Fifty-one of the 77 provinces were shrouded with 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 particulate matter 2.5 micrometres and less in diameter (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 governmeny-set safe threshold is 37.5µg/m³.
Forty-six provinces were shrouded with orange (initially unsafe) levels of PM2.5 ranging from 40.2 to 74.7µg/m³.
In ascending order of pollution they were - Mae Hong Son, Nan, Lampang, Samut Songkhram, Maha Sarakham, Rayong, Prachuap Khiri Khan, Udon Thani, Si Sa Ket, Kamphaeng Phet, Phitsanulok, Phetchaburi, Ubon Thani, Uttaradit, Surin, Chanthaburi, Khon Kaen, Chon Buri, Phrae, Phichit, Trat, Nong Bua Lam Phu, Chai Nat, Bangkok, Nakhon Sawan, Uthai Thani, Phetchabun, Nonthaburi, Ratchaburi, Nakhon Nayok, Samut Sakhon, Sukhothai, Pathum Thani, Kanchanaburi, Buri Ram, Loei, Tak, Nakhon Ratchasima, Sa Kaeo, Chaiyaphum, Lamphun, Chachoengsao, Suphan Buri, Samut Prakan, Nakhon Pathom and Ayutthaya.