A team led by NYU Grossman School of Medicine researchers measured real-time and gravimetric PM_2.5 concentrations and particle composition from area samples collected in the subways of Philadelphia, Pennsylvania; Boston, Massachusetts; New York City, New York/New Jersey (NYC/NJ); and Washington, District of Columbia. A total of 71 stations across 12 transit lines were monitored during morning and evening rush hours (all pre-COVID).

The researchers found an elevation in the PM_2.5 concentrations across these subway systems in the major urban centers of Northeastern United States during rush hours. Concentrations in some subway stations suggest that transit workers and commuters may be at increased risk according to US federal environmental and occupational guidelines, depending on duration of exposure. This concern is highest for the PM_2.5 concentrations encountered in the PATH-NYC/NJ transit system.

Among the 13 underground stations tested in New York, the investigators found particle concentrations that ranged anywhere from 2 to 7 times that of outdoor air samples. As detailed in the open-access paper, published in the journal Environmental Health Perspectives, one underground platform on the PATH line connecting New Jersey and Manhattan (Christopher Street Station) reached up to 77 times the typical concentration of potentially dangerous particles in outdoor, aboveground city air. This figure is comparable to sooty contamination from forest fires and building demolition, the study authors said.

While none of the other stations’ readings reached the severe levels of contamination seen in New York’s worst transit lines, underground subway stations within each of these cities still showed at least twice the airborne particle concentrations as their respective outside samples at morning and evening rush hours.

Our findings add to evidence that subways expose millions of commuters and transit employees to air pollutants at levels known to pose serious health risks over time.

—lead author David Luglio, a doctoral student at NYU Grossman School of Medicine

According to the findings, the PATH New York–New Jersey system had the highest airborne particle concentration at 392 micrograms per cubic meter (μg/m³), followed by the MTA New York at 251 μg/m³. Washington had the next highest levels at 145 μg/m³, followed by Boston at 140 μg/m³. Philadelphia was comparatively the cleanest system at 39 μg/m³. By comparison, aboveground air concentrations for all measured cities averaged just 16 μg/m³.

Mean (horizontal lines) gravimetric-based PM_2.5 concentrations on underground subway platforms in four urban transit systems. The open circles represent the individual measurements. Filtered-based PM_2.5 was collected for 30 to 60 min at 10L/min. Luglio et al.

The Environmental Protection Agency advises that daily exposures at fine particle concentrations exceeding 35 μg/m³ pose serious health hazards.

Besides the Christopher Street PATH Station, the most polluted stations in the Northeast included Capitol South in Washington, Broadway in Boston, 2^nd Avenue on the F line in New York City, and 30^th Street in Philadelphia, according to the findings.

Further analysis of air samples showed that iron and organic carbon, a chemical produced by the incomplete breakdown of fossil fuels or from decaying plants and animals, composed three quarters of the pollutants found in the underground air samples for all measured subway stations. Although iron is largely nontoxic, some forms of organic carbon have been linked to increased risk of asthma, lung cancer, and heart disease, the study authors say.

Co-senior study author Terry Gordon, PhD, a professor in the Department of Environmental Medicine at NYU Langone, noted that further research is needed to assess potentially higher risk for transit workers who spend far longer periods of time in the stations than riders.

Dr. Gordon cautioned that the researchers did not measure riders’ short-term exposure to the airborne substances, which would more closely mimic their experiences dashing to catch a train at the last minute. In addition, it remains unclear whether the steep drop in New York subway ridership due to the coronavirus disease (COVID-19) pandemic has influenced the metro’s air quality, he added.

Dr. Gordon says he plans to investigate sources of subway station air contamination, such as exhaust given off by diesel maintenance locomotives; whipped-up dust from the remains of dead rodents; and poor ventilation as potential culprits. He also encourages researchers and transit authorities to examine why some systems are less polluted than others in a bid to adopt practices that might relatively quickly make stations safer for riders.

Funding for the study was provided by National Institute of Environmental Health Sciences grants P30 ES000260, P30 ES009089, and T32 ES007324.

Resources

• David G. Luglio, Maria Katsigeorgis, Jade Hess, Rebecca Kim, John Adragna, Amna Raja, Colin Gordon, Jonathan Fine, George Thurston, Terry Gordon, and M.J. Ruzmyn Vilcassim (2021) “PM_2.5 Concentration and Composition in Subway Systems in the Northeastern United States” Environmental Health Perspectives 129:2 doi: 10.1289/EHP7202