Chiang Mai returned as the world's most polluted city again around noon yesterday after the government's ban on open burning came to an end on Tuesday.
According to www.airvisual.com, Chiang Mai ranked as the worst city on the US Air Quality Index (AQI) at 159 as of 12.17pm. An AQI level above 100 is considered unhealthy.
The AQI measures a broad spectrum of air pollutants including PM2.5, PM10, and carbon dioxide.
The highest level of the PM2.5 pollutant in the province was reported near Mae Taeng Hospital in Mae Taeng district where it reached 513.3 microgrammes per cubic metre of air. The safe limit of PM2.5 is 50 μg/m3.
Exposure to PM2.5 is known to cause short-term adverse health effects such as eye, nose, throat, and chest irritation. In addition, it is also linked with long-term health conditions such as asthma and cardiovascular disease.
It has been reported that residents have resumed burning crops to prepare for the new cultivation season ahead of the rains.
The move comes after the two-month prohibition period of open burning ended in the province on Tuesday.
According to air quality checks by the Pollution Control Department at 9am yesterday, the AQI level fluctuated between 63 and 129 during a 24-hour period. Meanwhile, the PM2.5 level moved between 41 and 62 μg/m3 and PM 10 was recorded to be between 63-86 μg/m3.
Despite the return of a dangerous level of air pollution, Chiang Mai's governor, Supachai Eiamsuwan, said that haze from forest fires has eased.
It's only the locals now contributing to pollution by setting fires on their farms. For this reason, fire buffer areas must be designated and approved by officials.
Mr Supachai added that authorities are still working on setting up forest fire buffer zones and monitoring.
Meanwhile, Chiang Mai deputy governor, Komsan Suwan-ampa, said 154 fire hotspots have been recently discovered in the province following the end of the ban.
The provincial governor wants locals to inform authorities before proceeding with slash burning, Mr Komsan said.