Dioxins and Dioxin-like Compounds

Dioxins And Dioxin-like Compounds

Dioxins and dioxin-like compounds (DLC) are by-products of various industrial processes, and are commonly regarded as highly toxic compounds that are environmental pollutants and persistent organic pollutants (POPs). They include:

• Polychlorinated dibenzo-p-dioxins (PCDDs), or simply (but inaccurately), dioxins. Technically PCDDs are derivatives of dibenzo-p-dioxin. There are 75 PCDDs, and seven of them are specifically toxic.
• Polychlorinated dibenzofurans (PCDFs), or simply furans. Technically PCDFs are derivatives of dibenzofuran. There are 135 congeners (derivatives differing only in the number and location of chlorine atoms). Whilst they strictly speaking are not dioxins, ten of them have "dioxin-like" properties.
• Polychlorinated biphenyls (PCBs), which also are not dioxins, but twelve of them have "dioxin-like" properties. Under certain conditions PCBs may form dibenzofurans through partial oxidation.
• Finally, dioxin may refer to dioxin proper, the basic chemical unit of the more complex dioxins. This simple compound is not persistent and has no PCDD-like toxicity.

• Polychlorinated dibenzo-p-dioxins

• Polychlorinated dibenzofurans

• Polychlorinated biphenyls

• 1,4-dioxin

Because dioxins refer to such a broad class of compounds that vary widely in toxicity, the concept of toxic equivalence (TEQ) has been developed to facilitate risk assessment and regulatory control. Toxic equivalence factors (TEFs) exist for seven congeners of dioxins, ten furans and twelve PCBs. The reference congener is the most toxic dioxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) which per definition has a TEF of one.

In reference to their importance as environmental toxicants the term dioxins is used almost exclusively to refer to the sum of compounds (as TEQ) from the above groups which demonstrate the same specific toxic mode of action associated with TCDD. These include 17 PCDD/Fs and 12 PCBs. Incidents of contamination with PCBs are also often reported as dioxin contamination incidents since it is this toxic characteristic which is of most public and regulatory concern.