Sample Linearized Dose-Response Curves for Threshold and Nonthreshold Responses

If two chemicals have the same potency, one can still be considered more toxic than the other, as is evident in fignre 3.2. Even though the lines are parallel, a higher percentage of responses for chemical Y occurs at every dose level than for chemical X. For example, at a dose of 1000, there are no responses seen for chemical X almost 20 percent responses are reported for chemical Y at this dose level. 

Another factor that complicates how to interpret potency concerns dose-response curves with different slopes, such as the two chemicals shown on figure 3.3. Such disparate shapes can resnlt when the mechanism of action between the two chemicals is different. If asked to determine whether chemical X or Y is more toxic, how could you respond It depends on the dose At a dose of 2000 (B on figure 3.3), the two chemicals have the same toxicity the same dose leads to equal frequency of responses for both chemicals. However, at low doses such as 700 (A on figure 3.3), chemical Y is far more toxic—more than double the responses of chemical X for the same dose. The situation is reversed at high doses (C on fignre 3.3)—chemical X is more toxic than chemical Y. If asked which chemical is more potent, yon conld trnthfully answer that you couldn t tell however, you could say which is more toxic at a given dose level. 

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Figure 3.2. Sample Linearized Dose-Response Curves for Threshold and Nonthreshold Responses...

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