Severe smog was recorded at air quality monitoring stations in five upper northern provinces on Tuesday, the Department of Pollution Control (DPC) reported.
The department reported that severe PM2.5 pollution was detected by eight air quality monitoring stations in Chiang Rai, Chiang Mai, Phayao, Mae Hong Son and Nan.
The eight stations in "red" danger zones are in Chiang Rai, tambon Wiang in Muang district where the pollution was measured at 102 micrograms per cubic metre (µg/m³) of air and tambon Wiang Phang Kham in Mae Sai district at 145 µg/m³; in Chiang Mai, tambon Chang Phueak where the PM2.5 level was recorded at 104µg/m³, the Sri Phum area of Muang district at 97µg/m³ and the Chiang Dao district at 180µg/m³; in Mae Hong Son, the pollution hit 109µg/m³ in Muang district; in Phayao, the dust surged to 116µg/m³ in Muang district; and in Nan, the pollution rose to 99µg/m³ in Chalerm Prakiat district.
Smog was also lingering over other northern provinces, including Lampang, Kamphaeng Phet and Uttaradit.
PM2.5 readings above 50µg/m³ are deemed hazardous to health by the government, about twice the international standard.
Pralong Dumrongthai, the PCD director-general, said that in the summer when there is little wind and rain, the dust tends to hover over specific areas.
The problem is exacerbated by slash and burn farming, which has continued despite repeated warnings and threats of tough legal action against offenders, he added.
The southwestern monsoon was blowing fine particulate matter from neighbouring countries into Thailand and had worsened the dust pollution in vast areas of the upper North, he said.
The department would ask those countries to reduce forest burning activities on their side of the border.
The department would also spray water to increase humidity in the air. The fine mist would cause the dust to drop to the ground. Water trucks and planes would be used in the operation, Mr Pralong said.
The DPC chief said Chiang Mai provincial governor Charoenrit Sanguansat would ask the air force to help with this.
He also said that In Phrao district of Chiang Mai there was a shortage of the fire extinguishers needed to battle bushfires burning deep in the mountains.