A trans-national air pollution crisis affected several countries in Southeast Asia from February to September 2019, including Brunei, Indonesia, Malaysia, the Philippines, Singapore, Thailand, and Vietnam.

It was the latest occurrence of the Southeast Asian haze, a long-term issue that occurs in varying intensity during every dry season in the region.

It was mainly caused by forest fires resulting from illegal slash-and-burn clearing performed on behalf of the palm oil industry in Indonesia, principally on the islands of Sumatra and Borneo, which then spread quickly in the dry season.

Most of the hotspots for the northern countries of South East Asia (Thailand, Myanmar, Cambodia, Laos, Vietnam, and the Philippines) occurred from January to May 2019, particularly in March and April 2019.

These fires clear land for palm oil plantations, and are known to be started by smallholding subcontractors who supply large companies that claim to discourage the practice but admit the chain of custody is a "complicated web.

Most of these banks are from Asia, including Singapore, Malaysia and Indonesia, that have funded billions of dollars towards deforestation for oil palm and pulp and paper industries.

[13] On 7 September 2019, Brunei experienced haze, which its government attributed to hot spots across the Indonesian border in the Kalimantan provinces of Borneo.

[26] On 17 September 2019, based on the initial report of regional offices of the Environment Management Bureau (EMB), it was confirmed by the agency and the authorities of General Santos that prolonged hazy skies were experienced in Mindanao and the Visayas, particularly Cebu City.

[43][44][45] This resulted in the National Environment Agency (NEA) forecasting healthy PSI readings since the haze first hit Singapore two weeks ago.

[48] Furthermore, in July, around 2,318.88 hectares (5,730.1 acres) of forest and farmlands in Pa Phru Kuan Kreng of Nakhon Si Thammarat Province and surrounding areas in southern Thailand have been destroyed by fires.

[51][52][53] In August, all of the involved countries reached an agreement to prevent land and forest fires that are causing transboundary haze pollution with the hope to achieve a haze-free Association of Southeast Asian Nations (ASEAN) by the year 2020.

[54] The Department of Environment, Parks and Recreation of Brunei (JASTRe) set to introduce a law that will tackle "rampant open burning" in the country to mitigate bush and forest fires.

[58] Malaysian Maritime Enforcement Agency (MMEA) has dispatched two flights using the Bombardier CL415 aircraft carrying 198,000 litres of water to fight forest fire in Miri.

[59] On 19 August 2019, Datuk Amar Douglas Uggah Embas, the Deputy Chief Minister of Sarawak, announced that principals would be given discretion on closing their schools if API reading became 'unhealthy'.

[60] On September 10, the National Disaster Management Agency (Nadma) sent 500,000 face masks to Sarawak amid worsening trans-boundary haze in the state.

[61] The state of Johor's Education Department closed down all schools and kindergartens in Pontian, Muar and Tangkak on September 15 because of the haze, affecting over 64,000 students.

[68] The Malaysian Energy, Science, Technology, Environment and Climate Change Ministry said they wanted concerted efforts taken in accordance with the ASEAN Agreement on Transboundary Haze Pollution ratified by member countries.

[74] Meanwhile, the Ministry of Education said that air purifiers are ready should the haze worsen, with closure of schools when the Pollutant Standards Index reaches 300 and above.