Smoke haze from Indonesia was at unhealthy levels in six southern provinces on Thursday and the problem was especially serious in Hat Yai district of Songkhla province.
The Pollution Control Department reported that unhealthy haze exceeded the safe level of 120 microgrammes per cubic metre of air in Narathiwat, Pattani, Satun, Songkhla, Surat Thani and Yala provinces.
The level of particulate matter measured in the past 24 hours peaked at the very unhealthy amount of 360 microgrammes in Hat Yai district of Songkhla, followed by 276 in Satun, 211 in Pattani, 199 in Surat Thani, 180 in Yala and 125 in Narathiwat.
Smoke haze also blanketed Phuket province but its level was at 101 microgrammes.
As smoke haze levels increased in all the seven southern provinces, the department advised local residents to wear face masks, stay indoors and close their doors and windows.
Two Thai AirAsia flights, to and from Hat Yai, were delayed for about two hours because of the thick haze on Thursday morning.
Seven flights to Koh Samui were diverted early Thursday, while Bangkok Airways cancelled 20 flights to the Surat Thani island after 4.30pm.
Government spokesman Sansern Kaewkamnerd said rainmaking flights would begin in the South to help clear thick smoke haze being blown in from Sumatra in Indonesia.
Songkhla deputy governor Anuchit Trakulmuthuta said primary and secondary schools could close if the pollution level exceeds 350 microgrammes.