Science Questions

How long does something need to be in contact with the floor to pick up germs?

Sun, 26th Feb 2012

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Kathy Bateman asked:

Hello Naked Scientists!


I listen to your podcast here in Austin, Texas, USA.


Around here parents have a "two second rule" for food that has fallen on the ground. The idea is that if the food item has been on the ground for less than 2 seconds it has not had enough time to get any germs on it and is therefore still ok to eat. How long does something need to be in contact with the floor to pick up germs?


Thanks!I love your show!


Kathy Bateman

Austin, Texas


Chris -   Letís poll the panel here.  Dave, would you pick up and eat something off the floor if itís been there for fewer than 5 seconds?

Dave -   I'm not sure if 5 seconds is important.  It rather depends on the floor. If itís a very clean floor then I would think itís safe.

Chris -   Hannah, your view?

Hannah -   I think I'm a bit of a grotbag.  Mine is the 30-second rule.

Chris -   Oh, you go for as long as 30 seconds!  Well, Paul Dawson, who is a researcher at Clemson University has a very nice paper on this in the journal of Applied Microbiology.  He recruited his own students to do the experiment.  They tested things like Salmonella being placed on a surface to look at how long the Salmonella will remain viable after a surface is contaminated.  The answer is for 4 weeks.  So basically, if there's contamination on the surface, it remains viable for a long time so if you drop something on it, you could pick something up.

Then they did the experiment with food items being dropped on to a surface and removed within a certain threshold length of time, and the answer was that 99% of the bacteria that got transferred at any time all transferred instantaneously.  So, there is no 5-second, 2-second, 30-second rule even for grotbags like you Hannah, as you call yourself.  The answer is that instantaneous contact will transfer microorganisms and if there are pathogens there which can survive for a long time, they will end up on the food and depending upon how pathogenic they are and whether or not you then re-sterilise the food, perhaps with temperature for instance, then you could get something from it. 

So, if in doubt, throw it out Ė I think is the bottom line.


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I don't get to excited about the "30 second rule"...  but rather what it looks like.  If a piece of food is covered with grime, I'll either rinse it, or toss it.  If it looks clean, then it can't be that bad. 

So, I'd be more concerned about how sticky the item is, rather than how long it had been dropped for. 

It might also depend on where one is.  At home, vs in a busy public place that might have exposure to a greater number of germs.

CliffordK, Thu, 1st Mar 2012

food + floor+ incubation time?
CZARCAR, Fri, 2nd Mar 2012

That's IT! I'm only eating the food AFTER it's been on the floor for 4 weeks, to give the salmonella a chance to die!  ;) wolfekeeper, Fri, 2nd Mar 2012

post contamination, can i pressure cook it & reeadibilize it? CZARCAR, Mon, 26th Mar 2012

Cooking will kill most of the bugs.
It will not necessarily destroy all of the toxins such as Staph Aureus Toxin, and Botulinum Toxin also takes significant cooking to destroy.

However, it normally takes time to build up organic toxins, so brief environmental exposure normally would not cause a buildup of toxins. 

Light surface contamination would be the easiest to destroy with cooking as the outer layers are often easiest to cook. 

Chemical contamination also may not be destroyed with cooking. CliffordK, Mon, 26th Mar 2012

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