Monitoring results recorded at 35 stations in Hanoi from Dec. 29 to Jan. 5 showed the air quality index (AQI) had dropped from bad to worse, according to the city's Department of Natural Resources and Environment.

The AQI is a metric used by multiple 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.

At 9 a.m. Wednesday, the air quality index reached 163 downtown, a level requiring the elderly and those with heart and respiratory ailments in the area to stay indoors.

IQAir, a Swiss air quality technology and monitoring company, said Hanoi's AQI climbed to an "unhealthy level" of 185 on Wednesday morning, making it the fifth most polluted city in the world.

The city's level of PM2.5 or super fine particles, a fraction of the width of a human hair released from vehicles, industry and natural sources like dust, was 121.3 micrograms per cubic meter (μg/m3) of air Wednesday.

The World Health Organization Air Quality Guideline recommends an annual mean exposure threshold of 10 μg/m3 to minimize health risks, with Hanoi's level 10 times higher. Vietnam’s national PM2.5 threshold is 25 μg/m3.

Health experts recommended people remain indoors in the early morning and evening, the most polluted times of the day. Schools are required not to let students participate in outdoor extracurricular activities. People have also been advised to wear masks and eyeglasses to reduce exposure to pollutants.

Major reasons for declining air quality

Officials from the department said Wednesday that due to seasonal transition, the capital would be blanketed by thick fog in the early morning and late afternoon. As a result, air circulation would be restricted which, in turn, would prevent the diffusion of pollutants.

Adding to the problem is Hanoi's plain topography, along with fast-developing industrial zones in neighboring provinces Hung Yen, Bac Ninh, Ha Nam, Hoa Binh, Vinh Phuc, and Thai Nguyen, home to a large number of foreign-invested enterprises in Vietnam.

Amid foggy weather conditions, particulates cannot escape and are trapped at ground level, which causes poor air quality.

In addition, the department ascribed the deteriorating air quality to high traffic volume, trash overload, sidewalk paving and rising construction and production activities.

Last month, over 100 tons of uncollected garbage were left to stack up in the capital after sanitation workers went on strike over late salary payment, which led to the increase in burning of domestic waste and straw.

High demand for travel in the lead up to Tet, Vietnam’s biggest holiday, has resulted in increasing numbers of private vehicles on the road, causing dirty air in the capital, the department stated.

From now to March many adverse meteorological weather conditions would appear, causing deterioration of air quality and affecting people's health, it further predicted.

Air pollution is not new to Hanoi, but it remains as pressing an issue as ever.