The thick, smoky haze caused by farmers burning off secondary growth is causing health problems in the northern provinces and on Monday prevented four flights from landing in Chiang Mai.
Staff of Chiang Mai airport said two flights from Bangkok were forced to turn back and two other flights from the South were diverted to Chiang Rai between 11.40am and 12.30pm.
Thai Airways flight TG106 from Suvarnabhumi airport and Thai Lion Air flight SL8504 from Don Mueang airport were forced to return to the capital.
The other two flights, PG241 of Bangkok Airways from Koh Samui in Surat Thani and FD3161 of Thai AirAsia from Phuket were diverted to land in Chiang Rai.
Particulate matter was measured at 258 microgrammes per cubic metre of air in Muang Chiang Mai district on Monday. Health authorities said more than 37,000 people have sought treatment for respiratory problems at community clinics.
Dapong Ratanasuwan, minister of natural resources and environment, made an aerial inspection over Chiang Mai on Monday and said the smoke haze was caused by farmers burning off secondary growth. The problem was worst in Mae Chaem district, where fires were observed in at least 10 locations, he said.
He discussed solutions to the problem with officials and they said measures imposed to restrict the practice had been unsuccessful and not strictly enforced at local levels.
In the North, the worst smoke haze was reported in Mae Hong Son province where particulate matter was measured at 303 microgrammes per cubic metre of air on Monday, followed by Chiang Rai with 300mcg, Chiang Mai with 258mcg, Phayao 251mcg, Lamphun 180mcg, Lampang 161mcg, Phrae 137 mcg, Nan 133mcg and Tak 132 mcg. All these levels were harmful to health. The maximum safe standard is 120mcg, he said.