Scientists have revealed the world’s most polluted cities where residents are most at risk of breathing in toxic particles.
Topping the 100-strong list is Vietnam’s capital Hanoi, the only city with air pollution levels described as ‘very unhealthy’.
In the densely populated capital, levels of a major pollutant linked to heart disease, asthma and even cancer are 40 times higher than recommended guidelines.
Other cities in the top 20 include Istanbul in Turkey, Delhi in India, Beijing and Wuhan, both in China, and Dubai in the United Arab Emirates.
Only two British cities make the list: Birmingham, which ranks 57th, and London, which ranks 89th.
Hanoi in Vietnam is the most polluted city in the world, followed by Tehran in Iran, Bishkek in Kyrgyzstan and Kolkata in India
The ranking is provided by IQAir, a Swiss air quality technology company specializing in protection against airborne pollutants.
It is based on data from thousands of air quality monitoring stations and air quality sensors operated by government agencies, research institutions and more.
“The immediacy of the rankings can highlight the impact of events such as forest fires, biomass burning, industrial activity, sandstorms and inversion weather events on a city’s air quality,” IQAir said in a statement.
According to the company, Hanoi is plagued by a pollutant known as PM2.5 – tiny particles or droplets less than 2.5 micrometers in diameter that are invisible to the naked eye and are inhaled without even realizing it.
Research has shown that these cancer-causing microparticles are small enough to enter the bloodstream through the lungs.
In Hanoi, the PM2.5 concentration is as much as 41.8 times the annual air quality guideline of the World Health Organization (WHO), according to IQAir.
Hanoi tops IQAir’s air monitoring website in the table of the world’s most polluted cities, with air pollution levels described as ‘very unhealthy’
Hanoi is often listed as one of the most polluted cities in the world, partly due to widespread construction and emissions from the huge number of motorcycles and cars that pass through the capital every day.
But other serious air pollution in the city includes nitrogen dioxide (NO2), a reactive gas formed by motor vehicle emissions that irritates the respiratory system.
Outdoors, NO2 contributes to the formation of ozone (O3), the main component of smog, which is also linked to health problems such as chest pain, bronchitis and asthma.
In recent years, Hanoi has often been cited as one of the most polluted cities in the world, partly due to widespread construction and emissions from the huge number of motorcycles and cars that pass through the capital every day.
Carbon emissions from coal-fired power stations in the north and agricultural combustion are exacerbating the problem.
“I have had to wear a mask every time I went outside in the past few days because the air quality was so bad,” said office worker Nguyen Minh Huong.
‘It’s hard to breathe. I sneeze all the time, so I’ve had to limit my time outside,” Huong told AFP.
In total, 19 of the 20 most polluted cities are in Asia (Mexico City being the exception), which experts describe as ‘not surprising’.
“This is unfortunately not at all surprising and is supported by a lot of evidence, such as surface measurements, satellite data and models,” Philip Stier, professor of atmospheric physics at the University of Oxford, told MailOnline.
Bishkek, the capital of Kyrgyzstan, is currently number 3 on the list. In the photo: Victory Square near Kyrgyz Range, Bishkek
Beijing, China (pictured) is currently number 9, just ahead of Wuhan (10) on IQAir’s list. In both cities, air pollution is described as ‘unhealthy’
‘This is mainly due to a combination of very limited air pollution regulations, the use of outdated equipment with inadequate air pollution mitigation and high population density.’
Professor Alastair Lewis from the University of York’s National Center for Atmospheric Science agreed, saying he was ‘not particularly surprised’.
‘Other factors that could affect places in Asia include the burning of crop waste – this could have a huge impact on cities like Delhi,’ he told MailOnline.
‘There are also wildfires, and many countries with high pollution are all close together.
‘And of course the transport fleet in Hanoi itself – cars, buses, trucks – may contain some very old vehicles with poor or possibly no emissions controls.’
However, Professor Lewis also said he was surprised to see Birmingham as Britain’s highest city.
Pictured: The skyline of Tehran, Iran with skyscrapers, pollution and snow-capped Alborz mountains in the background. Tehran’s air pollution described as ‘unhealthy’
‘This is likely to be influenced by some specific weather conditions as central London is generally the most polluted city in Britain, although much cleaner than many cities further up the list,’ he told MailOnline.
IQAir’s rankings are updated at least once an hour as new air quality monitoring station data becomes available, meaning these current rankings are a snapshot.
However, the cities that are in the top 20 at any point do not move up and down the list significantly – and Hanoi is firmly at the top.
“This ranking is likely to change from day to day depending on the weather, which is a major factor that can worsen air quality when wind speeds are low,” Professor Lewis added.