Scientists have poured cold water on the idea that you need hot water to get you clean.
Advice to wash your hands with warm, soapy water has little basis in fact, a US study found.
Using cool water is just as efficient at removing germs.
The temperature required to wipe out the bacteria would cause severe burns.
Researchers at Rutgers University in New Jersey concluded that repeatedly washing your hands in hot water can damage the skin, making it harder to remove germs and possibly letting more in.
The study, published in the Journal of Food Protection, overturns some of the existing guidance on cleanliness. Washing your hands for as little as ten seconds is effective, it found, while using antibacterial soap is little more effective than normal soap at removing E.coli a potentially deadly bug that causes food poisoning.
Co-author Donald Schaffner, a professor of food science, said: People need to feel comfortable when they are washing their hands but as far as effectiveness, this study shows the temperature of the water used didn't matter.'
Researchers used a harmless strain of E.coli to test the effectiveness of hand washing by 21 volunteers. Using water at a temperature ranging from 38C (100F) to 15C (60F), there was no significant difference in the removal of bacteria. The findings back those from a series of smaller studies.
Schaffner said: This study may have significant implications towards water energy, since using cold water saves more energy than warm or hot water.'
Men spent longer lathering their hands than women, but to little avail. Researchers found that washing for only ten to 20 seconds eliminates most bacteria.
This is said to be because microbes that are easy to remove are lifted from the hands in less than 30 seconds, with those embedded in deeper layers or pores, or biochemically attached to the skin not able to be removed no matter how long someone keeps trying. The study concluded that the amount of soap used makes no difference.
The scientists say their findings should lead to a change in US Food and Drug Administration guidance on hand washing. Schaffner said: "The policy should only say that comfortable or warm water needs to be delivered. We are wasting energy to heat water to a level that is not necessary."
However, Dr Sally Bloomfield, a microbiologist at the London School of Hygiene and Tropical Medicine, said warm water could still make a difference, even if it does not kill more bacteria.
"When you want to decontaminate your hands by washing them, the soap chemically detaches the dirt and the organisms, which then come off after you rinse them," she said. "Using warm water can make the soap more effective at removing dirt because it promotes this chemical reaction."
Many British experts advise using warm water, although the NHS Choices website advocates either warm or cold.
Dr Lisa Ackerley, food safety adviser for the British Hospitality Association, said: "Warm water is good because it helps the soap to lather and it's the action of washing soap off which helps to get hands clean."
© Daily Mail