Urban heat traps haze particles in Singapore's central region

Singapore's central region has been trapped under a blanket of haze for more than a week, with air quality plummeting to unhealthy levels starting September 4. Since then, the area has consistently recorded the worst readings across the island's five monitored zones—north, south, east, west, and central. On the morning of September 15, the Pollutant Standards Index in central Singapore hit 154, a threshold that signals genuinely hazardous conditions. The same morning, other regions registered readings between 104 and 128. By evening, the central region had eased slightly to 133, while the west and east hovered around 114 to 116. Only the north and south dipped back into moderate territory, at 92 and 94 respectively.

The central region encompasses Thomson, Marymount, Sin Ming, Ang Mo Kio, Bishan, Serangoon Gardens, MacRitchie, and Toa Payoh—a densely built landscape of concrete and steel. This geography, it turns out, is precisely the problem. Koh Tieh Yong, an adjunct associate professor in meteorology and climate science at the National University of Singapore, explains that the concentration of buildings and urban infrastructure creates a heat island effect. The concrete structures warm the air above ground, causing hot air parcels to rise and churn, generating turbulence. When winds carry fine particulate matter—PM2.5—from Indonesian forest fires across the strait, that turbulent air acts like a trap, catching and holding the pollutants close to the ground rather than allowing them to disperse. The result is a localized pocket of severely degraded air quality hovering directly over the city's core.

Steve Yim, a professor and director of the Centre for Climate Change and Environmental Health at Nanyang Technological University, adds another layer: Singapore's dry conditions may be amplifying the effect. Under such circumstances, the particles accumulating in the central region could originate not only from transboundary haze but also from local emissions and urban pollutants, compounding the problem. The distinction matters because it suggests the issue is not purely a matter of external fire smoke being funneled into one zone—local sources are contributing to the trap as well.

The health implications are immediate and concrete. A PSI reading above 100 signals unhealthy air. Healthy individuals are advised to reduce prolonged or strenuous outdoor activity. Those with chronic heart or lung disease are instructed to avoid such activities altogether. On September 15 morning, when all five regions of Singapore registered unhealthy air quality simultaneously for the first time since 2019, the entire island faced these constraints at once. Koh notes that if thicker haze sweeps across Singapore, the entire nation could slip into unhealthy or worse conditions—a scenario that has now already occurred.

The National Environment Agency took to social media on September 15 to address a secondary issue: confusion over competing air quality measurements. Some residents had been comparing Singapore's official PSI readings with alternative indices from websites like aqicn.org and IQAir, which often showed worse air quality than the government's 24-hour PSI. The agency clarified that there are no international standards for computing air quality indices; each country develops its own system based on local conditions. Singapore's PSI is calculated from six pollutants—PM10, PM2.5, sulphur dioxide, nitrogen dioxide, ozone, and carbon monoxide—using specialized analysers placed across the five regions. These instruments employ internationally recognized methods to ensure scientific rigor. Other websites, the agency noted, may rely on lower-cost sensors or a mix of public and private data sources, making direct comparison unreliable. The clarification underscores how technical the measurement of air quality actually is, and how easily public perception can diverge from official data when multiple sources exist.