Thick smoke covered roads as Thai farmers burned fields to clear their crops - despite warnings that the toxic practice is killing residents.

Footage shows air pollution reducing visibility along a road in the Nam Phong district of Khon Kaen province after workers set alight the nearby fields on January 4.

Some 40 acres of farmland were reported to have been set ablaze, while local officials were at the scene to keep the fire from spreading to residential areas.

Swathes of blackened land were also seen following the agricultural burning.

PM2.5 fine dust levels varied across the province, with some regions registering dangerous levels, according to local media.

The provincial authority led by Khon Kaen Governor Kraisorn Kongchalad had previously encouraged farmers to use alternative methods, as the slash-and-burn method harmed the environment and worsened the seasonal air pollution problem.

Bungling government officials in Bangkok have failed to tackle the issue other than floating measures last year to introduce subsidies for farmers who use costlier - but environmentally friendly - methods of clearing crop waste from the land.

Critics say that politicians are at the behest of food conglomerates who need a cheap and quick turnaround of commodities to cater to the growing demand for food from a developing population with a rising GDP.

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.

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.'