Ask the Doctors by Eve Glazier, M.D. and Elizabeth Ko, M.D

It’s the Soap, Not Water Temperature, That Kills Viruses

Hello, dear readers, and thank you for joining us for the continuation of our monthly letters column. We received so many questions related to the coronavirus and COVID-19 that we needed to address them.

-- A reader from Great Barrington, Massachusetts, asked if water temperature plays a role in killing the virus when we wash our hands. “I have been diligently washing my hands, but sometimes I don't wait for the water to come out hot,” he wrote. “Does washing with cold water and soap kill the germs, or does the water have to be hot?”

The answer is that the water you wash your hands with can be any temperature at all. Although warm water may be more comfortable, it’s the properties of the soap -- not the water temperature -- that breach the outer coating of the virus and kill it.

-- Along those same lines, a reader from Santa Rosa, California, urged people to consider turning off the tap during the 20 seconds they are lathering up. “Hand washing is critical for health now, but water conservation never fails to be significant,” she wrote.

-- A reader from Arkansas asked if using the microwave is a good way to destroy virus particles on surfaces. “I feel that the best way for the virus to invade my home in this rural and isolated area of Arkansas is through the mail and newspaper, and I have been running them through the microwave for 30 seconds,” he wrote. “Is the microwave killing the virus?”

A microwave oven works by exciting the water molecules in a food or beverage, which raises its temperature. The actual radiation doesn’t have a direct effect. Since viral particles are made up of proteins and fats, the microwave will not destroy them.

However, the most recent research shows that the major source of transmission of the novel coronavirus is through respiratory droplets, which are emitted while breathing, sneezing, coughing, speaking or laughing. And while viral particles have been found to persist on paper for up to 24 hours and on plastic for up to three days, the risk is quite low. There are no documented cases of that kind of transmission. If you’re still worried, you can use hand-washing or gloves to protect yourself until 24 hours have passed.

-- A reader from Indiana, who, like so many of us, is running low on hand sanitizer, asked about going DIY. “Will the homemade wipes be as effective in killing germs as the prepackaged brands?” she wrote. As long as the alcohol content in your formula is at least 60%, the minimum needed to kill most germs, your home brew will be effective. The only way to achieve this is by using isopropyl or rubbing alcohol that is 99% alcohol volume. Be sure to rub all hand sanitizers into your hands until they are completely dry, from 30 to 60 seconds.

Thank you again to all of our readers for your kind thoughts and well wishes. We hope you and your families stay safe and well.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)