Soap, Duration and Water Temperature: What Matters and What Doesn’t When It Comes to Good Hand-Washing

In the fight against the COVID-19 pandemic, our best weapon is proper hand-washing.

The novel coronavirus (known more formally as SARS-CoV-2) that is responsible for COVID-19 spreads two ways: directly, through droplets launched by coughs; and indirectly, through transfer from infected surfaces — where the coronavirus can last for up to three days. Because they’re how we physically interact with the outside world, our hands are ready conduits for the virus to enter our bodies.

By now, we’ve all heard that to mitigate this risk, we should wash our hands for 20 seconds and avoid touching our faces, which hold the key entry points (eyes, nose and mouth) where the virus can sneak into our bodies. But how important is that 20-second mark, really? Do we need to use hot water? We asked the experts what science says about what matters — and what doesn’t — when it comes to proper hand-washing.

Twenty Seconds of Soap

Soap is essential. Many pathogens, including the coronavirus, have an outer membrane made of a double layer of fatty molecules (a lipid bilayer) that is studded with proteins they use to infect cells. Soap can break down this membrane, killing bacteria and deactivating viruses (they can’t technically be killed, since they’re not alive to begin with).

At the same time, soap works to trap and remove pathogens, along with oils and other debris, from the skin’s surface. “The pure act of getting your hands sudsed up and really scrubbing them — it’s a physical act,” explains Kristen Gibson, University of Arkansas System Division of Agriculture microbiologist who specializes in the transmission of food-borne pathogens like norovirus.

But soap’s effects are not instantaneous, which is why the Centers for Disease Control and Prevention (CDC) recommends washing your hands for 20 seconds. But why 20 seconds?

Evidence suggests that 20 seconds of lathering removes more germs than shorter lathers, but there are relatively few studies that directly evaluate how different handwashing times impact health. “The short answer is, there’s not a lot of evidence that shows that, say, 14 seconds is different than 16 seconds,” says Donald Schaffner, a researcher at Rutgers University who studies microbes and handwashing.

That said, longer wash times are not necessarily better. A meta-analysis Schaffner co-authored in 2011 found that longer wash times of 120 seconds actually removed fewer pathogens than 30-second washes. “There’s usually diminishing returns after a certain period of time,” says Gibson.

Even in the midst of a pandemic, 20 seconds can feel like a long time. “People probably don’t do that most of the time,” says Schaffner. “I know, despite being a quote-unquote ‘hand-washing’ expert, I don’t do that most of the time.” People typically wash their hands for far shorter periods of time — around six seconds.

In these less-typical times, memes that encourage the longer hand-washes have themselves gone viral. While the CDC recommends humming “Happy Birthday” twice as a timer, people have found many other song options to keep themselves entertained during their 20-second scrub. (I personally have adopted the chorus of “Africa” by Toto as my hand-washing timer.)

Hot or Cold?

While time at the sink is important, technique matters, too.

“If I’m kind of slapdash, it doesn’t really matter how long I wash my hands,” says Schaffner. After all, soap cannot do its job if it’s not actually present. Be mindful to scrub between your fingers and under your nails, which are easy to miss. “My feeling is that individual variation in technique is a bigger driver than the exact amount of time,” he adds.

Other hand-washing factors are comparatively less important. When deciding between using hot or cold water, “the short answer is, water temperature doesn’t matter,” says Schaffner. There is no difference in how many microorganisms remain, so use whatever feels good for you. “If the water temperature is comfortable, what that means is that you’re going to do the best, most careful job,” he says. Likewise, the amount of soap you use need not be precise, provided you have enough to get a nice lather going.

After you rinse, be sure to dry your hands with a clean towel, which can further remove pathogens. Frequent hand-washing can damage the skin, making it harder to wash and more susceptible to colonization by pathogens, so moisturize regularly.

Like all habits, how you wash your hands can be hard to change, but the pandemic can provide a strong impetus to develop best practices for hand hygiene that you carry into the future, says Gibson. “This is really what [you] should be doing in normal life, not just under these extreme circumstances.” The important thing is the willingness to actually do it, she says.

And there’s the rub: “It really comes down to people and their willingness to actually do it,” says Gibson. “You can say all day what to do, but if someone doesn’t choose to adopt that hand-washing practice, it doesn’t matter.”

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