Seth Wenig/Reuters

The results of a microbial analysis.

Today, in questions New Yorkers don't want asked, let alone answered: What is the microbial content of the air in the subway system? How much of that musty subway breath is made up of organic material like fungi, bacteria, and fragments of our own bodies?

Remarkably, a whole century of straphanging has passed without much research on the subject. "There are lots and lots of regulations and pieces of information on aerosol chemicals and aerosolized particles in our society," says Dr. Norm Pace, a professor of biology at the University of Colorado-Boulder and one of the authors of a new study. "There's no assessment of microbiological air quality."

"A lot of people think the subway air is pretty dank," he adds, "and that was the basic question: are the microbes we encounter of concern?"

In short, no. New Yorkers can breathe easy. The study [PDF], to be published in Applied and Environmental Microbiology, reports that the air inside the stations, in microbiological terms, is remarkably similar to that above ground. "Our survey finds that the microbiota encountered in the NYC subway is fairly mundane," the researchers, from the University of Colorado, report, "essentially a mix of outdoor air with an overlay of human-associated microorganisms typical of the skin."

Wait, an overlay of human skin? Well, yes: mostly from the foot, hands, arms and head. It's likely a result of our collective "convective plume," the cloud of skin particles that rises from our bodies when we emit heat. It sounds gross, but it's not unusual: by way of reassurance Pace says that the proportion of skin microbiota is almost certainly higher in your office building.

The other distinguishing organic feature of the air down there is a doubled density of fungi, which the researchers speculate is a result of the wood rot from thousands of subway ties. Again, they say, nothing to be concerned about.

Indeed, after taking several cubic meters of air from seven stations, three times, over the course of 16 months, the surprising finding is how similar the microbial content of the subway is to the air in the streets. It's a testament to the subway's air circulation pumps that despite the traffic of 1.6 billion people every year (and an sizable number of rats), you're breathing just about the same stuff at Canal Street as you are on the elevated platforms of Brooklyn or the Bronx. There wasn't even a difference between platform air in winter and summer.

The researchers used a method called fluid impinging. By transferring the contents of subway air into water, they captured 70 to 80 percent of airborne microorganisms. Culture samples (think petri dishes), by contrast, collect less than one percent that amount of data. 

While disappointed with the lack of microbial differentiation, Pace said subway air is still unusual for its high levels of metal dust. (This has been the focus of several studies, and some controversy.) "It's loaded with iron aerosols," Pace says. "I wondered if the folks spending all day in the subway system would have trouble going through metal detectors." With their fluid samples turned black, the team had to use magnets to study the microbes without interference.

Top image: New Yorkers waiting for a 7 train. Seth Wenig/Reuters.

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