Deterministic Responses from Radiation Exposure

The Japanese atomic-bomb survivors also are a potentially important source of data on the dose-response relationship for severe hereditary responses. However, no evidence for inherited genetic effects has been observed in spite of nearly 50 y of study. In the absence of data in humans, estimates of the frequency of radiation-induced hereditary responses have been based primarily on data from studies in mice. 

In all studies of the relationship of radiation-induced stochastic responses to dose, the derivation of MLEs (mean values) of the dose-response relationships has been emphasized (NAS/NRC, 1988b 1990). Furthermore, for purposes of radiation protection, MLEs of the dose-response relationships, rather than UCLs, have been emphasized in extrapolating the observed dose-response data to lower doses beyond the range of observation (NCRP, 1975 1999b). 

The use in radiation protection of MLEs of the relationships of stochastic responses to dose, rather than UCLs, is justified on the grounds that the probability of a response in most individuals is not likely to be significantly underestimated. Even if the probability were underestimated, the current framework for radiation protection 

The conclusion about the uncertainty in the dose-response relationship for radiation stated above takes into account the uncertainty in extrapolating the data at high doses and dose rates in the Japanese atomic-bomb survivors to the lower doses and dose rates of concern in routine exposures of the public. The issue of extrapolation to low doses and dose rates is a matter of considerable controversy and is an important source of uncertainty (NCRP, 1997 2001). Forpurposes of radiation protection, the frequency of responses at low doses and dose rates generally has been assumed to be a factor of two less than MLE of the frequency of responses in the Japanese atomic-bomb 

In spite of uncertainties in the dose-response relationship for radiation discussed above, it is generally believed that radiation risks in humans can be assessed with considerably greater confidence than risks from exposure to most hazardous chemicals that cause stochastic effects. The state of knowledge of radiation risks in humans compared with risks from exposure to chemicals that cause stochastic effects is discussed further in Section 4.4.2. 

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In setting limits on exposure intended to prevent the occurrence of deterministic responses, the safety and uncertainty factors that are applied to the assumed thresholds for hazardous chemicals that cause deterministic effects usually are considerably larger (by at least a factor of 10) than the safety factor normally applied to the thresholds for deterministic responses from exposure to radiation. Furthermore, the assumed threshold usually is more conservative for hazardous chemicals than for radiation (i.e., a lower confidence limit of the threshold often is used for... 

For the purpose of developing a risk-based hazardous waste classification system, prevention of deterministic responses should be of concern only for hazardous chemicals, but not for radionuclides. Deterministic responses from exposure to radionuclides can be ignored because radiation dose limits for the public intended to limit the occurrence of stochastic responses are sufficiently low that the doses in any organ or tissue would be well below the thresholds for deterministic responses (see Section 3.2.2.1). 

In contrast, risk management for substances that cause deterministic effects must consider unavoidable exposures to the background of naturally occurring substances that cause such effects. Based on the assumption of a threshold dose-response relationship, the risk from man-made sources is not independent of the risk from undisturbed natural sources, and the total dose from all sources must be considered in evaluating deterministic risks. In the case of ionizing radiation, thresholds for deterministic responses are well above average doses from natural background radiation (see Section 3.2.2.1)... 

Given the models for estimating external or internal radiation doses in specific organs or tissues, the following sections consider the responses resulting from a given dose by any route of exposure. As is the case with hazardous chemicals, both stochastic and deterministic radiation effects can occur. 

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