5. To date, PBT risk assessment methods for wildlife and humans have been based on 
assessment of dietary exposures involving concentrations in aquatic food organisms, principally 
fish and shellfish. Although this results in common BAFs/BSAFs for aquatic life and wildlife, 
BAFs/BSAFs specific for wildlife species are measurable and could be modeled to reduce 
uncertainties associated with the present dietary exposure methodology. 
6. Determination of sensitive species, critical end points for population sustainability, and 
residue-based toxicology data are largely generic considerations which can be assembled on a 
national basis. However, bioaccumulation, exposure conditions, and chemical mass balance are 
more site-specific considerations. 
7. The conceptual model applies to deterministic or probabilistic analyses. 
8. Toxicity risks are linked to changes in chemical loadings or remedial actions and vice versa, 
so that tiered assessments are possible with choice, in the problem formulation, of steady state as 
either a specific assessment condition or as a reference condition for time-dependent risk 
assessment. 
9. Spatially explicit risk assessments require spatially explicit exposure and bioaccumulation 
analysis with feedback (right to left) to population model responses through the residue based 
toxicology model (Figure 11). Spatially explicit risk analysis for PBTs can be conceptualized, in 
accordance with Figure 11, as multiple fish or wildlife tracks based on different exposure levels, 
a common toxicity model, and the potential for different population level responses (assuming 
discrete populations, or meta-populations exposed within the assessment region). 
10. The third dimension in the conceptual model (stacked boxes) may be viewed as the multi¬ 
stressor component, particularly for chemical mixtures. Each chemical has to satisfy the risk 
assessment requirements in a parallel manner with a joint toxicity model (such as toxicity 
equivalence) linking each chemical’s contribution to the net exposure. 
Research Needs 
For this discussion, the five different paths in Figure 11 (box-arrow-box components) that 
involve the generation and use of data to accomplish components of risk assessments will be 
examined for research needs. Separate but parallel chains of the five paths emerge for aquatic 
and wildlife species from the conceptual model for PBTs. The five components are: 1) the 
relationships between chemical loadings to the ecosystem and exposures via food, water, and 
sediment; 2) the relationships between concentrations in food, water, and sediment to 
concentrations in tissues of organisms at risk; 3) the relationships between concentrations in 
organisms and incidence of effects; 4) the relationships between incidence of effects and 
population changes; and 5) the relationships between specific population changes and community 
structure and function. The chemical loading, fate and transport, and exposure path (mass 
balance model) is not a direct NHEERL research concern but must be consideral as a reference 
point for integration of effects research with exposure research. 
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