70% ethanol (or just ethanol in general) is harmful to bacteria and not human cells because human cells are protected by the epidermis, or skin.

The skin secretes oils naturally, and these oils and cells help to serve as a barrier to a lot of different molecules, including ethanol. If the ethanol were to come into direct contact with human cells, cell death would also occur.

In fact, you'll see the skin is included in the innate immune system, which is the body's first line of defense against pathogens and any other toxins. Cells can also secrete extracellular proteins and sugars (polysaccharides) that can serve as a barrier against ethanol - this would be especially useful in soft tissue such as the mouth and the intestinal tract, where you need to be able to prevent bacteria from crossing over into the bloodstream but need the bacteria present for good digestion. Fun fact - that's also why antibiotics can hurt your stomach; your stomach can't function as well without its beneficial microbiome.

Yes! 70% ethyl alcohol [ethanol] affects human cells! Vodka has about 40% alcohol, and drinking too much of it can damage your liver and cause other serious problems to your body.

Think of tiny microbes covered with 70% alcohol, as compared with just having some 70% alcohol on your hand, which is covered with layers of skin that protect the cells under the skin.

Our blood cells would probably burst open if we put them into 70% alcohol. Here's a similar experiment, where the red blood cells burst open [= 'hemolysis]:

click here

"I had today an experiment that we put 95% alcohol to the blood which made it completely transparent so hemolysis must have occurred. I started to think about the reasons."

We're so much bigger than microbes, so it takes much more alcohol, and much more time, for the alcohol to get into our bodies the way it gets quickly into tiny microbe bodies.