Signs in many bathrooms across the country recommend washing hands in hot or warm water. In fact, if Americans could be persuaded en masse to use a comfortable water temperature when washing their hands, it could prevent the annual greenhouse gas emissions totaling the equivalent of the United States’ lead industry or the entire output of a small nation such as Barbados.
So say four Vanderbilt University researchers led by Amanda R. Carrico, research assistant professor at the Vanderbilt Institute for Energy and Environment. Assisting with the study were Micajah Spoden, research analyst at Vanderbilt; Michael Vandenbergh, director of the Climate Change Research Network at Vanderbilt and David Daniels Allen Distinguished Chair of Law; and Kenneth A. Wallston, professor of psychology at the Vanderbilt School of Nursing.
“Although the perception that hot water is more hygienic is based in some factual evidence … there are few, if any, hygienic benefits of using warm or hot water to wash one’s hands,” Carrico and her colleagues write. “It is true that heat kills bacteria; however, the level of heat required to neutralize pathogens is beyond what is considered safe for prolonged human contact.”
In addition to global warming emissions, the use of elevated temperatures can cause skin irritation, which can lead to more bacterial colonization, not less.
Some soap may emulsify better with warmer water, but for most routine hand washes any such gains are irrelevant if hand washers follow a proper regimen of scrubbing, rinsing and drying, according to research reviewed for the study.
Other kinds of heated water use have been targeted by global warming researchers as potential ways to reduce greenhouse gas emissions, including laundry and dish washing. For the study on hand washing, Carrico and her collaborators found that 69 percent of 510 people surveyed believed that hand washing with warm or hot water is more effective than room-temperature water.
“The average respondent reported using warm or hot water 64 percent of the time,” Carrico wrote. “Although the choice of water temperature during a single hand wash may appear trivial, when multiplied by the nearly 8 billion hand washes performed by Americans each year, this practice results in in over 6 MMt (million metric tons) of CO² eq (equivalent emissions) annually.”
The temperature used during hand washing would be a good candidate for a public information campaign, say the authors of the report.
“Multiple federal and state organizations recommend using elevated water temperatures, in some cases citing its superior cleaning ability as the reason,” they write.
“There is no doubt that the intention of these organizations is to promote a behavior that is vital to protecting public health; however, the preponderance of evidence … suggests this recommendation provides no health benefit.”
Carrico, Spoden, Vandenbergh and Wallston suggest the public be encouraged to use a “comfortable” water temperature when washing their hands. They acknowledge that in colder climates warm water may be most comfortable.
“Although the degree of temperature was not measured in the current study, these data suggest that those who hold accurate beliefs do choose against selecting warm or hot water at the faucet on many occasions.”
The study was published in the July 2013 issue of International Journal of Consumer Studies.