It’s one of those personal-care policies we don’t really think to question: Always wash your hands with hot water. It makes sense, as heat is typically considered an enemy of bacteria. The Food and Drug Administration even advises that food service kitchens should have handwashing stations with water temperatures between 38 and 42°C, or 100 and 108°F [PDF].
But according to a new study, we may be risking scalding our hands for very little payoff. Cold water may be just as effective for reducing the number of germs as hot water.
In a new report published in the Journal of Food Protection, staff at Rutgers University-New Brunswick looked at several variables in handwashing technique, including lather time, use of antibacterial soaps, and water temperature. A total of 20 volunteers washed their hands three times using water that was either 15, 26, or 38°C, with the process repeated 20 times over a six-month period. Each time, researchers looked for Escherichia coli before and after cleaning.
The result? There was no real difference in the reduction of E. coli between handwashings at 15°C (59°F) and 38°C (100°F). Participants got roughly the same amount of pathogens scrubbed off regardless of how hot the water got.
Instead of water temperature, the study’s results suggest you might want to pay closer attention to lather time. A 20-second scrub with regular soap was shown to be more effective in reducing germs than the baseline 5-second lather. This is in line with the Centers for Disease Control's guidelines, which recommend the same 20-second duration, or the time needed to hum “Happy Birthday” to yourself twice.