Revealed: The most polluted cities in the world - with Hanoi topping the list

Scientists have revealed the most polluted cities in the world where inhabitants are most at risk of inhaling toxic particles. 

At the top of the 100-strong list is Vietnamese capital Hanoi, the only city with an air pollution level 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 are on the list – Birmingham which is ranked at 57th, and London, which comes in at 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 has been provided by IQAir, a Swiss air quality technology company specialising in protection against airborne pollutants. 

It's based on data from thousands of air quality monitoring stations and air quality sensors operated by governmental bodies, research institutions and more. 

'The immediacy of the ranking is able to highlight the impact of events like wildfires, biomass burning, industrial activity, sandstorms and inversion weather events on the air quality in a city,' IQAir said in a statement. 

According to the firm, 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 inhaled without even realising.

Research has shown that these cancer-causing microparticles are small enough to enter the bloodstream through the lungs. 

In Hanoi, PM2.5 concentration is a whopping 41.8 times the annual air quality guideline given by the World Health Organization (WHO), IQAir said. 

Hanoi topped air monitoring website IQAir's table of the world's most polluted cities, with an air pollution level described as 'very unhealthy'

Hanoi has frequently been listed among the world's most polluted cities, due in part to widespread construction and emissions from the huge number of motorbikes and cars that crisscross the capital every day

READ MORE: Even 'SAFE' levels of air pollution can trigger heart attacks

Graph shows three measures of nitrogen dioxide, a pollutant primarily produced by engines and burning of fossil fuels

But other severe air pollution in the city includes nitrogen dioxide (NO2), a reactive gas formed by emissions from motor vehicles that irritates the airways. 

Outdoors, NO2 contributes to the formation of ozone (O3), the main ingredient in smog, which is also linked with health problems like chest pain, bronchitis and asthma. 

In recent years, Hanoi has frequently been listed among the world's most polluted cities, due in part to widespread construction and emissions from the huge number of motorbikes and cars that crisscross the capital every day.

Carbon emissions from coal plants to the north and agricultural burning exacerbate the problem.

'I have had to wear a mask whenever I've gone out over the last few days as the air quality has been so bad,' said office worker Nguyen Minh Huong.

'It's hard to breathe. I sneeze all the time, so I have had to limit my time outside,' Huong told AFP.

In total, 19 of the top 20 most polluted cities are in Asia (Mexico City being the exception), which is described as 'unsurprising' by experts. 

'This is unfortunately entirely unsurprising and backed up by many lines of evidence, such as surface measurements, satellite data and models,' Philip Stier a professor of atmospheric physics at the University of Oxford, told MailOnline. 

Bishkek, the capital of Kyrgyzstan, is currently ranked number 3 on the list. Pictured, Victory Square near Kyrgyz Range, Bishkek

Beijing, China (pictured) is currently at number 9, just ahead of Wuhan (10) on the list by IQAir. Both cities have air pollution described as 'unhealthy' 

'This is primarily due to a combination of very limited air pollution regulations, use of outdated equipment with insufficient air pollution mitigation and a high population density.' 

Professor Alastair Lewis at the University of York's National Centre for Atmospheric Science, agreed, saying he was 'not particularly surprised'.  

'Other factors that can affect places in Asia are the burning of crop waste – this can have a huge impact on cities like Delhi,' he told MailOnline.

'There are also natural fires as well, and many high pollution countries all in close proximity to each other. 

'And of course in Hanoi itself the transport fleet - cars, buses, lorries can include some very old vehicles with poor or possibly no emissions control.' 

However, Professor Lewis also said he was surprised to see Birmingham as the highest UK city. 

Pictured, the skyline of Tehran in Iran with skyscrapers, pollution and snow capped Alborz mountains inthe background. Tehran's air pollution is described as 'unhealthy' 

'This is is likely affected by some specific weather conditions since central London is in general the most polluted city in UK, although very much cleaner than many cities higher up in the list,' he told MailOnline. 

IQAir's ranking updates at least once an hour as new air quality monitoring station data becomes available, meaning this current ranking is a snapshot. 

However, the cities in the top 20 at any one time are not moving up and down the list considerably – and Hanoi is placed firmly at the top. 

'This rank order likely moves around day to day depending on weather which is a big factor that can worsen air quality if wind speeds are low,' added Professor Lewis.