Just how bad is the air you're breathing while waiting underground for SEPTA's subways?

8th Street

Nearly 130,000 Philadelphians rode the subway every weekday in 2023 — something that helps air quality by cutting down on pollution from traffic. But how much pollution are riders exposed to in the enclosed space of underground subway stations?
How bad is air quality in underground SEPTA stations?
Pollution levels are much higher in underground stations than aboveground, research shows.
- Alison McCook
- Charmaine Runes
Published Dec 19, 2024
Villanova researchers led by environmental scientist Kabindra M. Shakya have been measuring pollution levels in Philadelphia subways for years. Although daily levels vary, they have shown that average air pollution was significantly higher in subway stations — up to nearly eight times higher — compared to the same locations above ground.
One of the metrics Shakya and his team used to assess air quality was PM2.5, also known as “soot.” These tiny particles come from different sources and are small enough to get into the bloodstream via the lungs.
Because large exposures to PM2.5 can cause problems, especially in people with heart and lung diseases, the U.S. Environmental Protection Agency has set an upper outdoor limit of 35 micrograms per cubic meter (µg/m3) over 24 hours. Which means that, over a 24-hour period, the average outdoor concentration of PM2.5 shouldn’t exceed that amount.
The researchers found that average soot levels at 12 SEPTA stations (four on the Market-Frankford Line and eight on the Broad Street Line) exceeded the EPA’s daily outdoor limits in 2018 and 2019. Note that the researchers measured PM2.5 indoors and not over a 24-hour period, making it difficult to directly compare their findings to the EPA limit. Keep scrolling to see readings for each station.
When the researchers returned to 15th Street station in 2022, PM2.5 levels topped 120 μg/m3.
While there was a lot of variability between stations, stops on the Market-Frankford Line had higher concentrations of PM2.5 compared to those on the Broad Street Line.
The soot levels there were higher than those measured by other scientists at subways in other cities around the world, including Singapore (24 μg/m3) and Athens, Greece (88 μg/m3). Still, they fell below measurements taken from subways in other U.S. cities, such as New York (548 μg/m3), Washington, D.C. (341 μg/m3), and Boston (327 μg/m3).
So what does this mean for commuters?
Shakya hopes these data don’t scare off the average commuter from taking the subway, as the exposure to pollution is for such a short time that the risk of problems is “very low.” He said he “won’t hesitate” to keep taking the subway, which cuts down on overall pollution levels by reducing traffic. Still, people with lung conditions may want to consider wearing a mask.
The study focused on exposures to commuters, but transit workers may face bigger risks from pollution because they can spend long hours underground, depending on their schedules.
“If I had to work [in the subway], then maybe I would need to start using a mask,” Shakya said.
To reduce pollution in the subway many interventions are possible. The researchers suggest better ventilation and air circulation, as pollution would dissipate aboveground, as well as regularly cleaning the platforms, tracks, and walls of the tunnels. Stations in other cities have also installed screens at the end of platforms to block pollution in tunnels from reaching platforms.

Ronald Drake, an occupational safety officer at SEPTA, said that he and his team regularly check air quality on subway platforms, and in the two years since he joined the transit system, the levels have never exceeded the indoor pollution limits for workers that have been set by the Occupational Safety and Health Administration.

Like Shakya, he cautioned commuters not to let the seemingly high numbers of PM2.5 deter commuters from using the subway. “Higher doesn't necessarily mean hazardous.”

Methodology

We used PM2.5 readings from a 2020 study by Villanova University researchers; we also referenced a 2024 paper by a different team of researchers that included one of the authors from the 2020 study.

We took the average of PM2.5 readings between two measurement periods. In the 2020 study, researchers measured aboveground and belowground PM2.5 on weekdays during two springtime periods: March 4 to 9 in 2018, and Feb. 1 to April 12 in 2019. In 2018, they recorded air pollution levels during peak and off-peak hours, but in 2019, they only looked at off-peak hours.

They assessed underground air quality by standing in the center of the underground platform for 30 minutes with a backpack containing the measurement equipment. Then, the researchers would take SEPTA to the next station and repeat the process. Every station had four to six replicate measurements.