TOXICITY, BIOAVAILABILITY AND METAL SPECIATION

1. Environmental toxicology emphasizes the difference from traditional toxicology in which pure compounds of interest are added to purified diets, or injected into the test animals. When the objective is to stu dy the fate and effects of trace elements in the environment, knowledg e of the speciation of the elements and their physico-chemical forms i s important. 2. Cadmium salts such as the sulfides, carbonates or oxid es, are practically insoluble in water. However, these can be converte d to water-soluble salts in nature under the influence of oxygen and a cids. Chronic exposure to Cd is associated with renal toxicity in huma ns once a critical body burden is reached. 3. The solubility of As(III ) oxide in water is fairly low, but high in either acid or alkali. In water, arsenic is usually in the form of the arsenate or arsenite. As( III) is systemically more poisonous than the As(V), and As(V) is reduc ed to the As(III) form before exerting any toxic effects. Organic arse nicals also exert their toxic effects in vivo in animals by first meta bolizing to the trivalent arsenoxide form. Some methyl arsenic compoun ds, such as di- and trimethylarsines, occur naturally as a consequence of biological activity. The toxic effect of arsenite can be potentiat ed by dithiols, while As has-a protective effect against the toxicity of a variety of forms of Se in several species. 4. Selenium occurs in several oxidation states and many selenium analogues of organic sulfur compounds exist in nature. Selenium in selenate form occurs in alkali ne soils, where it is soluble and easily available to plants. Selenite binds tightly to iron and aluminum oxides and thus is quite insoluble in soils. Hydrogen selenide is a very toxic gas at room temperature. The methylated forms of Se are much less toxic for the organism than s elenite. However, the methylated Se derivatives have strong synergisti c toxicity with other minerals such as arsenic. 5. Aquatic organisms a bsorb and retain Hg in the tissues, as methylmercury, although most of the environmental Hg to which they are exposed is inorganic. The meth ylmercury in fish arises from the bacterial methylation of inorganic H g. Methylmercury in the human diet is almost completely absorbed into the bloodstream. The nervous system is the principal target tissue aff ected by methylmercury in adult human beings, while kidney is the crit ical organ following the ingestion of Hg(II) salts.