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WALKING into a room adds 37million bacteria to the air an hour, research shows. This is because we deposit bacteria as well as stirring up existing organisms such as fungal particles, mostly from the floor. Around 18 per cent of bacteria in a room comes from humans as opposed to plants and other sources, Yale University found. Researchers measured levels over eight days � four when the ground-floor room was empty. Prof Jordan Peccia said: "We live in this microbial soup, and a big ingredient is our own micro-organisms. Floor dust turns out to be the major source of bacteria that we breathe. Mostly people are re-suspending what's been deposited." The US team found carpeted rooms hold most bacteria � but that does not mean they are a hazard as less than 0.1 per cent of common microorganisms are infectious. Understanding its movement in rooms is crucial for improving air quality, Prof Peccia explained � adding: "All those infectious diseases we get, we get indoors."