Washing your hands for 20 seconds with soap is a tried and tested method to reduce your chance of contracting an illness and limiting the spread of disease to others. A new physics paper has now demonstrated that that timeframe is not an arbitrary number but there are, in fact, good physical principles behind the 20-second approach.

Published in the journal Physics of Fluids, the approach is the first step into a deeper understanding of the mechanics of handwashing, but at this point, it doesn’t take into account chemical and biological factors. By simulating handwashing from a physics perspective, researchers could estimate how long it takes to remove viruses and bacteria from the skin. 

The paper simplifies hands to the rough imperfections of the skin, demonstrated as a simple curve. Our skin is a series of equal peaks and valleys. In those valleys, viruses and bacteria can gather before getting inside the body via the eyes, nose, and mouth, or before moving to another host or surface. So how much energy from the flow of soapy water is necessary to get those pathogens up and out of their cozy valleys?

Whether the water will push the pathogens out of the valleys depends on the strength of the flowing liquid, and that strength depends on the speed of the hands moving. A stronger flow kicks the particles out more easily. The model suggests that 20 seconds of vigorous movement is enough to dislodge any potential pathogens. 

"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," explained author Paul Hammond.

The result from this simple model could lay the foundation for a more complex analysis, one that could integrate the ability of soap of disrupting bacterial biofilm and reduce pathogens' adhesion to the skin.

Adding this complexity will not necessarily make a new and improved version of hand washing, but it might lead to the production of more effective and environmentally friendly soaps. "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," Hammond said.

The concept of handwashing to stop the spread of diseases was first described and tested in 1847 by Hungarian physicist Ignaz Semmelweis who demonstrated that "hand hygiene" dramatically reduced the mortality of patients in medical settings, in particular in childbirth. At the time, this was a subversive idea. That doctors themselves were spreading disease did not go down well, and Semmelweis was both derided and his ideas were rejected, leading him to have a nervous breakdown. His colleagues committed him to a Viennese asylum where he died most likely from beatings by the guards just two weeks later.

Semmelweis's then-revolutionary ideas have saved billions of lives since, and handwashing is still the simplest, most effective preventative measure we have to stop the spread of disease.