Q: Does the temperature of the water matter when washing your hands?
A: No. The FDA Food Code for retail food operations mandates that handwashing sinks provide water at 100°F or higher, and this guideline is often interpreted to mean that such temperatures are best. But there’s no good evidence to support the idea that hotter is better. Rather, a few studies have found that water temperature has no significant effect on reducing microbes under normal handwashing conditions.
The latest study, in the Journal of Food Protection in June 2017, involved 20 volunteers who, after having their hands contaminated with a nonpathogenic strain of E. coli bacteria, washed their hands under different conditions, including varying water temperatures (100°, 80°, and 60°F). As in previous studies, the cool water was just as good as warmer water in reducing microbial load.
Hotter water does cut through oil on your hands faster, so they may feel cleaner. But very hot water can also damage skin, making it more susceptible to colonization by bacteria, which are then harder to remove. Hotter water also uses more energy. So washing with cooler water is not only as effective, it’s also more energy-efficient.
The new study also tested the optimal amount of soap to use (one pump of foam soap was just as effective as four pumps) and lather time (10 to 20 seconds of rubbing soapy hands together, followed by a 10-second rinse, was better than five seconds of scrubbing followed by rinsing). Lathering beyond that had no additional benefit, possibly because microbes that are not already washed off by then are so embedded in the skin that they will not be removed no matter how long you wash your hands. The study further confirmed that antibacterial soap is not significantly better at removing bacteria than plain soap.
Also see 6 Tips for Smart Handwashing.