December 19, 2013 - It doesn't kill germs better than cooler water, but turning tap temperatures high, the U.S. burns carbon equal to the emissions of Barbados.
It's cold and flu season, when many people are concerned about avoiding germs. But forget what you think you know about hand washing, say researchers at Vanderbilt University. Chances are good that how you clean up is not helping you stay healthy; it is helping to make the planet sick.
Amanda R. Carrico, a research assistant professor at the Vanderbilt Institute for Energy and Environment in Tennessee, told National Geographic that hand washing is often "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case where people act in ways that they think are in their best interest, but they in fact have inaccurate beliefs or outdated perceptions."
Carrico said, "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."
She explained that boiling water, 212F (99.98C), is sometimes used to kill germs-for example, to disinfect drinking water that might be contaminated with pathogens. But "hot" water for hand washing is generally within 104F to 131F (40C to 55C.) At the high end of that range, heat could kill some pathogens, but the sustained contact that would be required would scald the skin.
Carrico said that after a review of the scientific literature, her team found "no evidence that using hot water that a person could stand would have any benefit in killing bacteria." Even water as cold as 40F (4.4C) appeared to reduce bacteria as well as hotter water, if hands were scrubbed, rinsed, and dried properly.
In fact, she noted that hot water can often have an adverse effect on hygiene. "Warmer water can irritate the skin and affect the protective layer on the outside, which can cause it to be less resistant to bacteria," said Carrico.