The Pollution Control Department is warning residents of Bangkok and its neighbouring provinces to brace for a drop in air quality, as changing wind patterns on Wednesday and Thursday are expected to trap ultra-fine dust particles over the city.
Siwaporn Rangsiyanon, the department's deputy spokesperson, said air quality in the capital and its vicinity has improved over the past several days, due to tighter controls on open-air burning and vehicle emissions.
A change in wind direction also helped clear the smog from the city, she said.
According to the department, 25 provinces in the North and Northeast reported PM2.5 -- particulate matter measuring less than 2.5 microns across -- levels of over 50 microgrammes per cubic metre (µg/m³) on Monday.
In the North, the concentration of fine dust pollutants hovered between 34-73µg/m³, while in the Northeast, the levels ranged between 40-91µg/m³.
In the West, PM2.5 levels hovered between 15-64µg/m³, while all provinces in the East and West reported PM2.5 levels below 50µg/m³.
Meanwhile, in Bangkok and its surrounding provinces, PM2.5 levels varied between 2-46µg/m³.
That said, Ms Siwaporn said a change in wind patterns will cause PM2.5 levels to shoot up tomorrow and Thursday. As such, the public is advised to avoid outdoor activities, especially the elderly, children, pregnant women and individuals with underlying medical conditions.
In response to the recent drop in air quality, the director of Logistics Research and Development Institute at the University of Thai Chamber of Commerce, Wanchai Rattanawong, called on the government to urgently replace Bangkok's ageing public bus fleet, saying their emissions are a major contributor to the capital's dust pollution.
According to Assoc Prof Wanchai, out of more than 3,000 public buses serving the capital, only 400-500 run on liquefied natural gas, while the rest are in a sorry state.