In radiology, the threshold dose is the smallest dose of radiation that will produce a specified effect. In the larger context of toxic exposure, threshold dose refers to the dose below which no harm is done. There is a threshold below which relatively little damage occurs from exposure, and above which the damage increases dramatically. For noncarcinogenic toxins, there does seem to be a safe dose for some substances, a threshold dose below which no harm is done. With carcinogens, however, just one change in the genetic materials of one cell may be enough to cause a malignant transformation that can lead eventually to cancer. Although there is some evidence that repair mechanisms or surveillance by the immune system may reduce the incidence of some cancers, it is generally considered prudent to assume that no safe or threshold dose exists for carcinogens.