Frank Bures: Is hot water best for washing hands? The answer may surprise you ...

May 4, 2019

Someone asked me what is the best water temperature for washing your hands. Instinctively, probably because of what mom always said, I said “I believe hot,” but I needed to do what we always told our kids to do when they asked a question: look it up. So I did. Lo and behold, it’s not hot.

What is the point medically to wash hands anyway? It is to avoid spreading germs and the diseases they cause. Duh. Hand transmission of malevolent microbes is one of the best ways to spread sickness. Washing is not only to eliminate the grime of the day.

I checked the internet site of the Centers for Disease Control and Prevention or CDC (which sounded like the best authority) and read the entry called Show Me the Science. It says: “The temperature of the water does not appear to affect microbe removal; however, warmer water may cause more skin irritation and is more environmentally costly.”

Their definition of microbes is tiny living organisms that may or may not cause disease. Germs are types of microbes that can cause disease, which is an important distinction. One of the first, if not the first, demonstration that hand washing prevented infection occurred in Vienna, Austria, in 1847. Ignaz Semmelweis was the doctor/professor in charge of the maternity hospital in Vienna. Childbed fever caused by bacteria, also called puerperal sepsis, killed many women who had just delivered babies. At that time nobody washed their hands between patients or after coming back from anatomy labs. Really. He made hand washing mandatory about 1847, and reduced maternal mortality dramatically. When he was forced to leave in 1850, the return to cessation of hand washing was associated with increased deaths once more.

A very well designed hand washing study from Rutgers University was published in the Journal of Food Protection in 2017. Twenty volunteers (often medical students who will do almost anything for $10), repeated different hand washing scenarios 16 times each for a total of 320 washes (no report on dishpan hands though). In each case the tested amount soap used was 0.5, 1, and 2 milliliters/ml; water temperature was 15 degrees C, 26, and 38; and lather time was 5, 10, 20, or 40 seconds.

Each day the volunteers soaked their hands in liquid solution mixed with a harmless strain of E. coli bacteria related to the bowel bacteria from the same tribe, and let their hands air dry. They then washed their hands with predetermined amounts and types of soap and water temperatures for different lengths of time. Finally, their hand culture samples were collected and quantified.

The only variable that mattered in reducing bacterial counts was the length of time they lathered up, not including wetting and rinsing times. The participants had less bacteria after a 10-second wash than 5 seconds, even less after a 20-second scrubbing, which matches the CDC recommendation of 20 seconds, or the time it takes to sing Happy Birthday twice. (You can substitute the words “wash your hands now, please do” if it bothers you that it’s not your birthday.)

The study’s lead author felt that the FDA regulations (for public health food handling recommendations) need only say that water with a comfortable temperature should be used. “We are wasting energy to heat water to a level that is not necessary.” My kind of guy. Also, turn off the water while scrubbing! In other international settings it has been shown using any water available, contaminated or not, with soap spreads less disease than not washing. You might say, use whatever water is “handy.”

The above referenced CDC site says, “The temperature of the water does not appear to affect microbe removal.” Voila! They also point out that the FDA issued a final rule in September of 2016 that 19 ingredients in common “antibacterial” soaps were no more effective than plain soaps, and are no longer marketed to the public as such. That doesn’t apply to hand sanitizers, wipes or antibacterial products used in medical settings.

So, looking up the answer worked, and showed that maybe Dr. Mom was not exactly correct, although not entirely wrong. Consider this a handy bit of information for many occasions. That 20-second bit of manual soapy labor can save lives in a place where there is Ebola virus, cholera, etc. And there are more of those than we spoiled Americans know. Check out the WASH emergency program, which stands for water, sanitation, and hygiene, in 23 countries. Does anyone have any suggestions for another tuneful ditty to hum besides Happy Birthday?