Scientists Test the 5-Second Rule

Can we really protect ourselves from germs by snapping up our fallen snacks within five seconds? Two scientists at Rutgers have tested the rule and say the answer is a resounding “…sort of.” They published their findings in the journal Applied and Environmental Microbiology.

The five-second rule, for those of you not familiar (what, did you skip elementary school?), states that food dropped on the floor is still safe to eat as long as you pick it up within five seconds. The origins of the rule are murky, but study co-author Donald W. Schaffner attributes it to Genghis Khan. Speaking to the New York Times, Schaffner said the legendary warlord once claimed that food was safe to eat for a full five hours after it had fallen to the ground. As centuries passed and we learned about germs, that five-hour estimate became a lot more conservative.

Five hours, five seconds—does timing really matter? Schaffner and his co-author Robyn C. Miranda decided to find out. They dropped four foods (watermelon, bread, bread with butter, and strawberry gummies) on four different surfaces (stainless steel, ceramic tile, wood, and indoor/outdoor carpet) for four different periods of time (less than one, five, 30, and 300 seconds) and measured the amount of bacteria each sample collected. They were looking specifically at the bacterium Enterobacter aerogenes, which causes all kinds of nasty infections.

They found that there might be something to that five-second rule after all, or at least the idea of retrieving downed morsels as soon as possible. The longer the food samples sat on the ground, the more bacteria they attracted. But bacteria did manage to find them all, even the one- and five-second samples. Even the speediest hand couldn’t snatch a gummy from the jaws of instantaneous bacterial invasion.

Interestingly, the authors say, time may be the least important part of the equation. Two other components had a huge impact on a sample’s vulnerability.

The first variable is the type of food, specifically its moisture content. It’s biology 101: Bacteria love water. It’s why your dish sponge starts to reek if you don’t regularly squeeze it out. It’s also why bacteria swarmed the wet watermelon while paying little attention to the gummy strawberries.

The other thing that made a difference was the surface onto which the food had fallen. Tile and stainless steel were the grossest, while carpet was relatively clean, but each sample’s bacterial content was ultimately determined by interactions between all three factors (time, food, and surface).

Now, for the five-second question: Does any of this matter? Should we give up on every Dorito we ever drop again? That’s really up to you. Honestly, there’s already bacteria all over everything you will ever touch (not to mention the teeming bacterial ecosystem that lives in your mouth), and most of it is completely harmless.

“I’ve eaten food off the floor,” Schaffner told the Times. Still, he recommends considering the water content of your lost bite. “If I were to drop a piece of watermelon on my relatively clean kitchen floor, I’m telling you, man, it’s going in the compost."

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