If you've ever dropped food – and, let's face it, at some point you have – you have probably at some point exclaimed "five-second rule" like you are citing legal precedent as you try and scoop your chicken korma back onto your food-less plate.
The rule, according to Urban Dictionary, is an "unwritten law" that food can be picked up within five seconds and safely consumed. "The reasoning behind this," they write, "is that dirt and germs take six seconds to transfer from one surface to another."
In some countries, like the UK, three seconds is sometimes cited. What isn't cited a whole lot is evidence. Yet the three/five-second rule has been tested a number of times, including by the Mythbusters on pop-sci show Mythbusters.
In the episode, the Mythbusters placed bacteria contact plates onto the various floor surfaces of their workshop for five seconds, before incubating the plates at body temperature and seeing what developed. They found that the bacteria grown was highly variable depending on where they dropped it, as well as – somewhat surprisingly – that samples taken from the toilet seat were surprisingly clean compared to other surfaces.
In order to test the five-second rule, and discover whether you can avoid picking up bacteria by quickly picking up your food, the team decided to take a clean surface and contaminate it as evenly as possible themselves, by soaking it in beef broth for five days. They then placed wet pastrami and dry crackers onto the surface, for two or six seconds.
While there were differences in the amount of bacteria picked up by the pastrami and the crackers, no significant difference was noticed between the food that was on the floor for two seconds and the food that was on the floor for six.
"As far as I can tell time didn't seem to be a factor," Jamie Hyneman concluded, "because all the samples seem to show about the same amount of contamination no matter how long they sat on the floor."
They then tried placing bacteria contact plates onto their pre-prepared surfaces for two and six seconds, to attempt to eliminate variables other than time. Again, no difference was found and the myth was busted.
More scientific studies have been conducted, broadly backing up these findings. One study in 2006 tested the transfer of Salmonella Typhimurium bacteria from wood tile and carpet surfaces to bologna, and bread. They found that the bacteria could survive for up to four weeks in high enough numbers to contaminate the food that touched it and that the transfer took place "almost immediately on contact."
"Consequently, this study concludes that proper and diligent sanitation of food contact surfaces is needed to reduce cross-contamination to food," the authors wrote in their study, "because even very short contact times result in the transfer of large numbers of bacteria."
A new rule, should you need one, is: don't drop your food on the floor.