Hazardous levels of PM2.5 were limited to northern provinces on Wednesday morning, with with the worst air pollution still in Chiang Rai's Mae Sai district, according to the Pollution Control Department.
The department reported at 10am on Wednesday that unsafe levels of particulate matter 2.5 micrometres and less in diameter (PM2.5) ranged from 58 to 320 microgrammes per cubic metre of air over the past 24 hours.
The lowest level was in Muang district of Phrae and the worst pollution was still in Mae Sai district of Chiang Rai. The government-set safe threshold is 50μg/m³.
Pinsak Suraswadi, director-general of the department, said seriously high levels of PM2.5, between 92 and 320μg/m³, were detected in Chiang Mai, Chiang Rai, Lampang, Lamphun, Mae Hong Son, Nan and Phayao provinces.
The causes were still extensive outdoor burning and the large number of fire hotspots in neighbouring countries, coupled with stagnant air.
Smog would be at its worst in the upper North, especially in border provinces, from Thursday to next Wednesday, Mr Pinsak said.
In the Northeast, PM2.5 reached an unhealthy level only in Muang district of Bueng Kan, 62μg/m³.