Singapore’s PSI Hits Unhealthy Level as Haze Returns — But the Bigger Threat May Still Be Blowing In

SINGAPORE — Singapore’s air quality slipped into the unhealthy range on Friday morning, September 4, as worsening regional haze pushed the 24-hour Pollutant Standards Index in the central region to 101 — and weather conditions suggest the threat may not be over yet.

At 7am, the National Environment Agency’s air-quality readings showed a 24-hour PSI of 101 in central Singapore, officially crossing the threshold from “Moderate” to “Unhealthy.”

The rest of Singapore remained within the moderate range at that time: the west recorded a PSI of 87, the east 86, the north 76, and the south 74.

Under Singapore’s air-quality scale, a PSI reading between 101 and 200 is classified as unhealthy.

But the number that residents may want to watch most closely throughout the day is not necessarily the 24-hour PSI.

Why the 1-Hour PM2.5 Reading Matters

The 24-hour PSI is designed to show Singapore’s overall air quality over a longer period and forms the basis of NEA health advisories.

It is calculated using six pollutants, including PM2.5, PM10, ozone, sulphur dioxide, nitrogen dioxide and carbon monoxide.

During a haze episode, however, PM2.5 — microscopic particles measuring 2.5 micrometres or less — is typically the main pollutant of concern.

Unlike the 24-hour PSI, the one-hour PM2.5 reading provides a more immediate snapshot of current air conditions, making it particularly useful for people deciding whether to exercise or undertake other outdoor activities.

The health concern comes from the particles' extremely small size, which allows them to penetrate deeply into the respiratory system.

NEA advises healthy people to reduce prolonged or strenuous outdoor physical exertion when the PSI is in the unhealthy range.

Children, pregnant women and elderly people should minimise prolonged or strenuous outdoor exertion, while people with chronic heart or lung conditions are advised to avoid such exertion.

Singapore's Ministry of Health notes that short-term exposure to haze generally does not cause major health problems among healthy people, although some may experience irritation of the eyes, nose or throat.

People with existing heart or lung conditions should keep their medications readily available and seek medical attention if they feel unwell.

Singapore Had Been Moving Closer to the Threshold

Friday morning's PSI reading did not emerge without warning.

The Straits Times reported that central Singapore's PSI stood at 90 at 1am, before rising to 99 by 6am and crossing the unhealthy threshold an hour later.

It marks the first time Singapore's PSI has reached unhealthy levels because of haze since October 2023, according to the newspaper.

Authorities had already been preparing for such a scenario.

Singapore's inter-agency Haze Task Force, comprising 28 public agencies, previously said its members were ready to activate their respective response plans if air quality deteriorated into the unhealthy range.

Healthcare providers, schools, workplaces and other agencies have also been preparing measures aimed at protecting vulnerable groups if conditions worsen.

NEA has said it will issue daily haze advisories once the 24-hour PSI reaches the unhealthy range.

Why the Haze Is Back

The bigger story is unfolding hundreds of kilometres away.

Persistent dry weather has intensified forest and land fires across parts of Sumatra and Kalimantan in Indonesia, creating large amounts of smoke that can be transported across national borders depending on wind direction.

Singapore's Meteorological Service warned on September 1 that dry conditions were expected to persist through much of the first half of the month.

Dense smoke haze had already been observed over southern and central Sumatra and various parts of Kalimantan on most days since around August 21, according to the agency.

With prevailing winds blowing mainly from the southeast or southwest, Singapore could experience further hazy conditions if the fires intensify.

The official regional haze portal has also reported unhealthy to hazardous air-quality conditions at monitoring stations in parts of West and Central Kalimantan, Sumatra, Sabah and Sarawak, while warning of a continuing risk of severe transboundary haze while dry weather persists.

Indonesia Is Fighting a Much Larger Fire Crisis

The scale of the fires is becoming increasingly significant.

Reuters reported that Indonesian authorities were inspecting 19 companies linked to fires detected within concession areas covering about 11,047 hectares, as the government stepped up efforts to control wildfires across Sumatra and Kalimantan.

Nearly 95,000 hectares burned in Indonesia during July alone, according to Reuters, while firefighting operations have involved tens of thousands of personnel, volunteers and aerial resources.

The human impact is also growing.

Around 1.4 million students across six Indonesian provinces had been moved to remote learning because of dangerous wildfire haze as of September 2, Reuters reported.

Malaysia has also been hit hard.

The Straits Times reported that air quality deteriorated dramatically in parts of Sarawak, with Serian registering an Air Pollutant Index reading of 408 at 8pm on September 3, while several other areas recorded very unhealthy conditions.

That regional picture matters for Singapore because haze severity does not depend only on how many fires are burning.

Wind direction can determine whether smoke is carried toward Singapore or away from it.

That means air quality can change substantially even when the underlying fire situation remains severe.

Could Singapore's Haze Get Worse?

There is no certainty that Friday morning's PSI reading will continue climbing.

Weather changes, shifting winds and rainfall could improve Singapore's air quality.

But the conditions surrounding Singapore remain favourable for continued haze episodes.

The Meteorological Service expects rainfall during the first half of September to remain below average across most of Singapore, while dry weather is also expected to persist over much of the surrounding region.

Daily temperatures are forecast to reach around 33°C to 35°C on most days, with temperatures possibly climbing slightly above 35°C on some days.

Those conditions mean the regional fire situation — and particularly where the smoke travels — will remain critical.

For Singapore residents, the most practical approach is to monitor both the 24-hour PSI for broader health guidance and the one-hour PM2.5 readings before deciding on immediate outdoor activities.

And while a PSI of 101 represents only the first point inside the unhealthy range, events elsewhere in Southeast Asia suggest the more important question is not simply how high Singapore's PSI climbed Friday morning.

It is whether changing winds will bring more of the region's smoke toward the island in the days ahead.