Up to five million people use the tube every day. What are they breathing in?
Air quality on the London Underground is a hot topic in the press. "The Dirtiest Place in the City," reckons the Financial Times. "30 times worse than congested roads above," concludes the Independent. "More toxic than a BRICK FACTORY," shouts the Daily Mail. A few days ago I read an article which opens with “Have you ever touched the air vents behind the seats on the Central Line?”. Dusty would be an understatement.
But it's not just the mainstream media who are obsessed with the tube's air quality. Scientists have also taken a keen interest in the network's pollution levels. Often, their findings are the basis for the media reports, albeit in an exaggerated form. In this article, we'll take a brief look at the evidence to see just how dirty the air on the tube really is.
What are the dust particles on the tube?
Of course, the tube network has its fair share of regular dust, from passengers' clothing fibres, skin cells and hair. But it also contains particles you wouldn't find in any great amounts at home. As trains brake, friction between the wheels and rails, and the brakes and the wheels, produces iron-rich dust. This is then suspended in the air and pushed through the tunnels of the underground network.
The concern is with particles under 2.5 micrometers in diameter (also called PM2.5), which is about a 20th the width of a human hair. These can readily get into the lungs and bloodstream. Such particles are present in trains and at the stations, but can also be found above-ground in ambient air, where increased concentrations are usually measured beside busy roads.
Are these dust particles dangerous?
A University of Cambridge study, which sparked much media interest in this topic, found that London's underground dust contained high levels of iron oxide called maghemite. These particles can enter the lungs and the bloodstream. PM2.5 particles have been linked to adverse health affects in other environments, but no study has yet found a direct link between the tube's environment and a measurable risk to health. What the Cambridge study confirmed, is that maghemite particles linger in the underground network for long periods due to a lack of sufficient ventilation. But, it is not known if maghemite particles are more dangerous than roadside dust.
What other studies have been done?
Not all news headlines are generated by proper scientific study. Some news organisations conducted their own dust measurement “studies” at underground stations, taking particle counters to measure concentrations of PM2.5. From a scientific perspective, this is problematic. You can't just wander onto a platform, take a reading and draw conclusions. Tests must be rigorously controlled. For example, such reports rarely disclose which particle counter is used, and with what accuracy. More importantly, particle counter accuracy depends on the way it was calibrated, and this can significantly change the readout. Maybe *insert tube station you love to hate* is dustier than a brick factory in your one-off measurement… but that's not in any way conclusive.
Where are you most exposed?
Not all stations and lines are equal when it comes to dust particles. The Victoria line has come up in peer-reviewed research articles as the dustiest, followed by the Northern and Bakerloo lines. This matches what we might expect, when we consider that the Victoria line is almost entirely underground, and therefore among the least ventilated lines.
For similar reasons, the District and Hammersmith and City are the least dusty overall. These lines are just below the surface, and are often open to the skies. Ventilation rates are naturally much better.
How can dust be reduced?
Some dust monitoring reports are available on the TfL website, which also states they are funding more research and rolling out new tunnel cleaning regimes. TfL has reached out to various research teams to tackle the air quality issues, and the research is ongoing. Enhanced cleaning of the tunnels does increase the air quality, and two tunnel cleaning teams are operating across the London underground network.
Scientific research also shows that installing platform screen doors (such as those at the new Elizabeth Line stations and on the newer section of Jubilee line) and improving ventilation rates can reduce dust concentrations, especially at deeper stations.
Would wearing a face mask help?
A number of studies exist on how masks can help with reducing personal exposure to dust. Surgical masks will reduce overall exposure to some extent but N95 (FFP2) masks are most effective against air pollution particles.
So, is the tube really "30 times worse than congested roads above?"
Tube air is dusty, and there is no doubt that chronic exposure to high PM2.5 is harmful. The key word here, though, is exposure, which is related to the amount of time we are in the presence of harmful particles. Using the tube is often faster than driving, cycling or walking, hence exposure time per journey is relatively low.
If we focus solely on the concentrations of PM2.5 then, yes, you might experience higher levels on some underground lines and stations than on the roads above. But this is only part of the story. Those who drive, cycle or walk have to contend with additional vehicle pollutants such as nitrous oxide. Plus, being stuck in traffic, directly behind another car's exhaust would likely increase your exposure.
In short, there is no evidence that choosing to travel by London Underground is more harmful overall than using other transport options.
