The city’s worsening air quality saw its average PM2.5 level last year rise to 46.9 micrograms per cubic meter of air from 40.8 in 2018, according to a report released this week by Switzerland-based air quality monitor, IQAir AirVisual.

The 2019 World Air Quality Report says that for the first time, Hanoi has overtaken China’s Beijing, the world’s most suffocating capital just a few years ago. The Chinese capital city has improved to ninth on the list with its average PM2.5 level at 42.1.

IQ AirVisual, which uses data from governments, companies, civic society groups and ground-based and real-time monitoring stations, surveys air quality in over 3,000 cities globally by measuring PM2.5 levels.

The capital city ranking compares annual PM2.5 averages in 2019 among 85 capital cities available in the report's dataset.

India's Delhi was the world's most polluted capital city with its PM2.5 level reaching 98.6 micrograms per cubic meter, followed by Bangladesh's Dhaka (83.3) and Mongolia's Ulaanbaatar.

PM2.5, also described as super fine particles, is a fraction of the width of a human hair, which is released from vehicles, industry and natural sources like dust. The World Health Organization (WHO) Air Quality Guideline recommends an annual mean exposure threshold of 10 μg/m3 to minimize health risks.

Hanoi ranked 150th in the list of world’s most polluted cities and was the sixth most polluted city in Southeast Asia.

Meanwhile, Ho Chi Minh City, Vietnam’s southern metropolis, ranked 609 in the cities list.

HCMC has its seen air quality improve over the past year, with the average PM2.5 level reaching 25.3 micrograms per cubic meter of air. In 2018, the southern Vietnamese metro was the 15th most polluted city in Southeast Asia, with PM2.5 level soaring to 26.9, more than two times the WHO threshold.

Vietnam ranked 15th in the list of 98 countries and territories with the worst air quality in the world with an average PM2.5 level of 34.1 and second in Southeast Asia behind Indonesia.

"Vietnam's rapid development and urbanization poses severe challenges to managing its PM2.5 pollution," the report said.

"Rapid development coupled with weak emission standards for power plants, vehicles and industries and a high and rising share of coal in power generation contribute to high air pollution levels in bigger cities. Vietnam's coal consumption doubled and oil consumption increased by 30 percent over the past five years," the report said.

Worsening air pollution in Hanoi and HCMC, Vietnam's two biggest metropolises, has become a top concern in the country of 94 million people, making headlines repeatedly.

Officials have said the low quality of air in the cities is caused by construction, a growing number of cars and motorcycles and heavy industry, including steel works, cement factories and coal-fired plants.

The Minister of Natural Resources and Environment, Tran Hong Ha, last year called for urgent measures to reduce air pollution, from improving monitoring system to ending people's use of coal stoves.

Research done by Vietnamese experts showed that Vietnam suffered between $10.8- $13.2 billion worth of economic losses associated with ambient air pollution each year, equivalent to about 5 percent of the country’s GDP.

Vietnam ranked 4th in the number of pollution-linked deaths in the Western Pacific region, with an estimated 71,365 Vietnamese people losing their lives to pollution, including 50,232 to air pollution in 2017, the latest year for which data was available, according to the Pollution and Health Metrics report by the Global Alliance on Health and Pollution last year.