Metals, as Contaminants

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Metals, as contaminants

Metals, objects made from metal, and chemical compounds containing metal are pervasive in the home, workplace, and environment . Although metals play an essential role in modern human society, there are many instances where they occur as unneeded and harmful contaminants. Heavy metals such as mercury , cadmium , lead , uranium , chromium, manganese, nickel , thallium, and bismuth are all industrially useful, but they are also potentially harmful at high concentrations in the environment or when found in food or drinking water. Environmental contamination with heavy metals can result from many activities including mining, large-scale combustion of coal in heating or power plants , mineral and metal processing, and use of fungicides and pesticides.

The metal mercury (Hg), a useful component of many products including thermometers, pesticides, drugs, paints, and batteries, is unusual in that it is a liquid at ordinary temperatures. It forms a variety of useful salts and organic compounds. Misuse of products containing mercury can lead to serious health problems. Disorders caused by mercury depend on the form of mercury ingested and the method of exposure. Ingestion of mercury salts can lead to kidney damage and death. Exposure to mercury vapor can lead to inhalation or absorption through the skin. Brain function is affected, and loss of memory, depression, anxiety, and other personality changes may result. Ingestion of organic mercury compounds used in pesticides can result in permanent neurological damage. One very serious mercury pollution incident occurred in the 1950s at Minamata Bay, Japan, where releases of mercury in effluent from a manufacturing plant were ingested by fish. People eating the fish developed serious, and in some cases fatal, neurological maladies. Infants and children were particularly vulnerable to this "Minimata disease."

Lead (Pb) has been used for many purposes for thousands of years. However, toxicity has been largely unnoticed or ignored until recent decades. Lead uptake by the body is quite slow, but its rate of excretion is even slower. Thus in cases of long term exposure, lead levels in the body gradually increase. There are many sources of contamination. Lead water pipes were once used to carry domestic water. Until recently, lead-containing solder was use to join copper piping in homes. Lead contamination of drinking water resulted, and symptoms of lead toxicity were noted in those who used the water. Until recently, white lead pigment was widely used in house paint. Deteriorating and chipping paint poisoned occupants, particularly children, who accidentally ingested or inhaled dust and fragments. Symptoms of lead toxicity can be both chronic and acute. Weakness, loss of appetite, anemia , vomiting, and convulsions have all been reported. Lead causes lesions in the central nervous system and serious long-term damage.

Cadmium (Cd) is useful both as an element and in compounds. The element is a lustrous silvery metal that is usually associated in nature with zinc. It is used to plate steel, copper, brass, and other alloys and serves to retard corrosion. Cadmium oxide is used in nickel-cadmium storage batteries and as a pigment in paints and inks. Although metallic cadmium is not dangerous under ordinary conditions, cadmium ions, produced when the metal is attacked by mineral acids, are very toxic. Toxicity symptoms resemble those of mercury.

Uranium (U) with an atomic number of 92, is one of the heaviest of all elements. Once thought to be rare, it is now known to be widely distributed in the Earth's crust. Its concentration in sea water exceeds those of mercury and cadmium and is at about the same level as copper and lead. Interest in uranium and its use expanded dramatically with the development of atomic power and nuclear weapons . Environmental and health concerns have centered on the radioactive nature of uranium, but chemical toxicity is also a potential problem. Uranium contamination surrounding mining and mill tailings in southwestern United States has been a major concern for Native Americans living nearby.

Each of the heavy metals has its own specific toxic effects on human health and the environment. Before this fact was generally realized, mining, smelting and manufacturing activities often resulted in appalling instances of environmental contamination. While greater concern has resulted in improvement, serious problems remain in instances where proper control has not been practiced.

[Douglas C. Pratt Ph.D. ]