Effect in the Case of Repeated Incorporation

The lethal dose is only one characteristic of the different toxic effects. The toxic effect starts, however, at clearly smaller doses in the body and follows a substance-specific dose-response curve. All substances show more or less similar behavior in the case of low doses with decreasing amounts of the substance smaller effects are observed, and finally, with a substance-specific dose, no effects occur. For all substances, except in the case of some special properties like genotoxic effects, the shapes of the dose-response curves are similar. 

Plotting the dose of an administered substance against its effects in a double-logarithmic manner, a characteristic S-shaped curve results. In the median region the curve shows a linear relationship between dose and effect (see Fig. 2.6). 

Except in the case of accidents, a single exposure to chemicals is unusual longterm use with repeated exposure is typical. Therefore, the toxic effect of repeated substance incorporation of small amounts over a longer duration is more important for a realistic assessment of chemicals at the workplace. 

The dose below which no biologically relevant effect can be found is called the no observed effect level , abbreviated to NOEL. The term no observe adverse effect level (abbreviated to NOAEL) is used for the dose at which no relevant health effects can be observed. Here, all effects of a substance have to be considered, and 

Known substances with very steep curves are phosgene, hydrocyanic acid, nitrogen dioxide, and hydrogen sulfide. Relevant effects can be seen where the NOAEL for these substances is only slightly exceeded, while in other cases lethality is observed only where the NOAEL is exceeded tenfold. 

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