It’s a question that has stirred up many a heated debate: Is it okay to eat food that has fallen on the floor? Many people abide by the “five-second rule,” which maintains that anything is fair game if you pick it up within that time frame. Some allow 10, 20, even 30 seconds to lapse before relegating the food to the trash bin. And legend has it that Genghis Khan abided by a 12-hour rule. But others argue that the rule is an urban myth—that no dropped food is safe. Who’s right?
The five-second rule has actually been put to scientific test. In an often-cited, though unpublished, study from 2003, a high school student interning at the University of Illinois found that gummy bears and fudge-striped cookies placed on ceramic floor tiles that had been inoculated with E. coli picked up the bacteria in less than five seconds. That is, germs can hitch a ride on food upon contact, it seems, so it doesn’t matter how quickly you try to grab it. On the other hand, the student also found that most floors—in university buildings—were cleaner than expected.
Subsequently, in a 2007 study published in the Journal of Applied Microbiology, researchers at Clemson University doused floor surfaces (tile, wood, and carpet) with Salmonella and then dropped bologna and bread on them for five, 30 or 60 seconds. Within five seconds, the food picked up 150 to 8,000 bacteria (the least from carpet, the most from tiles). What’s more, they found that bacteria can live on dry surfaces for several weeks at levels high enough to quickly transfer to food. According to the lead author, "while the bacteria found on most surfaces are harmless, food dropped on surfaces contaminated with pathogens will pick up those bacteria immediately.”
The latest study, published online in Applied and Environmental Microbiology in September 2016, went the full nine yards in testing the five-second rule. Researchers from Rutgers University dropped four foods (watermelon, plain bread, buttered bread, and hard gummy candy) onto four surfaces (stainless steel, ceramic tile, wood, and carpet) that were contaminated with bacteria and then allowed to dry. They allowed the foods to stay in contact with the surface for four periods—less than one second, five seconds, 30 seconds, and 300 seconds (5 minutes). Each scenario was tested 20 times.
Not surprisingly, watermelon, because it is very moist, became most contaminated at all time intervals, while the fewest bacteria transferred to the gummy candy. Once again, carpet had the lowest transfer rates. The study’s conclusion: “Although we show that longer contact times result in more transfer, we also show that other factors including the nature of the food and the surface are of equal or greater importance. Some transfer takes place ‘instantaneously’ at times less than one second, disproving the 'five-second rule.' "
Bottom line: Use common sense. Occasionally eating food that was briefly on the floor is not likely to make you sick. But it depends on what you drop and where. There’s a big difference between picking up a cracker from a just-cleaned dry kitchen floor (probably safe) versus the floor by the cat litter box (not). Or between a reasonably clean living room carpet (probably okay, though a little fuzzy) and a public bathroom (obviously not). On the other hand, since it’s hard to judge just how clean a floor is—it may look spotless but still harbor bacteria—you shouldn’t make eating off it a habit. And if you’re immune-compromised or in frail health, it’s best to follow the “zero second” rule. Keep in mind, too, that microbes are not just on floors. In fact, kitchen counters can be even more contaminated than the floor.
Originally published June 7, 2013.