London Underground study discovers previously unknown air pollutants

They believe that the particles – a type of iron oxide called maghemite – have been largely ignored in the past due to their diminutive size. But a new type of pollution analysis using magnetism has discovered the particles in Underground ticket halls, platforms and operator cabins.

Since it takes time for iron to oxidise into maghemite, the results suggest that pollution particles are suspended for long periods, due to poor ventilation throughout the Underground, particularly on station platforms.

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. However, it is not clear whether these particles pose a health risk.

The researchers believe that periodic removal of dust from Underground tunnels, as well as magnetic monitoring of pollution levels, could improve air quality throughout the network.

Multiple studies have shown that air pollution levels on the Underground are higher than those in London more broadly, and beyond the World Health Organisation’s 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.

“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 Cambridge researcher Professor Richard Harrison.

“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.”

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 researchers analysed 39 dust samples from the London Underground, provided by Transport for London (TfL).

The 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.

They 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 per cent of the pollution particles in the Underground are iron-rich, but the Cambridge team were 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.

“The abundance of these very fine particles was surprising,” said lead author Hassan Aftab 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 characterisation methods could be useful in future studies.

The researchers added 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.

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.