Five seconds is all it takes
The earliest research report on the five-second rule is attributed to Jillian Clarke, a high school student participating in a research apprenticeship at the University of Illinois. Clarke and her colleagues inoculated floor tiles with bacteria then placed food on the tiles for varying times.
They reported bacteria were transferred from the tile to gummy bears and cookies within five seconds, but didn't report the specific amount of bacteria that made it from the tile to the food.
But how much bacteria actually transfer in five seconds?
In 2007, my lab at Clemson University published a study -- the only peer-reviewed journal paper on this topic -- in the Journal of Applied Microbiology. We wanted to know if the length of time food is in contact with a contaminated surface affected the rate of transfer of bacteria to the food.
To find out, we inoculated squares of tile, carpet, or wood with salmonella. Five minutes after that, we placed either bologna or bread on the surface for five, 30, or 60 seconds, and then measured the amount of bacteria transferred to the food. We repeated this exact protocol after the bacteria had been on the surface for two, four, eight, and 24 hours.
We found that the amount of bacteria transferred to either kind of food didn't depend much on how long the food was in contact with the contaminated surface -- whether for a few seconds or for a whole minute. The overall amount of bacteria on the surface mattered more, and this decreased over time after the initial inoculation. It looks like what's at issue is less how long your food languishes on the floor and much more how infested with bacteria that patch of floor happens to be.
We also found that the kind of surface made a difference as well. Carpets, for instance, seem to be slightly better places to drop your food than wood or tile. When carpet was inoculated with salmonella, less than 1% of the bacteria were transferred. But when the food was in contact with tile or wood, 48%-70% of bacteria transferred.
Last year, a study from Aston University in the UK used nearly identical parameters to our study and found similar results testing contact times of three and 30 seconds on similar surfaces. It also reported that 87% of people asked either would eat or have eaten food dropped on the floor.