Stochastic Human Exposure and Dose

SHEDS Stochastic Human Exposure and Dose Simulation... 

Approaches for aggregating exposure for simple scenarios have been proposed in the literature (Shurdut et al., 1998 Zartarian et al., 2000). The USEPA s National Exposure Research Laboratory has developed the Stochastic Human Exposure and Dose Simulation (SHEDS) model for pesticides, which can be characterized as a first-generation aggregation model and the developers conclude that to refine and evaluate the model for use as a regulatory decision-making tool for residential scenarios, more robust data sets are needed for human activity patterns, surface residues for the most relevant snrface types, and cohort-specific exposure factors (Zartarian et al, 2000). The SHEDS framework was used by the USEPA to conduct a probabilistic exposure assessment for the specific exposure scenario of children contacting chromated copper arsenate (CCA)-treated playsets and decks (Zartarian et al, 2003). 

Monte Carlo analysis is a specific probabilistic assessment method that can be used to characterize health risks and their likelihood of occurrence based on a wide range of parameters (Shade and Jayjock 1997). The U.S. EPA s Stochastic Human Exposure and Dose Simulation (SHEDS) model allows for the quantification of exposures based on a probabilistic assessment of multiple exposure pathways and multiple routes of exposure (Mokhtari et al. 2006 US EPA 2003b). Additional applications of probabilistic techniques wiU be discussed in the section below on conducting an uncertainty analysis of reconstructed exposure values. 

Mokhtari, A., Christopher Frey, H., and Zheng, J. (2006). Evaluation and recommendation of sensitivity analysis methods for application to stochastic human exposure and dose simulation models. J Expo Sci Environ Epidemiol 16, 491-506. 

SHEDS Stochastic human exposure and dose simulation... 

The plume will travel downwind and the concentration of radioactive materials will tend to decrease as it travels further from the plant. As the concentration of radioactive materials in the plume decreases, the dose rate to the affected population will also decrease. Thus, those who are further away from the plant will generally be at less risk of deterministic (early) health effects. While the exposures further from the plant are small, they all add to the chance of getting cancer (stochastic effects). Since the total amount of human exposure is larger further from the plant (large number of people exposed to small amounts of radiation), this is where most cancers will occur. Following the Chernobyl release the vast majority of the excess thyroid cancers caused by the accident occurred between 50 and 350 km from the plant. 

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. 

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... 

The acceptable risks for substances that induce stochastic responses discussed in this Section are values in excess of unavoidable risks from exposure to the undisturbed background of naturally occurring agents that cause stochastic responses, such as many sources of natural background radiation and carcinogenic compounds produced by plants that are consumed by humans. This distinction is based on the assumption of a linear, nonthreshold dose-response relationship for substances that cause stochastic responses and the inability to control many sources of exposure. Risk management can address exposures to naturally occurring substances that induce stochastic responses, but only when exposures are enhanced by human activities or can be reduced by reasonable means. 

In many respects, the foundations and framework of the proposed risk-based hazardous waste classification system and the recommended approaches to implementation are intended to be neutral in regard to the degree of conservatism in protecting public health. With respect to calculations of risk or dose in the numerator of the risk index, important examples include (1) the recommendation that best estimates (MLEs) of probability coefficients for stochastic responses should be used for all substances that cause stochastic responses in classifying waste, rather than upper bounds (UCLs) as normally used in risk assessments for chemicals that induce stochastic effects, and (2) the recommended approach to estimating threshold doses of substances that induce deterministic effects in humans based on lower confidence limits of benchmark doses obtained from studies in humans or animals. Similarly, NCRP believes that the allowable (negligible or acceptable) risks or doses in the denominator of the risk index should be consistent with values used in health protection of the public in other routine exposure situations. NCRP does not believe that the allowable risks or doses assumed for purposes of waste classification should include margins of safety that are not applied in other situations. 

Exposure to radiation can cause detrimental health effects. At large doses, radiation effects such as nausea, reddening of the skin or, in severe cases, more acute syndromes are clinically expressed in exposed individuals within a short period of time after the exposure such effects are called deterministic because they are certain to occur if the dose exceeds a threshold level. Radiation exposure can also induce effects such as malignancies, which are expressed after a latency period and may be epidemiologically detectable in a population this induction is assumed to take place over the entire range of doses without a threshold level. Hereditary effects due to radiation exposure have been statistically detected in other mammalian populations and are presumed to occur in human populations also. These epidemiologically detectable effects—malignancies and hereditary effects—are termed stochastic effects because of their random nature. 

Effects on humans are separated into deterministic effects caused by severe exposures to high doses and stochastic effects (mostly cancer) caused by much lower exposures. Data on deterministic effects come from side effects of radiotherapy, exposure of the early radiobiologists, atomic bomb effects in Japan, and a few severe accidents. Data on stochastic effects are mostly based on epidemiological studies on the survivors of the atomic bomb detonations in Japan, on patients exposed to medical treatments, and on industrial exposures to workers. Animal studies are also used to evaluate human effects. 

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. 

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