Chiang Mai, Thailand, Named the World’s Most Polluted City

Chiang Mai, Thailand, has topped the list of the world’s most polluted cities with an AQI of 209 (Very Unhealthy), with PM2.5 concentrations far exceeding safety standards. This pollution is caused by a combination of wildfires in northern forests, open-air burning of crop residues in farmlands, and the dry season’s temperature inversion and basin topography, which hinder the dispersion of pollutants. The city is shrouded in thick haze, and visibility has dropped significantly.