The question of whether using a shared bar of soap can transmit pathogens is common, often causing concern in public and shared living spaces. While transferring illness through a communal hygiene product might seem logical, the actual risk is extremely low, provided proper handwashing technique is used. The safety of sharing soap is largely supported by scientific evidence, which focuses attention on the mechanical action of washing and how soap functions.
How Soap Works Against Microorganisms
Soap, whether solid or liquid, is composed of unique molecules known as surfactants, which are the fundamental agents of cleanliness. These molecules have a dual nature: one end is attracted to water (hydrophilic) and the other is attracted to oils and fats (hydrophobic). This structure allows soap to act as a bridge between water and the greasy film on skin where microorganisms reside.
When soap is lathered, the hydrophobic ends attach to the oils, dirt, and pathogens present on the hands. The soap molecules then surround these contaminants, forming tiny spheres called micelles, which suspend the grime away from the skin. Running water and friction during a thorough wash then flush these micelles and their trapped contents down the drain.
This process is a highly effective form of mechanical removal, though it does not necessarily “kill” the microbes. Soap can also directly disrupt certain pathogens, as the surfactant molecules dissolve the fatty lipid envelopes surrounding some viruses, such as influenza and coronaviruses, effectively inactivating them. Washing for at least 20 seconds ensures that both the chemical disruption and the mechanical removal processes are maximized.
Differentiating Transmission Risks by Soap Type
The perceived hygiene difference between soap types is a main factor driving consumer preference, but studies suggest the transmission risk remains minimal for both.
Bar Soap
Bar soap often harbors bacteria on its surface, as it collects a thin film of moisture, skin cells, and microbes from the user’s hands. In one study, researchers contaminated bar soap with up to 70 times the bacteria typically found on used soap, including E. coli and Pseudomonas. Volunteers who washed their hands with this heavily contaminated soap showed no detectable transfer of the test bacteria to their skin after rinsing. The rinsing action, combined with the soap’s alkaline environment and its removal properties, is sufficient to flush away residual contamination before transfer to the next user.
Liquid Soap
Liquid soap dispensers are considered more sanitary because the soap remains sealed inside a container until a single-use dose is dispensed. The product inside the reservoir is virtually sterile, eliminating the risk of surface contamination seen with bar soap. However, the external pump nozzle itself can become a fomite, a contact surface that harbors pathogens transferred from unwashed hands. A more significant risk arises when unsealed dispensers are improperly refilled by topping off a partially empty container, which can introduce water and contaminants that allow bacteria to grow in the bulk soap.
Factors Affecting Germ Viability on Soap
The environment created by the soap, particularly its inherent alkalinity and tendency to dry out between uses, is hostile to the extended survival of pathogens. The primary factor influencing transmission risk is the pathogen’s resilience and environmental conditions, particularly moisture.
Enveloped viruses, which rely on a delicate fatty outer layer, are highly susceptible to soap’s chemical action and drying. Non-enveloped viruses, such as Norovirus, are far more resilient and can survive on surfaces for days or even weeks. While soap does not chemically inactivate Norovirus as easily as enveloped viruses, the mechanical scrubbing and rinsing action is the most effective method for physically removing these hardier particles from the skin.
Bacteria like Staphylococcus can survive for several weeks on damp surfaces, and fungi, which exist as tough spores, can also persist in moist areas. However, these organisms require a viable dose to cause infection. The soap’s primary function is to remove them from the skin during the wash, and the risk of contracting an infection through shared soap is mitigated. This is because the subsequent wash by the next user effectively cleans the surface and removes the potential infectious dose.