Seasonal air pollution has long afflicted Thailand, like many countries in the region, as colder, stagnant winter air combines with smoke from crop stubble burning and car fumes.

On Thursday morning, the Thai capital was the sixth most polluted major city in the world, according to IQAir.

Level of PM2.5 pollutants -- cancer-causing microparticles small enough to enter the bloodstream through the lungs -- hit 122 micrograms per cubic metre.

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

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

And by Thursday morning, 194 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 BMA authority closed over air pollution.

Another 58 schools out of the 156 under the Office of the Basic Education, a central government body, had also decided to close by Thursday.

There are several other schools in the capital under different authorities, and private establishments, but figures for them were not available.

- Vulnerable children -

Children are especially vulnerable to the impacts of air pollution, but rights advocates warned that closures disproportionately affect the most vulnerable students.

"School closures should be a last resort," said Severine Leonardi, UNICEF Thailand deputy representative.

"There really needs to be a wake-up call on the need to invest in the education system and protect children," she told AFP.

Authorities encouraged people to work from home this week, but the scheme is voluntary and has just 100,000 registered participants in a city of some 10 million.

Officials have also limited access for six-wheel trucks in parts of the capital until late Friday.

The government has announced incentives to stop crop stubble burning and is even trialling a novel method to tackle air pollution by spraying cold water or dry ice into the air above the smog.

But the measures have had little impact so far, and opposition politicians have accused Thai Prime Minister Paetongtarn Shinawatra -- currently in Davos for the World Economic Forum -- of failing to take the issue seriously.

"While the prime minister is breathing fresh air in Switzerland as she tries to attract more investment to Thailand... millions of Thais are breathing polluted air into their lungs," Natthaphong Ruengpanyawut, leader of the People's Party, wrote on Facebook.

Clean air activists have been pushing for legislation that could pass later this year.

"You really need comprehensive legislation on all the different dimensions of the crisis," said Guillaume Rachou, executive director at Save the Children Thailand.

"It's difficult but I think with the Clean Air Act, we're getting there," he told AFP.