‘Five-Second Rule’: Does it Work?

How many times have you told your kid (or yourself), that it was okay to eat something dropped onto the floor because of the "five-second rule?" Strictly speaking, items dropped on a contaminated surface will immediately pick up bacteria. "At the end of the day, this is a polite social fiction we employ to allow us to eat lightly contaminated foods," says Jorge Parada, an infectious diseases expert at Loyola University.

Parada suggests a new saying, "When it doubt, throw it out." And this is particularly important for the elderly or people with compromised immune systems. As for cleaning off a pacifier dropped on the ground with your mouth? "That's double dipping," he says. "You are exposing yourself to bacteria and you are adding your own bacteria to that which contaminated the dropped item."

However, Parada admits to using the five-second rule "on occasion" and concedes that sometimes, it's a matter of common sense. "Maybe the dropped item only picks up 1,000 bacteria, but typically the innoculum, or amount of bacteria that is needed for most people to actually get infected, is 10,000 bacteria--well, then the odds are that no harm will occur."

So, how can you tell if a piece of food is okay? Microbiologist Mary Pickens, PhD, recommends evaluating the amount of time the food spent on the floor and the quality of the surface. "In a normal, clean, dry house, the five-second rule may not be scientific, but it is pretty reasonable," she tells .

"If you live near a cow barn, and are tracking in manure, that's different." She explains that wet surfaces will breed more bacteria than dry and microbes may proliferate dangerously on food that's been left out for an hour or more.

"I believe it's okay to eat a certain amount of dirt and it's good to have some exposure to bacteria because it builds up your immune system." says Pickens. "And, you are constantly getting exposure to all sorts of germs from your hands." In addition to considering the immune system of the person who will be eating the item, the quality of the surface onto which it was dropped, and how long it stayed there, Pickens adds one more personal criteria: "In my house, if I drop something and it has too many cat hairs on it, I throw it out," she laughs.