I don't have a citation, but I don't think it's filtering (in either direction) very well at all. With a cloth or paper mask, I suspect a lot of my exhales are slipping out of the sides and out the top. Instead, I think the mask is catching droplets and absorbing some of the moisture.
I think of it like a device that is acting the way your hand does when you cover your mouth when you sneeze or cough. Your hand isn't filtering in any real way, but you are keeping droplets from flying as far as they otherwise would.
I think a good filter would require something much more like a WWII gas mask or a forced air respirator in a hazmat suit, etc...
That's not surprising considering the link I posted. My question is whether the mask protects the wearer, and if not, why would the filtration only work in one direction?
Thinking about it some more, I suspect the disconnect between us is that you think the primary way of getting sick is breathing the airborne virus. Whereas I think the primary way of getting sick is touching your eyes (or picking your nose). So you want the filtering to be good, but I just don't want infected snot droplets to be blown everywhere.
Now, I'm sure someone will throw an obnoxious "citation needed" reply...
Are you really wearing a mask which is tight enough that _all_ of your respiration is going in and out through the material? If not, your argument/question about filtration isn't very relevant.
Either a mask filters effectively, or it doesn't[1]. Intuitively I don't understand why a mask would only filter air in one direction.
[1] https://scholar.google.com/scholar?q=.%2C+ACS+Nano+10.1021%2...