Transboundary Haze in Malaysia: Causes, Health Effects and the Regional Response

Every time the monsoon winds change direction during the dry season, the skies over the Klang Valley, Sarawak and much of the Peninsula can turn grey in a matter of days — the sun dims, the smell of smoke seeps into homes, and phones buzz with API alerts. This is transboundary haze: pollution not fully produced within the country, yet breathed in every year by millions of Malaysians.

Why does Malaysia’s sky turn grey almost every year?

The recurring thick haze is not an ordinary natural phenomenon. Its main cause is large-scale land and forest fires in the region. Open burning and forest fires are indeed listed as one of the major sources of air pollution in Malaysia, alongside industry, vehicles and power generation.

However, the worst haze episodes usually involve smoke carried by the wind from beyond the border. According to an analysis by the Center for Strategic and International Studies (CSIS), peatland fires in Indonesia and Malaysia account for a large share of transboundary haze, particularly as a result of slash-and-burn land-clearing techniques in Sumatra and the Kalimantan region.

Peatland is a cause that is hard to control: once drained and set alight, it smoulders deep into the ground, producing thick smoke that can drag on for weeks and is difficult to extinguish.

Where does this haze come from?

The main source is Indonesia’s two large islands — Sumatra and Kalimantan (Borneo) — where land is cleared for agriculture, including plantations, using burning methods that are cheaper than mechanical clearing.

Weather factors make matters worse. The El Nino phenomenon brings longer, hotter dry seasons, making peatland and vegetation more flammable and allowing fires to spread. CSIS notes that the 2023 haze episode, for example, was driven by an El Nino phenomenon that affected Indonesia, Malaysia and Singapore.

Its economic and health impacts are not small. The 1997 haze crisis — among the worst ever recorded — sent smoke as far as Malaysia, Singapore, Brunei, Thailand and the Philippines, and was estimated to have caused losses of around US$9 billion, according to CSIS.

This article sets out the facts about the sources and the regional response without pointing fingers at any particular party; the actual cause of each episode depends on field investigations and satellite data for the year in question.

How does Malaysia measure air quality?

The figure you see most often during haze is the API — the Air Pollutant Index. It is maintained by the Department of Environment (DOE) as an indicator of air-quality status in a given area.

The API is calculated based on five main pollutants, and the highest sub-index among the five determines the final API value:

• Sulfur dioxide (SO2)
• Nitrogen dioxide (NO2)
• Carbon monoxide (CO)
• Particulate matter smaller than 10 microns in diameter (PM10)
• Ground-level ozone (O3)

According to DOE documentation, the concentration of these pollutants is monitored by 52 automatic air-quality monitoring stations (Continuous Air Quality Monitoring, CAQM) across Malaysia, supported by 14 manual sampling stations, sited in industrial, urban, suburban and rural areas. (This network may have grown since that document — confirm the latest figure with the DOE.)

What does each API reading mean for your health?

The DOE maps each API range to a health status and an advisory. The table below summarises the official scale:

API range	Status	Health effect	Advice
0–50	Good	Low pollution with no adverse effects	No restrictions on outdoor activities
51–100	Moderate	Moderate pollution, no adverse effects	No restrictions on outdoor activities
101–200	Unhealthy	Worsens the condition of high-risk groups (heart and lung patients)	High-risk groups should limit outdoor activities; the public should reduce strenuous activity
201–300	Very Unhealthy	Affects exercise tolerance and public health	The elderly and high-risk groups should stay indoors; patients are advised to see a doctor
> 300	Hazardous	Hazardous to high-risk groups and public health	High-risk groups are barred from outdoor activities; the public should avoid outdoor activities
> 500	Emergency	Hazardous to high-risk groups and public health	The public should follow the instructions of the National Security Council and media announcements

A note on the ranges: the DOE’s official table openly lists the top bands as ”> 300” (Hazardous) and ”> 500” (Emergency). In practice, this means the Hazardous band covers 301–500 and Emergency begins above 500.

Note that “high-risk” means children, the elderly and those with heart or respiratory problems — they are affected earlier and more severely than healthy adults.

When are schools closed, and what happens at the emergency level?

For parents, the most pressing question is usually about schools. Under the reported standard operating procedure, schools in an area are typically closed when the API reading reaches 200. In September 2019, for example, dozens of schools in Selangor were ordered to close as the haze worsened.

The actual decision is made by the authorities based on the DOE’s current readings, so parents are advised to monitor the official readings and state education department announcements, not just the state of the sky they can see.

At the most extreme level — an API above 500 — an Emergency status is declared, and the public is advised to follow the instructions of the National Security Council and mass-media announcements.

Does Malaysia have its own Transboundary Haze Act?

This is a common misconception. Malaysia does not have a Transboundary Haze Act in force. The “Transboundary Haze Pollution Act 2014” that is often cited actually belongs to Singapore, not Malaysia.

In Malaysia, a similar draft act was once prepared during the Pakatan Harapan government, but it was shelved in 2020 and remains under review to this day. The relevant minister explained the main challenges: enforcement requires the cooperation of neighbouring countries, involves questions of the sovereignty of the source country of the pollution, and evidence for prosecution is hard to obtain because it involves sensitive cross-border data. Singapore’s experience also shows that a domestic act is difficult to enforce against entities operating outside the country concerned.

The government’s official position is that legislation alone will not solve this problem, and diplomatic engagement with ASEAN neighbours is given priority.

What is the ASEAN Agreement on Transboundary Haze Pollution?

Because the smoke crosses borders, the main solution is also regional in nature. The ASEAN Agreement on Transboundary Haze Pollution was adopted on 10 June 2002, and by 20 January 2015 it had been ratified by all ASEAN member states.

The agreement aims to prevent and monitor transboundary haze pollution arising from land and/or forest fires. It is implemented through several governance bodies, including the Conference of the Parties (COP), the Technical Working Group (TWG) and the Ministerial Steering Committee (MSC). ASEAN also adopted the ASEAN Cooperation Roadmap towards Controlling Transboundary Haze Pollution (the year of adoption needs to be confirmed; commonly cited as 2016).

Malaysia is an active party to this agreement, with the DOE implementing it through several key strategic components and taking part in regular regional meetings.

What’s next

• During the dry season: check the DOE’s official API reading for your area before planning outdoor activities, and pay particular attention to children, the elderly and respiratory patients.
• If the API approaches 200: expect possible school closures and monitor state education department announcements.
• To understand the governance: follow developments on whether Malaysia will eventually enact its own haze act, and the outcomes of meetings under the ASEAN Agreement.
• Further sources in this knowledge base: refer to related articles on the Department of Environment and the ASEAN Agreement on Transboundary Haze Pollution when available.

Note: This article is an AI draft and has not undergone expert human review. Figures and policy thresholds can change — confirm current readings and the latest SOP with official DOE sources before acting.