Story at a glance

  • Public transportation systems in the U.S. are seeing elevated air pollution levels, especially underground.
  • Prolonged exposure can cause adverse health effects.

New research looks at the air pollution levels found within the bustling subway systems in densely-populated Northeastern U.S. cities, with data suggesting that they are higher during rush hours than anticipated.

This could put both passengers and transit system workers at an increased risk for adverse health outcomes resulting from increased exposure to high levels of air pollution.

Published Wednesday in the journal Environmental Health Perspectives, the study collects and analyzes particulate matter levels (denoted as PM2.5) from the subways of Philadelphia, Pa., Boston, Mass., New York City,  and Washington, D.C, surveying a total of 71 stations within these systems during morning and evening rush hours––the times where the highest volume of commuters is reported.

The samples were collected with micro aethalometers, which are small devices used to monitor air quality. 

Each sample was collected on subway platforms five to 10 minutes prior to commuters reboarding trains, measuring the particulate matter concentrations. Researchers also collected samples from inside select trains as well. All of the data was collected in 2019, except for samples from Philadelphia, which were collected in 2015.

The results indicate that air pollution measurements were substantially higher in belowground subway stations than seen in corresponding aboveground stations, with on-train concentrations of air pollution also showcasing high volumes of air pollution.

The PATH transit system between New York City and New Jersey was found to have the highest volumes of air pollution, with the MTA-New York City rail system coming in as a close second. 

Some of the most common substances recorded in transit stations were sodium, manganese, and barium, which were commonly seen in select PATH stations.

During rush hour, the underground subway stations targeted in the NYC/NJ’s MTA and PATH subway systems had significantly worse air quality, in terms of PM2.5, than the targeted subway stations in Boston, Philadelphia, and Washington, DC.,” the report summarized.

The Washington, D.C., metro system, alternatively, recorded large quantities of sodium, copper, and zinc.

The particulate matter quantities found during rush hour in the New York City MTA and PATH system represent extreme levels and are considered hazardous to human health, warranting further investigation.

“Our findings suggest that, at least in the northeastern U.S. transit systems included in our study, commuters are exposed to poor air quality during their time spent in underground subway stations,” the authors write. “Moreover, exposures in at least some underground stations may be high enough to increase the risk of the adverse health effects associated with PM2.5, even if they occur for relatively short periods of time.”

The U.S. Environmental Protection Agency (EPA) warns of the myriad health problems that can arise from an elevated exposure to air pollution, such as asthma, decreased lung capacity, and irregular heartbeats. 

As for the broader environment, elevated particulate matter levels can damage the homeostasis of ecosystems and environments by depleting nutrients in soil and contributing to acid rain. 

Terry Gordon, a co-author of the study and professor at New York University’s Langone, told CBS that his team was “surprised” at the high levels of pollution in subway systems across Northeastern cities.

“No subway system in the world, nobody has reported values as high as what we found in Christopher Street Station on the PATH system," he said. 

Published on Feb 11, 2021