A dust particle in the middle of the room takes many hours to settle on the floor or other horizontal surface. During that air time, it frequently changes direction and rises and falls many times. Every time someone walks on a dusty floor, lots of dust is put into the air. Most will eventually settle on the floor again, but some ends up on other horizontal surfaces. But the dust that ends up on the fllor again is likely to put in the again by people walking.
