Air pollution on the London Underground: The effects and what is being done

DateJan 31, 2020
AuthorGeoff Eyre-Walker

Air pollution in the London Underground

Air pollution has experienced an upturn in topicality recently, due to attention provoked by the introduction of the Ultra-Low Emission Zone policy by the London Mayoral administration and that the Supreme Court ruling three times last decade (2015, 2016, 2018) that the UK Government is in breach of EU Directive 2008/50/EC on ambient air quality and cleaner air for Europe1,2,3,4,5,6,7,8. The Extinction Rebellion ‘The Air We Grieve’ protest this December has also garnered publicity9.

Publications on the effects on health regarding air pollution have drawn further scrutiny to the subject, and recent studies have now queried the severity of air pollution in rapid transit systems such as the London Underground. A Transport for London (TfL) commissioned study by the Committee On The Medical Effects Of Air Pollutants (COMEAP) stated that previous studies in other subways around the world have shown that gaseous pollutants like nitrogen dioxide (NO2) and volatile organic compounds (VOCs) tend to be in similar concentrations to local ambient environments but also reported that particulate pollution can be up to 30 times higher in tube stations than on busy roads above ground in London10. The particulate pollution is that of less than 2.5 microns (PM2.5) and is considered to be particularly harmful to health due to it being capable of entering the blood system due to its extremely small diameter (approximately 30 times smaller than a human hair)11,12.

To compound this, the Underground has been monitored and found to have higher particulate matter concentrations in comparison to other subway networks in other cities around the world, possibly due to its comparatively deep tunnels and poor ventilation related to its old infrastructure10. The Underground contains the highest exposure levels of PM2.5 in comparison to other modes of transport such as cycling, bus and driving13. On average 4.8 million journeys are taken on the London Underground every 24 hours, although it is not stated how frequently the average individual uses the system nor the duration of the journeys they make10. However, it is apparent that many people are exposed to the networks air pollution.

The stations and lines most severely affected

Concentrations of particulate matter has been shown to be considerably higher in the London Underground in comparison to the wider UK rail network. Smith et al. (2020) indicate that the primary PM2.5 on the underground is iron oxide (47% of PM2.5 composition)13. Particulates in the Underground come from the mechanical wear of train components, brake blocks, rail wear, and also textile fibres10. Additionally, there is pollution that is not just from within the Underground, but also from ambient air which enters via station and tunnel entrances/exits10. The air turbulence created by the movement of trains suspends these particles in the confined air of the Underground and therefore exposes staff and passengers to inhalation of the particulates10,13.

Particulates under 2.5 microns are more abundant at deeper stations with greater distance from above-ground sections of track. The Victoria line has been found by a recent study to have the highest median concentration of PM2.5 from the London Underground lines, with the Northern line also having comparatively high concentrations13. Conversely, District and Docklands Light Railway were recorded to have the lowest and second lowest median concentrations respectively13. It is perhaps worth noting that the Victoria and Northern lines are two relatively enclosed lines while Dockland Light Railway and District lines have large sections out of tunnel13. Vauxhall and Stockwell were the top two in the mean PM2.5 concentrations at stations results but when PM2.5 concentrations were weighted with passenger populations then Oxford Circus and Waterloo were the most severely affected respectively13. This suggests that passenger exposure levels are relatively high at Oxford Circus and Waterloo, which are the second and third most used stations respectively, and that these stations maybe of primary concern relating to human health.

Despite the elevated levels of PM2.5 air pollution in the London Underground in comparison to London ambient air, the aforementioned COMEAP study concluded that the risk to the public is not serious or substantial, concurring with a 2003 Institute of Occupational Medicine (IOM) report, but that TfL should work to reduce PM on the Underground10. In specific relation to workers on the network, monitoring by TfL has shown that the long-term (8 hours) dust exposure levels of staff and operators are four times lower than the Health and Safety Executive (HSE) workplace exposure limits of 4mg/m3 and therefore concentrations are well within these guidelines, although the toxicity of the particulate matter composition in the Underground is relatively unknown10,14.

Transport for London’s action plan

To improve air quality, TfL in 2017, launched a new air quality action plan for the Underground15. This action plan outlined operational changes in the form of more frequent and widespread monitoring and cleaning. The improved air quality monitoring programme within the Underground network will comprise monitoring taking place at more than 12 stations, and will involve staff and simulated passenger journeys to monitor how dust concentrations change at various times and locations. Testing apparatus installed will take much larger samples than previously to inform London Underground's strategy for minimising dust.

Technological improvements are also included in the action plan. One of the policies in place is for all new trains to have regenerative/rheostatic braking technology, which will reduce friction between the rails and the wheels during braking and therefore reduce particulate matter produced15. To reduce passenger and staff exposure to air pollution, sealed windows on trains and the expansion of platform safety doors are proposed. Additionally, TfL intend to increase the use of a replacement fuel for diesel-powered generators that are used for maintenance and construction. This fuel is expected to significantly reduce carbon monoxide, nitric oxide and other nitrogen oxide emissions. Furthermore, the authority wants to use more dust suppressant to reduce the suspension of particulates in the air. The COMEAP statement has identified that an expanded cleaning programme was launched in 2017 with the use of specialist cleaning equipment, and that it was found that cleaning entire sections of tunnel was found to reduce respirable dust concentrations (44% reduction) on station platforms than cleaning only stations and platform approaches (8% reduction) and thus indicating initial progress from Transport for London’s action plan10.

Groundsure Energy and Transportation report includes full route and operational times for the London Underground and Docklands Light Railway alongside our proprietary modelled depth data. Stations and proposed tube extensions are included. Click here to find out more about this report.


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