Exhaust from traffic, a construction boom, the burning of crop waste and pollution from factories that is trapped in the city have all been blamed.
People have also been asked not to burn incense and paper for Chinese New Year.
Bangkok’s air quality had fallen to harmful levels with the quantity of tiny unsafe dust particles, PM2.5, exceeding what is deemed safe at test sites in 41 areas around the sprawling city, the Department of Pollution Control said.
PM2.5 are microscopic particles, smaller than 2.5 micrometers in diameter, that are small enough to penetrate into the lungs and pass into other organs.
The Air Quality Index (AQI) is used by cities to determine levels of air pollution. The World Health Organisation says AQI levels under 25 are acceptable for humans to breathe regularly. AQI levels in Bangkok have been measured at 175.
There are fears about how it will affect tourism in Thailand, where earnings from foreign visitors make up around a fifth of GDP.
Bangkok governor Asawin Kwanmuang said about 50 drones would be deployed to spray water mixed with molasses to catch the particles.
The authorities have also tried to produce artificial rain and sprayed water from fire engines on congested streets.
Military-appointed Prime Minister Prayut Chan-o-cha said everybody should contribute towards efforts or the junta would enforce stern pollution-control measures.
Prayut warned that if the smog did not clear, he would issue orders prohibiting people from driving alone, banning diesel vehicles and enforcing road-space rationing.
The former general also said the authorities faced punishment if they failed to control pollution.
The debate has become politicised ahead of the much-delayed March 24 general election.
“I don’t see serious measures being taken,” tweeted Chaturon Chaisang of the Thai Raksa Chart party, which is linked to the populist former prime minister Thaksin Shinawatra. The party has launched a “measure the pollution” app and other parties have handed out masks.
Some university students are currently preparing for national exams and allowed to continue their studies. The government said the situation would be reassessed on Sunday to determine if schools would stay closed next week.
Private international schools in the capital are not compelled to follow the order.
Rasami British International School, Harrow International School Bangkok and Bangkok Patana, despite being far from the city centre, opted to close for the two days.
The extension of the BTS overhead train has helped reduce some congestion.