Are 20 seconds of handwashing really necessary? 

Physics says yes

No shortcuts, please: modelling supports the public-health directives to wash hands for 20 seconds. Credit: David Cliff/NurPhoto/Getty

FLUID DYNAMICS

 

20 AUGUST 2021

A simple model suggests that there’s no fast way to rid hands of virus-sized particles.

Handwashing must be done at a minimum speed to dislodge viruses and bacteria, and it should last roughly 20 seconds, on par with the time that public-health experts recommend, according to an analysis of the fluid dynamics of soaping up.

The simple act of handwashing masks some complex physics. Two rough surfaces — hands — slide past one another, separated by a thin layer of water and soap.

To illuminate the physical details, Paul Hammond, a consultant based in Bourn, UK, turned to a 135-year-old branch of fluid dynamics called lubrication theory, which excels at describing the physics of thin layers of fluids wedged between surfaces. Hammond used its formulas to devise a simple model that could be used to estimate how long it takes to dislodge any virus-sized particles.

The results confirmed that it does indeed require about 20 seconds of hand-rubbing to knock off pathogens. Although the analysis did not take the chemistry and biology of handwashing into account, the author says the results are an encouraging starting point for further study.