Toxic smog was recorded in 39 areas of greater Bangkok on Wednesday morning, trapped by stagnant air, with no relief expected before next Monday.
The worst pollution was in Samut Sakhon's Muang district and Bangkok's Bang Khunthian district.
The Pollution Control Department reported that levels of particulate matter 2.5 micrometres or less in diameter ranged from 59-142 microgrammes per cubic metre of air during the past 24 hours, breaching the 50mcg Thai safety threshold.
Pollution was at a 12-month high of 142mcg on Rama II Road in Muang district of Samut Sakhon province on Wednesday morning. Kanchanaphisek Road in Bang Khunthian district of Bangkok also posted a red alert with 90mcg of PM2.5.
They were just two of the 39 areas with unhealthy fine particulate levels.
Breathing the air was also hazardous in Bangkok's Bang Na sub-district of Bang Na district, Khlongchan sub-district of Bang Kapi district, Din Daeng sub-district and on Din Daeng Road in Din Daeng district, Rama IV Road in Pathumwan district, Intharaphithak Road in Thon Buri district, Lat Phrao Road and in Phlab Phla sub-district in Wang Thonglang district, and Phaya Thai sub-district of Phaya Thai district.
Elsewhere, dense smog was detected in tambon Nakhon Pathom of Muang district in Nakhon Pathom province; tambon Bang Kruai of Bang Kruai district in Nonthaburi province; tambon Khlong Nueng of Khlong Luang district in Pathum Thani; tambon Song Khanong and tambon Talad of Phra Pradaeng district, tambon Pak Nam of Muang district, and tambon Bang Sao Thong of Bang Sao Thong district in Samut Prakan; and tambon Om Noi of Krathum Baen district in Samut Sakhon.
The Pollution Control Department said the pollution was made worse by stagnant air over the greater city area, which was forecast to continue until Monday.