The Bangkok Metropolitan Administration (BMA) clarified that an unpleasant odour detected in many parts of the capital last night was caused by a sudden change of weather mixed with chemical reactions involving pollutants in the air.
Pornphrom Vikitsreth, an adviser to Bangkok governor Chadchart Sittipunt, made the remarks on Thursday after many citizens complained about a burning smell and smog blanketing the city on Wednesday night.
Mr Pornphrom said the wind direction had shifted and was coming in from the east rather than from the Gulf of Thailand as usual.
Many smog hotspots were also detected in nearby provinces over the past 24 hours.
The fluctuating climate aggravated by thunderstorms and high air pressure from the Northeast led to the increase in dust accumulation, he said.
This was compounded by the high humidity, which exacerbated the level of PM2.5 pollution derived from nitrogen and ammonia, Mr Pornphrom said.
The haze was also likely caused by gases, including nitrogen oxide and sulphur dioxide, that react to the sunlight and turn into dust, he said, adding this has been a problem for several months.
Sulphur dioxide is produced by coal, fuel, metal-smelting facilities and oil refineries, whereas nitrogen oxide is produced in combustion processes, partly from nitrogen compounds in fuel, but mostly by a direct combination of atmospheric oxygen and nitrogen in flames.
Nitrogen oxide turns into a more dangerous reddish-brown-coloured nitrogen dioxide gas when it hits the ozone, causing irritation to the eyes, nose and throat, he added.
According to the Geo-Informatics and Space Technology Development Agency (Gistda), the level of PM2.5 fine dust rose to high-risk levels in 47 districts of Bangkok at midnight on Wednesday.
Don Muang peaked at 143.5 microgrammes per cubic metre, followed by Laksi (143 µg/m³), Bang Sue (139 µg/m³), Chatuchak (128.1 µg/m³) and Phaya Thai (127 µg/m³).
High dust levels were also detected in many other provinces in the Central Region.