Chiang Mai in northern Thailand ranked the most polluted city in the world for four days in a row as the hazardous haze clouded the city harming thousands of locals.

The level of PM2.5 dust particles in the northern city exceeded the safe point, reaching 183 micrograms per cubic metres by 2 pm on March 10. It was the worst in the world in the morning before the smog eased in the afternoon.

Dr Aphinan Tantiwut at Lanna Hospital said PM 2.5 dust particles could damage respiratory and cardiac systems, and they could also cause skin irritation and allergy.

He said the number of patients affected by the pollution had increased by 10%. More than 30,000 people reportedly sought treatment at the hospital in recent weeks.

The pollution in the north is believed to be caused by open burning resulting in wildfires in Myanmar forests and Thailand farms.

A task force was deployed to control the fire and haze, while a group of rangers were ordered to extinguish the wildfires.