How many seconds and with what intensity to wash your hands to kill germs

The importance of hand washing became known at the end of the 19th century among doctors who until then did not even consider the practice of entering an operating room. But over time, in addition to being integrated into hospitals, after verifying that the lack of hygiene caused deaths due to the transmission of germs, the healthy custom began to spread to the general population at the beginning of the 20th century. With the current COVID-19 pandemic, there have been many recommendations from specialists in this regard, so A group of physics experts set out to describe the most effective way to wash hands to prevent the transmission of infections.

The study was carried out by a team of researchers from UK company Hammond Consulting Limited and was published by the revista Fluid Physics. Consists of description and physical demonstration of the effectiveness of handwashing in disease control.

The specialists carried out a hand washing simulation and they estimated the timescales in which particles, whether viruses or bacteria, are removed from hands.

The mathematical model works in two dimensions, with one corrugated surface passing next to another corrugated surface, and a thin film of liquid in between. The wavy surfaces represent the hands because they are rough at small spatial scales.

Particles get trapped on rough surfaces of hands in potential wells. In other words, it is as if they are at the bottom of a valley and, so that they can escape, the energy of the water flow must be high enough to move them up and out of the valley.

The strength of the flowing liquid depends on the speed of the moving hands. This means that a higher flow rate will remove particles more easily.

“Basically, the flow tells you about the forces exerted by the particles,” the study author said, Paul Hammond. “Then you can calculate how the particles move and find out if they are removed.”

The analysis compared the hand washing process with that performed, for example, to remove a stain from a shirt: the faster the movement, the more dirt is likely to come out.

“If you move your hands too gently, too slowly, the forces created by the flowing fluid are not large enough to overcome the force held by the particle,” explained Hammond.

Even when the particles are removed, this process is not quick. Common guidelines for hand washing, such as those from the U.S. Centers for Disease Control and Prevention (CDC), suggest at least 20 seconds under the tap.

The results of the Hammond model coincide with this time: it takes about 20 seconds of vigorous movement to dislodge potential viruses and bacteria.

The author clarified that the model does not take into account the chemical or biological processes that occur when using soap. However, knowing the mechanisms that physically remove particles from your hands can provide clues to formulating more effective and environmentally friendly soaps.

“Today we need to think a little more about what happens to chemicals from washing when they go down the sewers and enter the environment,” said Hammond, who said his analysis responds to important questions on the matter and lays the foundation for the future. research.

According to a study by the Pan American Health Organization (PAHO) disinfecting hands with soap and water reduces childhood diarrhea by 50% and respiratory infections by 25%.

Experts recommend incorporating hand washing into everyday life for everyone, and the main thing is not to wash your hands every two seconds, but rather do it at the right time and in the right way.

Appropriate times are when returning from the street, before eating and cooking, before breastfeeding, after using the bathroom or changing diapers, after blowing your nose, sneezing or coughing, after visiting someone sick, in contact with residue, in contact with animals, after touching raw food and before touching cooked food.

In addition, specialists emphasize that before washing your hands, you must remove rings, bracelet and watch because these objects accumulate microorganisms that do not come off in the wash and therefore the hands will not be completely free of germs and viruses.

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