Why 70% Isopropyl Alcohol Is a Better Disinfectant Than 99% Isopropyl Alcohol When It Comes to COVID-19

Medical experts explain the science behind this disinfecting solution.

As the coronavirus pandemic continues to spread across the globe, it's of the utmost importance to keep ourselves and those around us healthy right now. That's why we've stocked up on hand sanitizer, disinfectant wipes, and other sanitizing products to prevent the spread of COVID-19. But as the virus continues to spread as the months go by, we're being reminded that not all the products on the market are equally effective.

Woman washing hands with antibacterial sanitizer
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One common misconception is that the higher the percentage of alcohol you're using for disinfecting, the greater the chances are that you're actually clearing your skin and surfaces of worrisome germs. Surprisingly though, the numbers don't always add up. Here's what you need to know.

What is the difference between the two?

"Isopropyl alcohol 99 percent is pure isopropanol and isopropyl alcohol 70 percent is pure isopropanol diluted with 30 percent purified water by volume," explains Dr. Lucky Sekhon, fertility specialist and board certified OBGYN in New York City. "Isopropyl alcohol 70 percent, or isopropyl alcohol 99 percent diluted to 70 percent with purified water, kills organisms by denaturing their proteins. A 70 percent isopropyl alcohol solution dissolves their lipids and is effective against most bacteria and fungi and many viruses."

Here's why 70 percent is a better disinfectant.

In terms of disinfecting, higher concentrations of alcohol are less effective at killing bacteria. "Certain bugs, such as bacteria, are better eliminated with the use of a less concentrated isopropanol, because higher concentrations cause an external injury that forms a protective wall and shields these organisms," explains Dr. Grigoriy Mashkevich MD, facial plastic surgeon at Specialty Aesthetic Surgery. "On the other hand, most viruses are sensitive to various alcohol concentrations, including in the 90 percent range. Remember that hand-washing remains the best strategy when it comes to disinfecting surfaces."

Sachin Nagrani, MD and medical director for Heal, adds that while any isopropanol percentage of 60 percent or more is effective against the common cold, flu, or coronavirus, "99 percent isopropanol evaporates very quickly which doesn't allow it to penetrate cell walls and kill bacteria, and therefore isn't as good for sanitizing hand."

And why a more powerful percentage can be less effective for certain viruses.

Sure, it seems counterintuitive, but in some cases, including in terms of preventing the spread of COVID-19, 99 percent isopropanol is too powerful of an alcohol solution. Simply put, it breaks down the outside of the cell before it can penetrate the germ in question. On the other hand, 70 percent alcohol is the perfect concoction of alcohol and water to cross a cell membrane, thereby attacking the entire cell and killing the bacteria.

However, this rule of thumb is dependent on the virus in question. Viruses with an envelope structure, such as the common cold and coronavirus, can be broken down by alcohol solutions of 60 percent or higher. But there are other viral structures, like the norovirus, which cannot be killed with the use of any concentration of alcohol. That's where hand washing reigns supreme as the most effective method for removing every form of virus and bacteria.

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