英语六级阅读练习：灰尘背后的那些事儿02

　　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.

　　Here in Arizona, says Beamer, where we leave our windows open most of the year and have an arid climate, we would probably have a higher ratio. Industrial centers or sooty cities have plenty of dust too, though for different reasons.

　　arid adj. 干旱的;不毛的

　　ratio n. 比例

　　Theres not much to fret about in simple particles of dirt or organic materials such as pollen , but lead, arsenic and DDT can be a more serious matter. About one-third of the arsenic in the atmosphere comes from natural sources volcanoes principally. The rest comes from mining, smelting, burning fossil fuels and other industrial processes. Even in relatively low concentrations, arsenic is not without risk, especially to small children who play on the floor and routinely transfer things from their hands to their mouths. The same is true for lead, which comes less from wall paint the source most people would expect than from auto exhaust, smelting and soil deposits. Lead loading on floors is a key determinant of blood-lead levels in children, Layton and Beamer wrote in their paper.

　　fret about 因 焦急;为 烦恼

　　particle n. 颗粒;微粒

　　pollen n. 花粉

　　low concentration 低浓度

　　determinant adj. 决定性的 n. 决定因素