The elderly and children were urged to stay inside, with some five million people in eight northern provinces affected by the haze, the health ministry said.

Tourism officials were also worried that holidaymakers would be deterred by images of the smoke and residents wearing surgical masks.

Air quality is measured in micrograms per cubic metre, with 120 considered hazardous. Measurements of 240 and 290 micrograms per cubic metre were recorded in the provinces on Chiang Mai, Mae Hong Son and Lamphun, Kasem said.

"If the situation does not get any better, we will have to invoke emergency law, which will control people's activities," he said.

If an environmental emergency were declared, authorities would be able to evacuate towns and villages as well as stop farmers from setting fires. Cabinet was due to meet in two days to decide what measures to take.

Visibility was down to one kilometre (just over half a mile) in the north of Chiang Mai province, while residents in Chiang Mai city donned masks, with some reporting health problems.

"I run every day, but I don't think I will run today. Everyone is wearing masks and there is eye irritation" said Viparwan Chaiprakorb, 48, who works for a non-profit foundation in Chiang Mai.

"I can still see a lot of tourists coming in buses (but) it doesn't look good. If this goes on it is not going to help so much," she told AFP by telephone.

Chiang Mai province, home to hundreds of temples and a popular base for adventure tourism, is one of Thailand's top tourist destination, and welcomed 5.5 million visitors in 2006, a 39 percent increase from 2005.

"Absolutely (the haze) will affect tourism because it happened so suddenly," said Kajohnwit Boonsom, vice president of the Chiang Mai Tourism Business Association.

"Although there was not a significant number of immediate cancellations, it will affect tourists who are still making their plans."

The health ministry said it had already distributed 130,000 masks, with another 170,000 being passed out Tuesday.

Kasem said northern army units were working with the forestry department to control the forest fires, which began in late February and have been reported in about 1,340 locations.

The fires in northern Thailand, as well as neighbouring Laos and Myanmar, were caused by farmers trying to clear land and by people burning the forest to make scavenging for wild mushrooms easier, Kasem said.

All flights from Chiang Mai to Mae Hong Son town were suspended for the second day running Tuesday because of bad visibility. Other flights in the region were operating as usual.

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The Air We Breathe at TerraDaily.com