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When the results of these tests are plotted on a graph, with dose along the bottom (horizontal axis) and response up the side (vertical axis), a dose-response curve is obtained. (It is still called a curve even if it is a straight line!) For example, the dose could be milligrams of chemical per kilogram of body weight per day (mg/kg/day) and the response could be the percentage of animals that got a certain type of cancer, or suffered a certain degree of liver damage.

The question of whether there is a threshold dose, below which there is no toxic effect (no observed effect level, or NOEL) is controversial, especially for carcinogens. Because many carcinogens act by damaging DNA, and once this damage has been done it is permanent and by definition results in an increased risk of cancer, many argue that there is no such thing as a safe dose of these carcinogens. Also, because cancer is such a serious disease, and so many gaps exist in our knowledge of how carcinogens act, it makes sense to take a precautionary approach. In the Carcinogens Approved Code of Practice the HSE says, "the risk of cancer from exposure to a substance cannot in most cases be presumed to be zero except by eliminating exposure." NOELs are also used in setting occupational exposure limits.

"We assume that every molecule of benzene to which an individual is exposed has some finite risk, albeit small, of producing a mutation that may result in acute myelogenous leukaemia" (B. Goldstein and H. Kipen in Levy and Wegman).

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