At the end of last year, the Financial Times published an article which would unnerve many of the 4.8m morning commuters using  the tube: “London underground: The dirtiest place in the city”. The article went on to detail that in some sections of tunnel, Londoners are experiencing pollution levels of PM2.5  – 10x the World Health Organisation’s safe limit. But what does this actually mean, and is it as serious as people think? Firstly, what is PM2.5?

Figure 1: Courtesy of US EPA

PM2.5 stands for particulate matter that has a width of 2.5 microns which is equivalent to a twentieth of the diameter of a human hair as shown in Figure 1.  And what is particulate matter (PM)? Unlike nitrogen dioxide, particulate matter is not a gaseous pollutant and is instead made up of miniscule particles. Above ground these tiny particles can be caused by the wear of internal combustion engines, brake pads and tires. However, organic carbon is the largest constituent of above ground PM2.5 (35%). Underground, PM is a combination of dust, skin and clothing fibre, but it is principally the wear of train wheels and brakes, and rail grinding that result in PM2.5 meaning it is mostly made up of iron oxide (47%) (COMEAP, 2018). Are levels really that high? On the surface, London experiences an average PM2.5 of 13.3µg/m3. The London Underground, with its old tunnels that are deep, narrow, difficult to clean and poorly ventilated, experience levels much higher. For example, in a recent 2018 study, Hampstead station was found to be registering PM2.5 levels of 492µg/m3 (COMEAP, 2018). Out of all the London Underground  tube lines, the Victoria line fared worst of all, averaging readings of 381µg/m3, which is not surprising given its complete lack of natural ventilation. The other deep lines on the network also did badly compared to the sub-surface lines, as shown in Figure 2.

Figure 2: COMEAP, 2018

And is it actually bad to human health?  Yes. Due to its size PM2.5 can have negative effects on human health causing respiratory and cardiovascular issues such as aggravated asthma and lung cancer. There is convincing evidence available linking short-term exposure to PM with respiratory related mortality, and the risk increases as PM gets as small as 2.5µm (WHO, 2013). When inhaled, PM2.5 can travel further into the lungs, even crossing the lung boundary to enter the bloodstream and giving it access to the whole body including the brain (Fu et al, 2019).  Healthy adults are less likely to experience serious health ramifications, but  groups with pre-existing lung or heart disease, the elderly and the young are all the more susceptible to these issues (WHO, 2018). What does “safe” mean? The WHO safe limit (although no exposure to PM is technically safe (WHO, 2018)) is an annual mean of 10mg/m3 for PM2.5. This means that 10 out of the 11 tube lines are registering above what has been deemed as safe to human health. TfL are aware of the issue and have reassured us that the tube is “absolutely safe” given the limited time passengers spend underground, and that they are compliant with the UK’s Health and Safety Executive workplace exposure limits (HSE, 2018). So, I shouldn’t get the tube anymore? As TfL have pointed out, minimal exposure to these levels of PM2.5 is safe – but  it’s also not great. London’s public transport is a point of pride and has so many benefits when compared to other methods of travel such as personal cars.  People should continue to use it but also need to be aware of the potential health effects. If you are considering the alternatives, cycling in the city is becoming increasingly popular and is a great way to travel around and explore new neighbourhoods. In Lambeth, there are several cycle networks which extend out across the city, and Lambeth Council also offer a “Try Before You Bike” scheme and offer  mentoring to help you get over those initial road worries. We should never become complacent and should be constantly striving for better conditions for our natural and urban environments. If you’re concerned about these  issues then start a conversation about where you want to see change. You could join ‘Talk London’ and voice your opinion to the Mayor's Office; take your concerns directly to TfL via Twitter or join a local campaign group aimed at improving air quality in the area. If you’d like further information regarding a variety of environmental issues then feel free to get in touch.