Dust is not mostly human skin, but does consist of some pretty gross stuff.
This is one of those “Eww! Gross!” factoids that sound very scientific, but isn’t really true. Sometimes a specific percentage of dust is said to be skin, usually about 70 or 80 percent, but unless you’re a molting bird or reptile (or you work in Dr. Frankenstein’s laboratory), very little of your environment is composed of dead body parts.
There are far more common sources of dust pollutants, including animal dander, sand, insect waste, flour (in the kitchen), and of course lots of good, old-fashioned dirt.
Every time we open a window or a door, we stir up and move around tiny, airborne particles that eventually settle around the house. Humans do shed dead skin, but most of it is carried away by water when we shave or bathe, ending up not on our floors but in our sewers. Now don’t you feel better?