PM2.5 levels were above the safe threshold in 55 provinces on Wednesday morning, the Centre for Air Pollution Mitigation reported.
Fine-dust particle levels that exceed 50 microgrammes per cubic metre (µg/m³) are considered unsafe.
The mitigation centre’s tally excluded provinces in the South.
The 55 provinces with unsafe levels of PM2.5 dust as of 7am on Wednesday were: Bangkok, Pathum Thani, Nonthaburi, Nakhon Pathom, Samut Sakhon, Samut Prakan, Chiang Rai, Chiang Mai, Nan, Mae Hong Hon, Phayao, Lamphun, Lampang, Phrae, Uttaradit, Sukhothai, Tak, Phitsanulok, Kamphaeng Phet, Phichit, Phetchabun, Nakhon Sawan, Uthai Thani, Sing Buri, Lop Buri, Saraburi, Ang Thong, Suphan Buri, Ayutthaya, Kanchanaburi, Ratchaburi, Samut Songkhram, Prachuap Khiri Khan, Prachin Buri, Sa Kaeo, Chachoengsao, Chon Buri, Rayong, Chanthaburi, Trat, Bung Kan, Nong Khai, Loei, Udon Thani, Nakhon Phanom, Nong Bua Lamphu, Sakon Nakhon, Khon Kaen, Kalasin, Roi-et, Chaiyaphum, Ubon Ratchathani, Si Sa Ket, Nakhon Ratchasima and Buri Ram.
By region, PM2.5 levels were 65-137 µg/m³ in the North; 48-170 µg/m³ in the Northeast; 50-98 µg/m³ in the Central and the West; 59-86 µg/m³ in the East; and 17-33 µg/m³ in the South.
In Bangkok and nearby provinces, the PM2.5 range was 55-131 µg/m³.
Chiang Mai, meanwhile, was in the top five globally on one air pollution index.
As of 9.26am on Wednesday, the northern city was the 5th most polluted major city worldwide, with an air quality index (AQI) of 181, according to website www.iqair.com.
Above Chiang Mai were: Baghdad, Iraq with 230 AQI; Lahore, Pakistan with 224 AQI; Dubai, United Arab Emirates with 193 AQI; and Delhi, India with 185 AQI.
