radguide

23 Radiation for Radionuclide Users Dose vs. Dose Rate Radiation effects in humans have been shown to be directly dependent upon the total dose received. For many types of effects, however, the rate at which a given dose is imparted has also been shown to be important. This has been particularly evident for non-stochastic effects. For example, a dose of 300 rads to the skin, given within one hour (i.e., an acute exposure), will likely exceed the threshold for erythema. If the same dose is spread over a period of five years (i.e., a chronic exposure), erythema will not occur. The most likely explanation for such results is that spreading the dose over longer periods allows cellular repair mechanisms sufficient time to operate, thus minimizing the effects of the radiation damage. This repair phenomenon is also believed to operate, to some extent, for stochastic effects such as cancer, although the current conservative philosophy is to assume that the risk of such effects depends solely on the total dose. In other words, for the previous example, the risk of skin cancer would be presumed to be identical in both exposure situations. Portion of Body Irradiated The effectiveness of a given dose of radiation in producing biological damage in humans is also dependent upon the portion of the body irradiated. This is due to the differences in the radiosensitivities of the various tissue types and organs within the body. For example, a given dose to the eye is more likely to result in an adverse health effect than is the same dose to the hand. Similarly, a given dose to the whole body has a greater potential for causing an adverse health effect than does the same dose to only a portion of the body. Dose Equivalent Although the biological effects of radiation are dependent upon the absorbed dose, some types of particles produce greater effects than others for the same amount of energy imparted. For example, for equal absorbed doses, alpha particles may be 20 times as damaging as beta particles. In order to account for these variations when describing human health risk from radiation exposure, the quantity, dose equivalent , is used. This is the absorbed dose multiplied by certain “quality” and “modifying” factors (QF) indicative of the relative biological damage potential of the particular type of radiation. The special unit for dose equivalent is the rem (Roentgen Equivalent Man). The SI unit for dose equivalent is the sievert (Sv). The relationship of these units to those of absorbed dose is as follows: rem = rad x Radiation Weight Factor sievert = gray x Radiation Weight Factor