Chiang Mai and Chiang Rai were hit on Friday by air pollution caused by haze and weather conditions.
Levels of fine particles known as PM2.5 were 110 in tambon Chang Pheuk and 94 in tambon Sriphum in Muang district in Chiang Mai, according to readings taken at 4pm by the Pollution Control Department.
Muang and Mae Sai districts in Chiang Rai also had PM2.5 readings above the safe maximum of 50, at 129 in Muang district and 359 in Mae Sai.
Chiang Mai governor Charoenrit Sa-nguansat warned residents in the municipality against outdoor activities and urged them to wear protective masks when going outside, according to the Thai News Agency.
Provincial authorities said small particles whipped up by forest fires were trapped by low pressure over the north of the country.
Levels of small particles in the province had risen rapidly this week, said Saratcha Suriyakul Na Ayutthaya, director of the Natural Resources and Environment Office in Chiang Rai.
Although Chiang Rai had few hot spots, the southeasterly wind blew small particles from neighbouring countries into the province, he wrote on the Chiang Rai Public Relations Office Facebook account.
He did not mention the countries by name. Chiang Rai borders Myanmar and Laos.
Chiang Mai saw 672 hot spots on Friday, a big jump from 78 on Thursday, according to a satellite image from the Geo-Informatics and Space Technology Development Agency (Gisda).
Chiang Rai had 192 hot spots on Friday, compared to 15 on Thursday, Gisda reported.