Characterization of Exposure

The modalities of exposure for which there are at least s(xne hiunan data include the part of the body exposed to radiation, the dose to an organ or tissue, tl e distribution of the dose over time (dose-rate and fractionation), and the radiation quality defined in terms of LET. 

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Scientific information for the process of establishing OELs may come from human or animal data obtained using different methods, from studies of acute, subacute, and chronic toxicity through various routes of entry. Human data, which is usually the best source, is not easily available, and frequently it is incomplete or inadequate due to poor characterization of exposure and clear dose-response relationships. Human data falls into one of the following categories ... 

No chronic inhalation exposure (365 or more days) MRL was derived for trichloroethylene because the available chronic-duration data were limited by lack of adequate characterization of exposure conditions or quantitation of results, or because the existing studies had end points that were not suitable for derivation of an MRL. 

The second phase of ecological risk assessment, the analysis phase, includes two principal activities characterization of exposure and characterization of ecological effects (Figure 28.1). 

There is a continuing need for validated biomarkers of exposure that provide information on the frequency, duration, and intensity of an exposure, as well as a better understanding of distribution, metabolism, and excretion within the individual. Likewise, continued development of analytical methods (e.g. Monte Carlo) that provide a broad characterization of exposure and dose-response relationships should be encouraged. 

It is also important to recognize that, wherever possible, characterization of exposure and associated uncertainties as a basis to define critical data gaps necessarily involves an iterative process, whereby the assessment is refined upon acquisition of additional data or evolution of methodology. 

It is essential to provide decision-makers with an assessment of the overall degree of uncertainty in the assessment outcome. Therefore, if the uncertainties for an assessment have been analysed at different tiers (qualitatively, deterministically and probabilistically), it is necessary to find a way of presenting the results together and arriving at an overall characterization of exposure. This is difficult and subjective but unavoidable, since it is never possible to quantify all uncertainties objectively. Few assessments have attempted to do this in a systematic way, and it would be premature to give firm recommendations. However, a combined presentation should include ... 

As mentioned before, the ecological risk assessment is characterized by a problem formulation process, analysis containing characterizations of exposure and effects, and a risk characterization process. Several outlying boxes serve to emphasize the importance of discussions during the problem formulation process between the risk assessor and the risk manager, and the critical nature of the acquisition of new data, verification of the risk assessment, and monitoring. The next few sections detail each aspect of this framework. 

Characterization of exposure is a straightforward determination of the environmental concentration range, or if available, the actual dose received by the biota of a particular stressor. Although simple in concept, determining or predicting the environmental exposure has proven to be difficult. 

The first phase of the framework is problem formulation. Problem formulation includes a preliminary characterization of exposure and effects, as well as examination of scientific data and data needs, policy and regulatory issues, and site-specific factors to define the feasibility, scope, and objectives for the ecological risk assessment. The level of detail and the information that will be needed to complete the assessment also are determined. This systematic planning phase is proposed because ecological risk assessments often address the... 

The second phase of the framework is termed analysis and consists of two activities, characterization of exposure and characterization of ecological effects. The purpose of characterization of exposure is to predict or measure the spatial and temporal distribution of a stressor and its co-occurrence or contact with the ecological components of concern, while the purpose of characterization of ecological effects is to identify and quantify the adverse effects elicited by a stressor and, to the extent possible, to evaluate cause-and-effect relationships. 

Characterization of exposure (half of the analysis phase shown in Figure 3) evaluates the interaction of the stressor with the ecological component. Exposure can be expressed as co-occurrence or contact, depending on the stressor and the ecological component involved. An exposure profile is developed that quantifies the magnitude and spatial and temporal distributions of exposure for the scenarios developed during problem formulation and serves as input to the risk characterization. 

An uncertainty assessment is an integral part of the characterization of exposure. In the majority of assessments, data will not be available for all aspects of the characterization of exposure, and those data that are available may be of questionable or unknown quality. Typically, the assessor will have to rely on a number of assumptions with varying degrees of uncertainty associated with each. These assumptions will be based on a combination of professional judgment, inferences based on analogy with similar chemicals and conditions and estimation techniques, all of which contribute to the overall uncertainty. It is important that the assessor characterize each of the various sources of uncertainty and carry them forward to the risk characterization so that they may be combined with a similar analysis conducted as part of the characterization of ecological effects. 

The implications of adverse effects at spatial scales beyond the immediate area of concern may be evaluated by considering ecological characteristics such as community structure and energy and nutrient dynamics. In addition, information from the characterization of exposure on the stressor s spatial distribution may be useful. Extrapolations between different temporal scales (e.g., from short-term impacts to long-term effects) may consider the stressors distribution through time (intensity, duration, and frequency) relative to ecological dynamics (e.g., seasonal cycles, life cycle patterns). 

Exposure profile — The product of characterization of exposure in the analysis phase of ecological risk assessment. The exposure profile summarizes the magnitude and spatial and temporal patterns of exposure for the scenarios described in the conceptual model. 

Exposure assessment a quantification of contaminant release, migration and fate characterization of exposure pathways and receptors and measurement or estimation of exposure point concentrations (USEPA, 1989c). 

As important as the hazard identification for a substance is the characterization of exposure to human beings. All known conditions of use have to be compiled to estimate the most realistic exposure of a substance to the environment including an estimate of the intake of a substance via inhalation, dermal contact, oral intake (Table 8). A major result of a risk assessment is the comparison of exposure levels to NOELs. 

As seen in Figure 3-10, very little quantitative information is available regarding the health effects in humans exposed to selenium compounds via inhalation. The only quantitative inhalation studies in humans that relate selenium exposure levels or selenium body levels to health effects following inhalation exposure are epidemiological cancer studies. Fatalities following inhalation exposure to selenium compounds have not been reported. Despite the large number of cases of reported inhalation exposures in occupational settings, characterization of exposure concentrations and the selenium... 

The purpose of this paper is to describe MEPAS, a computer-based methodology for health impact estimation developed to support DOE site prioritization. MEPAS takes a physics-based approach based on characterization of exposures resulting from source-to-receptor transport at DOE sites. It is currently being used in DOE s Environmental Survey effort aimed at identification and prioritization of DOE sites. A Preliminary Summary Report has been issued for major DOE production sites (3), and a Final Summary Report for all DOE sites will be avail-able in 1990. 0097-6156/90/0431-0193 06.00/0... 

Animal studies involving exposures to aerosols should be designed in collaboration with scientists knowledgeable in aerosol generation, aerosol physics, and atmospheric quantification of vapors and aerosols to ensure accurate characterization of exposure atmospheres. The exposure conditions in the animal studies should mimic exposures encountered in the workplace (such as proper ratios of vapors to aerosols). 

A risk assessment requires a characterization of exposure setting. In this step, the assessor characterizes the exposure setting with respect to the general physical characteristics of the site and the characteristics of the populations on and near the site. Basic site characteristics such as climate, vegetation, ground-water hydrology, and the presence and location of surface water are identified in this step. Populations are also identified and are described with respect to those characteristics that influence exposure, such as location relative to the site, activity patterns, and the presence of sensitive subpopulations. The characteristics of the current population, as well as those of any potential future populations are considered. 

Controlled human exposure studies are essential to establish the health consequences of PM exposures. These studies are typically designed to study inhalation of size-defined PM under highly controlled conditions that will allow the characterization of exposure-response relationships. Since humans are exposed to the pollutant of interest, specifically size-fractionated PM, causahty can be established easily and the confounding effects of other pollutants can be minimized (Devlin et al. 2005). Another advantage of such clinical studies is the abihty to select subjects with a known clinical status (i.e., healthy vs. a specific disease, typically cardiovascular or pulmonary disease) and observe the pathophysiological responses of interest. However, controlled human exposures have some limitations (Utell and Frampton 2000). For both practical and ethical reasons, clinical studies are restricted to exposure concentrations and durations that will only ehcit transient responses in human subjects. These studies involve a small number of sample subjects, which excludes susceptible populations at higher risk. Chronic exposure or high PM concentration related health effects are not attainable by the clinical studies. These experiments are also very costly to perform. 

The analysis phase is directed by the products of the problem formulation and is further divided into characterization of exposure and characterization of effects steps. Exposure characterization uses the properties of the chemicals and the receiving environment to estimate the extent to which the organisms will be exposed. Effects characterization determines the effects of varying dose or other measures of exposure on organisms, populations or communities. 

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