"When I was driving to work this morning, I could see only 100 to 200 metres ahead of me," Taewan Dumronghud, a station manager for Thai Airways, told Reuters by telephone from Mae Hong Son near the Myanmar border.

"We can only hope that the rains will come sooner and wash it away," Taewan said, whose car was covered in ash at the airport.

The haze also disrupted flights to Chiang Mai on Sunday when air quality levels reached their worst in Thailand's second largest city.

Thailand's mountainous north is a popular destination for adventure tourism. The haze-affected areas are located near the borders of Myanmar and Laos -- the so-called Golden Triangle once famed for its opium poppy fields.

Weather experts said unseasonably cold weather had exacerbated the problem by pushing the smoke down into valleys and other low-lying areas.

"Sixty percent of the haze covering the region comes from burning of farm waste after harvest, and the other 40 percent from forest fires," Anuwat Kunarak, director of the region's Environment Management Office, told Reuters.

"It's cheaper for farmers to get rid of the waste by setting it on fire and then switching to a new cash crop," he said, adding the haze was the worst recorded by his office in 14 years.

Chiang Mai has been draped in a choking, eye-watering haze since last Friday, triggering health warnings for children and the elderly to stay inside or use surgery masks.

Healthy adults were urged to stop all outdoor exercise.

Many residents complained of burning eyes, coughing and sore throats from the smoke.