The researchers carried out a new type of pollution analysis, using magnetism to study dust samples from Underground ticket halls, platforms and operator cabins.
Some of the particles are as small as five nanometres in diameter: small enough to be inhaled and end up in the bloodstream, but too small to be captured by typical methods of pollution monitoring. However, it is not clear whether these particles pose a health risk..
The team found that the samples contained high levels of a type of iron oxide called maghemite. Since it takes time for iron to oxidise into maghemite, the results suggest that pollution particles are suspended for long periods, due to poor ventilation throughout the Underground, particularly on station platforms.
Other studies have looked at overall pollution levels on the Underground and the associated health risks, but this is the first time that the size and type of particles have been analysed in detail. The researchers suggest that periodic removal of dust from Underground tunnels, as well as magnetic monitoring of pollution levels, could improve air quality throughout the network. Their results are reported in the journal Scientific Reports.