London's busy tube network is riddled with toxic air particles that could pose a real threat to passengers' health, scientists have warned.
Specks of metal thrown up by braking trains enter people's bloodstream after being inhaled into the lungs, new research shows.
They come from the grinding of wheels and rails - and have been linked to cancer, dementia and cardiovascular disease.
The Tube handles up to five million passenger journeys every day. At peak times, there are more than 543 trains travel around the capital.
Some are 20,000 times thinner than a human hair - so small that the scale of the problem is likely to have been underestimated.
The first analysis of its kind identified high levels of an iron oxide called maghemite - a mineral that only develops over time.
Results suggest the fragments are suspended for long periods due to poor ventilation throughout the Underground - particularly on station platforms.
Lead author Hassan Sheikh, a final-year student, said: "The abundance of these very fine particles was surprising.
"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."
The Cambridge team used magnetism to study dust samples from ticket halls, platforms, and operator cabins.
They measured as little as five nanometres in diameter - too tiny to be captured by typical techniques.
They called for periodic removal of dust from tunnels and magnetic monitoring to improve air quality.
London's underground system carries five million passengers per day. Pollution levels are higher than those on the streets - beyond experts' recommended limits.
Senior author Professor Richard Harrison said: "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.
"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."
The study in Scientific Reports is groundbreaking, according to the scientists. Standard air filters cannot capture ultrafine particles - or discover what they contain.
Mr Sheikh said: "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.
"The Underground is a well-defined micro-environment, so it's an ideal place to do this type of study."
Transport for London provided 39 dust samples from the Piccadilly, Northern, Central, Bakerloo, Victoria, Northern, District and Jubilee lines.
They were collected in 2019 and 2021. Major stations included King’s Cross St Pancras, Paddington and Oxford Circus.
The researchers used magnetic fingerprinting, 3D imaging and nanoscale microscopy to characterise their structure, size, shape, composition and properties.
Earlier studies have shown 50 percent of pollution particles in the Underground are iron-rich.
The much closer examination found they range in diameter from five to 500 nanometres - with an average of 10.
They also joined together, forming larger clusters between 100 and 2,000 nanometres wide.
The researchers said further investigation was needed to discover if maghemite particles pose a direct health risk. Characterisation methods could be useful in future studies.
Mr Sheikh said: "If you are 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."
Due to poor ventilation, iron-rich dust can be resuspended in the air when trains arrive at platforms. This makes the air quality on platforms worse than in ticket halls or in operator cabins, he explained.
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.
Prof Harrison said: "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."
