Riding the subway might feel like a quick escape from city traffic, but for Philly commuters, it's not a break from pollution. The Conversation reported on a recent study that revealed air quality in Philadelphia's SEPTA subway system is far worse than the air on the city streets just above them.
What's happening?
Researchers from Villanova University studied air quality at the 15th Street-City Hall station, one of the busiest stops on Philadelphia's Market-Frankford and Broad Street Lines. The results weren't pretty: air inside the station was significantly more polluted than the sidewalks above.
Black carbon, a harmful pollutant linked to fossil fuel combustion, was found at levels 10 times higher underground. Ultrafine particles, which are so small they can penetrate deep into the lungs, were also nearly twice as concentrated in the station compared to street level.
The culprit? A mix of factors, including aging subway infrastructure, poor ventilation, and pollution from train braking systems, which release particles into the air that passengers can often smell. These pollutants linger in confined subway spaces, creating an invisible health hazard for daily commuters.
Why is the subway air quality concerning?
Breathing in polluted air isn't just gross — it raises real concerns for public health. Black carbon and ultrafine particles, which are significantly more concentrated underground, have been linked to asthma, heart attacks, and lung cancer. Because of their small size and ability to get deep into the respiratory system, they can also lead to oxidative stress and contribute to chronic diseases.
However, it's still unclear exactly what the long-term health impact of subway air pollution is. Kabindra Shakya and Aimee Eggler, two of the study's lead authors, explained this will be the focus of the next phase of their research.
They also emphasized that while their findings highlight the need for better air quality underground, they aren't suggesting that commuters ditch the subway altogether.
Air pollution levels vary across stations and times of day, and the average commuter spends only short periods in these spaces. For those concerned, simple precautions like wearing an N95 or surgical mask can help reduce exposure.
What's being done about it?
This isn't just a Philly problem — other cities with subways, like New York, Boston, and Washington, D.C., are dealing with similar air-quality issues. Experts are pushing for better air monitoring and ventilation in subway stations, with ideas like platform screen doors to help filter out harmful particles, and regular cleaning to keep dust in check. These solutions would make a big difference for both commuters and workers.
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New York City MTA communications director Tim Minton told The Cool Down after a similar study in NYC that "particles in subway cars are mitigated by MERV-8 and MERV-9 filters in all train cars ... that turn over fresh filtered air every 3-4 minutes — filtration and turnover comparable to hospitals and offices."
The main point Minton stressed was that air quality inside the subway cars themselves is filtered clean, and these studies focus only on air in the stations. He said that "any particles in stations are mitigated by mobile wash crews on a rotating basis" and that "cleaners clean the right of way within station limits of dust and debris," though Minton's office did not respond to a follow-up question about whether the MTA is exploring any additional new steps to improve air quality for those waiting on platforms. In the initial exchange, however, he raised an important reminder about the overall value of public transit.
"Every serious person knows transit is the antidote to climate change, the one reason NYC is the greenest city around, and an engine of equity for people of all communities who need an affordable, safe way to get to jobs, schools and opportunities of every kind," Minton said.
Globally, cities are already trying out new ideas. In Montréal and Barcelona, they've made ventilation a top priority, continuously airing out their subway systems, though it's costly. Anne Tyska, SEPTA's corporate sustainability initiatives manager, worked with the researchers. She believes the key could also be in redesigning stations and train cars to improve airflow from the start.
The bottom line? Subways are vital for city life, but Philly — and other cities — need to step up their game to make them safer for everyone. Because a commute shouldn't come with a hidden health cost.
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