Thick smog blanketed Hanoi on Friday, obscuring buildings and leaving nine million residents choking on toxic air as the Vietnamese capital topped a list of the world's most polluted major cities.

Authorities urged people to wear masks and limit time outdoors, but commuters told AFP they were struggling to breathe through the putrid smog, now a fixture of the winter months in the city.

According to IQAir, levels of PM2.5 pollutants — cancer-causing microparticles small enough to enter the bloodstream through the lungs — hit 227 micrograms per cubic metre, 15 times the World Health Organization's recommended maximum daily average exposure.

Hanoi topped the Swiss monitoring company's ranking of the world's most polluted major cities during the morning, before falling back down.

The city was rated among the world's top 10 polluted capitals by IQAir in 2023.

Office worker Tran Quynh Lan told AFP that her struggle to breathe through noxious haze while commuting on a motorbike had forced her to switch to buses and taxis, despite the increased cost.

"The air quality has been so extremely bad that I really don't feel I can breathe easily in the open air. I have to wear masks all the time," she said.

The WHO says that a number of serious health conditions are linked to air pollution exposure, including strokes, heart disease and lung cancer.

Experts say pollution in Hanoi is a result of widespread construction, as well as emissions from the huge number of motorbikes and cars that crisscross the capital every day.

Carbon emissions from coal plants to the north and agricultural burning exacerbate the problem.

"The source of pollution emissions changes little every day," said climate expert Huy Nguyen.

Due to current unfavorable weather conditions, "pollutants seem to be locked in a giant atmospheric glass cage that they cannot escape and they accumulate day after day", Huy told AFP.

He said Hanoi residents need to "wait for a strong northeast monsoon with rain and strong convection" for the pollution situation to improve. 

Rain does not typically arrive in the city until March.