It has happened to us all – dropping that last potato chip on the carpet or upturning that fresh-from-the-oven cake on the floor. Momentary horror gives way to a bit of despair till someone yells “five-second rule”. Often, that someone is the voice in your head. Suddenly, you find yourself able to scoop up that chip or cake and it is edible again.

The pop science maxim called the five-second rule is an understanding that food dropped on the floor or surface that it should normally not touch is safe to eat if taken off that surface within five seconds because this is too short a time for potentially harmful microbes to attach themselves to the food.

Even intuitively, this doesn’t make sense.

And yet, the five-second rule has been scientifically studied and disproved many times. In 2003, high-school student Jillian Clarke investigated the scientific validity of the five-second rule during her internship at the University of Illinois at Urbana-Champaign.

Clarke found that 70% of women and 56% of men were familiar with the rule and applied it to food that slipped through their fingers. Women were more likely to pick food off the ground and eat it than men. Cookies and candy were more likely to be picked up and eaten than cauliflower or broccoli. Most importantly, she found that food that fell on a floor containing microorganisms can be contaminated in five seconds or less.

In 2007, scientists from Clemson university in South Carolina also conducted tests to determine that applying the five-second rule will not actually prevent the food from gathering bacteria. The five-second rule has been studied so much that it literally gets an annual debunking. Even NASA engineers have tested the theory.


So that five-second rule holds but with a whole bunch of caveats. So many, that the rule may be the exception.

Rutgers University has scientifically reiterated that the fallen morsel is probably full of bacteria, no matter how quickly it was retrieved from the ground. In a recent paper that was published in the journal Applied and Environmental Biology, professor and specialist in food science Donald Schaffner showed that the transfer of bacteria to food depends on moisture, type of surface as well as contact time.

Schaffner and his team inoculated four surfaces – stainless steel, ceramic tile, wood and carpet –with a type of bacteria that normally occurs in the human gut – Enterobacter aerogenes, which is a nonpathogenic “cousin” of Salmonella. They

Tested each surface with watermelon, bread, bread and butter and gummy candy. for contact times of less than one second, five seconds, 30 seconds and 300 seconds.

They repeated each of the 128 scenarios 20 times to get 2,560 measurements. The results were not surprising, Watermelon had the most contamination and gummy candy had the least. According to Schaffner, transfer of bacteria from surfaces to food appears to be affected most by moisture since they move with the aid of moisture. What was unexpected was the finding that carpets had a very low transfer rate, which Schaffner attributes to the topography of the surface.

The experiment shows that the five-second rule only holds in the sense that the longer the contact time between food and an unclean surface, the greater is the danger of contamination. But, as Schaffner points out, the rule is an oversimplification since bacteria can contaminate instantly.

But then is the five-second rule really about health and cleanliness? As Monica Hesse writes in the Washington Post, “Eating off the floor is less about dirt and more about desire – how much we are willing to deceive ourselves in the reckless pursuit of something forbidden.”