The human hands bear more than 100 square inches of surface area.

That’s a lot of room for germs to hold on to, and people may not always use the best methods to eliminate disease-causing microbes from their hands.

A study published in this month’s issue of Emerging Infectious Diseases, the journal of the Centers for Disease Control and Prevention, noted some hand sanitizers on the market contain only 40 percent of the active ingredient ethanol and do not reduce the number of bacteria on hands.

Sanitizers that contain 60 to 95 percent ethanol fulfill their germ-killing claims, but only work on hands that are not visibly soiled, according to CDC hand hygiene guidelines.

The study has drawn attention to the age-old mothers’ admonition to ‘wash your hands.’

“Our policy on hand sanitizers is that soap and water are preferable,” Mary Bengtson, medical director of Student Health Services, said. “Hand washing would always take priority over sanitizers.”

Still, sanitizer, at the proper concentration, can be better than nothing.

“If we can get people to do either one, it’s an improvement,” Jason Troiano, staff physician at Student Health Services, said. “The most common things we see here — strep throat, influenza and the common cold — could be prevented by washing or sanitizing hands frequently.”

Germs include both bacteria — simple, single-celled organisms — and viruses, little packages of genetic information in a protein or lipid envelope that cannot reproduce without help from another cell.

Soaps and hand sanitizers attack both kinds of germs by disrupting cell membranes or viral envelopes and by denaturing proteins.

“Cell membranes dissolve in ethanol,” Jonathan Olson, professor of microbiology, said. “Soap is a long-chain lipid molecule, and it will also disrupt bacterial membranes. They lose their cellular contents and die.”

To destroy a membrane, ethanol has to be fairly concentrated. The study in Emerging Infectious Diseases showed that products with less than 60 percent ethanol smeared bacteria around without killing them.

A few hardy viruses, such as the norovirus that struck campus in 2003, may survive concentrated ethanol or soap, Frank Scholle, a virologist and professor of microbiology, said.

But what soap doesn’t kill directly, it washes away, and that’s where it has an advantage over no-rinse sanitizers.

“Soap is a surfactant,” Olson said. “It lifts bacteria off the surface of your hands so you can wash them off.”

Antibacterial soap is another story altogether. It contains a compound, triclosan, that blocks bacterial metabolism.

“Bacteria cannot become resistant to soap or ethanol. But triclosan inhibits bacteria from making phospholipids, molecules they need to build their cell membranes,” Olson said. “Now that you’re acting on an enzyme, that’s something that can mutate, and you can get resistance.”

This is one of the reasons antibacterial soap is not widely recommended except in health care facilities or under special circumstances, such as prior to surgery, according to a 2001 special issue of Emerging Infectious Diseases. Some strains of E. coli bacteria have already evolved resistance to it.

Hand-washing with regular soap, warm water and a lot of lathering may be the best way to keep hands clean … but it’s not always convenient, and that’s where alcohol-based hand sanitizers come in.

Freshteh Seyed, nurse supervisor at Student Health Services, said the clinic provides Purell hand sanitizer gel, a product with 62 percent ethanol, for staff to use between patients if they don’t have time to wash their hands.

The bottom line from health professionals and microbiologists alike is: use some sense, and, if in doubt, use soap and water.

“Common sense is really the best thing for getting around campus,” Scholle said. “If you shake hands with a whole bunch of people, don’t put your fingers in your mouth. Especially if it’s flu season.”