Going back to my pharmacy days, the standard was 70%. I did a little quick research and it appears 70% is sufficient but I am sure one can find something to contradict it. More is not always better.
"70% will kill 99.99% of everything on the skin. The only reason to go with 91% is if you are cleaning something that can't get wet. Actually 70% isopropyl is a better disinfectant. Alcohol works as a disinfectant by denaturing proteins, a small amount of water makes it more efficient."
From the CDC:
CDC ALCOHOLLots to read if one is interested.
I have some alcohol swaps that are 70% and that is what is used prior to injections and by the health care industry.
