Thailand’s Chiang Mai was ranked the world’s most polluted city on Friday, with authorities urging people to work from home to avoid the hazardous air. Smoke from forest fires and farmers burning crop stubble has blanketed the popular tourist destination in recent weeks. Prime Minister Prayut Chan-O-Cha held video talks with the leaders of neighbouring Myanmar and Laos to discuss the problem, which affects large areas of southeast Asia every year. Thailand has been choking on heavy air pollution since the start of the year, caused in part by seasonal agricultural burning. Nearly two million people have needed hospital treatment for respiratory conditions caused by air pollution this year, according to the public health ministry. On Friday morning, the air monitoring website IQAir ranked Chiang Mai as the most polluted major city in the world, above regular hotspots such as Delhi and Lahore. Levels of the most dangerous PM2.5 particles — so tiny they can enter the bloodstream — were more than 66 times the World Health Organization’s annual guideline, according to IQAir. Chiang Mai provincial governor Nirat Pongsittitavorn issued a statement urging people to stay indoors and work from home to “protect themselves and reduce the health impact” from PM2.5 particles. Forest fires have contributed to the problem. The latest, in Chiang Rai province, northeast of Chiang Mai, began on Thursday and has affected 96 hectares 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 percent,” 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 discussion of cross-border pollution at the next summit of 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.