Classification of toxic responses

Many chemicals that are acutely toxic are not chronically toxic, and vice versa. For example, pure vitamin D exhibits high acute toxicity. However, low repeated doses (such as in the normal intake of milk) are not only nontoxic but also essential to good health. For chemicals that are both acutely and chronically toxic, the mechanisms of the two types of toxicity are often different. For example, acute toxicity from a large dose of chloroform is caused by effects on the central nervous system that cause dizziness and narcosis. However, ingesting water containing trace concentrations of chloroform over a lifetime results in liver damage and cancer (Stewart, 1971). 

Estimates of the toxicity of noncarcinogens are based on the concept of their threshold the dose below which there are no short- or long-term effects on the organism. The lack of effect below the threshold dose can be understood in terms of its molecular basis. If millions of receptor biomolecules are available for a given function (e.g., nerve transmission, transport of oxygen), the binding of a toxic chemical to a small number of these receptor molecules does not produce a measurable toxic effect. A good example is the inactivation of a very small number of molecules of acetylcholinesterase by an insecticide such as parathion. 

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