Toxicity 
Two gaps relate to the needs for toxicity data: 
1. Critical to the idea of "risk-based" criteria is the need to understand the relationship between 
stressor intensity and responses affecting survival and reproduction (i.e., dose-response 
relationships). Although acute toxicity tests for aquatic life and wildlife usually produce dose- 
response relationships, most chronic exposure test and reproduction tests are designed to estimate 
effects thresholds (e.g., no observed adverse effect levels [NOAELs]) rather than describe dose- 
response relationships. Although some fish reproduction tests have been conducted with 
sufficient number and spacing of concentrations that dose-response relationships could be 
estimated, almost no avian reproduction testing has been done to quantify dose-response 
relationships. This is a large data gap, and OW presently may have no mechanism for generation 
of new toxicity data. 
2. Because of their hydrophobicity, a majority of PBTs accumulate in the benthos of freshwater 
and marine systems. From benthic environments, PBTs transfer to higher tropic levels where 
adverse effects to wildlife may occur. Despite the acknowledged effects of PBTs at higher tropic 
levels, a gap exists in our knowledge of whether or not this class of chemicals also causes 
significantly adverse effects to benthic organisms, populations, and communities. 
Bioaccumulation data for benthic organisms from contaminated sites around the country indicate 
exposure is occurring but we are not certain of the effects. To insure benthic ecosystems and 
resources are fully protected this gap should be addressed. 
Other gaps relate to the extrapolability of toxicity data among species and endpoints: 
3. Compared to aquatic organisms, the database of chronic exposure tests of effects on avian and 
mammalian survival and reproduction is much more limited, with much of the testing of non¬ 
pesticide chemicals done without standardized procedures. Consequently, the database for 
making interspecies extrapolations for wildlife is insufficient to significantly improve methods 
based on comparative toxicity relationships among species. This research program will be 
adding little new data for improving species sensitivity relationships for wildlife. Also, previous 
analyses of avian toxicity databases demonstrated no relationship between acute and chronic 
toxicity measurements for birds. 
4. Virtually unexplored are the TKTD determinants for interspecies and inter-elfect 
extrapolations of potency ratios required for PBT mixture toxicity risk assessment using a toxic 
units model approach, such as the additive TCDD toxicity equivalence model. 
Several gaps relate to the need for improved toxicity models and databases: 
5. Although models for predicting effects from fluctuating exposures models are proposed for 
development for metals and ammonia, a gap exists in this capability for organic chemicals. 
6. Residue-based toxicity data bases need to be advanced and evaluated for applicability to 
aquatic ecological risk assessment requirements for PBTs. 
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