Aerial footage shows an illegal agricultural fire razing a field in northern Thailand.

Drone video shows thick white smoke shrouding the farmland in Chiang Mai province on April 21.

The blaze caused air pollution in the Mueang Chiang Mai district to register an 'unhealthy' level of 201 on the air quality index (AQI), with PM2.5 levels reaching 80 microgrammes per cubic metre - 16 times the WHO's safe level of five microgrammes.

Siwss air company IQAir reported that Chiang Mai was the worst in the world for air pollution that day.

The fire comes after a hotspot broke out at the Chiang Mai Business Park on April 19, and another at the central Chiang Mai on April 20.

The Thai Meteorological Department reported that a low-pressure area was prevailing over northern Thailand and causing hot weather reaching 42 degrees Celsius in the provinces of Lamphun, Lampang, and Sukhothai.

Dangerous levels of deadly PM 2.5 particles - which cause cancer and other debilitating illnesses - are recorded every year in Thailand between December and April when farmers across Southeast Asia burn waste crops to clear their land.

The rudimentary method, known as 'slash and burn', is quicker and cheaper than modern machinery, leaving relatively poor landowners unwilling to heed environmental and health concerns over economic necessity.

Despite paying lip service for several years, politicians have failed to stop individuals from burning their land due to it being cheaper and easier than modern farming methods.

In March 2023, the Public Health Ministry of Thailand reported that around 1.3 million people in the country had been unwell due to the rising levels of air pollution. Officials said 200,000 residents were hospitalised in a single week.

Researchers from Peking University in Beijing, China, found that exposure to PM 2.5 air pollution is associated with an increased risk of lung cancer.

In December, Dr Krittai Thanasombatkul from Chiang Mai - one of the world's worst cities for air pollution - died of lung cancer. He had previously blamed PM 2.5 microdust particles for his condition.

The New York State Department of Health says PM 2.5 also increases the risk of heart disease, asthma, and low birth weight.

Medics from South Korea published research in the National Institute of Health in the United States that said the pollutant increases the risk of stroke, ischemic heart disease, chronic obstructive pulmonary disease (COPD), lung cancer, and other illnesses such as immune deficiency and even obesity.