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The two WHO formulations (ethanol and IPA) make up the volume with distilled or boiled water. But is this water essential for the best efficacy of the cleanser or is it only to bulk it out?

In other words is dilute alcohol (75 - 80%) a better hand sanitizer than 99% (with minor additions of H2O2 and glycerine)?

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  • What is the difference between best efficacy by volume expansion and "bulking it out"? The second question is different from the first.
    – Thomas
    Sep 22, 2020 at 19:24
  • I meant no difference, merely I didn't want to repeat myself. Neither did I intend the two versions of the question to have a different meaning. I could have asked "Does the water play an active role in alcohol based hand sanitisers or does it just make up the quantity?"
    – Peter Jennings
    Sep 22, 2020 at 20:31

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Higher concentrations of alcohol are actually less effective (that Q&A is specific for isopropyl, but the same is true for ethanol, their mechanism of sterilization is the same), see also here.

Summarizing from those answers: higher concentrations of alcohol don't penetrate into cells as well.

This page from the CDC also discusses optimal alcohol concentrations for disinfection and gives some references.

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    Thanks, those are references my web search didn't throw up.
    – Peter Jennings
    Sep 22, 2020 at 20:32
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    Just a couple of extra points that might be useful: The optimal sterilizing activity for ethanol is a concentration of around 70% by volume. Adding water also have the extra benefit that it slows the rate of evaporation, increasing the contact time.
    – D Greenwood
    Sep 23, 2020 at 13:33
  • Looks as if the virucidal effectivity, at least for lipid viruses, of alcohol should not decrease with higher alcohol concentration because it does not have to go inside the virus -- destroying the lipid membrane is probably the effective mode of operation. (But make sure exposure is sufficient in spite of evaporation, as D. Greenwood says). Just in case somebody talks about the novel Coronavirus...
    – Peter - Reinstate Monica
    Sep 23, 2020 at 14:29
  • @Peter-ReinstateMonica The recommended concentrations are still the same for viruses (the CDC source I linked says 60-80%); yeah, I think D Greenwood's reasoning is one of them, not sure about others. Overall, alcohols are not nearly the most potent disinfectants, so it's important to use them in ideal form.
    – Bryan Krause ♦
    Sep 23, 2020 at 14:33

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