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33 Radiation for Radionuclide Users F. Radiation Safety For Laboratory Use of Radionuclides Philosophy of Current Radiation Safety Practice Current regulatory limits for radiation dose have been conservatively set to prevent all acute effects of radiation dose and to limit the risk of chronic effects, such as cancer, to very low levels. As a primary objective, then, radiation safety practice attempts to ensure that all doses are below these limits. The accomplishment of this objective, however, is not the ultimate aim of current radiation safety practice. An overriding principle of radiation protection philosophy is that all doses must be maintained “as low as is reasonably achievable” ALARA. Thus, even if a given dose to an individual is within regulatory limits, it will not be acceptable if the dose could have been limited further by “reasonable” means. This unit describes the standard practices and procedures which constitute the means for ensuring doses are ALARA. External Radiation Protection High energy beta particles, gamma rays and x-rays are often referred to as “penetrating” radiations because of their ability to pass through considerable thicknesses of matter. Because of this ability, penetrating radiation can originate from sources external to the body and still impart a significant dose to living tissue. Radionuclides which emit such radiations thus pose an “external” radiation hazard. Examples of such radionuclides which are used in radiotracer research are the beta emitter, 32 P, and the gamma/x-ray emitter, 125 I. Certain gamma emitters (e.g., 137 Cs, 60 Co) which are routinely encapsulated as sealed sources can also pose significant external hazards. On the other hand, such commonly used radionuclides as 3 H, 14 C, and 35 S are primarily internal radiation hazards since the beta particles they emit do not have sufficient energy to penetrate the skin (although if applied directly to the skin’s surface in sufficiently large quantities, 14 C or 35 S can cause damage to living skin cells). The dose resulting from a given exposure situation depends upon the duration of the exposure, the distance from the source to the tissue, and the strength of the source. For external exposures, these factors can be controlled to determine the total dose received. Minimizing Exposure Time In general, the dose (D) received from a particular exposure situation is the product of the dose rate ( ) and the exposure time (t): D = ×t Eq. 8