Living the Scientific Life (Scientist, Interrupted)

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Have you ever heard of the five-second rule, where you can pick up food that has fallen on the floor within five seconds and eat it without risk of illness? Do you follow it? In 2003, a then-high school science intern at the University of Illinois, Jillian Clarke, conducted a survey and found that slightly more than half of adult men and 70 percent of adult women knew about the five-second rule and many said they followed it. Clarke then conducted an experiment to find out if various food became contaminated with bacteria after just five seconds on the floor.

For performing this first test of the five-second rule, Clarke was awarded the 2004 Ig Nobel Prize in public health by the Annals of Improbable Research.

Clarke’s study inspired another research group at Clemson University to investigate several questions regarding the five-second rule: Does the type of contaminated surface affect the numbers of bacteria collected? How many bacteria does a food item collect in just five seconds? Does it collect more if it sits on the contaminated surface longer? Does it collect enough to make you sick?

To answer these questions, Paul Dawson and his colleagues conducted several experiments of floor-to-food contamination. They chose Salmonella as their pathogenic bacteria; they examined several surfaces; tile, wood flooring and nylon carpet; and the test foods were slices of bread and bologna.

First, the researchers determined how long the bacteria could live on these test surfaces. To do this, they applied a broth containing several million live bacteria per square centimeter to their test surfaces — typical numbers for badly contaminated food — and found that, after 24 hours, thousands of Salmonella survived per square centimeter on both the tile and wood surfaces, while tens of thousands survived on the carpet. Surprisingly, hundreds of Salmonella were still alive after 28 days.

Next, Dawson and his colleagues placed test food slices onto Salmonella-covered surfaces for varying lengths of time, and counted how many live bacteria were transferred to the food. Slices of bologna and bread left for five seconds took up between 150 to 8,000 bacteria from surfaces that had been contaminated eight hours earlier. However, left for a full minute, the food slices collected about 10 times more bacteria from the tile and carpet, and a lower number from the wood surface.

Dawson and colleagues found that the type of contaminated surface did affect the number of bacteria that the food slices took up, and that the length of time that the food remained on the contaminated surface did affect the numbers of bacteria they absorbed. Apparently, this amount of bacteria is potentially enough to cause illness in people; the infectious dose — the smallest number of bacteria that can actually cause illness — is as few as 10 for some Salmonellas.

But consider that Clarke, the original investigator, found that bacterial contamination was so low on the floor at the University of Illinois that it couldn’t be measured, unlike the levels of contamination that the Clemson group were using for their studies. So the likelihood that a cookie, quickly picked off the floor and consumed, can make you ill, is somewhat remote, but it is a factor worth considering if you are in an area where there could be significant levels of bacteria present.


Dawson, P., Han, I., Cox, M., Black, C., Simmons, L. (2007). Residence time and food contact time effects on transfer of Salmonella Typhimurium from tile, wood and carpet: testing the five-second rule. Journal of Applied Microbiology , 102(4), 945. DOI: 10.1111/j.1365-2672.2006.03171.x [Original paper]


  1. #1 Vivian Aranha
    May 9, 2007

    Its a different story altogether – if the cookie for example gets contaminated at the first place… Think of it as this way… how many types of bacterias are there that will harm you. Our body has such a strong system that most of the bacteria will be killed by the liquids generated in the body…

    I am a strong follower of the 5 second rule….

  2. #2 Brian Thompson
    May 9, 2007

    I’d like to see this test performed at a variety of contamination levels aside from “badly contaminated” – as you mentioned, most floors do not house millions of salmonella bacteria per centimeter, so it seems reasonable that the test ought to be conducted on more normal settings.

  3. #3 John Gardner
    May 9, 2007

    The MythBusters did a 5 second rule thing too. While not the most scientifically or statistically minded show, they do interesting things.

    The concluded the myth was “busted”, that there was no difference between 2 seconds and 6 seconds, but i don’t remember that they tried for minutes or anything. The episode also did a comparison of a dog’s mouth being cleaner than a human’s, as well.

  4. #4 Robert Dobolina
    May 9, 2007

    As a teen, I worked in a restaurant. There, it was a “six-second” rule. The problem with this is that the floors of a kitchen restaurant are the most putrid, disgustingly-filthy floors that I know of. I had a feeling that even a minute amount of time was enough exposure for a food-item to catch some bacteria. Not all food dropped on the floor is scaldingly-hot enough to kill bacteria, certainly not on contact, and cooling quickly doesn’t help either I suppose. Eat well!

  5. #5 Dave Munger
    May 10, 2007

    Finally, a reasonable test of the five-second rule. I always thought it was odd that the Mythbusters tested 2 versus 6 seconds.

    The point of the 5-second rule is to compare something that has been sitting on the floor a very short time versus a very long time. 5 seconds compared to 1 minute is good, but I’d also like to see a comparison to 10 minutes, or an hour.

  6. #6 daedalus2u
    May 10, 2007

    What cured me of the 5 second rule, or the 5 minute rule was having children who would drop food. You have to give them food they can hold so they can learn to feed themselves, and so they are going to drop it. If you replace it with something new each time, you will soon run out of any finite amount of food.

    If I was going to give my children food that had fallen on the floor, I wasn’t going to treat myself differently.

    I do look, and remove any bits of debris, but that is it. And no, I don’t think of my floors as clean enough to eat off of.

  7. #7 "GrrlScientist"
    May 10, 2007

    dave; ewwww, would you eat a slice of bologna that had been sitting on the floor for an hour?

    for the record, i do eat cookies and chocolate that has fallen on the floor and been there longer than five seconds.

  8. #8 Original Lee
    May 10, 2007

    I didn’t follow the 5-second rule for a very long time because we had dogs, and I very rarely had the opportunity to beat them to the fallen food item. However, we are dogless at the moment and do have small children who drop food, so we now follow this rule at home. We do not follow this rule in any public setting.

    I’ll have to confess I horrified my next-door neighbor, who never had any kids, by picking up a dropped pacifier from the driveway, popping it into my mouth, and then giving it back to my then-infant son.

  9. #9 The Scarlet Pervygirl
    May 11, 2007

    Something to consider when deciding whether or not to follow the five-second rule might be that there are other things than just bacteria on floors and the ground. I’m sure trace amounts of chemicals–floor cleaner, asphalt, pesticides carried in on shoes from outdoors–probably aren’t so great for people to eat, either.

    In that respect, porousness and moisture of food might make a real difference. Eating a Skittle dropped on the floor might be better than eating a slice of banana.

  10. #10 Pierce R. Butler
    May 11, 2007

    With a reasonably alert & agile critter or two in the house, food on the floor for >5 sec. will no longer be there to be picked up.

  11. #11 Kim
    May 13, 2007

    I’ve always thought it wasn’t really about the length of time something’s been on the floor, but rather just a saying when something gets dropped. We tend to shout “five-second rule! seven-second rule! ten-second rule! fifteen-second rule!” if it isn’t getting picked up. Not really about the -time- spent on the floor, but more to convey the “eh, it’ll be fine” sentiment.

  12. #12 daedalus2u
    May 13, 2007

    I am having extreme difficulty posting any comments.

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