The density measured at a local pollution control office in Chiang Rai on Wednesday was 132 mg/cubic metre exceeding the recognised safety level of 120 mg/cubic metre for more than two days, officials said.

Chiang Rai governor Sumeth Saengnimnual said he had officially asked local authorities in Laos and Burma to make sure their communities did not light unnecessary fires, while local fire services have been placed on alert. Volunteer firemen are now stationed at eight districts where wildfires and manmade fires are frequent.

Deputy Public Health Minister Pansiri Kulanartsiri said the protective masks would be distributed to residents, especially young children and elders, in eight northern provinces affected by the smog.

Chiang Mai is the province hit second hardest by the smog, due to its geographical structure.