The instruction, introduced Saturday, include close supervision of air quality, wearing masks, improving personal hygiene and staying indoors on days with high air pollution levels.

This is the first set of instructions issued by Vietnam's top health agency, after continuous reports of very poor air quality in Hanoi and Ho Chi Minh City since September.

As of 11 a.m. Monday, Hanoi's Air Quality Index (AQI) was recorded at 158, with a PM2.5 level of 69.5 μg/m3, by multiple air monitoring stations in different parts of the city, according to Switzerland-based air quality monitoring facility IQAir AirVisual.

Vietnam-based air monitoring service Pam Air recorded AQI levels of between 105 and 166 and PM2.5 levels between 45.1 and 67 μg/m3. Vietnam's national standard for PM2.5 level is 50 μg/m3.

The capital city experienced days of very high air pollution last week, with AQI levels reaching above 200 at some points.

The AQI is a metric used by 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. An AQI level above 200 is deemed very unhealthy for humans.

Hoang Duong Tung, President of Vietnam Clear Air Partnership, said last week that Hanoi's air quality has been returning to 'hazardous levels' since December 8, and has only gotten worse since.

"It can be seen that the air quality in Hanoi this year has been continuously polluted. I think it is necessary to seriously talk about the sources of waste and seek emergency measures to tackle air pollution," he said.

The Hanoi administration was not taking the matter seriously, adopting the approach that air pollution was a natural problem that people have to live with, instead of serious health issue that humans are responsible for, he added.

While air pollution is not a new problem in Hanoi, it has gathered increasing urgency of late. Heightened levels of pollutants and smog have been seen in recent months, including a five-year high in September.

Officials have 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. However, they have not outlined any comprehensive plan to deal with the causes of pollution.

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.