Air pollution soared as rogue farmers began their annual crop burning in Thailand.

Thick smoke drifted from burning rice fields and shrouded buildings and roads in northeastern Nakhon Ratchasima province on December 17.

Local authorities dispatched fire trucks as the blaze spread from farmlands toward nearby residential areas.

Dr Taweechai Wisanuyothin, director of the Office of Disease Prevention and Control Region 9, said: 'Smoke from industrial factories and power plants, vehicle exhaust, and open burning contribute to PM 2.5 fine dust. This particulate matter can enter the bloodstream through the respiratory system and infiltrate internal organs.

'Therefore, it is important to continuously monitor at-risk groups, including pupils, the elderly, pregnant women, people with underlying health conditions, and those who work or exercise outdoors.'

Swiss air quality company IQAir today reported that PM2.5 pollutant levels reached 106 AQI, which was 'unhealthy for sensitive groups'.

Scientists found that smog is at its worst in the country when farmers burn waste crops to cheaply clear their land between late October and April. The agricultural process known as 'slash and burn' is quicker and easier than crop rotation fields and leaves a nutrient-rich layer of ash that helps to fertilise new crops.

However, the side-effect is severe air pollution full of toxic PM2.5 particles that cause respiratory ailments. The pollution has also worsened in recent years due to the rising demand for food from rapidly developing cities in Southeast Asia.

The lack of wind and rain during the same period, combined with a winter low-pressure system in which cold air is trapped closer to the ground by warm air above, prevent the smog from being dispersed naturally by wind.

Thailand's government has been criticised for failing to tackle the country's air pollution crisis, with bizarre measures such as spraying water into the air, placing giant purifiers on roads and even using cloud-seeding planes to cause downpours. There have also been claims that they are under pressure from powerful food conglomerates to allow the intensive farming methods.

Data scientist engineer Worasom Kundhikanjana said that the pollution will return next year without draconian policies banning agricultural burning.

She said: 'PM 2.5 pollution has a seasonal trend in Bangkok and the northern provinces, but has only recently received public attention. During this winter haze, the harmful particle pollution level is high throughout the day, including on weekends.

'Unfortunately, spraying water into the air does not appear to be effective, since the volume of water is minuscule compared to actual rain.

'With the start of the monsoon season, the weather in Bangkok will get better, and public attention on this issue will likely fade. However, without immediate policy measures, the problem will come back again next winter.'