A wildfire tore through a forest as it raged on a mountain for four days in northern Thailand.

The inferno broke out on Doi Nang mountain in the Chiang Dao Wildlife Sanctuary in Chiang Mai province on April 21.

Local authorities and the Protected Areas Regional Office 16 on Thursday, April 25, dispatched more than 50 firefighters and volunteers to battle the fire, which has been spreading for four days due to dry conditions and strong winds.

Helicopters were also deployed to dump water on parts of the mountain inaccessible to ground personnel.

Authorities said the fire was under control on April 26 morning. They said more than 1,000 rais (395 acres) of forest were razed.

Athapol Charoenshunsa, director general of the Department of National Parks, Wildlife, and Plant Conservation, said in a statement: 'We plan to indefinitely close Chiang Dao Wildlife Sanctuary until we can address the issues of illegal burning, forest encroachment, poaching, and organising accommodations and homestays. 

'Due to illegal burning, the forest has been ravaged by wildfires. There is a risk of losing the ecosystem and biosphere reserves, including rare and valuable plant species.'

Kritsayam Kongsatri, director of Protected Areas Regional Office 16, said: 'Doi Luang Chiang Dao in the Chiang Dao Wildlife Sanctuary is a vulnerable area that requires extra monitoring due to its significance in geology, plants, wildfires, history, and culture. UNESCO has declared it a biosphere reserve.

'This year's forest fires are still not as severe as in previous years, so we have dispatched patrol teams to strictly monitor forest entry. 

'If there are more forest fires or illegal burning, we will report them to the Ministry of Natural Resources and Environment for consideration. 

'By closing Doi Luang Chiang Dao, community tourism areas will also be affected. Therefore, we ask for cooperation from the locals to minimise the impact of the closure.'

Swiss air monitoring company IQAir reported that the air quality index in Chiang Mai on April 26 reached an 'unhealthy' 172 AQI, while the PM 2.5 fine dust level was at 95.4 µg/m³.

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.