Dear Science

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,

Grimy

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,

Science

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