It's a widely repeated factoid that dust consists primarily of human skin. For instance, one can find this piece of information in the first paragraph on the wikipedia page about dust. But Paloma Beamer, a dust expert at the University of Arizona, disputes this claim. From NPR.org:
Beamer says there are really only two places dust can come from: outdoors and indoors. We are an important part of the process of getting the outdoor stuff indoors. We bring it with us when we enter a house — through "soil particles that come in on your shoes," says Beamer, or tiny particles suspended in the air when we open the door and walk in.
Then there's the indoor component of dust. "Like pieces of your carpet fiber or your furniture, your bedding, or anything like that that starts decaying," she says. Then there are organic contributors. "Skin flakes and the dander off your pets, and other insects or bugs that might be in the home."
Now, as anyone who's looked under a sofa knows, there's dense dust and there's fluffy dust.
"A lot of the fluffy things, I think, tend to do more when you get a lot of fibers. In my house, it comes from cat hair," Beamer says.
Beamer's interest in dust stems comes from her effort to measure people's exposure to toxic substances. In a recent paper in the journal Environmental Science and Technology, she calculates the proportion of dust that's from indoor sources, compared with the amount from outdoor sources. She figures that one-third comes from indoor inorganic sources like carpet fibers. "Two-thirds comes from both soil tracked in, and the outdoor air particles," Beamer says.
I'm inclined to think Beamer is right. I find it hard to imagine that the volume of dust in my house comes primarily from the dead skin cells of my wife and I.