Do you have pets?
Is your PC in a room with a thick carpeting?
Do you live in a rural area with dirt or gravel roads?
Does anyone in your house smoke indoors?
Does the PC sit close to the kitchen where a lot of cooking/frying/sauteing takes place?
Do you have pets? Yes
Is your PC in a room with a thick carpeting? No
Do you live in a rural area with dirt or gravel roads? No
Does anyone in your house smoke indoors? No
Does the PC sit close to the kitchen where a lot of cooking/frying/sauteing takes place? Kind of. Its a fair bit away but there are no walls in the way.
Sounds like pets are the culprit. Looks like either cat hair or a dog with very fine undercoat.
I have 2 big german shepherds and man, they shed all the time.
This will just be something you have to account for and regularly clean unless you can isolate the PC in a room where pets do not frequent and doors always remain shut. Fan filters will be your friend, like you have found out.
Everybody has some problems with dust. High frequency with most PC parts work like static electricity and fans tend to ingest dust and deposit it in low pressure areas inside the case. Industrial dust collectors work on exactly same principle, fans draw air in and high frequency electrified grid collects it.
i used to service some PCs and electronics from hospitals and found a lot of dust in them.