The committee identified aspects of uncertainty and variability likely to have important scientific and policy implications for the potential health effects attributable to waste incineration. One overarching issue is how uncertainty and variability can influence the utility of estimates of health effects of waste incineration or of alternative technologies for waste management. As uncertainty and variability become larger, it becomes more difficult for interested or affected parties to decide how to interpret results and assign relevance to the magnitude of estimated risk. If the range is too large, different people might base their interpretation of the results on their prior opinion of waste incineration. That is, those who favor use of incineration technology might tend to focus on results in the middle range (for example, the median or mean of either variability or uncertainty distributions) of postulated effects. Those who oppose the technology might tend to focus on any results that suggest harmful effects (for example, the upper 5-10% of the possible range of outcomes). When the uncertainties or variabilities are large, there can be a large difference between those two parts of the range of possible outcomes.

To give some perspective on how uncertain and variable information can influence the characterization of health impacts, Figure 8-1 provides a schematic of the major components that must be characterized to assess possible health effects. Listed next to each component are the major types of information or models needed to map the output from one logical stage of this system into the next. Two important issues are evident. First, there are large variations in the precision and accuracy2 of the information needed to characterize the sequence of steps as listed on the left of the figure. Second, this process is open, so that each component is not solely influenced by the previous one. For example, the concentration of dioxin congeners in the atmosphere near an incinerator is not linked solely to emissions from the incinerator, but may also be attributable to other sources in and out of the region. And a health effect might be connected to the facility, but understanding the etiology of any disease requires consideration of a variety of potential factors. As has been pointed out by Oreskes et al. (1994) such open-ended systems models—which are common in earth sciences, economics, engineering, and policy-making—cannot be fully verified or validated, because the operative processes are always incomplete. Nevertheless, such models can be confirmed and can be used to put bounds on the likely range of outcomes; in this sense, they offer something of value to the policy-making process.

The following five factors determine the reliability of a health-risk assessment: specification of the problem (scenario development), formulation of the


Precision refers to the agreement among individual measurements of the same property of the sample. Accuracy refers to the agreement of a measurement (or an average of measurements of the same property) with an accepted reference or true value.

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