Levels of particulate matter in the world’s metro systems far exceed recommended limits—and the deeper you go, the worse it gets.
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.
A 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.