This is also very true for the air filters on your home furnace/heat pump, if you have one. Even if you aren't running the heat, if you are running the furnace/heat pump's fan to circulate air periodically (which you should be doing to avoid stagnation), air is still passing through the filter. I live in the Seattle area, and our furnace filter comes out absolutely filthy black after every bout of wildfire smoke. It nasty.
I think it's great! How long ago did you make it? Perhaps it's time for an updated version for this modern age. ;)