Transboundary Haze

Transboundary haze is the smoke pollution that reaches Singapore when land, forest, and peat fires elsewhere in Southeast Asia — most often Sumatra and Kalimantan — send plumes across national borders. Unlike everyday urban emissions from vehicles and industry, haze is episodic and seasonal: it is most likely during the Southwest Monsoon dry months, when south-easterly to south-westerly winds can carry dense smoke toward the island (NEA, accessed Aug 2026; NEA/MSS, accessed Aug 2026). Singapore cannot extinguish the fires itself; its response rests on monitoring and health advisories at home, diplomatic and ASEAN cooperation abroad, and an extraterritorial criminal statute aimed at entities whose conduct contributes to haze pollution in Singapore.

Causes, seasonality, and why peat matters

Transboundary haze affecting Singapore is driven by smoke from land and forest fires in the surrounding region, particularly peatland and vegetation fires in fire-prone parts of Sumatra and Kalimantan. Drier and warmer conditions make those fires more likely to start and harder to put out; peat fires can smoulder below the surface for long periods and release large volumes of fine particles (NEA/MSS, accessed Aug 2026; NEA FAQs, accessed Aug 2026). Whether that smoke reaches Singapore depends on wind direction and strength as well as how close and how extensive the fires are. The usual risk window is the Southwest Monsoon period — roughly June to September, sometimes extending into October — when prevailing winds over southern ASEAN more often blow from the south-east or south-west (NEA, accessed Aug 2026; NEA/MSS, accessed Aug 2026). Climate drivers such as El Niño and a positive Indian Ocean Dipole can intensify and prolong that dry season; Singapore's driest year on record, 1997, coincided with both, and Meteorological Service Singapore warned in May 2026 that expected El Niño and a potential positive IOD would raise haze risk again for June–October 2026 (NEA/MSS, accessed Aug 2026). Domestic industrial and vehicle emissions remain Singapore's main day-to-day air-pollution sources; haze is the intermittent regional overlay on that baseline (NEA, accessed Aug 2026).

Measuring haze: PSI, PM2.5, and official readings

The 24h PSI hazardous band is above 300. The 24h PSI is not the 1h PM2.5 reading: NEA uses the former for daily health advisories and the latter for immediate decisions.

Singapore's official air-quality information comes from the National Environment Agency (NEA), which runs a network of analysers measuring six criteria pollutants — PM10, PM2.5, sulphur dioxide, nitrogen dioxide, ozone, and carbon monoxide — and reports results for five regions (North, South, East, West, Central) (NEA FAQs, accessed Aug 2026). The 24-hour Pollutant Standards Index (PSI) is computed from those pollutants over a rolling 24-hour average; a sub-index is calculated for each pollutant and the highest becomes the PSI value. Bands run Good (0–50), Moderate (51–100), Unhealthy (101–200), Very Unhealthy (201–300), and Hazardous (above 300) (NEA FAQs, accessed Aug 2026; haze.gov.sg, accessed Aug 2026). PM2.5 — fine particulate matter with diameter under 2.5 micrometres — is the predominant pollutant during haze episodes and has been included in PSI computation since April 2014; NEA also publishes 1-hour PM2.5 concentrations with bands (Normal ≤55 µg/m³, Elevated 56–150, High 151–250, Very High ≥251) for immediate outdoor decisions, while the 24-hour PSI and its health advisory are meant for next-day planning (NEA FAQs, accessed Aug 2026). Visibility and smell are poor proxies: humidity can degrade visibility without a matching pollution spike, and burning odours come from gases that are not the pollutants used for the index (NEA FAQs, accessed Aug 2026). Third-party sites that recompute indices with different methods are not comparable to Singapore's 24-hour PSI; NEA data on the NEA website, haze microsite, myENV app, and data.gov.sg are the official source (NEA FAQs, accessed Aug 2026).

Major episodes Singapore remembers

Haze has marked Singapore's regional environment for decades. During the 1997–98 Southeast Asian haze, Singapore's PSI peaked at 226 — the modern-era record before 2013 (BBC, accessed Aug 2026). In June 2013, smoke from fires in Sumatra produced the worst air-pollution episode then recorded in Singapore: a 3-hour PSI reading of 401 on 21 June 2013, widely reported as an all-time high, with schools closed for holidays, mask shortages, and sharp diplomatic friction with Indonesia (BBC, accessed Aug 2026). Reporting conventions have changed since that episode — PM2.5 entered the PSI in 2014, and the public tools now emphasise 24-hour PSI plus 1-hour PM2.5 rather than a 1-hour or 3-hour PSI — so historical peaks must be read with the index definition of the day (NEA FAQs, accessed Aug 2026). In September 2015, under El Niño conditions, the 24-hour PSI entered the Hazardous range for the first time since 2013, reaching 268–320 as of 9am on 25 September 2015 after schools had already been closed for a day and outdoor sports venues shut (CNA/TODAY, accessed Aug 2026). Further significant episodes, including September 2019, appear in NEA's haze-period charts used to explain how volatile 1-hour PM2.5 can be even when the 24-hour PSI moves more slowly (NEA FAQs, accessed Aug 2026). Each bad season reinforces the same political point: Singapore's air quality can be dictated by land-use and fire practice outside its borders, which is why ASEAN cooperation and domestic statute both feature in the response.

Health impact and how people are told to respond

Short-term exposure to unhealthy daily average PSI levels over several days can cause eye, nose, and throat irritation in healthy people and can aggravate existing heart or lung disease; symptoms may appear up to one to three days after exposure (NEA FAQs, accessed Aug 2026). Children, the elderly, pregnant women, and people with chronic lung or heart disease are treated as more sensitive groups in the Ministry of Health advisory tied to the 24-hour PSI bands: as air quality moves from Unhealthy through Very Unhealthy to Hazardous, advice escalates from reducing or avoiding prolonged outdoor exertion to minimising or avoiding outdoor activity altogether (NEA FAQs, accessed Aug 2026). NEA's practical guidance during episodes includes closing windows when outdoor air worsens, reducing indoor pollutant sources, using wet cleaning rather than dry dusting, and optionally using suitably sized portable purifiers with high-efficiency particle filters; N95 (or equivalent) masks are framed as optional protection for several hours outdoors when PSI is Hazardous for healthy people, or when Very Unhealthy for vulnerable groups who must still go out — not as routine kit for short commuting or indoor use (NEA FAQs, accessed Aug 2026). Because Singapore's haze exposure is intermittent rather than year-round, NEA notes that there is little robust evidence on long-term effects from this specific short-burst pattern, even though continuous multi-year exposure to high fine-particle pollution elsewhere is linked to cardiovascular and respiratory harm (NEA FAQs, accessed Aug 2026).

Critical perspectives: ASEAN Agreement on Transboundary Haze Pollution

The ASEAN Agreement on Transboundary Haze Pollution (AATHP) was signed in Kuala Lumpur on 10 June 2002 by the then ten ASEAN member states and entered into force on 25 November 2003 (ASEAN AATHP text, accessed Aug 2026). It is a regional treaty committing parties to cooperate on preventing and monitoring land and forest fires and the resulting haze, and ASEAN has described it as the first regional arrangement binding a group of contiguous states to tackle haze from such fires (ASEAN AATHP text, accessed Aug 2026). Indonesia — the source country for most of the fires that affect Singapore — delayed ratification for more than a decade after signature; Indonesian lawmakers approved ratification in September 2014 and the instrument of ratification was deposited with the ASEAN Secretariat on 20 January 2015, completing membership of all then ASEAN states. The agreement operates inside the ASEAN consensus and non-interference tradition: it creates a cooperative framework and institutional machinery rather than a strong supranational enforcement court, so effectiveness still depends heavily on domestic fire suppression, land governance, and political will in fire-source jurisdictions. Singapore treats the AATHP as necessary but not sufficient — hence the parallel domestic statute that can reach corporate conduct linked to fires abroad.

Singapore's Transboundary Haze Pollution Act

Singapore's Transboundary Haze Pollution Act 2014 (Act 24 of 2014) is administered by NEA and provides for investigation and prosecution of entities inside or outside Singapore whose conduct causes or contributes to transboundary haze pollution in Singapore (NEA legislation index, accessed 11 Aug 2026; NEA EPD Annual Report 2015, pp. 5 and 11, accessed 11 Aug 2026). NEA's 2015 report records the first implementation of the Act after the 2015 haze episode, including preventive measures notices served on six foreign entities; its 2016 report records NEA's continuing use of the Act to seek information and impose penalties on companies regardless of where they are based or incorporated (NEA EPD Annual Report 2015, accessed 11 Aug 2026; NEA EPD Annual Report 2016, pp. 64–65, accessed 11 Aug 2026). The Act is deliberately corporate-facing and extraterritorial: it creates liability risk for plantation and related entities linked to fires abroad, not a replacement for Indonesian firefighting. Readers should not confuse this Singapore statute (often abbreviated THPA) with the ASEAN AATHP; they share a subject and a similar short label but are different instruments.

Domestic preparedness: Haze Task Force and public tools

Operational readiness inside Singapore sits with the Haze Task Force (HTF): 28 government agencies chaired by NEA, with tiered action plans keyed to air-quality readings and forecasts to protect public health, especially vulnerable groups (NEA FAQs, accessed Aug 2026; NEA/MSS, accessed Aug 2026). During Southwest Monsoon months when haze risk is higher, NEA publishes a forecast range for the next 24-hour PSI with an accompanying health advisory; live 1-hour PM2.5 and 24-hour PSI figures are pushed through the NEA website, haze.gov.sg, myENV, and NEA social channels (NEA FAQs, accessed Aug 2026). The same seasonal climate that shapes Singapore's monsoon pattern therefore also shapes the annual preparedness cycle. NEA stated in May 2026 that the HTF plans remained ready even after several quieter recent dry seasons associated with La Niña rainfall, underlining that absence of recent severe haze is not the same as absence of risk (NEA/MSS, accessed Aug 2026). Broader domestic climate and greening goals are covered separately under green plan 2030; haze remains the clearest case where Singapore's environmental outcomes are structurally regional rather than purely national.