The PM2.5 levels consistently exceeded national standards (50 μg/m3) on Tuesday and Wednesday, reaching above 100 μg/m3 at times. The Air Quality Index (AQI) on those two days ranged 101-200, with a station in Bac Tu Liem District recording AQI levels as high as between 201-300.

AQI levels above 100 are considered unhealthy. 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.

From Tuesday to Thursday, PM2.5 levels were often highest between 12 a.m. and 8 a.m., peaking at 6 a.m. on Wednesday and Thursday and at 12 a.m. on Tuesday (above 100 μg/m3). Some stations in Pham Van Dong, Hang Dau and Minh Khai streets and the U.S. Embassy in Vietnam recorded AQI levels above 200 during these times.

As of noon Friday, the AQI level in Hanoi was recorded at 122 and the PM2.5 level at 44.2 μg/m3, according to environment monitor app AirVisual. Meanwhile, Vietnam-based environment monitoring app, PamAir, recorded AQI levels between 77 and 158, using data from stations throughout the city.

There was little wind between midnight and dawn, which allowed heat inversion to happen more easily, which could explain the heightened levels of pollutants, the environment agency said. It said the situation would continue in the coming days as the weather becomes dry and less windy. It advised citizens to refrain from exercising outside in the morning, keep windows closed and use air masks outside.

Hanoi's air quality has been a topic of discussion in recent months, with the capital experiencing heightened levels of pollutants and even smog at some points.

Officials have said the low quality of air in Hanoi is caused by construction, a growing number of vehicles and heavy industry, including steel works, cement factories and coal-fired plants.

The city of eight million people has more than five million motorbikes and 550,000 cars, and the number of private vehicles is increasing at a rate of 4.6 percent a year.