The soot lined walls, the hazy platforms, the unbelievably high PM2.5 concentrations - how has the London Underground reached this point?
The Metropolitan line was the first underground tube system in the world, opening in 1863. The age of the London Underground is a factor in why it is one of the most polluted tube systems globally, but age plays a small role in pollution differences between tube lines. To understand why, we must examine the sources of the pollution.
There are 5 key sources of air pollution in the London Underground (1):
Wear of the train (wearing of wheels and brakes)
Non-train sources (rail wear and rail grinding)
Station sources (escalators)
External sources (air being drawn into station via tunnels, station entrances and vent shafts)
All of these sources produce little specks of pollution that sit and recirculate at tube stations (which is the soot you can see). These sources have different chemical compositions but due to the prevalence of steel in the underground system, iron accounts for most of the PM2.5 (2).
Key factors influencing pollution levels between London Underground lines
Train model and surrounding equipment used
Passenger numbers and train frequency
Distance from above ground stations
Pollution created varies by line due to differences in train model and surrounding equipment. Faster (3) and heavier (train weight and passenger numbers) tubes (4) increase PM production while creating finer particles. As tubes arrive at the station, heavy braking produces particles and the speed of the tube kicks up older particles sitting on the track and platform. On older and poorly ventilated lines, there would likely be more of these particles present at the track.
The depth of the line is another factor, likely as deeper lines have less sufficient air flow causing particles to accumulate. However, research has shown that distance from an above ground station may be a more significant factor. The Victoria line is one of the worst for pollution, likely due to it not having any above ground stations and having a poor ventilation system, rather than simply being one of the deepest lines (1). Particles build up in the line and constantly recirculate within the tunnels and platforms.
So how do TFL deal with this? There is a cleaning programme to remove pollution but (REF) highlights it is complex and slow (5). Ventilation is mostly driven by the movement of trains inducing airflow which draws outdoor air into the station. In deeper lines, there are ventilation fans and shafts in the tunnels which extract air (6). Additionally there are emissions vacuums which capture a portion of aerosols (2). There is also the cleaning of the platforms, tube rails and trains themselves to remove dust (7).
The age and distance covered by the tube network make keeping it clean no easy feat. We are still at an early stage of understanding how the factors and influences in the two lists above impact each other. With a greater understanding, more ways to mitigate the pollution will likely come to light.
1) Smith, J. D., Barratt, B. M., Fuller, G. W., Kelly, F. J., Loxham, M., Nicolosi, E., Priestman, M., Tremper, A. H., & Green, D. C. (2020). PM2.5 on the London Underground. Environment international, 134, pg.105188. https://doi.org/10.1016/j.envint.2019.105188
2) Sheikh, H. A., Tung, P. Y., Ringe, E. (2022). Magnetic and microscopic investigation of airborne iron oxide nanoparticles in the London Underground. Sci Rep 12, pg.20298. https://doi.org/10.1038/s41598-022-24679-4
3) Abbasi, S., Olander, L., Larsson, C., Olofsson, U., Jansson, A., Sellgren, U. (2012). A field test study of airborne wear particles from a running regional train. Proceedings of the Institution of Mechanical Engineers, Part F: Journal of Rail and Rapid Transit. 226(1), pg.95–109. doi:10.1177/0954409711408774
4) Sundh, J., Olofsson, U., Olander, L., & Jansson, A. (2009). Wear rate testing in relation to airborne particles generated in a wheel-rail contact. Lubrication Science. 21(4), pp.135–150. https://doi.org/10.1002/ls.80
5) Kumar P, Zavala- Reyes JC, Kalaiarasan G, et al. (2023) Characteristics
of fine and ultrafine aerosols in the London underground. Sci Total
Environ. 858, pp.159315. doi:10.1016/j.scitotenv.2022.159315
6) TFL (2021). FOI request detail: Air refresh. https://tfl.gov.uk/corporate/transparency/freedom-of-information/foi-request-detail?referenceId=FOI-0219-2122#:~:text=The%20ventilation%20of%20the%20London,piston%20works%20in%20a%20cylinder.
7) Lydall, R. (2023). Cleaning the London Underground: the unseen army of modern-day, minimum wage Tube ‘fluffers’. Evening Standard. https://www.standard.co.uk/news/transport/cleaning-london-underground-tube-fluffers-tfl-air-pollution-b1079662.html#