The London Underground – the world’s first subway – was opened all the way back in 1863. Today, as a train network throughout Greater London, it is polluted with ultrafine metallic particles small enough to end up in the human bloodstream.
Hassan Sheikh, a researcher at the Centre for Risk Studies at the University of Cambridge, and colleagues analyzed the structure, size, shape, chemical composition and magnetic properties of particles from 39 London Underground dust samples.
Samples were collected in 2019 and 2021 from platforms, ticket halls and train operator 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 authors of a new study just published in the journal Scientific Reports found that the iron-bearing particles in their samples primarily consisted of an iron oxide known as maghemite, which ranged in diameter from five to 500 nanometers and had an average diameter of 10 nanometers. Some particles were arranged into larger clusters with diameters between 100 and 2,000 nanometers.
The authors suggest that magnetic monitoring approaches – similar to those used in this study to characterize air pollution particles – could complement traditional air pollution monitors that may underestimate the number of ultrafine airborne particles in the London Underground due to their low weight. These particles are so small that they are likely being underestimated in surveys of pollution in the train system.
What have we known until now?
Previous studies reported that half of the air pollution particles in the London Underground are magnetic iron-bearing particles produced by contact between train wheels and brakes with train tracks and that carrying higher loads and traveling at higher speeds can produce finer particles. The Cambridge researchers carried out a new type of pollution analysis, using magnetism to study dust samples from Underground ticket halls, platforms and operator cabins.
The authors of the latest study suggest that the abundance of ultrafine maghemite in London Underground dust samples could be caused by iron-containing compounds from wheels, brakes and tracks being exposed to air for longer periods of time due to poor ventilation and then resuspended in the air when trains arrive at platforms.
They suggest that amount of resuspended dust could be reduced by washing the tracks and tunnel walls, using magnetic filters in ventilation systems or placing screen doors between platforms and trains.
Other studies have looked at overall pollution levels on the Underground and the associated health risks, but this is the first time that the size and type of particles has been analyzed in detail. The researchers suggest that periodic removal of dust from Underground tunnels, as well as magnetic monitoring of pollution levels, could improve air quality throughout the network.
Given the magnetic nature of the resuspended dust, the researchers suggest that an efficient removal system might be magnetic filters in ventilation, cleaning of the tracks and tunnel walls or placing screen doors between platforms and trains.
Multiple studies have shown that air pollution levels on the Underground are higher than those in London more broadly, and beyond the World Health Organization’s (WHO) defined limits. Earlier studies have also suggested that most of the particulate matter on the Underground is generated as the wheels, tracks and brakes grind against one another, throwing up tiny, iron-rich particles.
Why test this in the London Underground?
“Since most of these air pollution particles are metallic, the Underground is an ideal place to test whether magnetism can be an effective way to monitor pollution,” said Professor Richard Harrison from Cambridge’s Department of Earth Sciences, the paper’s senior author. “Normally, we study magnetism as it relates to planets, but we decided to explore how those techniques could be applied to different areas, including air pollution.”
“Normally, we study magnetism as it relates to planets, but we decided to explore how those techniques could be applied to different areas, including air pollution.”
Professor Richard Harrison from Cambridge’s Department of Earth Sciences
Pollution levels are normally monitored using standard air filters, but these cannot capture ultrafine particles, and they do not detect what kinds of particles are contained within the particulate matter.
“I started studying environmental magnetism as part of my doctoral studies, looking at whether low-cost monitoring techniques could be used to characterize pollution levels and sources,” said lead author Sheikh from Cambridge’s Department of Earth Sciences. “The Underground is a well-defined micro-environment, so it’s an ideal place to do this type of study.”
“The abundance of these very fine particles was surprising,” said Sheikh. “The magnetic properties of iron oxides fundamentally change as the particle size changes. In addition, the size range where those changes happen is the same as where air pollution becomes a health risk.”
While the researchers did not look at whether these maghemite particles pose a direct health risk, they say that their characterization methods could be useful in future studies. “If you’re going to answer the question of whether these particles are bad for your health, you first need to know what the particles are made of and what their properties are,” said Sheikh.
“Our techniques give a much more refined picture of pollution in the Underground,” said Harrison. “We can measure particles that are small enough to be inhaled and enter the bloodstream. Typical pollution monitoring doesn’t give you a good picture of the very small stuff.”
The researchers say that due to poor ventilation in the Underground, iron-rich dust can be resuspended in the air when trains arrive at platforms, making the air quality on platforms worse than in ticket halls or in operator cabins.