The Filthy Truth About Subway Air

A spokesperson for Transport for London, which operates the Tube, says safety is the body’s “top priority” and that staff have been working for years to reduce dust on the system. “This includes the use of industrial backpack dust [vacuum] cleaners, which are one part of our multimillion-pound Tube cleaning program,” she adds.

The big unknown is whether all of this particulate matter is actually causing health problems for people. Millions of commuters use metro systems, in many cases for multiple hours a day, five days a week, for years on end. And thousands of transport workers spend even longer in the tunnels. But there are no widespread signs of severe or acute health problems among these populations, even if pollution levels in subways exceed recommended limits. Could there be more subtle, chronic effects, however—impacts on lung, brain, or heart function?

“It’s certainly not something we can rule out,” says Matthew Loxham, an air pollution toxicologist at the University of Southampton. “It’s just on the basis of current evidence there doesn’t seem to be a clear and obvious risk to health, at least in the groups that have been looked at.” He coauthored a review of evidence on the health risks, published in 2019, that came to this conclusion. He is not aware of any new evidence that has really changed the picture since.

The fact that metals are often prevalent in metro system particulates, especially iron, is potentially a concern, he adds, since metals are generally considered toxic. Particulate components are also sometimes soluble, meaning that the material can dissolve in people’s lungs and make its way to their cells.

“That’s bad, but at the same time it’s possible that those soluble components are easier to get rid of than a solid particle,” says Loxham, indicating that some lumps of matter could simply become lodged in people’s lungs, which may or may not cause health issues in the future.

He adds that high levels of iron can be a problem—but then again our bodies deal with iron all the time; it’s a key part of hemoglobin in our blood, and so we have mechanisms of regulating it. It’s just not possible to be sure about the significance of any of these processes with regard to metro particulates at this point, he stresses. And tying occasional exposure to high levels of PM in metro systems to a specific negative health outcome is very difficult—it would be wrong to jump to conclusions.

People concerned about pollutants in underground railways could try wearing well-fitted filtering masks. Where possible, metro operators might consider installing screen doors along platforms to lower the amount of dust blown toward commuters by arriving trains. But even this approach has caveats. A study on the use of such screens in Seoul found that it tended to increase PM exposure inside trains even though it sometimes reduced exposure on platforms.

It’s hard to say if there really is a health risk, says Gordon. But he emphasizes the need for further study, including long-term research that tracks the health of transportation workers over many years—even into their retirement.