Air pollution in the Thai capital forced the closure of more than 350 schools on Friday, city authorities said, as Bangkok was ranked the world's seventh-most polluted major city by air quality monitor IQAir.

Seasonal air pollution has long afflicted Thailand, like many countries in the region, but this week's hazy conditions have shuttered the most schools since 2020.

"Bangkok Metropolitan Administration has closed 352 schools across 31 districts due to air pollution," the authority said in a message shared on its official LINE group. 

On Thursday, more than 250 schools in Bangkok were closed due to pollution, as officials urged people to work from home and restricted heavy vehicles in the city.

Air pollution hits the Southeast Asian nation seasonally, as colder, stagnant winter air combines with smoke from crop stubble burning and car fumes.

By Friday, the level of PM2.5 pollutants — cancer-causing microparticles small enough to enter the bloodstream through the lungs — hit 108 micrograms per cubic metre, according to IQAir.

The World Health Organization recommends 24-hour average exposures should not be more than 15 for most days of the year.

Bangkok authorities said this week schools in areas with elevated levels of PM2.5 could choose to close.

By Friday morning, 352 of the 437 schools under the Bangkok Metropolitan Authority had shut their doors, affecting thousands of students.

The figure was the highest since 2020, when all schools under the city authority closed over air pollution.

Interior Minister Anutin Charnvirakul on Thursday ordered a ban on stubble burning — intentionally burning leftover crops to clear fields — with those responsible risking legal prosecution. 

Prime Minister Paetongtarn Shinawatra, who is currently attending the World Economic Forum in Switzerland, called for tougher measures to tackle pollution on Thursday, including limiting construction in the capital and seeking cooperation from nearby countries.

Neighbouring Vietnam and Cambodia's biggest cities also ranked in IQAir's top 10 most-polluted major cities globally on Friday, with Ho Chi Minh City reaching second and Phnom Penh fifth.

Cambodia's Environment Ministry spokesman Khvay Atitya told reporters on Thursday that the air quality in the country was within safe levels.

"Other countries have their own standards. Cambodia has our own standard to determine the air quality," he told reporters, adding that authorities had not issued any emergency measures.