Investigators have long recognized the importance of certain elements, commonly called "minerals," in the diet of humans and animals and in the soil that supports plants, in that these elements are essential for the life or optimum health of the organisms. Deficiencies of 20 to 24 elements in animals and man and of 13 to 18 elements in plants have been recognized. At the same time, an understanding of the responses of these organisms to the insult of toxic concentrations of these and other elements also has been of interest. More recently, concern has arisen regarding the effects of an organism's exposure to the more subtle chronic and subchronic concentrations of certain elements that industrial and other human activities are releasing into the environment.