Analysis of Cumulative Toxicity

The cumulative dose-effect curve is used extensively to characterize chemical toxicity. It provides a foundation for aU forms of chanical risk assessment including human health risk assessment and ecological risk assessment. Because this relationship will be encountered again and again when evalnating risk, several important features of the cumulative dose-effect curve are worth anphasizing  

It concerns the frequency of occurrence of a single toxic effect of a specified magnitude, referred to as a quantal effect or endpoint. Examples are death, a specific birth defect infertility, cancer, liver disease, anemia, kidney failure, or a cardiac arrhythmia. It does not concern different magnitudes of the same toxic effect nor does it involve a spectrum of toxic effects. 

It is a statistical relationship that describes the incidence of a specified toxic effect as a function of the dose that a test population is exposed to. 

A key feature of a quantal dose-effect relationship is a statistic the median dose, variously annotated as the TDjg or LDjg, which marks the middle of the exposed population with respect to its susceptibility to the quantal toxic effect. Half the population manifests the specified toxic effect at doses less than the TD50, and half the population manifests the specified toxic effect at doses greater than the TD50. 

The median dose is distinct from the mean dose. The mean dose is the average dose to which a population is exposed. In practice, the mean dose rarely coincides with the median dose. 

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