I just made a replacement for my cabin microfilters on the 97 528i this afternoon. I'll post photos tomorrow.
I got a home furnace filter that is made up of a layer of thin foam sandwiched between to thin sheets of porous gauze like plastic and then sandwiched between two sheets of a plastic grid with about 1" openings. This was about 18 bucks and will make 2 filters. The package claims it to be washable,filter factor of 8 MAV whatever that is, filter to .08-.10 microns ............. and is special foam sandwiched between a positive and negative charged sheet.
What I did was to take the plasic frame that the original charcol filter is glued into and ripped the filter material (charcol+poly fuzz) out. I used a bic lighter to burn/singe the stray fuzz away from the plastic frame and cut the furnace filter to fit and dropped the whole thing back in the filter box.
I ran the car around and the blower seems quieter and the heater and AC come right to life when going from way cool to butt hot and back again. Probably make the blower motor last longer since its not sucking a golf ball through a garden hose. I'm sure it will stop most of the big chunks from landing in the vent system and as far as purified air- Here in LA you're only good till you open the door or crack the window and then its like you are down wind from a biohazard and no cheezy filter is gonna bail you out