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Monday’s medical myth: the three second rule (when food falls on the floor)

As a food microbiologist, I have always been amazed at people’s belief in the three- or five-second rule. It goes something like this: if you retrieve food dropped on the floor or another surface within…

It doesn’t really matter how long food is on the floor, it’s likely to collect the same amount of bacteria. Flickr/bark

As a food microbiologist, I have always been amazed at people’s belief in the three- or five-second rule. It goes something like this: if you retrieve food dropped on the floor or another surface within three or five seconds, it won’t yet be contaminated with bacteria.

Okay, it might just be wishful thinking. Or a handy excuse not to throw otherwise good food in the bin. But it really doesn’t add up.

Let’s look at the facts. Disease-causing bacteria (known as pathogenic bacteria) and other microorganisms (such as viruses) are potentially everywhere and can be remarkably virulent. So logically, it would be an easy task for a microorganism to attach itself to a surface, especially to a moist piece of food.

A 2007 study Journal of Applied Microbiology paper from a team at Clemson University in the United States, tested the five-second rule on tile, wood and carpet. They contaminated the three surfaces with a high level of Salmonella Typhimurium and looked at the rate in which the bacteria transferred to bread and sausages, over a period of 24 hours.

The amount of time food spends on the floor makes little difference to the transfer of bacteria. Kris Griffon

They found the most significant variable in the transfer rates from all three surfaces was not the length of time it had contact with the food. The three testing times (five, 30 or 60 seconds) made little difference in the rate of bacterial transfer.

The length of time the bacteria had been on the surface prior to contact with the food mattered more. Four hours after contamination, the same amount of bacteria remained on the carpet, while rates of bacteria on the tile and wood were slightly lower.

But another study, on bacteria in the manufacturing environment, found that the longer the food was exposed to a contaminated surface, the more bacteria it accumulated. As did an investigation on transfer between meat surfaces.

Overall, a comprehensive review on bacterial attachment to surfaces concluded that moisture, pressure and contact time increased the likelihood of bacterial transfer.

Watch what you touch

Countless studies have reported that pathogenic bacteria and viruses have a long life on inanimate objects, such as paper and public telephones and in various public places such as classrooms, homes, offices, shops, playgrounds and other environments.

This is of particular concern because if transfer rates to food are similarly high, then there’s potential for contamination of food from bacteria and other microorganisms present on inanimate objects.

Let’s look at a specific personal inanimate object, regarded as indispensable in modern society, handled frequently during the day, held close to the face and placed on many surfaces – the mobile phone. Many may not consider their mobile phone as a source of microbial contamination and disease potential, but studies have shown otherwise.

A 2011 study of Ghana university students found all 100 mobile phones inspected were contaminated with bacteria and many contained recognisable pathogens. One-quarter of the mobiles had Bacillus cereus (responsible for food poisoning) and one-fifth had Proteus mirabilis (which can cause urinary tract infections).

Similarly, the London School of Tropical Medicine and Hygiene recently reported that one in six mobile phones in the United Kingdom is contaminated with faecal matter, including pathogenic E. coli.

One in six UK mobile phones are contaminated with faecal matter. quinn.anya

Consequently, while some people hold onto the belief that food is safe to eat after falling on the floor, the take-home message here relates more to mobile phone surfaces than floor surfaces. Don’t eat food that has fallen on your mobile phone (unless you’ve just cleaned it!) and don’t eat food with your fingers if you’ve held your mobile phone.

My advice? Give your mobile phone a wipe down with a moistened cloth containing an antibacterial chemical when you get home each day. That way, you can minimise the chances that pathogenic bacteria, viruses or other microorganisms from your day’s outing aren’t transferred to your home’s inanimate surfaces.

And for those who hold on to the three-second rule, there’ll be fewer germs to contaminate your food!

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9 Comments sorted by

  1. k d

    logged in via Twitter

    Well, blow me down. I always thought it was the 45 minute rule. The things you learn.

  2. Steve Pratt

    logged in via email

    Don't forget the 2004 Ig Nobel Prize winning research by Jillian Clarke which concluded - among other things - that "cookies and candy are much more likely to be picked up and eaten than cauliflower or broccoli.".

    I think this gives the greatest insight into people's application of the 5-second rule...

  3. tqft

    logged in via Twitter

    Mythbusters results were similar as well IIRC

    1. John Aldhouse

      IT Consultant

      In reply to tqft

      Yes, exactly what I was thinking. The Mythbusters results were quite definitive. Here's a link to the precis:

      The influential factor was not the length of exposure, but the type of food material exposed, particularly its stickiness.

      However, John Hartland's point is the relevant one. Nobody actually "believes" the five second rule (or three seconds, or whatever), it's the equivalent of saying "excuse me" after a belch.

  4. John Harland

    bicycle technician

    Do we have any data on what proportion of people really believe the 5-second rule?

    My own encounters with it substantiate Steve Pratt's comment. It has been a way for people who do know better, to excuse themselves to other people who know better. They do it when they pick up a tasty morsel that they have dropped and that they know may be contaminated - but are prepared to risk.

    It is a way of making clear that it is something that they themselves dropped and an excusing of their ill manners in picking it up and eating it in public.

    Pathogens do cause illness. They also keep our immune system working properly instead of fighting against us when it has too little to do.

    We have come around to recognising the value of dirt in growing the immune system of a child. Some people are still shocked by the same strategy in adults.

    We need social excuses, such as the 5-second rule, to reduce the shock to others.

  5. Sue Ieraci

    Public hospital clinician

    Surely the real question is not "how many organisms are on the food" but "what are the health outcomes"?

    The advantage of any kind of "rule" is that it gave people an excuse not to be too paranoid about "germs". Do we really need to live in a totally sanitised society? Surely, now that we have advanced diagnostic methods and population research, we can know whether pitting stuff off the floor or handling a mobile phone results in a greater degree of infectious disease, or not. That is the point, after all.

    I take the author's point, however, that the floor is probably the least of our worries when it comes to pathogens that are transmitted between people. Easily transmitted viruses like cold or flu, or even bowel organisms, are more likely to be present on things we handle. But even then, does sanitising them lead to a reduction in infectious diseases? If not, then why bother about how many organisms there are?

    1. Michael James

      Research scientist

      In reply to Sue Ieraci

      Cold and flu viruses do not survive in air for long at all (and likewise on dry surfaces), and even less in dry air. Almost all transmission is due to direct droplet transfer from one person to another (and can happen at quite a distance given the force of sneezes and coughs).

      But, if I recall correctly, face masks like the Japanese wear on public transport etc. apparently do not protect as much as expected. I am guessing that is because they only protect against largish droplets. You need hepafilters for true protection.

  6. Michael James

    Research scientist

    "They contaminated the three surfaces with a high level of Salmonella Typhimurium..."

    That is the first level of non-reality in this report. Likewise with mobile phones and those nasty pathogenic bacteria, including E. coli, on them.

    Nasty bacteria are everywhere but not often in quantities that pose a problem, and especially for ingestion which copes with almost anything until quite high thresholds are reached. Seriously, only an immunocompromised individual is going to get sick from licking…

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    1. Sue Ieraci

      Public hospital clinician

      In reply to Michael James

      Has the author received funding from a company that makes alcohol-based sanitiser?