Just one person in a room adds 37 million bacteria to the air every hour, according to a study published in the journal Indoor Air.
Most of the bacteria are stirred up from the floor, where they were left behind by the room's prior occupants.
"We live in this microbial soup, and a big ingredient is our own microorganisms," Jordan Peccia, associate professor of environmental engineering at Yale and the principal investigator of the study, said in a press release.
"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."
Not long ago research revealed what lives in your belly button, so the overall amount of bacteria is astounding.
This latest study is the first to quantify how much a lone human presence affects the level of indoor biological aerosols (microorganisms).
Peccia and his team measured and analyzed biological particles in a single, ground floor university classroom over a period of eight days: four days when the room was periodically occupied, and four days when the room was continuously vacant. At all times the windows and doors were kept closed.
The HVAC system was operated at normal levels. Researchers sorted the particles by size.
The scientists found that "human occupancy was associated with substantially increased airborne concentrations" of bacteria and fungi of various sizes. Occupancy resulted in especially large spikes for larger-sized fungal particles and medium-sized bacterial particles.
The size of bacteria and fungi-bearing particles is important, because size affects the degree to which they are likely to be filtered from the air or linger and recirculate.
"Size is the master variable," Peccia said.
The scientists determined 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.
Peccia said carpeted rooms appear to retain especially high amounts of microorganisms, but noted that this does not necessarily mean rugs and carpets should be removed. At least it's good news that few of the microorganisms commonly found indoors are infectious, he said, but added, "All those infectious diseases we get, we get indoors."
Given that most Americans spend more than 90 percent of their time inside, it's no wonder we're often sick. And it seems like a vicious cycle, since we're inclined to spend more time indoors if we do feel lousy.
Photo: E. Coli. Credit: Rocky Mountain Laboratories, NIAID, NIH.