Forty-eight of the 77 provinces were blanketed in unsafe levels of ultra-fine dust on Tuesday morning, 21 of them reporting red-level air pollution.
The Geo-Informatics and Space Technology Development Agency (Gistda) at 9am reported red levels of particulate matter 2.5 micrometres and less in diameter (PM2.5), ranging from 75.9 to 145.2 microgrammes per cubic metre of air, over the previous 24 hours.
On Monday morning hazardous levels of PM2.5 were between 38.1 and 112.3µg/m³ in 41 provinces. The safe threshold set by the government is 37.5µg/m³.
On Tuesday, the smog had expanded to cover 48 provinces.
The 21 red-zoned provinces where air pollution was a serious threat to health were, in descending order - Samut Sakhon (145.2µg/m³), Samut Songkhram (135.7µg/m³), Nakhon Pathom (120.1µg/m³), Nonthaburi (116.3), Bangkok (111.8), Ang Thong 104.4), Samut Prakan (103.4), Pathum Thani (102.5), Ratchaburi (101.4), Lop Buri (98.0), Sing Buri (96.7), Ayutthaya, 94.7), Chai Nat (94.3), Saraburi (94.1), Phetchaburi (93.0), Suphan Buri (90.8), Kanchanaburi (83.2), Chachoengsao (81.7), Uthai Thani (80.7), Prachin Buri (77.9) and Nakhon Sawan (75.9).
Twenty-seven provinces reported orange levels of PM2.5, meaning an initial degree of health harm.
They were Chon Buri, Prachuap Khiri Khan, Nakhon Nayok, Nakhon Ratchasima, Rayong, Sa Kaeo, Tak, Sukhothai, Chanthaburi, Phetchabun, Kamphaeng Phet, Chaiyaphum, Khon Kaen, Lamphun, Uttaradit, Phitsanulok, Phichit, Maha Sarakham, Nong Bua Lamphu, Kalasin, Buri Ram, Ubon Ratchathani, Trat, Surin, Phrae, Loei and Roi Et.
The map from Gistda shows orange and red levels of PM2.5 covering mainly the Central Plains on Tuesday morning.