I used to have a PC case with a cooling fan filter -- by the time I retired that PC, the dust was nearly an inch thick. I had forgotten that it had a filter, an

I used to have a PC case with a cooling fan filter -- by the time I retired that PC, the dust was nearly an inch thick. I had forgotten that it had a filter, and "out of sight, out of mind". It wasn't even a HEPA filter, just a fine mesh.