Yummy or yuck? Part 1: The 5 second rule

Yesterday, Fynn dropped her teething biscuit on the shopping centre floor. I glibly picked it up, blew on it and said “5 second rule” before handing it back to her to munch on some more. My friend commented “she’s building a nice strong immune system” and we went on with our shopping.

But I did notice a few death stares from some of the other mothers in the vicinity. You know, the moms with bottles of steriliser in their nappy bags. The ones whose kids’ clothes are always pristine. Who never feed their kids junk food. The kind of mother I could never be.

But I suppose I should at least try not to give my child severe diarrhoea, so I decided to do some research. Turns out I’m not the only one. There are some actual studies into the validity of the 5 second rule, with an Ig Nobel prize having been awarded in 2004 for Jillian Clarke’s work on the topic.

What is the 5 second rule?

In case you’re not one of the 70% of women or 56% of men familiar with the 5 second rule, Wikipedia (the font of all knowledge, don’t you know), describes it as follows:

… the five-second rule states that food dropped on the ground will not be significantly contaminated with bacteria if it is picked up within five seconds of being dropped. Some may earnestly believe this assertion, whereas other people employ the rule as a polite social fiction that allows them to still eat a lightly contaminated piece of food, despite the potential reservations of their peers.

So essentially, if you’re too lazy to get your child a replacement food item, or if you are particularly fond of the item you dropped, you call on the 5 second rule to magically protect you from floor bacteria.

Germs are quick

So the bad news first: Germs are speedy little blighters.

All the studies I could find (academic and otherwise) concluded that within five seconds food stuffs dropped on the floor are contaminated. Although there are variations, the general gist is that that food that falls on the floor will be get germs on it really quickly.

Ok, so maybe it’s just me but “duh!” Surely the relevant questions are “how contaminated” and “contaminated with what”?

Are some floors better than others?

There are a number of variables at play here.

When Mythbusters explored this issue in 2005, they found that the moisture levels, surface geometry and location of the dropped food affected contamination, but that a 2 or 6 second drop time didn’t. I wouldn’t generally rely on Mythbusters as a source, but their results seem to be supported by the more academic studies.

The type of surface definitely seems to play a role. While James found (to her supervisor’s surprise) that some floors are surprisingly clean. (Well, the floors at the University of Illinois, I can’t vouch for the floors in my house!), this is apparently due to the lack of moisture on the floor generally. Carpets or rougher textured surfaces like tiles are more risky than wooden or other smoother floors. Dawson’s team found in their 2006 study that while even a short exposure on any of the tested surfaces could result in Salmonella contamination, food dropped on carpet or tile had much more contamination especially when the longer the food was left on the surface for up to a minute.

But while the University floors may be remarkably clean, many others are surprisingly germ-ridden. No surprise that public restroom floors are icky, but this did shock me: a 2007 study by the Hygiene Council (sponsored by Lysol) in the US showed that the floor in front of your sink has twice as many bacteria per square inch than the dustbin! And the kitchen sponge has 60 times more bacteria than the pet’s food bowl.

Although this doesn’t exactly answer the question of “how contaminated” it brings us a little closer to the answer. And since some serious bacteria, like Salmonella, only require a small load to make you ill, the answer seems to be tending towards “Don’t eat off the floor”

Which bugs are we talking about?

It’s quite easy to get all worked up about the masses of germs in everyday places and on everyday items (cellphones, handbags etc) but the majority of the bugs on these items are relatively harmless.

The studies into the 5 second rule seem to have used eColi and Salmonella as their contaminants of choice. In the James study, brief contact with a tile contaminated with eColi resulted in significant food contamination. Salmonella contaminated surfaces also led to food contamination in under 5 seconds in the study by Dawson. It makes sense for the researchers to use these bacteria, since these are the bad guys one would really choose to avoid. But how likely is it that my local restaurant has harmful eColi on the floor?

I wasn’t able to come up with an easy answer to this question. Most of the reports on eColi outbreaks that I scanned linked the outbreaks to consuming food products that were contaminated at source (beef, sprouts, spinach etc). I’d be interested to hear from the public health folks out there on this point.

“Don’t eat off the floor”

So where does this leave me?

I guess after all that reading, I’ll be less blasé about letting my kids eat off the floor (and using the 5 second rule myself). Not that I plan on freaking out about it, because, let’s be honest, crawling babies put stuff off the floor in their mouths ALL THE TIME and unless you’re prepared to lock them in a sterile room, there isn’t much you can do to stop it.

In fact, the one thing that I have always been quite paranoid about has been the baby playing with the cat food bowls, and if anything, I’m going to be less stressed about that than I was! (Although to be fair, my concern has always been worms not bacteria, so maybe not even that.)

But hang on, what about my friend’s comment about the floor food building up my baby’s immune system?

Turns out that’s not so simple either. In part 2. I’ll report back on my research into whether kids should eat dirt to build their immune systems.


Qualifier 1: If your child is immune compromised or you live or work in an environment that has a high risk level then clearly none of this applies.

Qualifier 2: I am not a microbiologist, doctor or public health practitioner. I am just a plain old mom with questions. This post represents what I have gleaned from my research which has often included journalistic reports on the research, as I either don’t have access to the actual research or the scientific literacy to understand it. I am clearly open to additions, and corrections!



The mother of two girls (Rose, 6, and Fynn, 11 months), Mombot is a feminist and human rights activist based in Cape Town, South Africa. She has a fairly laid back approach to parenting if you ignore the regular rants about the proliferation of the colour pink, the lack of diversity amongst "girls' " toys, the scarcity of good role models for girls in the media etc etc etc.

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  1. I seem to remember that a couple of factors affect bacterial life-cycles, like moisture etc – so I tend to worry most about cutting boards and how long food is in the fridge. I guess it comes down to one’s own personal experiences of what’s led to illness, the information you grow up with/learn, and your level of energy to freak out about stuff (with the latter usually directly proportional to a kid’s sleep non-cycle). My biggest neurosis with my kid is currently 6-monthly deworming – pork tapeworms are the single biggest cause of acquired epilepsy in southern Africa. Joyful, right!

  2. While I don’t feel comfortable just letting my baby stick stuff in his mouth that has been on the floor outside the (a) home, I’m not worried about our own floor.
    The reasons:
    1) we keep it pretty clean and it is indeed a smooth (fake) wood floor.
    2) he crawls around on that floor. He sticks those hands in his mouth anyway. Maybe some people think I should tie his hands behind his back or just not let him down on the floor ever at all (or maybe make him wear a muzzle?) (and wash his linens twice daily while we’re at it and dust 3 times a day), but… I don’t really have any appropriate words to describe what I think about people like that.

    I know that later on there will be sandboxes and eating of grass and whatever, but I can at least make sure that he will only play in sandboxes that are free of catpoop and the like. I hope. I managed to grab him before he could lick the wheels of his stroller today…

  3. I agree with Norah — there really needs to be a distinction between “home” and “elsewhere” in the use of the 5-second rule. In general I wouldn’t let my kid have something which had been dropped on a restaurant or shopping center floor (or even a university!) nor would I eat myself. But our floor at home — even though it’s not-very-clean carpet? Why would that possibly have eColi or salmonella on it? It probably DOES have bacteria, but like Norah said, she’ll probably end up getting exposed to those bacteria whether I let her have that cracker she just dropped or not. And I tend to agree with your friend that exposure to bacteria can actually be beneficial in the long run (for the immune system, “gut flora” etc) so long as they’re not, like, Ecoli and salmonella…

  4. The thing with floors, even in our own homes, is that we walk on the with the same dirty shoes that just trumped through the dog park (and hence the dog shit). I know a few people who have no shoes in the house rules which I am finally beginning to understand.

  5. It also depends what exactly you’ve dropped on the floor. If it’s a cookie, I just blow it off. If it’s something sticky or wet, I throw it out. Probably the biggest thing I think about is how much dust/hair is on what I’ve just picked up. (And that’s when I remember that I should be vacuuming more!)

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