London Underground air is more polluted than we thought

The atmosphere below ground on the planet’s oldest metro network contains ultra fine metallic particles small enough to enter the bloodstream, and so tiny they could easily have been missed — or underestimated — in previous air quality studies. 

Researchers at the University of Cambridge carried out a pollution analysis of the system using a very different approach, applying magnetism to dust samples from ticket halls, platforms and operator cabins. High levels of an iron oxide, maghemite, were identified, which takes a long time to form through oxidisation, suggesting particles had been suspended in the setting for extended periods of time due to poor ventilation. This was particularly apparent in platform samples.

Some particles were as small as five nanometres in diameter, increasing the likelihood of inhalation and absorption into the bloodstream. However, at this size they are easily missed in typical pollution monitoring processes. Notably, though, the public health risk from these is not known at the time of writing. Nevertheless, previous investigations have pointed to direct links between Underground air quality and medical conditions, but have failed to assess particle size and type in real detail. 

Proposed solutions to the problem include periodic dust removal from stations, and magnetic monitoring of pollution levels, which scientists believe could tackle the issue. This is the latest evidence to show that air quality on the London Underground is worse that the city itself, and pollution levels far exceed those stipulated by the World Health Organisation (WHO), with the grinding together of wheels, brakes and tracks leading to a significant amount of particulate matter being released into the atmosphere.

‘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 Professor Richard Harrison from the University of Cambridge’s Department of Earth Sciences, the paper’s senior author. ‘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 abundance of these very fine particles was surprising,’ added Hassan Sheikh, lead author from Cambridge’s Department of Earth Sciences. ‘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… 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.’

Last month, the University of Surrey’s Global Centre for Clear Air Research (GCARE) published recommendations for improving the London Underground’s air quality. Find out what they were here.

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