Moms always told us to wash our hands with plenty of soap and hot water before sitting down to dinner, and we would begrudgingly do it after a long day of making mud pies. Though parents were right to emphasize adherence to some standard of personal hygiene, a recent study has shown that it doesn’t matter what the temperature of the water is when washing away harmful bacteria.
For the research, which appeared in the Journal of Food Protection, scientists put high amounts of non-pathogenic bacteria on the hands of 20 subjects myriad times over six months. After each application, they were asked to wash their hands in either 60°, 79°, or 100° water with corresponding .5ml, 1ml, or 2ml of soap. Cultures were then taken and compared, and the researchers found that washing your hands for 10 seconds in hot or cold water results in the elimination of the same amount of bacteria, with no difference in how much soap is used.
"People need to feel comfortable when they are washing their hands, but as far as effectiveness, this study shows us that the temperature of the water used didn't matter," said study co-author Donald Schaffner, Ph.D., distinguished professor and extension specialist in food science.
The biggest difference they found, though, is in the amount of energy wasted to heat up water that is not giving you any cleanliness advantage, he says: "This study may have significant implications toward water energy, since using cold water saves more energy than warm or hot water. Instead of having a temperature requirement, 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."