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Human health risk assessment3 is one of the most powerful tools used by EPA in making regulatory decisions to manage threats to health and the environment. Historically, the analysis and consideration of uncertainty in decision making at EPA has focused on the uncertainties in the data and the analysis used in human health risk assessments, including the underlying sciences that comprise the field, such as exposure science, toxicology, and modeling.

The typical goal of a human health risk assessment is to develop a statement—the risk characterization—regarding the likelihood or probability that exposures arising from a given source, or in some cases from multiple sources, will harm human health. The risk characterization should include a statement about the scientific uncertainties associated with the assessment and their effect on the assessment, including a clear description of the confidence that the technical experts have in the results. Such information is provided to decision makers at EPA for consideration in their regulatory decisions (EPA, 2000; NRC, 2009). That information is also made available to the public. Uncertainties in the health risk assessment could stem, for example, from questions about how data from animals exposed to a chemical or other agent relate to human exposures or from uncertainties in the relationship between chemical exposures, especially low-dose chemical exposures, and a given adverse health outcome.

The statutes and processes guiding decision making at EPA make it clear that uncertainties in data and analyses are legitimate and predicable aspects of the decision-making process. Congress, the courts, and advisory bodies such as the National Research Council have recognized the inevitability of uncertainties in human health risk assessment and environmental decision making and, in some instances, have urged EPA to give special attention to this aspect of the process. The origins, necessity, and legitimacy of uncertainty as an aspect of EPA decision making, therefore, have both legal and scientific underpinnings. Those bases are discussed below.

To fulfill its mission, EPA promulgates regulations to administer congressionally mandated programs. Although the statutes that govern EPA do not always contain explicit references to uncertainty in human health risk assessments, a number of statutes clearly imply that the information available to EPA may be uncertain and permit the agency to rely on uncertain information in its rulemaking. In other words, the need for EPA to consider and account for uncertainty when promulgating regulations is implicit in the statutes under which the agency operates. For example, the statutes related to air4 and water5 recognize and allow for uncertainty in decision making by calling for health and environmental standards with an “adequate” or “ample” margin of safety. Other statutes require judgments relating to the “potential” for environmental harm and “a reasonable certainty that no harm will result.”6 Congress's recognition of the uncertainty inherent in factors other than human health risks is also evident, for example, in such statements as “reasonably ascertainable economic consequences of the rule.”7 Such statements indicate a recognition by Congress that, at the time of a regulatory decision, data and information may be incomplete, controversial or otherwise open to variable interpretations, or that use of the available data and information may require assumptions about future events and conditions that are unknown or uncertain at the time of the rulemaking. Although the statutory language may seem vague and incomplete, the fact that such language was incorporated into a law by Congress indicates a recognition that EPA should have the discretion to interpret the statute and to develop approaches informed by agency experience, expertise, and decision-making needs.

Furthermore, some provisions do explicitly mandate that EPA discuss uncertainties in reports to Congress and other entities. For example, the Clean Air Act (CAA)8 requires EPA to report to Congress on “any uncertainties in risk assessment methodology or other health assessment technique, and any negative health or environmental consequences to the community of efforts to reduce such risks.” The Clean Water Act (CWA) Amendments9 require EPA to specify, in a publicly available document, “to the extent practicable … each significant uncertainty identified in the process of the assessment of public health effects and studies that would assist in resolving the uncertainty.”

In addition, several statutes contain provisions that amplify and clarify legislative objectives with respect to the uncertainties associated with assessing human health risk. For example, the 1996 Food Quality Protection Act (FQPA) specifies the following for pesticide approvals: “In the case of threshold effects … an additional ten-fold margin of safety for the pesticide chemical residues shall be applied for infants and children.”10 The 1996 amendments to the Safe Drinking Water Act (SDWA) are similarly explicit about the presentation of human health risk estimates and uncertainty in those estimates:

The Administrator shall, in a document made available to the public in support of a regulation promulgated under this section, specify, to the extent practicable—

(i)

Each population addressed by any estimate of public health effects;

(ii)

The expected risk or central estimate of risk for the specific populations;

(iii)

Each appropriate upper-bound or lower-bound estimate of risk;

(iv)

Each significant uncertainty identified in the process of the assessment of public health effects and studies that would assist in resolving the uncertainty; and

(v)

Peer-reviewed studies known to the Administrator that support, are directly relevant to, or fail to support any estimate of public health effects and the methodology used to reconcile inconsistencies in the scientific data.11

Even from EPA's earliest days the courts have upheld the agency's legal authority and its need to account for uncertainty in its decision-making process. For example, in 1980 the U.S. Court of Appeals for the District of Columbia Circuit accepted and expanded on EPA's need to account for uncertainty by upholding an EPA decision related to the then new National Ambient Air Quality Standard (NAAQS) for particulate matter.12 In affirming EPA's regulation under the CAA, the court pointed to “the Act's precautionary and preventive orientation”13 and noted that “some uncertainty about the health effects of air pollution is inevitable”14 and that “Congress provided that the Administrator is to use his judgment in setting air quality standards precisely to permit him to act in the face of uncertainty.”15

As a discipline, science treats uncertainty as a natural and legitimate part of measurement methodology and, therefore, as an expected aspect of technical data used in decision making. To make scientific progress, “uncertainty, discrepancy, and inconsistency” are often necessary to point the way to new lines of experimentation and new discoveries (Lindley, 2006). However, the variability in the state of understanding and uncertainty is quite different from the more binary and absolute world of regulatory and courtroom decisions surrounding the EPA's regulatory decisions. To try to bridge that gap, the scientific community has endeavored to provide regulatory decision makers with a more comprehensive view of the estimates of risks, and a number of scientific reports have been published that focus on regulatory decision making and, to some extent, on the uncertainties inherent in the information that supports EPA's human health regulatory decisions and on the implications of that uncertainty.

Over the past three decades two core themes, which were outlined in the germinal report Risk Assessment in the Federal Government: Managing the Process (hereafter the Red Book) (NRC, 1983), have governed core aspects of human health risk assessment and decision making at EPA: (1) the special meaning of “uncertainty” in relation to human health risk assessment and decision making; and (2) the interface between risk assessment and risk management, that is, the interface between estimating risks and the decision about how to manage those risks. When discussing uncertainties, the Red Book (NRC, 1983) focused on uncertainty in risk estimates and, to some extent, in economic analyses, stating that “there is often great uncertainty in estimates of the types, probability, and magnitude of health effects associated with a chemical agent, of the economic effects of a proposed regulatory action, and of the extent of current and possible future human exposures” (p. 11; emphasis added). The report highlights the need to include uncertainties in characterizations of health risks, stating, “The summary effects of the uncertainties in the preceding steps are described in this step” (p. 20; emphasis added). It also highlights the need to communicate uncertainty and the paucity of guidance on how to do so, stating, “The final expressions of risk derived in [the risk characterization] will be used by the regulatory decision maker. … Little guidance is available on how to express uncertainties in the underlying data and on which dose—response assessments and exposure estimates should be combined to give a final estimate of possible risks” (p. 36; emphasis added).16

Over a decade later, Science and Judgment in Risk Assessment: Managing the Process (NRC, 1994), in response to its charge, focused on estimates of human health risks and statistical methods to quantify the uncertainty in those estimates. In line with those reports, EPA has focused a great deal of attention on methods for quantifying and expressing the uncertainty in health risk estimates.

Understanding Risk: Informing Decisions in a Democratic Society (hereafter Understanding Risk) (NRC, 1996), discussed the uncertainty inherent in estimates of health risks in the broad context of regulatory decisions, including those made by EPA. That committee wrote, “Significant advances have been made in recent years in the development of analytical methods for evaluating, characterizing, and presenting uncertainty and for analyzing its components, and well documented guidance for conducting an uncertainty analysis is available” (p. 108). The committee focused, however, “on the role of uncertainty in risk characterization and the role that uncertainty analysis can play as part of an effective iterative process for assessing, deliberating, and understanding risks when discussing uncertainties” (p. 108). It also proposed that “[p]erhaps the most important need is to identify and focus on uncertainties that matter to understanding risk situations and making decisions about them,” and emphasized “the critical importance of social, cultural, and institutional factors in determining how uncertainties are considered, addressed, or ignored in the tasks that support risk characterization” (p. 108). In other words, Understanding Risk highlighted the subjective nature of interpreting uncertainty in human health risk estimates and how that subjectivity—which is influenced by social and cultural factors such as public values and preconceptions—can affect how the uncertainty is characterized.

The final report of the Presidential/Congressional Commission on Risk Assessment and Risk Management (1997a) recommended a risk management framework geared toward environmental risk decisions (see Figure 1-1). The three main principles of the framework are (1) putting health and environmental problems in their larger, real-world contexts,17 (2) involving stakeholders “to the extent appropriate and feasible during all stages of the risk management process” (p. 6), and (3) providing risk managers (referred to as the decision makers in this report) and stakeholders with opportunities to revisit stages within the framework when new information emerges. When discussing uncertainty in health risk characterizations for routine risk assessments, the report recommended using qualitative descriptions of uncertainty rather than quantitative analyses, because it “is likely to be more understandable and useful than quantitative estimates or models to [decision makers] and the public” (p. 170). In Volume 2 of its report, the commission further discussed uncertainty analyses, focusing on the uncertainties in both health risk estimates and economic analyses (Presidential/Congressional Commission on Risk Assessment and Risk Management, 1997b). Studies have shown, however, that different people interpret qualitative descriptions differently (Budescu et al., 2009; Wallsten and Budescu, 1995; Wallsten et al., 1986). See Chapter 6 for further discussion.

Science and Decisions: Advancing Risk Assessment (NRC, 2009) emphasized that risk assessment is an applied science used to help evaluate risk management options, and, as such, assessments should be conducted with that purpose in mind. It further stated that “descriptions of the uncertainty and variability inherent in all risk assessments may be complex or relatively simple; the level of detail in the descriptions should align with what is needed to inform risk-management decisions” (p. 5). The report recommended that EPA adopt a three-phase framework and employ a “broad-based discussion of risk-management options early in the process, extensive stakeholder participation throughout the process, and consideration of life-cycle approaches in a broader array of agency programs” (p. 260). Phase III of the framework, the risk-management phase, includes identifying the factors other than human health risk estimates that affect and are affected by the regulatory decision. These include the factors discussed above that EPA is required by certain statutes to consider, such as technologies and costs. The report does not, however, discuss the uncertainties in those factors and how any such uncertainties should affect EPA's decisions.

More recently, A Risk-Characterization Framework for Decision Making at the Food and Drug Administration (NRC, 2011) emphasized that “risk characterization should be decision-focused” and describe “potential effects of alternative decisions on health rather than on comparing different health and environmental hazards” (p. 21). The report discussed the factors that, in addition to human health risks, are sometimes considered in the decisions of the Food and Drug Administration (FDA): social factors, political factors, and economic factors.18 Those factors often are not independent from the estimates of human health risks. The report briefly discussed uncertainty, focusing on the uncertainty in the characterization of risk, not the uncertainty in the other factors that play a role in FDA's decision making.

Over the past several decades EPA's science advisory boards (SABs) have also addressed the importance of considering uncertainties in risk assessment and decision making. For example, the SABs have emphasized its importance in radiological assessments (EPA, 1993, 1999b), the CAA (EPA, 2007), microbial risk assessments (EPA, 2010b), expert elicitation (EPA, 2010b), and a comparative risk framework methodology (EPA, 1999a).

These reports have contributed greatly to the science of risk assessment, uncertainty analysis, and environmental regulatory decision making. Although a number of those reports discuss the factors beyond the estimates of health risks that play a role in regulatory decision making, when discussing the analysis and implications of uncertainty on decision making they focus on the uncertainty in the estimates of human health risks. The reports typically do not discuss the uncertainty inherent in the other factors that are considered in regulatory decisions. This report broadens the discussion of uncertainty to include the uncertainty in factors in addition to human health risk assessments.