Authorities on Friday urged people in the northern Thai city of Chiang Mai to stay indoors and work from home as air pollution spiked to hazardous levels.
Smoke from forest fires and farmers burning crop stubble has blanketed the popular tourist destination in recent weeks.
On Friday morning, the air monitoring website IQAir ranked Chiang Mai as the most polluted major city in the world, ahead of 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.
Thailand has been choking on a spike in 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.
Officials previously warned Bangkok residents to stay indoors and work from home in February as the capital was covered with harmful haze.