After researchers found New York’s MTA and PATH stations are the most polluted among northeastern train systems, researchers are saying straphangers and workers could keep doing something most are already acquainted with during the COVID-19 pandemic: wearing a mask.

Researchers from New York University found New York and New Jersey had the highest amount of a toxin called particulate matter 2.5—a microscopic particle tiny enough to spread deeper into your respiratory system or seep into your bloodstream. PM2.5, a pollutant often measured in air quality research, is linked to asthma as well as lung and heart problems.

Scientists collected 300 air samples in New York, Philadelphia, Washington D.C., Boston, and along PATH and Long Island Rail Road lines from 71 stops. The PATH had the highest concentrations of PM2.5, followed by the MTA’s subway stops, Washington D.C., Boston, and Philadelphia, according to the study published in Environmental Health Perspectives. 

“New York City had, in general, sometimes double the amount of air pollution in the subway stations than the other cities,” said senior author Terry Gordon, an environmental medicine professor at NYU Langone, told Gothamist/WNYC. The Christopher Street PATH station was ranked the worst for PM2.5 at 1,499 of the particulate matter per micrograms of cubic meters, which was 1.5 times that of the highest sample found in the MTA system. Gordon said lead author and PhD candidate David Luglio had to test the station multiple times to confirm the data because it was so surprising.

“These levels were shocking,” Gordon said.

More research would be needed to assess the air quality differences between transit systems, and researchers said they did not collect all factors that could determine this. But the samples collected across the Northeastern cities were all two to seven times what is considered healthy under the Environmental Protection Agency’s standards.

Subway riders can expect to have a short-term exposure to the pollution. But the risk is mitigated by the reality that commuters may not be on the platform for an extended amount of time and—during the COVID-19 pandemic—most people are wearing masks.

“That’s actually helping even if it’s a cloth face mask,” Gordon said.

Ilias Kavouras, a City University of New York professor at the Department of Environmental, Occupational, and Geospatial Health Sciences, echoed that advice.

“We’re very familiar with that right now,” Kavouras said. “We might as well continue.”

Gordon said he would still go to the polluted Christopher Street station if he needed to, but he is also interested to see if people will continue to wear a mask after the COVID-19 pandemic, in order to negate the health impacts from air pollution.

Workers should be the most concerned since they’re in subway stations for longer periods of time, Gordon and Kavouras said, and NYU’s researchers hope to work with transit authorities to solve the problem.

Kavouras, who studies air pollution and its impacts on human health, said the findings were an “alarm” to indicate more research should be conducted on the source of pollution and exposure to transit workers by fastening air quality measures to their person. The latter is a research method called personal monitoring.

The Port Authority of New York and New Jersey’s executive director Rick Cotton said at a Thursday board meeting that officials are “obviously concerned” and will be looking closely at the study.

“We are totally committed to protecting the health and safety of our workers, we are totally committed to protecting the health and safety of PATH riders, and we will dig into this, come to conclusions, and if necessary develop an appropriate action plan to address it,” Cotton said at the meeting.

The MTA said that it would review the study’s findings, but previous air quality tests found no health risks due to air pollutants. The authority also said the study looked at a small fraction of MTA stations. Gordon said their take may be reasonable, but there is no evidence of zero health risks.

So what should transit and public health officials do to mitigate the risk? That’s more complicated.

Gordon says finding the source of emissions and eliminating that source is the best route. Most of the particles were made up of iron, presumably from friction between the wheels of the train and the rail, as well as carbon from the part of the train that rubs against the third rail. Diesel-powered trains do operate in the subway, but mostly overnight for repair or maintenance work.

“Eliminating a diesel engine might be easy,” Gordon said. “Eliminating the particles given off by the wheels rubbing up against the rails, that would not be very, very easy to to mediate.”