The Filthy Truth About Subway Air

Levels of particulate matter in the world’s metro systems far exceed recommended limits—and the deeper you go, the worse it gets.
train station in New York
Photograph: Gary Hershorn/Getty Images

There are people in this world who, out of sheer curiosity, carry around scientific instruments so they can measure levels of potentially harmful airborne particulates—tiny clumps of matter that may be breathed in. “We’re sort of air pollution nerds, right?” says Terry Gordon, an environmental health scientist at New York University.

Some years ago, a colleague of his got a shockingly high reading on a particulate monitor when he entered a subway station in New York. “He thought it was broken,” recalls Gordon. But it wasn’t. That reading inspired a much-discussed study, published in 2021, on particulate concentrations in various subway stations in New York, New Jersey, Philadelphia, and other locations in the northeastern US.

It’s just one of numerous recent papers that have documented particulate pollution in subway and metro systems around the world—reflecting a growing concern that city commuting could carry a health risk. Earlier this month, prosecutors in Paris opened a criminal investigation over allegations that air pollution in the capital’s metro was endangering people’s lives. Not only that, the operator of the underground railway system there, RATP, has been accused of deliberately underreporting pollution levels—which it denies.

The fact that particulates are present in metro systems, often at concentrations many times those found at street level, is undeniable. The rubbing of metal wheels on tracks, or brakes on wheels, shears off tiny metal particles that get kicked up into the air as trains move. The question is how the dusty tunnels of the world’s metro systems compare on this point—and whether science reveals any genuine health risks for people who travel or work in these environments. Long-term exposure to particulate matter is known to be linked to a variety of heart and lung problems, as well as premature death.

Gordon, though, was surprised to hear about the legal case in Paris. “Paris is nothing compared to London,” he says. And no metro stations anywhere, he adds, are as particulate-prone as those in and around New York—at least according to his research.

His study of the northeastern US contains some of the highest air pollution measurements ever recorded for subway stations: around 1,700 micrograms (μg) of particulate matter measuring 2.5 microns in diameter or less—what’s known as PM2.5—per cubic meter (m3) of air, for example, at two stations, one in Manhattan and one in New Jersey. The reading represents average levels during two relatively short, hour-long windows. PM2.5 is considered particularly dangerous, since these very fine particles can travel deep into people’s lungs and possibly also their bloodstreams. The World Health Organization advises that average exposure to PM2.5 over a day not exceed 15 μg/m3.

The Port Authority Trans-Hudson (PATH)—the transit system those stations in Manhattan and New Jersey are part of—says that the air quality at the stations, when independently evaluated, was within guidelines set by the US Occupational Safety and Health Administration. “We will continue to take the appropriate measures to ensure that PATH system air quality remains within regulatory guidelines,” PATH says.

separate study, published last year, detailed PM2.5 levels inside New York City subway trains along the full length of nine lines, as well as on 341 platforms at 287 stations. The average concentrations on platforms ranged from around 120 to 300 μg/m3, though they varied and some had readings as high as roughly 600 μg/m3.

A spokesperson for New York’s Metropolitan Transportation Authority says: “We have conducted previous air quality testing on subway trains operating in our system and found no health risks,” adding that the safety of customers and employees is the Authority’s highest priority.

In contrast, an official study on concentrations in the Paris metro published last year found PM2.5 concentrations of around 55 μg/m3—much lower, but still roughly three times outdoor levels in Paris. This level had gradually decreased between 2010 and 2018, from 70 μg/m3. And a study in Stockholm found that weekday PM2.5 levels between 7 am and 7 pm averaged 260 μg/m3, while separate research in Seoul found PM2.5 levels of around 129 μg/m3 on metro platforms.

It’s crucial to note that these studies differ in multiple ways. The scientists used different monitors, took measurements at different times of day, and calculated averages for time periods of varying length—so it’s not possible to directly compare these results. Not only that, seasonal differences can affect readings, as noted in one study about particulate concentrations in Shanghai’s metro system. The researchers found much higher PM readings during the fall and winter, on average, which was likely due to the winter smog that affects much of China.

But these studies certainly provide a sense of the elevated particulate levels in underground rail networks on the whole. And older, deeper metro stations—including some in New York, for instance—tend to have slightly higher overall readings.

Among the most-studied metro systems in the world is the London Underground, known affectionately as the Tube. It is also the world’s oldest metros—part of it dates back to 1863. Hassan Aftab Sheikh, a PhD candidate at the University of Cambridge, has recorded particulate levels at multiple stations and notes that the deepest, oldest lines tend to be the most polluted. Average particulate background levels hover around 300-400 μg/m3, but this can spike to around 1,000 μg/m3 when a train rushes into a platform, blowing dust up from the ground and around the rails, he notes. Such heightened levels soon lower again.

In a study published last year with colleagues, he described the relatively high levels of iron oxide in particulates on the Tube. The chemical composition of the London material suggests it has likely been there for many years. “It’s not the usual iron oxide,” says Sheikh. “Because it’s been in the system so long, it’s been further oxidized.”

This contrasts somewhat with the Paris metro system, where multiple lines have trains with rubber wheels. Sheikh also notes that Paris’s network is younger and suggests it is better ventilated than London’s.

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.