Dear Science,

What is the action in hand-washing that cleans or kills bacteria (aside from specifically antibacterial soaps) and viruses? I always figured that you weren't killing germs so much as cleaning your hands of oil and grease that bacteria could stay moist and viable in, so that even germs that don't go down the drain dry out and die as your hands dry off, but I don't actually know. With everyone freaking out about swine flu again, it seems like a good time to ask.

Thanks in advance,


Anyone suffering in quiet fury over the health-care debate raging right now in the U.S. must take a bit of delight in this: Most of the competently nasty bacteria and viruses can sit on skin, tables, doorknobs, and toys for days, weeks, months, or, in rare cases, years. With or without grease and oil, they're ready to lie in wait. Friction is the big way hand-washing helps with nasty microbes on the skin. The most evil bugs tend to find their way onto only the surface of the skin, sitting on the top of grease, dirt, and layers of dead skin cells. Deeper in the skin, and down the hair follicles, are the friendlier bacteria—the skin flora that helps protect us from the nastier, transient colonizers. Washing your hands with soap and water decreases the number of the nasty bacteria on the surface of your skin by about tenfold in the first 15 seconds, a hundredfold in 30 seconds, and a thousandfold in a minute. (Washing your hands for more than a minute at a time is just plain old crazy.) Ten thousandfold! That certainly should be enough to protect you from an unfortunate weekend of diarrhea, right? Nope. Hands coming from a recent sojourn on a cab floor or Metro bus, at a doctor's office, or with a young child can easily have tens of millions of bacteria clinging onto the surface. A minute of hand-washing (and who is patient enough to wash their hands for a full minute each time? Try it; you'll never make it without a timer) merely reduces that to a thousand or so.

If you really want to get your hands clean, you'll need another step. Alcohol gels, like Purell, do a much better job of killing microbes. The alcohol in them acts like a superpowerful magnet for the water in bacteria and viruses, sucking them—and their proteins—dry and horribly and permanently crinkling them into oblivion in the process. (The proteins in every living thing rely upon captured water to hold their shape. With no water, there is no shape and no function.) The alcohol gels, provided the alcohol is at least 60 percent, have been shown in study after study to do the best job of sterilizing hands.

Cleanly Yours,


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