David Pope from a friend of mine. I have always used cheap Vodka to clean... I overheard someone speaking about using 90% alcohol to disinfect. I told them they didn’t want to do that. Thought I’d pass the knowledge on in case someone else was under a similar impression. The reason isn’t obvious, so it’s an easy mistake to make.

Isopropyl alcohol, particularly in solutions between 60% and 80% alcohol with 20 – 40% water, is rapidly antimicrobial against bacteria, fungi, and viruses. Once alcohol concentrations drop below 50%, usefulness for disinfection drops sharply. Interestingly, higher concentrations of alcohol don’t generate more desirable bactericidal, virucidal, or fungicidal properties.

The presence of water is a crucial factor in destroying pathogenic microorganisms with isopropyl alcohol. Water acts as a catalyst and plays a key role in denaturing the proteins of cell membranes. Isopropyl alcohol, typically used at 70%, penetrates the cell wall or membranes more thoroughly and permeates the entire cell or viral particle and causes coagulation of proteins. Proteins are essential to life processes, and coagulation results in the death of the microorganism. Extra water content also slows evaporation, thereby increasing surface contact time and enhancing effectiveness.

That is a point not to be missed. Microorganisms die exponentially, and adequate contact time is necessary. I remember in my first microbiology lab at Texas Tech; we were instructed to use disinfectant on the lab benches and to not wipe it off. Let it dry naturally, or work on a wet lab bench. So, allow adequate contact time so that the disinfectant can work.

Returning to my point, isopropyl alcohol concentrations over about 90% are not as effective because they coagulate surface proteins instantly. As a result, a protective layer is created which protects other proteins from further coagulation. This may induce a dormant state in the microorganism and fall short of killing it.

So, 70% isopropyl alcohol is what you want to use.