Several million people ride the New York City subway system every single day. As you might imagine, it's not quite the picture of cleanliness. According to research conducted in 2007 and 2008 by biologists from the University of Colorado, 15% of the matter analyzed in New York City subway system air consisted of human skin. Most of this skin came from the heads and heels of riders, but smaller portions originated from riders' belly buttons, ears, underarms, and rear ends.
The air is just one place to find pollutants in the subway. Christopher Mason of Cornell University tested samples from every subway station in New York City to map the bacteria. His study, published in 2015, states that traces of disease like the bubonic plague, meningitis, and staph infections were present. "Our data show evidence that most bacteria in these densely populated, highly trafficked transit areas are neutral to human health, and much of it is commonly found on the skin or in the gastrointestinal tract," Mason said. "These bacteria may even be helpful, since they can out-compete any dangerous bacteria." Watch the video below to learn more about Mason's research.