Assessing suspension and infectivity times of virus-loaded aerosols involved in airborne transmission

Abstract

Airborne transmission occurs through droplet-mediated transport of viruses following the expulsion of an aerosol by an infected host. Transmission efficiency results from the interplay between virus survival in the drying droplet and droplet suspension time in the air, controlled by the coupling between water evaporation and droplet sedimentation. Furthermore, droplets are made of a respiratory fluid and thus, display a complex composition consisting of water and nonvolatile solutes. Here, we quantify the impact of this complex composition on the different phenomena underlying transmission. Solutes lead to a nonideal thermodynamic behavior, which sets an equilibrium droplet size that is independent of relative humidity. In contrast, solutes do not significantly hinder transport due to their low initial concentration. Realistic suspension times are computed and increase with increasing relative humidity or decreasing temperature. By uncoupling drying and suspended stages, we observe that enveloped viruses may remain infectious for hours in dried droplets. However, their infectivity decreases with increasing relative humidity or temperature after dozens of minutes. Examining expelled droplet size distributions in the light of these results leads to distinguishing two aerosols. Most droplets measure between 0 and 40 µm and compose an aerosol that remains suspended for hours. Its transmission efficiency is controlled by infectivity, which decreases with increasing humidity and temperature. Larger droplets form an aerosol that only remains suspended for minutes but corresponds to a much larger volume and thus, viral load. Its transmission efficiency is controlled by droplet suspension time, which decreases with increasing humidity and decreasing temperature.

Risk assessment for long- and short-range airborne transmission of SARS-CoV-2, indoors and outdoors

“…This result, albeit simple in appearance, results from a subtle effect of Lagrangian turbulence. This has important implications in determining infectivity times: as settling is not the relevant physical mechanism for aerosol transport, the infectivity of particles between 20 and 100 μm could be underestimated by models in which the air is still ( 10 ). We are not aware of previous simultaneous measurements of gas and particles in the same controlled flow.…”

“…Initially, transport of water molecules from the droplets to the surrounding air is diffusive so that the drop squared radius decreases linearly in time. After a very short time, the droplet stabilizes at an equilibrium radius at which the viral particle is surrounded by proteins and water ( 10 , 34–37 ). The influence of droplet chemical composition on the equilibrium radius is still poorly understood ( 38–40 ).…”

“…Clinicians often use an incorrect definition, still reported by the World Health Organization: aerosols would be particles smaller than 5 μm that settle slowly enough to be transported over a few meters ( 6 , 8 ). As a consequence, problematic criteria are introduced, such as the distance after which a particle launched with an initial velocity in a fluid at rest stops ( 9 ), or the settling time of a single particle dropped from head height in still air ( 10 ). An aerosol is a locally homogeneous phase constituted of solid or liquid particles suspended in a gas, either by thermal fluctuations (for very small particles) or by turbulent fluctuations.…”

Measuring mutual diffusion coefficients in aqueous binary mixtures with unidimensional drying cells

“…For instance, it can be used to probe more complex systems such as protein solutions like saliva, as we recently demonstrated. 3…”

“…This is notably important when assessing water evaporation from aerosols of physiological fluids. 3 In practice, this evaluation is compromised by the coupling of diffusion with the other mass transport phenomena, convection, which can be either forced or resulting from the diffusion process itself such as in the case of phase change or natural convection. For instance, any concentration gradient can lead to a density gradient and thus to gravity-induced convection.…”