The 5 second rule is an informal rule of thumb known to many people around the world. Essentially, the “rule” states that dropped food can be picked up and eaten, as long as it is removed from the floor within five seconds.
In other areas, the 5 second rule is known as the 3 second rule, suggesting that some people may be more concerned about contamination than others. Unfortunately, in both cases the “rule” is not an accurate measurement of potential contamination, as food is perfectly capable of picking up harmful bacteria like salmonella in an instant.
Several things are at issue with the five second rule. The first is that harmful bacteria are often invisible, meaning that an apparently perfectly clean floor could actually be teeming with bacteria. The second is that harmful bacteria are everywhere, especially on floors, as they are tracked by shoes, feet, and pets. Other substances like carpet can harbor bacteria for weeks. Since one never knows what might be lurking on the floor, the 5 second rule really only applies if you have a microscope and some swabs handy.
The second issue is that bacteria might wait to adhere to foods. Bacteria is transferred through direct contact, and food will be unsafe after one second, let alone five, if the floor is harboring bacteria. It is certainly true that more bacteria will be present after a longer spell on the floor, but the 5 second rule is not necessarily a good gauge of potential food safety.
It should come as no surprise that several universities have conducted studies testing the 5 second rule. They discovered that dry foods like cookies and crackers may not be as likely to pick up bacteria after brief contact, but that wet foods such as ice cream are almost certain to be contaminated. Dry flooring like wood or stone is also less likely to harbor bacteria than carpeting, although no floor is completely free of bacterial guests.
As a general rule, the 5 second rule should not be observed when food is dropped on the floor, no matter how tempting it might be. Foods which can be washed could potentially be washed and then consumed, but other foods ought to be thrown away, as the risk of bacterial contamination is too great. While some bacteria might just make the consumer feel a little ill, others can lead to serious systemic infections which could be deadly. “She lived by the 5 second rule” is not exactly the inscription one wants on a tombstone, after all.