The filter is often found near the vent hole to equalize the pressure between the inside of the disk, and the atmosphere. The disk does not 'breath' per say, but there is a hole, and very often a small filter to prevent the dust from entering when pressure changes (heat, cool, heat, cool...). I too have disassembled several disks, some have the filter, some do not.

eniac

I keep a vacuume cleaner in my building room (it has the hose adaptor that is supposed to be used for cleaning draperies). During the builing process (including sanding) I vacuume the floor and my clothing and arms occasionally. I fill at least 2 large vacuume bags a year. I have alergies but have not been affected by sanding particles or fumes from CA glues.
I have been building for over 40 years, still have the same lungs and eyes. My nose still works too. Had a prosectomy for cancer (must have used the wrong hose somewhere along the line).
When painting, I use a mask because the paint gets stuck on my nose hairs and I don't know whether to pick the paint out of my nose or pick the glue off my fingers.
If I had to build wearing goggles and rubber gloves I'd change my hobby and go fishing. Paint, especially auto paint and epoxies are treated with rubber gloves and proper breathing equipment. If I spelled vacuume incorrectly, I apologize.