Travellers on London’s Tube network could end up with metallic particles from the polluted air in their bloodstream, a study has found. 

The tiny metal substances hanging around ticket halls, platforms and train driver cabins have been analysed for the first time by a team of University of Cambridge researchers.

They found high levels of a type of iron oxide called maghemite, which they said suggests pollution particles are suspended for long periods due to poor ventilation, particularly on platforms.

Some of the particles have a diameter of just five nanometres, making them small enough to be inhaled and end up in passengers’ and workers’ bloodstreams, University of Cambridge researchers found.

About 3.5 million daily journeys are made on the London Underground on weekdays.

The samples were collected in 2019 and 2021 from locations including Oxford Circus, King’s Cross St Pancras and Paddington stations.

The researchers did not look at whether the metal particles pose a direct health risk, but said their methods could inform future studies.

Professor Richard Harrison, one of the senior authors on the study, said: “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.

“Our techniques give a much more refined picture of pollution in the Underground.

“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.”

Transport for London’s chief safety, health and environment officer Lilli Matson, said: “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.”

TfL had developed a number of “innovative” cleaning methods, she said, including the use of industrial backpack dust cleaners.