Big thank you to @EricCarroll for pointing out this new WHO document on SARS-CoV-2 transmission.
This document is pretty complex, in-depth, dense, and I still expect it to evolve as we learn along the way. They have some of the correct people to be working on this, for once. Hello Lidia Morawska signing off on it at the beginning of the forward.
First, a tldr. If you don't care about how it came to be, or the science, and just want to know the outcome, here it is:
https://partnersplatform.who.int/tools/aria
Go to the calculator, enter your data, and come out with a probability of infection in a given situation along with the number of expected secondary infections from that interaction.
Here's the document itself if you want to follow along:
https://iris.who.int/bitstream/handle/10665/376346/9789240090576-eng.pdf?sequence=1&isAllowed=y
Disclaimer - This is evolving science.
I'm going to split this up in a thread, because I took a lot of notes of what stood out to me on a first read, and I hope to come back to it, and use it as a general reference moving forward.
Next up is a pretty in-depth discussion of aerosolized particles. Salts, mucous and membranes! 😬
Honestly, my wife was a little grossed out by some of this, and she's in biology, so, if you're squeamish about it, might want to skip this one.
"Humans emit hundreds of aerosolized particles of different sizes during exhalation, and even more when speaking and singing [69]. Those aerosolized particles are water-based solutions of salts, containing mucus, proteins and may contain other material present on the surface of the respiratory tract including infectious pathogens. Thus, viruses on the surface of the respiratory tract can be released in the exhaled particles out of a person’s mouth and nose [70]–[72]."
"Particle aerosolization mainly results from an air- stream passing sufficiently quickly over the surface of a liquid to create separation. Several physiological phenomena contribute to aerosolization of respiratory fluid [73]. When exhaling, respiratory fluid blockages formed in the bronchioles burst and produce aerosolized particles released at the next exhalation – this is called bronchiole fluid film burst (BFFB)[74]. The vocal cord vibration during vocalization also aerosolizes the fluid by bathing the larynx, while the interactions between the tongue, teeth, palate, and lips aerosolize the saliva during speech articulation. Before being emitted, the particles undergo processes in the respiratory tract which change their size distribution. Furthermore, the respiratory tract contains relatively small-scale, curved viscous and viscoelastic films, which wrinkle during exhalation and thereby break up, leading to aerosol production. Emitted particles range in diameter from 0.01 and 1000 μm depending on the generation mechanism, respiratory and vocalization activity, age and site of origin [52], [74]–[76]. The size distribution is further affected by the quasi- instantaneous evaporation process particles have undergone after leaving the body. Particles of diameter smaller than 100 μm are likely to become airborne and remain suspended in the air from seconds to hours, because of their reduced size and settling velocity compared to larger ones [77]. The volumetric particle emission concentration can be estimated considering the specific anatomical processes originating in the aerosol, the bronchial region, the larynx, and the oral cavity. The aerosol concentration size distribution for speaking and coughing can therefore be modelled as a tri-modal lognormal distribution known as the Bronchiolar/Laryngeal/Oral (B.L.O.) tri-modal model [78]."
