Particulate pollution from vehicles and smokestacks is a familiar sight to city dwellers, in the form of that lacy black film of soot that settles on urban snowbanks after a blizzard. But a new study has found troublingly high levels of the very fine particulate pollution known as PM2.5 — named for the size of its particles, which penetrate deep into the lungs — in an unexpected place: inside dozens of subway stations throughout the Northeast. And one New York City stop, the Christopher Street PATH station, turns out to have the worst air by far, with particulate pollution on the platform registering 77 times the concentration found in aboveground air — a public-health experience that’s less like breathing in the usual city dinginess and more like inhaling wildfire smoke on your daily commute.
Researchers from NYU’s Grossman School of Medicine measured PM2.5 levels on the platforms of 70 underground stations in New York City, Boston, Philadelphia, and Washington, D.C., systems, as well as stations for the Long Island Rail Road and the PATH train. Only 13 New York City stations were analyzed as part of this study, but the department had previously conducted a much larger survey of MTA stations in 2014 with similar findings. This new body of research focused on reevaluating the MTA subway stations with the worst readings from the 2014 study while attempting to get comparable data on other systems. Two New York region systems registered highest for PM2.5, with the PATH stations at an average of 392 micrograms per cubic meter, and MTA subway stations coming in second at 251 micrograms per cubic meter. Only Philadelphia’s was relatively low, with only 39 micrograms per cubic meter — which still exceeds the EPA’s clean-air levels. The data was collected before the coronavirus pandemic, and researchers say they’re not sure how almost a year of reduced service might have changed the air quality.
This doesn’t mean people should avoid the subway, cautions David Lugilo, a doctoral student at NYU Grossman School of Medicine who served as lead author on the study. “I ride the subway every day. I think subways are a very good thing for cities because they reduce traffic on the road, which reduces greenhouse gases,” he says. But it’s important to know that there is a risk, especially to certain groups of subway riders, he notes. “The air pollution in these subway stations likely poses a significant health risk to individuals with preexisting respiratory health conditions such as asthma, for commuters and workers alike.” While no one debates the superior air quality inside subway cars — it’s refreshed 18 times per hour — the air on the platform hasn’t been as carefully evaluated, says Lugilo, which is where MTA representatives say they will look more closely. “We have conducted previous air-quality testing on subway trains operating in our system and found no health risks. However, we will thoroughly review this study, as the safety of customers and employees is always our highest priority,” said the MTA’s Tim Minton. He added that the system is piloting additional filtration technology for subway cars as the system prepares to move back to 24-hour service as the city reopens. For those who don’t spend a lot of time on the platform in general, the news is less alarming; the study says adverse health effects are not likely with 15 minutes or less of exposure, and average MTA wait times are far below that for rush-hour trains, when a majority of passengers are riding transit.
The fine-particle pollution in our subways is not the same stuff that comes out of automobile tailpipes. It’s largely created by the friction of steel wheels against steel rails, and a “black carbon” generated by the constant grinding of brakes. (Some subway systems have rubber wheels on concrete tracks, and they reduce but do not fully eliminate the problem.) In London, an investigation spurred by Mayor Sadiq Khan’s air-quality initiatives caused the Financial Times to declare the Tube the “dirtiest place in the city” in 2019. Many of the world’s busiest transit systems have made major changes to address the issue over the last decade: London has undertaken an aggressive cleaning regime, Barcelona is experimenting with new station designs, and Seoul has installed over 800 air-quality monitors throughout its subway system to share data publicly with passengers. A 2019 study that surveyed various mitigation efforts like fans and filters deemed platform screen doors the most effective solution. These doors, which completely enclose the platform and open only when the train has completely stopped at the station, would protect passengers from airborne pollution and also prevent people from jumping or falling onto the tracks. Installing them in a system the size of New York’s would be an immense task, and they were considered and then rejected as part of the L train modernization work. While not common in the U.S., many cities, including Singapore, have them systemwide.
But even platform doors would not universally protect workers, whom Lugilo says are most at risk from high underground-pollution levels. What’s dangerous about the pollution in New York City subways in particular is that it includes elevated levels of iron, manganese, and chromium, heavy metals that are far more destructive than the usual fossil-fuel emissions to the lungs, and more likely to cause heart attacks and strokes. Nearly 140 MTA workers have died from COVID-19, a disease that multiple studies have confirmed to be more severe for people exposed to high levels of particulate matter. “Subway workers have had some of the highest rates of mortality,” says Lugilo. “I wouldn’t be surprised if the air pollution had something to do with it.” Luckily, high-quality masks, something we hear about a lot these days, can be extremely effective at keeping out both viruses and PM2.5. If you plan to be spending a lot of time underground, Lugilo does suggest wearing one — which is, for better or for worse, what MTA workers and commuters will be doing for a long time.