Footage from March 4 shows forest fires that have been burning on mountainsides in northern Thailand.

The blaze has been spreading since last weekend in Lampang province, close to Chiang Mai.

Fires have sent the PM.25 levels of air pollution in the region soaring, with the toxic smog passing the safe threshold.

On Wednesday, neighbouring Chiang Mai was the third most polluted city in the world.

Officials said that forest fires and agricultural burning had increased the levels of harmful PM 2.5 particles in the air.

The AirVisual website ranked Chiang Mai the third most polluted city in the world after Lahor in Pakistan and New Delhi in India.

Major General Thanadpol Kosaisewee, deputy chief of a command tackling air pollution in the region, said 928 critical areas had been detected across the region.

Smoke from mountain widfires had blanketed the city. Cooler temperatures and a lack of wind and rain had meant that it had lingered in the atmosphere.

Footage from March 4 shows forest fires that have been burning on mountainsides in northern Thailand.

The blaze has been spreading since last weekend in Lampang province, close to Chiang Mai.

Fires have sent the PM.25 levels of air pollution in the region soaring, with the toxic smog passing the safe threshold.

On Wednesday, neighbouring Chiang Mai was the third most polluted city in the world.

Officials said that forest fires and agricultural burning had increased the levels of harmful PM 2.5 particles in the air.

The AirVisual website ranked Chiang Mai the third most polluted city in the world after Lahor in Pakistan and New Delhi in India.

Major General Thanadpol Kosaisewee, deputy chief of a command tackling air pollution in the region, said 928 critical areas had been detected across the region.

Smoke from mountain widfires had blanketed the city. Cooler temperatures and a lack of wind and rain had meant that it had lingered in the atmosphere.