Soap: How Much Cleaner Does It Actually Make Your Hands?

Many of us wash our hands with soap and water, but how much work is the soap really doing?

It’s common knowledge that washing your hands often and well is the best way to prevent disease transmission. Many of us are accustomed to using soap during handwashing as a matter of course — it’s there in public bathrooms, it’s in our homes, it’s in the office kitchen. Then there are those miscreants among us who seem satisfied simply to rinse with running water before going back to their business. Who are these germ-mongerers, that they think they can ignore the very clearly labeled (and fragrant!) sudsy agents the rest of us use with such diligence?
Before we get too carried away in our indignation, it’s worth pointing out that soap is neither the holy elixir we sometimes think it is, nor do the vast majority of people actually use it as fastidiously as they should. Below, what science has to tell us about the real value of soap.
How effective is soap over plain old water? It works, but all else being equal, water has a greater marginal effect. Health professionals recommend handwashing before eating, after using the bathroom, changing a diaper, and other situations in which you might come into contact with harmful bacteria. Germs cling to our hands a lot more easily than we give them credit for, and almost no amount of soap will remove them if other aspects of your handwashing technique aren’t up to snuff. On the bright side, combining good technique with water alone can actually remove a significant share of germs from your hands.
This has been proven in countries where access to soap is limited. In rural Bangladesh, where diarrhea among children is a widespread problem, scientists examined the effectiveness of four different forms of hygiene on incidences of diarrhea. Some study participants were observed preparing their family’s meals without washing their hands after using the bathroom. Others were observed washing one hand using water only; still others were seen washing both hands with water; and lastly, scientists saw some food preparers wash at least one hand with water and soap. While taking detailed notes on the manner and opportunities for handwashing, researchers also conducted monthly diarrhea tests on the children in each household in the study.

Here’s what they found:

In households where food was prepared without washing hands, children had diarrhea in 12.5% of monthly assessments compared with 8.3% in households where one hand was washed with water only, 6.9% where both hands were washed with water only, and 3.7% where at least one hand was washed with soap. Food preparers commonly washed one or both hands with water only, but fieldworkers observed food preparers washing at least one hand with soap in only three households (1%).

Through the use of water alone on both hands, the rate of diarrhea was cut by nearly half. Not bad for a little H2O. Adding in soap had a predictable effect, cutting the prevalence of diarrhea again by another 3.2 points, but the gains from soap clearly weren’t as high as from scrubbing with water. So, while avoiding soap if it’s available is still a missed opportunity to remove germs, rinsing isn’t so pointless, either. Maybe we should withhold our judgment.

Is soap always clean? This may be disappointing to diehard germaphobes, but it’s possible for soap to be crawling with bacteria as much as anything else. If you’re storing your soap improperly, such as leaving it in a wet puddle on the edge of your sink, it gives bacteria a fertile place to multiply. When you use it, you basically wind up transferring germs from the soap directly to your hands.

In a thorough study of soap contamination, one team of U.S. researchers found that even among test subjects with great handwashing technique — more on that in a minute — soap that was already contaminated wound up increasing the number of bacteria on the subjects’ hands after washing. The scientists tested three types of soap dispenser, in both lab and real-world settings. Of the three variants, the dispensers that were refillable from a giant bottle of liquid soap were by far the filthiest, leading to a 26-fold increase in handwashers’ bacteria levels. Modular dispensers that relied on sealed refills stayed clean even after a year of use. In short, both the nature of the dispenser as well as the cleanliness of the soap itself can have a major impact on how clean your hands are after washing.

How helpful is antibacterial soap, anyway? In a head-to-head test of antibacterial and regular soap, antibacterial soap has an inherent advantage. One study has shown that a 15-second handwashing session with regular soap successfully reduced E. coli by 1.72 log10, compared to 2.90 log10 for antibacterial soap. But after doubling the time spent washing, the amount of bacteria removed skyrocketed (for antibacterial soap, the figure was 3.33 log10). Increasing the volume of soap used seemed to help in the case of antibacterial soap, but there seemed to be a ceiling for regular soap beyond which more time and more soap did virtually nothing. Why?

The level of bacterial reduction caused by non-antimicrobial soap is due to its surfactants, which physically remove bacteria. Once maximum removal is achieved, soap amount and wash time do not improve surfactancy. Antimicrobial soap provides both surfactancy and biocidal modes of action.

In other words, regular soap simply causes bacteria to loosen their grip on your hands, to be rinsed away. That helps explain why using water alone still seems to work just fine, as long as you rub your hands together vigorously. By contrast, antibacterial soap has additives that are designed to kill bacteria outright.

What does it mean to have “good handwashing technique”? As you might have guessed already, washing your hands means more than slapping on a bit of soap, lathering up and then rinsing off. Anyone can “wash their hands” with soap and water and still come away with even more bacteria than when they started. The real secret to cleanliness, it seems, is not only whether you use soap, but how hard you scrub, and for how long. The way your soap is stored and dispensed also matters, although in public environments, that’s much less under your control. While health officials recommend washing for anywhere from 20 to 30 seconds, they should consider themselves lucky if people’s entire bathroom trips last that long. Realistically, you might shoot for around 15 seconds of washing — which, as it happens, isn’t much longer than the current average (with soap, it hovers around 13 seconds; without it, it’s about 11).

Brian Fung is a former technology writer at National Journal.

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