Metals have been used for centuries and are fundamental to major industries, yet some have the potential to damage human health and disturb the balance of environmental systems if they are allowed to reach excessive concentrations in air, water, soil, or food.
Only a few metals are important environmentally: those most likely to cause concern include copper, cadmium, mercury, tin, lead, vanadium, chromium, molybdenum, manganese, cobalt and nickel. In addition, the metalloids (including antimony, arsenic and selenium), which have some metallic properties, may cause environmental problems; uranium, plutonium and other actinides also have metallic properties, and are a cause for concern.
A metal is regarded as toxic if it injures the growth or metabolism of cells when it is present above a given concentration. Almost all metals are toxic at high concentrations, and some are severe poisons even at very low concentrations. Copper, for example, is a micronutrient, a necessary constituent of all organisms, but if the copper intake is increased above the proper level, it becomes highly toxic. Like copper, each metal has an optimum range of concentration, in excess of which the element is toxic. The toxicity of a metal depends on its route of administration and the chemical compound with which it is bound. The combining of a metal with an organic compound may either increase or decrease its toxic effects on cells. On the other hand, the combination of a metal with sulphur to form a sulphide results in a less toxic compound than the corresponding hydroxide or oxide, because the sulphide is less soluble in body fluids than the oxide. Toxicity generally results: when an excessive concentration is presented to an organism over a prolonged period of time; when the metal is presented in an unusual biochemical form; or when the metal is presented to an organism by way of an unusual route of intake. Less well understood, but perhaps of equal significance, are the carcinogenic and teratogenic properties of some metals.