Stochastic 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. 

Several series of measurements are to be compared as regards the standard deviation. It is of interest to know whether the standard deviations could all be traced to one population characterized by a (Hq no deviation observed), and any differences versus a would only reflect stochastic effects, or whether one or more standard deviations belong to a different population Hi difference observed) ... 

Swiss case The following means were found 20.32, 20.43, 20.34, 20.60, 20.35, 20.36, 20.45, 20.40, 20.30, and 20.31. The number of tubes with fill weights below the -5% limit was 4, 1, 0, 0, 4, 1, 3, 3, 3, and 3, for a total of 22, and none below the -12.5% one. Twenty-two tubes out of 500 tested correspond to 4.4%. Since the limit is 5% failures, or 2.5 per 50, fully six out of 10IPC inspection runs at u = 50 each did not comply. At a total batch size of 3000 units, eventually 1 /6 of all packages were tested. Evidently, unless the filling overage is further increased, a sampling rate of well above 10% is necessary to exclude these stochastic effects, and so the 10 inspections were combined into one test of n = 500. 

Fleischmann, M., M. Labram, C. Gabrielli, and A. Sattar, The measurement and interpretation of stochastic effects in electrochemistry and bioelectrochemistry, Surface Science, 101, 583 (1980). 

Health effects from exposure to radiation fall into two categories stochastic (based on probability) and acute. Stochastic effects typically take several years to materialize (e.g., cancer appearing 20 years after an exposure) while acute effects such as nausea or reddening of the skin may take only weeks, days, or even hours to materialize. Stochastic and acute effects are described in more detail in the following sections. First, however, a brief discussion describes how radiation damages human tissue and why exposure may produce one or a combination of the described health effects. 

Stochastic radiation effects are typically associated with those that occur over many months or years (i.e., are typically chronic instead of acute). Chronic doses are typically on the order of background doses (0.3 rem [0.003 Sv] or less) and are not necessarily associated with larger doses that could result from a terrorist attack with radiological weapons. However, stochastic health effects are defined here as effects that occur many years after chronic or acute exposure to radiological contaminants. Stochastic effects are categorized as cancers and hereditary effects. Because no case of hereditary effects (e.g., mutation of future generations) has been documented, this discussion focuses on cancer risk. 

Specific health effects resulting from an acute dose appear only after the victim exceeds a dose threshold. That is, the health effect will not occur if doses are below the threshold. (Note that this is significantly different from the LNT model used to predict stochastic effects.) After reaching the acute dose threshold, a receptor can experience symptoms of radiation sickness, also called acute radiation syndrome. As shown in Table 3.2, the severity of the symptoms increases with dose, ranging from mild nausea starting around 25-35 rad (0.25-0.35 Gy) to death at doses that reach 300-400 rad (3-4 Gy). Table 3.2 shows that the range of health effects varies by both total dose and time after exposure. 

It is also unlikely that the doses associated with a dirty bomb will produce even the milder acute effects. Although the observation of acute radiation syndrome may be unlikely after a dirty bomb explosion, doses should be kept ALARA to limit the potential for acute and stochastic effects. The entire range of acute radiation syndrome effects will be observed after an attack with a nuclear weapon, as described in Chapter 5. 

We emphasize that any utilization of Eq. (5) already rest upon a number of (often reasonable) assumptions. Equation (5) represents an ordinary deterministic differential equation, based on assumption of homogeneity, free diffusion, and random collision, and neglecting spatial [102] or stochastic effects [103]. While such assumptions are often vindicated for microorganisms, the application of Eq. (5) to other cell types, such as human or plant cells, sometimes mandates careful verification. 

Some radiation effects result from nonlethal damage to a single cell. These effects are called stochastic. They have the property that there is no threshold for these effects to occur. It is the probability of occurrence rather than its severity which increases with dose. The causation of some cancers may be rooted in a stochastic effect. 

UNSCEAR. Sources and effects of ionizing radiation. United Nations Scientific Committee on the Effects of Atomic Radiation. 1993 Report to the General Assembly. Annex F Influence of dose and dose rate on stochastic effects of radiation, 1993. 

Carcinogenic risks are considered to be stochastic effects—those for which the probability of an effect occuring, rather than its severity, is regarded as a function of the dose without threshold. 

The basic assumption of the International Commission on Radiological Protection (ICRP) is that for stochastic effects, a linear relationship without threshold is found between dose and the probability of an effect within the range of exposure conditions usually encountered in radiation work. However, ICRP cautions that if the dose is highly sigmoid, the risk from low doses could be overestimated by linear extrapolation from data obtained at high doses. Furthermore, ICRP... 

Here, we shall discuss the implications of cosmological expansion for the searches of a quantum-gravity-induced refractive index and a stochastic effect. We will consider Friedman-Robertson-Walker (FRW) metrics as an appropriate candidate for standard homogeneous and isotropic cosmology. Let R be the FRW scale factor, and a subscript 0 will denote the value at the present era. Ho is the present Hubble expansion parameter, and the deceleration parameter qo is defined in terms of the curvature k of the FRW metric by k ( 2[Pg.588]

There would be similar cosmological corrections to the stochastic effect. 

Combined strategic and operational model 2 Genetic algorithm and simulation model to analyze stochastic effects... 

Risk Index for Mixtures of Hazardous Substances. For the purpose of developing a comprehensive and risk-based hazardous waste classification system, a simple method of calculating the risk posed by mixtures of radionuclides and hazardous chemicals is needed. The method should account for the linear, nonthreshold dose-response relationships for radionuclides and chemical carcinogens (stochastic effects) and the threshold dose-response relationships for noncarcinogenic hazardous chemicals (deterministic effects). 

Risk index for mixtures of substances that cause stochastic effects (carcinogens). The risk index for mixtures of substances that cause stochastic effects (radionuclides and chemical carcinogens) takes into account the risk in all organs or tissues, and it assumes that the risk in any organ is independent of the risk in all other organs. The risk index for mixtures of substances causing stochastic effects can be represented as ... 

Use of the composite risk index in classifying waste. Given the risk indexes for mixtures of substances causing stochastic or deterministic effects calculated using Equations 1.5 and 1.6, respectively, the composite risk index for all hazardous substances is calculated using Equation 1.4. This procedure assumes that induction of stochastic effects is independent of exposures to substances causing deterministic effects, and vice versa. 

In the hazard identification process for chemicals that cause stochastic effects described above (EPA, 1987a), the weight-of-evidence classification is determined primarily by observations of tumors in animals or humans. Other information about the properties of a chemical, structure-activity relationships for other chemicals that cause stochastic effects, and the influence of a chemical on the carcinogenic process often is limited and plays only a modulating role in the weight-of-evidence classification based on tumor findings. 

Dose-Response Relationships. The primary objective of this study is to set forth the foundations of a risk-based waste classification system that applies to hazardous chemicals and radionuclides. Most aspects of the risk assessment process that provide the basis for establishing this system are conceptually the same for chemicals and radionuclides, although the specific data (e.g., solubilities) may differ. One important exception is the assumed relationship of the probability of a response to a unit dose of a substance that causes stochastic effects, which is called the dose-response relationship There are important conceptual differences in the way this relationship has been defined and used for hazardous chemicals and radionuclides, and these differences could pose a major impediment to development of a risk-based waste classification system that applies to both types of substances on a consistent basis. These differences are elucidated in the following section. 

In general, the relationship between dose and response can be represented by a variety of functional forms. At low doses of substances that cause stochastic effects, the dose-response relationship usually is assumed to be linear and, thus, can be expressed as a single probability coefficient. This coefficient is frequently referred to as a risk (or potency factor or unit risk factor or slope factor) in the literature. However, it is really the response (consequence) resulting from a dose of a hazardous substance, and it should not be confused with risk as defined and used in this Report. 

The benchmark dose method can also be applied to chemicals that cause stochastic effects (Section 3.2.1.3.3). This is indicated by the projected linear response at doses below LEDi0 in Figure 3.6. 

Dose-Response Assessment for Chemicals That Cause Stochastic Effects. For hazardous chemicals that do not have a threshold in the dose-response relationship, which is currently believed to... 

Although dose-response assessments for deterministic and stochastic effects are discussed separately in this Report, it should be appreciated that many of the concepts discussed in Section 3.2.1.2 for substances that cause deterministic effects apply to substances that cause stochastic effects as well. The processes of hazard identification, including identification of the critical response, and development of data on dose-response based on studies in humans or animals are common to both types of substances. Based on the dose-response data, a NOAEL or a LOAEL can be established based on the limited ability of any study to detect statistically significant increases in responses in exposed populations compared with controls, even though the dose-response relationship is assumed not to have a threshold. Because of the assumed form of the dose-response relationship, however, NOAEL or LOAEL is not normally used as a point of departure to establish safe levels of exposure to substances causing stochastic effects. This is in contrast to the common practice for substances causing deterministic effects of establishing safe levels of exposure, such as RfDs, based on NOAEL or LOAEL (or the benchmark dose) and the use of safety and uncertainty factors. 

Because of the statistical and biological problems inherent in the identification of a true no-effect level in any study of dose-response, most mathematical models for chemicals that cause stochastic effects have eliminated the concept of a threshold dose below which no... 

UCL takes into account measurement uncertainty in the study used to estimate the dose-response relationship, such as the statistical uncertainty in the number of tumors at each administered dose, but it does not take into account other uncertainties, such as the relevance of animal data to humans. It is important to emphasize that UCL gives an indication of how well the model fits the data at the high doses where data are available, but it does not indicate how well the model reflects the true response at low doses. The reason for this is that the bounding procedure used is highly conservative. Use of UCL has become a routine practice in dose-response assessments for chemicals that cause stochastic effects even though a best estimate (MLE) also is available (Crump, 1996 Crump et al., 1976). Occasionally, EPA will use MLE of the dose-response relationship obtained from the model if human epidemiologic data, rather than animal data, are used to estimate risks at low doses. MLEs have been used nearly universally in estimating stochastic responses due to radiation exposure. 

The benchmark dose method is particularly useful when the mode of action of a chemical that causes stochastic effects is thought to be nonlinear. In these circumstances, the response is assumed to decrease more rapidly than linearly with decreasing dose. Alternatively, the mode of action may theoretically have a threshold for example, the carcinogenicity of a substance may be a secondary effect of its toxicity or of an induced physiological change that is itself a threshold phenomenon. 

Fig. 3.9. Biologically-based model of the cancer induction process used to estimate the dose-response relationship of chemicals causing stochastic effects (Andersen etal., 1987).

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. 

Given the different approaches to dose-response assessment and the different measures of response normally used for radionuclides and chemicals that cause stochastic effects, estimates of responses from exposure to the two types of substances clearly are not equivalent, and the correspondence of the estimated frequency of responses to the frequency that might actually be experienced differs substantially. Specifically, if the results of experiments indicating chemical-induced stochastic responses in animals are assumed to be indicative of stochastic responses in humans, estimates of responses for chemicals could be considerably more conservative (pessimistic) than estimates for radionuclides. This difference is primarily the result of... 

Approaches to Risk Management for Radionuclides and Hazardous Chemicals That Cause Stochastic Effects... 

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