Genotoxic carcinogens can lead to DNA mutations with the potential to cause
cancer. Typically, a series of mutation events are needed before malignancy
occurs so a single, small exposure may not result in disease. Also, cells have
an armoury of defence mechanisms which, to a degree, counter the effects of
mutagens. Distinguishing the point at which exposure to a carcinogen increases
mutation rates beyond the background level is challenging. In fact, there is now
general agreement that, for genotoxic carcinogens, no specific threshold can be
identified. However, NOAELs (No Observed Adverse Effect Levels) may be used in
the process of establishing a dose-response relationship. These denote the