London Underground pollution includes particles small enough to be inhaled

Particles of five nanometres in diameter were detected in a study using a new type of analysis

About 3.5 million journeys are made on the London Underground every weekday. Getty Images

Pollution on the London Underground tube system includes metallic particles small enough to enter the bloodstream, a study released on Thursday has found.

University of Cambridge researchers discovered high levels of a type of iron oxide called maghemite, suggesting pollution particles are suspended for long periods because of poor ventilation, particularly on platforms.

Using a new type of pollution analysis, they inspected dust samples from London Underground ticket halls, platforms and train drivers' cabins.

Some particles were found to have a diameter of just five nanometres, making them small enough to be inhaled and to enter passengers’ and workers’ bloodstreams.

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.”

In the study, samples were collected in 2019 and 2021 from some of the busiest tube stations, such as Oxford Circus, King’s Cross St Pancras and Paddington.

About 3.5 million journeys a day are made every weekday on the London Underground.

Researchers did not look at whether maghemite particles pose a direct health risk, but they believe their methods could be useful in future studies.

“Our techniques give a much more refined picture of pollution in the Underground,” said Prof Richard Harrison from the university's Department of Earth Sciences, a senior author of a paper produced from the study.

“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.”

Pollution levels are normally monitored using standard air filters, which cannot capture ultrafine particles or identify particles.