Estimating the Public Health Benefits of Proposed Air Pollution Regulations (2002) / Chapter Skim
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5 Uncertainty
5 Uncertainty
Pages 126-152
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From page 126... ...
A third barrier is that projected health benefits are often reported as absolute numbers of avoided death or adverse health outcomes without a context of population size or total numbers of outcomes. Areas of uncertainty include air-quality mo(leling, population demographics and heterogeneity, intersubject variability, health and exposure baselines, compliance with control measures, effectiveness of controls in reducing pollutant emissions, validity andprecision of concentration-response functions and use of alternative models (linear, nonlinear)
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From page 127... ...
The chapter outlines a revised approach that would reflect overall uncertainty more realistically, in part by using probabilistic expressions of expert judgment. The chapter also briefly reviews the history of probabilistic uncertainty assessment in EPA health benefits analyses under the Clean Air Act.
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From page 128... ...
EPA reports each numerical health benefit estimate in the form of a probability distribution and summarizes the distribution by reporting its mean and 5th and 95th percentiles. The distribution assigns a nonzero probability to every possible value including the null hypothesis of no benefit.
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From page 129... ...
The incorporation of additional sources of uncertainty would widen the distribution. EPA correctly notes that incorporating only the uncertainty from random sampling error in concentration-response function estimates into its primary health benefits analyses "omits important sources of uncertainty, such as the contribution of air quality changes, baseline population incidences, projected populations exposed, transferability ofthe concentration
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From page 130... ...
3-26~. Ancillary Uncertainty Analyses EPA assesses all other uncertainties in a second part of each health benefits analysis.
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From page 131... ...
Current baseline incidence rates might not approximate baseline incidence rates in the year 2030. Projected population and current demographics used to derive incidences might not approximate future-year populations and demographics.
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From page 132... ...
For both calculations and analyses, assumptions or sources of uncertain quantities are varied and the mean ofthe health benefit probability distribution is recomputed. In all cases, the alternative calculations and sensitivity analyses are conducted for only one source of uncertainty at a time.
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From page 133... ...
EPA's decision to incorporate only one source of uncertainty, the random sampling error in the estimated concentration-response function, into the probability distributions resulting from its health benefits analyses is worth reconsidering. The committee agrees with the agency's judgment that its current practice produces health benefits probability distributions that give "a misleading picture about the overall uncertainty in the estimates" (EPA 1 999a, p.
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From page 134... ...
3-26~. The committee agrees with EPA's statement that it would require expertjudgmentto specifyprobability distributions for many ofthe uncertain components of the health benefits analyses.
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From page 135... ...
Incorporation ofthose uncertainties as probability distributions into the primary analysis would likely change the expected value and widen the resulting probability distribution for each health benefit. The result will include more of the uncertainty in the health benefits assessment.
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From page 136... ...
In a probabilistic uncertainty analysis, EPA could assign appropriate weights to various threshold models. For PM-related mortality in the Tier 2 analysis, the committee expects that this approach would have resulted in only a slight widening of the probability distribution for avoided mortality ant} a slight reduction in the mean ofthat distribution, thus reflecting EPA's views about the implausibility ofthresholds.
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Here a probabilistic weighting of alternative lag structures based on expert judgment might have led to a more appreciable widening of the health benefit probability distribution. Although EPA considered!
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From page 138... ...
. These assessments use the same methods as those the agency uses to combine health benefits probability distributions with probability distributions for valuation ofthose benefits as was done for the prospective analysis ofthe 1990 Clean Air Act Amendments (EPA 1 999b)
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From page 139... ...
Adding further sources of uncertainty to the analysis might cause the mean to rise or fall but would further widen the probability distribution. In 1994, the National Research Council recommended that EPA conduct formal uncertainty analyses, including probabilistic assessment of uncertainties that "cannot be quantified on the basis of data" (NRC 1994, p.
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"Evaluation of the probability distributions of the 22 influential parameters of the model was performed on the basis of available statistical data,
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From page 141... ...
Almost all expressed concern that most risk assessments and health benefits analyses tend to underestimate uncertainties and leave decision-makers with a false sense of confidence in estimates of risk. In addition, most of the reports suggested that failure to address model uncertainty adequately is often a major issue.
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From page 142... ...
/ 5,'''"''j ,7. J _ J ,: -0.2 0.0 0.2 0.4 0.6 O.8 1.0 Percent Increase in Mortality per 10 p9/m3 PM'o FIGURE 5-2 Posterior probability distributions for estimated short-term mortality effects of PM~o with and without adjustment for copollutants.
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From page 143... ...
The committee recommends that EPA begin to incorporate additional sources of uncertainty into the probability models it uses in its primary health benefits analyses. Furthermore, the committee recognizes that decisionmakers will need to be informed about how and why uncertainty was added to the health benefits analysis and how, in turn, this uncertainty might be communicated to the public This process will use probability distributions to replace model components that are treated as known fixed values.
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From page 144... ...
EPA should also consider conducting analyses to determine the sensitivity ofthe final results to the specification of reasonable alternative probability distributions for the uncertainty sources in the primary analyses. The need for sensitivity analyses will be particularly great for distributions that are based solely or largely on expert judgment.
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From page 145... ...
Graphs will be especially helpful as the incorporation of additional uncertainties results in asymmetrical health benef~t probability distributions (Read and Morgan 1998~. In presenting a probability distribution for each health benefit produced by a primary analysis, EPA should emphasize even more than it has in the past the sources of uncertainty that remain unaccounted for in the primary analysis.
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From page 146... ...
CONCLUSIONS · In its primary analyses of health benefits, EPA reports the uncertainty as a probability distribution. Only one source of uncertainty, the random sampling variability ofthe estimated concentration-response function, is given with an emphasis on the mean ofthe probability distribution.
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From page 147... ...
· As EPA begins the transition to incorporate additional sources of uncertainty into its primary health benefits analyses, it should continue the sensitivity analyses it has traditionally conducted. These analyses should tee expanded, however, to consider sources of uncertainty jointly rather than singly.
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From page 148... ...
As a result, primary health benefits analyses that more fully and accurately portray the uncertainties might not be considered useful. It is unrealistic for EPA to defer decisions until it can make them on the basis of perfect science.
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From page 149... ...
1 999a. Final Tier2 Rule: Air Quality Estimation, Selected Health and Welfare Benefits Methods, and Benefit Analysis Results.
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From page 150... ...
1995. Environmental and Health Benefits of Cleaner Vehicles and Fuels.
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1981. A framework for assessment of health risks associated with national ambient air quality standards.
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From page 152... ...
1994. Uncertainty analysis of health risk estimates, Appendix F
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