Choking smoke from forest fires and slash-and-burn farming in Thailand, Laos and Myanmar has caused the worst pollution incident in northern Thailand in 14 years, leading authorities to declare a state of emergency in the tourist town of Chiang Mai.

The thick haze has disrupted flights and, as air quality reached dangerous levels, health workers distributed 300,000 masks to residents, advising the elderly and the very young to remain indoors.

Air quality is measured in micrograms of particles per cubic metre of air. A rating of 120 is considered unhealthy, and 300 dangerous. Pollution levels in Chiang Mai were reported to be 382.7, with visibility down to half a mile.

Thailand’s second city is a popular backpacker destination, famous for its adventure activities, but a local guide, Nirin Sittardan, says the pollution has brought tourism to a standstill.

“I hate to say it, but if you’re planning on coming to Chiang Mai, you should forget it for at least a month,” he told The Sunday Times. Meteorologists warn that it could be weeks before the haze dissipates.

John Telfer, of the tour operator Explore, said: “We will be reviewing the situation over the next two days. We will continue to take advice provided by the Foreign Office. Currently, the region is deemed safe, with no warnings, and is open to foreign visitors. And as far as cancellations go, normal booking conditions still apply.”

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