Laboratory studies examining the toxicity of dioxins are available for only a limited number of species and very little data are available for wildlife. Laboratory studies show that dioxins are AhR agonists, which cause a wide spectrum of adverse effects in many vertebrate species, with embryos, fetus, and newborn being especially vulnerable to exposure during gestation and lactation. Dioxins are particularly potent developmental toxicants at low concentrations and can disrupt the development of the endocrine, reproductive, immune, and nervous system of the offspring of fish, birds, and mammals when exposed from conception through postnatal or post hatching stages. Outside the laboratory, it has not been possible to demonstrate a clear cause/effect relationship between biological response and the exposure to dioxins.
Mammals, birds, and fish vary among species in their sensitivity to dioxins. It has been suggested that, on average, humans are among the more dioxin-resistant species, but the human data are too limited to be conclusive. The most sensitive fish species are the salmonids, while the most sensitive avian species belong to the order of galliformes. The most sensitive mammalian species tested so far are the mink and the guinea pig. Aquatic algae and plants, invertebrates, and amphibians are much more tolerant of dioxins than fish, birds, and mammals. The observed lack of sensitivity of certain taxonomic groups is consistent with the view that the AhR is not present in these organisms.
The organisms at the top of the food chain, such as raptors and dolphins, generally accumulate relatively higher levels of dioxins as compared to lower-trophic-level organisms. However, localized dioxin hot spots cannot be ruled out as a source for dioxins in some individual animals with high levels, particularly those collected in urban/ industrial environments. This is because dioxins are highly persistent in soil and sediment, and may still be present at high levels in some areas even though regulatory initiatives are in place to limit releases of dioxins to the environment.
Most existing contaminated sites are the result of historical rather than current emissions, arising from a time when there were fewer environmental controls.
Future work on dioxin ecotoxicology needs to take into account the effects of bioaccumulation of dioxins from the physical environment and from food. As with general toxicity, bioaccumulation appears to be low for congeners with fewer than four chlorine atoms or greater than six, because of rapid metabolism or elimination of sparingly chlorinated forms and poor bioavailability or cell membrane permeability of highly chlorinated forms. Similarly, only congeners with four to six chlorine atoms appear to biomagnify up the food chain.
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