Major Gaps in Knowledge

While scientific interest regarding the ecological effects of dioxins stems from interest in human heath, limited progress has been made in assessing dioxin risks to animals and plants. In fact, current information is insufficient to provide a thorough description of dioxin risks to wildlife. Consequently, the few available environmental quality guidelines for dioxins are based on limited ecotoxicity data. While the most sensitive and ecologically important endpoints for mammals and birds are associated with reproduction, there is a lack of reproduction bioassays and toxicokinetic information for most taxa and species to establish well-defined dose-response relationships.

Furthermore, it is generally acknowledged that assessing the probability of an individual organism experiencing harm (e.g., reproductive impairment or mortality) is not useful for wildlife assessment; risk is more appropriately assessed at the population, rather than the individual, level. Population endpoints, however, tend to be difficult to assess, requiring the use of dynamic population models covering effects on survival, breeding success, and immigration. In general, well-validated population models do not yet exist for the majority of species, and it is difficult to estimate the extent of mortality or reproductive failure that could be incurred. Since population models are rarely available, it is more common for wildlife risk assessments to define the 'no observed adverse effect level' (NOAEL) for endpoints such as mortality or reproductive effects in individuals, and to assume that these may be used to set levels that will protect the whole population. Still, few NOAEL values for the dioxins are available, and those that are available are associated with high degrees of uncertainty.

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