Q. Does the widespread use of hand sanitizers risk breeding resistant bacteria?

A. Alcohol-based hand sanitizers containing 60 percent to 95 percent alcohol do not increase the chances of producing resistant bacteria, according to research cited by the Centers for Disease Control and Prevention, but some other kinds may do so.

Furthermore, even the recommended kinds of sanitizers do not eliminate all germs, the C.D.C. warns in its guidelines for sanitizer use. Soap and water are still more effective than hand sanitizers for removing or inactivating certain kinds of germs, like Cryptosporidium, norovirus and Clostridium difficile, especially when hands are grimy, not just contaminated.

Hand sanitizers that are not based on alcohol — notably those that rely on substances called chlorhexidine and triclosan — are both less effective and slower to act than alcohol, other studies have found, and both of these agents do present a risk of producing bacterial resistance.

Triclosan especially may increase the risk of antibiotic-resistant E. coli and salmonella, a 2006 review concluded, possibly presenting a public health hazard with widespread use.