A longer version of the short Facebook post. Three parts:
Masks on the docks
Masks everywhere across the USA
The physics of masks
Masks on the docks
At the risk of sounding like a NextDoor whiner. I’m pissed. The docks are narrow; typically the total width of the passageway between flowers is 5 feet. Hard to keep 6 feet from strangers, then. And many of our residents have significant health risks: they’re elderly or have infirmities that put them at risk. Our oldest residents are well into their 90s.
So, it makes enormous sense that we have a mask-wearing and social-distance protocol. Actually: it’s now not significantly different from the statewide mandate. What makes me feel disheartened and betrayed is that the entire set of signs on Issaquah Dock calling for compliance - and reminding people that the docks are private property, and that we have immuno-compromised residents - have been ripped down. All the signs, both the official signs from the Harbor and the unofficial ones that residents added. All of them.
What bothers me is that some people are aggressively asserting that they won't wear masks in our confined space.
Not a surprise: the same people destroyed and removed the Black Lives Matters signs people had posted. From people’s flower boxes and from the closed poetry box in front of one home. Really! It’s as though there’s a set of people who will not do even the slightest thing to care for strangers.
Yes, there are people, even in bucolic Marin, who think that they can assert some imaginary right to put the health of others in danger.
Masks everywhere
Despite this, the otherwise good news on masks is that Marin County is, in general, a place of high adherence to mask-wearing protocols. The New York Times this week reported on a nationwide survey on mask-wearing intents. The survey got over a quarter of a million responses (and I’m imagining there’s a lot to be challenged in the survey’s methods, including that people often report their behaviors as better than they are, but that’s a different subject). 250,000+ responses are so many that you can zoom in to quite small localities. Here’s the national map. Few surprises, to be honest. (Texas looks stronger than I’d anticipated.)
Nifty! Now, you zoom in to Northern California, and can see that the Bay Area looks like it has good mask protocol compliance!
And further, into the dark-looking area of northern Sausalito, Richardson Bay / Marin City … wait, that looks great! The hover-over text suggests that 91% of people self-report as always wearing masks. And the algorithm predicts - as you can read - that there’s a 90% chance everyone will be wearing masks in five random encounters. That, unlike the hoodlums invading our docks, feels like the Sausalito dock residents I know and care for.
And now, because I’m a physicist by trade and training …
The physics of masks
The virus mostly is a respiratory pathogen. It exits the body of the infected person via droplets. Some are small and some are large. Since all these droplets emerge from the moist passages of the human carrier’s body, each of the droplets carries about the same density of virus per volume. The idea of similar concentrations means that droplets that are small as they leave the respiratory system carry far less virus. Large ones carry more. (This is an r^3 relationship, where r is the radius of the drop: the volume of the drop - and thus the number of viruses - rises as the third power of r.)
After exiting the mouth or nose, two things start happening. The droplets both start falling with gravity and evaporating in the air. The virus is believed to become concentrated as this evaporation occurs: it’s the water that’s leaving the droplet. (There's no evidence I know of that the viri themselves can propagate airborne without water.)
How they fall: Bigger ones fall faster (because their weight is again proportional to r^3, whereas the drag from the air that slows their descent is proportional to the area, an r^2 dependence); ones that start out small don’t call, but also don't carry enough viral load. The medium size ones stay airborne as aerosols - a diffuse mist of microdroplets with a higher viral concentration. These would not be as effectively blocked by cloth masks as larger ones.
How they evaporate: the evaporation rate is approximately proportional to surface area: r^2 dependency. It will also depend on temperature and humidity: higher temperatures and lower humidity cause faster evaporation. So the droplets that are smallest evaporate slowest but those that start out small have little viral content.
It seems there is some class of Goldilocks droplets by size that are large enough to carry a significant viral load and small enough to stay in the air for a long time.
Droplets that start out small may linger in the air but not carry enough viral load to be dangerous.
The medium size ones fall more slowly, but evaporate and some stay airborne as aerosols - a diffuse mist of microdroplets with a higher viral concentration. These would not be as effectively blocked by cloth masks as larger ones.
How they evaporate: the evaporation rate is approximately proportional to surface area: r^2 dependency. It will also depend on temperature and humidity: higher temperatures and lower humidity cause faster evaporation. So the droplets that are smallest evaporate slowest but, again, those that start out small have little viral content.
It seems there is some class of Goldilocks droplets by size that are large enough to carry a significant viral load and small enough to stay in the air for a long time.
Masks generally have a blocking mesh you can think of as effectively linear to r.
Conclusion from physics: the infected person absolutely should wear a mask, since it'd block nearly all the droplets and certainly an extremely high percentage of those carrying a significant viral load.
Since few people know if they're infected, all people should wear masks. People in constant contact with others of unknown status should wear N95 masks, since those are the only masks sufficient to protect against inhaling microdroplets.
Now let’s deal with the whiners.
You’ll hear some people arguing against cloth masks “because cloth masks are not basically useless”. This is incorrect. What they're good at: they prevent the infected person from spraying droplets. What they're not very good at: preventing an uninfected person from inhaling microdroplets (but they're still a lot better than nothing). Alas, due to government incompetence, we don't have enough N95 masks to go round, we have to ration them. And most people don't know if they're infected. So: cloth masks for everyone N95 masks for all at risk.
You’ll hear some people saying blood oxygen levels drop too low. Either they’re lying, or their lung function needs help. Surgeons and dentists and dental assistants and biolab techs and metal workers and dozens of other professions require mask wearing for hours. (I mean: can you imagine open-heart surgery and the whole team not masked?)
You’ll hear some people saying masks aren’t effective because you can still smell things. That’s nonsense. The smells (kitchen or toilet smells) are mostly tiny molecules: methane and hydrogen sulphide and things like that. These are unbelievably small: 0.13 nanometers for each arm of Hydrogen Sulfide; simple esters, like those in perfumes, are larger: benzyl octanoate is (I believe) less than 0.2 nm long. The Coronavirus itself is about 120 nm in diameter - 1,000 times larger. And it’s contagious vectors are microdroplets - typically 10 microns or larger in diameter - that’s vast.
Here are those numbers again, using the nanometer as a measure - 1 nm / one nanometer = one millionth of a millimeter
Hydrogen sulfide / rotten eggs small, also present in farts and overcooked cabbage: 0.13 nm
Esters used in perfume: 0.5 - 1nm (I used benzyl octanoate as my metric)
Coronavirus, naked (not in droplet): 120 nm
Aerosol droplet: 10 microns, 10,000 nm
Typical droplet as exhaled / expelled: 100 microns - 500 microns: 100,000 nm - 500,000 nm
Wear masks. All the time.