Residents wear masks to protect themselves against soaring levels of air pollution as they travel home in Bangkok,Thailand, this evening (January 21).

Unhealthy levels of air pollution and toxic smog have been choking the country for several weeks.

Thick layers of air pollution have blanketed the capital Bangkok and Chiang Mai in the north of the country for several days.

The levels of the dangerous PM2.5 particles in the air soared past 100 micrograms per cubic metre - far higher than the safe amount of 50.

The pollution is caused by a combination of farmers burning waste crops to clear their fields in a process known as 'slash-and-burn' or fire-fallow cultivation as well as high numbers or fume-belching cars on the roads. Low pressure, dry air and a lack of wind and rain mean that the pollutants are suspended in the atmosphere instead of being naturally dispersed.

Residents wear masks to protect themselves against soaring levels of air pollution as they travel home in Bangkok,Thailand, this evening (Jan 21).

Unhealthy levels of air pollution and toxic smog have been choking the country for several weeks.

Thick layers of air pollution have blanketed the capital Bangkok and Chiang Mai in the north of the country for several days.

The levels of the dangerous PM2.5 particles in the air soared past 100 micrograms per cubic metre - far higher than the safe amount of 50.

The pollution is caused by a combination of farmers burning waste crops to clear their fields in a process known as 'slash-and-burn' or fire-fallow cultivation as well as high numbers or fume-belching cars on the roads. Low pressure, dry air and a lack of wind and rain mean that the pollutants are suspended in the atmosphere instead of being naturally dispersed.