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
From what I understand, if the solution is **too dilute (under 60% or whatever)**, it doesn’t have enough chemical “ammo” to *quickly* “burn” through cellular membranes and denature the proteins inside. It works against some germs, but not the more resilient ones.
If the solution is **too concentrated (90% and higher)**, it actually has *too much* chemical ammo; instead of dissolving/permeating through the membrane, it coagulates the outer cellular membrane, which in turn creates a sort of protective shell that slows the rate at which the alcohol can attack the cell. Additionally, these higher concentrations evaporate more quickly, thereby reducing contact time.
That **sweet spot of 70%** is just beefy enough to tear through the cellular membrane and fuck shit up–but *isn’t beefy enough* to chemically sear the membrane into a protective shell. Since water evaporates more slowly than alcohol, having a higher percent of water means it will evaporate more slowly, thereby increasing contact time. IIRC, this is the concentration that is well suited for killing spores.
Lastly, it’s worth noting that isopropyl alcohol and ethyl alcohol are different alcohols, and each one behaves better (or worse) against certain germs. Here’s the CDC’s page on [chemical disinfectants](https://www.cdc.gov/infectioncontrol/guidelines/disinfection/disinfection-methods/chemical.html).
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