Friday Weird Science: The 5 second rule!

Mar 25 2011 Published by under Friday Weird Science

“5 Seconds!!!”

I don’t know about you, but I have always been a big fan of this rule. If you haven’t heard of it, it’s when you drop some food on the floor or ground or somewhere else that is probably pretty gross, and if you pick it up before 5 seconds have passed, it’s still considered “safe” to eat (thought whether or not it’s POLITE to eat it, is probably another issue). This is a great rule. I mean, what if you JUST bought that piece of chocolate?! What if you really NEED that piece of chocolate!? Desperate times, 5 second rule. In my previous experience, the 5 second rule doesn’t apply to things that are liquid, or if you can see anything fuzzy on the food when you pick it up.

I’ve got various friends who adhere to this rule more or less. For some, it’s the ten second rule. For others, it’s the two second rule. For most, it’s only if the food dropped is dry and there’s no dirt or cat litter or whatever sticking to it. For some, hey, those carpet fuzzies come right off, you know?

And so, I went about my merry way, telling myself (as all of us who practice this rule tell ourselves) that bacteria can’t POSSIBLY get on my food within 5 seconds. Right? RIGHT?!?!!?

Sigh. Science. Most of the time it’s awesome, but today it has shattered my treasured 5 second rule.

Dawson et al. “Residence time and food contact time effects on transfer of Salmonella Typhimurium from tile, wood, and carpet: testing the five-second rule”. Journal of Applied Microbiology, 2007.
(These authors are hilarious and awesome because they cited Wikipedia for the definition of the 5 second rule. Hopefully they went in and edited it when they were done to show their findings.)

Now the 5 second rule is not necessarily a bad thing. After all, it reduces food waste (especially if you just dropped an entire sandwich). But what if there’s piles of bacteria on that dropped food and you don’t know it? That’s what we’re here to find out.

The authors took two classically droppable foods: bread and bologna (I wish they hadn’t chosen bologna, because that means I have to attempt to spell bologna correctly for the entire rest of the post! My life is so hard, you guys), and three common surfaces, tile, wood flooring, and carpet. They took the surfaces, cleaned them, and sterilized them before use, and then carefully rubbed some Salmonella bacteria all over them. They then put the surfaces in a controlled environment, and dropped bologna on them at 0, 2, 4, 8, and 24 hours after the bacteria had been applied. The bologna was dropped onto the surface and remained for 5, 30, or 60 seconds after dropping (some people have odd notions of time, after all). They note that they did not apply extra pressure (no grinding it in or anything).

They then analyzed the bologna for bacteria. And the results were a little surprising, to me at least.

Above are the results for bacteria amounts on bologna exposed to hardwood flooring and tile. You can see that the biggest effect on how much bacteria got on the food was how long the bacteria had been sitting there. Fresh bacteria = very germy bologna. You can also see that the amount of time the bologna was on the wood or the tile didn’t really help (there’s no significant effect of bologna exposure time on wood or tile for how much bacteria got on there). For wood and tile, the 5 second rule doesn’t really apply. Booo.

However, CARPET looked quite different.

First of all, the bacteria levels were very different. While the wood and tile levels went low quickly and then kind of leveled off, the carpet levels did more of a constant decline. But what really comes out is the bologna effect (hehehe, someone needs to come up with something awesome in astrophysics and call it the “bologna effect”. Really, this would be GREAT). You can see that the residence time of the bologna on the carpet (how long it stayed there) significantly influenced how much bacteria it picked up. So it appears, for CARPET, the 5 second actually has some effect on how much bacteria gets on there! Of course, no data were taken on whether the bologna acquired lint, cat fur, or other icky things commonly found in carpets.

Finally, we know that bologna is a rather damp substance. What about foods that are drier? To address this, the authors dropped some bread on the tile (they note it was white bread, effects of wheat bread dropped on tile have not yet been studied. It is also important to note that the bread was NOT buttered and therefore could not have fallen butter-side down). They got similar effects for bread, the time that the bread was on the floor didn’t affect how much bacteria got on it (except for the 8 hour condition, which appears to be a little wonky).

So it looks like bacteria not only don’t care about YOU, they don’t give a sh** about the five second rule. It also looks like the type of food you drop (bologna vs bread) is less important than how much bacteria is already there and waiting for you.

One thing to keep in mind here. Just because there’s bacteria on your food, doesn’t mean you will get sick. To be honest, there’s bacteria in MOST of your food, MOST of the time. On your hands, on your kitchen sink, on your fork. Our bodies are generally pretty good at fighting this stuff off. So just because it’s there, doesn’t mean you’ll get sick. Most likely, you won’t, it takes some pretty significant bacterial presence to make you ill. But still, now I’m looking at my dropped food on the floor a whole new way. 🙁

Dawson, P., Han, I., Cox, M., Black, C., & Simmons, L. (2006). Residence time and food contact time effects on transfer of Salmonella Typhimurium from tile, wood and carpet: testing the five-second rule Journal of Applied Microbiology DOI: 10.1111/j.1365-2672.2006.03171.x

15 responses so far

  • Janne says:

    I remember this paper from before, and I vaguely remember that either they, or some other group, first tried the rule using "in vivo" surfaces - unprepared, actual floor surfaces.

    The results, if I remember correctly, were that the 5-second rule didn't apply - but mostly because they never managed to get any significant bacterial contamination from any surface. So they resorted to actively contaminating surfaces in order to get any effect at all.

    Which would mean that the 5-second rule really is incorrect but in a good way: you can wait much longer than that without incurring any health risk.

  • Christina Pikas says:

    When the Mythbusters tested it, I think they also found that the 5 second rule didn't matter. If there's bacteria there, it transfers immediately. Too bad!

  • Reyna says:

    How many bacteria were applied to the various surfaces? Is this a number that would be reasonable to expect on a household floor? The Mythbusters episode on the 5 second rule applied what seems like a very large dose of bacteria.
    Here they use an overnight culture (they say up to 10^8, but S. Typhimurium in stationary phase is more like 10^9 per ml). That's a lot of bacteria to start with. I don't think my floor at home is that dirty. At least not dirty enough to transfer an infective dose onto my food.

  • CHRIS says:

    I wish they tested things we actually use the 2/5/10 second rule for. Not many people would eat a moist food that had fallen on the floor. It's pretty obvious that it'll grab the germs on contact. But, what about testing what really counts - M&M's, chips...dry stuff that doesn't cling to the germs. We all know lunch meat on the floor is nasty, but I sure would love to know how dirty the habit is in the real world!

  • JB says:

    For dog owners, worrying over the 5 second rule is like contemplating the sound of one hand clapping.

  • They also tested this, and found similar results, on the short-lived Food Network series "Food Detectives." IIRC, in their experiment, they food picked up bacteria pretty quickly, but if you *left* the food on the gross surface for a long time, it didn't pick up more bacteria than it had after only a few seconds. So, no 5-second rule, but 5 seconds is no different from 10 minutes, in terms of how much bacteria it *does* pick up.

  • FiSH says:

    What I learned from this is not to rub bacteria all over my floors before I eat. I would like see them drop some food (of various textures so to speak), on various actual surfaces and see what turns up. I doubt they would identify very much that's particularly toxic. Although I do have certain friend's homes where that might not be the case, and different rules certainly apply there. I think that there are a variety of qualifications of the 5 second rule based on the cleanliness of the location. In any case, not all bacteria are bad for us anyway (does that sound like a rationalization?).

  • Absolutely - many people have tried to work on this problem in the past. My take on it was that it's a duel between Bacteria, and Us. They'll always win.

  • Nick says:

    Currently enjoying the Skep/Speak broadcast with the Briansx2

  • Aya says:

    I have never believed that the five second rule was about bacteria transferal, I thought it was more about trying to make eating food you really wanted to eat but dropped more socially acceptable.

    If you saw someone pick food off the floor and eat it with no comment, you assume bad things about them (I would never eat from the floor in public unless I can quote 5 second rule out loud). If however they cited the rule, they inform you that this is not their general way of life, they have considered cleanliness and made a decision about it and they can then gauge by the reactions of those around them whether their actions are acceptable before carrying through with them.

    My decisions about eating food from the floor are based on growing up with a mother who never stopped me from eating dirt because 'it was good for my immune system' I didn't get sick then and thanks to being exposed to all kinds of bacteria from it, i'm (usually) not going to get sick now.

  • lilly says:

    i did an experiment on the five second rule and it doesn't work

  • ilana says:

    cool! i am going to do an expermient on the five second rule1

  • OREODUDE2014 says:


  • Shanoah says:

    I am going to be doing this for a science fair experiment to see if there is a lot more bacteria on it then we say and think.

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