Why is 70% alcohol solution a much better disinfectant than a 90% alcohol concentration? Wouldn’t a higher concentration mean better?
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All disinfectants (bleach, alcohol, ect.) have a “soak time” to be effective at killing viruses and bacteria. This is the amount of time the surface being disinfected has to remain moist and undisturbed to penetrate the cell walls and actually break down the virus or bacteria.
90% alcohol evaporates quickly and does not have the time to fully “soak” into the cells it’s trying to destroy. That’s also why using a single wipe of a bleach wipe doesn’t actually do anything. (Most I’ve seen require enough wipes to keep the surface wet for at least 30 seconds.)
If you’re just removing grime you can see, the concentration doesn’t matter. It’s the microscopic grime that takes time to kill.
The pancake example currently at the top of this thread is perfect!
90% evaporate too fast, germ(infacting agent) can still survive after all the solution evaporated. 70% evaporate at slower rate so germ will die before the solution completed its evaporation.
Other than what the top comment explained, 70% is the magic number if you don’t want your disinfectant to evaporate before it actually disinfects. Those who have problems with dry skin also use 60% if needed.
Absolute alcohol has a very low flash point and it’s easy to accidentally ignite it from a naked flame used to sterilize needles and scalpels, so 70% was considered to be the best compromise. Also it doesn’t evaporate too quickly so when it’s sprayed onto a surface it has sufficient contact time to kill pathogens.
Alcohol is like Hulk who beats up bad guys. And water is the vehicle Hulk uses to reach thebad guys.
With 90% alcohol and 10%water, alcohol can’t reach the bacteria to beat them up. But 70%alcohol and 30% water can reach all the bacteria
I use 70% alcohol for my mcology hobby. I’ve also used 90% alcohol. The rule of thumb is that things need to soak for three minutes and if you spray it on, 90% evaporates much too quickly. Other than that some are commenting on the mechanism of action where 90% causes cells to coagulate and protect themselves but I don’t know about that. IMHO 90% would work just as well if you reapplied it and waited the three minutes.
90% alcohol evaporates quicker than 70% alcohol, so the 70% alcohol is able to stay on the bacteria longer and attack it more
90% evaporates too quickly to be as effective – it will still disinfect, just not as completely. The 70% stuff takes a bit longer to evaporate, so longer exposure time means more time to do its job and therefore more effective.
Cells are hydrophilic. This means the molecules they are made of attract water. So when the cells come into contact with alcohol mixed with water, the cells will act like a sponge and absorb the water, and also brings with it the alcohol.
If it’s just alcohol, the sponge effect won’t work so well on the cells.
It’s all about fire hazard vs evaporation rate vs protein disintegration.
70% is the most suitable condition. Additionally, if you use about 90% alcohol- proteins will denature real fast and make a protective outer layer which will effectively decrease the efficiency.
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