Size distribution of virus laden droplets from expiratory ejecta of infected subjects

Abstract

For rebooting economic activities in the ongoing COVID-19 pandemic scenario, it is important to pay detailed attention to infection transfer mechanisms during interaction of people in enclosed environments. Utmost concern is the possibility of aerosol mediated infection transfer, which is largely governed by the size distributions of virus laden droplets, termed as virusols in this work, ejected from humans. We expand on the well-known theory of Poisson fluctuations which acts as statistical barrier against formation of virusols. Analysis suggests that for viral loads < 2 × 10⁵ RNA copies/mL, often corresponding to mild-to-moderate cases of COVID-19, droplets of diameter < 20 µm at the time of emission (equivalent to ~ 10 µm desiccated residue diameter) are unlikely to be of consequence in carrying infections. Cut-off diameters below which droplets will be practically free of contamination, are presented as a function of viral loading. The median diameters of virus laden polydisperse droplet distributions will be 1.5 to 20 times higher depending upon the geometric standard deviation. The studies have implications to risk assessment as well as residence time estimates of airborne infections in indoor environments. Additionally, it will be also helpful for performance evaluation of sanitization and control technologies to mitigate infection risks in workplaces.

Figure 1

Fraction of virus-laden droplets formed from the ejected droplets, as a function of its size and viral load in the fluid.

Figure 2

Smallest droplet diameter likely to be contaminated as a function of viral load in ejecta.

Figure 3

Virusol fraction of lognormally distributed ejecta droplets as a function of viral load in patients.

Figure 4

Virusol size-distribution for different propensity parameter, ${\mu }_G=\frac{\pi }{6}{d}_G^3{C}_v$.

Figure 5

Variation of median size of virus-laden droplets (Virusols) relative to the original droplets with respect to propensity parameter $\left({\mu }_G\right)$ for different dispersity measure (${\sigma }_G$ of droplets).