Worsening dust pollution was forecast for the capital and nearby areas on Thursday with the wind that has dispersed much of the smog expected weaken in the afternoon.
Unsafe PM2.5 levels were reported in the morning in Bangkok's Bang Khunthian district and areas in four nearby provinces.
Chayapol Thitisak, director-general of the Department of Disaster Prevention and Mitigation, said levels of particulate matter 2.5 micrometres or less in diameter (PM2.5) has ranged from 53 to 75 microgrammes per cubic metre of air in the 24 hours since Wednesday morning.
Unsafe levels were reported on Thursday morning at Kanchanaphisek Road in Bang Khunthian district of Bangkok; tambon Khlong Nueng of Khlong Luang district in Pathum Thani province; tambon Song Khanong of Phra Pradaeng district in Samut Prakan; tambon Om Noi in Krathum Baen district and on Rama II Road in Muang district of Samut Sakhon province; and in tambon Nakhon Pathom of Muang district in Nakhon Pathom province, he said.
The Pollution Department said the level of hazardous dust was alleviated by strong winds in the past 24 hours. However, the situation would probably worsen again in the afternoon because the weather office had forecast that winds would ease off.