Wash your hands for 20 seconds; Here's the reason

Ground Report | New Delhi: Wash your hands for 20 seconds; Although the efficacy of handwashing in preventing the spread of pathogens and diseases is proven, the physics behind it has hardly been studied. But now a researcher at Hammond Consulting Limited in Cambridge (UK) is presenting in the journal Physics of Fluids a simple model that captures the mechanics behind the process.

By simulating hand washing, the time scales in which particles such as viruses and bacteria are eliminated during the process were estimated. The mathematical model operates in two dimensions, representing the hands (rough at small spatial scales) as wavy surfaces, together with a thin film of liquid between them.

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Wash your hands for 20 seconds

Particles get trapped on rough hand surfaces in potential wells. In other words, they are at the bottom of a valley, and for them to escape, the energy of the water flow must be high enough to drive them up and out of that valley.

The force of the liquid flowing between the hands depends on the speed of the hands during movement. A stronger flow removes particles more easily. "Basically, the flow tells you about the forces exerted by the particles," explains author Paul Hammond, "so you can calculate how the particles are moving and find out if they are removed."

The researcher compares the process to remove a stain from a shirt: the faster the movement, the more likely it is to come off. "If you move your hands too gently, too slowly, the forces created by the fluid are not great enough to overcome the force held by the particle," says Hammond.

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In any case, even if the particles are removed, the process is not fast. The usual recommendations from health authorities, such as the Centers for Disease Control and Prevention (CDC) in the US, indicate that you should spend at least 20 seconds under the tap.

Chemical or biological processes

The results of the Hammond model match that time: it takes about 20 seconds of vigorous movement to dislodge potential viruses and bacteria from the hands.

This model has not taken into account the chemical or biological processes that occur when using soap. However, knowing the mechanisms that physically remove particles from your hands can offer clues to formulating more effective and environmentally friendly soaps.

"Nowadays, we have to think a little more about what happens to chemicals from washing when they go down the drain and enter the environment," recalls Hammond, who acknowledges that this study does not reflect all the complexity of washing hands-on, but it does answer important questions and lay the groundwork for future research.

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