Physicists have applied mathematical equations for the assessment of coronavirus survival on surfaces. This included an examination of the drying time of respiratory droplets from COVID-19-infected subjects. This related to a differing range of surfaces, with samples taken from six cities located in different parts of the world, including North America, Asia and Australia.
The research focus was to look at the droplets expelled from the mouth or nose, as a person speaks, coughs or sneezes. If a person is infected with the coronavirus, then a proportion of these droplets will contain viral particles (virions). The droplets act to protect the relatively fragile enveloped virus and also affect the aerodynamics, in terms of how far the droplets are likely to travel and with the direction they will take.
The typical droplet size, when viewed at any position, is approximately similar to the width of a strand of hair. As these tiny droplets settle on surfaces, some of the viral material will remain after the droplet has evaporated and become attached to the surface, leading to the prospect of a person becoming infected by touching the contaminated surface and then touching their nose or mouth.
The model developed drew on interface science (a field that considers the boundary between two spatial regions occupied by different matter). This approach enabled the scientists to run calculations on how fast a given droplet will take to dry. The model could be adapted according to variations with ambient air temperature, different surface materials, and variations with indoor relative humidity.
The model showed, for example, how a higher ambient temperature dried out a droplet faster compared with conditions where there is cooler air. A faster air drying time reduced the possibility of virus survival. This approach also showed that where humidity is higher, the additional moisture enabled the droplet to remain on a surface for longer. The consequence of this was that virus survival was greater.
The findings were then examined alongside epidemiological data relating to the infection rates in the cities studied. It was found that the model had good predictive power in that with cities with a higher COVID-19 infection rates, the ambient conditions were such that the drying time for respiratory droplets lasted a relatively longer time compared with droplets produced in cities with lower rates of infection.
The study also revealed the types of surfaces are a factor for consideration, in relation to both virus survival and viral viability. From this, surfaces like computer and phone screens, cotton, and wood were at a greater risk compared to surfaces formed from glass or steel. The key differentiating factor here was the degree to which surfaces are hydrophilic. These types of surfaces enable droplets to evaporate faster and hence they present a lower risk.
The research is published in the Physics of Fluids, titled “Likelihood of survival of coronavirus in a respiratory droplet deposited on a solid surface.”
