Although the FDA mandates warm water for those in food service, the CDC says any temperature is fine, so it's a bit of a contested subject.Heat is known to kill bacteria, but the temperature and duration of exposure required would seriously damage human skin, so that argument for warm water is a bust. Additionally, some advocate for washing in cool water because it uses less energy and is therefore more environmentally friendly.Anyway, no matter the temperature, washing your hands thoroughly with soap for 20 seconds is good practice.