The Five-Second Rule

Thanks to Seinfeld, we know that double-dipping is bad for us. But what about the five-second rule? Conventional wisdom suggests it’s safe to eat food dropped on the floor – as long as you pick it up in less than five seconds. Presumably that’s not long enough for the food to get contaminated.

In 2003, high-school student Jillian Clarke conducted a series of experiments which discredited the five-second rule. During a summer internship at the University of Illinois, Clarke swabbed bacteria onto 1-inch squares of floors in a variety of locations on campus and then placed gummy bears and cookies on the tiles for varying lengths of time. The results showed bacteria were transferred to the food in less than five seconds and more bacteria were transferred from smooth tiles than from rough tiles. In a separate and more surprising finding, Clarke also found that women were more likely than men to eat food that’s been on the floor.

Clemson Professor Paul Dawson – the one who proved double-dipping is bad – expanded on Clarke’s study by spreading salmonella bacteria on tile, carpet and wood. After five minutes, they placed bologna and bread on the surfaces for 5, 30 and 60 seconds, measuring the amount of bacteria transferred to the food in each case. They then repeated the process after the bacteria had been on the surface for two, four, eight and 24 hours.

Dawson found that it was safer to drop your food on carpet than wood or tile. Less than 1% of the bacteria on the carpet was transferred but 48%-70% of bacteria was transferred from wood and tile. He also found the food was almost as contaminated in 5 seconds as it was after a full minute. Finally – and most significantly – the number of bacteria on the surface decreased over time so there was less bacteria to contaminate the food.

Donald Schaffner, a food microbiologist at Rutgers University, decided to take the experiment one step further by investigating the type of food. Schaffner found that dry foods like bread or cereal were relatively safe but wet foods were more likely to pick up bacteria. In his own words,

if it’s a big sloppy wet piece of watermelon that drops in my kitchen, where I walk and my dogs walk, no, I’m definitely not going to eat that, because I know the risk is much higher.

Despite all of the science, most people go with their gut. 87% of people Dawson surveyed said they would eat food dropped on the floor, or already have done so.

To which, I say “yuck.”

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2 Responses to The Five-Second Rule

  1. mcylk August 13, 2018 at 9:51 am #


    As always I enjoy your periodical musings.

    I would like to see a study on the probability of finding pathogens on any random surface. The studies you reference all assume that harmful bacteria and viruses exist on all surfaces, but do they?

    I watch my grandchildren place their peanut butter and jelly sandwiches on the carpet while playing, then after the dog comes over and sniffs it, they pick it up and finish it. And if I say anything like using a napkin or paper towel, their mother looks at me, rolls her eyes, and says, “Papa, leave it alone. They’re okay.” And they are. I just wouldn’t do it.

    Years ago, I witnessed a surgical resident harvest a leg vein for a patient undergoing coronary artery bypass grafting. The resident carried the vein around the table instead of handing it across the table. Part of the way around, he dropped it. Everybody froze. This was a patient who had had previous heart surgery and had already used other veins, the mammary and radial arteries, and this vein laying on the floor was all there was. The surgeon said, “Pick it up, we’ll rinse it off, and let’s culture the floor so we know what we’ll have to deal with later.” The culture came back sterile and the patient healed without a problem and was discharged.

  2. Chrystelle August 19, 2018 at 3:52 pm #

    I used to apply the 5-second rule mostly while cooking. However I took a Microbiology lecture and lab class last Spring, and there is no way I could ever do something even remotely close. But quite frankly, the worst was the lab experiment in regards to the potential presence of bacteria on people’s phones.
    People should definitely think twice before using them while eating.

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