Most provinces in the North were covered with dangerous red-coded levels of PM2.5 on Monday morning, with Chiang Mai rated the city with the fourth worst air pollution in the world.

The Geo-Informatics and Space Technology Development Agency (Gistda) reported at 8am on Monday that 11 provinces were facing red (seriously harmful) levels of PM2.5, ranging from 82.1 to 158.8 microgrammes per cubic metre of air over the past 24 hours. The safe threshold is at 37.5µg/m³.

The worst level, 158.8, was measured in Chiang Rai, followed by 141.9 in Chiang Mai, 136.4 in Lamphun, 131.6 in Phayao, 128.2 in Mae Hong Son, 118.6 in Lampang, 102.3 in Phrae, 94.9 in Nan, 87.8 in Sukhothai, 85.5 in Uttaradit and 82.1 in Tak. All of them are northern provinces.

Orange (initially unsafe) levels of PM2.5 were detected in 46 other provinces, mostly in the Northeast and the Central Plains. They ranged from 38.2 to 67.6µg/m³.

Provinces in the East and the South had moderate and good air quality. Phuket had the best air quality with 15.3µg/m³ of PM2.5, followed by Phang Nga with 15.7µg/m³. Both are in the South.

IQAir rated Chiang Mai as the major city with the world's fourth worst air pollution on Monday morning after Delhi, Lahore and Kathmandu.