Dosimetry and Bioaccumulation 
Two gaps relate to the development of PBTK models for improving understanding of dose- 
residue-response relationships and extrapolation among species or life stages: 
1. Although the mercury-loon project (B3) will develop a PBTK/TD model in kestrels for use in 
estimating the toxicity of mercury to piscivorous birds, the toxicokinetics of mercury are not 
representative of other PBTs. This will limit the general q^plicability of these models to other 
chemicals. 
2. Another gap for developing avian PBTK models is the paucity of information on avian 
physiology and metabolism for setting parameters in the models. 
Several gaps relate to improvements needed in bioaccumulation information: 
3. A method for determination of rates of metabolism for PBTs is needed in order to allow 
accurate predictions of bioaccumulation with aquatic food chain models. If the research 
proposed to demonstrate the feasibility of determining rates of metabolism from field data fills 
this important risk assessment need, additional studies will be required to provide the metabolism 
data for all PBTs of concern in divCTse food webs. 
4. Existing BAFs, BSAFs, and food chain models are based on whole adult organisms and thus 
may not be sufficient when dose to ELSs and/or specific tissues must be evaluated. ELS 
dosimetry-based BAFs and PB-TK models are needed to fill this gap. The metabolism rate gap 
extends to bioaccumulation of PBTs like the PAHs in embryo-larval stages of fish with potential 
vulnerability to photo-induced toxicity. Proposed photo-induced PAH toxicity research provides 
a beginning for filling this large gap. Additional research will be required to establish a general 
ELS toxicity risk assessment capability equal to that available for juvenile or adult organisms 
exposed to a wide variety of chemicals. 
5. Very few bioaccumulation data sets are of sufficient quality to validate the uses of BAFs and 
BSAFs, especially when extrapolated across species and/or ecosystems. The intent of proposed 
NHEERL research is to maximize the capability for extrapolation of BAFs and BSAFs for PBTs. 
However, consistent data gathering efforts sponsored by Offices interested in the application of 
BAFs and BSAFs for criteria development and risk assessment are needed in order to provide 
measures of uncertainty involved in such extrapolations as well as bioaccumulation model 
predictions performed without calibration (site-specific measurement of BAFs and BSAFs). 
6. Comprehensive and toxicity hazard assessment compatible BAFs, BSAFs, and food chain 
models are needed to meet the requirements of joint action toxicity models such as for TCDD 
toxicity equivalence or photo-induced PAH toxicity. 
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