Singapore’s air quality deteriorated on September 19, as the 24-hour Pollutant Standards Index in the central region crossed into the unhealthy range at 109, driven by smoke plumes drifting from forest fires in Indonesia.
Central Region PSI Crosses into Unhealthy Territory Once Again
The air quality across Singapore took a turn for the worse as the 24-hour Pollutant Standards Index in the central region climbed to 109 by 7pm on Sept 19, crossing past the threshold into the unhealthy band. According to the daily haze advisory issued by the National Environment Agency, conditions in the surrounding western (95) and eastern (94) regions also inched toward the unhealthy range, while readings in other areas of Singapore increased steadily but remained in moderate levels. In its daily haze advisory on Sept 19, the NEA said the 24-hour PSI forecast is set to be within the high moderate to mid unhealthy
range, even following afternoon showers over the central and western parts of Singapore.
Air quality is considered unhealthy when the 24-hour PSI is in the range of 101 to 200, during which the National Environment Agency 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.
The 24-hour Pollutant Standards Index in Singapore’s central region crossed into the unhealthy band at noon on Monday, September 14, reaching 102. The reading climbed further to 105 by 1 pm, while PSI levels across the other four regions remained in the moderate range, between 81 and 93. According to official readings from the National Environment Agency, the 24-hour PSI as at noon was 91 in the east and west, 82 in the south and 79 in the north. The PSI crossed into the unhealthy range two weeks ago, hitting 101 in central Singapore on the morning of Sept 4. In the following days, readings in the central, eastern and western parts of Singapore occasionally crossed into the unhealthy range. PSI readings across the island have fallen back into the moderate spectrum and have remained there since Sept 11. The recent developments mark the first time the PSI in Singapore has hit unhealthy levels due to haze since October 2023, with the authorities earlier warning of the risk of haze between June and October.
Transboundary Smoke Drifts from Fires in Sumatra and Kalimantan
The latest deterioration comes as hotspots and moderate to dense haze continue to be detected across parts of Indonesia, raising concerns that prevailing conditions could continue to affect Singapore’s air quality. The National Environment Agency said on Monday that it was closely monitoring air quality across Singapore as haze conditions developed in the region. According to the NEA, dry conditions have persisted. In its advisory on Thursday, NEA said that moderate to dense smoke plumes continued to be detected from fires in southern Sumatra and Kalimantan, with some haze from southern Sumatra drifting towards Singapore. This is due to ongoing forest and land fires in Sumatra and Kalimantan, Indonesia.
National Environment Agency
The central region includes the Thomson, Marymount, Sin Ming, Ang Mo Kio, Bishan, Serangoon Gardens, MacRitchie and Toa Payoh areas, according to the National Environment Agency’s website. The National Environment Agency website displays haze and air quality conditions through two readings: 24-hour Pollutant Standards Index and one-hour PM2.5 concentration readings.
Understanding the Metrics: PSI Versus Hourly PM2.5 Readings
The 24-hour PSI is calculated based on the daily average concentrations of six pollutants, including PM2.5, sulfur dioxide, carbon monoxide, and ozone. The PSI index has five categories: 0-50 is “good”; 51-100 is “moderate”; 101-200 is “unhealthy”; 201-300 is “very unhealthy”; and above 300 is “hazardous.”
Meanwhile, the one-hour PM2.5 concentration reflects current air quality and should be used when deciding whether to carry out outdoor activities in the next few hours. The one-hour PM2.5 index has four categories: 0-55 is “normal”; 56-150 is “elevated”; 151-250 is “high”; and 251 and above is “very high.”
The National Environment Agency advises the public to use hourly PM2.5 readings when deciding on immediate outdoor activities and the 24-hour PSI to plan activities for the day.
The National Haze Task Force revealed to The Straits Times on August 24 that Singapore has stepped up efforts to raise public awareness of increased haze risks and guide the public on health protection measures. Doctors have been reminded to advise vulnerable groups—including the elderly, pregnant women, children, and those with lung or heart disease—to take precautionary measures. In schools, teachers conduct visual checks on students’ health, and haze advisories have been sent to workplaces.