An individual\'s mere presence in a room stirs up 37 million bugs every hour — actually material left behind on the floor by previous occupants, says a new study. \"We live in this microbial soup, and a big ingredient is our own micro-organisms,\" said Jordan Peccia, associate professor of environmental engineering at Yale University and the principal study investigator. \"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,\" said Peccia, the journal Indoor Air reports. This is the first study that quantifies how much a lone human presence affects the level of indoor biological aerosols, according to a Yale statement. Peccia and his team measured and analysed biological particles in a single, groundfloor university classroom over a period of eight days - four days occupied, and four days vacant. At all times the windows and doors were kept closed. The heating, ventilation and air conditioning (HVAC) system was operated upon at normal levels. Researchers sorted the particles by size. Overall, they found that \"human occupancy was associated with substantially increased airborne concentrations\" of bacteria and fungi of various sizes. Researchers found that about 18 per cent 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 linked with humans, including the most abundant, Propionibacterineae, common on human skin.