The skylines of Singapore and Kuala Lumpur have spent much of September behind a grey-brown veil. Across the region, air quality monitors are telling the same story: Southeast Asia is in the grip of its worst transboundary haze episode since 2019, and the two capitals of the Malay Peninsula are breathing it in.
On September 15, Malaysia’s Department of Environment reported 23 areas with unhealthy Air Pollutant Index readings. Johan Setia in Selangor logged 166, Pasir Gudang in Johor 164, and Cheras in Kuala Lumpur 159 — all well past the 100 mark that triggers suspension of outdoor school activities under the National Haze Action Plan. Across the causeway, Singapore’s 24-hour Pollutant Standards Index breached 100 for the first time since October 2023, reaching 105 in the central region, and the city-state briefly ranked among the ten most polluted major cities in the world. At the start of the month the regional league table was even starker: Kuching first, Kuala Lumpur fourth, Singapore seventeenth.
Where the Smoke Comes From
The engine of the crisis sits across the water. Clusters of fires in southern and southeastern Sumatra and in Kalimantan — 324 hotspots were counted in Kalimantan at the end of August — are pouring moderate to dense smoke plumes into prevailing southeasterly and southwesterly winds, which carry them straight over the Strait of Malacca to the Peninsula and Singapore. Much of the burning is the old story of land clearing for plantations, made worse by drained peatlands that smoulder for weeks and release far more fine particulate matter than ordinary forest fires. Malaysian Borneo has taken the most brutal readings: Serian in Sarawak hit a hazardous API of 475, with Samarahan, Sri Aman and Kuching all above 300.
Volcanoes have muscled into the picture this month as well. Anak Krakatau went into a major eruption over September 4 to 6, in a booming episode that lasted more than a day, drove ash to around 50,000 feet, disrupted thousands of flights and measurably degraded air quality in parts of Indonesia, including Jakarta, while a sulfur dioxide cloud spread out from the Sunda Strait at high altitude. Claims quickly circulated that the ash had crossed to Malaysia. MetMalaysia’s assessment is more restrained: the plume has stayed concentrated around the Sunda Strait and nearby Indonesian areas, the risk of volcanic ash reaching Malaysia is low, and the poor visibility over Sarawak and the western peninsula is transboundary fire haze, not ash. Over Kuala Lumpur and Singapore, the blanket remains overwhelmingly peat-fire PM2.5 — but with the volcano still restless, ash riding a shifted wind is a wildcard the region’s forecasters are explicitly watching.
City sources — traffic exhaust, construction dust and local open burning — sit underneath the imported smoke and push readings higher still, which is why the Department of Environment has paired its monitoring with stepped-up enforcement against open burning at home. But the ASEAN Specialised Meteorological Centre left little doubt about the driver when it raised its haze alert for the southern region to Level 3 in late August, its highest tier, citing persistent hotspot clusters and dry conditions.
What It Does to the Lungs
The dominant pollutant in haze is PM2.5 — particles less than 2.5 microns across, small enough to slip past the nose and throat, lodge deep in the lungs and cross into the bloodstream. In the short term that means stinging eyes, sore throats, coughing and asthma flare-ups. Sustained exposure is associated with reduced lung function and added strain on the heart. Studies of the extreme 2019 Indonesian peatland fires documented what researchers called a catastrophic impact on urban air quality and health across the region, and the same smoke chemistry is in the air now.
The risk is not spread evenly. Children, the elderly, pregnant women and anyone with existing heart or lung conditions feel the haze first and worst. Health authorities on both sides of the causeway are giving the same practical advice: cut back on prolonged or strenuous outdoor activity while readings are elevated, keep well hydrated, wear a properly fitted N95 mask outdoors on bad days, run an air purifier indoors if you can, and keep regular medications within reach. The encouraging news is that for most healthy people the effects of a haze episode are temporary and reversible once the air clears. Anyone who develops breathlessness, chest tightness or a cough that will not settle should consult a doctor promptly rather than wait it out.
How Long the Cities Will Breathe It
The honest answer from the forecasters: weeks, not days. Singapore’s Meteorological Service warned at the start of September that hazy conditions could persist for two weeks, with only sporadic showers offering relief. The ASEAN centre’s outlook is drier still — below-normal rainfall across the southern region for September, a strong El Niño in place, and an escalated hotspot and haze situation expected for as long as the dry spell and the southeasterly winds hold.
Malaysia has not waited. A cloud-seeding campaign flew from September 3 to 5 over thirteen areas in Sarawak, Kedah and Perlis, doubling as a top-up for dams and water catchments ahead of more dry weather. Experts caution that artificial rain buys only temporary, local relief; it cannot put out peat fires burning in another country.
History points to the real off-ramp. Severe haze years like 2015 and 2019 — both, like this one, shaped by El Niño — only ended when the monsoon transition brought sustained rains to Sumatra and Kalimantan, typically from late October into November. Until then, residents of Singapore and Kuala Lumpur should expect the haze to come and go on the wind: clearer days when showers pass through or winds shift, unhealthy spikes when the plumes line up again. The masks, the purifiers and the daily glance at the API and PSI dashboards are likely to remain part of city life for several more weeks — and the pressure on regional governments to deal with the fires at their source, rather than their smoke, will only grow.
