"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."