Science

The Truth and Flaws of the Five-Second Rule

It doesn’t matter how quickly you pick up your food—bacteria transfer starts immediately. But the real question is: What bacteria were on the floor?

5 second rule.
How clean do you think that movie theater floor is?

lolostock/Thinkstock

When you drop a piece of food on the floor, the reflex to pick it up is usually pretty immediate. It’s only once you’ve retrieved it that the hindsight appraisal comes in: That wasn’t really on the floor that long … right? How badly did I actually want to eat that? When was the last time I cleaned?

If the retrieval was swift, the food sufficiently yummy, and the surface seemingly clean, most people probably feel pretty comfortable still eating said food item. This is made more socially acceptable thanks to the widespread acceptance of the “five-second rule”—if you picked it up quickly enough, it didn’t have time to get contaminated right?

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This is actually something scientists have studied. Back in 2007, researchers from Clemson University decided to see how well Salmonella typhimurium (one of the two major types of salmonella that cause food poisoning) could adhere to bologna and bread after it came in contact with carpet, wood, and tile floors. The results showed that bacteria transferred immediately, and the longer the food stayed on the floor, the more bacteria it picked up. This both proves and disproves the idea of the rule: It shows that even a near-immediate grab doesn’t make the food safe, but it is true that the longer you let it lie there, the worse things get. (The research also showed that the moist surface of the bologna made for the most fertile ground, and carpet transferred the fewest germs.) Another study, in 2014, also supported the idea that the longer the food stays on the floor, the more bacteria transfer (carpet again proving to be your best bet if you’re dining à la floor).

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This go round, researchers from Rutgers University did something similar, but a bit more comprehensive. They tested out four different surface types and foods, as well as four different contact times (one, five, 30, and 300 seconds), to produce a total of 2,560 measurements. Put another way, they spent two years researching whether eating food off the floor was a bad idea.

Their results were pretty familiar: Carpet transferred the least bacteria. (Steel transferred the most.) The wettest food, watermelon, was consistently the most well-infected (in fact, the wetness of the food proved to be the most consequential factor in how much bacteria were ultimately transferred). Bacteria adhered to the dropped food immediately, but the longer the food lingered, the more bacteria were transferred. As the researchers put it, “the 5-second rule is ‘real’ in the sense that longer contact time result in more transfer.”

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So, this new research reinforces past research. But when you’re standing there contemplating a recently rescued delectable morsel, you’re not really asking whether it has any bacteria on it. You’re asking whether eating it is going to make you sick.

So will it? Well, if you ate the food from their study, maybe, but that’s because they were testing their samples on surfaces that had been deliberately contaminated. But this study doesn’t really give a concrete answer to whether eating food off any old floor will make you sick. That’s because the big determining factor there is not just about how quickly it transferred, but also: What kind of bacteria are on your floor?

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Of course, researching the five-second rule won’t really help you here, and the bacteria that show up on any given floor is somewhat random. A small body of research does suggest that kitchen and bathroom floors don’t really have that much more bacteria than any other place in the home (sinks tended to be the most contaminated areas). A 2012 study also suggests bacteria found in the home tend to be pretty harmless—and most of them come from our own skin, rather than from, say, dirt. (The objects we handle all the time, like cellphones and sponges, tend to carry the most bacteria, anyway.) Outside of the home, one study found that office carpets actually hold less bacteria than the air (carpets: still the best option for food dropping!).

None of this can prove for certain that the particular floor you’ve dropped your food on is germ-free, but it does help show one reason why the five-second rule has lasted so long. In reality, even though bacteria can transfer immediately, eating a quickly rescued food item doesn’t actually make us sick most of the time. So we keep doing it.

Well, that or the simpler truth that if you took more than five seconds to pick something up, you probably didn’t want to eat it that badly in the first place.

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