As you know, an atom is 99.9999% empty space. So even if a respirator's edge is touching your face, why can't pathogens cross and sneak past the face seal? See the green arrows in my pictures below.
To wit, how can we guarantee that the gap between your face and a respirator's edge is smaller than any virus or pathogen?
If the mask isn’t sealed around your face, you’re just breathing through the edges, and it doesn’t filter much at all. Men with beards cannot get the full benefit of a mask, and many men have looked at shaving it off for the sake of their lungs.
3M writes in this handy but rather complex guide: “Do not use tight-fitting respirators or loose-fitting facepieces with beards or other facial hair or conditions that prevent direct contact between the face and the edge of the respirator.”
The left is a 3M P100 respirator.