Singapore has experienced hazy conditions for over a week after air quality in the central region first dipped to unhealthy levels on Sept 4, according to official records. The central region has consistently registered the highest Pollutant Standards Index and PM2.5 readings among the north, south, east, west, and central monitoring zones. The National Environment Agency defines the PSI as a measure of air quality, noting that readings exceeding 100 reflect unhealthy conditions.
Did You Know? Per the National Environment Agency website, the central region encompasses the areas of Ang Mo Kio, Bishan, MacRitchie, Marymount, Serangoon Gardens, Sin Ming, Thomson, and Toa Payoh.
Central Region Air Quality Readings and Regional Breakdown
On Sept 15 at daybreak, the PSI in central Singapore reached a high of 154, while other regions recorded readings between 104 and 128. By 5 pm that same day, the central region reading dropped slightly to 133, according to agency data. The west and east regions recorded unhealthy air with values between 114 and 116, while the north and south dipped to moderate levels at 92 and 94.
PM2.5 concentrations refer to fine particulate matter that acts as the dominant pollutant during periods of transboundary haze. Under these conditions, the National Environment Agency advises healthy individuals to reduce prolonged or strenuous outdoor activities, while those with chronic heart and lung disease should avoid them entirely. On the morning of Sept 15, all parts of Singapore registered unhealthy air for the first time since 2019.
Meteorological Factors and Urban Heat Effects
The Straits Times was told by Koh Tieh Yong, an adjunct associate professor in climate science and meteorology at the National University of Singapore, that the elevated readings in the central region are a result of dense building concentrations and heavy urbanisation. Urban heat from concrete structures warms the air near the ground, producing rising hot air parcels that increase air turbulence. As winds transport PM2.5 particles from forest fires in Indonesia toward Singapore, the turbulent air in the central region catches the pollutants more efficiently, concentrating them close to the ground.
Steve Yim, a professor and director of the Centre for Climate Change and Environmental Health at Nanyang Technological University, stated that the elevated PSI could also relate to dry conditions in Singapore. Under such dry weather, particles can originate from a combination of haze and local emissions or downtown pollutants. Koh added that if thicker haze blows over the island, the entire country could reach unhealthy air quality or worse.
Official Clarification on Air Quality Indices
In a Facebook post on Sept 15, the National Environment Agency addressed public confusion regarding how Singapore’s PSI compares with the US Environmental Protection Agency’s Air Quality Index system. The agency stated that while the local system references the US framework, the two are not identical. According to the agency, no international guidelines govern the computation of air quality indices, and each nation adopts a system based on local needs.
The National Environment Agency cautioned that readings provided by websites like aqicn.org or local platform IQAir adopt alternative computation methods and are not comparable with the 24-hour PSI. These sites may rely on a combination of low-cost sensors and published data, whereas Singapore’s official data relies on specialised pollutant analysers placed across five regions. These analysers continuously measure small particulate matter PM10 and PM2.5, sulphur dioxide, nitrogen dioxide, ozone, and carbon monoxide using internationally recognised methods to ensure reliability.
Frequently Asked Questions
What does a PSI reading over 100 mean?
Readings over 100 reflect unhealthy air quality. During these conditions, healthy people should reduce prolonged or strenuous outdoor activities, while individuals with chronic heart and lung disease should avoid them.
Why does the central region record higher PSI levels than other parts of Singapore?
According to Koh Tieh Yong, the central region is highly urbanised and surrounded by buildings. Urban heat from concrete structures creates rising air parcels that enhance turbulence, trapping PM2.5 particles more efficiently close to the ground.
How does Singapore measure its official air quality?
Air quality is measured using specialised pollutant analysers placed across the north, south, east, west, and central regions to continuously track six pollutants: PM10, PM2.5, sulphur dioxide, nitrogen dioxide, ozone, and carbon monoxide.
How will changing wind patterns or heavier transboundary smoke influence the entire island in the coming days?
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