Prime Minister Srettha Thavisin has defended a decision not to designate Chiang Mai a disaster area despite it being blanketed by smog, saying he was trying to protect tourism.

Mr Srettha said in an X message on Sunday that he made the decision after carefully consulting with all agencies, believing the status could discourage tourists from plans to visit Thailand, including the province.

"What will happen after the announcement is it will affect the number of foreign tourists when tourism has just recovered in the wake of Covid-19," he wrote.

He said travel health insurance purchased by tourists would not cover cases in disaster and emergency areas. "Chiang Mai province will definitely lose tourists who plan for short- and long-term stays," the prime minister said. "That is what we are worrying about."

As he posted the message, Chiang Mai remained the most polluted city in the world, according to IQAir. The weather monitoring website reported at 4.04pm that the air quality index in the province was 160, topping all major cities in the world. It had fallen to second place by 4.30pm.

Commenters reacted on the prime minister's X account, saying he should focus more on the health of people in Chiang Mai than on tourism.

The province has been choked by smog for days so that people are unable to see Doi Suthep from the city.