Fifty provinces were blanketed with hazardous levels of PM2.5 dust on Tuesday morning and 13 of them, mainly in the North, with seriously harmful levels of pollution.

The worst reading was in Mae Hong Son and the cleanest air was in Phuket.

The Geo-Informatics and Space Technology Development Agency reported at 9am that the highest level of PM2.5 was in the northern province of Mae Hong Son, at 159.1 microgrammes per cubic metre of air over the past 24 hours. The government-set safe threshold is 37.5µg/m³.

Mae Hong Son was among 13 provinces, mostly in the North, blanketed with red (seriously harmful) levels of PM2.5.

The others were Chiang Rai with 153.6µg/m³ , followed by Chiang Mai (149.5), Lamphun (130.9), Phayao (126.6), Nan (116.6), Lampang (111.8), Phrae (101.5), Tak (91.7), Uttaradit (89.5), Loei (83.1), Sukhothai (77.7) and Ubon Ratchathani (75.8).

Orange (initially unsafe) levels of PM2.5 were detected in 37 other provinces in the North, the Northeast and the Central Plain. The levels ranged from 37.7 to 73.9µg/m³.

Other provinces in the lower Central Plain, the East and the South had moderate and good air quality. Phuket had the best air quality with 19.6µg/m³ of PM2.5.