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case where people act in ways that they think are in their best interest, but they in fact have inaccurate beliefs or outdated perceptions."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, 212°F (99.98°C), 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 104°F to 131°F (40°C to 55°C.) 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 40°F (4.4°C) 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. Using hot water to wash hands is therefore unnecessary, as well as wasteful, Carrico said, particularly when it comes to the environment. According to her research, people use warm or hot water 64 percent of the time when they wash their hands. Using that number, Carrico's team calculated a significant impact on the planet.

"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," she said. That's roughly equal to the emissions of two coal-fired power plants, or 1,250,000 passenger vehicles, over the course of a year. It's higher than the greenhouse gas emissions of small countries like El Salvador or Armenia, and is about equivalent to the emissions of Barbados. If all US citizens washed their hands in cooler water, it would be like eliminating the energy-related carbon emissions of 299,700 US homes, or the total annual emissions from the US zinc or lead industries. The researchers found that close to 70 percent of respondents said they believe that using hot water is more effective than warm, room temperature, or cold water, despite a lack of evidence backing that up, said Carrico. Her study noted research that showed a "strong cognitive connection" between water temperature and hygiene in both the United States and Western Europe, compared to other countries, like Japan, where hot water is associated more with comfort than with health. The researchers published their results in the July 2013 issue of International Journal of Consumer Studies. They recommended washing with water that is at a "comfortable" temperature, which they noted may be warmer in cold months and cooler in hot ones.