Potentially toxic air pollution hazard identified on London Underground

Air pollution on the London Underground includes unexpectedly high levels of extremely fine and potentially toxic iron particles small enough to end up in the human bloodstream, according to a study by the University of Cambridge.

The scientists employed new magnetic detection technology to analyse samples from the ticket halls, platforms and trains of the UK capital’s metro system, often referred to as the Tube. They found high concentrations of a magnetic form of iron oxide called maghemite in submicroscopic particles down to 5 nanometres (billionths of a metre) in size — too small for normal pollution monitoring systems to detect.

“The abundance of these very fine particles was surprising,” said Hassan Sheikh, lead author of a paper releasing the results in the journal Scientific Reports.

Although the study did not look at the health effects of maghemite particles found in the Tube, other research suggested that they might be harmful, said Richard Harrison, senior scientist on the project.

Exposure to ultrafine particulate matter in the air has been linked to a range of health risks, from brain damage to lung and heart disease, but no medical study has looked specifically at their effects in underground railways.

“The abundance of ultrafine particles identified [in the Tube] could have particularly adverse health impacts as their smaller size makes it possible to pass from lungs to the bloodstream,” the researchers said.

The iron-containing particles are produced by contact between trains’ wheels and brakes and the rails. The study found that most of the particles were well oxidised, suggesting that they had been around for some time inside tunnels and along the train tracks before being blown into the air by moving trains.

The researchers said the exposure of staff and passengers to iron-rich dust could be reduced by washing the tracks and tunnel walls or even using magnetic filters in ventilation systems, though this would require new equipment. Installing more screen doors between platforms and trains, such as those on the Jubilee Line, would help too.

The study illustrated the increasing attention being paid to reducing air pollution from “non-combustion sources” now that less fossil fuel was being burnt in vehicle engines, heating systems and factories, said Harrison. “The Underground is an excellent place to study these non-exhaust pollutants,” he added.

London Underground provided samples for the research from multiple locations across the Tube network. “They contacted me this week to set up a meeting to discuss our findings, including the health effects and mitigation,” said Harrison.

Transport for London, which operates the Tube, said it was reviewing the results “to ensure it fully understands them and how to reduce the levels further”.

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

“We are going further and have developed a number of innovative new cleaning regimes,” she added. “This includes the use of industrial backpack dust cleaners, which are one part of our multimillion pound Tube cleaning programme.”