Trace elements are chemicals that are required by organisms in very small quantities for proper physiological and biochemical functioning. Trace elements commonly occur in organisms in concentrations smaller than about 0.001% of the dry weight (less than 10 parts per million, or ppm). Listed in alphabetical order, the most commonly required trace elements for healthy animal or plant nutrition are: boron (B), chlorine (Cl), chromium (Cr), cobalt (Co), copper (Cu), fluorine (F), iodine (I), iron (Fe), manganese (Mn), molybdenum (Mo), selenium (Se), silicon (Si), tin (Sn), vanadium (V), and zinc (Zn). Some organisms also appear to require aluminum (Al) and nickel (Ni).
All of the 92 naturally occurring elements occur ubiquitously in the environment, in at least trace concentrations. In other words, there is a universal contamination of soil, water, air, and biota with all of the natural elements. As long as the methodology of analytical chemistry has detection limits that are small enough, this contamination will always be demonstrable. However, the mere presence of an element in organisms does not mean that it is indispensable for healthy biological functioning. To be considered an essential element, three criteria must be satisfied: (1) the element must be demonstrated as essential to normal development and physiology in several species, (2) the element must not be replaceable in this role by another element, and (3) the beneficial function of the element must be through a direct physiological role, and not related to correction of a deficiency of some other element or indirect correction of a toxic condition.
Research into the physiological roles of trace elements is very difficult, because it involves growing plants or animals under conditions in which the chemical concentrations of food and water are regulated within extremely strict standards, particularly for the trace element in question. In such research, even the slightest contamination of food with the trace element being examined could invalidate the studies. Because of the difficulties of this sort of research, the specific physiological functions of some trace elements are not known. However, it has been demonstrated that most trace elements are required for the synthesis of particular enzymes, or as co-fators that allow the proper functioning of specific enzyme systems.
A principle of toxicology is that all chemicals are potentially toxic. All that is required to cause toxicity is that organisms are exposed to a sufficiently large dose. The physiological effect of any particular dose of a chemical is related to the specific susceptibility of an organism or species, as well as to environmental conditions that influence toxicity. This principle suggests that, although trace elements are essential micronutrients, which benefit organisms that are exposed within certain therapeutic ranges, at larger doses they may cause biological damages. There are many cases of biological and ecological damages being caused by both naturally occurring and human caused pollutions with trace elements. Such occurrences may involve the natural, surface occurrences or metal-rich minerals such as ore bodies, or emissions associated with certain industries, such as metal smelting or refining.