Fifty-one of the 77 provinces saw hazardous levels of ultra-fine dust on Monday morning, most of them in the Central Plains, the North and the Northeast.
The Geo-Informatics and Space Technology Development Agency (Gistda) reported the air pollution situation at 9am.
It said five central plains provinces were blanketed by red (seriously hazardous) levels of PM2.5.
The worst level was 93.4 microgrammes of PM2.5 per cubic metre of air over the past 24 hours in Lop Buri province.
Four others were Sing Buri (80.8µg/m³), Prachin Buri (79.3), Saraburi (78.7) and Lop Buri (78.6). The government-set safe threshold is 37.5µg/m³.
Forty-six provinces were shrouded with orange (unsafe) levels of PM2.5 ranging from 40.2 to 74.7µg/m³.
