Thick smoke from grass fires fill the air in Chiang Mai, northern Thailand, as the military battles to control the blazes.

The vegetation fire started in the Doi Suthep-Pui national park in Chiang Mai, northern Thailand last Saturday (March 14) afternoon.

Due to the dry air and lack of rain, the fire spread quickly across the hillsides, leaving firefighters and military units struggling to extinguish the flames.

The grass fires have added to already poor levels of air pollution in the region. Readings of PM 2.5 particles show there are 222 micrograms of the harmful dust per cubic metre in the atmosphere - far past the safe level of 50.

Thailand's Pollution Control Department said red alerts were still in place for Chiang Mai and Mae Hong Son provinces.

The thick smog is caused by grass fires and agricultural burning. Low pressure, lack of wind and low humidity then cause the particles from the fire to hang in the atmosphere, forming thick clouds which engulfed towns and cities.

Thick smoke from grass fires fills the air in Chiang Mai, northern Thailand, as the military battles to control the blazes.

The vegetation fire started in the Suthep-Pui national park in Chiang Mai, northern Thailand last Saturday (14/03) afternoon.

Due to the dry air and lack of rain, the fire spread quickly across the hillsides, leaving firefighters and military units struggling to extinguish the flames.

The grass fires have added to already poor levels of air pollution in the region. Readings of PM 2.5 particles show there are 222 micrograms of the harmful dust per cubic metre in the atmosphere - far past the safe level of 50.

Thailand's Pollution Control Department said red alerts were still in place for Chiang Mai and Mae Hong Son provinces.

The thick smog is caused by grass fires and agricultural burning. Low pressure, lack of wind and low humidity then cause the particles from the fire to hang in the atmosphere, forming thick clouds which enguld towns and cities.