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 there any scientific evidence in support of this claim?
The new 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. It 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.
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.”