If you’ve ever waited on a subway platform at Borough Hall in Downtown Brooklyn and assumed you were breathing gross air, you were right. And a new study sheds light on why it’s so polluted.

Scientists from NYU's Grossman School of Medicine wanted to see if air pollution was inferior at subway stations near river tunnels compared to stations just a few stops inland or near bridges.

It turns out air quality in river-tunnel stations is way worse, according to the study, which looked at dozens of stops. It’s so bad that an outside expert said the daily pollution levels measured in some of the river tunnel stations were similar to breathing air after a wildfire.

The U.S. Environmental Protection Agency sets a standard of daily average exposure to fine particulate matter — a type of air pollution — at 35 micrograms per cubic meter of ambient air. Some stations in the study had concentrations of fine particulate matter that were almost 10 times that daily standard.

MTA spokesperson Michael Cortez said applying these ambient standards to subway platforms is unfair.

“This study tested a hypothesis by only taking samples for short periods, at the most active time of day, which is not an accurate comparison to EPA standards for daily exposure limits,” said Cortez.

How the researchers measured air quality

The group studied air quality during rush hour on subway platforms at 59 transit stations around New York City (two sites were PATH stops in Jersey City). Specifically, they measured concentrations of one type of fine particulate matter, known as PM2.5. These air particles contain tiny traces of chemicals such as iron or carbon. Their width is “about 30 times smaller than that of a human hair,” per the New York State Department of Health.

That small size means they can penetrate the lungs and bloodstream when inhaled. These particles have been linked to a host of health conditions including heart attacks, asthma and even premature death.

The researchers found that air quality was worse at stops closest to river tunnels. There, PM2.5 levels were 80% to 130% higher compared to stations farther away. The scientists think this is due to limited ventilation within the tunnels under the rivers, which can allow particulate matter to accumulate over time.

That's a lot of particle pollution.
Paul Billings, American Lung Association

Of the top 10 most polluted stations, eight were river-tunnel stations. Broadway-Lafayette Street in NoHo, a station not near a river tunnel, had the highest amount of air pollution in the study, which the researchers chalked up to it being a hub for several train lines. Pollution levels were much lower at subway stops such as the Bergen Street F stop or at Delancey Street-Essex Street.

“That's a lot of particle pollution,” said Paul Billings, the national senior vice president of public policy at the American Lung Association.

He said that at the highest concentrations observed in the study, air quality was similar to what you’d find “in some of the most polluted cities in the world on very bad pollution days” or in the days after a wildfire.

New York City dwellers are no strangers to poor air, though overall quality has improved for the past two decades, according to city health department data. New Yorkers regularly inhale other forms of particle pollution: from cars, trucks and buses, to name just a few sources.

Billings said “the public should be concerned about particle pollution, period. It is a major health threat.”

How worried should subway riders be?

How concerned subway riders should be about particle pollution in their stations is not yet fully understood, said Steven Chillrud, director of the Exposure Assessment Facility Core of Columbia University's Center for Environmental Health and professor at the Lamont-Doherty Earth Observatory. He said more research is needed to fully understand its health impacts and advised anyone concerned to wear a proper fitting mask.

And Terry Gordon, a co-author of the study and a professor at NYU's Grossman School of Medicine, acknowledged that applying the EPA’s standards for ambient pollution to brief exposures on subway platforms was a “gray area.”

A view of the entrance to the Bowling Green Subway station in Lower Manhattan on Oct. 1, 2021. An NYU study recommends that stations near river tunnels, like Bowling Green, need better ventilation to fight air pollution.

Subway workers, who can be exposed to these pollutants for hours at a time, were likely at the greatest risk, said Gordon.

Cortez from the MTA said in an email that the agency has conducted previous air quality testing in the subway system and “found no health risks,” though he did not share results of those surveys with Gothamist.

He said the agency would “thoroughly review this study as the safety of customers and employees is always our highest priority.”

So what could improve air quality on the subway system?

Gordon said better ventilation was needed.

To achieve that, installing bigger fans and increasing the frequency of trains could help, said Robert E Paaswell, a distinguished professor of civil engineering at the City College of New York who previously served as executive director of the Chicago Transit Authority. But, he added, those upgrades could be expensive. And the MTA is facing a massive deficit.

Ultimately, Gordon said he hoped his study would encourage New Yorkers to put pressure on transit authorities to improve ventilation systems.

Billings at the American Lung Association echoed the sentiment, adding that high pollution levels “are not inevitable … Nor should we accept exposure to dangerous levels of air pollution.”