Sharing a bar of soap is generally safe for routine use by most individuals. The concern that bar soap is a reservoir for germs that will infect the next user is unfounded by scientific research. However, this safety comes with important caveats concerning storage and the specific context of its use, as the transfer of microorganisms is still possible.
The Science of Soap and Microbial Activity
Soap’s effectiveness is rooted in its chemical structure, which contains molecules called surfactants. These molecules have one end that attracts water and another that is attracted to oil and dirt. When mixed with water, the surfactants surround and trap dirt, oil, and microorganisms in structures called micelles.
The physical action of lathering and rinsing removes the germs from the skin and flushes them down the drain. While the alkaline nature of soap is hostile to the long-term survival of many pathogens, its primary role is removal, not killing. Studies demonstrated that even after intentionally contaminating hands with billions of bacteria, no detectable bacteria were transferred to a second user after washing. This shows the cleaning process effectively neutralizes the transfer risk.
Factors Determining Germ Transfer Risk
While the act of washing renders bar soap safe for general use, the soap’s surface can harbor transient bacteria between uses. This bacterial presence is typically concentrated on the slick, moist layer that forms when the bar is left wet. When a bar of soap is left sitting in a non-draining dish, the constant moisture encourages the growth of environmental bacteria.
Risk involves specific, highly pathogenic microorganisms and improper storage conditions. One study on university athletes found that sharing bar soap was associated with a higher likelihood of recurring Methicillin-resistant Staphylococcus aureus (MRSA) infections. This suggests that in environments with high exposure to serious pathogens, or for individuals with open cuts or compromised immune systems, the minimal risk increases. Rinsing the bar under water before and after use can help wash away the superficial layer of surface microbes.
Practical Alternatives and Safe Storage
For individuals concerned about sharing, particularly in high-traffic or communal environments, liquid soap remains the most hygienic alternative. Liquid soap dispensers eliminate skin-to-product contact, ensuring each dose is free from prior contamination. In hospital settings, liquid soap is preferred because studies show a significantly lower rate of contamination compared to bar soap.
For those preferring bar soap, proper storage is the most effective way to minimize microbial persistence. The most important action is ensuring the bar dries completely between uses. This is best achieved by storing the soap on a dish with drainage holes or slats, which allows water to escape and air to circulate around the entire surface. Keeping the bar out of the direct spray of water will also prolong its life and maintain cleanliness.