How Bad is Air Pollution on the London Underground?

Recently, a listener to Nick Ferrari’s weekday breakfast show on LBC called in to express concerns that far from protecting Londoners from air pollution, the Mayor of London’s new ULEZ plans may be driving people into ‘awful’ air quality on the tube, citing several personal experiences with airborne particulates on the Underground. The host was inclined to agree, prompting the caller: "You know if you're riding it for a long time, it almost gets like a choking sensation, do you know what I mean?" 

It’s been estimated that there are about 4.8 million journeys made on any given day using the London Underground, so it would seem paramount that passengers are not being made excessively sick by this mode of travel. Worryingly, however, there is very little research on the topic, with the last study of any considerable size taking place in 1998 – which, to remind some of our readers, is almost a quarter of a century ago! Such a paucity of data came as quite the shock to researchers from the government’s Committee on the Medical Effects of Air Pollution (COMEAP) when they began their investigation into Tube pollution last year. So, this report, published in 2021, is not as comprehensive, extensive or conclusive as one might hope but taken together with all of the other snatches of information that we have on the topic, gives us at least the beginning of a picture. 

In particular, the COMEAP’s investigation looked at the presence of particularly dangerous PM2.5 - i.e. particulate matter measuring less than 2.5 micrometres in diameter, small enough to breach the lung barrier. It’s been correlated with a range of nasty illnesses, including lung cancers and heart diseases. The problem, the report found, was that despite new ventilation systems being installed during the national lockdowns of 2020, the engineers failed to account for the fact that this supposedly fresh air was coming from one of the most air-polluted cities in western Europe – indeed, it was found to have the highest health costs from air pollution of any European city. This is partly because of the acceptable limits that British regulators have established, which are now in excess of the World Health Organisation’s guidelines by 500%! And of the whole of England, four out of the five highest concentrations of PM2.5 were in London and the South-East.  

But where are these particulates coming from? Well, the main culprits are the material shed when carriages move along their tracks (quite a common occurrence on the Underground, you’ll find), when brake-blocks rubs against the wheels, and that which is produced by the electrical connections between collector plates and live rails. The researchers located these sources by working backwards from the metallic compound particulates that they found in the atmosphere: iron, steel, copper, barium (from brakes), carbon and molybdenum (found in lubricants on wheels). 

The report finds that the deepest and the oldest lines are the most polluted, citing the Northern line on both counts. Higher and newer lines like the Circle, District, Hammersmith and City, and Metropolitan lines are (relatively) cleaner. But no matter which line you take, you’re not that much better off. The COMEAP state categorically that the concentrations of PM2.5 on the Underground are many times greater than any other mode of transport. 

One of the lead researchers for the project, Dr. David Green of the Centre for Environment and Health at Imperial College London, summarises the results of the study when it comes to PM2.5 as follows: “[Travelling on the tube] for one hour every weekday for 48 weeks a year (assuming 4 week’s holiday) on the Victoria would increase your annual exposure to PM2.5 by 6.8μm/m3. This compares to 0.3μm/m3 at a background site in London, 2.6μm/m3 on an average London Underground line or 1.2μm/m3 in a car.” 

When it comes to gaseous pollutants, the other focus of the study, the underground is home to concentrations of ozone (O₃) and nitrous oxides (N₂O), both of which inflame and bring on respiratory conditions. These concentrations are, however, showing long-term decreases, according to a 2020 DEFRA report – although London is, again, the sick man of Europe when it comes to the erosion of public health caused by these pollutants. And in this case, the tube is relatively free of the stuff when compared to travelling by car. Think about it, says Green: “You’re sitting directly behind the exhaust of the car, so you have a higher exposure than cyclists riding along the road or the pedestrians walking past. […] So, while the car isn’t worse than the tube in the case of PM2.5, it is much worse for other pollutants like nitrous oxides.” 

If you’re regularly travelling into and around London, then, you’re caught between a rock and hard place. Regardless of whether you’re driving or catching the tube, you’re going to be exposed to relatively high levels of air pollution, just of different types. Judging by the evidence, then, Nick Ferrari’s listener made a decent point: the extended Ultra Low Emissions Zone should be combined with improvements in the Underground’s infrastructure. Perhaps, some of the money generated by the fines might be ring-fenced for exactly this purpose. Whatever the outcome, it might be best to consider cycling or walking around the city the next time you’re in London.