Is warm water more effective than cold water for washing one's hands?

Question

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.

Answer 1

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).

Answer 2

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.

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).

Answer 3

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.”

Answer 4

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?

Soap may dissolve better in warm water than cold (this is perhaps especially true of the soap your grandmother used to use, when your mum was being taught how to wash her hands).

People may do a longer/more thorough job of washing their hands when the water is warm (luxuriating the warm water all over their hands and between their fingers etc, instead of removing their hand from the cold water asap).