EPA-Expo-Box (A Toolbox for Exposure Assessors)
- Fate & Transport
The goal of scenario evaluation is to estimate exposure or dose by establishing exposure profiles that link concentrations of a stressor in environmental media to the frequency and duration of a receptor’s contact with those media. The organizational construct used most often to analyze the link between source and receptor is an exposure scenario. Exposure scenarios provide a foundation that can be used by assessors as they:
- qualitatively characterize the conditions under which exposures are expected to occur;
- quantitatively estimate exposure, dose, and risk values;
- provide a context for quantitative estimates to risk managers, mainly through application of appropriate exposure and risk descriptors; and
- evaluate the relative impacts of different risk management decisions.
Although the full set of specific information needed to develop an exposure scenario depends on the purpose and scope of the assessment, as determined during planning, scoping, and problem formulation, an exposure scenario generally contains some information on the following components:
- Exposure Setting: The physical setting where exposure takes place.
- Characterization of the Stressor: Identification and characterization of stressors of concern, sources and releases, and concentrations in environmental media.
- Exposure Pathways: The pathway(s) of the stressor from source(s) to receptors(s), including its fate and transport through the environment, the routes to exposed individual(s), and the specific exposure location(s).
- Characterization of the Exposed Population: Identification of the individual(s) or population(s) exposed, and the receptor characteristics, activities, and behaviors (i.e., the exposure factor(s) that influence the frequency and duration of contact with the stressor).
- Intake and Uptake Rates: Exposure factor(s) that quantify the transfer of the stressor across biological boundaries.
Planning, Scoping, and Problem Formulation
Problem formulation is the process by which the assessor, in conjunction with risk managers and often various stakeholders, determines the purpose, scope, level of detail, and approach of an assessment. According to EPA’s Guidelines for Exposure Assessment (U.S. EPA, 1992), "In beginning the evaluation phase of any assessment, the assessor should have a scenario's basic assumptions (setting, scope, etc.) well identified, one or more applicable exposure pathways defined, an equation for evaluating the exposure or dose for each of those exposure pathways, and the data and information requirements pertinent to solving the equations." To arrive at these basic assumptions, the assessor usually considers a set of basic questions about the factors influencing an exposure assessment, like the ones presented below, and compiles the information available to inform the assessment. Generally, consultation with experts (e.g., statisticians, toxicologists) is necessary to address some of the questions in detail.
Planning, scoping, and problem formulation is often an iterative process and a step that will be revisited throughout the course of the exposure assessment as new information is collected and preliminary results are obtained.
Planning, scoping, and problem formulation is necessary to establish a clear purpose and scope of the assessment and to characterize the exposure setting and stressors of concern in sufficient detail to allow quantitative analysis and modeling. It also helps to determine whether a scenario evaluation approach is appropriate, which tier or type of scenario should be developed, what descriptor is most appropriate for the scenario, and which routes, populations, and media should be included in the scenario. The Screening-Level and Refined Module in the Tiers and Types Tool Set of EPA-Expo-Box provides additional information on the planning process and applying a tiered approach.
EPA’s Guidance on Cumulative Risk Assessment: Part 1. Planning and Scoping (11 pp, 121KB, About PDF) (1997b) notes that developing a conceptual model is a key part of the planning and scoping stage for an exposure assessment. A conceptual model (CM) is a diagram or written description of the predicted key relationships between the predicted responses of a population (or entity of concern) and its stressors laying out the environmental pathways and routes of exposure in the context of the assessment. The CM needs to distinguish between what is known or determined and what is assumed or based on default values. Also, it needs to include a discussion of uncertainties in the formulation of the assessment (U.S. EPA, 1997b).
|Level of Detail|
Overarching approach questions:
Detailed approach questions:
Several resources are available for the process of planning, scoping, and problem formation.
An exposure setting is the physical setting where an exposure of interest occurs and is defined by the boundaries of the analysis and the scope and geographic scale of the assessment. In addition, data are collected on physical characteristics that will affect the movement, transformation, and persistence of contaminants within the domain of the exposure scenario. Relevant information might include data on groundwater flow, soil type, surface water characteristics, meteorological conditions, and land use/land cover types, among others, as illustrated in the graphic below.
Stressors of Concern
A stressor is any biological, chemical, or physical entity that can cause or induce an adverse response in a human or ecological receptor. Traditional risk assessment has used a single-stressor approach—often because inadequate data for quantifying risks from multiple stressors are available or methodologies available for considering possible impacts from multiple exposures were limited—but some risk assessment tools and models that allow for the assessment of multiple stressors are now available. (See the Aggregate and Cumulative Module in the Tiers and Types Tool Set of EPA-Expo-Box for discussion and resources related to assessing exposure from multiple stressors.)
The process of identifying and characterizing stressors of concern includes a data collection step where sources and releases of stressors are identified and quantified, the degree of contamination in various media is measured or estimated, and the stressors themselves are evaluated to determine what factors influence their transport, transformation, fate, and toxicity. For example, some chemical stressors might break down in certain media, forming degradation products that are either more or less toxic than the parent compound.
Databases and other resources are available that describe the occurrence and characteristics of single stressors, classes of stressors (e.g., radiation, pesticides, dioxin-like substances, ozone-depleting substances, carcinogens, pathogens), and stressors associated with specific scenarios (e.g., drinking water contaminants, household product ingredients, common contaminants at hazardous waste sites) that might be of interest to assessors. These resources generally include available (and often limited) data on the physicochemical properties of stressors that affect their transport, transformation, and fate in environmental media as well as properties that are relevant to the toxicological potential of the stressor. Regulatory agencies also derive exposure levels for various stressors based on human health or ecological effects, and these values can be found in a variety of databases.
Considerations involved with developing exposure scenarios for specific chemical classes are described in the Chemical Classes Tool Set of EPA-Expo-Box.
Tailoring the Exposure Scenario
Generally, due to legal, regulatory, policy, or business reasons and/or resource considerations, an exposure scenario focuses only on certain elements of the overall situation that are of primary interest (e.g., dust exposures in children via the oral route of exposure, drinking water exposures over a lifetime). As a result there are a number of ways to tailor an exposure scenario to focus on a specific tier or type of analysis, exposure route, exposed population, exposure medium, or chemical class. The methods and resources available for tailoring an exposure scenario in these different areas are described in the other Tool Sets available in EPA-Expo-Box:
- Tiers and Types. Exposure scenarios can be developed to support different tiers and types of exposure assessments. Individual "tiers" correspond to iteratively more complex, and typically data-intensive, steps in the assessment. At each stage of a tiered exposure assessment, investigators evaluate whether the assessment results are sufficient to support the risk management decisions. The type of assessment might refer to whether the assessment is considering aggregate or cumulative exposures or whether is at the individual or population level.
- Exposure Routes. Exposure scenarios can be developed to focus on either one or many routes of exposure. The routes of exposure for which environmental exposure scenarios are commonly developed are inhalation, ingestion, and dermal contact.
- Exposure Media. Exposure scenarios can evaluate the releases of stressors to specific media, how stressors behave within those media and flow between different media compartments, and which media offer potential for contact of a receptor with stressor. Typical media compartments that are evaluated in environmental exposure assessments include air, water and sediment, soil and dust, food, aquatic biota, and consumer products.
- Exposed Populations. Exposure scenarios can be developed for one or more individuals within a population or a population as a whole. While some scenarios are developed to estimate general population exposures, scenarios can be refined to focus on specific population segments such as residential, consumer, occupational, and highly exposed populations. For each of these populations information on sources and releases, fate and transport, concentrations, and exposure factors relevant to the population is needed.