Singapore's air quality improves to moderate in 4 regions as haze eases

Singapore woke on the morning of September 15 to find all five of its regions breathing unhealthy air. The Pollutant Standards Index—the measure by which the National Environment Agency tracks what people are actually inhaling—had climbed into the danger zone everywhere at once. It was the first time in seven years that the entire country had recorded unhealthy readings simultaneously. The central region bore the worst of it, hitting 154 PSI, a level that prompts authorities to tell people to stay indoors.

By that afternoon, the situation had begun to shift. The south and north regions saw their readings drop back below 101—the threshold that separates moderate from unhealthy—sometime in the daylight hours. The east followed suit by 11 p.m. that same evening. But the central region held firm in the unhealthy range, and the west did not dip into moderate territory until midnight on September 16. When the clock struck twelve, four of Singapore's five regions had returned to moderate air quality, with PSI readings of 80 to 96. The center remained stubborn at 112.

The source of the pollution was not mysterious. Haze from burning in Kalimantan and Sumatra had been drifting across the region for weeks, but conditions had worsened sharply on September 14 when most of the island except the north crossed into unhealthy levels. The National Environment Agency attributed the overnight improvement to showers that had fallen over parts of the surrounding region, but warned that the underlying conditions had not fundamentally changed. Dry weather was forecast to persist across central and southern Sumatra and southern Kalimantan in the days ahead. The prevailing winds—blowing from the east-southeast and southeast—were expected to continue funneling smoke toward Singapore.

The fine particulate matter that makes up much of the haze, measured separately as one-hour PM2.5 readings, told a similar story of improvement. At 5 p.m. on September 15, all regions had shown elevated levels, with the central region reaching 96 micrograms per cubic meter and the south at 56. By midnight on September 16, those readings had normalized across the board, ranging from 29 in the north to 47 in the central region. Yet the 24-hour PSI—which averages six different pollutants over a full day—remained the more cautious measure, and it was the one that kept the central region in the unhealthy category even as the immediate air quality improved.

This episode marked a return to a problem Singapore had largely avoided for nearly three years. The last time unhealthy haze had affected the city-state was in October 2023. The authorities had warned in the months leading up to September that the combination of El Niño and the Indian Ocean Dipole would create warmer, drier conditions across the region—precisely the circumstances that drive transboundary smoke. Two weeks before the September 15 spike, on the morning of September 4, the central region had first crossed into unhealthy territory. In the days that followed, the eastern and western regions occasionally joined it. But nothing had prepared the island for the moment when all five regions would breach the threshold at once.

As of the evening of September 15, the National Environment Agency was forecasting that air quality would remain in the high-moderate to mid-unhealthy range for the next 24 hours. The improvement that had come overnight was real but fragile. The winds that had brought the haze were still blowing in the same direction. The fires that had produced it were still burning in the same dry landscape. Singapore's air would remain hostage to weather patterns and land management practices thousands of kilometers away.