Modeling exposure estimation concept

Exposure Estimation Concept 237 Model Utility Concept 238 REFERENCES 239... 

The explanation of the pharmacokinetics or toxicokinetics involved in absorption, distribution, and elimination processes is a highly specialized branch of toxicology, and is beyond the scope of this chapter. However, here we introduce a few basic concepts that are related to the several transport rate processes that we described earlier in this chapter. Toxicokinetics is an extension of pharmacokinetics in that these studies are conducted at higher doses than pharmacokinetic studies and the principles of pharmacokinetics are applied to xenobiotics. In addition these studies are essential to provide information on the fate of the xenobiotic following exposure by a define route. This information is essential if one is to adequately interpret the dose-response relationship in the risk assessment process. In recent years these toxicokinetic data from laboratory animals have started to be utilized in physiologically based pharmacokinetic (PBPK) models to help extrapolations to low-dose exposures in humans. The ultimate aim in all of these analyses is to provide an estimate of tissue concentrations at the target site associated with the toxicity. 

The basic concept is that estimated results for pesticide movements and exposure levels vary greatly with the model types and modeling philosophy. Before con-dncting a model exercise, a conceptual check of the model is needed to ascertain if the model contains aU relevant routes of exposure. A simple model, such as SCIES, is based on worst-case assumptions, and may be sufficient for inhalation risk assessment. More complicated simulation models, such as CONSEXPO and InPest, provide information on the amounts of pesticides on the room materials, as well as the airborne concentration, and they are appropriate for risk assessment via aU routes. Even in complicated models, each mechanistic model contains assumptions to simplify the process description of the pesticide movement in the real world . The underlying assumptions for each of the models, and the relevant processes they implicate, are criteria to consider when selecting an appropriate model. Therefore, the validity of the assumptions used for the assessment should be considered before using the model, and they should be well documented. A simple phrase such as, we used model xx to estimate an exposure level of yy, is inadequate for documentation purposes. 

Critical loads are quantitative estimates of the exposure to one or more pollutants below which significant harmful effects on specified sensitive components of the environment do not occur, at least according to present knowledge (Nilsson and Greenfelt, 1988). The exceedance of critical loads for impacts on soils and surface waters have been the basis of negotiations for emission reductions in Europe. The concept of critical loads has spread to Canada but not to the United States. The steady-state model for critical loads implies that only the final results of a certain deposition level are considered. The time required... 

Time-to-response models also have important implications in a consideration of possible threshold mechanisms since they encompass the concept of practical thresholds . Druckrey has pointed out that the latent period or time from exposure to the observance of tumors is inversely related to dose of carcinogen. Thus at very low doses, the median time to tumor appearance could extend beyond an individual s expected lifetime resulting in a practical threshold. It also has been noted, however, that the effect of dose on latent period may only occur at relatively high carcinogen concentrations and thus may not be appropriate for human risk estimates. ... 

Figure 9.4 Risk assessment for an aquatic environment based on a probabilistic procedure into which the concept of varying sensitivity in multispecies communities is incorporated (Nendza, Volmer and Klein, 1990). Exposure and effects are determined separately from experimental or, if not available, QSAR data. Physico-chemical data and information on bioaccumulation and biotransformation are the input for computer simulations of transport and distribution processes that estimate the concentrations of a potential contaminant in a selected river scenario, using, for example, the EXAMS model (Bums, Cline and Lassiter, 1982). For the effects assessment, the log-normal sensitivity distribution is calculated from ecotoxicological data and the effective concentrations for the most sensitive species are determined. The exposure concentrations and toxicity data are then compared by analysis of variance to give a measure of risk for the environment. Modified from Nendza, Volmer and Klein (1990) with kind permission from Kluwer Academic Publishers, Dordrecht.

Provided that the concrete is not water-saturated, it may be reasonable to assume that the initiation phase is considerably longer than the propagation period and that the end of the initiation period alone is a useful indicator of service fife. Clifton and Pommersheim have reviewed simple models based on this approach. For chloride-induced rebar corrosion, one of these is the use of Fick s second law of diffusion and the concept of a critical chloride concentration. Limitations and simplifying assumptions of this approach have been discussed in previous sections. Actual chloride concentration profiles can be measured on structures, to estimate parameters such as the diffusion coefficient used in the model. For carbonation, it has been proposed that the depth of carbonation is proportional to the square root of the exposure time. Again, the measurement of actual carbonation depth with time can be used to estimate a proportionality constant for a specific structure. 

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