Thailand's Prime Minister Gen Prayut Chan-o-cha (centre) points a leaf blower at the world's worst quality air in Chiang Mai on Tuesday. (AFP photo)

Prime Minister Gen Prayut Chan-o-cha flew into Chiang Mai early on Tuesday to review the crisis caused by a thick blanket of smog and toxic levels of PM2.5 dust that has been choking northern Thailand for several weeks.

He demanded a clampdown on crop burning.

"I want to reiterate that the problem of hotspots (crop burning areas) must be solved in seven days," said Prayut.

"Nobody should ignite fires in the forest," he said.

The city’s air quality index was 379 as he visited, the worst major urban reading globally and a level that’s hazardous, according to IQAir AirVisual pollution data.

Thai authorities blame crop burning to clear farmland, as well as wildfires in mountainous forests amid a drought and searing heat.

However, Olivier Evrard, a Thailand-based specialist for the Institute of Research for Development, said crop burning was not the only culprit.

"The government has encouraged the population to buy more vehicles and coal plants are still running at full speed," Evrard said.

Chiang Mai has set up a so-called safe zone for residents in a convention centre, while a university in Chiang Rai cancelled classes on Monday and Tuesday.

'The haze usually comes and goes within a week or two, but it’s been persistent this time -- it’s the worst so far,' Khuanchai Supparatpinyo, the director of Chiang Mai University’s Research Institute for Health Sciences, said in an interview. 'This can be quite dangerous, and pose health risks.'

The seasonal duration of the haze, which used to last for about three months, has now increased to six months, according to Chaicharn Pothirat, a lung disease specialist at Chiang Mai University's Faculty of Medicine.

The long-term effects include an increase in respiratory and cardiovascular diseases, he said, adding that he is sceptical the government has any "long-term plan" good enough to tackle the problem.

"They show reporters, they fly to Chiang Mai ... but ... it does not clearly improve the situation," Chaicharn told AFP.

Worsening haze could pose a challenge for the tourism industry.

Top Asian finance ministers and central bankers are due to have a summit in northern Thailand this week.