Is soap "self-cleaning" because it's soap?

Answers to your questions about the news.
Jan. 3 2007 7:11 PM

Is Soap "Self-Cleaning"?

An answer to the Explainer's Question of the Year.

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Two weeks ago, the Explainer offered up a list of questions that we never got around to answering in 2006, among them: "Why is smooth peanut butter cheaper than nutty?" and "Why is grilled chicken tasting increasingly rubbery and odd?" We invited Slate readers to let us know which unanswered question was most deserving of an answer. After a thorough analysis of the votes—of which there were thousands—three questions emerged as the reader favorites.

The first was about whether we're likely to have inhaled molecules from the body of Abraham Lincoln. This conundrum, it turns out, is a classic brainteaser often presented in college physics classes. For an in-depth discussion of the question, see page 32 of the book Innumeracy, by John Allen Paulos, or check out this episode of NPR's Morning Edition.

Daniel Engber Daniel Engber

Daniel Engber is a columnist for Slate

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The second question concerned the plight of a young man in a May-December relationship with a cocaine-snorting stripper, and concluded, "Can you give me some advice?" The query seemed to be outside the purview of this column. However, the Explainer was able to forward the question on to Slate's own advice columnist, who was more than happy to provide an answer.

Which brings us to the third reader-selected question, and the official Explainer Question of the Year:

How clean is bar soap in a public bathroom? Is it "self-cleaning," since it's soap? It seems like a health hazard to me.

It's dirty, but that doesn't make it a health hazard. Soap can indeed become contaminated with microorganisms, whether it's in liquid or bar form. According to a series of tests conducted in the early 1980s, bars of soap are often covered with bacteria and carry a higher load than you'd find inside a liquid dispenser. But no one knows for sure whether this dirty soap will actually transfer its germs to your hands during a wash.

In fact, what little clinical evidence there is suggests that dirty soap isn't so bad. A study from 1965 and another from 1988 used similar methodologies: Researchers coated bars of soap in the lab with E. coli and other nasty bacteria, and then gave them to test subjects for a vigorous hand-wash. Both teams found no transfer of contamination from the dirty soap. However, both studies were tainted by potential conflicts of interest: The first was conducted by Procter & Gamble, and the second came from the Dial Corp.

Still, there's no good evidence to contradict these studies, and it's likely that the bacteria on a dirty bar would just wash off when you rinsed your hands. In other words, you'd be cleaning the soap as you cleaned your hands. (Your hands would probably have been a lot dirtier than the soap to begin with.)

It's not even clear that you need clean water to get the benefits of a hand-washing. Recent hand-hygiene studies in the developing world have found that washing with soap and water reduces infections even when the water supply might be contaminated. Dirty water, like dirty soap, might not make washing less effective.

Even under the best conditions, washing your hands can actually increase the number of microorganisms present on your hands, thanks to contaminated surfaces near the sink, splashes of contaminated water, or improperly dried hands. (In general, it's safer to leave your hands unwashed than to leave them wet.) The hand-washing paradox might also result from soap-induced skin damage: Dry skin tends to crack and flake and may become more permeable to infectious agents. (You're more susceptible to this if you wash many times per day.)

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