“As soon as there’s a person in the room, you start to find things like staphylococcus or streptococcus, things that we all have on us,” said James Meadow, the paper’s lead author and a scientist at Phylagen, a company that researches the microbiomes of public spaces like hospitals and offices. (He was a researcher at the University of Oregon while conducting the experiment.) “In my office, when I walk across the room, I’m carrying behind me an invisible train of air. On a microscopic level, it might look something like an 18-wheeler going down a dusty road.”

For the study, the researchers put volunteers in an environmentally controlled chamber for several hours at a time, then collected and analyzed samples of air and dust from inside. The mix of bacteria collected, they found, was distinct from person to person, enough that “you can actually tell who a person was, if you know enough about their microbiome,” he said. Things like diet and travel can change this airborne microbiome slightly, but for the most part, it will remain consistent over time.

The results weren’t universal, though—some people, Meadow said, didn’t emit enough bacteria for the researchers to detect the differences in their microbial cloud. Every hour, the average person sheds millions of particles, “bacteria and skin cells and little pieces of fungi that break out of our hair,” he explained. “And there’s a lot of things that determine that. You might be giving off more or less, and it depends on what we do to ourselves—how you take care of your skin, whether one person fidgets more than the other.”

Nevertheless, both Gilbert and Meadow said that down the road, microbiome sequencing could play a role in forensics, helping law enforcement understand who was in a certain place a certain time. In the future, they may be able to match air samples to entries in a microbial database, similar to today’s system of cataloguing fingerprints.

Gilbert has already investigated this application. In one experiment, he and his colleagues sent two “burglars” into a home, then took samples from surfaces in the rooms that had been disturbed. Comparing the bacterial mix in those rooms to the rest of the house, they were able to isolate the burglars’ microbial signature—and, from there, to learn certain things about them.

For example, “we knew that one of them had taken migraine medicine. Their microbial signature was enriched with certain organisms that we only really find enriched on the skin of people who take migraine medicine,” he said. “So the medicine itself alters the type of microbes that can exist on your body and in your body.”

Meadow’s latest study, Gilbert said, offered a purer look at the microbes that people can leave behind. “When we look at a complex environment like a home or a hospital, we don’t see those individual variances. It’s too complex,” he explained. “So having these very controlled environments helps us to see the impact of these very small and subtle variables in people that may influence their cloud.”

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