Dioxins and dioxin-like compounds: toxicity in humans and animals, sources, and behaviour in the environment

Dioxins and dioxin-like compounds comprise a group of chemicals including polychlorinated dibenzo -p- dioxins (PCDD) and polychlorinated dibenzofurans (PCDF), as well as certain dioxin-like polychlorinated biphenyls (dl-PCB), and potentially others. They act via a common mechanism, stimulation of aryl hydrocarbon receptor (AH receptor, AHR), a vital transcription factor in cells. There are very high differences in potency among these compounds, i.e. in the ability to stimulate the receptor. This leads to ten thousand-fold or higher differences in doses causing similar toxic effects. Most of these compounds are eliminated very slowly in the environment, animals, or humans, which makes them persistent. They are much more soluble in fat than in water, and therefore they tend to accumulate in lipid or fatty tissues, and concentrate along the food web (bioaccumulation and biomagnifica-tion). PCDD/PCDFs are formed mostly as side products in burning processes, but PCBs were oils manufactured for many purposes. Because of toxicity and persistence, dioxin-like compounds have been regulated strictly since 1980s, and their levels in the environment and animals have decreased by an order of magnitude or more. Therefore the effects on wildlife have clearly decreased, and even populations at the top of the food web such as fish-eating birds or seals have recovered after serious effects on their reproductive capacity and developmental effects in their young especially in 1970s and 1980s. This does not exclude the possibility of some remaining effects. In humans the intake is mostly from food of animal sources, but because our diet is much more diverse than that of such hallmark animals as white-tailed eagles or seals, the concentrations never increased to similar levels. However, during 1970s and 1980s effects were probably also seen in humans, including developmental effects in teeth, sexual organs, and the