Natural Resources and Environment Ministry permanent secretary Jatuporn Buruspat said the record showed the range of dust particles was 47 – 257 microgrammes per cubic metre.
Firefighters were sent to control the fire spreading across Chiang Rai, Chiang Mai, Phayao and Mae Hong Son, which are the most affected provinces.
The Pollution Control Department is cooperating with the Asean secretariat on the issue and held discussions with representatives from Myanmar to create measures dealing with pollution across borders.
Prime Minister Prayut Chan-o-cha has ordered the arrest of people who violate the law by lighting fires in forests – the main cause of pollution in the region.
Government spokesperson Narumon Pinyosinwat that wind currents were another reason for PM2.5 dust particles still covering the northern region.