There are just too many things living on our hands to wash all of them off. Killing all the microbes on your hands has never even been the point of hand-washing. The point is to get as big a reduction in microbes as possible while balancing that effort with the demands of real life.

Your hands are never going to be clean

The Centers for Disease Control and Prevention says to wash your hands for 20 seconds. The World Health Organization says hand-washing should take 40 to 60 seconds. And it's complicated to determine who is right. The optimal length of time for hand-washing is likely to depend on many factors, including the type and amount of dirt on the hands and the setting of the person washing hands (temperature of water, washing technique, the type of soap, etc.)

The numerical oddities 

Food safety researchers have to think in orders of magnitude, by necessity. Instead of aiming to completely wipe out a population of microbes, they’re trying to achieve a relative decrease as measured in orders of 10. A 1-log reduction would be a 10-fold decrease — so, 90 percent fewer germs. A 2-log reduction is a 100-fold decrease — so 99 percent fewer germs — and 3-log is 1,000 times less, or 99.9 percent fewer. You get the picture.

Logarithmic reduction

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<figure> <img src="https://fivethirtyeight.com/wp-content/uploads/2020/03/GettyImages-1206913947-4x3-1.jpg?w=575">
</figure>Don Schaffner wants you to understand that your hands are never going to be clean. He should know - over the past 20 years, he has published multiple research papers investigating the minutia of hand-washing. (Work that is extremely relevant to the public interest all of a sudden.) And those decades of work have taught him that there are just too many things living on our hands to wash all of them off. In fact, he says, killing all the microbes on your hands has never even been the point of hand-washing. The point is to get as big a reduction in microbes as possible, while balancing that effort with the demands of real life.
Technically, Schaffner is a professor of food science at Rutgers University, where he studies food safety and the microbiology of the stuff we eat. But for our purposes, he's a hand-washing expert. And his expertise can help shed some light on a lot of simplified tips we've been hearing since the COVID-19 pandemic began. "Hand-washing isn't magic. It doesn't magically sterilize your hands," Schaffner told me. And that's an important thing to understand, because it helps explain one of the numerical oddities of the COVID-19 pandemic. The Centers for Disease Control and Prevention says to <a href="https://www.cdc.gov/handwashing/when-how-handwashing.html">wash your hands for 20 seconds</a>. The World Health Organization says <a href="https://www.who.int/gpsc/5may/Hand_Hygiene_Why_How_and_When_Brochure.pdf">hand-washing should take 40 to 60 seconds</a>. Who is right? And why? Oh, that's easy, Schaffner said. The answer is neither. And both. It's complicated.
By necessity, food safety researchers have to think in orders of magnitude. Instead of aiming to completely obliterate a population of microbes, they're trying to achieve a relative decrease as measured in orders of 10. This is called a logarithmic reduction. A 1-log reduction would be a 10-fold decrease - so, 90 percent fewer germs. A 2-log reduction is a 100-fold decrease - so 99 percent fewer germs - and 3-log is 1,000 times less, or 99.9 percent fewer. You get the picture.
"Which sounds good, right, except that in microbiology we may be dealing with millions of organisms," Schaffner said. A 2-log reduction on 1 million microorganisms still leaves 10,000 organisms left. "And in some cases with these organisms, it may only take a single organism to make you sick."
Measurements of the log reductions are how we know that washing with soap is more effective at removing microorganisms than simply washing with water. In pre-coronavirus studies, scientists found that <a href="https://www.ncbi.nlm.nih.gov/pubmed/18095447">washing with antimicrobial soap</a> - something containing the antibacterial chemical triclosan, say - was more effective than washing with plain old soap. <a href="https://academic.oup.com/cid/article/31/1/136/317796">Alcohol-based hand rub</a> was more effective than either of those.
In fact, it turns out that what you're washing with has a much bigger statistical effect than how long you're washing.
That's something Schaffner found out years ago when his team reviewed the literature on hand-washing techniques - oh, yeah, this is a whole field, people - to determine which findings held consistent across research by different groups and which were flukes in the data from one or two studies. <a href="https://meridian.allenpress.com/jfp/article/74/11/1875/170575/A-Meta-Analysis-of-the-Published-Literature-on-the">They ended up analyzing 25 publications</a>, which included more than 300 individual experiments.
"In this paper, we looked at wash times of 10 seconds, 15 seconds, 20 seconds, 30 seconds," he said. "I have some at 90 seconds and some at 120 and even one paper that studied 180 seconds. And if you look at the trend line, it's not like the longer you wash the better it gets."
In fact, the log reductions vary widely between trials that use the same time length. There are 30-second hand-washing trials that don't reduce the microbial load at all ... and ones with a reduction greater than 4-log. The best of the 120-second trials doesn't break 3-log. The 180-second trial barely gets better than a 1-log reduction. The difference probably has to do with other variables like specific soap formulation, the type and amount of microorganisms, and individual technique.
This uncertainty is a big part of why there's not consistency between expert recommendations for how long you should be washing your hands now, during this coronavirus epidemic. Even the CDC has acknowledged that this sort of thing is an issue: "The optimal length of time for hand-washing is also likely to depend on many factors, including the type and amount of soil on the hands and the setting of the person washing hands," it wrote on <a href="https://www.cdc.gov/handwashing/show-me-the-science-handwashing.html">a non-COVID-19 webpage</a>.
Neither the CDC nor the WHO returned requests for comment. (We hear they're a little busy.) But Schaffner's best guess is that the firm-sounding time recommendations are determined less by evidence and more by evidence-based estimating - picking times that aren't so short that the job doesn't get done and aren't so long that people give up.
And that's true about a lot of risk management, Schaffner told me. Another good example is the temperature you wash at. Namely, it doesn't matter. Again, the issue is about the soap and about how you're rubbing your hands around. That will work in cold water as well as warm. But the Food and Drug Administration's food code for restaurant safety still expects hand-washing sinks to be able to deliver very hot water. Science can give you a range of answers under different scenarios and likelihoods of accuracy. "But at the end of the day, some person somewhere has to write down a number," Schaffner said.
That's not to say that hand-washing isn't important. It is. It's just not something you can expect to be an exact science, with perfect results that are perfectly replicable. "I'm a big believer in hand-washing," Schaffner told me. "I think it's good. We should have been doing it before the pandemic. We should keep doing it after. And, heck, let's do it during the pandemic, too."
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CORRECTION (March 31, 2020, 12:46 p.m.): An earlier version of this article misstated how many organisms would be left after a 2-log reduction on 1 million microorganisms. It is 10,000, not 100,000.
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Don Schaffner wants you to understand that your hands are never going to be clean. He should know - over the past 20 years, he has published multiple research papers investigating the minutia of hand-washing. (Work that is extremely relevant to the public interest all of a sudden.)

Hand-Washing Technique Is Surprisingly Controversial

Your hands are never going to be clean

The numerical oddities

Logarithmic reduction

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