Scientific Review of the Proposed Risk Assessment Bulletin from the Office of Management and Budget (2007) / Chapter Skim
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4 Standards for Risk Assessment
4 Standards for Risk Assessment
Pages 35-77
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From page 35... ...
In defining special standards for influential risk assessments, OMB appropriately recognizes that risk assessments that have potentially greater impact should be more detailed, be better supported by data and analyses, and receive a greater degree of scrutiny and critical review than risk assessments likely to have smaller impacts. However, proposing different standards for general and influential risk assessments is problem 35
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Other issues associated with the standards are discussed in the remainder of this chapter. RANGE OF RISK ESTIMATES AND CENTRAL ESTIMATES One focus in the bulletin is the presentation of a range of risk estimates and a central estimate; statements on this topic in the bulletin and 1 See Appendix E, pp.
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From page 37... ...
reports have made relevant comments on this and related topics; selected comments from those reports are provided in Table 4-2. The committee agrees with OMB that in some cases "presentation of single estimates of risk is misleading" and that ranges of "plausible risk" should be presented; however, the challenge is in the operational definitions of such words as central, expected, and plausible.
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. Risk assessment for regu- A risk assessment should include "whenever The supplementary information expands on issues related to risk latory analysis possible, a range of plausible risk estimates, assessments used for regulatory analysis.
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If model uncertainty is present, this central model could reflect a weighted average of risk estimates from alternative models or some synthesis of "probability assessments supplied by qualified experts" (OMB 2006, p.
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and estimate or by a range of possibilities around the point estimate. But estimates that include a range of risk estimates wide range of uncertainties can imply that a disastrous consequence is ‘possible,' even when expert opinion is unanimous that the likelihood of disaster is extremely small.
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For example, would substituting the 2.5th percentile and 97.5th percentile from the exposure distribution in the dose-response model yield a plausible range? Without more guidance and operational definitions of terms, the bulletin's guidance on central estimates and plausible risk ranges is unclear.
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Risk communication is hampered by the use of vague or meaningless terms. For example, there is no such thing as the "average person." Is the average person male or female?
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UNCERTAINTY This section assesses the extent to which the proposed bulletin achieves its goal of enhancing technical quality and objectivity with respect to the treatment of uncertainty. Understanding the current state of best practice is a precondition for improving that practice.
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On the basis of an earlier U.S. Nuclear Regulatory Commission document, a PRA procedures guide was published in 2002; it included a chapter on uncertainty analysis (NASA 2002)
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Nuclear Regulatory Commission created an independent group of experts to review its "achievements and limitations." The report of that review group (Lewis et al.
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The Nuclear Regulatory Commission released the Fault Tree Handbook in 1981 (Vesely et al.
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. Current Good Practice in the Evaluation of Uncertainty in Risk Analysis Risk analysis typically involves substantial uncertainties, and the quantification of uncertainty has become an integral part of PRA.
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48 Review of the Proposed OMB Risk Assessment Bulletin BOX 4-1 Principles and Objectives of a PRA Standard (U.S.
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The U.S. Nuclear Regulatory Commission and EU studies applied performance-based weighting in addition to equal weighting, leaving the choice to the discretion of the problem-owner (Harper et al.
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. Uncertainty Addressed in Previous National Research Council Reports The standards and related discussion in the proposed OMB bulletin and supplementary information on uncertainty echo previous NRC reports.
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quantify the uncertainty in risk predictions." NRC (1994, Table 9-1) provided a taxonomy of uncertainty in risk analysis that emphasized parameter-value uncertainty and model uncertainty.
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Such a quantitative distribution of uncertainty can often be produced, but the numerical values of the uncertainty distribution may not be highly accurate. The qualifying "where feasible" is too vague to serve as technical guidance.
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That problem is a key limitation to the bulletin's call for model uncertainty analysis. The committee notes that the selection of the models considered for any averaging process should reflect candidate models that are plausible.
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Technical guidance is premature, but the issue of dependence modeling should be acknowledged and targeted for research. The ability to quantify and propagate uncertainty is still in development.
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Recent efforts have been made to identify measurable adverse effects or biologic changes that occur at a point in which they are minor, reversible, or subclinical and that do not 4 The proposed definition of adverse effect generally follows the approach of EPA. However, the distinction is that previous EPA guidance on this matter has been relatively flexible and could be adjusted or changed as science advanced.
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From page 56... ...
Adverse effects usually develop along a continuum, starting with uptake of a toxicant, distribution and metabolism, contact with a target organ, biologic change, physiologic response and repair, and clinical disease. Thus, with some doses and hosts, biologic changes occur, but the body has sufficient defense mechanisms for detoxification or adaptation, and there is little or no adverse cumulative effect, particularly at a low dose.
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Microbial risk assessment often focuses on the risk of infection rather than directly on the manifestation of adverse effects. Infection (replication of an organism in a host)
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From page 58... ...
OMB responded that although the bulletin does not speak to specific use of precursor effects, it does not preclude the use of a precursor of an adverse effect or other mechanistic data as the basis of a risk assessment. The committee nevertheless concludes that the bulletin's focus on the choice of an adverse effect and the description of what is and is not an adverse effect give a strong message for what would be considered acceptable and nonacceptable end points for toxicologic risk assessment.
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Readers who consult the risk communication literature will find that serving either purpose requires both formal analysis to ensure that defensible comparisons are being made and dedicated empirical research to ensure that the result is understood as intended. Readers of that literature will also find that poorly done risk comparisons can confuse, mislead, and antagonize recipients.
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. The second conceivable use of risk comparisons is to facilitate making consistent decisions regarding different risks.
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Unless such comparisons are developed in a scientifically sound and empirically evaluated way that addresses the values and circumstances of all recipients, risk comparisons should not be made. SUMMARY OF COMMITTEE COMMENTS ON INDIVIDUAL STANDARDS The proposed bulletin describes 16 standards for risk assessment; they are listed in Table 4-3.
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quantitative characterization of risk is provided, a The term plausible risk estimate is undefined in the bulletin. If a distribution is substanrange of plausible risk estimates shall be provided.
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. Component 4.a could, however, degrade risk analysis if it were nor exaggerating the nature and magnitude of interpreted so as to deprive decision-makers of important information on sensitive sub risks; populations on the grounds that such information may generate risk estimates considera b.
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Instead, risk communication is an text/perspective with other risks familiar to the essential element of all stages of risk assessment. Unless the risk communication occurs at target audience.
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Although it is important that there be communication between risk measures being considered, and assess, to the assessor and risk manager, risk management tasks and decisions should not be delegated extent feasible, countervailing risks caused by to the risk assessment. Furthermore, although risk assessors may evaluate various risk alternative mitigation measures; management options (for example, mitigation options for a Superfund site)
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As discussed in the section mates, when a quantitative characterization of on "Range of Risk Estimates and Central Estimates," the presentation of single estimates risk is made available. may provide an incomplete picture, and without proper definitions and context, use of the range or "central estimate" will be misleading.
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time scales, and the definition of adverse effects. However, the bulletin fails to define which comparisons are required.
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The bulletin does not constitute "technical guidance" and hence cannot "enhance the technical quality…of risk assessments" (OMB 2006, p.
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issue a "response-to-comment" document that is preferable to positions proposed by commenters is likely to expend excessive resources summarizes the significant comments received and might result in less time to devote to agency risk assessments, thus affecting the qual and the agency's responses to those comments; ity and output of agency products.
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2001. Probabilistic Accident Consequence Uncertainty Assessment Using COSYMA: Uncertainty from the Food Chain Module.
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1997. Probabilistic Accident Conse quence Uncertainty Study: Uncertainty Assessment for Deposited Ma terial and External Doses, Vols.
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1997. Probabilistic Accident Consequence Uncertainty Study: Early Health Effects Uncertainty Assessment, Vols.
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2001d. Probabilistic Accident Consequence Uncertainty Assessment Using COSYMA: Uncertainty from the Early and Late Health Effects Module.
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1997. Probabilistic Accident Consequence Un certainty Analysis: Late Health Effects Uncertainty Assessment, Vols.
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1996a. Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Vol.
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1979. Nuclear Regulatory Commission Issues Policy Statement on Reactor Safety Study and Re view by Lewis Panel: NRC Statement on Risk Assessment and the Re actor Safety Study Report (WASH-1400)
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Standards for Risk Assessment 77 Wittenberg, E., S.J. Goldie, B
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