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.