You may have been washing your hands wrong this whole time

Hand washing has always been important. Hopefully, that isn’t up for debate. Still, the past year and a half or so have really driven home that importance. Since the early days of the pandemic you’ve heard it time and time again: Wash your hands!

If you’re still unsure as to exactly how long you should be spending by the faucet, a new study from the American Institute of Physics has an answer for you. At least 20 seconds. 

Why 20 seconds?

It’s common knowledge that regular hand washing is the easiest and most efficient way to mitigate the spread of various diseases and infections (not just COVID-19). This latest research, however, is among the first to investigate the actual physics that occurs each time we clean our hands. Via a simple (in terms of physics) model study authors were able to simulate the key mechanics of hand washing.

This mathematical model consists of two dimensions: One “wavy surface” moving past a second wavy surface, along with a thin film of liquid between the two surfaces. That last sentence has probably left you confused, but all you really need to know is that the two surfaces represent hands, and the thin film of liquid recreates a running faucet. This setup allowed the research team to estimate the length of time it takes for a given particle, such as a bacteria or virus, to be washed off of a hand.

Now, the model showed that particles tend to get stuck within the rough surfaces of the human hand. Rough surfaces in this case refer to tiny indentations across human skin. Study authors liken this process to the particle being trapped at the bottom of a valley. When we wash our hands, there needs to be enough energy from the water flow to pick up the particle and wash it out of the “valley.”

It isn’t as simple as placing your hand under running water, though. Water flow and pressure during washing is determined by the speed of the moving hands. And, a stronger water flow means particles are removed more easily.

“Basically, the flow tells you about the forces on the particles,” explains study author Paul Hammond. “Then you can work out how the particles move and figure out if they get removed.”

Hammond uses the analogy of scrubbing a stain out of an article of clothing. The harder you scrub the faster the stain will disappear. “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,” he adds.

The right way to wash your hands

So, it’s definitely a good idea to add some pep to your hand washing. Don’t just idly leave your hands under the faucet. It just might rid you of that one lingering disease particle that’s holding on for its viral life. 

Even if you are moving your hands frantically it still takes some time for the particles to move on. The Centers for Disease Control and Prevention recommend that a handwashing session last at least 20 seconds.

This work reinforces that recommendation. The mathematical model showed that it takes at least 20 seconds of diligent hand washing for particles to dislodge from the skin. That means that you should probably be spending upwards of 30 seconds to a full minute under the faucet – especially if you aren’t moving your hands much.

Next on Hammond’s agenda is how soap plays into all of this. More specifically, if these new revelations regarding how disease-causing particles are removed from the hands may aid in the creation of new and more effective, 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 adds.

The full study can be found here, published in Physics of Fluids.