The city topped air monitoring website IQAir's table of the world's most polluted cities early Tuesday afternoon.
Levels of PM2.5 pollutants -- cancer-causing microparticles small enough to enter the bloodstream through the lungs -- were classified as "very unhealthy" and hit more than 24 times the World Health Organization's annual guideline.
In recent years Hanoi has frequently been listed among the world's most polluted cities, due in part to widespread construction and 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.
"I have had to wear a mask whenever I've gone out over the last few days as the air quality has been so bad," said office worker Nguyen Minh Huong.
"It's hard to breathe. I sneeze all the time, so I have had to limit my time outside," Huong told AFP.
Last month dozens of flights were affected when high humidity caused thick fog to envelop Hanoi, producing a spike in air pollution and causing visibility to plummet.
Weather forecasters have issued regular warnings of thick haze, especially in northeastern mountainous areas of Vietnam.
The latest World Bank report on air pollution says 40 percent of people in Hanoi are exposed to concentrations nearly five times greater than WHO guidelines.
The WHO says that a number of serious health conditions are linked to air pollution exposure, including strokes, heart disease and lung cancer.
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