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I was recently told that I should use hot water to wash my hands rather than using cold water because the hot water will kill germs better. The person who told me this isn't a very reliable source, so I'm not sure if I should believe them or not. It does seem like it makes sense, though.
Does washing your hands with hot water kill germs better than washing them with cold water?
Surprisingly, no. Hot water does kill bacteria, but what's comfortable for your hands is also pretty comfortable for bacteria. Most pathogens start to die off around 60°C to 70°C (140°F to 158°F)1, but water from the "hot" tap in a sink is typically below that (40° to 55°C or 104° to 131°F)2. In order to kill bacteria, the water would have to be way too hot for you to tolerate.
According to National Geographic, in Washing Hands in Hot Water Wastes Energy:
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.
This agrees with the World Health Organization's conclusion3:
Water temperature - Apart from the issue of skin tolerance and level of comfort, water temperature does not appear to be a critical factor for microbial removal from hands being washed. In contrast, in a study comparing water temperatures of 4 °C, 20 °C and 40 °C, warmer temperatures have been shown to be very significantly associated with skin irritation. The use of very hot water for handwashing should therefore be avoided as it increases the likelihood of skin damage.
There are a number of studies that have looked into water temperature and handwashing and concluded that there is little to no added benefit to washing with hotter water, for example:
Water temperature as a factor in handwashing efficacy.. Michaels, B., Gangar, V., Schultz, A., Arenas, M., Curiale, M., Ayers, T. and Paulson, D. (2002), Food Service Technology, 2: 139–149
The Environmental Cost of Misinformation: Why the Recommendation to Use Elevated Temperatures for Handwashing is Problematic. Carrico AR, Spoden M, Wallston KA, Vandenbergh MP. Int J Consum Stud. 2013 Jul 1;37(4):433-441. (This study examines beliefs about handwashing and unintended economic or climate effects, rather than water temperature directly.)
The CDC recommends using soap, scrubbing for at least 20 seconds, and rinsing under running water4. Based on the currently available scientific literature, it does not recommend using hotter water, as it does not seem to have an effect.
In short, scrubbing and using soap are far more effective for hand-washing than using hot water is.
They weren't entirely wrong. Saying that warm water kills germs better isn't true, but warm water is good for getting rid of dirt, grime, and oils. It works in the same way as dissolving sugar in water. Heat causes partials to move faster which increases the chance of a collision and therefore the rate at which molecules bond. So in the end, warm water is still the better option even if it doesn't kill germs. https://www.chemguide.co.uk/physical/basicrates/introduction.html#top https://www.chemguide.co.uk/physical/basicrates/temperature.html
