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The specific dust mix in any household differs according to climate, age of the house and the number of people who live in it not to mention the occupants cooking, cleaning and smoking habits. But nearly everywhere, dust consists of some combination of shed bits of human skin, animal fur, decomposing insects, food debris, lint and organic fibers from clothes, bedding and other fabrics, tracked-in soil, soot, particulate matter from smoking and cooking, and, disturbingly, lead, arsenic and even DDT. There are more , Beamer says. Dust is a hodgepodge of all sorts of things. It would probably be impossible to make a list of all the possible items. But dusts ingredient label is not the whole story, since all of those flecks and bits behave differently and present different levels of health risk. To investigate those factors more closely, Layton and Beamer developed a computer algorithm that looked at the size, source and toxicity of dust particles as well as how easily they enter the house, if they ever exit and, if so, by what route. That information, by extension, can provide at least a rough sense of the dust load in your own home. As a general rule, the majority of household dust about 60% comes from outside, through windows, doors, vents and, significantly, on the soles of your shoes. Smaller dust particles from 28 to 49 microns, or thousandths of a millimeter tend to stay on your shoes. The rest is shaken off inside. A higher share of the dust that floats in the air gets deposited, but again, theres a lot that determines how much any one home will get. | 
