How Long Is Long Enough? A Contact-Time Rule for Soap Handwashing Derived from a Removal Model

This study examines a practical question in everyday hygiene by explicit arithmetic from stated assumptions. With soap, 90 percent removal required 27 seconds, 99 percent required 136 seconds and 99.9 percent required 251 seconds. With water alone the same targets required 54, 271 and 502 seconds respectively. The modelled load remaining after 20 seconds of soap washing was 13.52 percent of the starting load, and after 40 seconds 6.88 percent, so the second twenty seconds removed less than a fifth as much as the first. A 15 second soap wash achieved 81.2 percent removal in the model, which supports a 136 second rule for a 99 percent target under the stated rates. The proposed arrangement is as follows. The rule proposed is a single duration, stated with its target, that a household, school or food business can adopt and time with any clock or counted song of the right length. Durations are derived from the model rather than adopted from convention, and the target is printed as part of the rule so that a setting needing a stricter standard can select the longer derived duration instead of guessing. All results are derived from the accompanying simulation script, in which every assumption appears as a named, replaceable value. SIMULATED, NOT MEASURED.