I found what I was referring to before:
“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 counter intuitive 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.
Secondly, 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.”
So if Covid-19 particles were 400 nano meters in size, N95 masks would not help.
But because they are anywhere from 5-120 nano meters in size, N95 masks work. But if the anti static feature wears off, then even an N95 mask is not going to keep out covid-19.