Study Finds Subway Pollution Is Too High—With a Disproportionate Effect on Black and Hispanic Riders

Most New Yorkers are aware of the pollution caused by cars and trucks. But while we pride ourselves on public transit alternatives, how many of us have considered the air quality on the subway? Recently, a postdoctoral researcher at the Columbia Climate School—Shams Azad—decided to find out just how polluted our subway cars and platforms are.

Azad and his colleagues took measurements of on-train air pollution for 19 subway lines and on-platform concentrations for 429 stations. He was able to estimate work-to-home commute times and distances by analyzing U.S. Census Bureau data for about three million working commuters across four boroughs.

In his analysis, Azad found the average particulate matter (known as PM_2.5) in the subway to be 10 times more polluted than the air above ground and seven to nine times higher than what the World Health Organization deems safe. These fine particles pose a significant public health problem for anyone who breathes them in—lodging themselves deep into the lungs, penetrating the blood stream and contributing to a number of health conditions. The full results of the study were published in PLOS One last month.

Azad, who focuses on the intersection of environmental pollution, health and justice, said these findings were especially alarming for Black and Hispanic populations, who were found to have a disproportionate rate of exposure. He shares more about his study and future goals for this work in the interview below.

(If you’re curious about your own commute, Azad and his colleagues also built an interactive platform where anyone can calculate their personal exposure for any origin and destination within NYC.)

Can you tell me a little about your background and research interests?

My journey into environmental research began with a deep concern for the challenges that vulnerable communities face due to environmental injustices. I completed my Ph.D. at NYU Tandon School of Engineering, where my work was mainly focused on exploring the dynamics of air pollution, extreme events and environmental justice.

Here at the Climate School, I’m continuing my work, and I’m committed to advancing our understanding of environmental justice, environmental pollution and environmental health.

Why did you start investigating subway pollution and how it affects different communities?

I started my subway work in 2021, when I discovered a study that showed alarmingly high levels of particulate matter concentration in subway stations. This finding deeply concerned me because over 3 million people use the subway on a daily basis, and a majority of those people do not know the air quality. So they are unknowingly exposed to the dangerous environment.

This motivated me to perform a comprehensive analysis on the air quality in the subway system and also assess its impact on the population. In 2021, I started collecting air samples from subway stations and also air samples inside of subway train cabins. I found that this underground environment in the subway system is highly polluted with particulate matter. Compared to air aboveground, the concentration of particles is at least 10 times higher. According to W.H.O. guidelines, the concentration should not exceed 15 micrograms per cubic meter for a 24-hour period. We found on average the concentration of particulate matter on the underground subway system is over 200 micrograms per cubic meter, which is very high. Inside the train is polluted too—around seven to nine times higher than the W.H.O. guidelines. So people who are traveling and who are taking a longer commute get highly exposed to these particles.

We also analyzed the composition of those particles and we found those particles are mainly constituted by iron, which is coming from most likely the friction between rails and wheels. Iron particles are generated and those particles cannot get out from this underground environment because of the poor ventilation system in these stations. Over time, the particles accumulate in the underground tunnels and stations.

Did you find any differences in exposure based on racial and socioeconomic groups?

I’ve only done this exposure analysis on work-to-home trips, because we know on the census-block level where people are living and where they are going to work. Combining these findings, we correlated daily time spent traveling on the subway for different racial and socioeconomic communities in NYC with pollution measures to identify any disparities in exposure levels.

We found that Black and Hispanic workers tend to live farther away from their workplace and white and Asian communities tend to live, on average, closer to their workplace. Black and Hispanic workers often need to change trains and they get exposed to more particles at the station and also on the train itself; on average, I found that the Black and Hispanic workers were exposed to 20 to 35% more than Asian or white workers. I have also done research on how this exposure varies based on income level, and I found the low-income communities are more exposed compared with affluent communities.

Were you surprised by any of your findings?

I’m very surprised by these findings. Based on the level of concentration, the most polluted stations are the 181st and 168th streets on the 1 train, which are very close to Columbia University. There are a few stations in the downtown and midtown area that are also highly polluted.

It’s interesting to note that the stations outside of Manhattan are mostly aboveground and generally less polluted. Black and Hispanic communities tend to live near those stations, but they still get exposed more. Even though their nearest station might not be heavily polluted, they typically have longer commutes, often transferring at more polluted stations in Manhattan. This longer subway journey leads to higher exposure for these communities.

What do you plan to work on in the future?

My preferred goal is to extend this work and to identify the health effects of exposure. I want to study the effects on subway workers, who spend a majority of their time in that environment. I want to identify what effect they have from this kind of exposure over a longer time.

I also want to understand the variability of the concentration of this pollution over time and over different seasons. So that can play a role in identifying which external factors—for example, temperature—come into the equation.

How has your research affected your own subway use?

I not only use the MTA but also the PATH train. So I also want to look into the PATH system because I feel it is also very polluted. But I am still riding the trains because, like many New Yorkers, I don’t have any other options.