Aluminum is a soft, non-magnetic and ductile metallic chemical element that visually resembles silver, both in its color and in its great ability to reflect light, and has a weight lower than those of other common metals, at approximately a third that of iron. It has a great affinity towards oxygen, which forms a thin protective layer of oxide on its surface when exposed to air. Aluminum is found in a wide variety of minerals and is the third most abundant element after oxygen and silicon in the earth's crust and the twelfth most common element in the known universe.
Aluminum is the second most commonly produced metal worldwide, although its output is only a small fraction of that of steel. While not nearly as strong or stiff or cheap as steel, its strength can be greatly increased by alloying with other metals. This ease of alloying, very light weight, high ductility, and high thermal and electrical conductivity result in its use in numerous applications, particularly where light weight and corrosion resistance are crucial. Nearly always used in alloys (mainly with copper, zinc, magnesium, manganese and silicon), major applications include aircraft and other transportation vehicles, cans and other packaging, window frames and other construction parts, electrical components and cooking utensils.
Despite its widespread occurrence, aluminum has no known biological function. Although it is generally recognized as being non-carcinogenic, there has long been concern about its role in Alzheimer's disease because small but significant quantities have been found in the brains of Alzheimer's patients. Most aluminum ingested will leave the body in feces, and most of the tiny amount that enters the bloodstream, will be excreted via urine; however, some apparently passes through the usually impervious blood-brain barrier to enter and remain in the brain.