
The PSI crossed into the unhealthy range just over 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 the island occasionally crossed into the unhealthy range.
The developments in September 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.
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