Why Is Rubbing Alcohol Used As A Germicide?

Rubbing alcohol kills germs by dissolving the fatty membranes that hold microbial cells together and denaturing (unraveling) the proteins inside them, destroying bacteria, fungi and many viruses from the inside out. A 60–90% solution (70% is the sweet spot) works best, because the water it contains helps the alcohol soak in before it evaporates.

Before any injection or blood test that you may have taken in the past, you have likely noticed the phlebotomist (the fancy word for the person that draws your blood) wipe the skin region to be injected with rubbing alcohol or a surgical spirit. Why is that?

It’s because alcohol is a germicide, meaning that it kills all the germs present on your skin. This property is why alcohol is added to sanitizers.

So… what makes alcohol an effective germ-killing liquid?

Alcohol Breaks Open And Destroys Microbial Cells

First, to understand how alcohol kills germs, we need to know exactly what alcohol is.

Alcohol is an organic solution that has both water and fat-loving properties. These properties are because alcohol has both an alkyl group (-CH₃) and a hydroxyl group (-OH). The alkyl group gives alcohol its fat-loving properties, making it a solution that can dissolve lipids. On the other end, the -OH group gives alcohol its water-soluble property.

The most commonly used alcohol is ethanol, also known as ethyl alcohol (the same stuff one might drink on a Friday night after work).

Because of the -OH group, ethanol is a polar molecule. The oxygen pulls electrons more strongly than the hydrogen attached to it, so the molecule ends up with a slightly negative end and a slightly positive end (a tiny built-in tug-of-war chemists call a dipole). The molecule itself carries no net charge, but that lopsided pull is what lets it grab onto the parts of a microbe it needs to.

These properties give alcohol its germicidal activity.

When alcohol is applied to the skin or any other surface, it dissolves the bacterial cell membranes, as they are made of lipids (fat molecules). It also denatures the bacteria’s proteins, meaning it unravels them from their carefully folded working shapes into useless tangles. It basically breaks down bacteria by dissolving their body’s components. Sounds gruesome!

Alcohol doesn’t just kill bacteria. It works the same way on fungi and on viruses that are wrapped in a fatty envelope, which is exactly why a quick squirt of sanitizer is so handy during cold and flu season.

What Is The Right Concentration Of Alcohol?

If you’ve ever read the contents of rubbing alcohol or sanitizers, you would have seen that 100% alcohol is not used. Instead, it is diluted with water because the water also helps in the disinfection process.

Let’s go back to our applying rubbing alcohol to the skin example. There is always some dirt on our skin that harbors more germs that are stuck to it. The water present in the rubbing alcohol bottle helps wash away the dirt that’s harboring the germs.

Apart from that, water helps in making the ethanol solution more permeable to germs. Additionally, ethanol evaporates very easily and water’s presence slows down the evaporation rate, increasing the contact time with the germs and making it more effective.

You’ll find rubbing alcohol with different concentrations, but 70% is usually the go-to. In fact, even at the laboratory I worked at, before doing any work that needed sterile conditions, I’d wipe down my workspace with 70% ethanol.

It’s important to not over-dilute the alcohol, as at concentrations below 60% it loses its germicidal strength. On the other end of the spectrum, using too much alcohol (90-100%) isn’t the most effective either. Part of the problem is that it evaporates before it has had time to act, but there’s a sneakier issue too: very strong alcohol denatures the proteins on the outside of a microbe so fast that it seals them into a hard shell, a sort of scab that shields the still-living interior from the alcohol. A little water slows that down just enough for the alcohol to reach all the way inside.

What Kinds Of Alcohol Are Used?

Rubbing alcohol consists of mainly two types; ethyl alcohol (ethanol) and isopropyl alcohol (isopropanol). Methanol can also be used, but that is extremely toxic to humans.

Ethanol is a compound you may be familiar with, as that’s the compound present in alcoholic beverages. Isopropanol, however, is not drinkable at all (not that the ethanol in hand sanitizers are) and is also a germicide.

Both work as effective germicides, and which one wins depends on the target. Isopropanol is a touch better at killing common bacteria such as E. coli and Staphylococcus aureus, while ethanol has the edge against viruses, especially the tougher ones that don’t have a fatty envelope. For everyday skin and surface use, though, the difference is small enough that you can reach for whichever bottle is closest.

When we apply any sanitizer or rubbing alcohol on our skin, we feel that slight cooling effect as it evaporates away. In doing so, it also dehydrates our skin by stripping away whatever oils and moisture were on it, leaving it dry. Both alcohols do this, which is why repeated use leaves your hands feeling parched. Isopropanol is also the go-to for cleaning electronics, not because it dries faster (it actually boils at 82.6 °C / 180.7 °F versus ethanol’s 78.4 °C / 173.1 °F, so it lingers a touch longer), but because it leaves almost no residue behind and doesn’t conduct electricity.

Where Else Is Alcohol Used As A Germicide?

Apart from applying alcohol on the skin before injections, it is also used as a surface disinfectant to clean floors and surfaces. Hospitals also use alcohol to wipe down small surgical instruments, thermometers and ventilator surfaces.

However, wiping down surgical instruments isn’t enough because there is one thing alcohol is not good at killing: spores. Spores have a hard coating, making it difficult for alcohol to penetrate and destroy them. So, on top of wiping down instruments with alcohol, they are also either UV or radiation sterilized.

Additionally, alcohol is a germicide, meaning that it kills the germs with which it comes in contact. It can’t prevent microbial growth that may arise after it evaporates away.

What Can Rubbing Alcohol Not Kill?

For all its talents, rubbing alcohol is not a cure-all. We’ve already met its biggest blind spot, bacterial spores, but there’s another important one: viruses that lack a fatty outer envelope. Since alcohol’s opening move is to dissolve those lipid coatings, naked viruses like norovirus (the infamous “stomach flu” bug) shrug it off. That’s why hand sanitizer is no match for a norovirus outbreak, and why health agencies tell you to wash with soap and water instead, where the friction physically scrubs the virus off your skin.

Two other questions come up a lot, so let’s settle them. Is rubbing alcohol an acid or a base? Neither, really. Isopropyl alcohol is essentially neutral, with a pH usually somewhere between 6 and 8, so it won’t eat through surfaces the way a strong acid or alkali would. And is rubbing alcohol the same thing as hydrogen peroxide? No, though it’s an easy mix-up since both live in the bathroom cabinet as disinfectants. They’re completely different chemicals: alcohol denatures proteins and dissolves membranes, while hydrogen peroxide is an oxidizer that attacks germs by ripping electrons away from them. Hydrogen peroxide is usually slower to act, but it can handle a few things alcohol can’t.

Conclusion

Now that you know why rubbing alcohol is such a good germicide, you still shouldn’t go rubbing your hands with it all the time. Remember, too much of it causes dry skin. That’s why some sanitizers have added moisturizers and emollients that trap the moisture and keep it in the skin.

Rubbing alcohol isn’t the only chemical used as a germicide. Other chemicals like hydrogen peroxide, sodium hypochlorite (bleach) and formaldehyde are also excellent germicides. However, alcohols are usually the preferred choice, as they are cheaper and less harsh for everyday use.