Choking|Chiang Mai

The air in Chiang Mai is particularly harmful after midnight, says a recent study.

The research found a high concentration of polycyclic aromatic hydrocarbons (PAHs) in Chiang Mai's air from midnight to 3am each day, even though there are few vehicles on the streets.

"The level of PAHs during those hours is even higher than at rush hours," Dr Siwatt Pongpiachan of the Prince of Songkla University reveals.

PAHs are a group of over 100 different chemicals formed during the incomplete burning of charcoal, oil, gas, garbage and other organic substances.

Pollution spike

"We have detected 15 PAHs such as Benzopy-rene that are harmful to people's health in Chiang Mai," Siwatt says.

Chiang Mai's natural landscape is one factor in the pollution spike after midnight, Siwat explains. "Warm air rises during the day, carrying dust and airborne particles with it. But the natural amphitheatre formed by the mountains means that when the temperature drops at night, the dust or particles settle back on the town."