Fine particulate matter (PM2.5) pollution is on average four times worse in the London Underground than on busy roads above ground, a new study has found.
Scientists from King’s College London undertook the research using a device placed inside a backpack, which measured PM2.5 on journeys on the Tube as well as walks around central London to compare the findings.
The results have been published in Environment International and found that PM2.5 in the London Underground (mean 88â€¯Î¼gâ€¯/m3, median 28â€¯Î¼gâ€¯/m3) was greater than at ambient background locations such as Hyde Park (mean 19â€¯Î¼g/â€¯m3, median 14â€¯Î¼g/â€¯mâˆ’3) and roadside environments such as Oxford Street (mean 22â€¯Î¼g/â€¯m3, median 14â€¯Î¼g/â€¯m3).
The scientists say PM levels in the London Underground are higher because of a combination of factors including the design of the network, which means dirty air is drawn in through tunnels and station entrances.
The piston effects of the trains themselves are big contributors, along with the wheels and brakes of the trains as they move through the tunnels.
The London Underground’s 426 escalators also add to the problem.
The most polluted route was the Victoria line, with a median level of 361â€¯Î¼gâ€¯/m3. A stretch of the line between Brixton and Pimlico peaked at a massive 885â€¯Î¼gâ€¯/m3.
The District Line had the lowest levels of PM2.5, with a median figure of 4Î¼gâ€¯/m3.
Researchers also took readings at stations as they waited for the trains, and said that District Line stations Vauxhall, Stockwell and Pimlico were the most polluted on the network.
Exposure to PM2.5 has been linked to a myriad of health impacts. However, the composition of PM2.5 found on the London Underground contains much more iron oxide than above ground PM2.5, where organic carbon is the largest contributor.
The King’s scientists say there is now a clear need for well-designed studies to better understand the health effects of underground exposure.
Transport for London (TfL) said the public should express caution at the findings.
Lilli Matson, TfLâ€™s chief safety, health & environment officer said: ‘We welcome this research and will continue to engage with academics conducting further research to gain a better understanding of the health risks associated with air on the Tube.
‘The particulates found Underground are very different to those found on the surface, consisting predominantly of iron oxide rather than traffic pollutants. Particulates found in air above ground are known to be carcinogens, whereas those on the Tube are not known to have that effect.
‘We spend around Â£60m every year cleaning our trains, stations and tunnel and are committed to maintaining the cleanest air possible for our staff and customers.’
There have been several studies around the world on PM concentrations in underground stations.
Average PM2.5 concentrations of up to 100â€¯Î¼g/â€¯m3 have been reported in subway systems in Helsinki, Los Angeles, New York, Mexico, Paris, Shanghai and Taipei.
Concentrations in excess of 100â€¯Î¼g/â€¯m3 have also been reported in the subway systems of many cities: Barcelona (125â€¯Î¼g/â€¯m3, Stockholm (258â€¯Î¼gâ€¯/m3, and Seoul (129â€¯Î¼g/m3)