The moisture from breathing also plugs them up and makes them stop working effectively. It discharges the electrostatically charged material used to attract the particulates. An N95 can only be worn once for a limited amount of time before it loses its ability to filter out the very small particles in the air.
More than that, there is an actual pumping action. When you inhale, a wet mask sucks up, forcing to draw through it... but on the exhale, the mask pushes away, allowing the exhaust to largely escape around the edges of the mask. So as far as the mask is concerned, there is greater draw than push, encouraging migration through the wet, body-temperature medium of the mask.
When painting once the mask gets wet, you can literally taste the paint coming through the mask.
As for the n95 (and like) respirators, they were designed to be a cheap, disposable respirator, as an upscale joe-homeowner or now-and-then solution instead of buying a real respirator with filter cartridges designed for industrial use, and with interchangeable filter sets to operate in varying environments, and meant to be cleanable, renewable, repairable, and used all day long, indefinitely.
Everybody acts like they're all that, but they were meant to be a disposable piece of junk that will work for a minute.
