The number of particles smaller than 10 microns (PM10) in the air in Mae Hong Son yesterday reached 246 micrograms per cubic metre (mpcm).
The level exceeded the safe limit of 120mpcm, according to the Pollution Control Department.
Other upper northern provinces also saw increased PM10 levels.
Chiang Rai's Mae Sai district reported 161 mpcm and a station at the Chiang Rai Natural Resource and Environment Office reported 148 mpcm.
Chiang Mai City Hall's station reported 154 mpcm and Yupparaj School's station reported 149 mpcm while Lampang had 141 mpcm, Phayao 134 mpcm, Phrae 127 mpcm and Nan 122 mpcm.