I have given this a lot of thought.

A filter mask becomes more efficient as time passes due to blockage of infiltration/exfiltration paths. In this fashion, an N95 mask degrades to an N99 level mask and it becomes more difficult for the user to get enough air across the filter media to support life so they end up needing a new mask that has less restriction.

As the user wears the mask, most of the blockage and increase in filtering efficiency will occur on the face side of the mask and will be a result of condensation of droplets from the user's respiration. The filter media will become blocked from the inside out. There will be little if any measurable restriction added by the pathogen load of the unfiltered air on the outside of the mask. The distribution of pathogens in the air or the dust load would need to be pretty high for the outside of the mask to ever cause the mask to be restricted.

The main problem with the outside of the mask will be one of accumulation of pathogens on the outside.

I believe that this is the case for use of a mask by an uninfected person.

If the mask user is infected then the mask still becomes blocked on the face side of the mask, increasing its filtering efficiency, but that side also contains the pathogen load that that mask needs to prevent escaping into the room air.

Since the blockage in any case will be mainly from condensation and not from any biological load or dust load then it should be a simple matter to sterilize the masks in a microwave with a small amount of water to create steam. It may even be that the breath condensed on the mask will be sufficient.

I doubt in any case that a pathogen can pass through a mask and I will tell you why.

The path of infection for most people wearing a mask is mostly likely to be infection due to an ill-fitted mask that allows air movement with no filtering such as through hair, or in small spots where a good seal is not possible.

Now the why - filter media must have porosity and permeability. It is designed to create a tortuous path through the media so that anything moving in the connected air spaces has a high probably of being trapped in a void. If there are lots of twists and turns the resistance to flow increases (geologists call it the formation factor - a measure of the tortuosity of the path through the media) and turbulence causes an increase in the likelihood that a particle will contact the inside of the pore path and become trapped. Think of it like water in a river. As the river winds across the landscape, there are points in the stream where flow is weaker and the particle load settles out to form sandbars. There are also points in the stream where the current is faster and larger particles can pass.

The path through filter media is the same. When something encounters an area of low air velocity it becomes stuck. When turbulence increases it becomes more likely to collide with the sides and become stuck. Most of the pathogen load will likely be found very close to the source side. I don't expect much to be able to penetrate to the center of the mask.

That is also why condensation ends up rendering the mask useless. Water droplets coalesce and eventually you have blocked too many of the potential paths through the filter media.

Anyway, this got long winded. Sorry.

I hope some of this makes sense and if anyone has anything to add or discuss hit me up.