London underground polluted with metal particles entering the blood

London underground polluted with metal particles entering the blood.


According to the University of Cambridge, the London Underground is polluted with ultrafine metal particles small enough to end up in the human bloodstream. These particles are so small that they are probably underestimated in pollution surveys on the world’s oldest subway system. Researchers have carried out a new type of pollution analysis , using magnetism to study dust samples from subway ticket offices, platforms and operator booths. The team found that the samples contained high levels of a type of iron oxide called maghemite. As iron takes time to oxidize into maghemite, the results suggest that the particles, produced by the pollution, remain suspended for long periods due to poor ventilation in the subway, particularly on station platforms.


Some particles are five nanometers in diameter – small enough to be inhaled and end up in the bloodstream, but too small to be captured by typical pollution monitoring methods. However, it is unclear whether these particles pose a health risk.

Other studies have looked at overall subway pollution levels and the associated health risks, but this is the first time that the size and type of particles have been analyzed in detail. The researchers suggest that regular dust removal from subway tunnels and magnetic monitoring of pollution levels could improve air quality across the network. The findings are reported in the journal Scientific Reports .

The air in the tube is more polluted than the air in all of London

Numerous studies have shown that air pollution levels in the Underground are higher than in London overall and beyond the limits set by the World Health Organization (WHO). Previous studies have also suggested that most of the particulate matter in the subway is generated by the scrubbing of the wheels , rails and brakes, which produce tiny, iron-rich particles. “As most of these air pollution particles are metallic, the underground is an ideal place to test whether magnetism could be an effective way to monitor pollution,” said Professor Richard Harrison of the Department of Earth Sciences at Cambridge, author of the work. “We normally study magnetism in relation to planets, but we decided to explore how these techniques could be applied to different fields, including air pollution.” Pollution levels are normally monitored using standard air filters, but these are unable to capture ultra- fine particlesand they do not detect what types of particles are contained in the particulate matter.” I began studying environmental magnetism as part of my PhD, trying to figure out if low-cost monitoring techniques could be used to characterize the levels and sources of pollution,” said lead author Hassan Sheikh from Cambridge’s Department of Earth Sciences. “The subway is a well-defined microenvironment, therefore an ideal place for this type of study”.


Magnetic filters for ventilation

In collaboration with colleagues from Cambridge’s Department of Materials Science and Metallurgy, Sheikh and Harrison analyzed 39 dust samples from the London Underground, supplied by Transport for London (TfL). Samples were collected in 2019 and 2021 from railway operators’ platforms, ticket offices and cabins on the Piccadilly, Northern, Central, Bakerloo, Victoria, Northern, District and Jubilee lines. The sampling included major stations such as King’s Cross St Pancras, Paddington and Oxford Circus. The researchers used magnetic fingerprinting (which allows for the simultaneous measurement of multiple tissue properties in a single acquisition), 3D imaging and microscopy at the nanoscale to characterize the structure, size, shape, composition and magnetic properties of the particles contained in the samples. Previous studies have shown that 50% of particulate pollutants in the underground are iron-rich, but the Cambridge team were able to look at the details much more closely. They found a high abundance of maghemite particles, ranging in diameter from 5 to 500 nanometers and with an average diameter of 10 nanometers. Some particles formed larger clusters with diameters ranging from 100 to 2,000 nanometers.

“The abundance of these very fine particles was surprising,” Sheikh said. “The magnetic properties of iron oxides change fundamentally as the particle size varies. Furthermore, the size range in which these changes occur is the same size range in which air pollution becomes a health risk.

Researchers argue that due to poor subway ventilation, iron-rich dust can be resuspended in the air as trains arrive at the platforms, making the air quality on the platforms worse than that in ticket offices or operator cabins. . Given the magnetic nature of the resuspended dust, the researchers suggest that effective removal could be magnetic filters in the ventilation, cleaning the tracks and tunnel walls or placing shield doors between platforms and trains.



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