an Omenti minipost
Before diving in consider the bad guy. Viruses are really small. Viruses (thousands) are transported in droplets. Large droplets (0.3 micron to millimeter size) come from talking, singing, sneezing and so on. There are also laerosol droplets under a tenth of a micron that also are byproducts of talking, singing, breathing and so on. The large drops fall to the ground fairly quickly, but can shoot a fair distance. The aerosols can hang around in still air for a long time.. sometimes hours.
N95s aren’t a sieve of very fine holes - or even a series of sieves. They consider of several layers of fibers… sort of a gauze of fibers - I was reminded of cotton candy (spun maple candy if you’re Canadian). The goal is not to block particles, but rather get them to stick to a fiber. The neat bit of physics is micron and smaller sized particles stick to about anything due to the van der Waals force (there’s an attractive force at small distances like these. It’s one of the reasons why IC manufacture must be done in exceptionally clean areas.) Effectively a cotton candy mass of sticky spiderweb strands.
All you have to do is get the particles to touch a strand.
BIg particles .. larger than a micron .. have a lot of momentum and behave like objects we’re used to seeing fly through the air. Their trajectory is mostly a straight line. The fact there are multiple layers mean one eventually strikes a fiber - the really big ones (millimeter size) are blocked by several fibers like a conventional mask.
The tiny aerosol particles are small enough to be bounced around by Brownian motion following a random path that traces out a slowly expanding region. Their random path give them a high probability of bumping into a fiber.
The tough ones are the intermediate size — say 0.3 to 0.5 microns. They don’t move in straight lines like the large particles and don’’t bounce around like the aerosols. Instead they follow the airflow and smoothly streamline around the fibers (technically they have a very small Reynolds number). Mmany could get through .. perhaps more than a good standard mask would permit. But the N95 has a trick up its sleeve (if a mask could have a sleeve) The fibers are made of a special material that holds an electrostatic charge for a long time… you’ve probably rubbed a balloon and watched it collect small bits of paper. These particles (and the others) feel the charge from a distance a bit larger than the space between fibers and are attracted.
The mask is a roach motel for virus carrying droplets.
The N95 rating means about 95% of these intermediate particles are trapped by the mask along with nearly 100% of the larger and smaller particles. There are N97 and N100 masks, but they’re much more expensive,
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Why you don’t want to wear one.
They require a tight fit that isn’t easy to achieve and they aren’t comfortable, Beards are out. They have a port that lets you exhale.. they only protect you rather than others. The important feature of mask wearing is not so much to protect yourself, but to protect others.
They have a limited shelf life (the electrostatic charge wears off)
You should only use them once and then throw them.
If there were enough to go around the trick would be to wear a good fitting cloth mask over the N95 to protect others.
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