It's long been claimed that eating food off the floor is fine as long as you pick it up — and eat it — in under five seconds. 

But the so-called "five second rule" has come under scrutiny from researchers at Rutgers University, who found it can take as little as one second for food to become contaminated, depending on the surface it lands on.  

Scientists tested the contamination time of watermelon, bread and butter, plain bread and gummy candy when they fell on stainless steel, ceramic tile, wood and carpet. The surfaces were coated with Enterobacter aerogenes, a microorganism similar to Salmonella.

After testing each combination of food, surface and exposure 20 times, experts came to the conclusion that the wetness of the food and topography of the surface play a significant role in how quickly bacteria can transfer from the floor to food.

“One of the most interesting results is that there is a high cross-contamination from any surface to watermelon (wet food). It picks up bacteria in less than one second,” said lead author Donald Schaffner. 

He added: “Surfaces that are more flat — tile and stainless steel — also had much higher rates of bacterial transfer than carpet or wood.”

However, scientists claim that eating a cookie with a bit of dust on it is not likely to cause much harm to a person with a healthy immune system. But there is always a risk of picking up more dangerous bacteria, for example, in hospitals.

“We have advanced the science of cross-contamination and made people aware that the ‘5 second rule' is not true,” Schaffner added. “Hopefully people will make better decisions about eating foods that have fallen on the floor.”

In the future researchers will continue investigating predictive food microbiology, handwashing and microbial cross-contamination.

-Dmitry Belyaev