Age at exposure versus years of exposure

The pattern of incidence rates according to age for many forms of cancer has been found to be in reasonable accord with the equation or some modification of it: It = btk, where It is the incidence rate at age t, and b and k are constants. An alternative equation postulates that the risk of cancer is determined not by the age of a person but by the length of time exposed to a carcinogenic agent: It = b(t-w)k, where t-w represents the "effective exposure" between first exposure and clinical evidence of cancer. Mesothelioma rates in asbestos insulation workers were strongly related to time from onset of exposure regardless of age at first exposure. However, the same pattern was not evident for lung cancer mortality in the same workers compared with blue collar worker controls from the American Cancer Society Cancer Prevention Study I. Lung cancer mortality by attained rates and by duration of smoking were shown for current smokers of cigarettes only for the Cancer Society study, classified by age at which they started smoking. Lung cancer results were also given for men who never smoked regularly.