This is a great question! Hand sanitizers are
actually only effective at killing germs in
certain situations. It is not effective if your
hands are visibly dirty. In order for hand
sanitizer to effectively kill germs, the sanitizer
needs to contain above 60% of some kind of
alcohol, and that the skin needs to be exposed for
at least about 15 seconds to the alcohol in order
for it to be effective. Typically, alcohols such
as isopropanol, ethanol, or n-propanol are used as
the active ingredients in hand sanitizers.
Furthermore, it turns out that certain bacteria
(the non-spore forming kind) are more susceptible
to hand sanitizers than others.
So what is the mechanism that kills these
bacteria? Alcohols kill germs by destroying
the cell membranes and denaturing proteins of
bacterial cells. Because of this, gram-negative
bacteria (e.g. E. coli and salmonella) are more
susceptible to sanitizers, since they have a thin
peptidoglycan cell wall surrounded by an outer
membrane, which can be dissolved by alcohols.
Gram-positive bacteria (e.g. Staphylococcus) have
a thicker peptidoglycan cell wall and are less
vulnerable to alcohol-based sanitizers, but can
still be killed by these sanitizers. There are
some non-alcohol based sanitizers that are more
effective against gram-positive bacteria, such as
Benzalkonium chloride (BAC) based sanitizers.
But if you get a choice between washing your
hands and using a hand sanitizer, you should
probably opt for washing your hands with soap for
at least 20-30 seconds. Why? The smaller the
pathogen of interest (e.g. viruses are much, much
smaller than bacteria), the easier it is for it to
get trapped in the small crevices of your skin.
Vigorous hand washing seems to be more effective
at dislodging these germs, since in addition to
the surfactants in the soap and the friction from
rubbing your hands, there is the rinsing step,
which does not occur when using hand sanitizer.
Regular and frequent hand-washing is one of the
most effective ways to reduce your chances of
getting a cold or flu!
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