London Underground is polluted by metallic particles small enough to enter bloodstream, say University of Cambridge researchers

The London Underground is polluted with ultrafine metallic particles that are small enough to end up in the human bloodstream, University of Cambridge scientists have discovered.

They say it is not clear whether these particles pose a health risk.

And they suggest that periodic removal of dust from Underground tunnels, as well as the magnetic monitoring of pollution levels, could be used to improve air quality throughout the network.

The scientist carried out a new type of pollution analysis, using magnetism to study dust samples from ticket halls, platforms and operator cabins on what is the world’s oldest metro system.

The samples contained high levels of a type of iron oxide called maghemite.

It takes time for iron to oxidise into maghemite, which suggests pollution particles are suspended for long periods due to poor ventilation throughout the Underground, particularly on station platforms.

They found particles as small as five nanometres in diameter, which is small enough to be inhaled and end up in the bloodstream, but too small to be captured by typical methods of pollution monitoring.

“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 Hassan Sheikh, from Cambridge’s Department of Earth Sciences, who is lead author of a study published in the journal Scientific Reports.

Other researchers have looked at overall pollution levels on the Underground and attempted to assess the associated health risks, but this work is the first time that the size and type of particles have been analysed in detail. Multiple studies have shown that air pollution levels there are higher than those in London generally and beyond the World Health Organization’s defined limits.

Earlier research has suggested 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.

“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 Prof Richard Harrison, from the Department of Earth Sciences and 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.”

Standard air filters are usually used to monitor pollution levels, but these fail to capture ultrafine particles or detect what kinds of particles are contained within the particulate matter.

“I started studying environmental magnetism as part of my PhD, looking at whether low-cost monitoring techniques could be used to characterise pollution levels and sources,” said Hassan. “The Underground is a well-defined micro-environment, so it’s an ideal place to do this type of study.”

Working with colleagues from Cambridge’s Department of Materials Science and Metallurgy, Hassan and Prof Harrison analysed 39 dust samples from the London Underground, provided by Transport for London and collected in 2019 and 2021 from platforms, ticket halls and train operator cabins on the Piccadilly, Northern, Central, Bakerloo, Victoria, District and Jubilee lines.

Major stations such as King’s Cross St Pancras, Paddington and Oxford Circus were involved in the sampling.

The research team used magnetic fingerprinting, 3D imaging and nanoscale microscopy to characterise the structure, size, shape, composition and magnetic properties of particles in the samples.

Half of the pollution particles on the Underground were shown by earlier studies to be rich in iron, but the Cambridge study examined these in much more detail, finding a high abundance of maghemite particles that ranged in diameter from five to 500 nanometres, and had an average diameter of 10 nanometres.

Some particles formed larger clusters with diameters ranging from 100 to 2,000 nanometres.

“The abundance of these very fine particles was surprising,” said Hassan. “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.”

Future studies would be needed to assess whether these maghemite particles pose a direct health risk.

“Our techniques give a much more refined picture of pollution in the Underground,” said Prof 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.”

Due to poor ventilation, 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.

Since this resuspended dust is magnetic, the researchers suggest that an efficient cleaning system could include magnetic filters in ventilation, cleaning of the tracks and tunnel walls and the placing of screen doors, as on some newer lines, between platforms and trains.

About three and a half million journeys are made daily on the Tube.

Transport for London’s chief safety, health and environment officer Lilli Matson said: “Safety is our top priority and we have been working for many years to improve air quality on the Tube, and will continue to do so.

“We periodically collect samples of Tube dust and analyse its content to track levels of potentially harmful materials, including iron, chromium and nickel. Analysis has shown that quantities of these materials are well below the legal limits in environments such as the Tube.

“Our monitoring has shown that dust levels on the Tube remain well below limits set by the Health and Safety Executive, but we are going further and have developed a number of innovative new cleaning regimes.

“This includes the use of industrial backpack dust cleaners, which are one part of our multi-million pound Tube cleaning programme.”

The research was supported in part by the European Union, the Cambridge Trust and Selwyn College, Cambridge.