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Exposure Assessment

Exposure refers to a measurable contact of an agent with a target or receptor for a specific duration of time.[172, 173] In the broadest terms, agents can be biological, physical, chemical, social, or psychological, and can produce both adverse and beneficial impacts to the target. EPA has historically focused on minimizing negative impacts, but in a sustainability context assessing exposures that result in positive impacts is also relevant to evaluating tradeoffs. For human exposures, receptors can be individuals, populations, subpopulations, or life-stages of interest. For ecological systems, receptors can be individuals, populations, species communities, or ecosystems that include both wildlife and vegetation. For exposure to occur, the agent and the receptor must intersect in both space and time.

Exposure assessments characterize and predict this intersection by estimating the magnitude, frequency, and duration of exposure.[174] Exposure assessments also describe the number and characteristics of the population exposed (e.g., vulnerable communities, ecosystems, or endangered species). They describe the sources, routes, pathways, and uncertainty in the assessment. Exposure assessments describe the environment as well as characterize and link the processes that impact the transport and transformation of agents from their source through contact with human or ecological receptors. These assessments are a central component in understanding environmental systems and how they change when intended or unintended perturbations occur.

Exposure Assessment

How can Exposure Assessment Contribute to Sustainability?

Exposure assessments are often used as part of risk assessment. They describe the ways that humans and ecological receptors interact and provide the information needed to forecast and trace exposures, identify important future states of the system and the implications of those future system states (futures methods), describe the current state of the system, and evaluate the impact of decisions on both current and future system states.

Information generated through exposure assessments can be used to identify sustainability metrics that reflect the important processes in the system and that also serve as effective indicators of important changes in the system. Exposure assessments provide inputs to many of the other analytical tools in this document such as life-cycle assessment, health impact analysis, benefit-cost analysis, ecosystem service valuation, and environmental justice analysis.

What are the main steps in an Exposure Assessment?

Exposure assessments range from simple assessments (single agents/single receptor) to complex cumulative assessments that evaluate exposures involving multiple harmful and beneficial agents, (multiple pathways/multiple receptors). For complex ecosystem assessments, the dynamic nature of the natural environment must also be taken into account. The form and scope of an assessment will depend on the overall situation, the resulting exposure assessment question, the decision objectives, and in some cases, regulatory or statutory requirements; general steps in an exposure assessment are:

  • Step 1—formulate problem, including articulating the overall concern, the management or sustainability options, and the resources available to assess and manage the problem. Problem formulation should be done through stakeholder engagement and collaboration to ensure that the assessment is relevant to the specific problems being addressed. In a sustainability context, this step is crucial and may be more complex because each problem needs to be considered in the context of the overall system;
  • Step 2—define the target population and/or environment of concern;
  • Step 3—identiffall the agents of concern;
  • Step 4—identifyrelevant sources, fate and transport, routes and pathways of exposure for the target population and/or environment;
  • Step 5—quantify temporal and spatial distributions of agents in the environment and similar distributions for the target populations;
  • Step 6—quantify exposures or exposure distributions including the frequency and duration of exposure;
  • Step 7—estimate the contribution of each source and pathway;
  • Step 8—characterize results and their inherent uncertainty; and,
  • Step 9—communicate information from the assessment to decision-makers and all stakeholders. Again, in the context of sustainability this becomes a very important step given the various mitigation tradeoffs that should be considered.

Exposure assessments rely heavily on models to characterize the movement, transformation, removal, distribution, and interaction of agents and receptors within a system. Models enable the use of a systems approach covering a broad range of issues and allow for both prospective and retrospective assessments. Additionally, models provide the opportunity to evaluate the effectiveness of multiple management options, as well as unintended perturbations to the system.

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What are the strengths and limits of Exposure Assessment in sustainability context?

Applying exposure assessment tools to sustainability will require the development and application of 21st century tools. This includes full development and evaluation of predictive environmental and exposure models for chemical, biological, physical, and geophysical data. These models must be capable of predicting exposures to agents for which little or no data exist. Also, these models must be interoperable such that multiple models can be seamlessly used within a larger systems context that integrates across the environmental, social and economic pillars.

Exposure assessment also requires the development of newer computational and sensing technologies that are sufficiently advanced to support the development of observational exposure networks, in order to more comprehensively describe the environment and populations in both time and space. Additionally, enhanced computational techniques to manage and analyze large complex data sets resulting from increased sensor networks need to be developed. Finally, methods for assessing exposures to multiple agents simultaneously must be improved.

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How is Exposure Assessment used to support EPA decision-making?

Currently exposure assessments are used primarily as inputs for risk assessments when determining human or ecological risks and represent one of the four major steps in the risk assessment process. In this process, exposure assessments are used to estimate exposure or dose, which is combined with dose-response data to estimate risk.

Exposure assessments also provide information on the populations exposed as well as the sources, routes, and pathways for exposure. When used as part of an environmental justice analysis, exposure assessments can help identify vulnerable populations and evaluate environmental inequities. This information can be used to determine the most effective ways to reduce risk by minimizing exposure to harmful agents. By considering agents with positive impacts, exposure assessments can also be used to understand how to maximize benefits while minimizing risks. Exposure assessments could also be used to monitor status and trends by applying appropriate exposure indicators and monitoring changes in them over time. Using exposure assessments in this manner could help evaluate emerging risk concerns, identify unintended changes in the environment, and evaluate the impact of regulatory or societal decisions.

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Where can I find more information about Exposure Assessment?

  • Information on conducting exposure assessments as part of the risk assessment process can be found in EPA (US Environmental Protection Agency) 1992. Guidelines for (Human) Exposure Assessment and Monitoring. This document is currently being updated. [174]

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Illustrative Approach applying Exposure Assessment

  • The Future of Radiation Protection: 2025

    Source: EPA Office of Air and Radiation[255]
    Suite of sustainability tools: futures methods; green accounting; risk assessment; exposure assessment; collaborative problem-solving
    The Future of Radiation Protection: 2025 (PDF) (81 pp, 901K) is a report on challenges the radiation protection community will confront over the generation ahead. It is also a handbook with exercises that people in the field of radiation protection can use to develop better responses to those challenges. It is a product of a project carried out by the Institute for Alternative Futures with support from the US EPA. The project involved hundreds of people inside and outside the radiation protection community during a three-year period between late 1999 and early 2002.

    The project reached conclusions that are themselves challenging. The bottom line is that the challenges ahead are so numerous and serious that they cannot be dealt with successfully through business as usual. A major shift in perspective and approach is needed:


  • From                

    To

    Exclusive focus on current issues, programs, budgets

    Greater attention to the full range of radiation-related challenges facing society, leading to major changes in current priorities

    Tacit assumption that the future will be much like the present

    Realization that the future is likely to be much worse than the present if business-as-usual continues

    Radiation protection defined primarily by a focus on “Legacy” issues

    Assessment that Legacy issues will decline in importance and that future needs center primarily around developing more preventative approaches to 4 Key Sectors: Energy, National Security, Industrial & Consumer, and Health

    Radiological attacks and other terrorist acts viewed as possible but not given a high priority

    Radiological attacks and other terrorist acts considered highly credible and on a high priority

    Reactive responses to problems after they become serious

    More anticipatory, preventative approaches to problems

    Conflicts between deeply entrenched positions

    Emphasis on good science and shared principles for working toward better positions

    Limited emphasis on public information and involvement due to habits of secrecy from the Cold War era

    Primacy of transparency and public right-to-know; emphasis on public education and as much access as feasible to credible, usable information

    Radiation protection as a community onto itself

    Integration of radiation and environmental protection through shared principles for guiding action, combined databases, and risk harmonization

  • Innovative Tools Help EPA Scientists Determine Total Chemical Exposure

    Source: EPA Office of Research and Development [256]
    Suite of sustainability tools: exposure assessment; risk assessment
    Everyday activities – actions as simple as biting into an apple, or walking across a carpeted floor – may expose people to a host of chemicals through a variety of pathways. The air we breathe, the food and water we consume, and the surfaces we touch all are the homes of natural and synthetic chemicals, which enter our bodies through our skin, our digestive systems, and our lungs.

    This makes determining how (and how much of) certain chemicals enter our bodies challenging. In most cases, there is not one single source for any given chemical that may be found in our bodies. Using sophisticated computer models and methods, EPA scientists have developed an innovative set of tools to estimate total exposures and risks from chemicals encountered in our daily lives.

    “The traditional approach of assessing the risk from a single chemical and a single route of exposure (such as breathing air) may not provide a realistic description of real-life human exposures and the cumulative risks that result from those exposures,” said EPA scientist Dr. Valerie Zartarian. “Risk assessments within EPA are now evolving toward the ‘cumulative assessments’ mandated by the Food Quality Protection Act and the Safe Drinking Water Act.”

    Moving the science of chemical risk assessments forward to where it’s possible to evaluate total risks from exposures to a wide variety of chemicals requires several key pieces of information. You need to know what chemicals are found in the environment, their concentration levels in the environment, and how they come into contact with humans. You also have to know how they enter the body, and what they do after that.

    EPA’s Stochastic Human Exposure and Dose Simulation (SHEDS) model addresses the first part of this problem. SHEDS can estimate the range of total chemical exposures in a population from different exposure pathways (inhalation, skin contact, dietary and non-dietary ingestion) over different time periods, given a set of demographic characteristics. The estimates are calculated using available data, such as dietary consumption surveys; human activity data drawn from EPA's Consolidated Human Activities Database; and, observed chemical levels in food, water, air, and on surfaces like floors and counters.

    The data on chemical concentrations and exposure factors used as inputs for SHEDS are based on measurements collected in EPA field studies and published literature values. "EPA’s observational exposure studies have also provided information and data to help define the processes simulated in the model, and evaluate or "ground-truth" SHEDS model estimates", said Zartarian, "who co-developed the model with Dr. Jianping Xue, Dr. Haluk Ozkaynak, and others."

    “The concept of SHEDS is to first simulate an individual over time,” she explained. “The model calculates that individual’s sequential exposures to concentrations in different media and across multiple pathways, and then applies statistical methods to give us an idea of how these exposures might look across a whole population.”

    The story of how chemicals enter the human body doesn’t end there, however. The exposure estimates that SHEDS generates are now being used as inputs for another kind of model – a physiological based pharmacokinetic (PBPK) model, which predicts how chemicals move through and concentrate in human tissues and body fluids.

    Using PBPK models, scientists can take the estimates of chemical exposures across multiple pathways generated by SHEDS and examine how these will impact organs and tissues in the body, and determine how long they will eventually take to be naturally processed and expressed.

    Together, these two models provide scientists with a much more accurate picture of the risk certain chemicals pose to human health – a picture they’ve been able to confirm by extensive comparisons against real-world data, such as duplicate diet and biometric data collected by the US Centers for Disease Control and Prevention in the National Health and Nutrition Examination Survey (NHANES), which collects biomarker data from 5,000 people each year. When EPA researchers have compared the SHEDS-PBPK exposure and dose estimates with the physical NHANES data, they’ve found that the model’s predictions line up very closely with the observations in the survey.

    “The real-world grounding gives you a lot of confidence in the exposure routes modeled in SHEDS and PBPK,” said Rogelio Tornero-Velez, an EPA scientist who has helped develop the Agency’s PBPK models used in the study.

    SHEDS has already been used in developing EPA’s regulatory guidance on organophosphate and carbamate pesticides, and chromated copper arsenate, a chemical wood preservative once used on children’s playground equipment. Now, EPA researchers are using the coupled SHEDS-PBPK models to examine a relatively new class of chemical pesticides called pyrethroids to determine whether they pose any risk to human health and the environment.
    EPA scientists are continuing to refine the SHEDS and PBPK models used in these studies, adding functions and testing them against real-world data. For policy makers, these models will serve as invaluable tools in making decisions meant to protect human health and the environment from the risk of exposure to harmful chemicals.

    “The science and software behind SHEDS and PBPK are substantial,” said David Miller, who has worked on the project from EPA’s regulatory perspective. “They provide exposure and risk assessors both within and outside EPA with a physically-based, probabilistic human exposure model for multimedia, multi-pathway chemicals that is in many ways far superior to those that are presently in routine use.”

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