Stronger is better, right? Concentrated dish soap degreases better. Higher proof rum will get you drunk faster. You even scoop your ice cream from the part of the tub with the most fudge and nuts. But when it comes to rubbing alcohol, higher concentrations aren’t always better after all.
Drugstores tend to carry rubbing alcohol in two varieties: a 70% version (for disinfecting), and another version at 91% or 99% for…well, it may not be immediately clear. But if you’re going to get some isopropyl alcohol or some drugstore-grade ethyl alcohol, you need to know the difference.
Use 70% when you want a disinfectant
The 70% alcohol mixtures (whether isopropyl or ethyl) are the best kind for disinfecting, and if you look at the fine print on the label, they’ll often say that’s what they’re for.
The other 30%, by the way, is water. Our best understanding of how alcohol kills germs, the CDC points out, is that it denatures proteins. Proteins are made of strands of amino acids, and they can get out of formation—essentially, loosening and tangling up—when we cook them or treat them with certain chemicals. Proteins denature more readily in the presence of water, so the thinking is that the extra water helps the alcohol to be able to break down the proteins in viruses, bacteria, and fungi.
Whatever the reason, experiments have shown that the concentrations of alcohol that can kill microbes tend to fall between 50% and 90%, and 70% isopropyl rubbing alcohol falls perfectly in the middle of the range. The 99% stuff does not.
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Use 99% when you want a solvent
So why is 99% isopropyl alcohol (or even 91%) sold in drugstores? It’s for cleaning things when you don’t care about killing germs, but when you do care about moisture. For example, if you’re trying to make sure electronic components are clean and dry, 99% alcohol is perfect. For the same reason, it’s also used to clean skin oils off of fingernails before applying nail polish.
High concentration alcohols are also good as solvents, for making certain kinds of dyes, inks, and so on. For example, 99% isopropyl alcohol would be my pick to scrub Sharpie marks off a dry-erase board. Rubbing alcohol also features as a solvent in this craft that makes watercolor-look tiles by diluting Sharpie ink. In short, 99% is a great pick for anything that rubbing alcohol can do, besides disinfecting.