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I know that it is common to say, "use hot water when washing your hands" or when you've got a cut, "wash your hands with warm water," etc. I was wondering, why is this the case? Since bacteria grow in warm environments, you would think that it might be beneficial for bacteria. Is this just another myth or is this actually true?

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The bacteria wouldn't see any benefit from the warm water in the ~30-60 seconds you're washing your hands, neither would hot water "sterilize" your hands at temperatures you could tolerate. The reason you wash your hands with hot water is because the hot water+detergent soap mix is better for removing oil and dirt than cold water+detergent, which is supposedly where bacteria reside.

Interestingly, there was a study published saying that washing with hot water doesn't have much of an effect on bacterial load. Hot water for handwashing--where is the proof? (Laestadius and Dimberg, 2005). I'd be interested to look more into their methods but the paper is behind a paywall and I don't have access at home.

Also, this paper, Water temperature as a factor in handwashing efficacy by Michaels et al. is open and shows no evidence that hot water is any better than cold water for removal of microbes.

[W]ater temperature exhibits no effect on transient or resident bacterial reduction during normal handwashing with bland soap.

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    $\begingroup$ Perhaps warm water just feels nicer while washing, thus encouraging longer washing. I'd suspect this should then increase the removal of microbes. $\endgroup$ – edA-qa mort-ora-y Dec 17 '14 at 18:18
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    $\begingroup$ @edA-qamort-ora-y I'm sure you're right and that this is a published result - I'll have a look if my journal access is working. $\endgroup$ – Chris H Dec 18 '14 at 14:29
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The goal of hand washing is to remove surface debris, including foreign pathogens. Most things that most people encounter on a daily basis dissolve more easily in hot water than in cold water. Thus, hot water around 100° F is used to facilitate this debris removal process, although the ideal culture temperature for E coli and a raft of related bacteria is around there too.

That being said, from the wikipedia on hand washing:

Contrary to popular belief however, scientific studies have shown that using warm water has no effect on reducing the microbial load on hands

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    $\begingroup$ "Everything that can dissolve in water dissolves more easily in hot water than cold water." That's not actually true. Gasses are much more soluble in cold water but even some solids, such as sodium sulphate dissolve better in cold water. (I concede that the removal of adsorbed gasses and sodium sulphate are not major factors in hand-washing. :-) ) $\endgroup$ – David Richerby Dec 17 '14 at 17:00
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    $\begingroup$ @DavidRicherby I changed the wording a little to reflect your comments, but I am curious about what you said. Do you have a reference? $\endgroup$ – tel Dec 18 '14 at 3:59
  • $\begingroup$ @DavidRicherby I would also like to see any source for a solid dissolving better in cold water $\endgroup$ – user2813274 Dec 18 '14 at 5:03
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    $\begingroup$ 2012books.lardbucket.org/books/… The curves with a negative slope are the examples you seek. $\endgroup$ – Stefan Dec 18 '14 at 5:06
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    $\begingroup$ Another indirect but common example of a solid being more soluble at low temperatures is calcium carbonate, which is only weakly soluble in pure water but with carbon dioxide dissolved in the water the solubility increases. Remove the CO2 by heating (remember gas solubility decreases with increasing temperature) and the CaCO3 precipitates out. $\endgroup$ – Chris H Dec 18 '14 at 14:28
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It makes no difference at all. To sterilize your hands you would need to wash them with water at around 115 degrees C at 1.2 atmospheres of pressure for over 30 minutes – that's obviously not going to happen. The bacteria that normally reside on your skin can easily tolerate tap water, no matter whether the faucet is set to maximum hot or maximum cold, or just average. Even if the temperature of the water could appreciably slow down bacterial growth (which it can't), once you've washed your hands your skin temperature goes back to as it was before the washing in a minute or so.

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  • $\begingroup$ Besides that: The bacteria living on our skin are usually not threat for us and belong to our normal bacterial environment. We learned to live with it and to control it. $\endgroup$ – Chris Dec 17 '14 at 7:34
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    $\begingroup$ The fact that you can't wash your hands in water hot enough to sterilize them doesn't mean that hot water has no benefits. (Though the other answers suggest that it doesn't.) $\endgroup$ – David Richerby Dec 17 '14 at 17:02

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