What is dust, exactly, and why might it be bad for me?

Dust is made up of particles of all sorts of things. In places where people live, a great deal of dust comes from flakes of dead skin, which are being shed all the time. Dust mites, tiny microscopic creatures that feed on this dead skin, make up dust, too (including their waste and tiny skeletons). Particles of the environment contribute to dust as well: grit from the sidewalk, salt from the sea, dry earth, pollen from plants, pet dander, molds, and smoke from burning materials. And Earth gets 10 tons of dust from outer space every day, from the meteors that burn up as they enter our atmosphere. Sometimes these ingredients cause allergic reactions, such as sneezing and coughing.