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 Most metals occur in nature in rocks, ores, soil, water, and air, levels areusually low and widely dispersed. In terms of human exposure and toxicological significance, it is anthropogenic activities that are most important because they increase the levels of metals at the site of human activities. The toxic effects of metals usually involve interaction between the free metal and the cellular target. These targets tend to be specific biochemical processes and/or cellular and subcellular membranes.
lead :   is one of the most ubiquitous of the toxic metals. Exposure may be through air, water, or food sources. Inorganic lead may be absorbed through the GI tract, the respiratory system, and the skin. Ingested inorganic lead is absorbed more efficiently from the GI tract of children than that of adults, readily crosses the placenta, and in children penetrates the blood-brain barrier. Initially, lead is distributed in the blood, liver, and kidney; after prolonged exposure, as much as 95% of the body burden of lead is found in bone tissue

The main targets of lead toxicity are the hematopoietic system and the nervous system. Several of the enzymes involved in the synthesis of heme are sensitive to inhibition by lead, although clinical anemia occurs only after moderate exposure to lead, biochemical effects can be observed at lower levels

The nervous system is another important target tissue for lead toxicity, especially in infants and young children in whom the nervous system is still developing. Even at low levels of exposure, children may show hyperactivity, decreased attention span, mental deficiencies, and impaired vision. At higher levels, encephalopathy may occur in both children and adults.
Mercury: Mercury exists in the environment in three main chemical forms: elemental mercury (Hg0), inorganic mercurous (Hg+) and mercuric (Hg2+) salts, and organic methyl mercury (CH3Hg) and dimethyl mercury (CH3HgCH3) compounds. Elemental mercury, in the form of mercury vapor, is almost completely absorbed by the respiratory system, whereas ingested elemental mercury is not readily absorbed and is relatively harmless. Once absorbed, elemental mercury can cross the blood-brain barrier in to the nervous system. Most exposure to elemental mercury tends to be from occupational sources. Organic mercury primarily affects the nervous system, with the fetal brain being more sensitive to the toxic effects of mercury than adults.
Cadmium: Cadmium occurs in nature primarily in association with lead and zinc ores and is released near mines and smelters processing these ores. Industrially cadmium is used as a pigment in paints and plastics, in electroplating, and in making alloys and alkali storage batteries (e.g., nickel-cadmium batteries). Environmental exposure to cadmium is mainly from contamination of groundwater from smelting and industrial uses as well as the use of sewage sludge as a food-crop fertilizer. Grains, cereal products, and leafy vegetables usually constitute the main source of cadmium in food. Acute effects of exposure to cadmium result primarily from local irritation. The main organ of damage following long-term exposure is the kidney, with the proximal tubules being the primary site of action.

Treatment of Metal Poisoning. Treatment of metal exposure to prevent or reverse toxicity is done with chelating agents or antagonists. Chelation is the formation of a metal ion complex, in which the metal ion is associated with an electron donor ligand.

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