characterization. These sections are followed by a discussion of issues relevant to specific inorganic and organic chemicals, including toxicity assessment.

HAZARD ASSESSMENT AND CHEMICAL SELECTION

To date, EPA has conducted two rounds of assessments to identify chemicals to regulate in the Part 503 rule. Round 1 was conducted to identify an initial set of chemical pollutants to regulate, and Round 2 was conducted to identify additional pollutants for regulation. Standards for the Round 2 pollutants have not been established, but EPA is considering regulation of dioxins (a category of compounds that has 29 specific congeners of polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans, and coplanar polychlorinated biphenyls) for land application. Therefore, although evaluation of EPA’s dioxin risk assessments for biosolids is outside the scope of the committee’s charge, the committee believes that evaluating the selection of dioxins for regulation is within the charge.

Round 1 Pollutant Selection

EPA used a two-stage process to select its initial set of contaminants to regulate under the Part 503 rule. First, a list of chemicals was subjected to a hazard screening. Second, chemicals found to represent a potentially significant risk were subject to formal risk assessment.

In 1984, using available data on effects in humans, plants, domestic animals, wildlife, and aquatic organisms and frequency of chemical occurrence in biosolids, EPA identified 200 potential chemicals of concern in biosolids. A panel of scientific experts selected 50 chemicals of potential concern for evaluation by EPA. A screening process was then used to select 22 pollutants for potential regulation (Table 5–1). The process involved developing environmental profiles for each pollutant for which data were readily available on toxicity, occurrence, fate, and pathway-specific hazards. When relevant, aggregate cancer risks from exposure via several pathways were assessed. Risks posed by some of the pathways subsequently analyzed in the risk assessment were not used in the screening process (pathways 11–14, see Table 5–4 in summary of exposure pathways).

To determine whether a full risk assessment was warranted for a particular chemical via a specific exposure pathway, a hazard index was calculated for each contaminant and pathway that had sufficient data (EPA 1985). This index is the ratio of the estimated concentration of the pollutant in the envi-



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