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Air in the London Underground is far more polluted than beside busy city roads

London is taking great — and appropriately aggressive — strides to help pedestrians and cyclists breathe a bit easier in areas with hazardous air quality caused by vehicle emissions. In the British capital alone, 9,000 premature deaths per year have been directly linked to air pollution alongside a range of serious related health woes, a situated deemed by many officials as bona fide public health emergency.

In response, Mayor Sadiq Kahn has, among other things, established the United Kingdom’s first ultra-low vehicle emission zone in the congested heart of London. (Kicking off in April, the super-strict emissions standards would impose hefty charges on certain motorists and has, unsurprisingly, generated considerable uproar.)

What’s more, Kahn just unveiled the Breathe London initiative in which roaming Google Street View cars are equipped with air quality sensors. The hope is that the sensors will help to better identify air pollution hotspots than fixed monitors. “After today we’re going to have the largest and most comprehensive air quality lenses and monitors of any city in the world,” Kahn tells BBC London. “And it’s really important for us to know what’s going on — how bad is the nitrogen dioxide, the nitrogen oxide, the particulate matter — so we can then take action to clean up the air.”

While all this attention to street-level air pollution is very much needed, a new report shows that the air quality beneath the streets within the vast London Underground subway system isn’t all that much better. In fact, commuters are confronted with even filthier air — albeit a different kind of filthy — while riding the Tube than they are walking down some of London’s most traffic-riddled streets.

The new Transport for London (TfL)-commissioned report from the Committee on the Medical Effects on Air Pollutants (COMEAP) found that at some Tube stations, the concentration of airborne particulate matter was up to 30 times higher than around traffic-clogged London roadways. Furthermore, Tube riders are exposed to the same concentration of particulates in just one hour of subterranean commuting than if they walked for an entire day alongside London’s busiest streets. This is particularly true of the Underground’s deepest lines.

Elaborates the Guardian:

Tests found that the Northern Line had the highest concentration of PM2.5 (tiny particles linked to health problems) with the air on platforms at Hampstead station — the deepest on the tube network at 60 metres (200 feet) below ground level — recording an average 492 micrograms per cubic metre (μg/m3) of air, compared with an annual average of 16 μg/m3 from a roadside monitoring site in the capital.

Tube stations that run closer to street level or that are near open-air stations, common the further you travel out into the ‘burbs, experienced significantly lower concentrations of PM2.5.

Commuter walking on platform, London Underground A new report finds that the deeper you go in the London Underground, the worse the air quality gets. This is particularly true of the Northern (black) Line, which is particularly old and particularly busy. (Photo: Oli Scarff/Getty Images)

Older, deeper and sootier

The reason why the air quality is so poor within the 11-line London Underground when compared to other subways is relatively straightforward: the Tube is deeper and more poorly ventilated than its peers. Established in 1863, the Underground is also the world’s oldest metro system, and is currently the world’s 11th busiest with roughly 1.4 billion annual passengers, trailing the Paris Metro, the Mexico City Metro, the Hong Kong MTR and the New York City Subway.

What’s not clear is exactly how potentially dangerous the air in the Underground is for commuters. No alarm bells, at this point anyway, have been sounded. COMEAP is, however, urging TfL to further investigate: “Given that there is strong evidence that both long- and short-term exposure to particle pollutants in ambient air are harmful to health, it is likely that there is some health risk,” reads the report, noting that the particulates circulating around Tube platforms are heavier and more metal-based than the carbon-based pollutants that the city are trying to minimize above ground.

In the meantime, COMEAP is urging Londoners to continue to use the Tube as they normally would given that the jury is still out when it comes to conclusive evidence regarding how toxic the air within the Underground actually is.

“We’ve got all this information about the health impacts of the particles above the surface,” explains COMEAP chair Frank Kelly. “Below ground, we know we have a higher mass but of a different type — we don’t yet have the research into the level of the toxicity, and hence the heath risk. He adds: “You’re down there for a short period of time — passengers should just use the Tube as usual until we have better understanding of the risks.”

The Guardian notes that the TfL will continue to monitor air quality and test dust samples for toxicity. “Although the report emphasises further monitoring and research is needed, it is an important contribution to the study of health effects associated with dust exposure,” says Peter McNaught, director of asset operations at TfL. “We are committed to maintaining the cleanest air possible for our staff and customers when using the Tube.”

train pulls into Knightsbridge Tube station London transport officials are encouraging commuters to ride the Tube as normal despite the release of an eyebrow-rising report detailing air quality levels in the world’s oldest metro system. (Photo: Oli Scarff/Getty Images)

The mechanical friction factor

While the Evening Standard notes that the findings from COMPEAP are gleaned from the first official report to consider air pollution in the Underground in nearly 20 years, other more recent studies have delved into the matter.

A 2017 study conducted by the University of Surrey found that particulate levels were found to be eight times as high as those on city streets. In reaction to this, Feargus O’Sullivan of CityLab wrote: “The pollution caused by motor vehicles may be a menace to health, but when it comes to exposure and potential health effects, it seems you’re worse off underground.”

O’Sullivan does, however, make an important distinction. The air-polluting dust that plagues the Tube does not, unlike the air on the streets, originate from motor exhaust. London’s subway trains are, after all, electrified. Rather, it comes from “mechanical abrasion between rails, wheels and brakes” — that is, large particles are dispersed into the air due to friction between the tracks and the trains themselves. (Exhaust fumes that have been blown down from street level, where they become trapped and continually re-circulate, are also present at Tube stations.)

“It’s not the case, therefore, that electric subway trains are especially noxious sources of pollution. It’s simply that the enclosed, poorly ventilated subterranean environment means that it’s harder to disperse the concentrations of particulate matter,” elaborates O’Sullivan, noting that while particulate matter caused by mechanical friction must be present to some degree in all subway systems, the Tube’s age, depth, lack of ventilation and the narrow design of its tunnels leads to alarmingly higher concentrations of PM2.5.

For those worried about the potential health risks of riding the Tube, Chris Large, a senior partner at Global Action Plan, suggests that commuters simply get off a stop earlier than they normally would and hoof it the rest of the way, preferably, of course, on quieter and less congested streets. He also suggests that plenty of exercise, a healthy diet and avoidance of nicotine products can help minimize any potential health impacts on the human body caused by air pollution. He does not recommend Tube riders start wearing protective face masks in light of the recent report.

“We closely monitor dust levels on the Tube and, through a wide range of measures, ensure that particle levels are well within Health & Safety Executive guidelines,” assures McNaught. “We have already enhanced our sampling regime by including tests for additional metals and we will continue to investigate ways we can keep dust and particles to an absolute minimum.”

Matt Hickman ( @mattyhick ) writes about design, architecture and the intersection between the natural world and the built environment.

Air in the London Underground is far more polluted than beside busy city roads

A new report commissioned by Transport for London brings bad news to those who thought they were avoiding air pollution by taking the Tube.