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How close is too close & when should I wear a mask?

Please note: that I am not a doctor. I am a software guy who does a lot of machine learning, statistics, and debugging. The observations I provide here are backed with the provided references. Nothing on this post should be considered medical advice and it is best to follow the guidelines of the Centers for Disease Control and the guidance of your local county health authority. Unless you live in a community where people think it is a matter of “individual freedom” to have to wear a mask. In that case, fuck individual freedom and just wear a mask until this whole COVID thing is properly addressed by an effective vaccination.

Walking into Bianchini’s Market in San Carlos today Torri and I were subject to some very poor user interface on two accounts. The first was the placement of hand sanitizer just inside the entrance and not enough to the left. This caused an immediate back up and the abrupt halt of the flow of people we were in. The second was the woman in front of me who (having stopped abruptly) now turned to me after about 10 seconds and with the clear tone of judgement in her voice asking me if I could back up. No please mind you.

So, in the face of outward hostility which is clearly being driven by fear, I decided it’s time to write a post about the way SARS-CoV-2 spreads and the relative risks we all face in day to day life. First of all let me say that for people who don’t do well with nuances, just fucking wear a mask all the time; and, please don’t hang out talking to people whom you don’t live with. For the rest of us, let me see if I can tease out the nuances in a way that makes sense.

If you have any symptoms like coughing, or sneezing, or a runny nose, probably best to stay home and isolate yourself until these symptoms go away. If you have to go out, please wear a mask at all times in all places because you might be very contagious. If you see what I am talking about here you understand why the person who has COVID but no symptoms is such a problem. They are spewing viral load every time they breath.

Experts estimate that as few as 1000 SARS-CoV2 infectious viral particles are all that will be needed. That’s the bad news. The good news is that you’re not going to contract Covid-19 if a single particle falls on you. And, while Scientist don’t know exactly how many particles it takes — 900? 1,500? — we can use a number like 1000 to calculate our risk in a variety of situations. 

Successful Infection = Exposure to Virus x Time

  • Outdoors while constantly moving and keeping >= 2 meters distance from people you don’t live with: you probably don’t need to wear a mask most of the time. If you stop to talk to people, you probably want to wear a mask, but just being out there exercising, you are likely going to be just fine, and so are they. Doctor’s Rasmussen and Kasten both agree that a “perfect storm” of events has to happen for a virus to jump from an infected passerby outdoors to you. The particles — enough of them to be able to kickstart an infection — have to spray out of the passerby with enough force to make their way over to you. The virus inside the particles has to survive while sunlight, humidity, wind, and other forces work to decay and disperse them. The particles have to land right in your upper throat or respiratory tract — or on your hands, which you then use to touch your eyes, nose, or mouth — and they have to get past all the barriers to infection in the respiratory system, like nose hairs and mucus. Then the viral particle have to dock up with your cells’ ACE-2 receptors and use them to enter the cells.
  • Indoors with people you don’t live with: just wear a mask. This is especially true if exercising (and yes, I am specifically calling out Gyms who are allowing optional mask situations). Inside viral load has an easier time building up and a harder time being destroyed by sunlight.

Now another bit of nuance here, and why you don’t need to yell at people who get close to you indoors for 5 to 10 minutes: Inside, while wearing a mask, and absent any sneezing or coughing, you are unlikely to spread this viral load to your nearby neighbor. It is simply a function of not enough viral load (mask is catching most of it) and not enough time. Assuming a rate of 20 viral particles coming into the environment from the breathing (not talking, just breathing) of that infected person standing too close to you; and, EVEN IF (very unlikely) every single one of those viral particles managed to wind up in your lungs, you would need 50 minutes of exposure. 20 * 50 = 1000 viral particles = you are infected.

Spread the word. Wearing masks is good sense to protect others in these times of COVID, especially with so many people out there who seem to be symptom free and yet contagious. You should always do it inside when you are with people whom you don’t live/quarantine with. You shouldn’t worry so much when outside taking walks, riding bikes, running, or engaging in other forms of exercise.

If you want even more detailed nitty gritty on all this, take the time to read Comparative Immunologist and Professor of Biology (specializing in Immunology) Erin Bromage’s most excellent post on the topic.


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