Washing your hands for 20 seconds with soap is a tried and tested method to limit the spread of an illness infection and others. A new physics paper has now shown that time is not an arbitrary number but, in fact, there is a good physical principle behind the 20-second method. Published in the journal Physics of Fluids, it is the first step in a deeper understanding of the mechanics of hand washing, but at the moment, it does not take into account chemical and biological factors.
By mimicking handwashing from a physics point of view, researchers can estimate how long it takes to remove viruses and bacteria from the skin. The paper simplifies the hand for rough imperfections of the skin, appearing as a normal curve. Our skin is a series of equal peaks and valleys. In this valley, viruses and bacteria can accumulate in the body through the eyes, nose, and mouth before entering or before moving to another host or surface. So how much energy is needed from the flow of soapy water to get those pathogens out of their comfortable valleys?
Whether the water will push the germs out of the valley depends on the energy of the fluid flowing, and that energy depends on the speed of the hand movement. A stronger flow makes the particles come out more easily. The model suggests that 20 seconds of vigorous movement is sufficient to remove any potential pathogens. Author Paul Hammond explains, “If you move your hands too slowly, too slowly, relative to each other, the forces created by the flowing liquid are not enough to overcome the force holding the particle.”
The results of this simple model could lay the groundwork for a more complex analysis, which could consolidate the ability of soaps to disrupt the bacterial biofilm and reduce the adherence of pathogens to the skin. Adding this complexity will not necessarily create a new and improved version of hand washing, but it could lead to more effective and environmentally friendly soap production.
“Nowadays, we need to be a little more thoughtful about what happens when fume chemicals go down into the plughole and enter the environment,” Hammond said. The concept of handwashing to stop the spread of the disease was first described and tested by the Hungarian physicist Ignaz Semmelweis in 1847 who showed that “hand hygiene” dramatically reduced patient mortality in medical settings, especially in childbirth. At the time, it was a destructive idea.