ters of tissue to interact with DNA in cells deep within the body. Radiation from high-energy beta-emitting radionuclides, on or close to the skin, can penetrate the skin's outer layer of dead and aging cells to reach the actively dividing cells beneath the outer layer. These exposures have the potential to cause local skin injuries and effects according to the penetrating range. Such manifestations of acute radiation-induced health effects can occur alone, in combination with each other, and with non-radiation-induced trauma, including thermal burns, or other serious medical conditions. Combined injuries of these types tend to be synergistic, that is, the combination can have more of an effect on the health of the exposed person than the sum of the effects of the individual contributors.

Accidents involving humans, medical experience, and studies with animals indicate that doses of radiation must exceed a threshold to cause the various types of acute (deterministic) health effects (injuries) that have been described. Thresholds for several radiation effects of interest are presented in Table 2-3.

If the dose is received instantaneously or within a short time, the threshold for early radiation effects may be rapidly reached or exceeded, resulting in acute effects. This can occur in the event that a high dose from a source outside the body (e.g., nuclear weapon detonation) is received at a high dose rate. If, however, the same total dose is accumulated over a longer period of time (i.e., is fractionated or protracted), the types of deterministic health effects due to the exposure are likely to be fewer in number and less severe. For a given total dose, the effects of protracted or fractionated doses are less than those of acute doses for two reasons: (a) the numbers of cells being killed by the radiation over time will be less than the numbers of new cells being produced in the body's tissue systems during the same period; and (b) because repair of radiation injury occurs within most cells. Doses of radiation can be accumulated over long periods as the result of repeated exposures to radiation outside the body, and when long-lived radionuclides (as opposed to short-lived [rapidly decaying] radionuclides)