I’ve come across this fact stating something along the lines of ‘a 40% alcohol solution is ineffective at killing germs, but anything between 60% – 90% is.” Why is that?
The fact I’m referring to can be found [here](https://www.insider.com/does-alcohol-kill-germs), [here](https://www.cdc.gov/infectioncontrol/guidelines/disinfection/disinfection-methods/chemical.html#Alcohol) and [here](https://blog.gotopac.com/2017/05/15/why-is-70-isopropyl-alcohol-ipa-a-better-disinfectant-than-99-isopropanol-and-what-is-ipa-used-for/)
In: Biology
Alcohol [needs TIME to kill](https://www.researchgate.net/profile/Aidan_Nikiforuk/publication/311322233/figure/fig1/AS:434882271813636@1480695421701/Standard-Ebola-Kill-Curves-Reduction-in-Ebola-virus-titres-when-the-a-Mayinga-b_Q320.jpg), it’s not instantaneous. All chemicals need time, actually. A lot of the easy germs die within seconds, but some of the more resistant ones can survive for some minutes. For 70% rubbing alcohol, 10 minutes is the recommended time, as most of the germs will be dead by then for that concentration.
So what happens is, if you less alcohol (and more water) in the mixture, 60% 50% etc., then you need more than 10 minutes for the same kill power.
And the problem is that this stuff evaporates; try to pour some rubbing alcohol on your hands, it will evaporate probably well before 10 minutes, your hands will be dried up by the 2 minute mark and the killing will stop at 2 minutes instead of 10, leaving some germs alive.
If you have more concentration of alcohol, 90%, it kills a bit faster, say 8 minutes, but alcohol evaporates A LOT faster than water, so your hands will be dry within 30 seconds, which is basically worse.
Sweet spot between “hands wet the longest time” and “killing power” is 70%.
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