Hanoi in a shroud of haze in the morning of November 12, 2019. Photo by VnExpress/Gia Nghia.

The high level requires an emergency health warning to the entire population, it said. All people living in the city, regardless of their conditions, are susceptible to be affected by noticeable health issues due to the hazardous air quality.

At 556 Nguyen Van Cu Street in Long Bien District, the air quality index (AQI) reached 344 at 5 a.m. and only dropped to 270 at 8 a.m. AQI at Minh Khai air monitoring station in Bac Tu Liem District was 213, and at Pham Van Dong in Cau Giay District 201. All other stations recorded an average AQI of over 170, with many stations staying above 200 for many hours.

AQI levels above 100 are considered unhealthy.

Many independent air quality analysis systems, PAMAir and AirVisual, also recorded similar results throughout the city. The Nguyen Che Nghia station in Hoan Kiem District recorded 327, De La Thanh station in Dong Da District 326 and Ho Tay station in Tay Ho District 317.

Air quality at many places in Hanoi was at hazardous level (purple) according to PAMAir.

"This is the first time I've seen such hazardous air quality in Hanoi," Hoang Duong Tung, President of Vietnam Clear Air Partnership, who has spent many years researching on air quality in Vietnam.

With such levels, I think Hanoi should immediately notify its citizens so that they can protect themselves," Tung said.

Vietnam Environment Administration identified the main cause of the pollution as temperature inversion. The large gap between day and night temperature cause a difference in temperature at ground level in comparison to higher altitudes, creating an inversion. It makes air pollutants, especially the PM2.5 fine dust stuck at the lower altitudes, slowly worsening the air quality and damaging population.

Tung did not accept the explanation.

"The usual reason of weather inversion cannot explain such pollution. It is possible that a strong pollutant source in the vicinity is causing this."

Earlier, the environment administration had reported that Hanoi's air quality was worsening in the first days of the month, with its PM2.5 levels rising.

The PM2.5 levels consistently exceeded national standards (50 μg/m3) on Tuesday and Wednesday last week, reaching above 100 μg/m3 at times, it said.

Air pollution is not new in Hanoi, but it remains as pressing an issue as ever. Heightened levels of pollutants and smogs have been experienced by Hanoi citizens in recent months, with a five-year high peak in September.

City authorities have also blamed the low air quality on large-scale construction, high rate of individual vehicles and heavy industry activities, such as steelworks, cement factories and coal-fired plants.