A food scientist evaluates the risk of eating off the floor.

By Chelsey Hamilton
June 16, 2016
Credit: Getty Images

If you're a believer in the 5-second rule, you've accepted the popular wisdom that it takes longer than that for germs to transfer from the floor to a dropped piece of food. And as long as you pick up a stray blueberry straight away, you'll likely gobble it down.

But Paul Dawson, PhD, a professor of food science at Clemson University, wasn't convinced five seconds was the "critical threshold that separates an edible morsel from a case of food poisoning." And in a recent article on CNN.com, he describes an experiment he ran in his lab to test the theory.

Dawson's team started by contaminating squares of tile, carpet, and wood with Salmonella. Five minutes later, they placed bologna and bread on each surface for 5 seconds, 30 seconds, and 60 seconds; then checked to see how many bacteria ended up on the food in each scenario. The researchers repeated the same protocol after the bacteria had been on the squares for longer periods of time—2, 4, 8, and 24 hours.

Their findings, published in 2006 in the Journal of Applied Microbiology, revealed that the amount of time the food was on the squares had zero effect on how germy it got. What mattered more was how contaminated the squares were when the food touched them, Dawson explained. The quantity of bacteria on each surface decreased over time.

Another factor to consider when you're debating what to do with that dropped sandwich: the type of surface it landed on. In Dawson's lab, when the bread and bologna touched the inoculated squares of tile and wood, the transfer rate was over 60% (the range was 5% to 68%). But when the bread and bologna touched the carpet square, less than 1% of the bacteria transferred to the food, he said.

So does this mean it's okay to pop a chip if it fell on a (very clean-looking) carpet?

Any time you eat off the floor there's risk involved, Dawson points out. "Certain types of bacteria are extremely virulent, and it takes only a small amount to make you sick," Dawson writes. But that said, the chances that these bacteria are on most surfaces is low.

"The odds are in your favor that you can eat [a fallen] morsel and not get sick," he explains. "But in the rare chance that there is a microorganism that can make you sick on the exact spot where the food dropped, you can be fairly sure the bug is on the food you're about to put in your mouth."