radguide

28 Radiation for Radionuclide Users correspondingly greater amount of dose than the body as a whole. The dose committed to a particular organ or tissue depends, in part, upon the time over which these areas are irradiated by the radionuclide. This, in turn, is determined by the radionuclide’s physical and biological half-lives (i.e., the effective half-life). The biological half-life of a radionuclide is defined as the time required for one half of a given amount of the radionuclide to be removed from the body by normal biological turnover. Exposure Limits Concern over the biological effects of ionizing radiation began shortly after the discovery of x-rays in 1895. From that time to the present, numerous recommendations regarding occupational exposure limits have been proposed and modified by various radiation protection groups, the most important being the International Commission on Radiological Protection (ICRP). These guidelines have, in turn, been incorporated into regulatory requirements for controlling the use of materials and devices emitting ionizing radiation. Basis of Recent Guidelines In general, the guidelines established for radiation exposure have had as their principal objectives: (1) the prevention of acute radiation effects (e.g., erythema, sterility, etc.); and (2) the limiting of the risks of late stochastic effects (e.g., cancer and genetic damage) to “acceptable” levels. Numerous revisions of standards and guidelines have been made over the years to reflect both changes in the understanding of the risk associated with various levels of exposure and changes in the perception of what constitutes an “acceptable” level of risk. The current annual occupational dose limits have been established at a level where the risk of death from radiation induced cancer should not exceed the risk of accidental death to an average worker in a safe, non- nuclear industry. Current guidelines for radiation exposure are based upon the conservative assumption that there is no safe level of exposure. In other words, even the smallest exposure is assumed to have some probability of causing a late effect such as cancer or genetic damage. This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulatory limits, but of also maintaining all exposures “as low as is reasonably achievable” ( ALARA ). This is a fundamental tenet of current radiation safety practice and is a regulatory requirement to be followed by all occupational users of radioactive materials and radiation producing devices.