I live near a volcano, and I'd say most of the "dust" in our home is actually very fine volcanic ash.

There's definitely some skin cells and pet dander in there. There's just no way that those things are a majority of what we sweep up every few days, because our collective mass cannot possibly be dwindling that quickly.

A quick search suggests that one square inch of skin has 19 million skin cells. At a rate of 1000 per hour, it would take 19,000 hours (791.6 days) for one person to shed enough cells to equal one square inch of skin. Two humans live in my household, so we'll say for us together it would take roughly a year.

I'm sweeping up multiple cubic inches of dust multiple times a week. If dust were "mostly" skin cells, we should only need to dust a little bit once a year.