On Saturday morning the air quality index (AQI) reached 172 at the monitoring station on Pham Van Dong Street, 173 at the French embassy and 177 at the station on Hang Dau Street.

Monitoring app AirVisual showed 216 in the downtown West Lake area and 196 in the Sai Dong area in Long Bien District.

AQI values from 150 to 200 are considered unhealthy.

According to the Vietnam Environment Administration, the high figures last week were mostly due to PM2.5 fine dust.

"Of all urban areas, the 24-hour average value of PM2.5 was highest in Hanoi, exceeding the permitted limit on six days during the period," the agency's report stated.

The central cities of Da Nang and Hue both had PM2.5 values below the national average.

The northern province of Phu Tho also had unhealthy air quality on four out of the seven days, while in Ho Chi Minh City it was unhealthy only on one day.

The report also showed that air pollution is worst from late night to morning.

"On days when the air quality is at unhealthy or very unhealthy levels, people should limit outdoor activities, always wear anti-dust face masks and not open doors," the environment administration said.

According to Assoc Prof Nghiem Trung Dung, former head of the Hanoi University of Science and Technology’s school of environmental science and technology, the main cause behind the current air pollution in Hanoi is the weather.

"Pollution sources such as traffic and construction sites can basically be considered unchanged in the short term. However, in recent days the meteorological conditions have not been conducive to diffusion."

Besides, the northeast monsoon possibly brings dust from other areas, contributing to increased pollution in the city, and secondary dust forming from other pollution sources could increase PM2.5 values, he said.

"However, a full, accurate and quantitative evaluation needs enough reliably measured data."

But Hoang Duong Tung, president of the Vietnam Clean Air Partnership, stressed that weather and temperature inversion only helps increase or decrease pollution and is not the cause.

"What we need to do is to determine if the source is traffic, production activities or neighboring provinces."

He emphasized the need to quickly carry out studies to obtain specific data.

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

City authorities have blamed the low air quality on large-scale construction, large number of private vehicles, intensive industrial activity like steel and cement production, and coal-fired power plants.