By Serene Inulia Kong Kwai Yan

Transboundary haze is an annual environmental and public health issue in Southeast Asia (SEA), with initial cases recorded as early as 1972. It typically occurs during the dry season between June and October, with the highest risk in August and September. The characteristic “smoke-haze” refers to a suspension of smoke, dusty moisture and hazardous fine particulate matter that creates a fog-like layer. It is primarily caused by illegal slash-and-burn farming, forest and peatland fires in Indonesian hotspots, namely Sumatra and Kalimantan. Prolonged dry seasons, El Niño and the positive Indian Ocean Dipole (IOD) exacerbate fire risks. Southwest monsoon winds then carry toxic smog across national borders, specifically into Peninsular Malaysia, East Malaysia and Singapore. The resulting crisis affects the health of around 75 million people and the economies of up to 6 Association of Southeast Asian Nations (ASEAN) member states. This article aims to explore the question: when haze crosses national borders, who bears the health burden, and how do we prevent it?

Understanding the regional crisis

For centuries, slash-and-burn agriculture was conducted by the indigenous populations of Sumatra and Kalimantan as a sustainable form of swidden cultivation. However, in the 1970s, it evolved into a commercial tool, leading to destructive burning. In particular, between 1976 and 1989, the Transmigration Program, which aimed to relocate families out of overpopulated Java to the outer islands of Sumatra, Kalimantan and Sulawesi, increased demand for land to construct settlements and cultivate crops. During dry seasons, fires tend to spread uncontrollably into adjacent forested or protected zones. Peatland degradation is particularly concerning because burning permanently lowers the water table, turning previously wet soil into highly flammable “carbon bombs” which burn underground for weeks.

Forest fires release massive amounts of carbon soot, ash and fine particulate matter into the atmosphere. The smoke contains a high concentration of particulate matter, mainly particles less than 2.5 microns in size (PM2.5). Beyond the obvious health effects, it reduces visibility and disrupts economic activity.

Responsibility for the crisis, however, extends beyond its host, Indonesia. Corporate investments, agribusiness financing and plantation ownership link Malaysian and Singaporean commercial interests to Indonesian agricultural operations. For instance, Malaysian and Singapore businesses have been identified by Greenpeace as among the top 20 contributors to this effect, leading to national calls for stricter legislation.

Although major episodes of fire and transboundary haze occur regularly, the blaze of 1997-1998, which affected Brunei Darussalam, Indonesia, Malaysia, Philippines, Singapore and Thailand, is to date the most damaging in recorded history. More than USD 9 billion of damage was recorded in terms of economic, social and environmental losses, while around 1-2 billion tonnes of carbon was released. It was also associated with an estimated 19,800 to 48,100 premature deaths in Indonesia, illustrating the devastating consequences of severe haze.

Health, social and economic impacts

Recurrent but intermittent transboundary haze has been proven to have deleterious effects on health, economies and environments. In the short term, seasonal haze exacerbates physical symptoms, such as sore throat or dry mouth, nose discomfort, eye discomfort, headache, shortness of breath and skin irritation. It has also been associated with an increase in both mild (upper respiratory tract infection, rhinitis) and severe (bronchitis, emphysema) respiratory symptoms. Fine particulate matter is particularly concerning because it penetrates deep into the lungs and may contain carcinogenic substances. For instance, walk-ins for asthma, allergies and respiratory issues increased by as much as 20% in Singapore during September-October 2026 haze.

The health burden is not distributed equally. At the highest risk are the elderly, young children, people working in urban environments or outdoors, or patients with preexisting respiratory conditions are at risk of worsening. This reflects the “inverse care” phenomenon: populations with least resources to protect themselves (such as staying indoors and purchasing high-quality masks) are the most affected by public health threats. 

Aside from respiratory effects, long-term exposure to air pollution is also linked to increased risk of cerebrovascular disease, like migraines and stroke, and cardiovascular disease. However, the underlying pathological mechanism remains unclear. In Singapore, haze has also been attributed to psychological stress.

Moreover, haze has a significant economic and social impact. Costs invoked include absenteeism, medical leave and lost income opportunities. Poorer visibility also affects the transport and tourism sector by causing flight cancellations, airport closures and a decline in tourist arrivals. In the workforce, productivity decreases by disrupting manufacturing operations. Macroeconomic estimates note that 1 µg/m³ increase in PM2.5 reduces workforce productivity by 0.6% to 1.9%. 

Ultimately, costs are borne by individuals, healthcare systems, businesses and governments across the region, including countries that did not cause the fires.

Regional responses

Addressing transboundary haze demands cooperation between countries with interconnected environmental and economic interests. The 1992 Rio Declaration on Environment and Development declared three principles relevant to this challenge: nations have the right to exploit their own resources, but without causing environmental damage beyond their borders; the polluter should bear the cost of pollution; sustainable developmentment requires better understanding of the problems.

In 2002, the ASEAN Agreement on Transboundary Haze Pollution (AATHP) was signed by member states as a regional response, outlining discussion, monitoring and information-sharing mechanisms. Nevertheless, this approach is limited due to its reliance on the ‘ASEAN Way’ diplomacy, which dictates a non-interference approach to other country’s internal affairs. As a result, enforcement is restricted, empowering non-action or non-cooperation. Its effectiveness also depends on ciuntdies’ willingness to participate. For example, despite become a signatory in 2002, Indonesia was the final member state to ratify the AAHTP treaty in 2015. In 2026, Indonesia’s delay in accepting emergency response and firefighting assistance has setback cross-border cooperation.

Domestic legislation is therefore an important avenue for Malaysia and Singapore to address corporate responsibility. As a case in point, Singapore uses its Transboundary Haze Pollution Act 2014 to hold companies accountable for conduct outside its borders that causes local haze. This model is being examined in Malaysia by the Ministry of Natural Resources and Environmental Sustainability (NRES) to study the viability of a domestic transboundary haze act.

Governments have also implemented measures to reduce immediate public exposure. In 2026, Malaysia rolled out school closures across several states and federal territories due to unhealthy Air Pollution Index (API) readings; while in Singapore, government medical subsidies for citizens seeking treatment for haze-related illnesses were activated.

In 2026, Malaysia and Singapore also participated in bilateral engagement as part of the regional response. Namely, Malaysian and Indonesian prime ministers communicated on the need for immediate action as well as discussed joint firefighting assistance and mitigation, while Singapore affirmed their readiness to assist. 

On a national level, civil society plays an important role in urging action. Greenpeace Malaysia, an environmental group, has recently advocated for stricter cross-border corporate accountability.

Conclusion

Considering its catastrophic consequences, Indonesia, Malaysia and Singapore have a vested and common interest to reduce the frequency and severity of seasonal transboundary haze. Multilateral and national discourse should not be dominated by political interests, but rather prioritize victims affected by haze disasters. Efforts should be not just reactive, but also preventive, such as incentivizing a shift towards alternative agricultural practices. Ideally, regional cooperation, like through ASEAN, should be encouraged to achieve an effective coordinated response.