was to develop and use five attributes that contribute to the likelihood that a particular PCCL contaminant or group of related contaminants could occur in drinking water at levels and frequencies that pose a public health risk. In this regard, the committee devised a scoring system for each of these five attributes whereby the highest-priority PCCL contaminants are selected in conjunction with expert judgment for inclusion on a CCL. The five attributes are divided into health effect and occurrence categories. For health effects, the committee identified severity and potency as key predictive attributes. Prevalence, magnitude, and persistence-mobility comprise the occurrence attributes.

It is important to note that the committee spent a great deal of time deliberating on the number and type of contaminant attributes that should be used in the prototype classification algorithm approach recommended for use (in conjunction with expert judgment) in the development of future CCLs. Ultimately, the committee decided that these five contaminant attributes constitute a reasonable starting point for the U.S. Environmental Protection Agency (EPA) to consider, especially since they were subsequently found to aptly demonstrate the utility of the recommended CCL development approach (see Chapter 5). Furthermore, the metrics and related considerations presented in this chapter for scoring each attribute should similarly be viewed as illustrative. Thus, the committee does not explicitly or implicitly recommend these five (or necessarily a total of five) attributes or the related scoring metrics as being ideally suited for direct adoption and use by EPA. Rather, should EPA choose to adopt a classification approach for the development of future CCLs, the committee recommends that options for developing and scoring contaminant attributes should be made available for public and other stakeholder input and undergo scientific review.

Severity

The question of severity may be stated simply as, How bad is the health effect? In terms of this report, severity can be scored using the most sensitive health end point for a particular contaminant (e.g., the health effect that occurs at the lowest dose compared to other health effects reportedly caused by the contaminant) and considering vulnerable subpopulations (see Chapter 2). In other words, for the most sensitive health end point, what is the anticipated clinical magnitude in affected individuals? The committee recommends that the assessment of severity



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