Dust can be made up of anything from dead skin to space rocks.
Outdoors, dust contains just about anything that can be broken down into small bits and moved around by air currents. Some common things in this dust are bits of sand and dirt, pollen grains from flowers, and spores made by molds and other fungi. Many animals (like the elephant in the photo at right) use this dust for bathing, possibly to remove parasites.
House dust has "outdoor dust," plus a lot of other things. The exact contents in dust probably vary from house to house. House dust typically contains bits of dry skin, which are always falling off people and pets. Dust usually also has tiny fibers of human hair, pet hair, and lint from rugs and clothing.
And some dust is from space. Little rocks are always falling to Earth. Some of them have been around since before the age of dinosaurs, and they are still being blown around the world, settling on our dressers.
Photo by Ariadne Van Zandbergen/Alamy