“In handwashing experiments with 21 volunteers, Rutgers University researchers found no significant difference in cleaning power between water that was 60, 79 or 100 degrees Fahrenheit. They also found that lathering hands for just 10 seconds was sufficient to remove germs….
That 10 seconds, however, applies only to time spent lathering, or rubbing hands together with soap, Schaffner notes. “The time you spend turning on the tap, putting soap in your hands, and rinsing afterward, those don’t count.”
He also points out that this is the minimum amount of time the authors are recommending for hand washing—and that some circumstances may call for longer washes. “If you just changed a diaper or you’ve been in the garden or you’re cutting up a raw chicken, don’t think you’re good to go after 10 seconds if you can still see or feel something on your hands,” he says. “By all means, keep lathering.””
So if your hands are really dirty then forget the cold water, forget the 10 seconds. Warmer is better. Longer is better. And do you really know how dirty your hands are?