There's a widespread belief that food dropped on the floor won't risk contamination if you pick it up within three seconds (or, depending on who first told you, five seconds). As RMIT research officer Philip Button points out in an article at The Conversation, the reality is rather more complicated than that.
Picture by Onay Davus
Existing studies suggest that a wide variety of factors, including moisture, the pressure on food, the length of time bacteria has been on an existing surface and the length of time food has contact with the surface all influence the rate at which bacteria is transferred.
Button also notes that worries about food on the floor may be misplaced: because bacteria can also survive on other objects such as mobile phones, the risk of food getting contaminated by you is potentially just as high or higher as what the floor can inflict. Washing your hands before cooking and eating thus makes sense (and we've got a detailed guide on how to do it properly).
I tend to fall in the "let's not be overly precious about our exposure to dirt" camp, partly because it always seems to me that the people who obsessively use hand sanitiser are the ones who catch everything going around. In your own house, you'll know how clean the floors are, and whether that banana you've dropped will be fine after a quick rinse. "When it doubt, don't" is a good rule, but taken to extremes it's also a wasteful one.
Monday’s medical myth: the three second rule (when food falls on the floor) [The Conversation]