In this case, if millions of bacteria are exposed to triclosan, it kills almost all of them. But if one of those bacteria has a genetic mutation that enables it to survive the lethal chemical, then the application of triclosan will select for that individual. Only bacteria resisting the chemical survive.

Descendents of resistant bacteria also carry the mutation, which leads to the generation of large populations of resistant organisms. "This is exactly what has happened with antibiotics, and why strains of bacteria like MRSA (Methicillin-resistant Staphylococcus aureus) are no longer susceptible to many commonly used antibiotics," he adds.

McCann says studies have shown sanitizers that use alcohol -- ethanol and/or isopropyl-- are more effective at killing microorganisms and inactivating viruses than triclosan. Further, there does not seem to be a mechanism by which bacteria and other organisms can evolve resistance to alcohols -- another reason to use these products.

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