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When washing my hands, I was always told to use warm soapy water.

However, is there any chemical or biological reason why warm soapy water is more effective to cold soapy water when trying to sanitise your hands?

For example, a reason given for using warm water was that it opens your pores.

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  • I was told the same thing, the reason being, it opens your pores. Not sure if thats true, but thats what I was told.
    – JD Isaacks
    Apr 26, 2011 at 18:57
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    Superior in what way? That it cleans better?
    – Sklivvz
    Apr 26, 2011 at 18:57
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    I was told in Red Cross training that the #1 factor in determining the effectiveness of hand washing is the friction. That's why surgeons scrub and scrub and scrub.
    – Apreche
    Apr 27, 2011 at 11:37
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    Of course, those natural oils also protect your hands during the winter, so if warm water removes them more effectively, it means you're more likely to get cracked and bleeding hands in cold weather.
    – Kyralessa
    Apr 27, 2011 at 13:35
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    btw, pores don't open/close (Source)
    – Oliver_C
    Apr 27, 2011 at 15:48

4 Answers 4

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According to

Hot Water for Handwashing - Where is the Proof?

The initial experiment involved testing with bland non-antimicrobial soap at 5 temperatures from 4.4°C (40°F) to 49°C (120°F). Independent of soil or bacterial type (resident or transient) there was no significant difference in efficacy attributed to water temperature.

[...]

In the second experiment antimicrobial soaps were used having different antimicrobial active ingredients, at each of two water temperatures, 29.5°C (85°F) and 43°C (110°F).

In this experiment, even though slightly higher efficacy was seen with antimicrobial soaps at higher temperatures, overall, there was no statistical difference in efficacy ... at the two water temperatures.

Concomitant to the increase in efficacy at higher temperatures was a consistent trend for increases in measures of skin damage, such as skin moisture content, transepidermal water loss and erythema. This was also found not to be statistically significant.

[...]

As has been shown by many previous researchers, overall handwashing effectiveness is more dependent on the vigorousness of execution than details such as the type of soap, the length of handwash or in this case water temperature.

For the complete text of the paper go here (it references more than 50 publications).

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    So... quite possibly, using warmer water works better in many cases as a secondary effect, because colder water makes the handwashing more perfunctory?
    – Ben Barden
    Jun 1, 2017 at 13:42
  • @BenBarden bah, I was just getting ready to point that out. stop beating me to the punch :P
    – dsollen
    Jun 1, 2017 at 14:50
  • At those auto faucets at airports and such, the water gets so hot that it becomes too uncomfortable for me to wash properly. So that works both ways. Goldilocks.
    – user11643
    Jun 1, 2017 at 15:42
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It is to do with the surface tension of the water. The hotter the water the lower the surface tension hence the easier it is to wash dirt away (obviously you still need to scrub your hands, but as the surface tension is lower dirt should come away easier).

To quote a physics site:

The surface tension of water is 72 dynes/cm at 25°C . It would take a force of 72 dynes to break a surface film of water 1 cm long. The surface tension of water decreases significantly with temperature as shown in the graph. The surface tension arises from the polar nature of the water molecule. Hot water is a better cleaning agent because the lower surface tension makes it a better "wetting agent" to get into pores and fissures rather than bridging them with surface tension. Soaps and detergents further lower the surface tension.

enter image description here

EDIT

I should probably also mention that soap reduces the surface tension a lot more than heating the water will. (Source) Unfortunately I don't have a graph for this.

EDIT

Added a link to the patent for detergents to show that the point of detergents is to reduce the surface tension (Page 4) to improve the wetting ability of water and hence improve the washing efficiency (Page 5).

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    This doesn't really make sense. How much of cleaning relies upon "getting into pores and fissures"? Skin, unless extremely oily, is not highly hydrophobic, and if it is extremely oily, the pores and fissures are full of oil. Seems to me that something else is going on.
    – Rex Kerr
    Apr 27, 2011 at 14:37
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    @Rex try pushing your finger against a glass window, do you see bubbles? I suspect there is a layer of air under the water even though it is not instantly obvious as bubbles,I suspect if I had a microscope I would be able to see it quite well even though I can't with my naked eye. Are you trying to debunk surface tension?
    – Ardesco
    Apr 27, 2011 at 15:16
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    @Ardesco - Surface tension is a property of the self-interaction of water (or whatever solution you have). Surface tension causes weightless droplets in a vacuum to form into spheres. This is related to but not the same thing as as entrapment of dirt and oils in micelles, or the solubility of hydrophobic compounds in water (see en.wikipedia.org/wiki/Surfactant for example). Glass-finger contact is a red herring since both are nominally solids; one needs gas or liquid to fill in the spaces. Check out en.wikipedia.org/wiki/Hydrophobe and the Cassie-Baxter state.
    – Rex Kerr
    Apr 27, 2011 at 17:39
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    Beware: that graph is exaggerating. The graph appears to show that surface tension halves with increasing heat, but if you read the numbers it's only decreasing from about 74 to about 68.
    – ChrisW
    Jun 7, 2011 at 12:26
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    What is to do with the surface tension of the water? This answer seems to start out by assuming hot/warm water is actually more effective than cold water, but that is exactly the claim that is being questioned! This seems like a theoretical argument for, or explanation of, a phenomenon that doesn't actually seem to be observable (or at best, barely so) in real handwashing situations.
    – paradisi
    Jun 1, 2017 at 15:05
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A recent scientific study showed that cold water works just as well. This is not the most recent study, but here is an article from The New York Times:

In a 2005 report in the Journal of Occupational and Environmental Medicine, scientists with the Joint Bank Group/Fund Health Services Department pointed out that in studies in which subjects had their hands contaminated, and then were instructed to wash and rinse with soap for 25 seconds using water with temperatures ranging from 40 degrees Fahrenheit to 120 degrees, the various temperatures had “no effect on transient or resident bacterial reduction.”

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    As noted in other comments to other answers, no difference in controlled conditions, when the amount of time is artificially equalized, but the temperature may have an effect on the behavior of the hand-washer, specifically impacting how much time one takes for the task of washing.
    – PoloHoleSet
    Jun 1, 2017 at 16:13
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Results of a study by the Journal of Food Protection:

They asked 20 people to wash their hands 20 times each with water that was 15 degrees, 26 degrees or 38 degrees.

Volunteers were also asked to experiment with varying amounts of soap - ranging from 0.5mm to 2mm. Before they started the tests, their hands were covered in harmless bugs.

Researchers say there was no difference in the amount of bugs removed as the temperature of the water or the amount of soap changed.

See Washing hands in cold water 'as good as hot' - http://www.bbc.co.uk/news/health-40118539

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