[Dr Hullett is a longtime friend, colleague, and RR reader. As systems engineers and developers, we have traveled much of the long road together. He has given permission to publish this personal correspondence.]
Wayne Hullett, PhD
The coronavirus particles are too small to be seen. A doctor on Youtube suggested thinking about coronavirus particles as the glitter worn by some people at festive events. If you have ever gotten any glitter on you, then you know how difficult it is to get off. As George has pointed out, the source of this glitter is the lungs of people who have the disease, most of whom do not yet have symptoms and thus probably do not know they have it. The glitter exits their bodies with normal breathing, talking, coughs and sneezes, and becomes an aerosol in the air. It can float in the air for several minutes before it settles on the floor, on nearby surfaces, or gets inhaled into the lungs of someone nearby. Those who inhale enough of it also become infected, and after an incubation period, they too become a source of the glitter. Anyone who touches a surface that has accumulated some glitter and then touches their mouth, nose or eyes also lets it into their system and can also become infected, thus becoming another source. This is how it spreads exponentially.
Here is a good discussion of the size of virus particles (the glitter) and how well mask materials trap them.
Three common materials and the amount of virus particles they trap are:
Hankerchief 28%
Surgical Mask 80%
N95 Mask 96%
There are two reasons to wear a mask: 1. to keep particles out and 2. to keep particles in. If you wear a mask to keep from catching the virus, you would want an N95 mask, which requires some care and training in fitting. This is what our doctors and nurses wear when treating coronavirus patients. And the patients (I hope) wear surgical masks to protect the medical personnel.
Now, I am guessing (I am an engineer, not a medical doctor), that the body's immune system can deal with a small number of particles, which is why the N95 masks, at 96% effectiveness) seem to work. Also, older people's immune system does not work as well as when they were young, which would explain the increased mortality with age.
The best reason to wear a mask, it seems to me, is to keep the particles IN; to keep from spreading them. A surgical mask (the blue pleated mask you see in hospital TV shows and any street scene of an Asian country) traps 80% of the particles. This, to me, is huge. If only 20% of the particles are getting into the air and on surfaces, then much fewer people will get infected. I am not sure whether it is 1 for 1 (80% fewer infections), but if the body's immune system is able to handle a small amount of particles, then common sense tells me that there will me many fewer infections, thus reducing the impact on the hospital system and buying us more time to develop an effective treatment or vaccine, resulting in fewer deaths.
So, while social distancing is effective, the next best thing we can do is for EVERYONE, especially the younger who may have only a mild case and not even know they are carriers, to wear surgical masks most of the time. Older folks should stay at home, and wear N95 masks when they have to emerge to buy groceries.
I am troubled by the CDC party line that masks do not help. I think they issued that statement because there is a shortage of masks, and by claiming that they do not help they hope to avert a rush to buy and horde masks, thus denying them to the medical professionals who have a greater need. Even the Surgeon General was defending that lie on TV today. I think the correct approach is to not lie, but to tell the truth and to cure the shortage. There are around 330 million people in the country, and the masks are single use, so I am thinking that we need a production capacity of somewhere between 100 million and 500 million surgical masks per day, and maybe 10% to 20% of that of N95 masks. Considering production capability this country developed in WWII, I would think that is readily achievable, for as long as it takes to develop and get approval for a vaccine. They should be on the supermarket shelves next to the bread and milk (and beer), and cost less than a penny each. In the interim, the CDC and Surgeon General could help not by lying but by showing us how to make the most effective homemade masks from commonly available materials. There are already a number of Youtube videos that show how to make masks of various qualities.
So, the key, for most people, is not to wear the mask to keep from getting the virus, but to keep everyone else from getting it from you, because you may not know you are a carrier. Protect your fellow citizens by wearing a mask.
**** [In my reply I included the following.}
Thanks much Wayne, I will definitely post this. My take on masks may be a bit more broader than yours. Not only do they reduce the fraction/size of the exhaled aerosols, but also capture a significant fraction (perhaps not as great as the exhaled part) of the infectious aerosol particles during inhalation. Bottom line, a wide use of masks will help in two ways to reduce the ‘reproduction’ factor/rate among a population of infected pre-symptomatic and vulnerable uninfected people.
rebanesruminations.com 
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