In case of cancers, there is no upper theoretical level, and dose responses are usually read off from either animal studies or epidemiological studies assuming a linear statistical model. Hence, for cancer, the lowest level at which cancer is found to occur in animal studies (where the data are then extrapolated to human equivalent levels of exposure or dose), the slope factor (called q1* or q1 star) are used to estimate the probable cancer effects and the dose or threshold is assessed as such. See the following figure: