A spokesman for Transport for London, which operates the Tube, said safety was the body’s “top priority” and staff had worked for years to reduce dust on the system. “This includes the use of industrial backpack dust [vacuum] cleaners, who are part of our multi-million pound tube cleaning program,” she adds.
The big unknown is whether all of these particles actually cause health problems for people. Millions of commuters use subway systems, in many cases for several hours a day, five days a week, for years. And thousands of transport workers spend even more time in the tunnels. But there are no widespread signs of serious or acute health problems among these populations, even though 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 based on the current evidence that there doesn’t seem to be a clear and obvious health risk, at least in the groups that have been looked at.” He co-authored a review of the evidence on health risks, published in 2019, which came to this conclusion. He is not aware of any new evidence that has really changed the situation since.
The fact that metals are often present in particles in the subway system, particularly iron, is potentially concerning, he adds, because metals are generally considered toxic. The particulate components are also sometimes soluble, meaning the material can dissolve in people’s lungs and travel to their cells.
“It’s bad, but at the same time, it’s possible that these soluble components are easier to get rid of than a solid particle,” Loxham says, indicating that some pieces of material could just lodge in people’s lungs, which which may or may not cause health problems in the future.
He adds that high iron levels can be a problem, but again, our bodies deal with iron all the time; it is a key component of hemoglobin in our blood, and so we have regulatory mechanisms. It’s simply not possible to be sure how important any of these processes are with respect to subway particles at this point, he points out. And it’s very difficult to link occasional exposure to high levels of PM in subways to any specific negative health outcome – it would be wrong to jump to conclusions.
People concerned about pollutants in underground railroads could try wearing tight-fitting filter masks. Where possible, subway operators could consider installing screen doors along platforms to reduce the amount of dust blown towards commuters by arriving trains. But even this approach has caveats. A study of the use of such screens in Seoul found that they tended to increase PM exposure inside trains, although they sometimes reduced exposure on platforms.
It’s hard to say if there really is a health risk, Gordon says. But he points to the need for more in-depth studies, including long-term research that tracks the health of transport workers over many years, even into retirement.