A trace element is a chemical element whose concentration in an organism or a substance is extremely low. Some trace elements play a crucial role in biological and other processes, and an essential trace element is one that is required for one or more physiological or biochemical processes.
In the case of the human body, for example, the four elements oxygen, hydrogen, carbon and nitrogen constitute about 96 percent by weight. Calcium accounts for an additional approximate 1.5 percent and phosphorous for another one percent. With the other main elements, potassium, sodium, chlorine, sulfur and magnesium, eleven elements account for about 99.85 percent of total body weight. The remaining 0.15 percent consists of trace elements, mostly of iron, cobalt, copper, zinc, manganese, molybdenum, iodine and selenium.