The decimal reduction time: how long it takes to reduce the microbial count by 90%. Soap use shortens T90 in this model.

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🧼 Hand Washing — Germ Removal Kinetics

Interactive log-reduction model of hand-washing hygiene. Soap use and coverage thoroughness set a decimal reduction time T90, showing why a full ~20 second wash removes far more microbes than a quick rinse.

Washing & Cleaning2DEasy60 FPS

About this simulation

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.

🔬 What it shows

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.

🎮 How to use

Drag Wash Duration, Soap Use and Coverage Thoroughness to see T90, the log-reduction achieved so far, and the relative count remaining update live.

💡 Did you know?

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.

Frequently asked questions

Why is microbial removal modelled as a log-linear decay?

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.

What is T90?

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.

Why does coverage thoroughness matter separately from duration?

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.

Why is roughly 20 seconds commonly recommended?

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.

⚙ Under the hood

Simulate the log-reduction of surface microbe count during a hand wash, following the well-known hand-hygiene pattern where soap and thorough coverage roughly linearise log10(count) against wash duration, and see why the commonly recommended 20-second wash is a genuine inflection point, not an arbitrary number.

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2D · HTML5 Canvas 2D · 60 FPS target · runs fully client-side, no install