Dioxins and dioxin-like compounds

Dioxins and dioxin-like compounds (DLCs) are a group of chemical compounds that are persistent organic pollutants (POPs) in the environment. They are mostly by-products of burning or various industrial processes or, in the case of dioxin-like PCBs and PBBs, unwanted minor components of intentionally-produced mixtures.

Some of them are highly toxic, but the toxicity among them varies 30,000-fold. They are grouped together because their mechanisms of action are the same. They activate the aryl hydrocarbon receptor (AH receptor), albeit with very different binding affinities, leading to high differences in toxicity and other effects. They include:

Polychlorinated dibenzo-p-dioxins (PCDDs), often referred to simply as dioxins. PCDDs are derivatives of dibenzo-p-dioxin. There are 75 PCDD congeners, differing in the number and locations of chlorine atoms, and 7 of them are specifically toxic, the most toxic being 2,3,7,8-tetrachlorodibenzodioxin (TCDD).
Polychlorinated dibenzofurans (PCDFs), often referred to as furans. PCDFs are derivatives of dibenzofuran. There are 135 isomers; 10 have dioxin-like properties.
Polychlorinated biphenyls (PCBs), derived from biphenyl. 12 PCBs have dioxin-like properties. Under certain conditions, PCBs may form dibenzofurans through partial oxidation.
Polybrominated analogs of the above classes may have similar effects.
"Dioxin" can also refer to 1,4-dioxin or p-dioxin, the basic chemical unit of the more complex dioxins. This simple compound is not persistent and has no PCDD-like toxicity.

Dioxins have different toxicities depending on the number and positions of the chlorine atoms. Because dioxins refers to such a broad class of compounds that vary widely in toxicity, the concept of toxic equivalency factor (TEF) has been developed to facilitate risk assessment and regulatory control. TEFs exist for seven congeners of dioxins, ten furans, and twelve PCBs. The reference congener is the most toxic dioxin (TCDD), which per definition has a TEF of one. In essence, multiplying the amount of a particular congener with its TEF produces the amount toxicologically equivalent to TCDD, and after this conversion, all dioxin-like congeners can be summed up, and the resulting toxicity equivalent quantity (TEQ) gives an approximation of toxicity of the mixture measured as TCDD.

Dioxins are virtually insoluble in water but have a relatively high solubility in lipids. Therefore, they tend to associate with organic matter such as plankton, plant leaves, and animal fat. In addition, they tend to be adsorbed to inorganic particles, such as ash and soil.

Dioxins are extremely stable and consequently tend to bioaccumulate in the food chain. They are eliminated very slowly in animals; for example, TCDD has a half-life of 7 to 9 years in humans. Incidents of contamination with PCBs are often reported as dioxin contamination incidents, since these are of most public and regulatory concern.