Smoke from forest and peatland fires burning across Indonesian Borneo has driven air quality in parts of Malaysia to hazardous levels, triggering a sharp surge in respiratory illness, shutting hundreds of schools, and prompting emergency measures across the region. The crisis — described as the worst transboundary haze episode since 2015 — has now spread beyond the island of Borneo to affect peninsular Malaysia, Brunei, Singapore, and parts of the Philippines.

The fires are concentrated in Kalimantan, the Indonesian portion of Borneo — the world's third-largest island, shared between Indonesia, Malaysia, and Brunei — as well as in Sumatra. Between January and July alone, more than 200,000 hectares burned across both islands, with nearly 95,000 hectares lost in July alone. Fire hotspots in West Kalimantan nearly doubled on Monday, and cities within Indonesian Borneo have recorded some of the worst air in the world: Pontianak, the West Kalimantan provincial capital, registered an air quality index of 851 on Tuesday, while Palangkaraya in Central Kalimantan reached 675 — both far exceeding the 300 threshold considered hazardous. Across the border, six areas in the Malaysian state of Sarawak recorded readings above 300 on Malaysia's Air Pollutant Index, with Kuching, the state capital, hitting 407. Analysts say the fires are largely set deliberately by plantation companies and smallholders clearing rainforest and carbon-rich peatland for palm oil, farming, and pulp and paper production — a practice that is illegal in Indonesia, where police have arrested 72 people suspected of arson.

The human cost is severe and mounting. Indonesia's deputy health minister reported that some 13,500 people were treated for respiratory infections in July and August, with approximately 5.5 million people directly exposed to toxic smoke. In Malaysia, asthma cases climbed from 219 in the week before August 23 to 1,811 in the following week. In Sarawak, nearly 600 schools closed on two days in August, affecting around 200,000 students, and Independence Day outdoor celebrations were cancelled. Residents describe skies that have not been blue for over a month, while medical professionals are pre-emptively increasing medication dosages to keep asthma patients out of hospital.

The crisis extends beyond public health. Indonesia has temporarily closed nine national parks — including the ecologically significant Mount Leuser National Park in northern Sumatra and Bukit Baka Bukit Raya in Kalimantan — to redirect firefighting helicopters toward the six worst-affected provinces. Wildlife is also at serious risk: orangutans, an endangered species native to Borneo and Sumatra, are losing habitat and food sources to the flames. At least one weakened individual has been rescued and transferred to a rehabilitation centre run by wildlife foundation YIARI.

Researchers warn that the worst may still be ahead. The fires are being intensified by an El Niño weather pattern — a naturally occurring climate cycle that reduces rainfall and raises temperatures across Southeast Asia — now threatening to reach