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.
Humans have been exposed more and more widely to metallic contaminants in his environment, resulting from the products of industry. There are three main sources of metals in the environment. The most obvious is the process of extraction and purification: mining, smelting, and refining. The second is the release of metals from fossil fuels (such as coal or oil), when these are burned. Cadmium, lead, mercury, nickel, vanadium, chromium, and copper are all present in these fuels, and considerable amounts enter the air, or are deposited in ash, from them. The third and most diverse source is production and use of industrial products containing metals, which is increasing as new applications are found. The modern chemical industry, for example, uses many metals or metal compounds as catalysts; metal compounds are used as stabilizers in the production of many plastics; and metals are added to lubricants, and so find their way into the environment.
Metals follow many pathways and cycles in the environment, and some of them undergo transformations in the process - like the conversion of inorganic mercury to the more toxic methyl form, and the subsequent accumulation of the latter by fish. Where metal-rich mine drainage enters fresh waters there are often obvious ecological effects, including a great reduction in the invertebrate fauna and the absence of fish. Some plants and invertebrate animals also accumulate metals of potentially toxic levels. Once toxic concentrations have been reached, it may take a long time to reduce them to non-toxic levels. Pathways within man and other targets are also crucially important. The rates and mechanisms of absorption and excretion, and the extent to which metals are deposited in such tissues as bone or the kidney cortex and then only slowly removed, need to be known if risks are to be assessed. The biological half-life of methyl mercury in man, for example, is about 70 days, that of cadmium around 20 years, and that of lead only a few weeks in blood and soft tissue, but at least ten years in bone. The risk from cadmium appears limited largely to groups of people consuming food produced in areas where the soil or irrigation water is contaminated, although there is concern about rising cadmium levels in the environment. Mercury is a problem where populations eat large amounts of fish taken from contaminated waters. Lead is the most widespread potential hazard, since quite small increases in lead consumption can raise blood lead levels to the point where biochemical changes are detectable. Children appear to be more sensitive to exposure to heavy metals than adults, and are consequently the focus of concern. Today, increasing emphasis is being placed on the carcinogenic effects of metals. Chromium, nickel, lead, and cadmium are all proven or suspected causes of certain cancers associated with industrial processes. Large doses of cadmium and nickel are teratogenic in animals, but this effect of metals is not well established in man.