A study into pollution levels on the London Underground has found ultrafine metallic particles small enough to end up in the human bloodstream, although the study also notes that it’s not clear whether these particles pose a health risk.

Researchers from the University of Cambridge carried out a new type of pollution analysis, using magnetism to study dust samples from Underground ticket halls, platforms and operator cabins.

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. The team found that the samples contained high levels of a type of iron oxide called maghemite. Since it takes time for iron to oxidise into maghemite, the results suggest that pollution particles are suspended in the air for long periods.

Transmission electron microscopy image of magnetic nanoparticle aggregates. Credit: Hassan Sheikh

The study says this could be due to poor ventilation, although it could just as easily be due to the dirt being disturbed by the piston effect caused by the trains pushing air through the tunnels and becoming airborne.

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 have been analysed in detail.

Some of the particles are as small as five nanometres in diameter: small enough to be inhaled and end up in the bloodstream, but too small to be captured by typical methods of pollution monitoring.

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.

“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 lead author Hassan 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.”

Working with colleagues from Cambridge’s Department of Materials Science and Metallurgy, Sheikh and Harrison analysed 39 dust samples from the London Underground, provided by Transport for London (TfL).

The researchers used magnetic fingerprinting, 3D imaging and nanoscale microscopy to characterise the structure, size, shape, composition and magnetic properties of particles contained in the samples. Earlier studies have shown that 50% of the pollution particles in the Underground are iron-rich, but the Cambridge team was able to look in much closer detail. They found a high abundance of maghemite particles, ranging in diameter from five to 500 nanometres, and with an average diameter of 10 nanometres. Some particles formed larger clusters with diameters between 100 and 2,000 nanometres.

Although the researchers did not look at whether these maghemite particles pose a direct health risk, they say that their characterisation methods could be useful in future studies.

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.

Their results are reported in the journal Scientific Reports.

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

This article was published on ianVisits


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