Left: two hands, germ dots fade as coverage is washed. Right: live log10(relative count) vs time.
This simulation models the decimal-reduction pattern widely used in hand-hygiene science: log10 of the relative surface microbial count falls roughly linearly with wash duration once soap and mechanical rubbing are active. The time for a 90% (one order of magnitude) reduction, T90, shortens with more soap use, and a coverage-thoroughness factor leaves any unwashed skin fraction untouched — capturing why real hand-washing effectiveness depends on technique, not just the clock.
Two hands with germ dots that fade on a log scale as washing proceeds, alongside a live log10(relative count) vs time chart — the standard way hand-hygiene studies present their results.
Drag Wash Duration, Soap Use and Coverage Thoroughness to see T90, the log-reduction achieved so far, and the relative count remaining update live.
Because the reduction is logarithmic, the first few seconds of a wash remove a much larger absolute number of microbes than the last few seconds — but the guideline ~20 second duration is chosen because the curve is still usefully steep there, unlike a 5 second rinse.
Hand-hygiene studies commonly report results as log10 reduction versus time because surface microbial removal — like many disinfection and cleaning processes — follows a roughly exponential decay, which appears as a straight line on a log10 scale.
T90 is the decimal reduction time: how long it takes to reduce the microbial count by 90% (one log10 unit). A shorter T90 means faster removal — soap use in this model directly shortens T90 by helping lift microbes off the skin surface.
Washing for a long time does little good on skin the rubbing motion never reaches — the coverage slider represents the fraction of hand surface actually washed, and leaves the remainder at its starting count regardless of total duration.
In this model's log-linear pattern, the curve is still dropping steeply around 15-20 seconds of soap-assisted washing; stopping much earlier gives up most of the achievable reduction, which is the practical reasoning behind widely cited hand-hygiene duration guidance.