Dioxins and Related Compounds

The term “dioxins” refers to a group of environmental pollutants that are often produced as byproducts of industrial or combustion processes.

Dioxins are produced naturally as a byproduct of calamities like forest fires and volcanoes, though their production through these sources is relatively negligible.  Industrial activities as well as uncontrolled combustion like burning of household waste contribute to a large extent to the total dioxins in the environment.

The World Health Organization has listed out 7 polychlorinated dibenzo-p-dioxins (PCDDs), 10 polychlorinated dibenzofurans (PCDFs), and 12 polychlorinated biphenyls (PCBs) as dioxins or dioxin-like compounds. 

History has recorded a number of events of dioxin contamination, right from the late 1800s. These include the health effects due to the use of Agent Orange, the explosion of the chemical plant in Seveso, Italy and the alleged poisoning of the then Presidential candidate and later President of Ukraine, Viktor Yushchenko.

A person is usually exposed to dioxins through intake of contaminated food.  When dioxins enter the body, they remain in the fatty tissues over prolonged periods of time.  Dioxins exert their effects through activation of a particular receptor on body cells called the Aryl hydrocarbon Receptor (AhR).  

The toxic effects of the various dioxins and dioxin-like compounds vary. Thus, to compare their toxicity, a ranking called the Toxic Equivalency Factor (TEF) is assigned to each compound. The ranking compares the toxicity of the particular compound to that of TCDD – or 2,3,7,8-tetrachlorodibenzo-p-dioxin, which is regarded as the most toxic dioxin. TEF is calculated as the ratio of the half the maximal effective dose (ED50) for TCDD to the ED50 for the dioxin or dioxin-like compound of interest. 

The TEF of TCDD is 1 and for other compounds is consequently less than 1.  Another compound, 1,2,3,7,8-pentachlorodibenzo-p-dioxin (1,2,3,7,8-PeCDD) has a TEF same as that of TCDD.

If multiple dioxins and dioxin-like compounds are present in a mixture, then the total toxic equivalency (TEQ) for the mixture is calculated. TEQ is calculated by multiplying the amount of each compound present by its TEF and adding the products together.