why do alcohols decimate microbes at 70% and not at 90 or 100%?

why do alcohols decimate microbes at 70% and not at 90 or 100%?
the germs evolve?
because they do
if you mean why do we use 70% isapropyl alcohol instead of a stronger %, it's because it works merely as well as a high % and is safer and cheaper. Alcohols still kill germs at high %, but more is not always better. Children hold been poisoned by cooling alcohol baths and that front to finding a lower strength that would still kill germs and pose smaller quantity risk.
Some water is required for the ethanol (EtOH) to disinfect. Absolute EtOH will dehydrate the microbes hurriedly, and the EtOH will not be able to enter/kill the cell effortlessly.
Studies have shown that the combination of 70% isopropyl alcohol and 30% hose down does a better job of bloodbath bacteria than other combinations. I must mention that 70% alcohol can be contaminated beside certain microbes and that care must be taken to prevent that contamination of the alcohol remaining contained by the bottle.