Physics of virus transmission by speaking droplets

Some clarity from Netz and Eaton (open source) on a scientifically contentious politicized issue:

To make the physics of person-to-person virus transmission from emitted droplets of oral fluid while speaking easily understood, we present simple and transparent algebraic equations that capture the essential physics of the problem. Calculations with these equations provide a straightforward way of determining whether emitted droplets remain airborne or rapidly fall to the ground, after accounting for the decrease in droplet size from water evaporation. At a relative humidity of 50%, for example, droplets with initial radii larger than about 50 μm rapidly fall to the ground, while smaller, potentially virus-containing droplets shrink in size from water evaporation and remain airborne for many minutes. Estimates of airborne virion emission rates while speaking strongly support the proposal that mouth coverings can help contain the COVID-19 pandemic.

From the text of the article:

A few examples are instructive. In the absence of water evaporation, droplets placed initially at z0 = 1.5 m (the average height above ground for the mouth of a standing human adult) with radii of 1, 10, or 100 μm will require 1.3 × 104 s (∼3.5 h), 130 s, and 1.3 s, respectively, to fall to the ground.

After an analysis of the number of emitted virions while speaking (Table 1):

Overall, the above analysis strongly supports the concept that simply speaking can be a major mechanism of person-to-person COVID-19 transmission and that covering the mouth in public, as suggested by the work of Anfinrud and coworkers (11⇓–13) and others (10, 17), could help to more rapidly contain and potentially end the pandemic.

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