About 40% of the country was blanketed with thick and harmful levels of ultrafine dust on Sunday afternoon, according to Geo-Informatics and Space Technology Development Agency (Gistda).
The public organisation reported via its website pm25.gistda.or.th that 31 out of 77 provinces in the country, mostly in the Central Plain, were shrouded with unsafe levels of particulate matter 2.5 micrometres and less in diameter (PM2.5).
The levels ranged from 38.3 to 75.6 microgrammes of PM2.5 per cubic metre of air over the past 24 hours. The safety threshold is at 37.5µg/m³.
At 3pm the worst level, 75.6µg/m³, was recorded in Samut Prakan, followed by 73.3µg/m³ in Ang Thong, 72.9µg/m³ in Bangkok and 72.0µg/m³ in Nonthaburi.
Lower but still unsafe levels of PM2.5 were also reported in Sing Buri, Pathum Thani, Nakhon Pathom, Samut Sakhon, Ayutthaya, Lop Buri, Suphan Buri, Ratchaburi, Kanchanaburi, Kalasin, Petchaburi, Saraburi, Nakhon Sawan, Uttaradit, Uthai Thani, Chai Nat, Udon Thani, Sukhothai, Nong Khai, Phitsanulok, Roi Et, Prachuap Khiri Khan, Chachoengsao, Nakhon Nayok, Phichit, Ubon Ratchathani and Prachin Buri.
People in the 31 provinces were advised to avoid outdoor activities. The Meteorological Department attributed the air pollution to stagnant air.