Where does all the dust inside a watch actually come from?

gremlins

Most of what you're seeing is probably not 'dust' in the household sense. It's a mix of dried lubricant migrating, tiny bits of gasket material, and yes, some skin flakes that get in when you open the caseback. Skeleton watches just show it all because there's no dial hiding it. A watchmaker once told me a lot of 'dust' is actually metal particles from the movement wearing in, which sounds alarming until you realize it's microscopic and normal.

Seconding the lube thing. Old oil dries out and turns into little amber crusties that flake off. Looks exactly like dust under a loupe.

so what you're saying is the watch is slowly cannibalizing itself. cool cool cool

Not cannibalizing, just breaking in. But honestly a chunk of it is environmental. Every time you set the time with the crown pulled out, there's a tiny gap. Dust in the air finds gaps. Skeleton cases also tend to have more seams and openings than a solid dial watch.

Seasoning is a generous word for it, but yeah, that's a real ingress path. Not a huge deal in normal use, but it adds up over years. The bigger contributor is usually whoever last had the caseback off. Even careful hobbyists leave a speck or two.

Honestly this is why I lurk here and stick to quartz. No oil, no metal shavings, no existential dread about my watch slowly turning into a snow globe.

Quartz has its own dust drama, don't kid yourself. But fair.

OP, if it genuinely bothers you, a competent watchmaker can blow it out and re-lube during a service. But I'd resist the urge to crack it open yourself just to clean it. That's how you turn three specks into a confetti party.

confetti party is now my term for a botched DIY service. thank you for this gift

One more source nobody mentioned: the crystal itself. Some aftermarket sapphires shed tiny particles at the edges, and AR coating breakdown can look like dust too. Probably not your issue but worth ruling out