Human Presence Adds 37 Million Bacteria an Hour to the Air of a Room

Posted on March 29, 2012

Yale University engineers have found that a person's mere presence in a room can add 37 million bacteria to the air for every hour they are in the room. Most of this is material that is stirred up from the floor, which indicates that much of the bacteria we inhale when we are indoors is stuff kicked up from the floor. This is the first study that quantifies how much a lone human presence affects the level of indoor biological aerosol.

Jordan Peccia, associate professor of environmental engineering at Yale and study leader, says, "We live in this microbial soup, and a big ingredient is our own microorganisms. 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."

Peccia and his research 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 researchers found that human occupancy was associated with substantially increased airborne concentrations of bacteria and fungi of various sizes. Human occupancy resulted in especially large spikes for larger-sized fungal particles and medium-sized bacterial particles. They also found that carpeted rooms appear to retain especially high amounts of microorganisms.

Researchers also found that about 18% of the all the bacterial emissions in the room came from humans, as opposed to plants and other sources. The most abundant was Propionibacterineae, a bacteria which resides on human skin. The researchers say that less than 0.1 percent of the microorganisms commonly found indoors are infectious.

The research was published online in the journal, Indoor Air.

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