Foam soaps are increasingly replacing traditional liquid soaps in hospitals, nursing homes, schools, the food industry and other public spaces, but the trend may carry a risk, a small study suggests.
Foam soaps may not be as effective as liquid soaps in eliminating bacteria that can lead to infection, the authors say.
"In this pilot study, when standard hand washing techniques were used - washing wet hands with one pump of foam soap for six seconds and drying with a paper towel for four seconds - the foam soap was not as effective as the liquid soap in eliminating the hand bacterial load," Dr. Ozlem Equils at UCLA's School of Medicine and the Miora Educational Foundation in Encino, California, told Reuters Health by email.
Equils and colleagues tested two common brands of foam and liquid detergent-based soaps that are available in neighborhood grocery stores.
When volunteers washed with foam soap, the average bacterial colony count on each hand went from 3.6 to 2.6, on a scale from 1 to 4 - a difference that could have been a coincidence. With liquid soap, the colony count went from 3.8 to 1.2 - a statistically significant drop, according to a report in the American Journal of Infection Control.
Each group had five healthy test subjects. Two more sets of experiments were conducted with additional volunteers and yielded similar results.
The research team suggests foam soap may be less effective than liquid soap because it comes out of the pump as a lather, whereas the liquid soap lather is built up in the process of hand washing. Also, the amount of soap in foam is markedly less in a single pump than is found in its liquid counterpart.
Dr. Guenter Kampf, a hand hygiene expert at the University of Ernst in Germany, told Reuters Health that because the study was small and the methods weren't rigorous, more robust research is needed to confirm the findings.
In the meantime, he said, "For domestic use it may not make a difference whether a foam or liquid soap is used because cleaning of the hands is the main purpose of washing them."
The US Food and Drug Administration "has recently banned the marketing of over-the-counter consumer antiseptic wash products containing antibacterial chemicals due to the concern over emerging antimicrobial resistance," Equils noted.
The FDA has said it's not clear that anti-bacterial soaps available to consumers are any more effective at preventing sickness than regular soap and water. The two brands of soaps in this study did not claim to be antimicrobial.
As for alternatives to potentially ineffective foam soaps, Kampf said, "The most effective formulations are alcohol-based hand rubs or gels. That is why they are used in health care."
The second most effective type of product will be antimicrobial soaps closely followed by plain soaps, he said. "But it should always be kept in mind that the efficacy depends on the antimicrobial substance, its concentration, the volume of the product used and the application time. That is why a general assessment has limitations."
The genesis of the study was the collaboration between Nicolette Dixon, an undergraduate biochemistry major at Washington State University in Pullman and lead author on the paper, and The Miora Educational Foundation, which connects high school students with mentors in healthcare and STEM fields.