Everyone has heard of the 5-second rule. You know the one: if a food item drops on the floor and you pick it up within five seconds, it’s still perfectly safe to eat. Undoubtedly invented by a child or teenager anxious to eat the last bite of dessert he accidentally dropped on the floor, the 5-second rule has been accepted and employed by kids and adults just about everywhere.
You’ll be dismayed to find out, however, that the rule does not have much scientific credence. You read that right — that Hershey's Kiss you dropped on the floor while you were reading this article? Not as clean as you may assume it to be after having picked it up and popped it in your mouth a mere three seconds after it fell on the floor. Study after study proves that eating off the floor probably isn't a good idea.
Most recently, food science researchers at Rutgers University found that moisture, surface type and contact time all contribute to cross-contamination. In some cases, the transfer of bacteria from floor to food can begin in less than one second. They tested four surfaces (stainless steel, ceramic tile, wood and carpet) and four foods (watermelon, bread, bread and butter, and gummy candy). They looked at four different contact times — less than one second, five, 30 and 300 seconds.
The 2016 study results found that watermelon had the most contamination, gummy candy the least, and carpet had very low transfer rates compared to stainless steel and tile, while wood was more variable. The researchers said that the 5-second rule is "real" only in that the longer amount of time you wait, the more bacteria can transfer.
“The five-second rule is a significant oversimplification of what actually happens when bacteria transfer from a surface to food,” said study co-author Rutgers professor and extension specialist in food science, Donald Schaffner. “Bacteria can contaminate instantaneously.”
But the Rutgers study is hardly new news.
In 2003, high school student Jillian Clarke also disproved the 5-second rule while doing an internship at the University of Illinois. She found that food picked up E. coli bacteria as soon as it was dropped on a contaminated surface. (Interestingly, but not surprisingly — at least to me — she also discovered that women are more likely than men to eat food that fell on the floor.) The motivated student’s research earned her an Ig Nobel Prize at Harvard University in 2004, awarded to scientists whose research “first makes you laugh, then makes you think.”
In 2007, researchers at the University of Clemson at South Carolina took the research a step further to determine if leaving food on the floor longer actually meant more germs would attach to its surface, and if different types of floors carried more or less germs. Their findings? Not pleasant. They found that bacteria such as salmonella can thrive on floor surfaces like hardwood, tile and carpet for as long as four weeks! They also found that food dropped on these surfaces can pick up anywhere from hundreds to thousands of bacteria. When left for an even longer period of time, say a minute? The number grew to 10 times that amount. Enough to make you stop and think before eating that precious potato chip.
Another interesting (and particularly unsavory) point to note: Dr. Charles Gerba, a microbiologist at the University of Arizona, found in his studies that 93 percent of our shoes contain fecal bacteria on them. That’s because we’re walking everywhere in them — in the grocery store, the parking lot, even the public restroom. And if you wear your shoes in your house, where do you think that fecal bacteria is landing? You bet — right on your kitchen floor. Yet another reason to toss that tasty snack that landed on the floor, no matter how good it’ll taste.
Bottom line: Though you may not like it (and you may hear your mother in your ear telling you not to waste food), better to toss the fare from the floor into your garbage can than into your mouth.
Editor's note: This story was originally published in May 2013 and has been updated with new information.