The northeast monsoon is predicted to arrive in the northern part of Vietnam, including Hanoi, this weekend and that will help improve the air quality in the city, said Le Thanh Hai, former deputy head of the Vietnam Meteorological and Hydrological Administration.

Hanoi's current low air quality is the result of little air circulation in the atmosphere during the transition between summer and fall.

"Air masses in the city are staying unchanged in fixed positions instead of spreading in various shapes and sizes," he said as explaining the high pollution indexes measured in the air of Hanoi in recent days.

Temperature inversion also contributed to the capital’s low air quality as thermal radiation is dispersed from the ground into the atmosphere, which causes fog at lower levels, he noted.

Wednesday marked the fourth day in a row that Hanoi's air quality stays at the "unhealthy" level as its automatic air monitoring system recorded the PM2.5 level at 90-140 micrograms per cubic meter of air, the "dangerous level."

The World Health Organization Air Quality Guideline recommends an annual mean exposure threshold for PM2.5 of 10 micrograms to minimize health risks.

PM, or particulate matter, refers to a mixture of solid particles and liquid droplets found in the air. PM2.5, also described as super fine particles, is a fraction of the width of a human hair, and is released by vehicles, industry and natural sources like dust.

At the station at Hang Dau Street in Hoan Kiem District in the downtown, the level even climbed to 150 and stayed there for three straight days.

The monitoring system operated by the U.S. embassy in Dong Da District, meanwhile, measured the PM2.5 at 149 on Wednesday afternoon after recording it at 183 on Monday.

Explaining the differences, an official with the Vietnam Environment Administration from the Ministry of Natural Resources and Environment, said the monitoring stations are placed at different positions, some at heavily polluted areas while others are set at areas with a lot of trees. Furthermore, each monitoring system could use different tools for measuring, he added.

AQI level between 101 and 150 are deemed "unhealthy for sensitive groups" while higher levels indicate that the air is unhealthy for everyone.

IQAir AirVisual, a Switzerland-based air quality monitoring facility that generates data from public, ground-based and real-time monitoring stations, recorded Hanoi's Air Quality Index (AQI) level at 152 on Wednesday morning, making it the seventh most polluted city in the world. It used data from three monitoring stations in the city.

At one point on Tuesday morning, Hanoi's AQI level was recorded at 185, only better than Malaysia's Kuching.

IQAir AirVisual also forecast that Hanoi's subpar air quality would last until the weekend.

Nguyen Ngoc Hong, head of the occupational lung disease at the Vietnam National Lung Hospital in Hanoi, said the number of patients hospitalized in the past week has increased and most of them had lung problems.

"For now we can’t tell if these patients have been affected by the current air pollution in Hanoi or not, but we guess that there might be a connection ," Hong said, urging old people, children and those having chronic lung and heart diseases to avoid going outside as much as possible.

In case they must go out, they should use quality face masks and change the masks frequently in order to prevent pollutants from entering their lungs, he suggested.

Hanoi, which has eight million people and more than five million motorbikes and 550,000 cars, was ranked the second most polluted city in Southeast Asia by the World Air Quality report put out by IQAir AirVisual earlier this year.

Vo Tuan Nhan, Deputy Minister of Natural Resources and Environment, has blamed the city's worsening air quality on dense traffic, emissions from construction projects, industrial facilities and waste burning.