Southeast Asia Haze

Last updated May 29, 2026 · 7 min read

For most of the year the air across maritime Southeast Asia is good. Then, in the dry months, a brown pall can settle over Singapore, Malaysia, Brunei, and southern Thailand for days or weeks. This is "the haze" — smoke carried on the wind from fires hundreds of kilometres away. This guide explains what it is, why it happens, when to expect it, and how to read the numbers during an event.

What the haze actually is

The haze is wildfire smoke, dominated by fine particulate matter (PM2.5). Its source is land-clearing fires — much of it slash-and-burn agriculture and plantation clearing — in Sumatra (especially Riau province) and the Indonesian and Malaysian parts of Borneo (Kalimantan). Crucially, many of these fires burn in drained peatland. Peat is partially decomposed organic matter that smoulders rather than flames, producing enormous volumes of dense, particulate-rich smoke that is extremely hard to extinguish — fires can burn underground for months.

The season: dry months and El Niño

Haze season runs roughly June to October, during the southwest monsoon's drier phase. Its severity is closely tied to the El Niño–Southern Oscillation: in strong El Niño years the dry season is longer and more intense, peat dries out, fires spread, and the haze is far worse. The worst episodes on record — 1997–98, 2015, and 2019 — all coincided with El Niño conditions.

The major episodes

The 1997–98 haze was the first to draw sustained global attention. In June 2013, Singapore's 3-hour PSI hit a record of around 400 — the "Hazardous" band — as smoke from Sumatran fires blanketed the city-state. The 2015 episode was the most damaging: smoke persisted for weeks across the region, schools and airports closed, and later public-health analyses attributed tens of thousands of premature deaths across the region to that single season. A further significant episode followed in 2019.

Reading the numbers: why the 1-hour PM2.5 matters

Singapore reports the Pollutant Standards Index (PSI) as a 24-hour figure. During a fast-moving haze event, that 24-hour average lags reality — it can still read "Moderate" while the air outside has already turned acrid. That is why the NEA publishes the 1-hour PM2.5 reading alongside the PSI during haze, and keys its activity advisories to it. When haze is active, the 1-hour PM2.5 band is the number to watch, not the headline PSI.

The transboundary politics

Because the smoke crosses borders, the haze is also a diplomatic issue. ASEAN adopted the Agreement on Transboundary Haze Pollution in 2002, and Singapore passed its own Transboundary Haze Pollution Act in 2014, which allows the prosecution of companies whose fires cause haze in Singapore regardless of where the fire is lit. Enforcement remains difficult, and the underlying driver — the economics of burning to clear land — persists.

What to do during a haze episode

• Watch the 1-hour PM2.5 band, not just the 24-hour PSI, and follow the NEA's activity guidance for your band.
• Stay indoors with windows shut when readings are high; run an air purifier with a HEPA filter if you have one.
• Wear a properly fitted N95/KN95 if you must be outdoors for long — surgical and cloth masks do little against fine particulate.
• Prioritise the vulnerable — children, the elderly, pregnant people, and anyone with heart or lung conditions feel effects first.

Primary sources: Singapore NEA — Haze · ASEAN Specialised Meteorological Centre (ASMC) · WHO Air Quality Guidelines