Anonymous One and Two, here are some thoughts from people who are exactly in their area of expertise regarding the efficacy of N95 filtration on viruses:
“The science of mask functionality gets really small, really fast. The unit of measurement here is microns — 1/1000th of a millimeter.
The size-based argument against N95 laid out in this claim assumes mask filtering works something like water flowing through a net — particles in the water smaller than the net opening pass through, while larger items don’t.
But the physics involved don’t work like that at all.
The COVID-19 particle is indeed around 0.1 microns in size, but it is always bonded to something larger.
“There is never a naked virus floating in the air or released by people,” said Linsey Marr, a professor of civil and environmental engineering at Virginia Tech who specializes in airborne transmission of viruses.
The virus attaches to water droplets or aerosols (i.e. really small droplets) that are generated by breathing, talking, coughing, etc. These consist of water, mucus protein and other biological material and are all larger than 1 micron.
“Breathing and talking generate particles around 1 micron in size, which will be collected by N95 respirator filters with very high efficiency,” said Lisa Brosseau, a retired professor of environmental and occupational health sciences who spent her career researching respiratory protection.
Health care precautions for COVID-19 are built around stopping the droplets, since “there’s not a lot of evidence for aerosol spread of COVID-19,” said Patrick Remington, a former CDC epidemiologist and director of the Preventive Medicine Residency Program at the University of Wisconsin-Madison.
The N95 filter is indeed physically around the 0.3 micron size. But that doesn’t mean it can only stop particles larger than that. It works well for particles above that size, and actually snares particles below that size better than those at exactly the 0.3 level.
“N95 have the worst filtration efficiency for particles around 0.3,” Marr said. “If you’re smaller than that those are actually collected even better. It’s counterintuitive because masks do not work like sieving out larger particles. It’s not like pasta in a colander and small ones don’t get through.”
N95 masks actually have that name because they are 95% efficient at stopping particles in their least efficient particle size range — in this case those around 0.3 microns.
Why do they work better for smaller ones? There are a number of factors at play, but here are two main ones noted by experts:
The first is something called “Brownian motion,” the name given to a physical phenomenon in which particles smaller than 0.3 microns move in an erratic, zig-zagging kind of motion. This motion greatly increases the chance they will be snared by the mask fibers.
The second is the N95 mask itself uses electrostatic absorption, meaning particles are drawn to the fiber and trapped, instead of just passing through.
“Although these particles are smaller than the pores, they can be pulled over by the charged fibers and get stuck,” said Professor Jiaxing Huang, a materials scientist at Northwestern University working to develop a new type of medical face mask. “When the charges are dissipated during usage or storage, the capability of stopping virus-sized particles diminishes. This is the main reason of not recommending the reuse of N95 masks.”
I don’t know about you fellers, but I am going to wear a mask, an N95 or N99 if I can get them. It may be a false comfort. I may look silly wearing one. People might call me a sheep. That’s fine. I’d rather look like a fool for whistling while going past the graveyard that be the one getting whistled at. –Uchuck