US scientists say they have poured cold water on the theory that washing hands with hot water kills more germs than unheated water.
The small study of 20 people found using water at 15C (59F) left hands as clean as water heated to 38C (100F) .
The report, in the Journal of Food Protection, suggests this could help cut electricity bills in restaurants.
NHS advice recommends that people wash their hands in either cold or warm water.
In this study, scientists at Rutgers University-New Brunswick wanted to find out if popular assumptions about the benefits of warm or hot water and official guidance on hot water – given to the food industry in the US – held true.
They asked 20 people to wash their hands 20 times each with water that was 15C (59F), 26C (79F) or 38 degrees (100F).
Volunteers were also asked to experiment with varying amounts of soap.
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.
Prof Donald Schaffner said: “People need to feel comfortable when they are washing their hands but as far as effectiveness [goes], this study shows us that the temperature of the water used did not matter.”
However, the researchers accept their study is small and say more extensive work is needed to determine the best ways to remove harmful bacteria.
In the UK, NHS experts say people can use cold or hot water to wash their hands.
They say hands should be washed for at least 20 seconds and stress the importance of using enough soap to cover the whole surface of the hands.
Their guidance focuses on rubbing hands together in various ways to make sure each surface of each hand is clean.