Air Visual, an independent online air quality index monitor, said Hanoi's levels of hazardous small particles known as PM2.5 in the air were at an extreme high of 257 micrograms per cubic metre early on Friday.
"All incoming flights are being diverted to other cities, including Haiphong or Danang. None can take off either at the moment," an official at Noi Bai International Airport said, declining to be named.
Nearly 100 flights to and from the airport have been affected, state media reported, citing the airport authorities.
Vietnam's budget airline Vietjet on Friday said several of its flights scheduled to land in Hanoi had been diverted to land in Haiphong City.
Air pollution in Hanoi has constantly been ranked among the world's highest, with health authorities last month advising people to wear masks and limit outdoor activities.
