Metal nanoparticles in the underground air

Last week, we looked at some examples of how magnets are making their mark in modern science. And much like London buses, another one has come along straight after! Or should that be Underground trains? Researchers at the University of Cambridge have been using magnetic techniques to find that the London Underground is polluted with tiny metallic particles so small they can end up in the human bloodstream. Their results, published this week in Scientific Reports, are taken from samples collected at platforms, ticket halls, and train operator cabins from a range of popular stations like King’s Cross, and Paddington. Richard Harrison is with us to explain…

Richard - What we found is abundant nanoparticles of iron oxides within the London Underground, which we identified using a magnetic monitoring technique. And these are nanoparticles generated inside the London Underground when you have the metal of the train tracks and the metal of the wheels of the underground train rubbing past each other. Friction from that and also in the brake systems of these trains. If you imagine a brake disc, you have to replace that very frequently in your car. And that's because they get worn down and abraded over time. And that process of abrasion is a very powerful way of generating abundant nanoparticles of iron-rich materials. And that's what we can pick up and characterise very successfully using magnetic monitoring methods.

Chris - I was quite alarmed when I read your paper that the air quality underground is worse than the air quality overground and London is actually ranked as one of the world's worst cities for air pollution?

Richard - Yes, it's a major problem. And it is surprising because you think going into an enclosed environment like the London Underground, you might be avoiding a lot of the air pollution that you would see overground on the streets of London. But the pollution is still there. And what's critical is that it's also a very different type of pollution. Overground you are exposed to primarily the particulate matter that's being pumped out of the exhaust pipes of cars, whereas you have none of that in the underground system because everything's electrified. So instead you're much more dominated by these non-exposed emissions caused by abrasion processes between the wheels and the tracks and the brake systems and things like that.

Chris - Do you think that they have health harm implications? They're effectively rust particles if they're bits of iron oxide, aren't they?

Richard - Yeah, essentially. There's increasing work being done on this. These non exhaust emissions are less well studied than the exhaust emissions from a health perspective. And there is conflicting evidence about their long-term health impacts. But a few studies now are starting to emerge, focusing on these emissions. There was one recently based on particles from the London Underground showing an increased risk of pneumococcal infection in mice, for example, exposed to those sorts of particles. And there's been some extensive work on the fact that these very small particles of iron oxides can enter the human body and even enter the human brain where they've been suggested to be linked to diseases such as Alzheimer's.

Chris - Is this new pollution or are we just churning up and recycling old pollution? Because is this a legacy of underground journeys of days gone by and you are just detecting that every time a train goes whooshing through it throws more of it up? Or is this genuinely generated de novo by each train that puts its brakes on?

Richard - Well, probably a mixture of both, but one of the surprising results we found was that the particles we were observing using our magnetic methods are a very oxidised form of iron. When these nanoparticles are freshly generated, they would be metallic, but they oxidise on exposure to air. And the longer they hang around in the air, the more oxidised there will be. And so we were quite surprised to find that a lot of the particles we were seeing were this very oxidised form, suggesting that they'd been around for some time. It's hard to know exactly how long and that these particles were, as you suggest, being resuspended whenever a train comes through. So the particles will be being generated, but there appears to be quite a lot of legacy particles in the system as well that could be cleaned up.