Between September 12 and 30 PM2.5 -- described as superfine particles with 3 percent the diameter of a human hair -- levels were consistently above 50 μg/m3, according to a report from the Ministry of Natural Resources and Environment.

Vietnamese standards prescribe safe PM2.5 limits of 50 µg/m3 for 24 hours and 25 µg/m3 over a year. 

The World Health Organization (WHO) limits are 25 and 10.

PM2.5 particles are discharged by vehicles, industry and natural sources like dust and can easily pass through lung tissue and be absorbed into the bloodstream, causing adverse health effects.

In the period – the ministry only released data for the 19 days -- there were only five days when the air quality index (AQI) was within the safe level of 100.

On Wednesday morning Hanoi took the top spot in the world for worst air quality in a ranking of more than 10,000 cities worldwide, with an AQI of 180, a level where there's increased risk of adverse affects to the heart and lungs, especially among children, elders and people with respiratory or heart diseases.

Officials said the low quality of air in Hanoi is caused by construction, a growing number of cars and motorcycles 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.

The problem has been worse as Hanoi is on the transition from summer to autumn, and it is a phenomenon that happens every year, officials from the Department of Natural Resources and Environment said.

A temperature inversion also contributed, with thermal radiation dispersing from the ground into the atmosphere and causing fog at lower levels, while smoke caused by burning of straw on the city’s outskirts too playing a role in worsening air quality, they said.  

The average rainfall in September was the lowest in six years, resulting in an increase in pollution.

The environment ministry has urged the public, especially children, older people, pregnant women, and people with respiratory problems, to limit outdoor activities.

If they have to go out, people should wear masks and eyeglasses to reduce the exposure to pollutants, it said.

Ta Ngoc Son, an official of the Hanoi Environmental Protection Agency, said the city has been making efforts to improve air quality such as calling people not to burn straw on rice fields, grow more trees, limit individual vehicles and promote public transport.

Hoang Duong Tung, head of Vietnam Clean Air Partnership, said that improving air quality must be a long-term process. "It took Beijing years to escape the group of most polluted cities in the world. Cities cannot take actions today and expect their air quality to improve tomorrow," he said.