Revealed: the most polluted cities in the world – with Hanoi at the top of the list

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

Top 10 most polluted cities

  1. Hanoi, Vietnam
  2. Tehran, Iran
  3. Bishkek, Kyrgyzstan
  4. Calcutta, India
  5. Dhaka, Bangladesh
  6. Yangon, Myanmar
  7. Karachi, Pakistan
  8. Tashkent, Uzbekistan
  9. Beijing, China
  10. Wuhan, China

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 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.

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

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'

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'

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.

WHAT ARE THE EFFECTS OF THE MAIN AIR POLLUTIONS IN THE WORLD?

According to the Environmental Protection Agency, there are six major pollutants that can affect human health and well-being.

Particulate matter: Particulate matter is the term for a mixture of solid particles and liquid droplets that occur in the air.

These particles come in many sizes and shapes and can consist of hundreds of different chemicals.

Some are emitted directly from a source, such as construction sites, dirt roads, fields, chimneys or fires.

Fine particles (2.5 parts per million) are the leading cause of reduced vision (haze) in parts of the United States, including many of our beloved national parks and wilderness areas.

Carbon monoxide: Breathing air with a high CO concentration reduces the amount of oxygen that can be transported in the bloodstream to critical organs such as the heart and brain.

At very high levels, which is possible indoors or in other confined environments, CO can cause dizziness, confusion, unconsciousness and death.

Nitrogen dioxide: Nnitrogen dioxide mainly ends up in the air through the combustion of fuel. NO

It comes from emissions from cars, trucks and buses, power plants and off-road equipment.

Breathing air with a high concentration of NO can irritate the airways in the human respiratory system. Such exposure for short periods may aggravate respiratory diseases, especially asthma, leading to respiratory symptoms (such as coughing, wheezing or difficulty breathing).

Sulphur dioxide: The largest source of sulfur dioxide in the atmosphere is the combustion of fossil fuels by power plants and other industrial facilities.

Short-term exposure to SO can damage the human respiratory system and make breathing difficult. Children, the elderly and people suffering from asthma are particularly sensitive to the effects of SO.

Ground level ozone: The ozone layer in the lower part of the lower stratosphere, about 12 to 19 miles above the planet’s surface (20 to 30 km).

Although ozone protects us from UV radiation, when found at ground level it can cause health problems for vulnerable people suffering from lung diseases such as asthma.

It is caused by chemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) – which occur in exhaust gases – in the presence of sunlight.

Pipe: Major sources of lead in the air include ore and metal processing and piston-engine aircraft that fly on leaded jet fuel.

Other sources include waste incinerators, utilities and lead-acid battery manufacturers. The highest lead concentrations in the air are usually found near lead smelters.

Depending on the level of exposure, lead can adversely affect the nervous system, kidney function, immune system, reproductive and developmental systems, and cardiovascular system.

Infants and young children are particularly sensitive to even low levels of lead, which can contribute to behavioral problems, learning delays and lowered IQ.

Source: EPA