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A new study discovers just how bad the Underground’s air can be.

If you find yourself wheezing as you wander down a fume-filled London street, just remember that things could be worse. You could be on the city’s subway system.

According to a new study from the University of Surrey, London’s Tube riders experience worse air than those who travel by car. In the worst cases, particulate levels in the subway system can be as much as eight times higher than those experienced by drivers. 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.

This might be hard to swallow. London’s Tube trains, first electrified in the 19th century, are hardly thought of as the worst polluters of the transit world. It turns out that the particulates in the system don’t come from motors or exhaust (of which there is none) but from “mechanical abrasion between rails, wheels and brakes.” In other words, as the wheels rub the rails or on brake pads, they discharge some particulates into the air.

These emissions are common to most forms of wheeled transit. Studies have found, for example, that in urban areas, brake wear in motor vehicles contributes up to 21 percent of all particulate emissions. Remove vehicle exhaust from the equation, and the emission share for brake wear rises to 55 percent. 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.

These particulate emissions must be common across subway systems, but the design of London’s, with narrow tunnels and no air conditioning, could be making things worse. When your train pulls out of a London Underground station, you see the pipe-lined tunnel walls barely a foot or two beyond the window. It’s no wonder that particulates reach high concentrations, given the small space into which they are dispersed by train friction.

Those particulates could still stay in the tunnel even if trains had better filtered air. Significantly, on the lines where windows can be opened at the ends of the cars, such as the Northern and Victoria Lines, particulate levels inside carriages are higher. You might wonder why anyone would be foolish enough to open the window in a tight, sooty-looking tunnel anyway: Sometimes that’s the only way to get a cooling breeze flowing in summer. On lines without openable windows, the ventilation systems are so feeble that riders broil in hot weather.

Does this mean that the city’s focus on cutting car emissions is misplaced? Not in the least. Car passengers get relatively less exposure not because the mode is cleaner or their share of general emissions small, but because their air conditioners often keep them from needing to drive with the windows down. The study found that Londoners traveling by bus were also exposed to high particulate levels, up to 23 percent above what was found in cars. This is not because buses create more emissions, but because opening windows and doors allows more polluted air inside. If bus passengers are suffering health risks, cars are still the ones responsible for the pollutants damaging their health. (Even more so now that London’s bus fleet is steadily being electrified.)

There are still two important lessons to be learned from the study. First, as we steadily improve our standards for exhaust emissions, the share of pollution caused by mechanical abrasion will only rise. We’re going to need to focus on ways to reduce these emissions more in the future if we want to have genuinely clean transit.

Second—and it’s hard not to scream this one every single day—London’s public transit needs air conditioning. It needs it badly. The unbearably hot summer conditions on London public transit are already an annual scandal. Now it seems that the lack of temperature and air control isn’t just reducing Londoners to sweaty messes, it could also be shortening their lives. This city’s inhabitants—indeed Britons in general—may have perfected grumbling as a form of Olympic sport. Now, it’s time to stop just complaining and start pushing for real action.

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