Singapore's air quality enters unhealthy range as PSI exceeds 101 amid regional haze

On the morning of September 4th, Singapore's air quality crossed a threshold that had not been breached in nearly three years. The 24-hour Pollutant Standards Index, the standard measure by which the government tracks airborne pollution, climbed past 101—the point at which health authorities classify the air as unhealthy and begin advising people to stay indoors. By 10 a.m., the reading in central Singapore had reached 105. The number would tick down slightly to 104 by mid-morning, but the damage was done. For the first time since October 2023, Singapore was breathing unhealthy air.

The deterioration happened fast. At 1 a.m. that same morning, the PSI had sat comfortably in the moderate range, between 70 and 90. By 6 a.m., it had climbed to 99 in the central region. An hour later, it crossed 101. The culprit was smoke—dense plumes of it drifting across the region from forest and land fires burning in Sumatra and Kalimantan, in Indonesia. Those fires had been burning for weeks, their intensity amplified by a super El Niño weather pattern that had dried out the landscape and made vegetation tinder-dry. The Meteorological Service Singapore had warned on September 1st that the region would likely experience hazy conditions for the next two weeks as dry weather persisted, with only sporadic showers expected to offer any relief.

The smoke does not respect borders. Malaysia, geographically closer to the burning regions, was already suffering far worse. In Sarawak, one of the hardest-hit states, the town of Serian recorded an Air Pollutant Index reading of 408 at 8 p.m. on September 3rd—a number that places it in the very hazardous category, the worst classification on the scale. Four other areas in Sarawak had also recorded very unhealthy readings. Across Malaysia, hospitals were reporting a spike in asthma cases as people with respiratory vulnerabilities struggled with the degraded air.

Singapore's government had been preparing for this possibility. The Haze Task Force had intensified public awareness efforts in late August, knowing that the dry season in the region typically brings fire risk. Doctors had been instructed to counsel vulnerable populations—the elderly, pregnant women, children, and anyone with existing lung or heart disease—to take precautions. Schools were told to conduct visual checks on students' well-being. Workplaces received haze advisories. The National Environment Agency maintains two separate air quality measures on its website: the 24-hour PSI, which forecasts conditions for the next day and helps people plan outdoor activities, and the one-hour PM2.5 reading, which tracks fine particulate matter in real time and is meant to guide immediate decisions about whether to go outside right now.

Those fine particles—PM2.5—are the real threat. They are thirty times smaller than a human hair, small enough to lodge deep in the lungs and cross into the bloodstream. They come from the smoke of burning forests and vegetation. The 24-hour PSI itself is calculated from the average concentration of six different pollutants measured over the past day, including PM2.5, sulphur dioxide, carbon monoxide, and ozone. When the PSI reaches 101 to 200, the air is officially unhealthy; people are advised to reduce outdoor activity. Good air quality sits between 0 and 50. Moderate air sits between 51 and 100.

How much worse things get in Singapore depends on wind direction. The fires are burning hundreds of kilometers away. A shift in the prevailing winds could push more smoke toward the island or disperse it elsewhere. The meteorological forecast suggested that dry conditions would continue through at least the middle of September, which means the fires will likely keep burning, and the smoke will likely keep drifting. For now, Singapore's residents are being asked to stay inside, to keep their windows closed, to wear masks if they must go out. The unhealthy air is expected to linger.