New research suggests that hot water is not any better at washing away germs than cold water.
The U.S. Food and Drug Administration (FDA) state that warm water is more effective for removing germs during handwashing than cold water is, and they require the water temperature in restaurants, cafeterias, and other food service establishments to be 40°C, plus or minus 2 degrees (or between 100 and 108 degrees Fahrenheit).
The rationale for this is that hot water makes soap lather and helps to get rid of the germs. But is this scientifically proven?
Previous research has drawn attention to the fact that there is not scientific evidence to back up the claim that hot water is required to kill off germs during handwashing. And now, new research suggests that cold water might do the trick just as well as hot water.
The study was carried out by researchers at the Rutgers University-New Brunswick in New Jersey, and the results were published in the Journal of Food Protection.
Analyzing the effects of cold water versus hot water in handwashing
The new study examined the effect of various factors, such as soap volume, water temperature, lather time, and the handwashing efficacy of the soap as formulated on the product.
At the beginning of the study, the participants used 1 milliliter of non-antibacterial soap for a 5-second lather time at a water temperature of 38°C.
The bacterium examined was ATCC 11229, a nonpathogenic strain of Escherichia coli.
The researchers examined the effects of hot and cold water handwashing on 20 volunteers, consisting of 10 men and 10 women.
Each test was replicated 20 times over a period of 6 months. During this time, the participants washed their hands in water that was 16°C, 26°C, or 38°C.
The volume of soap used also differed, with participants washing their hands with 0.5 milliliters, 1 milliliter, or 2 milliliters of soap.
Results call for FDA policy change
Overall, using an antimicrobial soap did not prove to be that much more effective than using regular soap. Lather time, however, significantly improved efficacy in one scenario.
Importantly, water temperature did not have a significant effect on reducing bacteria. Whether it was 38°C or 16°C, the researchers did not detect any difference in bacteria reduction.
Additionally, the study revealed that even washing hands for as little as 10 seconds is effective enough for removing germs.
The findings are particularly important, given the FDA's regulations on water temperature for safe food handling and concerns around energy waste.
Donald Schaffner, the study's corresponding author, is a distinguished professor and extension specialist in food science. He explains, "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 didn't matter."
This study may have significant implications towards water energy, since using cold water saves more energy than warm or hot water.
There should be a policy change. Instead of having a temperature requirement, the policy should only say that comfortable or warm water needs to be delivered. We are wasting energy to heat water to a level that is not necessary."
The authors concede that more studies are required to determine exactly how much soap and what type is most efficient for removing harmful bacteria.