Dear Doctor:

I know that plain old soap and water are best for washing your hands, but sometimes it’s either do nothing, or use an alcohol-based hand sanitizer. So, I’m wondering, though they promise to kill “99.9 percent of germs,” just how effective are hand sanitizers with respect to disease-causing viruses like the flu?

Dear Reader: A hand sanitizer is a liquid made with a specific chemical composition to decrease the presence of potentially infectious agents like bacteria, fungi and viruses. You’ve asked about alcohol-based hand sanitizers, and since it’s widely agreed that formulations with an alcohol content of 60 percent or higher are the most effective, these are the ones we’ll be referring to in our answer. When it comes to specifics regarding which infectious agents these hand sanitizers kill or abate, though, things can become a bit murky.

A study done in Boston in 2005, said to be among the first into the efficacy of alcohol-based hand sanitizers, found no difference in the spread of respiratory infections between the families who used hand sanitizers and those in the control group who didn’t. Subsequent other studies over the years, as well as a repeat of the Boston study, found the same thing. As we’ve discussed here in previous columns, infections like the influenza virus are most often spread via the aerosolized droplets from an infected person’s cough or sneeze. These hang in the air, where they can be breathed in and thus cause a new infection.

Although hand sanitizers can neutralize an impressive range of microbes, they don’t work against everything. Even when you’ve used the right amount and the proper technique, germs like Cryptosporidium, norovirus and Clostridium difficile may persist. Studies have shown that hand washing with soap and water is most effective for these types of germs.

The federal Centers for Disease Control and Prevention agree with you that soap and water is the best hand-washing method. However, when you need a Plan B, do reach for the alcohol-based hand sanitizer. Those with an alcohol content lower than 60 percent, however, and those that are not alcohol-based don’t work as well for all germs. They may only reduce and not eliminate the germs on your hands and may encourage the infectious agents to develop resistance to the sanitizer.

As with hand washing, proper technique matters when you’re using a liquid hand sanitizer. Be sure to read the label of your particular product and use the full amount indicated. Dispense into one palm, then rub continuously to spread the liquid on all of the surfaces of your hands until the alcohol has evaporated, and your hands are dry.

When you do use a liquid hand sanitizer, it’s important to start out with clean hands. When hands are dirty or greasy, the products are less effective.