The risk of catching an illness from sharing a bar of soap is extremely low. Soap is a cleaning agent designed to lift and remove foreign materials from the skin, not to sterilize its own surface. Its primary function is to facilitate the mechanical removal of microbes, dirt, and oil during handwashing.
The Science of Soap and Germs
Soap’s effectiveness against microbes stems from its unique molecular structure, containing surfactants with both water-loving and fat-loving ends. The fat-loving portion penetrates the lipid membrane of many viruses and the cell walls of bacteria. This action causes the microbe’s protective shell to be pulled apart and destroyed. Surfactants also trap microorganisms within tiny bubbles called micelles. The mechanical action of rubbing your hands together and rinsing with water then washes these trapped particles down the drain. The environment of a typical bar of soap, which is often highly alkaline, is also hostile to the growth and survival of most pathogens.
Distinguishing Bar and Liquid Soap
The discussion often centers on the perceived hygiene difference between a bar and liquid soap, yet both rely on the same surfactant action to clean. Liquid soap is often perceived as more sanitary because it is dispensed individually, avoiding direct contact with the vessel. However, the dispensing mechanism or the reservoir of liquid soap can sometimes harbor bacteria if not cleaned properly. Conversely, while bacteria can certainly be found on the surface of a used bar of soap, the act of washing itself is what prevents transmission. Studies have shown that even when bar soap is intentionally contaminated, the organisms are not transferred to the hands of a subsequent user. The lathering process immediately dilutes and rinses away any surface microbes, making the bar functionally self-cleaning with each use.
Pathogens of Concern and Transmission Reality
Concern about shared soap is rooted in the fact that microbes can survive for a period on a damp surface. When a person washes their hands, they transfer a layer of skin flakes, oil, and transient bacteria onto the soap’s surface. However, the presence of a microbe on the soap does not equate to a pathway for infection. For an infection to occur, a sufficient quantity of the pathogen must be transferred from the soap to the hands, survive the washing process, and then enter the body through a mucous membrane or a break in the skin. The mechanical action of handwashing, which involves the soap’s surfactants and the flow of water, is highly effective at disrupting this chain of transmission. Any microbes picked up from the surface of the soap are immediately caught in the new lather and washed away before they can cause harm. This reality explains why decades of research have failed to link the shared use of soap to the spread of infectious disease.
Minimizing Theoretical Risk
While the risk of contracting an illness from shared soap is negligible, simple maintenance can further reduce any theoretical risk. The most important factor in keeping a bar of soap sanitary is allowing it to dry out completely between uses. Bacteria and fungi require moisture to multiply, and a dry surface limits their ability to survive. Using a soap dish that has draining slots is the best way to ensure the bar is not sitting in a pool of water. If a bar of soap has been sitting in a wet, slimy dish for an extended period and concern persists, a user can simply rinse the bar under running water before lathering. This swift rinse will wash away any surface film and trapped microbes, preparing the soap for its self-cleaning action during the actual handwashing process.