Nonstochastic effect

The response of humans to var> ing doses of radiation is a field tlmt has been widely studied. The obscr ed radiation effects can be categorized as stochastic or nonstochastic effects, depending upon tlie dose received and tlie time period over which such dose was received. Contrary to most biological effects, effects from radiation usually fall under tlie category of stochastic effects. The nonstochastic effects can be noted as having three qualities a minimum dose or tlucshold dose must be rcceii ed before the particular effect is obsen ed the magnitude of the effect increases as the size of the dose increases and a clear, casual relationship can be determined between the dose and the subsequent effects. 

If a person is exposed to a large amount of radiation (i.e., large radiation dose) delivered to the entire body, cells in tissues can be destroyed in large numbers. Because tissues have important functions, the destruction of significant numbers of cells can lead to impairment in one or more of these functions. The biological effects that arise when large numbers of cells are destroyed by radiation are called acute somatic effects if they occur in a relatively short period of time (e.g., within a few weeks) after brief exposure. Acute somatic effects are a subset of what is now formally called early and continuing deterministic effects (once called nonstochastic effects). 

Peel DM, Hopewell JW, Wells J, et al. 1984. Nonstochastic effects of different energy beta emitters on pig skin. Radiat Res 99 372-382. 

An important difference between stochastic and nonstochastic effects is that nonstochastic effects have a threshold stochastic effects do not. The threshold is a minimum radiation dose that has to be received in a relatively short time period for the effect to appear (Fig. 16.5). A dose below the threshold will not... 

An example of stochastic versus nonstochastic effects can be made using alcohol. If a person drinks 20 glasses of wine in a short period of time, it is... 

Figure 16.5 (a) The probability for a nonstochastic effect to occur versus dose D. (b) The probability for a stochastic effect to occur versus dose D. 

Since nonstochastic effects have a threshold, all that is needed to satisfy requirement 2 is to set the exposure limits below that threshold. For nonstochastic effects, the maximum allowed dose is set at 0.5 Sv (50 rem) for any tissue, except for the lens of the eye, for which the limit is set at 0.15 Sv (15 rem). For stochastic effects the limits are set at an acceptable level of risk. Ideally, the limit should be zero, since any exposure is supposed to increase the probability for stochastic effects to occur. Obviously, a zero limit is not practical. For stochastic effects the 10CFR20 sets the limiting exposure on the basis that the risk should be equal regardless of whether the whole body is irradiated uniformly or different tissues receive different doses. Recognizing the fact that different tissues have different sensitivities and, therefore, the proportionality constant between dose and effect is not the same for all tissues, the limit is expressed in terms of the effective dose equivalent (Ffg), defined as... 

Nonstochastic effects— Effects that can be related directly to the radiation dose received. 

The effects of exposure to significant doses of radiation can be both immediate and/or delayed. Stochastic effects are those where the probability of the effect (but not the degree) is related to the dose. Nonstochastic effects are those where the severity of the effect is related to the dose. In both cases, however, less exposure is safer. In species other than humans (but not in humans), it has been demonstrated that abnormalities of offspring are related to radiation exposure in parents. Radiation is also known to have a teratogenic effect on fetuses and embryos. 

The first assumption, the linear nonthreshold dose-effect relationship, implies that the potential health risk is proportional to the dose received and that there is an incremental health risk associated with even very small doses, even radiation doses much smaller than doses received from naturally occurring radiation sources. These health risks, such as cancer, are termed stochastic because they are statistical in nature i.e., for a given level of dose, not every person exposed would exhibit the effect. The second assumption means that when a stochastic effect is induced, the severity of the effect is not related to the radiation dose received. The third assumption implies that there are effects, termed nonstochastic effects, for which there is an apparent threshold i.e., a dose level below which the effect is unlikely to occur. An example of a nonstochastic effect is the formation of radiation-induced cataracts of the eyes. 

In the absence of convincing evidence that there is a dose threshold or that low levels of radiation are beneficial, the Commission believes that the assumptions regarding a linear nonthreshold dose-effect model for cancers and genetic effects and the existence of thresholds only for certain nonstochastic effects remain appropriate for formulating radiation protection standards and planning radiation protection programs."... 

Nonstochastic effects effects that can be related directly to the radiation dose received Nucleus central part of an atom that contains protons and neutrons... 

WSRC s assessment indicated that it was in compliance with this element except in the area of nonstochastic effects of radiation exposure. WSRC reported in May li9i9d 1(Reference 16) that subsequent changes to procedures and employee training programs eliminated all areas of noncompliance. A DOE review of programs and procedures such as Special Hazards Bulletin 7 (SHB-7), "Radiation Exposure Control" (Reference 17), indicated that WSRC was in compliance with the requirements of this section of the Order. 

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