Which is better – 70% or 95% ethanol?

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Rubbing Alcohol

I have gotten asked from some curious students why we use alcohols as antiseptics and why it is that most of these so-called rubbing alcohols are 70% ethanol or 70% isopropanol and not 95% or 100%. Even in our labs we use 70% ethanol as disinfectants but if you go and ask your seniors, they can rarely give you a better explanation than some variant of “it’s cheaper that way.” While that is also true, it is not really the primary reason for using diluted alcohol.

You would think that more concentrated the alcohol, the more lethal it will be, but the truth is that 70% ethanol is just much more efficient at killing bacteria than absolute (100%) ethanol. I know that sounds counter-intuitive but keep up with me for a minute and hopefully, by the end of it you will understand why.

Before we go any further, we must understand why alcohols like ethanol are even harmful to bacteria. Alcohols, to put it simply, ‘destroy’ proteins and enzymes within bacterial cells so that bacteria can’t carry out their essential survival functions like break down food, etc, which ultimately leads to their death. Notice that for alcohols to be effective they have to get inside the cells. And this is where our answer lies.

Alcohols are highly volatile substances – they evaporate easily at room temperature – compared to water. At absolute concentrations, or even 95% for that matter, alcohols evaporate before they even have time to enter the cell. They just end up precipitating proteins outside the cell, which is obviously not ideal if you are trying to kill some bacteria.

What you have to try to do is reach a happy mix in which the alcohol is concentrated enough to have an effect and, at the same time, stable enough so it has time to enter the cell. 70% seems to be that magical number. It is still able to denature proteins and because it’s diluted in water, which evaporates more slowly, it has enough time to enter the cell. That is basically why all rubbing alcohols are 70%.

Now you know why most rubbing alcohols are diluted to 70% and if you tried using 95% thinking that you could kill even more of those pesky little germs, well, you barely killed any.

Questions, thoughts, disagreements, complaints? Please leave a comment.

  1. If alcohol kills bacteria by destroying proteins in the cell wall, why doesn’t it destroy proteins in the walls of our skins cells?

  2. We had a very old bottle of isopropyl alcohol, 99%, exp 09/08, in a plastic container, which had only a very tiny amount of substance left in the bottom (it is now the end of 2013). I rubbed some on a tender section of skin where I had a small injury and noticed that my fingers turned white. I went to sleep and a few hours later, I woke up with the area where I had rubbed this old alcohol burning and stinging. I looked at it and had a seeping, weeping 2nd degree burn there, where the alcohol had burned through almost all of my layers of layers of skin.I have used fresh rubbing alcohol before with no ill effects but this old, nearly crystallized remains in the bottle severely burned my skin which is taking weeks to heal. We didn’t even know what had caused the burn until I thought about that alcohol and my husband rubbed a little on his skin, which immediately turned white and started burning his skin as well (he washed it off immediately.) What on earth was it that we rubbed onto our skin that caused such burning? Was it just very concentrated alcohol? What happens to isopropyl alcohol as it degrades over time in a plastic container?

  3. what percentage of bacteria does alcohol kill?

  4. dr. stan de loach

    how long does it take for rubbing alcohol isopropyl 70% to kill bacteria? I have read that it takes 5 minutes, but I cannot find the study..do you know it? thanks

    • From anywhere between 30 seconds to 120 seconds, and definitely within 5 minutes. Still, it is not a cut and dry answer because probably depends on 1) bacterial species, 2) temperature, etc.

      Here is the reference.
      “Disinfection, Sterilization and Preservation”, edited by Seymour S. Block pp. 230–240.

  5. So, why exactly does alcohol kill proteins and enzymes? If the answer is too complex, can you give me a resource?

  6. well, I agree with your logic, but to be more sound, can you please cite a scientific source?

    • Bereket, I had put together the information from knowing what I know about microbiology and from a bunch of different resources (even professors) to fill in the holes. So, I can not really give you a scientific source here. Sorry…
      I did, however, find a detailed info pack from WHO (World Health Organization) about similar stuff , which should hopefully convince you.

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