I have a collection of various 94+% masks that I amassed towards the beginning of the pandemic. About 6 or 7 types. Only 1 of them doesn’t fog my glasses in the winter (and it’s a freakin Honeywell respirator) That means all of the others are leaking, even when I wear them “properly”
Water vapor is a lot smaller than viral particles and is likely to pass through even a N95 mask, which is good, as you'd drown pretty quickly if it didn't.
I wouldn't read too much into fogging.
(That said, I do wish there were better options for people with giant noses like mine.)
I mean, they’re fogging because the top of the mask leaks, and the breath takes the path of least resistance and goes around rather than out of the mask.
Likely, but not necessarily. My silicone half-face (with disabled check valves) slightly fogs my glasses because enough water vapor is leaving the filter close to my glasses. I'm sure it's not leaking, because I can feel the silicone pull away from my face when it does.
For me, fogging my glasses means I have a leak and the air is being forced out gaps around my nose. Maybe under extreme weather conditions a perfect fit would still fog, but that's not what the op is talking about.
I think it's more about the direction of the water vapor than the fact that the masks can pass vapor. If there was no leakage one might expect vapor to leave through the mask uniformly or in the same pattern as it did without masking (e.g. mostly forward.)
