The physics of evaporating human-exhaled drops and possible implications for virus viability*

Human-exhaled drops are thought to be a main way to transport Sars-CoV-2 virus particles from COVID-19 infected persons to other persons, thereby rendering them possible to also become infected. The viability of virus particles in airborne drops depends on a number of physical properties, e.g., air temperature, exposure to ultraviolet light, and relative humidity of air.

Drawing on prior studies of sea-spray, the present talk focuses on the hygroscopic properties of human-exhaled drops and how these depend on relative humidity. As a human-exhaled drop partially evaporates, the concentration of electrolytes increases. It has been hypothesized that high salinity leads to rapid reduction in virus viability, i.e., under high salinity, the virus particles are breaking down and becoming less capable of causing infection.

The hypothesis is that (i) virus viability remains high for human drops in high relative humidity air (dilute salt solutions), (ii) virus viability is low for drops in intermediate relative humidities (drops are concentrated salt solutions), and (iii) virus viability is high for drops in very dry environments (electrolytes have effloresced to form salt crystals).

As human exhaled drops contain a complex mixture of electrolytes and organic material, this talk uses microscopy to examine the efflorescence process of different salts in human-exhaled drops. The goal is to determine a relative humidity range, that keeps drops as concentrated salt solutions. In the future, this may be used to guide hospitals and other public buildings to control relative humidity to values that are optimal for reducing virus viability in human-exhaled drops.