radguide

22 Radiation for Radionuclide Users between various cell types within the tissue, and the ability of the tissue to repair itself. The relatively high radiosensitivity of tissues consisting of undifferentiated, rapidly dividing cells suggests that, at the level of the human organism, a greater potential exists for damage to the fetus or young child than to an adult for a given dose. This has, in fact, been observed in the form of increased birth defects following irradiation of the fetus and an increased incidence of certain cancers in individuals who were irradiated as children. Human Health Effects The effects of ionizing radiation upon humans can be classified as being either stochastic or non-stochastic. Stochastic Effects Stochastic effects are those that occur by chance. They consist primarily of cancer and genetic effects. As the dose to an individual increases, the probability that cancer or a genetic effect will occur also increases. However, at no time, even for high doses, is it certain that cancer or genetic damage will result. Similarly, for stochastic effects, there is no threshold dose below which it is relatively certain that an adverse effect cannot occur. In addition, because stochastic effects can occur in unexposed individuals, one can never be certain that the occurrence of cancer or genetic damage in an exposed individual is due to radiation. Non-Stochastic Effects Unlike stochastic effects, non-stochastic effects are characterized by a threshold dose below which they do not occur. In addition, the magnitude of the effect is directly proportional to the size of the dose. Furthermore, for non-stochastic effects, there is a clear causal relationship between exposure and the effect. Examples of non-stochastic effects include sterility, erythema (skin reddening), and cataract formation. Each of these effects differs from the others in both its threshold dose and in the time over which this dose must be received to cause the effect (i.e., acute vs. chronic exposure). Factors Determining Health Effects The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including the size of the dose received, the rate at which dose was imparted, the specific tissues or parts of the body irradiated, and the type of radiation involved.