Brett Smith for redOrbit.com – Your Universe Online
You may have been told that using hot water to wash your hands is more effective at killing bacteria than cold water, but a new study from researchers at Vanderbilt University has found that using hot water may actually be a waste of energy.
Vanderbilt researcher Amanda Carrico told National Geographic reporter Brian Clark Howard that her team’s study looked at “a case where people act in ways that they think are in their best interest, but they in fact have inaccurate beliefs or outdated perceptions.”
“It’s certainly true that heat kills bacteria, but if you were going to use hot water to kill them it would have to be way too hot for you to tolerate,” Carrico added.
She explained that while heat is used to sterilize water, the temperature to do so would scald a human hand – meaning “hot” by most people’s standards isn’t too hot for some bacteria.
In the study, which was published in the International Journal of Consumer Studies, the researchers reviewed previously published literature and found “no evidence that using hot water that a person could stand would have any benefit in killing bacteria,” Carrico said.
The team found that even water as cold as 40 degrees F appeared to eliminate bacteria from the hands as well as hotter water – given that the hands were properly scrubbed, rinsed, and dried. In fact, some evidence pointed to hot water actually having an unhygienic effect.
“Warmer water can irritate the skin and affect the protective layer on the outside, which can cause it to be less resistant to bacteria,” Carrico said.
The study team also found people use warm or hot water 64 percent of the time they wash their hands. Based on that figure, the team found a significant negative impact on the environment.
“Although the choice of water temperature during a single hand wash may appear trivial, when multiplied by the nearly 800 billion hand washes performed by Americans each year, this practice results in more than 6 million metric tons of CO2 equivalent emissions annually,” Carrico said.
In most western countries, hot water heating makes up for 15 percent of home energy use and according to the American Council for an Energy-Efficient Economy (ACEEE), every 10 degree reduction in water temperature for a hot water heater will save 3 to 5 percent on water heating costs.
Carrico said she stumbled upon hand washing as a research topic after looking for easy ways to reduce greenhouse gas emissions.
“Sometimes simply educating people can go a long way toward changing behavior and reducing emissions,” she said, adding that, “some people may have a negative reaction” to changing their habits.
“With any change you are going to have some small backlash effect, but most often it is not going to make up for the benefits that come from broader public education,” Carrico said. “While behaviors like hand washing don’t account for a large source of emissions, they do play a role in meeting emissions targets and they are one more example of something people can do.”
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