The Truth Behind the Five-Second Rule Revealed

The bigger culprit here is not time but moisture.

Throughout our lives, we are dogged by a question that challenges our perceptions of time and space: Is it safe to eat food you’ve dropped if you pick it up quickly enough?

The good old five-second rule. It’s been the subject of household debates and innumerable science fair projects, with some claiming it’s real and others denouncing it as bunk. And now this famous rule of thumb is in the headlines again, with a new study putting it to a yet more rigorous test.

Why hasn’t science solved what seems like a simple question—or has it?

The new experiments, reported in the journal Applied and Environmental Microbiology, show that the five-second rule is really no rule at all. True, the longer food sat on a bacteria-coated surface, the more bacteria glommed onto it—but plenty of bacteria was picked up as soon as the tasty edibles hit the ground.

The bigger culprit here is not time but moisture. Wet food (watermelon in this case) picked up more bacteria than drier food, like bread or gummy candy. Carpeted surfaces transferred fewer bacteria to food than did tile or stainless steel, since it soaked up the bacterial solution the scientists applied. (But no, the scientists say, that doesn’t mean you should trade in your dishware for throw rugs.)

All of that is pretty intuitive, so why did we need a fancy experiment with more than 2,500 measurements of bacterial transfer rates?

Well, for starters, a lot of other folks are getting it wrong, says food scientist Donald Shaffner of Rutgers University, who conducted the study with his student Robyn Miranda. Amateur scientific studies and televised “investigations” have confused the issue by relying on experiments that don’t pass scientific muster.

In fact, to date there has been only one other rigorous inquiry into the five-second rule: a peer-reviewed study by Paul Dawson, a food scientist at Clemson University, in 2007. Dawson and colleagues likewise reported that food can pick up bacteria immediately on contact with a surface—but that study focused more on how long bacteria could survive on surfaces to contaminate food. Shaffner’s team decided to test a more robust variety of food under more diverse conditions.

So, if science has debunked the five-second rule, does that mean it’s unsafe to eat food that has hit the floor? That depends on the surface and what kind of bacteria you might pick up. “If you’re in a hospital and you drop something, you probably don’t want to eat it,” Dawson says.

Likewise, you certainly wouldn’t want to pick up Salmonella from a kitchen floor covered in chicken juice.

But in most cases, eating a cookie that has picked up a little dust and floor bacteria is not likely to harm someone with a healthy immune system. “Ninety-nine percent of the time, it’s probably safe,” he says. Practicing good sanitation by keeping floors and surfaces clean is the most important lesson in all of this.

Still, the five-second rule will likely endure. “People really want this to be true,” Shaffner says. “Everybody does this; we all eat food off the floor.”

Perhaps the value of the five-second rule (or the three-second rule, if you’re more uptight) lies more in psychology than microbiology. If nothing else, having a rule provides a socially acceptable excuse for our unsavoury behaviour. Just holler "Five-second rule!" before picking a cookie off the floor and popping it into your mouth, and everyone can have a good laugh. It’s a bit like calling shotgun before elbowing your way into the front passenger seat.

And that leaves us with another way to decide whether to eat that jelly bean you dropped: Just see if anyone’s looking.

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