Dangerously thick smog blanketed the upper northern provinces of Chiang Mai, Chiang Rai and Mae Hong Son, the Pollution Control Department reported on Thursday.
The levels of particulate matter 2.5 micrometres and less in diameter (PM2.5) ranged from 32-241 microgrammes per cubic metre of air in the North over the past 24 hours. The governmnent-set safe level is at 50mcg, far higher than the international standard.
A red alert signifying severe air pollution was posted for the three provinces.
PM2.5 was measured at 108mcg in tambon Wiang Phang Kham of Mae Sai district in Chiang Rai.
In Chiang Mai, PM2.5 rose to 101mcg in tambon Chiang Phuak of Muang district, 100mcg in tambon Sri Phum of Muang district, and 241mcg in tambon Muang Khong of Chiang Dao district.
In Mae Hong Son, the PM2.5 level reached 121mcg in tambon Chong Kham of Muang district.
Lower but still unsafe levels of fine dust were reported in Mae Chaem and Muang districts of Chiang Mai; Mae Mo and Muang districts of Lampang, Muang district of Lamphun, Muang district of Phayao, Chalerm Phrakiat district of Nan and Mae Sot district of Tak.
In the North, air quality was deemed good in Muang district of Uthai Thani (32mcg), Wichian Buri district of Phetchabun (36mcg) and Muang district of Nakhon Sawan (37mcg).
PM2.5 levels ranged from 44-48mcg in Kamphaeng Phet, Phichit, Phitsanulok and Uttaradit provinces.