Exposure assessments components

The migration and fate of contaminants in each transport pathway can be simulated using MEPAS components. The transport pathways are systematically integrated with an exposure assessment component that considers the type, time, and duration of exposure and the location and size of the population exposed. These various pathways and their interactions as considered by MEPAS are discussed in Droppo et al. (5). 

The component of an ecological risk assessment in which exposure is evaluated. This is parallel to the exposure assessment component of a human health risk assessment. 

Due to the fact that JP-8 contains hundreds of aliphatic and aromatic hydrocarbons, in addition to various performance additives, this complex mixture poses a serious challenge for risk assessment. Exposure assessment is complicated by the fact that JP-8 may be encountered as a vapor, aerosol, or liquid, and possibly as combustible products, and each physical state may contain different chemical entities. However, progress has been made in the identification of JP-8 components that may serve as reliable and predictable biomarkers of exposure, particularly for dermal exposures [12,35,81,82,83,84],... 

Table 8.1 Component groups for gasoline based on C- number and main chemical composition. Reprinted with permission from Foster KL, Mackay D, Parkerton TF, Webster E, Milford L (2005) Five-stage environmental exposure assessment strategy for mixture gasoline as a case study. Environ Sci Technol 39 2711-2718. Copyright 2005 American Chemical Society...

The Risk Assessment process includes four steps hazard identification, hazard characterization (related term dose-response assessment), exposure assessment, and risk characterization. It is the first component in a risk analysis process. 

Risk assessment An empirically based paradigm that estimates the risk of adverse effects) from exposure of an individual or population to a chemical, physical or biological agent. It includes the components of hazard identification, assessment of dose-response relationships, exposure assessment and risk characterization. 

According to the National Research Council of the National Academy of Science, risk assessment consists of four broad but interrelated components hazard identification, dose-response assessment, exposure assessment, and risk characterization, as depicted in Figure 24.1. The reader should, however, be aware that these risk assessment activities can provide research needs that improve the accuracy of estimating the risk or probability of an adverse outcome. 

Exposure assessment should be a component of population-based biomonitoring studies to facilitate interpretation of the data. Typically, large-scale biomonitoring studies do not evaluate potential sources of exposure. That often leads to the question, Where is the exposure coming from For some chemicals, exposure pathways may be well defined from previous... 

Risk assessment is frequently described as involving four components - hazard identification, exposure assessment, dose-response assessment, and risk characterization. Risk assessment may be an input to risk management. 

After these questions have been answered, the next step is to undertake a systematic analysis of the mixture itself, with the goal of identifying the toxicologically relevant components. For example, the concentration of compounds in the mixture could be compared to ecotoxicity data for those compounds to identify compounds most likely to contribute to mixture effects (exposure > sensitivity threshold), or grouping of compounds in the mixture according to modes of action. In the following steps, these issues are elaborated, starting with exposure assessment. 

This chapter will address the implications of the data presented in previous chapters for assessing the risks from environmental chemical exposures. WHO/IPCS has defined risk assessment as an empirically based paradigm that estimates the risk of adverse effects from exposure of an individual or population to a chemical, physical, or biological agent. As shown in Figure 21, it includes the components of hazard identification (Is there an adverse effect ), dose-response assessment (How severe is it ), exposure assessment (What is the level of exposure ), and risk characterization (What is the risk ) (NRC, 1983 IPCS, 2000). 

The exposure assessment characterizes the pathways, magnitude, frequency, and duration of human exposures from various sources. Chapter 5 provides an overview of these components and addresses the principles of exposure assessment in children. General principles of exposure assessment have been reviewed in a number of publications (USEPA, 1992a, 2005a IPCS, 1999a, 2000 Needham et al., 2005). This chapter will focus on the considerations that are important when applying the exposure data to a children s health risk assessment (see Box 2). 

Although the level of uncertainty of the model assumptions, structure and details was characterized as Low when considered in isolation, the uncertainty of other components (i.e. model extrapolations, chemical-specific exposure data, non-chemical-specific exposure data and exposure assessment result) was considered to be a function primarily of limitations (i.e. simplicity) of the conceptual model. 

It is important to note that it is not always necessary for all data for, and thus all components of, an exposure assessment to be of uniform quality, as defined by the four hallmarks. Often the magnitude and uncertainty of predicted exposure may be strongly determined by a small number of assessment inputs and be relatively insensitive to others. If it is concluded that the quality of an assessment needs to be improved, it is usually efficient to target efforts towards the data inputs that contribute most to the uncertainty of the output (presuming that uncertainty in this input can be reduced). However, a high level of transparency in discussion of key inputs is always desirable, such that an accurate assessment of uncertainty associated with each input can be made. 

As a first step in the risk assessment of chemicals, it is essential to have an insight into the magnitude and duration of exposure. Following the toxicological principle that dose determines the effect, one may assume that no exposure means no risk. In the case of chemical mixtures, a proper assessment of exposure assists in adequately interpreting the interacting effects of chemicals. So, exposure assessment is an essential component of any risk assessment study of mixtures, since it can be used to reduce uncertainty and provide data. 

Indirect exposure assessment, both human and environmental, starts with emission data and a prediction of the fate of chemicals in the environment and the resulting concentrations in different environmental compartments. Foster et al. (2005) outlined 5 steps in a strategy to conduct exposure assessment of complex mixtures, consisting of many different components, such as gasoline. These steps, as outlined below, are also relevant when assessing exposure to less complex mixtures. 

This section outlines the current scientific state of the art in the assessment of human health risks for chemical mixtures. It focuses on the gathering, assessment, and evaluation of effect data. The reader is referred to Chapter 1 for detailed information on exposure assessment of chemical mixtures. The section starts with an overview of methods commonly used to obtain effect data on chemical mixtures. This is followed by an overview of the current mixture approaches in human health assessments, that is, the whole mixture approach for common mixtures and the component-based approach. The section concludes with a paragraph on uncertainties in human health assessments of chemical mixtures. 

Common Whole Mixtures The procedures based on common whole mixture data (Figure 5.4) assume that the concentration ratios between the mixture components are more or less constant or that environmental transformations in the mixture can be accounted for in the exposure assessment, for example, used to evaluate PCBs (USEPA 1996 Cogliano 1998). Based on the origin of the effect data used, distinction is often made between 1) assessments using data directly on the mixture of concern and 2) assessments using data on a sufficiently similar mixture as a surrogate for the mixture of concern. Both types of assessments are discussed in more detail below. 

Exposure assessment The component of an ecological risk assessment that estimates the exposure resulting from a release or occurrence in a medium of a chemical, physical, or biological agent. It includes estimation of transport, fate, and uptake. 

This process measures the intensity, frequency, and duration of exposure to an agent. More precisely, it evaluates the potential exposure to each chemical of concern by the potential receptor populations or indicator species that are biotic components of any ecosystems in the site which may be affected. Two important components of an exposure assessment are (a) an exposure pathway analysis, and (b) the estimation of exposure point concentrations. To evaluate potential exposures, one requires concentrations of the chemicals of concern in the exposure media (e.g., surface water or sediments). 

Occupational pesticide exposure holds a peculiar status within the field of occupational health and safety, both from a scientific and regulatory perspective. Methods for personal monitoring of dermal exposure first arose in the context of pesticide applications in agriculture, pioneered by scientists in the USA Public Health Service (Batchelor and Walker, 1954 Durham and Wolfe, 1962). These methods gained worldwide recognition in the early 1960s, and remain a component of exposure assessment practice today. This work pre-dated most personal monitoring methods that were developed for industrial workplaces. 

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