It takes 20 seconds of vigorous handwashing to dislodge viruses and bacteria from your hands, according to a new study.
While many people have heard that 20 seconds is the recommended amount to spend handwashing, very few people actually know why.
But now a mathematical model has shown that it takes that long for bacteria to escape from the "valleys" in the surface of the skin that requires a heavy stream of water and fast scrubbing.
Other studies suggest only 5.3 per cent of people spend 15 seconds or longer washing, rubbing and rinsing, with the average time spent was six seconds.
The model captures the key mechanics of handwashing by estimating the time scales on which particles, like viruses and bacteria, could be removed.
The simplified display acts in two dimensions, with one wavy surface moving past another wavy surface, and a thin film of liquid between the two.
Particles are trapped on the rough surfaces of the hand in potential wells. If a person puts enough energy into the water flow, then it is easier for viruses and bacteria to be removed.
Dr Phil Hammond, with the American Institute of Physics, equated the process to scrubbing a stain out of a shirt: the faster the motion, the more likely it is to come out.
He said: "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."
The model does not consider chemical or biological processes that occur when using soap.
The study was published in the journal Physics of Fluids.
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