The composition of dust depends on where the dust is located and the habits of the people who live there. There’s a myth that most dust inside your house primarily comprises flakes of shed human skin, but that’s not true. Most of your skin flakes go down the drain when you wash.
Dust does have some nasty bits in it, though. There’s some skin, granted, but there can also be hair, animal dander and fur, soil, dead insects and their faeces, sand, food particles, pollen, bugs, fibres from clothing and linens, pollution and soot.