The drop in air quality across Bangkok over the past week is caused by an urban "dust dome" over the city, which formed as low atmospheric pressure and high-rise buildings trapped dust and other pollutants from the burning of agricultural waste, says Natural Resources and Environment Minister Varawut Silpa-archa.
The haze which has shrouded Bangkok over the past four to five days wasn't just caused by the build-up of microfine PM2.5 dust, as the concentration of larger PM10 pollutants and greenhouse gases has also soared.
These pollutants, the minister said, are the result of the improper disposal of farming waste around the city's northern outskirts.
The minister said he has asked provincial governors to ask farmers in their area to avoid burning farm waste.
He also asked them to consider imposing a ban on outdoor burning if the farmers refuse to cooperate.
Mr Varawut suggested farmers find a way to sell their agricultural waste instead of burning it.
Schools will be allowed to delay their reopening if the haze problem in their area continues to deteriorate, he added.
He also encouraged Bangkok residents to avoid outdoor activities and keep their masks on while outside, to prevent health problems from the ultra-fine PM2.5 pollutant as well as Covid-19.
Bangkok's Air Pollution Coordination and Resolutions Centre said on Thursday that PM2.5 levels in the capital hovered between 51-89 microgrammes per cubic metre (μg/m³); 69 areas in the city reported PM2.5 levels exceeding the safe threshold for exposure, which is set by the government at 50μg/m³.
Meanwhile, in an effort to improve air quality in Thailand, the Department of Pollution Control is considering lowering the "safe" threshold for PM2.5 exposure below the current level.
Department chief Attapol Charoenchansa said the department is looking at potentially lowering the safe threshold for PM2.5 exposure over a 24-hour period from 50μg/m³ to 30μg/m³.
The department said a new standard for air quality is likely to emerge within the next five years, as many of the government's active measures to combat air pollution have yielded positive results.
Mr Attapol said that tackling air pollution is one of the government's top agenda items, and a variety of agencies have been asked to implement measures to combat PM2.5 pollution at its source -- namely vehicle emissions and the burning of farm waste.
Measures which have been rolled out include an extension of the work-from-home policy, lowering the price of low-sulphur fuel in the capital and its vicinity, extensive monitoring of waste burning on farms, as well as offering higher prices for sugarcane products which were made in a sustainable manner, he said.
As a long-term measure, Mr Attapol said the government aims to apply the Euro-5 standard for vehicle emissions by 2024.
According to the department, the annual average level of PM2.5 last year stood at 23μg/m³ -- a slight decrease from the 2019 average of 25μg/m³.
The department also said the total number of fire hotspots detected dropped, from 90,455 in 2019 to 88,855 last year.
"We are seeing signs of improvement after the measures were rolled out. This year the number of days where [air quality] exceeded safe standards was less than 20% of the year," he said.