Residents wear masks to protect themselves against soaring levels of air pollution as they travel home in Bangkok,Thailand, this evening (January 21).<br /><br />Unhealthy levels of air pollution and toxic smog have been choking the country for several weeks.<br /><br /> Thick layers of air pollution have blanketed the capital Bangkok and Chiang Mai in the north of the country for several days. <br /><br />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. <br /><br />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.
