The smoke haze from Indonesia hanging over the lower South was at unhealthy levels on Wednesday morning, according to the Pollution Control Department.
As of 7am, the particulate matter reading in Hat Yai city of Songkhla province was measured at 185 microgrammes per cubic metres, far above the safety level of 120.
In tambon Sateng of Yala's Muang district, the particulate reading was 137.
Both Yala town and Hat Yai were blanketed with thick smoke with visibility less than 100 metres.
The 16th Regional Environment Office in Songkhla said this situation was likely to persist throughout the day. The elderly, children and those sufferring from respiratory problems are advised to wear face masks.