The net is riddled with toxic airborne particles that could endanger the health of passengers, according to new research.
Metal particles thrown up by braking trains enter people’s bloodstream after being inhaled into the lungs, scientists say.
They come from grindstones and splints – and have been linked to cancer, dementia and cardiovascular disease. Some are 20,000 times thinner than a human hair – so small that the magnitude of the problem was probably underestimated.
The first analysis of its kind identified high concentrations of an iron oxide called maghemite – a mineral that only evolves over time. The results suggest that the fragments linger for a long time in the subway, particularly on the platforms, due to poor ventilation.
Lead author Hassan Sheikh, a senior student, said: “The abundance of these very fine particles was surprising. The magnetic properties of iron oxides fundamentally change as the particle size changes.
“Furthermore, the scale at which these changes occur is the same at which air pollution becomes a health risk.”
The Cambridge team used magnetism to study dust samples from ticket halls, train platforms and operator cabins. They were only five nanometers in diameter – too small to be detected by conventional techniques.
The London Underground system carries five million passengers a day. Pollution levels are higher than on the roads – outside the limits recommended by experts.
Lead author Professor Richard Harrison said: “Since most of these air pollution particles are metallic, the subsurface is an ideal place to test whether magnetism can be an effective way to monitor pollution.
“Usually we study magnetism in relation to planets, but we decided to investigate how these techniques could be applied to different fields, including air pollution.”
The study in Scientific Reports points out that standard air filters can’t trap ultrafine particles — or detect what they contain.
Mr Sheikh said: “I started studying environmental magnetism as part of my PhD, exploring whether low-cost monitoring techniques could be used to characterize pollution levels and sources.
“The subsurface is a well-defined microenvironment, so it’s an ideal place for this kind of research.”
Transport for London (TfL) provided 39 dust samples from the Piccadilly, Northern, Central, Bakerloo, Victoria, Northern, District and Jubilee routes.
They were collected in 2019 and 2021. Major stations included King’s Cross St. Pancras, Paddington and Oxford Circus. Researchers used magnetic fingerprints, 3D imaging and nanomicroscopy to characterize their structure, size, shape, composition and properties.
Previous studies have shown that 50 percent of subway dirt particles are iron-rich. A much closer study found them to be 5 to 500 nanometers in diameter — with an average of 10. They also coalesced into larger clusters, ranging from 100 to 2,000 nanometers across.
The researchers said more research is needed to determine whether maghemite particles pose a direct health risk. Characterization methods may be useful in future studies.
Mr. Sheikh: “If you want to answer the question of whether these particles are harmful to health, you first need to know what the particles are made of and what properties they have.”
Poor ventilation can cause iron dust to become airborne when trains arrive at platforms.
As a result, the air quality on platforms is worse than in ticket halls or in operator cabins, he explained.
An efficient removal system is, for example, magnetic filters for ventilation, cleaning tracks and tunnel walls or placing screen doors between platforms and trains.
Prof Harrison said: “Our techniques give a much more accurate picture of subway pollution.
“We can measure particles that are small enough to be inhaled and enter the bloodstream. Typical pollution monitoring doesn’t give you a good idea of the very small things.
Source: Metro.co
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