Tourists in Phuket have been warned that a thick haze blanketing the island poses a health risk.
The smog has also disrupted some flights, said the Tourism Authority of Thailand.
Anyone with respiratory problems has been urged to wear face masks and local officials have advised people to stay indoors and not to take part in outdoor activities.
The haze, which has been caused by land clearing blazes in Indonesia, has prompted hospitals to issue free face masks.
Air pollution on Phuket over the last 24 hours has reached 136 micrograms per cubic metre on average, which is 16mg above the public safety limit.
The Thai foreign minister has called in the Indonesian ambassador Lutfi Rauf in Bangkok to discuss the forest fires.
A spokesman for the TAT in the UK said: "It is unusual for Phuket to be affected by these fires. Every year Thailand does experience a similar haze but normally much further south.
"We regret any disruption but obviously this is out of our control. We hope, weather permitting the haze will clear in the next couple of days."
