Analyses of Laboratory and Field Studies of Reproductive Toxicity in Birds Exposed to Dioxin-Like Compounds for Use in Ecological Risk Assessment
This report is intended to assist ecological risk assessors who must characterize risks to birds from exposure to dioxin-like chemicals. Those chemicals include the halogenated dibenzo-dioxins, dibenzo-furans, and biphenyls that have the same mode of action as 2,3,7,8-tetracholorodibenzo-p-dioxin. In particular, they include the coplanar PCBs, which account for most of the toxicity of PCB mixtures. They have been shown to severely affect birds in contaminated sites and regions by causing mortality, deformity and inhibited development of embryos and hatchlings.Effects of dioxin-like chemicals in the field may be assessed in multiple ways. The most accurate way is to perform tests of the mixture that occurs in the field. For example, one may collect contaminated fish from the contaminated site and feed them to birds or extract the contaminants and inject them into eggs. However, that approach is costly and time consuming. An alternative, where PCBs are the contaminants of concern, is to use toxicity data for the commercial PCB mixtures. However, the PCBs found in food items in the field are quite different from the original commercial mixtures. The last approach, presented here, is to measure or estimate the concentrations of individual congeners and relate them to appropriate toxicity data. This approach is made possible by the ability to convert the toxicity of all dioxin-like chemicals to common toxic equivalent concentrations (TEQ) and then adding the TEQ values to estimate the exposure to the mixture as an equivalent concentration of 2,3,7,8-tetracholorodibenzo-p-dioxin. This approach has its own uncertainties, but it has the advantage of allowing assessment of diverse dioxin-like chemical mixtures without testing.The exposure metric used in this report is :g/kg of egg as TEQ. The laboratory data are based on egg injections and the field data are based on measured egg concentrations. Most of the laboratory data are for domestic chickens, but ten other species of birds have also been tested. Chickens are the most sensitive avian species tested, but their sensitivity does not appear to be aberrant relative to other sensitive species.Multiple approaches are considered for estimating risks to a particular bird species or community. Common methods include using the most sensitive species to represent all species, using a similar species, or using the most sensitive species with an uncertainty factor. These approaches use only one effects datum, so the other available information is lost. The species sensitivity distribution (SSD) approach uses the distribution of effects concentrations for all species. Hence, just as conventional dose-response curves can be used to estimate the probability of effects for an individual human, the SSD can be used to estimate the probability of effects on a species. However, for these chemicals, effects levels for the most sensitive species are approximately equal to the 5-10% levels of the SSD which are commonly used as benchmark values. Hence, the methods are concordant for dioxin-like effects on birds.The TEQ concentrations in eggs in the field that induced death or developmental defects were generally lower than the corresponding laboratory values. The effects levels for chickens and the low end of the laboratory SSDs correspond to effects on 25-50% of species in the field. The difference is believed to be due to effects of non-dioxin-like co-contaminants in the field. However, other factors such as parental behavior may also be involved.Since death or developmental defects in embryos or hatchlings are the critical effects of dioxin-like chemicals in birds, the results presented in this report are believed to be useful for screening assessments. The screening benchmark for an assessment may be chosen from values presented here based on the assessment endpoints and the preferences of the assessors and risk managers. Use of these values for more definitive assessments must be based on the expertise of an assessor who is knowledgeable concerning the effects of these chemicals on birds. When practical, tests of site-specific mixtures should be conducted to provide a more accurate characterization of risk.