And it wastes more energy than cold.
Washing your hands in warm or hot water offers no hygienic advantage over cooler, more comfortable water, according to new research. A debate has raged over the merits of warm-water hand washing for a surprisingly long time, but a new study suggests that using cool water removes the same amount of bacteria as washing with hotter stuff. That could have real implications for energy conservation in the restaurant and food industry, where regulations have strongly recommended hot water for handwashing.
In the study, researchers from Rutgers University covered 20 participants' hands with a harmless strain of Escherichia coli bacteria multiple times over a six-month period. They were asked to wash their hands in water that was 60 degrees Fahrenheit, 79 degrees, or 100 degrees. They were also given three different volumes of soap: 0.5 milliliters, 1 milliliter, or 2 milliliters.
The researchers found no significant difference in the amount of bacteria removed at different water temperatures; they were all about the same. There was no cleanliness difference between the amounts of soap used, and antibacterial soap (which they also tested) didn't clean hands any better than the regular stuff, but they want to do more research on the type and amount of soap needed.
They also found that even just ten seconds of washing with soap and water of any temperature removes a significant amount of bacteria. For the record: The Centers for Disease Control and Prevention recommends using warm or cold(!) water and scrubbing your hands for at least 20 seconds which, as they helpfully inform us, is two repetitions of the "Happy Birthday" song. In one of the study's many test conditions, a lather time of 20 seconds blew a 5-second wash out of the water.
"I think this study indicates that there should be a policy change," Donald Schaffner, a professor of food science and one of the authors of the study, said in a statement. Many states interpret Food and Drug Administration guidelines as requiring foodservice workers to wash up in 100-degree water, when all they really say is that the sink must be able to provide water of at least 100 degrees. "Instead of having a temperature requirement, the policy should only say that comfortable or warm water needs to be delivered. We are wasting energy to heat water to a level that is not necessary."
Washing with warmer water might make intuitive sense, thanks to our association of "hotter" with "cleaner." But it's not like hotter water kills more germs; 100-degree water is hot enough to sting your hands and irritate your skin—but not hot enough to kill most microbes. (Neither is the 110-degree water required to wash dishes by hand in commercial settings.)
There's still some debate over the effects of water temperature on soaps and detergents, but Schaffner notes that in the past, health policy wasn't really in step with the science. The FDA has a conference scheduled for 2018 where it'll discuss potential revisions to its water-temperature policy. The rest of us, of course, can start now: Enjoy that cool, cool water on your soon-to-be clean hands.
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