Saturday, November 20, 2010

On Falling Food, Part II: The Five-Second Rule

Earlier this week my little sister, Ashley, asked me some questions about falling food. In yesterday's post I addressed the phenomenon of bread falling buttered-side down. Her other question concerned the colloquial rule about when food dropped on the floor becomes unsafe to consume.[1] The actual time span varies, depending on who you talk to, ranging from a two-second rule to a thirty-second rule. So is this a good rule of thumb, or are we setting ourselves up for multiple rounds of food poisoning?

Fact: even though there are millions or even billions of bacteria in a single gram of soil [2] (for comparison, a dollar bill weighs about a gram), most of them are unable to cause disease in a healthy human being. And of the few that can cause disease (such as Clostridium difficile, shown to the right), you're unlikely to consume enough that they can outcompete the bacteria already established in your gut to the point that they can make you ill. So if eating a whole gram of soil is unlikely to make you sick, how much is getting on your food when you drop it?

A number of different researchers have tested the time it takes bacteria to adhere to dropped food.[3] In general, their conclusions have been that germs are able to stick to food immediately and that a matter of seconds doesn't increase the bacterial load significantly. However, if you leave your food on the floor for minutes or (even worse) hours, then more bacteria are able to attach. And the bacteria that have already attached will increase their reproduction rate in response to the sudden increase in available nutrients.

More important variables to consider are: the shape of your food, the moisture content of your food, and the cleanliness of the surface you dropped it onto. First, shape: a crinkly potato chip or a round M&M will have less contact with the floor than a slice of bologna. Second, moisture: the buttered side of your bread will pick up more dirt and bacteria than it would have if the unbuttered side had landed down. Third, the floor: the floor of the bathroom or the floor of a family with indoor pets will harbor more germs than the living room floor of a family that vacuums the carpet on a regular basis.[4] If all three criteria seem safe to you, then by all means pick it up and eat it.[5]


[1] For fun, check out this t-shirt and this flowchart.

[2] See I am unsure what the numbers are for viruses, fungi, or protozoa. But again most of them won't be able to infect a healthy human being.

[4] However, since humans are carriers of their own diseases and since they do drop food on the kitchen floor (or splatter it while preparing it), it often has more harmful bacteria growing on it than an innocent gram of soil would. So avoid eating off of kitchen floors. See–-is-it-kitchen-safe/#more-878.

[5] But perhaps don't eat that mushy french fry you found wedged in the back seat while you were vacuuming your car.

Image attribution:

False-color scanning electron micrograph of Clostridium difficile is by Annie Cavanaugh, available here.


  1. Have you ever heard the theory that one of the reasons we have so many allergies nowadays is because we live to cleanly? People who live in the country are less likely to have allergies than those in the city. People with pets, less than those without. Et cetera. I think there's some truth to this, I just don't know how much.

  2. Yes, I have heard that. It's not entirely true because there is definitely a genetic component, but there is also evidence that people who over-sanitize are more prone to allergies and other autoimmune diseases (asthma, Crohn's disease, etc.).