Forest fires have contributed to the problem.
The latest, in Chiang Rai province, northeast of Chiang Mai, began on Thursday and has affected 96ha of forest.
Home to nearly 130,000 people, Chiang Mai is a gateway to Thailand’s hilly north, visited pre-pandemic by millions of tourists for its historic centre and laid-back atmosphere.
But Wittaya Pongsiri, vice-president of the Chiang Mai Tourism Business Association, said the pollution was putting visitors off.
“The number of tourists has dropped by 20 per cent,” he said.
After his talks with his Laotian counterpart Sonexay Siphandone and Myanmar junta leader Min Aung Hlaing, Prayut’s office said he would push for a discussion of cross-border pollution at the next summit of the regional bloc ASEAN.
The three leaders discussed the need to find ways to curb emissions from agriculture and industry, but did not agree on any concrete steps for action.
Officials previously warned Bangkok residents to stay indoors and work from home in February as the Thai capital was covered with harmful haze.