Metro systems are among our cities’ most polluted places: We asked an expert if we should be worried

Networks like the London Underground can be more polluted than the street above.
Networks like the London Underground can be more polluted than the street above. Copyright Pixabay
By Rosie Frost
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Many studies over the last few years have revealed high pollution levels on underground trains.

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Air pollution in the Copenhagen Metro is higher than on the most polluted stretch of road in the Danish city.

New measurements from the University of Copenhagen found ultra-small particle concentrations underground were 10 to 20 times higher than next to the city’s Town Hall Square.

“Our measurements show that the metro is probably the place in Copenhagen’s public space where you are exposed to the most concentrated air pollution,” says Professor Matthew Johnson, lead author of the study.

It is just one of many studies conducted over the last few years that look at pollution on underground train networks.

The French Agency for Food, Environmental and Occupational Health & Safety found metros in the country had three times the pollution of the outside air. Research on the London Underground discovered ultrafine metallic particles small enough to end up in our bloodstream.

So why is pollution such a problem in metro train systems and is it something we should be worried about?

Why does pollution build up in metro systems?

Professor Frank Kelly is the head of the Environmental Research Group at Imperial College London - a global centre dedicated to air pollution research.

He calls the issue with metro systems a “box problem”.

“Imagine that the pollution being generated is going into a relatively small volume of air, that's the underground system itself,” he explains.

AP Photo/Alastair Grant
A man sits on a bench waiting for a tube train at Westminster Underground station in London.AP Photo/Alastair Grant

“Whereas the pollution generated above ground is going into an enormous volume of air, so it gets diluted down quite quickly.”

A 2020 study surveyed the entire London Underground system. The bottom line of its findings was, the deeper the train line the worse the pollution is.

It all comes down to a lack of ventilation - a problem that many cities are aware of and are working hard to solve.

Where does pollution in the metro come from?

The majority of air pollution in metro systems is usually PM 2.5 (tiny particles with a diameter of less than 2.5 microns).

On the London Underground, this pollution is mainly made up of particles of metals - primarily iron and copper.

These are generated from the wheels running on the rails. A small amount of particulate pollution is also generated by the connection between the trains and the electrified rail.

But Professor Kelly says the pollution that builds up is very unique to the environment and is different in other metro systems. 

Some networks, for example, have rubber wheels which create their own form of pollution as they rub against the rails.

REUTERS/Sarah Meyssonnier
Passengers stand on a platform at Saint-Lazare metro station in Paris.REUTERS/Sarah Meyssonnier

Should we be worried about pollution in metro train systems?

Professor Kelly calls this the “million dollar question”.

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Most of the research done on the health effects of air quality has been conducted above ground. But the composition of the pollution below ground isn’t the same.

“The jury’s still out on whether it's very damaging to our health, [somewhat] damaging or not really of any consequence,” Professor Kelly says.

“Common sense would suggest that it probably is having some impact but we really can’t say to what extent.”

Studies are currently underway on the London Underground to determine the effect of tube journeys on people with sensitive lung conditions. They are also looking at the health records of the staff that work there like guards who stand on the platforms all day.

Professor Kelly says we probably won’t have the “big answer” for at least another year.

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So should we avoid metro trains because of the pollution?

For many, the only real alternative to catching a metro train is getting in their car.

“It’s pretty well proven that pollution above ground is bad for your health, we know that,” Professor Kelly says.

He adds that if you’re sitting in your car in traffic, unless you’ve got your air intake on recirculation, you’re actually being poisoned by the car in front.

And some of the most polluted parts of our cities are where there is double lane, slow-moving traffic. It's even worse on those roads that are frequented by heavy-duty, commercial diesel vehicles.

Vuk Valcic / SOPA Images/Sipa US via Reuters Connect
Increased traffic is seen on the roads in King's Cross, London.Vuk Valcic / SOPA Images/Sipa US via Reuters Connect

“Being in heavy traffic is not an alternative that one would want to choose to any other exposure such as the London Underground,” according to Professor Kelly.

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It's better, he says, to get where you need to go in the least polluting fashion.

Professor Kelly also stresses that networks like the London Underground are valuable public resources and operators such as Transport for London recognise the need to solve the problem.

“We need to keep it and just make it better,” he concludes.

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