Massage therapists know it’s important to clean all surfaces in their massage session rooms—but their session-room floors could be the biggest source of bacteria.

New research indicates a person’s mere presence in a room can add 37 million bacteria to the air every hour—material largely left behind by previous occupants and stirred up from the floor.

We live in this microbial soup, and a big ingredient is our own microorganisms,” said Jordan Peccia, associate professor of environmental engineering at Yale University and the principal investigator of a study recently published online in the journal Indoor Air. “Mostly people are re-suspending what’s been deposited before. The floor dust turns out to be the major source of the bacteria that we breathe.”

Many previous studies have surveyed the variety of germs present in everyday spaces. But this is the first study that quantifies how much a lone human presence affects the level of indoor biological aerosols, according to a university press reelase.

Researchers found that about 18 percent of all bacterial emissions in the room, including both fresh and previously deposited bacteria, came from humans, as opposed to plants and other sources. Of the 15 most abundant varieties of bacteria identified in the room studied, four are directly associated with humans, including the most abundant, Propionibacterineae, common on human skin.

Understanding the content and dynamics of indoor biological aerosols is helpful for devising new ways of improving air quality when necessary, Peccia said.

“All those infectious diseases we get, we get indoors,” he said, adding that Americans spend more than 90 percent of their time inside.

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