Lots of Words about Masks

Lots of words inbound!!!

Recently the CDC has reversed it's stance on masks. It's now believed that any mask is better than no mask and people are coming up with various homemade hacks in lieu of N95s, etc. There has been a relatively viral video I saw on Facebook of a man spraying some sort of aerosol through various masks, a few homemade which shows some penetration, and surgical and N95+ that shows no penetration. I believe their intent is to make people feel like homemade masks don't do anything. I would argue that homemade masks are still better than no mask at all. Another argument I've seen made is that since CoV is ~100 nm and N95s are rated for 300 nm, N95's *don't work* with CoV.

First of all, I love that standards, math, and statistics are all of a sudden all the rage to post about, but in the interest of keeping information accurate, here are a few things. Just because a particle is smaller than a hole doesn't mean it automatically flows through, much like how just because a basketball is smaller than a basketball hoop doesn't mean every shot will go through the basket. There is an associated percentage of shots that will go through depending on a number of variables. Viruses are not aimed like basketballs are and there's more than one hole so it's more like a random assortment of basketball tosses across a field of random baskets. In this case it's like tens of thousands of microscopic basketballs being randomly thrown at millions of baskets. In practice, the percent of these shots made is like the penetrance of a mask. The 95 in N95 is a standard that assures *95%* of particles over 300 nm are blocked. It is important to note that it is not 100%, so even the popular-culturally coveted N95 *is not perfect*, 5% of particles are still allowed through so the argument that N95s "don't work" is kind of silly because they never claimed to 100% work. In fact, no mask works 100%, even the more restrictive P100 mask only blocks 99.8% of particles, so there is still 0.2% uncertainty.

Now, also note that this efficacy is a percentage, so time is still very much a factor in how much exposure you actually receive. The longer you're exposed to the contagion, the more particles pass, so be fast about what you have to do in public areas even if you have a mask. Furthermore, the closer you are to a source, the higher the concentration of particles in the air, and therefore the more particles pass. I've been seeing people stop social distancing at the grocery just because they have a mask on but it's important to note that N95s do not perfectly protect you from all particles!

So continuing the basketball analogy, the size of the CoV argument is like changing the size of the basketball. Yes more balls will make it through baskets but there will still significantly less than 100% allowed through baskets. The viral video of the man spraying aerosol particles does not have any quantitative aspect so it is impossible to garner any insight into how effective any of the masks he tests are. Luckily, mask efficiency and penetration is actually a very well studied topic aside from randos on the internet.

Here (https://www.semanticscholar.org/paper/Do-N95-respirators-provide-95-protection-level-and-Ba%C5%82azy-Toivola/07457065ac5891d671768ea1d6232454b2f6409b) are some plots from a paper (https://www.ncbi.nlm.nih.gov/pubmed/16490606) that quantifies the penetratiom of various masks including N95 and surgicals and it is shown that though the masks are rated for larger particles, their efficacy is non-negligible against smaller ones.

This paper (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2662779/…) compares some popular washable fabric masks as well as a cotton handkerchief to NIOSH masks and shows that while protection is far from perfect, even the handkerchief provided ~30% protection in this experiment. (above)

The table below is from this paper which lists the efficacy of various household fabrics as masks. https://www.researchgate.net/publication/258525804_Testing_the_Efficacy_of_Homemade_Masks_Would_They_Protect_in_an_Influenza_Pandemic

So yeah, although they got it wrong at first, the CDC's current recommendation that anything is better than nothing is correct. Wear a mask. It not only protects you from the world, but if you happen to be infected and asymptomatic, it also protects the world from you.