Seasonal air pollution smog has begun choking parts of Thailand as farmers crops to clear their land.

Footage shows the haze shrouding roads as orange warnings were raised for heightened PM2.5 or fine particle levels in central Ang Thong province on December 12 morning.

Dust-measuring devices placed at the Ban Ro Intersection in the Mueang District showed PM2.5 readings reaching 'unhealthy' levels of 52 micrograms per cubic metre. The safe level is around 50mcg.

Scientists believe the country's air pollution is caused by a combination of still atmospheric conditions, agricultural burning and soaring numbers of cars and construction projects.

Researchers found that smog - caused by high levels of harmful PM 2.5 particles in the air - is at its worst in the country when farmers burn waste crops to cheaply clear their land between late October and April.