Humans bring millions of bacteria to indoor spaces, study says

At least, that's what researchers at UC Berkeley and Yale University have discovered about the presence of a person in a room.

"We live in this microbial soup, and a big ingredient is our own microorganisms," said Jordan Peccia, associate professor of environmental engineering at Yale and principal investigator of the study, which was published in the most recent edition of the journal Indoor Air.

"Mostly, people are re-suspending what's been deposited before," he said. "The floor dust turns out to be the major source of the bacteria that we breathe."

To understand the impact a person can have on indoor air quality, the researchers monitored a university classroom for eight days. For four of those days, the room was vacant. The other four had periodic occupancy.

While other studies have examined the bacterial contagion of objects or spaces in a room, this is the first to quantify the bacterial sloughing of the human presence.

This is the "first time anyone has quantified emissions of bacteria and fungi associated with human occupancy using modern DNA-based analysis methods," said William Nazaroff, co-author and environmental engineering professor at UC Berkeley.

The researchers found that the presence of a person correlated with significant spikes in fungi and bacteria circulating in the air. In particular, it was the large-fungal particles and medium-sized bacteria that were most prevalent.

They found that nearly one-fifth of all bacteria and fungi measured in the room came from human sources, as opposed to plants or other sources. And the most predominant bacteria was a common human skin critter called Propionibacteria.

The researchers noted that carpeted rooms were the most infested.

"Whenever you share occupied indoor spaces with others, you are exposed to bacteria and fungi associated with other current and probably recent occupants," Nazaroff said. "We have clues about factors that might influence levels; for example, higher occupant density implies greater exposure. However, we don't yet know whether there is any health significance associated with routine bacteria and fungi exposure indoors."

Co-author Peccia was a little more blunt.

"All those infectious diseases we get, we get indoors," he said.

The team's next step: Do the experiment overseas.

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