Why you should wash your hands for twenty seconds

Although most people understand that it’s important to wash your hands for sanitary reasons, few have ever looked at the physics of the task. A new study led by Paul Simon Hammond does just that.

Researchers from Hammond Consulting Limited created a model looking at the mechanics of hand washing. With their model, the scientists were able to estimate the time it takes to remove harmful material such as bacteria and viruses.

The model represents hands as wavy lines, because on microscopic levels, hands are quite rough surfaces. In the troughs of those wavy lines, microbes or particles can cling. This is why water has to be a certain force to remove contaminants from hands. 

“Basically, the flow tells you about the forces on the particles.Then you can work out how the particles move and figure out if they get removed,” explained Hammond.

Under certain circumstances, the contaminants will remain on the hands due to inadequate hand washing. 

“If you move your hands too gently, too slowly, relative to one another, the forces created by the flowing fluid are not big enough to overcome the force holding the particle down,” said Hammond.

It still takes time, not just a brief but vigorous scrub. The team’s new model and the CDC guidelines agree – you should wash your hands for at least 20 seconds. 

Although the research didn’t look at the role of chemicals such as different soaps, Hammond hopes the study might be helpful in designing environmentally friendly cleaners in the future, saying,

“Nowadays, we need to be a bit more thoughtful about what happens to the wash chemicals when they go down the plughole and enter the environment,” said Hammond.

The study is published by the American Institute of Physics in the journal Physics of Fluids. 

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By Zach Fitzner, Earth.com Staff Writer