Much of London Underground's pollution problem comes from the fact that many stations are deep and poorly ventilated. Toby Melville/Reuters

Pollution in some Underground stations is up to 30 times worse than what you’d find on the average London street, a new Transport for London study shows.

If you think London’s streets are polluted, wait until you go underground.

According to a new study commissioned by Transport for London, the air in parts of the city’s subway system is thick with harmful particulates, with some of London’s Tube stations registering levels of air up to 30 times worse than the average for a London street above ground.

In the worst cases of underground pollution, it’s possible that particulate pollution of this type is a feature of subway systems across the world. The study nonetheless makes two things clear. First, that London Underground’s particulate problem is notably worse than other city systems thanks to the network’s age and design. Second, that despite increasing awareness of the noxious effects of motor vehicle exhaust fumes, many harmful particulate emissions on the network do not come directly from fuel combustion.

So why are things so bad? Partly because much of the network is so deep and poorly ventilated—with many lines running far farther beneath the ground than equivalents in Paris, New York, or Berlin. As the network’s operation began as early as the 1860s, many stations were built without great thought to ventilation or concern about forms of pollution that, while harmful, are odorless and invisible.

Still, the picture isn’t universally bleak—there is, after all, considerable variance in particulate levels from station to station. Northwest London’s Hampstead Station is the worst, containing an average of 492 micrograms of PM 2.5 per cubic meter of air, compared with an annual average of 16 micrograms of PM 2.5 per cubic meter from a roadside monitoring site. The reason for this especially high level is easy enough to fathom: Hampstead is the deepest station below ground on the entire network, its platforms lying 192 feet below ground (and accessible only by elevator).

Not all stations are this bad. Further out in the suburbs, many London Tube lines go above ground, while in the center, the very oldest lines tend to be only just below the surface or partly open to the sky, making the tunnels far easier to ventilate.   

It still might come as a surprise that a system run entirely on electricity should deliver such poor air quality. In fact, many of the particulate emissions trapped below ground are not created by combustion at all. According to the study, some of the PM 2.5 in the system does indeed derive from exhaust fumes wafted underground. These concentrations then increase thanks to particulates emitted by wheel friction, brake pads, and even particles that come off clothing worn by passengers. The constant movement of trains ensures that these particulate concentrations are continually redistributed through the air.

These alarming discoveries might put you off going underground altogether. Indeed, we’ve known for a while that air on London’s tube trains is worse than that in private cars, where air conditioning acts as a filter. Now it turns out that London’s buses also have notably cleaner air. A Tube passenger on a crosstown journey, the study found, would likely experience particulate concentrations two-thirds higher than a bus passenger traveling the same route above ground—even when you take into account the far longer journey time by bus.

To its credit, TfL has already been trying to do something about the pollution. In 2017 the authority instigated a more thorough cleaning regime, using industrial vacuums and “magnetic wands” to clean the platforms and tunnels. It turns out that this cleaning of tunnels (rather than just platforms) is particularly vital to improving air quality. When an entire section of tunnel between two stations was cleaned, it reduced respirable dust by a significant 44 percent. When only platforms and the sections of tunnel nearest them were cleaned, respirable dust levels only dropped by 8 percent, with train movement possibly wafting particulates from the uncleaned sections back onto the platforms.

Given that the phenomenon was only investigated recently, it’s still too early to say how long the improvements gained by deep tunnel cleaning remain in effect. The findings are nonetheless likely to build pressure for a much longed-for (and much delayed) innovation that would make riding the Tube more pleasant as well as healthier: pumps that draw air away from platforms to ground level, which would clean the air and reduce the network’s sometimes painfully high summer temperatures.

The London Underground is still a work of wonder, circulating huge numbers of people across a massive area on trains that come at a frequency North American subway users can only dream of. And yet, the network’s poor air quality shows that there are disadvantages to being the first city to pioneer such a system.

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