The Pollution Control Department posted a red alert for hazardous levels of fine dust in Mae Hong Son and Phayao townships on Monday, and unsafe levels in Chiang Mai city, with the northern and northeastern regions facing a cool change this week.
Monitoring stations recorded levels of particulate matter 2.5 micrometres and less (PM2.5) in diameter above the government's safe threshold of 50mcg in three areas of Muang Chiang Mai.
Tambon Sri Phoum was measured at 84, tambon Chang Pheuk at 79 and tambon Suthep at 54 at 2pm, according to the department.
In Muang district of Mae Hong Son PM2.5 levels were measured at 93 and Phayao was even worse at 97, with the department warning the levels were hazardous to health.
The Meteorological Department forecast temperatures to drop in most parts of the country from Monday through to Thursday.
The northeastern region will see the mercury down 4-6°C, while people in the northern, central and eastern regions can look for a drop of 3°C.