Viruses, bacteria can stay on your hands if you don’t wash them for 20 seconds

WASHINGTON — Are you really giving your hands a full cleansing when it comes to scrubbing them in the sink? Scientists show that it truly does take 20 seconds of vigorous hand-washing to dislodge viruses and bacteria.

While many people have heard that 20 seconds is the recommended time for washing hands, very few people actually know why — and most people don’t go the distance. 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 percent of people spend 15 seconds or longer washing, rubbing and rinsing, with the average time spent being only six seconds.

Math model for hand-washing
One case of a particle’s movement. It begins as the red point, resting on the surface of the hand (black wavy line). It follows the blue path, escaping the hand and then moving freely through the hand-washing fluid. (Credit: Paul Hammond)

The model captures the key mechanics of hand washing 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, equates the process to scrubbing a stain out of a shirt: the faster the motion, the more likely it is to come out.

“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,” Hammond says in a statement.

The model does not consider chemical or biological processes that occur when using soap while hand-washing.

The study was published in the journal Physics of Fluids.

South West News Service writer Mark Waghorn contributed to this report.