Dose-Response Theories

While we're looking at the hormesis curve, we might as well take a look at the other theories of dose-response in comparison, as shown in Figure 2. The LNT is linear, as we would expect from its name, and indicates that all exposure is cumulatively dangerous. We often see this dose-response relationship for high levels of toxins such as lead. [It wouldn't surprise me if lead had some sort of a threshold also, but it is generally considered to have a cumulatively harmful effect.]

The threshold plot shows no effect from the agent at low doses; but at some threshold value of dose, the health effect becomes negative. We see this response to such substances as caffeine. (Even here we would likely get arguments that in small doses caffeine has beneficial properties.) When one starts searching for examples of threshold effects, the much more prevalent hormetic examples keep getting in the way.