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The air quality in the City Hall subway station in downtown Philadelphia is much worse than on the sidewalks directly above the station. That is a key finding of our published in the Journal of Exposure Science & Environmental Epidemiology.

We are an and a who assessed the air quality at the 15th Street-City Hall station in Philadelphia. We focused on this station because our previous study found it to have the among 12 Philly subway stations we measured on the busy Market-Frankford or Broad Street lines.

Most concerning, we found that there was about 10 times more black carbon inside the station than at street level. Black carbon, which is commonly called soot, is a major component of . It is emitted from incomplete combustion of fossil fuels and biomass burning, and inhaling it is associated with adverse health effects such as asthma, heart attacks and lung cancer.

Our findings suggest that the black carbon is being generated by the subway system itself. Graphite used on brake pads is .

We also found that levels of ultrafine particles, or UFP, were about 1.7 times higher underground than above ground. While fine particles are up to 2.5 microns in diameter—about than a grain of fine beach sand—ultrafine particles are 0.1 microns or smaller. These particles are concerning, as they .

Limited access to outside air, the frequency of trains, a large number of passengers and its location in the heart of Center City may be some of the reasons for the high concentrations of particulate matter at the 15th Street station.

Subway air quality has previously been investigated in Philadelphia and several other U.S. cities, including Boston, New York City and Washington, D.C. A study of found in New York and New Jersey to have the highest concentration of fine particles. That study also found a similar concentration of fine particles, mainly iron and carbon, in Philadelphia as we found.

Our study, however, measured more types and sizes of particulate matter.

Our study highlights the need for Philadelphia to monitor the air quality in its subways and to reduce air pollution exposure for commuters and subway workers.

Several major factors contribute to subway air pollution. These include the subway system's age, how worn the wheels and rails are, the frequency of trains, the use of a graphite lubricant on the brake pads, poor ventilation, station depth and the limits of access to outdoor air and outdoor traffic.

Having better ventilation, using platform screen doors and cleaning more often to minimize dust are a few ways to improve air quality.

While we believe air quality in the subway needs to be improved, our findings do not suggest that commuters should avoid taking the subway. Air pollution levels are highly variable across stations and over time, and commuters spend relatively short periods of time inside subway stations.

People with any health concerns, especially a lung condition, can get excellent protection by wearing an N95 mask or . Subway workers can use air purifiers to reduce their exposure to particle pollution.

We simultaneously measured the particle pollution in the underground subway platform and the pedestrian pathway above ground using three types of monitors. We took measurements for six hours a day, from about 9 a.m. to 3 p.m., on five weekdays during the summer of 2022.

The next step in this project is to expose lung cells in the lab to the air pollution particles from the 15th Street Station and measure the oxidative stress caused by the particles. This stress , including asthma, and lung cancers.

We will also examine the types of air particles, and the levels of various metals in them, to determine what about these particles from the subway causes stress to lung cells.

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