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Advances in Risk Assessment Since the Establishment of the Part 503 Rule

The committee’s review of the risk assessment used to support the Part 503 rule was carried out in the context of current and emerging practice in risk assessment. The committee determined that its review of the risk assessment should communicate the committee’s interpretation of how the risk-assessment process has evolved from the time the Part 503 rule was issued until present. Of particular interest to the committee were documents from EPA and the National Research Council (NRC) that propose and encourage methods that differ substantially from the methods used in the Part 503 risk assessment. This chapter provides a foundation and context for the following chapters.

This chapter first describes new approaches and considerations in risk assessment since the Part 503 rule (Standards for Use or Disposal of Sewage Sludge) was established in 1993 (40 CFR Part 503). It focuses on the changing priorities of cancer versus noncancer end points, acute versus chronic end points, probabilistic risk-assessment approaches, and the need to address aggregate exposures and cumulative risk. A brief description is then given of the changes in risk-assessment approaches of EPA over this period.

THE RISK-ASSESSMENT PROCESS

Risk assessment is a process for identifying potential adverse consequences along with their severity and likelihood. In contrast to other tools



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Biosolids Applied to Land: Advancing Standards and Practices 4 Advances in Risk Assessment Since the Establishment of the Part 503 Rule The committee’s review of the risk assessment used to support the Part 503 rule was carried out in the context of current and emerging practice in risk assessment. The committee determined that its review of the risk assessment should communicate the committee’s interpretation of how the risk-assessment process has evolved from the time the Part 503 rule was issued until present. Of particular interest to the committee were documents from EPA and the National Research Council (NRC) that propose and encourage methods that differ substantially from the methods used in the Part 503 risk assessment. This chapter provides a foundation and context for the following chapters. This chapter first describes new approaches and considerations in risk assessment since the Part 503 rule (Standards for Use or Disposal of Sewage Sludge) was established in 1993 (40 CFR Part 503). It focuses on the changing priorities of cancer versus noncancer end points, acute versus chronic end points, probabilistic risk-assessment approaches, and the need to address aggregate exposures and cumulative risk. A brief description is then given of the changes in risk-assessment approaches of EPA over this period. THE RISK-ASSESSMENT PROCESS Risk assessment is a process for identifying potential adverse consequences along with their severity and likelihood. In contrast to other tools

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Biosolids Applied to Land: Advancing Standards and Practices used for environmental evaluation and policy, the principal objective of the risk assessment and risk management approach is not to eliminate all risk but to quantify the risk and provide risk managers with tools to balance the level of risk against the cost of risk reduction, against competing risks, or against risks that are generally accepted as trivial or acceptable. Controlling the exposure of human populations to environmental contaminants in biosolids using a risk-based approach requires a definition of both an appropriate metric for assessing the impacts of contaminants on human health and a defensible process for assigning value to the predicted impacts. The end product of a risk-based approach to environmental management is either to identify an acceptable level of exposure or to prescribe the technical controls or political process needed to attain acceptable risk. Intervention can be achieved through technical or political controls. Components of the Risk-Analysis Process The NRC (1982, 1994) has divided and continues to divide the practice of risk analysis into two substantially different processes—risk assessment and risk management. Along with these processes are concurrent efforts to communicate and evaluate risk (NRC 1989, 1996). This section explores the evolution of the risk-assessment process over the last decade by considering the component steps in the process. Risk assessment is the process of selecting and quantifying the adverse consequences that result from an action, such as application of biosolids to soils, or from inaction. A risk assessment begins with efforts to identify the potential hazards associated with a chemical or microbial agent and its use or occurrence. Hazard identification addresses the potential for harm but not the likelihood of harm. Risk characterization establishes the significance of an identified hazard by quantifying the likelihood and severity of exposure scenarios linked to that hazard. As applied to toxic agents, risk characterization has five principal elements: (1) quantification of sources and environmental concentrations in exposure media; (2) quantification of exposure to the target population and distribution of the dose among the population; (3) characterization of a dose-response function for all potential toxic agents that have been identified; (4) estimates of the number of people affected and severity of consequences expected within the population at risk; and (5) an assessment of the magnitude and sources of uncertainty that limit the precision of the estimate of consequences. Risk management is the process of weighing policy alternatives and selecting the appropriate societal or institutional response. Risk management is

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Biosolids Applied to Land: Advancing Standards and Practices used to integrate the results of a risk characterization with social, economic, and political valuation to reach a decision. The goal of the risk-management process is to establish the significance of the estimated risk, compare the costs of reducing this risk with the benefits gained, compare the estimated risks with the societal benefits derived from incurring the risk, and carry out the political and institutional process of reducing risk. Linking the risk-assessment and risk-management processes are the concurrent efforts to evaluate and communicate risk. Risk evaluation is the process by which the risk-characterization and risk-management processes are reconciled with individual and societal valuations of risk (NRC 1996). A key step in this link is effective risk communication. According to the NRC (1989), risk communication has become more difficult in recent decades and common misconceptions often hamper communication efforts. In considering these issues, the NRC (1989) emphasizes that solving the problems of risk communication is as much about improving procedures as improving the content of risk messages. Figure 4–1 provides a view of how the risk-analysis process might proceed for assessing the health impacts of pollutants in biosolids. Each of the major steps in this process involves one or more actions that are listed to the right of each major step. Confronting Uncertainty and Variability An important and often ignored final step in the risk characterization process is the characterization of uncertainties. Important sources of uncertainty and variability in risk assessments involve the data and models used. With incomplete data and models used to characterize contaminant transport representing heterogeneous geographic and climate regions, the variability and uncertainty associated with the resulting risk estimates are large. In evaluations of uncertainty in risk assessment, Morgan et al. (1990) and Finkel (1990) distinguish among parameter uncertainty, model uncertainty, decision-rule uncertainty, and natural variability in any of the parameters and call for separate treatment of the different types of uncertainty. Probabilistic methods such as Monte Carlo analysis are available to evaluate uncertainty in parameters. According to Finkel (1990), model uncertainty derives from a number of actions, including the use of simplifications that might exclude relevant variables from the analysis; the use of surrogate variables that might not be appropriate for the variable of interest; the appearance of abnormal conditions that might occur in nature but that might not be appropriate in the model; and the use of incorrect model forms. Morgan et al. (1990) noted that

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Biosolids Applied to Land: Advancing Standards and Practices FIGURE 4–1 The major components of the risk-assessment and risk-management process.

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Biosolids Applied to Land: Advancing Standards and Practices relatively little research has been done on uncertainty or disagreement about what form of model to use. Decision-rule uncertainty applies to risk management and arises whenever ambiguity or controversy exists about quantifying or comparing social objectives. According to Finkel (1990, p. 16), “to take any actions using the outputs of a risk assessment, including the decision not to take action, one must be prepared to make a series of potentially controversial value judgments.” An important source of uncertainty in risk characterization is the development and application of dose-response models. Among the many issues that complicate the process of establishing a dose-response function is the variation in human susceptibility. In large heterogeneous populations, there are large variations in susceptibility to toxic effects. Those variations are due in part to variations in genetic predisposition to certain disease states, variations in age, and large variations in physical stresses and other chemical or non-chemical exposures that might be extant in the system of interest. NEW APPROACHES AND CONSIDERATIONS IN RISK ASSESSMENT This section reviews new approaches to risk assessment that were developed since the Part 503 rule was issued. A summary of key documents from the NRC, the Presidential/Congressional Commission on Risk Assessment and Risk Management, and EPA are provided. Then, consideration is given to how those documents have altered the standard practice in each of the key steps of the risk-assessment process. Recent Reports Define New Directions in Risk Assessment Among the reports that have had particular impact are two reports issued by the NRC. The first report, titled Science and Judgment in Risk Assessment, provided an update on the process of risk assessment and management (NRC 1994). This report made seventy-five specific recommendations, but among its overarching recommendations are those to address explicitly uncertainty and variability in risk assessment, to address multimedia exposures and cumulative intake through multiple exposure pathways, to and foster more interaction among risk assessors and risk managers. The second report, titled Understanding Risk, Informing Decisions in a Democratic Society (NRC 1996), used several case studies to evaluate the emerging trends in risk-assessment methodology.

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Biosolids Applied to Land: Advancing Standards and Practices The Presidential/Congressional Commission on Risk Assessment and Risk Management was created through the 1990 Clean Air Act amendments to make recommendations for improving the risk-assessment and risk-management process. In 1997, the commission issued Framework for Environmental Health Risk Management. The report emphasizes how to present a risk assessment and how to work with community concerns in an iterative fashion. It identifies a clear need to modify the traditional approaches used to assess and reduce risks. Traditional approaches rely on a chemical-by-chemical, medium-by-medium, risk-by-risk strategy. The report states the need to focus less attention on refining assumption-laden mathematical estimates of the small risks associated with exposures to specific chemicals and the need to focus instead on the overall goal of reducing risk and improving health status. There is strong emphasis on stakeholder participation. Stakeholders are groups who are potentially affected by the risk, groups who will manage the risk, and groups who will be affected by efforts to manage the source of the risk. Involving stakeholders throughout the risk-assessment process provides opportunities to gather information and to bridge gaps in understanding, language, values, and perspectives. Over the last decade, EPA issued a number of reports that are having an impact on the framework and process of regulatory risk assessment. Of particular note are the 1992 Habicht memo, which provides guidance to EPA managers on risk characterization (Habicht 1992); a journal report on benchmark dose (Barnes et al. 1995), which provides guidance for a more harmonized approach for addressing cancer and noncancer health end points; and the proposed guidelines for carcinogen risk assessment (EPA 1996a). The Habicht memo emphasizes the need to avoid point estimates of risk and to provide instead details on the scientific basis of decisions, including clear statement of assumptions and uncertainties. Barnes et al. (1995) recommend the use of the benchmark-dose approach as an alternative to using the no-observed-adverse-effect level. EPA’s proposed guidelines for carcinogen risk assessment put more emphasis on “margin of exposure” (relative to a benchmark dose), weight of evidence, and the use of uncertainty factors in the risk characterization process. Also of note is EPA’s (1997a) Exposure Factors Handbook, which provides a large compendium of information on human activities that relate to exposure—including time-activity data, exposure duration, consumption of homegrown food, and water ingestion. In addition, there is an ongoing effort to address aggregate exposures to the same substances from multiple sources and pathways and cumulative exposures and risk from mixtures. The 1996 Food Quality Protection Act (FQPA) explicitly calls for addressing aggregate exposure and cumulative risk in setting standards for pesticide residues in food.

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Biosolids Applied to Land: Advancing Standards and Practices From a risk assessment perspective, this report will clearly establish that biosolids are a complex mixture of chemical and biological agents, the exact composition of which can change from time to time and place to place. Moreover, it will never be possible to account for all the components of the mixture, although the stable components are well characterized. As discussed in detail in various sections of this report, considerable effort has been devoted to an enumeration of the hazardous constituents of biosolids. During the course of its study, the committee found that it remains necessary to conduct risk assessments on biosolids based on their component parts. Figure 4–2 provides a time line showing when a number of significant risk-guidance documents have been issued relative to the year when the Part 503 rule was issued. Advances in Hazard Identification Since EPA issued cancer and mutagenicity risk-assessment guidelines in 1986 (EPA 1986a,c), the types and reliability of methods used to identify potential hazard have advanced. In the 1986 guidelines, the stated goal of a hazard assessment was to provide a review of the relevant biological and chemical information on an agent that might pose cancer or other health hazards. At that time, the recommended elements of the hazard identification included (1) a summary of an agent’s physical-chemical properties and routes and patterns of exposure; and (2) a review of toxic effects, structure-activity indicators of toxicity, metabolic and pharmacokinetic properties, short-term animal and cell tests, long-term animal tests, and human studies. These elements have remained the core components of hazard identification, but the arsenal of methods, the reliability of techniques, and the relative emphasis on the various hazard identification elements have changed over the past decade. In particular, risk assessors can now make use of better markers of genetic damage (toxicogenomics) for rapid assessment, improved structure-activity relationships (SAR), and improved quantitative structure-activity relationships (QSAR). However, to date, these emerging methods have seen only limited use in regulatory risk assessment. Health-effects research has focused more on early indicators of outcome, making it possible to shorten the time between exposure and observation of an effect Use of measures of exposure as hazard indicators (e.g., Hertwich et al. 2001) has increased, and more-sophisticated measures of hazard such as the human toxicity potential have been developed. Human toxicity potential includes emissions, exposure potential, and toxic hazard indicators in a single measure of potential harm. It has been used as a cumulative-exposure screening tool for multiple chemical agents.

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Biosolids Applied to Land: Advancing Standards and Practices FIGURE 4–2 Time line showing year of release of a number of important risk guidance documents relative to the release of the Part 503 rule.

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Biosolids Applied to Land: Advancing Standards and Practices Public-health and environmental concerns about biosolids foster a need for hazard assessments that can address multiple and complex issues. Among these issues are health hazards from chemical mixtures and pathogens, as well as concerns about specific categories of chemical hazard, such as metals, persistent organic pollutants (POPs), and high-production-volume chemicals (HPVs). Recent advances in hazard assessment provide EPA with better tools for those issues. Community issues are not adequately addressed in the current risk-assessment paradigm (e.g., property intrusions, odor, and truck traffic). Other issues have been addressed in EPA programs but have not been explicitly addressed in the risk-management goals of the biosolids program. Those include potential health effects from added diesel exhaust and potential environmental effects from added nitrogen burdens, runoff, damage to endangered species habitat, and conversion of inorganic mercury to organic mercury in situ and in water bodies following runoff. Advances in the Dose-Response Characterization Process A number of important changes have been proposed and, in some cases, applied to dose-response characterization over the last decade. In 1993, the NRC considered the scientific basis, inference assumptions, regulatory uses, and research needs in risk assessment and focused on two dose-response issues—the use of maximum tolerated dose in animal bioassays and the use of two-stage models of carcinogenesis (NRC 1993). The report presented options for revising those default procedures. Recent EPA documents (EPA 1996a, 2001a) proposed that dose-response characterization be handled differently from that proposed in the 1986 risk-assessment guidelines (EPA 1986a). According to the 1986 guidelines, risk for carcinogens is modeled using potency—the increase of risk per unit increase of dose or exposure. Risk for noncarcinogens is addressed using a hazard index—the ratio of the predicted dose to the reference dose. More recently, efforts have been made to harmonize those two approaches by using a margin of exposure (MOE) to characterize risk for both carcinogens and noncarcinogens. MOE is the ratio of a dose derived from a tumor bioassay, epidemiologic study, or biologic marker study to an actual or projected human exposure. Changes in Dose-Response Methods Several proposals within and outside EPA have been made to modify the standard approach for building dose-response models on the basis of animal or human data. The most important and comprehensive proposal is EPA’s

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Biosolids Applied to Land: Advancing Standards and Practices 1996 proposed revisions to its carcinogen risk-assessment guidelines (EPA 1996a). These guidelines, which are still undergoing review and revision within EPA, propose a different weight-of-evidence classification and the option of using an MOE in place of potency to estimate risk. Risk-assessment literature has provided proposals for the use of time-to-tumor models (Krewski et al. 1983), Bayesian methods for constructing and revising dose-response models (Taylor et al. 1993; Evans et al. 1994; Wilson 2001), and meta-analysis. EPA’s Proposed 1996 Carcinogen Risk-Assessment Guidelines In 1996, EPA issued its proposed Guidelines for Carcinogen Risk Assessment (EPA 1996a) for a 120-day public review and comment period. EPA issued the guidelines as a replacement for the 1986 Guidelines for Carcinogen Risk Assessment (EPA 1986a). The revised guidelines were issued in part to address changes in the understanding of the variety of ways in which carcinogens can operate. For example, because many laboratories now use test protocols aimed at mode of action, the 1996 proposed guidelines provide a framework that allows for incorporation of all relevant biological information and flexibility to consider future scientific advances. In contrast to the single default dose-response relationship (the linearized multistage model for extrapolating risk from upper-bound confidence intervals) used in the 1986 cancer guidelines, the 1996 guidelines provide several options for constructing the dose-response relationship. Biologically based extrapolation, that is, extrapolation from animals to humans based on a similar underlying mechanism of action, is the preferred approach for quantifying risk. However, because data for the parameters used in such models are not likely to be available for most chemicals, the 1996 guidelines allow for alternative quantitative methods, including several default approaches. In the default approaches, dose-response assessment is a two-step process. In the first step, response data are modeled in the range of observation; in the second step, a determination is made of the point of departure (benchmark) or the range of extrapolation below the range of observation. In addition to modeling tumor data, the new guidelines call for the use and modeling of other kinds of responses if they are considered measures of carcinogenic risk. Three default approaches—linear, nonlinear, or both—are provided. Curve fitting in the observed range provides the effective dose corresponding to the lower 95% limit on a dose associated with a 10% response (LED10). The LED10 is then used as a point of departure for extrapolation to the origin as the linear default or for an MOE as the nonlinear default The LED10 is the standard point of departure, but other departure points can be used when the data justify it.

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Biosolids Applied to Land: Advancing Standards and Practices Other modifications of interest in the 1996 guidelines include the following: Emphasis is placed on all biological information rather than only tumor findings in the hazard-assessment phase of risk assessment. Mode of action is emphasized to reduce the uncertainty in describing the likelihood of harm and in determining the dose-response approaches. A weight-of-evidence narrative replaces the current alphanumeric classification categories (A, B1, B2, C, D, E) from the 1986 cancer guidelines. The narrative summarizes the key evidence, describes the agent’s mode of action, characterizes the conditions of hazard expression, and recommends appropriate dose-response approaches. The overall conclusion on the likelihood of human carcinogenicity is given by route of exposure. Only three descriptors for classifying human carcinogenic potential are now available—known/likely, cannot be determined, and not likely. In contrast to the 1986 guidelines that provide very little guidance for risk characterization, the 1996 guidelines provide direction on how the overall conclusion and the confidence of risk are presented for the risk manager and call for assumptions and uncertainties to be clearly explained. Time-to-Tumor Models Because dose-response functions for many chemical substances are derived from lifetime animal-feeding studies, results apply to lifetime risk of cancer. The most common dose-response model derived from such toxicological experiments describes the lifetime change in cancer incidence with dose. However, the stage theory of cancer and other diseases emphasizes that many harmful exposures can be more accurately characterized as reducing the time to tumor induction rather than increasing the lifetime risk of tumor (Armitage and Doll 1954). In a time-to-tumor dose-response model, important information is disclosed by the time it takes for a fraction of the test subjects to get tumors (Krewski et al. 1983). Some animal bioassay data indicate when individual bioassay animals died before scheduled terminal sacrifice and whether they died with or without tumors. In some human populations, time to tumor or other disease is also available. Use of time-to-tumor data in the analysis of the tumor dose-response relationship provides a credible estimate of the potency of the carcinogen by incorporating considerable information. These models are not common but have much potential when data are substantial.

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Biosolids Applied to Land: Advancing Standards and Practices Waste (OSW) develops guidelines for the land disposal of municipal and hazardous waste and the Office of Underground Storage Tanks (OUST) develops guidance for limiting the risks from leaks of underground storage tanks. OSWER provides technical assistance to all levels of government to establish safe practices in waste management. OSWER is also home to the Superfund program, which addresses health concerns of communities with abandoned and active hazardous waste sites and accidental oil and chemical releases. Superfund also encourages innovative technologies to address contaminated soil and groundwater. Office of Solid Waste (OSW) OSW is responsible for setting limits on the concentrations of chemicals that can be placed in municipal landfills. Limits are set through a risk-assessment process that identifies and evaluates multiple exposure pathways. OSW has identified a number of potential exposure pathways linked to landfills and uses multimedia risk assessments to link human exposure and health risk to chemicals in the landfill waste. The assessment is a forward-calculating analysis that evaluates the risks of multiple exposure pathways to human and ecological receptors. One of the pathways that the OSW landfill risk assessments addresses is the advection of chemicals out of the landfill due to forced convection that results from methane and carbon dioxide generation in the waste pile. Office of Underground Storage Tanks (OUST) OUST was created in 1985 to carry out a congressional mandate to develop and implement a regulatory program for underground storage tank (UST) systems. OUST works with EPA regional offices and state and local UST programs to promote the use of risk-based decision-making. In OUST, risk-based decision-making (RBDM) is a process by which decisions are made about contaminated sites using a site-specific assessment of the risk each site poses to human health and the environment In cooperation with the American Society for Testing and Materials (ASTM), OUST is evaluating whether its RBDM programs are achieving their stated agency management goals. Office of Emergency and Remedial Response (OERR) The EPA Superfund program is administered by the OERR. After a

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Biosolids Applied to Land: Advancing Standards and Practices hazardous waste site is listed on the National Priorities List, risk assessment has an important role in the characterization and cleanup of Superfund sites. OERR provides general tools and specific tools to assist in the major steps of the risk-assessment process. In 1989, Risk Assessment Guidance for Superfund (RAGS), Part A, was issued (EPA 1989). This document provides recommended algorithms and data for calculating potential exposures to chemical contaminants found at Superfund sites. In contrast to the OUST risk methods, RAGS are more generic in providing uniform national risk-assessment defaults. Additional RAGS documents were issued in 1991 in Part B (EPA 1991b), which provides guidance on using EPA toxicity values and exposure information to derive risk-based preliminary remediation goals, and Part C (EPA 1991c), which provides guidance on the human health risk evaluations of remedial alternatives. In 1998, OERR issued Part D (EPA 1998c), and in 1999, it issued a supplement to Part A (EPA 1999a). This document is of interest to biosolids risk assessors, because the supplement provides information to improve community involvement in the Superfund risk-assessment process. Specifically, the supplement suggests ways for Superfund staff and community members to work together during the early stages of Superfund cleanup; identifies where community input can augment and improve EPA’s estimates of exposure and risk; recommends questions that the site team should ask the community; and illustrates why community involvement is valuable during the human health risk assessment at Superfund sites. A review draft of Part E provides dermal risk assessment guidance (EPA 2001a). OERR has also developed probabilistic risk assessment guidance for Superfund (EPA 2001b). Office of Water (OW) EPA’s OW is responsible for all national water-quality activities, including the regulation of surface water and groundwater supplies to protect human health and the environment. OW is responsible for implementing the Clean Water Act, Safe Drinking Water Act, and portions of other environmental laws and treaties that apply to water quality. Several organizations make up the OW, including the Office of Wetlands, Oceans, and Watersheds; the Office of Science and Technology; the Office of Wastewater Management (which oversees EPA’s biosolids program); and the Office of Ground Water and Drinking Water. A major task of OW is to set drinking-water standards. Risk assessment provides a key input to this process. Since 1986, OW has more than tripled the number of contaminants for which it has published drinking-water stan-

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Biosolids Applied to Land: Advancing Standards and Practices dards, bringing the total to 94. A current challenge for OW in its effort to minimize health risks from water supplies is to find the appropriate balance between the risks from naturally occurring microbial pathogens and the chemical by-products of disinfection processes used to remove the pathogens. It is important to provide protection from these microbial pathogens while ensuring decreasing health risks to the population from disinfection by-products. As part of its effort to protect watersheds, OW has established the total maximum daily load (TMDL) program. A TMDL is a calculation of the maximum amount of a pollutant that a body of water can receive and still meet state water-quality requirements. TMDLs are determined in part by considering multiple sources of pollutants (from point, nonpoint, and background sources, including atmospheric deposition), seasonal variations, and margins of safety. The calculations of these programs provide benchmarks for the continuing evaluation of biosolids standards. Office of Prevention, Pesticides and Toxic Substances (OPPTS) EPA’s OPPTS develops national strategies for toxic substance control and promotes pollution prevention and the public’s right to know about chemical risks. OPPTS has an important role in protecting public health and the environment from potential risk from toxic chemicals and pesticides. OPPTS is dealing with issues such as endocrine disruptors and lead poisoning prevention. Within OPPTS, the Office of Pesticide Programs (OPP) regulates the use of all pesticides in the United States and establishes maximum concentrations for pesticide residues in food. As part of this effort, OPP is expanding access to information on risk-assessment and risk-management actions to help to increase transparency of decision-making and facilitate consultation with the public and affected stakeholders. OPP has a mandate under the FQPA of 1996 to address aggregate exposure and cumulative risk from multiple sources of pesticide exposure. To address that issue, OPP developed a framework for conducting cumulative risk assessments for organophosphates and other pesticides that have a common mechanism of toxicity (that act in the same way in the body). Through its cumulative risk-assessment framework, OPP will be able to consider whether the risks posed by a group of pesticides that act the same way in the body meet the FQPA safety standard of “reasonable certainty of no harm.” As part of that framework, OPP is developing new methods to assess cumulative risk, to assess residential exposure, and to aggregate exposures from all nonoccupational sources.

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Biosolids Applied to Land: Advancing Standards and Practices FINDINGS AND RECOMMENDATIONS The Part 503 rule risk assessments were carried out more than a decade ago. In this chapter, the committee considered the likely impact of changes in risk-assessment practice in general and in various EPA offices in particular on the risk-assessment process for biosolids. The committee found that the development of methods in the broader academic community and the evolution of risk-assessment methods within various EPA offices and programs provide important benchmarks for the committee’s assessment of the relevance and reliability of the Part 503 rule risk assessments. Of particular note are updates to the risk-assessment framework recommended by the NRC, the Presidential/Congressional Commission on Risk Assessment, and various EPA offices. The risk-assessment methods and policies practiced and advocated at EPA have changed significantly, although not at the pace recommended by the NRC and the risk commission. As a result, the Part 503 rule, which has not been modified to account for any new methods and policies, is now inconsistent with current NRC recommendations and EPA policies within various offices. Particularly relevant examples of the inconsistency are the absence of stakeholder participation and the lack of explicit treatment of uncertainty and variability. Recommendation: Because of the significant changes in risk-assessment methods and policies over the last decade, EPA should revise and update the Part 503 rule risk assessments. Important developments include recognition of the need to include stakeholders throughout the risk-assessment process, improvements in measuring and predicting adverse health effects, advances in measuring and predicting exposure, explicit treatment of uncertainty and variability, and improvements in describing and communicating risk. EPA should consider how the updated risk assessments would change the risk-management process. A similar approach can be taken with the issue of biological agent risks. In recent years, health-effects research has made use of large-scale studies of human health end points at multiple sites. Health-effects research has also focused on early indicators of outcome, making it possible to shorten the time between the exposure and the observation of an effect. In addition, more use has been made of meta-analysis, better modeling of dose-response relationships, and more sophisticated regression models. These improvements make possible more site-specific assessments of the impacts of biosolids land-application practices. Managing exposure of human populations to environmental contaminants using a risk-based approach requires an accurate metric for the impacts of

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Biosolids Applied to Land: Advancing Standards and Practices contaminants on human health and a reliable process for monitoring and recording the exposures within populations assumed to be at risk. Over the past decade, the practitioners of exposure assessment have made important improvements in methods to measure and model source-to-dose relationships. These improvements have been made through greater use of time-activity surveys, personal monitors, and biomarkers of exposure, and they have made it possible to confirm some of the exposures predicted in risk assessments. Recommendation: Many of the measures of risk used in developing the Part 503 rule guidelines cannot be monitored. Because of that inability to monitor, the committee acknowledges that EPA must perform theoretical risk assessments. Nevertheless, there is a continuing need to provide some measures of performance that can be monitored (e.g., concentrations of selected chemicals in exposure media, such as indoor air, house dust, or tap water of residences near land-application sites; and exposure biomarkers in the blood or urine of nearby residents). Recent improvements in health surveillance and exposure monitoring provide new opportunities for EPA to develop more explicit and measurable metrics of performance for biosolids land-application practices. Advancements in monitoring health outcomes and exposure have resulted in improvements in the description and communication of risk. In particular, improved exposure assessments have led to better exposure classification in health-effects studies. Better descriptions of risk are available, using benchmark dose and margin of exposure to communicate hazard and risk in place of risk of death, hazard quotients, or exposure-potency product relationships. There have also been improved methods for prioritizing compounds using measures of risk. Recommendation: In making revisions to the Part 503 rule risk assessment, EPA must strike a balance between expending resources to carry out site-specific data collection and expending resources to model and assess risk using existing information. In light of improvements in exposure and health monitoring, the committee encourages EPA to consider options carefully for collecting new data in support of risk-assessment assumptions before resorting to another risk assessment that relies only on existing data, models, and default assumptions. Among the data that would be of value are data on proximity of receptors to land-application sites; surveys of activities that could increase direct and indirect exposures; and samples of biosolids, air, vegetation, runoff, groundwater, and soil in environments surrounding land-application sites. In addition, EPA should conduct site-specific surveys of performance (e.g., monitor the extent to which rates and depth of application are consistent with risk-assessment assumptions) and scientifically relevant studies of health complaints.

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Biosolids Applied to Land: Advancing Standards and Practices Risk assessments make use of a number of assumptions to define chemical loading in biosolids that pose no undue risk to surrounding populations. Implicit in this process is the premise that these assumptions and the associated demographic and operational conditions will persist. However, there are no guidelines to ensure that these conditions persist. Recommendation: Because there are no guidelines to ensure that conditions assumed in the risk assessment actually transpire, the committee recommends that the Part 503 rule provide guidance for periodic reassessments that will be used to ensure that the demographic and operational conditions of biosolids land application are consistent with the assumptions of the applicable risk assessment. REFERENCES Armitage, P., and R.Doll. 1954. The age distribution of cancer and a multistage theory of carcinogenesis. Br. J. Cancer 8(March):1–12. Barnes, D.G., G.P.Daston, J.S.Evans, A.M.Jarabek, R.J.Kavlock, C.A.Kimmel, C.Park, and H.L.Spitzer. 1995. Benchmark Dose Workshop: Criteria for use of a benchmark dose to estimate a reference dose. Regul. Toxicol. Pharmacol. 21(2):296–306. Bogen, K.T., and R.C.Spear. 1987. Integrating uncertainty and interindividual variability in environmental risk assessment. Risk Anal. 7(4):427–436. Dockery, D.W., and J.D.Spengler. 1981. Indoor-outdoor relationships of respirable sulfates and particles. Atmos. Environ. 15(3):335–343. EPA (U.S. Environmental Protection Agency). 1986a. The Risk Assessment Guidelines of 1986. EPA/600/8–87/045. Office of Health and Environmental Assessment, U.S. Environmental Protection Agency, Washington, DC. August 1986. EPA (U.S. Environmental Protection Agency). 1986b. Guidelines for the Health Risk Assessment of Chemical Mixtures. EPA/630/R-98/002. Risk Assessment Forum, U.S. Environmental Protection Agency, Washington, DC. Fed. Regist. 51(185):34014–34025. (September 24, 1986). [Online]. Available: http://www.epa.gov/ncea/raf/rafguid.htm [December 27, 2001]. EPA (U.S. Environmental Protection Agency). 1986c. Guidelines for Mutagenicity Risk Assessment. EPA/630/R-98/003. Office of Research and Development, U.S. Environmental Protection Agency, Washington, DC. September 1986. EPA (U.S. Environmental Protection Agency). 1989. Risk Assessment Guidance for Superfund, Vol. 1. Human Health Evaluation Manual (Part A). Interim Final. EPA/540/1–89/002. Office of Emergency and Remedial Response, U.S. Environmental Protection Agency, Washington, DC. December 1989. EPA (U.S. Environmental Protection Agency). 1991a. Part V. Guidelines for Developmental Toxicity Risk Assessment. Notice. EPA/600/R/91/001. Fed. Regist.

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