Is the Shared Bar a Hazard? A Transfer Bound for Bar Soap Under Rinsing and Drying Conditions

This study examines a practical question in everyday hygiene by explicit arithmetic from stated assumptions. Under the full chain, a bar used again after 3 hours of draining delivered at most 5 organisms to the next user's hands before their wash, and at most 0.1 remained after the wash itself, against a typical pre-existing hand load in the model of 1000000. Removing the pre-rinse raised the bound tenfold to 50, and keeping the bar standing in water, so that no drying occurred, raised it to 500. The dispenser nozzle comparison delivered at most 500 organisms with no rinse available to the user. The single most valuable link in the chain was drying on a draining rack. The proposed arrangement is as follows. No new product is proposed. The deliverable is a usage rule that keeps the bound low, drain the bar, rinse it briefly before use, and a statement of the conditions, a perpetually wet bar sitting in a puddle in a shared dish, under which the bound rises enough that the familiar advice against sharing regains its force. All results are derived from the accompanying simulation script, in which every assumption appears as a named, replaceable value. SIMULATED, NOT MEASURED.