The northern provinces of Chiang Mai, Lampang and Phrae were reportedly covered with haze on Monday morning, mainly smoke from fires clearing forests for planting, Manager Online reported.
In Chiang Mai, thick dust and smoke particles blanketed the provincial capital, making Doi Suthep invisible from the town.
The poor visibility caused problems for aircraft landing and taking off at Chiang Mai International Airport.
The particle level in Chiang Mai town was reported at 185 microgrammes per cubic metre.
In Lampang, smoke from forest fires cast a thick haze, particularly in Mae Moh district where the particle level was 260 microgrammes per cubic metre. In Lampang town, the particle level was 238 microgrammes per cubic metre.
In nearby Phrae province, haze from forest fires which started to cause problems late last month has become more serious. The dust particle level in the provincial town this morning was reported at 215 microgrammes per cubic metre.
The health safety standard is below 120 .
Hot spots were detected throughout the province, particularly in Rong Kwang, Den Chai and Rong districts.