The Five-Second Rule Explored, or How Dirty Is That Bologna? 
 
 
 
 
 
By HAROLD McGEE 
Published: May 9, 2007 
 
	  
A COUPLE of weeks ago I saw a new scientific paper from Clemson University that struck me as both pioneering and hilarious. 
 
Accompanied by six graphs, 
two tables and equations whose terms include “bologna” and “carpet,” 
it’s a thorough microbiological study of the five-second rule: the idea 
that if you pick up a dropped piece of food before you can count to 
five, it’s O.K. to eat it.  
I first heard about the rule from my 
then-young children and thought it was just a way of having fun at 
snack time and lunch. My daughter now tells me that fun was part of it, 
but they knew they were playing with “germs.” 
We’re reminded 
about germs on food whenever there’s an outbreak of E. coli or 
salmonella, and whenever we read the labels on packages of uncooked 
meat. But we don’t have much occasion to think about the everyday 
practice of retrieving and eating dropped pieces of food.  
Microbes 
are everywhere around us, not just on floors. They thrive in wet 
kitchen sponges and end up on freshly wiped countertops.  
As I 
write this column, on an airplane, I realize that I have removed a 
chicken sandwich from its protective plastic sleeve and put it down 
repeatedly on the sleeve’s outer surface, which was meant to protect 
the sandwich by blocking microbes. What’s on the outer surface? Without 
the five-second rule on my mind I wouldn’t have thought to wonder. 
I learned from the Clemson study that the true pioneer of five-second research was Jillian Clarke, a high-school intern at the University of Illinois 
in 2003. Ms. Clarke conducted a survey and found that slightly more 
than half of the men and 70 percent of the women knew of the 
five-second rule, and many said they followed it.  
She did an 
experiment by contaminating ceramic tiles with E. coli, placing gummy 
bears and cookies on the tiles for the statutory five seconds, and then 
analyzing the foods. They had become contaminated with bacteria.  
For 
performing this first test of the five-second rule, Ms. Clarke was 
recognized by the Annals of Improbable Research with the 2004 Ig Nobel 
Prize in public health. 
It’s not surprising that food dropped 
onto bacteria would collect some bacteria. But how many? Does it 
collect more as the seconds tick by? Enough to make you sick?  
Prof. Paul L. Dawson and his colleagues at Clemson  have now put some numbers on floor-to-food contamination. 
Their 
bacterium of choice was salmonella; the test surfaces were tile, wood 
flooring and nylon carpet; and the test foods were slices of bread and 
bologna.  
First the researchers measured how long bacteria could 
survive on the surfaces. They applied salmonella broth in doses of 
several million bacteria per square centimeter, a number typical of 
badly contaminated food. 
I had thought that most bacteria were 
sensitive to drying out, but after 24 hours of exposure to the air, 
thousands of bacteria per square centimeter had survived on the tile 
and wood, and tens of thousands on the carpet. Hundreds of salmonella 
were still alive after 28 days. 
Professor Dawson and colleagues 
then placed test food slices onto salmonella-painted surfaces for 
varying lengths of time, and counted how many live bacteria were 
transferred to the food.  
On surfaces that had been contaminated 
eight hours earlier, slices of bologna and bread left for five seconds 
took up from 150 to 8,000 bacteria. Left for a full minute, slices 
collected about 10 times more than that from the tile and carpet, 
though a lower number from the wood. 
What do these numbers tell 
us about the five-second rule? Quick retrieval does mean fewer 
bacteria, but it’s no guarantee of safety. True, Jillian Clarke found 
that the number of bacteria on the floor at the University of Illinois 
was so low it couldn’t be measured, and the Clemson researchers 
resorted to extremely high contamination levels for their tests. But 
even if a floor — or a countertop, or wrapper — carried only a 
thousandth the number of bacteria applied by the researchers, the piece 
of food would be likely to pick up several bacteria.  
The 
infectious dose, the smallest number of bacteria that can actually 
cause illness, is as few as 10 for some salmonellas, fewer than 100 for 
the deadly strain of E. coli. 
Of course we can never know for 
sure how many harmful microbes there are on any surface. But we know 
enough now to formulate the five-second rule, version 2.0: If you drop 
a piece of food, pick it up quickly, take five seconds to recall that 
just a few bacteria can make you sick, then take a few more to think 
about where you dropped it and whether or not it’s worth eating. 
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