A number of you have asked me to write about masks. I'm not quite ready to do a deep dive on masks yet, but for tonight I'll share the results of a preprint* published today that provides some insights into how dangerous the indoor air is, and, indirectly, how useful masks might be in protecting against transmission.
Underlying this analysis is the question of how many virus particles need to be inhaled to cause an infection. To be sure of this we would need volunteers to be inoculated with the virus. We don't have that. However, animal studies suggest the median dose required is 280 “plaque-forming units,” (PFU) while 10 percent of people could get infected with as little as 43 PFU.
The authors of today's preprint estimate that it takes 10 virus particles to make one PFU, so this would mean that 10% of people could be infected by 430 particles while the median infectious dose would be 2800 particles. These authors, however, use data from the first SARS virus and the flu to estimate that the infectious dose would be between 100 and 1000 particles.
This preprint, released by physicists, virologists, and physicians from Amsterdam, used healthy people in an experiment to model how speaking and coughing release large droplets and aerosols in a closed environment.
98% of the spray from a cough goes into large droplets that are 100 to 1000 micrometers. The remaining 2% goes into smaller microdroplets. The smaller the microdroplet, the longer it can stay in the air, and it is very small microdroplets that aerosolize and become airborne.
Using the typical concentrations of SARS-CoV-2 in saliva and sputum, they estimated that if a single person coughed once in a small enclosed space that was 2x2x2 cubic meters (one meter is 3.3 feet), airborne virus particles would be so few and far between that one would have to spend 12 minutes in the environment to risk getting infected, and that the maximal number of viral particles one could inhale from that single cough would be 120, which is unlikely to be sufficient to get most people sick, but might infect a small proportion of people.
With lots of people in a closed environment, the probability of there being more than one cough, and perhaps many coughs, goes up. Limiting the number of people who enter an enclosed environment per unit time would decrease that probability. Ventilating the room would decrease the probability by increasing the likelihood that the few aerosolized droplets released from a given cough are removed from the room.
Saying the words “stay healthy” 230 times produces the same volume of aerosolized liquid as coughing once. So, people speaking for great lengths indoors can transmit as effectively as people occasionally coughing.
These results suggest that, while open air is quite safe (unless you are close enough for someone to cough on you), enclosed environments can be dangerous if people are coughing frequently or constantly speaking. To minimize transmission in such a closed environment you would want each cough that takes place in a 2x2x2 cubic meter environment to be matched by fewer than 12 person-minutes (the number of people times the number of minutes spent).
It would be silly to micromanage this on such modeling, but it supports the general principle that in enclosed environments you would want to keep out people who know they are coughing, while limiting the number of people to prevent overcrowding as well as the length of time for which people are staying indoors.
How protective would masks be? At a minimum, they would likely stop most of the 98% of liquid that yields large droplets of 100 to 1000 micrometers. I would have to do more research to estimate whether they might also prevent some aerosolization, as I imagine this depends on whether droplet sizes are finalized before they exit the mouth, or whether many aerosolized microdroplets are formed from larger droplets after exiting the mouth.
I will try to produce a deep-dive into masks within the next week. In the mean time, I believe the science supports using them in indoor public environments, around large crowds, and environments where people might cough in each other's faces. I also believe that limiting the number of people and the time spent within small spaces should be strongly protective as well.
Stay safe and healthy,
Chris
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I am not a medical doctor and this is not medical advice. I have a PhD in Nutritional Sciences and my expertise is in conducting and interpreting research related to my field. Please consult your physician before doing anything for prevention or treatment of COVID-19, and please seek the help of a physician immediately if you believe you may have COVID-19.
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*Footnotes
* The term “preprint” is often used in these updates. Preprints are studies destined for peer-reviewed journals that have yet to be peer-reviewed. Because COVID-19 is such a rapidly evolving disease and peer-review takes so long, most of the information circulating about the disease comes from preprints.