The following originally appeared on The Upshot (copyright 2016, The New York Times Company).
Our continued focus on this threat has long baffled me. Why are we so worried about the floor? So many other things are more dangerous than that.
I first became interested in the five-second rule years ago, when I was a co-author of a book on medical myths. We cited a number of studies showing that food that touched household surfaces — even for brief periods of time — could pick up bacteria or other harmful substances.
This most recent study was similar in that it tested a variety of foods, a variety of substances, for various periods. And, like those other studies, this one found that food touching the floor, even for a very short amount of time, could pick up bacteria.
There’s no magic period of time that prevents transmission. But even though I know bacteria can accumulate in less than five seconds, I will still eat food that has fallen on my kitchen floor. Why? Because my kitchen floor isn’t really that dirty.
Our metric shouldn’t be whether there are more than zero bacteria on the floor. It should be how many bacteria are on the floor compared with other household surfaces. And in that respect, there are so many places in your house that pose more of a concern than the floor.
Perhaps no one in the United States has spent more time investigating the occurrence of bacteria on public surfaces than Charles Gerba. He’s a professor of microbiology and environmental sciences at the University of Arizona, and he has published many papers on the subject.
In 1998, he and his colleagues investigated how well cleaning products could reduce coliform bacteria counts on household surfaces. As part of that research, they measured various locations in the house before any cleaning.
They found that the kitchen floor was likely to harbor, on average, about three colonies per square inch of coliform bacteria (2.75 to be exact). So there are some. But here’s the thing — that’s cleaner than both the refrigerator handle (5.37 colonies per square inch) and the kitchen counter (5.75 colonies per square inch).
We spend so much time worrying about what food might have picked up from the floor, but we don’t worry about touching the refrigerator. We also don’t seem as worried about food that touches the counter. But the counter is just as dirty, if not dirtier.
The same thing happens in the bathroom. I know a lot of people who are worried about the toilet seat, but it’s cleaner than all the things in the kitchen I just mentioned (0.68 colonies per square inch). What’s dirtier in the bathroom? Almost everything. The flush handle (34.65 colonies per square inch), the sink faucet (15.84 colonies per square inch) and the counter (1.32 colonies per square inch).
Things get dirty when lots of hands touch them and when we don’t think about it. We worry about the floor and the toilet seat, so we clean them more. We don’t think about the refrigerator handle or the faucet handle as much.
If we carry this logic out further, there are things we handle a lot and never really clean. One study, for instance, found that about 95 percent of mobile phones carried by health care workers were contaminated with nosocomial bacteria. Of those contaminated with staph aureus, more than half were contaminated with methicillin resistant bacteria (MRSA).
I see people pay for food every day and then eat what they’re handed with no concern that the food might have been contaminated. And the money and the hands that just held it could be much dirtier than the floor.
The dirtiest thing in your kitchen, by far, is likely to be the sponge you keep near the sink. Most people almost never wash or disinfect those sponges. Mr. Gerba found they had, on average, more than 20 million colonies per square inch.
All of this should remind you that it’s always a good idea to wash your hands before you eat. Hand-washing is still one of the best ways to prevent illness.
People react to news like this in one of two ways. One is to become paranoid about everything. Such people start to clean compulsively, worry about all the things they’re touching, and use hand sanitizer obsessively.
The alternative is to realize that for most of us, our immune systems are pretty hardy. We’ve all been touching this dirty stuff for a long time, without knowing it, and doing just fine.
I clearly fall into the latter group. If I drop food on the floor, I still eat it. I do that because the harm I might get from the floor is not worth my concern compared with many, many other things. You may feel differently. Either way, make an informed judgment based on relative risks, not on any arbitrary span of time that one thing has been touching another.