Is 70 or 91 percent alcohol a better disinfectant?

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The percentages refer to the relative amount of alcohol as compared to the amount of water. 70% alcohol is 30% water. 91% alcohol is 9% water. Which is a better disinfectant depends somewhat on how it is used, but 70% alcohol will tend to disinfect much more effectively than 90%+ alcohol. This is particularly true for the disinfection of surfaces, including human skin.

This may seem counter-intuitive, but the reason has to do in part with the fact that alcohol evaporates much more quickly than water. With the 70% solution, evaporation tends to be slower (due to higher water content), and therefore the alcohol will have more time to kill microorganisms. In other words, the 91% solution has a higher tendency to evaporate before all susceptible microorganisms are killed. This is the primary reason that 70% alcohol tends to be a more effective disinfectant for surfaces than 90-91% alcohol.

The 70% solution also helps to ensure that the alcohol enters the cells of microorganisms, rather than just rapidly disrupting the cell walls and doing nothing further, which is what tends to happen with 90%+ alcohol solutions.

Also, the greater relative concentration of water in the 70% alcohol solution means that more proteins--and more types of proteins--in microorganism cells will be denatured (i.e. destroyed). This is because water plays an important role in the destruction of proteins.

This should not be taken to mean that 91% alcohol is not an effective disinfectant; it is. But for maximum efficacy, I would recommend 70% alcohol. If disinfection is a primary concern, you definitely do not want to go above 91%, in any case.

So as we can see, one should not simply see the bigger number and make assumptions based on that.
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Why is 70 percent alcohol a better disinfectant than 100 percent alcohol?

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Is 91 percent Isopropyl Alcohol a better disinfectant than 3 percent Hydrogen Peroxide or is 3 percent Hydrogen Peroxide better?

Hydrogen peroxide 3% is a much better disinfectant. Hydrogen peroxide may be used in the oral cavity for disinfection. Isopropyl alcohol, also known as Rubbing Alcohol is only

Why is 100 percent of alcohol not used as a disinfectant?

Probably because 100% alcohol is very flammable, truly an explosion hazard, hard to keep from evaporating, and lower concentrations will do the job. The alcohols used in th

Why only 70 percent alcohol as disinfectant not more or less concentrated?

because the 100% alcohol is inhibited the protein synthesis but can not reach at the nucleic acid and it create an protective layer out side the cell membrane but 70% alcohol

What is 70 percent alcohol and what is 95 percent alcohol which is better to use and why?

70% alcohol means in hundred parts of it, 70 parts are pure alcohol, rest is water. 95% means 95 parts of pure alcohol in 100 parts, so 5 parts of water. It depends on t