The "five-second rule" is tailor-made for clumsy people. The conventional wisdom goes: If you drop food on the floor, it's fine to eat it if you pick it up really quickly. But alas, it turns out that wisdom is nothing but a myth. In a September 2016 study published in Applied and Environmental Microbiology, researchers found that bacteria begins to contaminate food the instant it hits the ground.
The Rutgers University researchers examined how quickly Enterobacter aerogenes bacteria (a common type that can cause a variety of diseases and infections) got onto food. They applied the bacteria to stainless steel, tile, wood, and carpet flooring, and then dropped watermelon, bread, bread with butter, and gummy candy on each surface. They varied how long the food stayed on the surfaces, whether it was less than one second, five seconds, 30 seconds, or 300 seconds.
The results will probably upset some germaphobes: Even though more bacteria got onto the food the longer it stayed on the germy surface, some transfer of bacteria took place "instantaneously" — sometimes in a fraction of a second.
The researchers also found that factors like the type of food dropped and the surface of the floor have just as much of an effect as the time spent on the floor. If you drop food on carpet, there's less of a risk of contamination than if you drop it on tile or stainless steel. Watermelon had more bacteria on it than the other foods, while the gummy candy had the least bacteria transferred despite its sticky surface. That's because moisture has a huge effect on contamination. "Bacteria don't have legs, they move with the moisture, and the wetter the food, the higher the risk of transfer," researcher Donald Schaffner said in a press release.
So even though keeping food on the floor for longer does result in more contamination, you're not necessarily "safe" if you obey the five-second rule. "The five-second rule is a significant oversimplification of what actually happens when bacteria transfer from a surface to food," Schaffner said. "Bacteria can contaminate instantaneously."