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A 70% solution is considered the most effective. Higher concentrations of alcohol (over 90% especially) evaporate more quickly, reducing contact time. Also, they work too quickly. By coagulating the outer proteins of bacteria instantly, they form a protective layer that actually stops the alcohol from penetrating further.

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Although I agree that over 90% is less effective (and your explanation is clearer than mine [1]), do you have a reference for 70% (and is it w/w or v/v?) is the most effective?

Last I read up on it, I found the number to be closer to the European hospital standard of 85%, although I don't have the reference in my bookmarks, nor do I recall if it was reflect of scientific consensus, if one even exists on the topic.

[1] https://news.ycombinator.com/item?id=17684070

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