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An understanding of our fragile environment can begin with a recognition of the importance of certain elements, commonly called "mineral substances" (such as iron and zinc), in the lives of humans and animals and in the soils that support plants. This recognition is well deserved because these elements are essential for the life or optimum health of an organism. Some elements such as carbon, hydrogen, oxygen, magnesium, potassium, and phosphorus are required in relatively large amounts by organisms. However, others are required in smaller quantities; these are referred to as trace elements. Diseases have been related to the deficiency of about 20 elements in animals and humans and to the deficiency of approximately 13 elements in plants.

At the same time, if these and other elements occur in quantities great enough, toxicity can result. An element, or any substance, that occurs in the environment and contains concentrations above what are considered to be background levels may be considered a contaminant. When contaminants occur at levels that are potentially harmful to organisms, they are labeled as hazards. Often the quantitative difference between essential amounts and toxic concentrations of these elements is very small. For example, the trace element selenium is required at a level of no less than 0.4 ppm in the diet of cattle but can be toxic at levels greater than approximately 4 ppm.

Elements that are required for survival by animals and plant are termed essential while those not required are nonessential. Trace essential elements such as fluorine, copper, selenium, molybdenum, can be hazardous to life forms if present at high levels. Nonessential heavy metals such as a arsenic, lead, mercury, cadmium, and chromium are usually toxic to organisms at much lover levels than trace essential elements. (From USGS Circular 1105.)

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