Exposure assessment scenarios

The models in the THERdbASE CD are Chemical Source Release, Instantaneous Emission, Chemical Source Release, Timed Application, Indoor Air (2-Zone), Indoor Air (N-Zone), Exposure Patterns for Chemical Agents, Benzene Exposure Assessment Model (BEAM), Source Ba.sed Exposure Scenario (Inhalation + Dermal), and Film Thickness Based Dermal Dose. 

The degree of confidence in the final estimation of risk depends on variability, uncertainty, and assumptions identified in all previous steps. The nature of the information available for risk characterization and the associated uncertainties can vary widely, and no single approach is suitable for all hazard and exposure scenarios. In cases in which risk characterization is concluded before human exposure occurs, for example, with food additives that require prior approval, both hazard identification and hazard characterization are largely dependent on animal experiments. And exposure is a theoretical estimate based on predicted uses or residue levels. In contrast, in cases of prior human exposure, hazard identification and hazard characterization may be based on studies in humans and exposure assessment can be based on real-life, actual intake measurements. The influence of estimates and assumptions can be evaluated by using sensitivity and uncertainty analyses. - Risk assessment procedures differ in a range of possible options from relatively unso-... 

Exposure assessments have become an essential element of contemporary risk assessment (NAS/NRC, 1983). The primary purpose of exposure assessment is to qualitatively and/or quantitatively determine exposure and absorbed dose associated with a particular use practice or human activity. Contemporary exposure assessors and risk managers place a high premium on accurate data obtained by monitoring chemical exposure scenarios and critical human activities or work tasks. 

For human health risk assessment, it is necessary to elaborate realistic scenarios. Knowledge of real scenarios where the contaminant is emitted to the environment will help to obtain information about the fate and transport of the contaminant once emitted to the environment and the route of exposure for the human beings living in this scenario of concern. There are different types of exposure, i.e., direct, indirect (as is the case of food contaminated by the air, water, or soil contaminated by the emission), occupational exposure, and consumer goods coming from outside the scenario of concern. Depending on the objective of the study, it will be necessary to consider in the exposure assessment one or more types of exposure. 

The exposure assessment in EUSES aims at reasonable worst-case, i.e., the exposure scenario was the worst scenario without being unrealistic and as much as possible using mean, median, or typical parameter values. If the outcome of the reasonable worst-case risk characterization indicates that the substance is not of concern, the risk assessment for that substance can be stopped with regard to the scenario considered. 

The existing substances ordinance only introduced obligations for substance manufacturers to provide information to the authorities, but not for substance consumers. This resulted in systematic gaps or practice-remote worst-case scenarios in exposure assessment. 

Many other OECD activities on hazard/risk assessment are undertaken within programs such as Existing Chemicals, New Chemicals, and Pesticides and Biocides, which deal with specific types of chemicals. The work on exposure assessment methods is undertaken by the Task Force on Environmental Exposure Assessment, consisting of experts. Most of the outcome of this work is published in the Series on Testing and Assessment or in Emission Scenario Documents, which are available at the OECD Web site (OECD 2006a). 

Under this project, an IPCS Harmonization Project Document on the Principles of Characterizing and Applying Human Exposure has been published (WHO/IPCS 2005). This document sets out the characteristics of exposure assessment models that should be described to aid in model selection by exposure assessors. The document summarizes current practice in exposure modeling and principles for evaluating exposure models, but does not provide a comprehensive list of existing exposure models. The focus of the document is on the discussion of general properties of exposure models and how they should be described. The characteristics of different modeling frameworks are examined, and 10 principles are recommended for characterization, evaluation, and use of exposure models in order to help model users select and apply the most appropriate models. The report also discusses issues such as validation, input data needs, time resolution, and extrapolation of the model results to different populations and scenarios. 

If actual or potential exposure has been identified, a quantitative exposure assessment is necessary. Exposure levels/concentrations for each potentially exposed population need to be derived from the available measured data and/or from modeling. A range of exposure values to characterize different subpopulations and scenarios may result. These results are taken forward to the risk characterization where they are combined with the results of the effects assessment in order to decide whether or not there is concern for the human population exposed to the substance. In some cases all three types of exposure estimates may contribute to an overall exposure value (combined exposure), which should be considered in the risk characterization. 

Exposure assessment should describe the exposure scenarios of key populations undertaking defined activities. Such scenarios that are representative of the exposure of a particular (sub)population should, where possible, be described using both reasonable worst-case and typical exposures. The reasonable worst-case prediction should also consider upper estimates of the extreme use and reasonably foreseeable other uses. However, the exposure estimate should not be grossly exaggerated as a result of using maximum values that are correlated with each other. Exposure as a result of accidents or from abuse shall not be addressed. 

In carrying out the exposure assessment the risk reduction/control measures that are already in place should be taken into account. Consideration should be given to the possibility that, for one or more of the defined populations, risk reduction/control measures which are required or appropriate in one use scenario may not be required or appropriate in another (i.e., there might be subpopulations legitimately using different patterns of control, which could lead to different exposure levels). 

The exposure assessment could be performed for a single exposure scenario, or be more comprehensive including several exposure scenarios. In some simations, the estimated exposure from a single scenario is taken forward to the risk characterization while for other purposes, the estimated exposures from various scenarios form the basis for an estimation of a combined exposure to the chemical under evaluation from all characterized exposure scenarios. 

It is essential to define the assessment scenario within which the assessment endpoint will be assessed. The assessment scenario should specify the spatial, temporal, and ecological boundaries within which the endpoint is assessed, since these have substantial implications for the structure of the assessment model and the scope of the input data. The assessment scenario should also describe those aspects of the ecosystem that are relevant to the assessment, that is, those aspects that have an influence on the mechanisms of exposure and effects that will be assessed. This step is important in all ecological risk assessments it places the assessment activity into the real context of an ecosystem, helps to prevent construction of inappropriate models, and helps with interpretation and communication of results. 

Exposure scenarios specify the conditions in which the chemical is to be used (including the protective equipment to be used, working practices, products it is incorporated into, how consumers use those products and how these are disposed), limiting the range of the conditions that must be considered in the exposure assessment. The exposure scenario is annexed to the chemical safety data sheet that is passed to downstream users of the chemical. Those downstream users are required to pass information back to their suppliers about their use of the substance, the exposure to the substance involved in that use and the practicality of specified risk reduction measures. If a use is significantly different from those identified in the CSA, the user must report this to the European Chemicals Agency and prepare its own CSA, or ask the manufacturer to revise the CSA to include that use. 

Tier I exposure assessments represent an unrealistic, worst-case scenario since maximum use rates and minimum preharvest intervals are rarely used in normal agricultural practice. In addition, all triazine herbicides are not used on 100% of all planted acres. Further refinements lead to a more realistic assessment and to a significant reduction in the exposure estimate. 

Mangels G] Waterborne Environmental, Inc. 2001. The development of MUSCRAT (multiple scenario risk assessment tool) a software tool for conducting surface water exposure assessments, http //www.waterborne-env.com/modeling/model down-load muscrat.html (accessed January 2, 2007). 

Because the objective of an exposure assessment is to characterize both the magnitude and the reliability of exposure scenarios, planning for an uncertainty analysis is a key element of an exposure assessment. The aims of the uncertainty analysis in this context are to individually and jointly characterize and quantify the exposure prediction uncertainties resulting from each step of the analysis. In performing an uncertainty analysis, typically the main sources of uncertainties are first characterized qualitatively and then quantified using a tiered approach (see chapter 4). In general, exposure uncertainty analyses attempt to differentiate between key sources of uncertainties scenario uncertainties, model uncertainties and parameter uncertainties (for definitions, see section 3.2). 

In exposure assessment, uncertainty arises from insufficient knowledge about relevant exposure scenarios, exposure models and model inputs. Each of these sources of uncertainty has factors that determine the magnitude of uncertainty and variation. For example, Mosbach-Schulz (1999) identified three factors related to the uncertainty and variability of input data ... 

The definition of the scope and purpose of each exposure assessment provides the specifications for building the exposure scenario, which represents the real-life situation that is to be assessed and provides the boundary limits of the assessment. As pointed out... 

The scope and purpose of the exposure assessment inform the formulation of one or more scenarios for which exposures are to be estimated. The exposure estimation approach should be capable of faithfully representing the key structural assumptions of the scenario, such as exposed population, agents to be considered, spatial and temporal scope, microenvironments and so on (see section 3.2.1 for a complete list). If the modelling approach omits any of the relevant elements, then the estimates could be biased. If the modelling approach includes irrelevant, unnecessary or superfluous elements, then, at best, the model is likely to be more cumbersome to deal with than it needs to be, or, at worst, biases may be introduced. 

Exposure assessment involves the specification of values for parameters, either for direct determination of the exposure or as input for mechanistic or empirical or distribution-based models that are used to fill the exposure scenario with adequate information. Numerical values for exposure parameters are obtained using various approaches, such as the USEPA s Exposure Factors Handbook (USEPA, 1997a), the European Union s (EU) Technical Guidance Document (EU, 2003), the German XProb project (Mekel, 2003) and the European KTL s ExpoFacts (Vuori et al., 2006). 

Adequate documentation of all aspects of the uncertainty analysis for the exposure assessment is also critical to ensure transparency to reviewers and stakeholders. This includes sufficient documentation to enable independent replication of results and necessitates detailed description of qualitative and quantitative aspects pertaining to data, scenarios, methods, inputs, models, outputs, sensitivity analysis and interpretation of results. 

Accuracy of information is a result of scientific expertise, the delivery of adequate, complete and unbiased information about results and residual uncertainties. The speed of release is influenced by the organizational culture, to what extent the process to find answers and to acknowledge uncertainties is developed. Empathy is related to the willingness to recognize the situation (the scenario) in which the persons/clients are found. The degree of openness corresponds to the information given about uncertainties and limitations in the exposure assessment, the restrictions with respect to selected scenarios, the model assumptions and... 

The presentation of results should support an unbiased understanding of the results of the exposure assessment to enable the members of the target groups to make informed and independent decisions. This requires a basic understanding of the exposure process (the model from source to dose/burden) and at least an intuitive competence to understand the quantitative data and results in nature and magnitude. The selected scenario, data, model assumptions, results and inherent uncertainties should be communicated in an understandable and scientifically accepted presentation format. Presentations should be tailored to address the questions and information needs of the different audiences. To handle the different levels of sophistication and detail needed, it may be useful to design a presentation in a tiered format where the level of detail increases with each successive tier (USEPA, 1997b). 

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