The study in the Journal of Food Protection compared what happened to bacteria on the hands of 10 men and 10 women when they washed their hands in water of varying temperatures – 15.5C (60F), 26.1C (79F) and 37.8C (100F). Each of the volunteers washed their hands 20 times over a 6-month period.
A type of E. coli bacteria was used but for safety reasons the strain selected was non-pathogenic.
The US researchers say there was no significant difference in how much bacteria were eliminated between those who washed their hands at the hottest temperature and those who used the cooler water.
The findings are in line with UK and international advice that washing your hands with soap in either warm or cold water amounts to good protection from food poisoning, colds and flu.
Professor Don Schaffner, a food science specialist at Rutgers University, who led the study, says finding a comfortable water temperature is key to effective hand washing. "The thing with heat, is to get the water hot enough to kill bacteria, you'd be blistering your hands," he tells us. "So, there's no amount of heat you can apply that would actually do anything to kill bacteria that you could possibly put your hands in."
Did the amount of soap matter?
The researchers also found no difference in bacterial removal between those who used either 0.5 ml, 1 ml or 2 ml of soap. "Now, what you shouldn't conclude from that is that this means you can just get away with not using any soap at all, or that you can get away with only using a very small amount of soap," says Professor Schaffner, who insists the findings do not support such a claim.
Antibacterial soap was more effective than ordinary soap, the study finds. However, Professor Schaffner says the difference between them was small.
Advice from health authorities in the UK is to wash hands for around 20 seconds – the time it takes to sing 'Happy Birthday' twice. The Rutgers study found that even washing hands for 10 seconds – half the time – removed a significant amount of bacteria.
"Whether you choose the bar of soap, whether you use a plain soap or whether you use an antimicrobial soap, the most important thing is to wash your hands," says Professor Schaffner.
Also, using cold water comes with an energy-saving advantage, he adds.
Quantifying the Effects of Water Temperature, Soap Volume, Lather Time, and Antimicrobial Soap as Variables in the Removal of Escherichia coli ATCC 11229 from Hands, Jensen D et al, Journal of Food Protection
Professor Donald W. Schaffner, extension specialist in food science, Rutgers University
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