The capital's Air Quality Index (AQI) was recorded at 136 on Tuesday evening by 11 air monitoring stations scattered throughout the city.

IQAir AirVisual, a Switzerland-based air quality monitoring facility that generates data from public, ground-based and real-time monitoring stations, recorded Hanoi's AQI level at 152 on Wednesday morning, making it the seventh most polluted city in the world. It used data from three monitoring stations in the city. 

IQAir AirVisual also forecast that Hanoi's subpar air quality would last until the weekend.

Throughout Sunday and Monday, the AQI levels remained consistently at around 170, according to multiple air monitoring stations.

At one point on Tuesday morning, Hanoi's AQI level was recorded at 185, only better than Malaysia's Kuching.

Cities' air quality index as measured by IQAir AirVisual,
September 17, 2019.

The AQI is a metric used by multiple governmental agencies to determine how polluted the air is. An AQI level above 100 is considered polluted or unhealthy for humans. Children, seniors and individuals with respiratory and heart diseases are recommended to avoid sustained and high-intensity outdoor exercises when AQI levels reach 150 or above.

Hanoi's ongoing low air quality is the result of little air circulation in the atmosphere during the transition between summer and fall, Le Thanh Hai, deputy head of the Vietnam Meteorological and Hydrological Administration, told VnExpress.

"Air could not rise high by convection, so there was no cloud formation to produce rain and cleanse the atmosphere. There was also no wind to push the polluted air away," he said.

Temperature inversion also contributed to the capital’s low air quality, Hai noted.

"At night, thermal radiation is dispersed from the ground into the atmosphere, which causes fog at lower levels," he added.

Echoing Hai, Hoang Duong Tung, president of the Vietnam Clean Air Partnership, said temperature inversion at night prevents air from rising up into the atmosphere and confines them to lower levels, which leads to air pollution.

Hanoi, which has eight million people and more than five million motorbikes and 550,000 cars, was ranked the second most polluted city in Southeast Asia by the World Air Quality report put out by IQAir AirVisual earlier this year.

Vo Tuan Nhan, Deputy Minister of Natural Resources and Environment, blamed the city’s worsening air quality on dense traffic, emissions from construction projects, industrial facilities and waste burning.