Thick smog caused by farmers clearing land with fire was creating serious health problems in northern Thailand, disrupting transportation and threatening to cripple the region\'s tourism industry,dpa cited medical officials and news reports as saying Friday. Dr Chaicharn Photilak of Chiang Mai Hospital said the number of patients suffering from respiratory problems was increasing, particularly among children and the elderly. \"The situation this year is worse than usual,\" he said. \"There is more burning (of farmland) in Thailand and Burma (Myanmar).\" He said most of his patients complained of severe coughing and blocked airways and advised residents of smog-hit areas to remain indoors. According to the Health Ministry, a total of 23,718 patients with respiratory problems was recorded at hospitals in eight northern provinces between February 19-25, a four-fold increase over the same period last year. The thickest smog was reported Thursday in Mae Sai district of the northernmost province of Chiang Rai, with small dust particles rose to 305.6 micrograms per cubic metre. Any more than 120 micrograms of small dust particles is considered a health hazard. Chiang Rai Governor Thanin Supasean handed out face masks to residents on Thursday. Northern residents said high prices offered for crops by Thai companies were encouraging Thai and Myanmar farmers to clear more forest land for the upcoming growing season. In Chiang Mai, where small dust particles were recorded at 200.72 micrograms per cubic metre on Thursday, Tour Guide Association chairman Somrit Haikham said tourism was threatened by the layer of smoke and called for a strict ban on setting fires for land-clearing. A Bangkok Airways flight to Lamphang, 450 kilometres north of Bangkok, had to be diverted to nearby Sukhothai Airport on Sunday due to poor visibility.