The Pollution Control Department reported unsafe smog levels in greater Bangkok on Wednesday morning, but the air in other regions was at good to very good quality.
Particulate matter 2.5 micrometres or less in diameter exceeded the government's safe threshold of 50 microgrammes per cubic metre of air over the past 24 hours in most areas of Bangkok, Nonthaburi, Samut Prakan and Samut Sakhon, the department reported at 7am.
PM2.5 was at 26-98 mcg/cm over greater Bangkok.
In the capital, PM2.5 peaked at 98 on Ma Charoen Road in Nong Khaem district, followed by 87 on Klong Thawi Wattana Road in Thawi Wattana district and 76 in Bang Kae district.
Elsewhere, PM2.5 levels averaged 12-46 in the North, 8-22 in the Northeast, 16-50 in the Central Plains and the West, 13-22 in the East and 3-26 in the South, according to the PollutionControl Department.