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The term ‘dioxin’ refers to three groups of substances: polychlorinated dibenzo-p-dioxins, (PCDDs), polychlorinated dibenzofurans (PCDFs), and dioxin-like polychlorinated biphenyls  (dl-PCBs).  Figure 4 provides a schematic view where the black balls represent carbon atoms, the red oxygen, and the orange chlorine atoms (these can be substituted by other halogenated elements, like bromine, fluorine and iodine to form dioxins). The possible combinations with chlorine atoms (congeners) are 75 for dioxins (PCDDs), 135 for furans (PCDFs), and 217 PCBs congeners. Of these chlorinated congeners, 29 are found to be toxic and therefore regulated in EU; 7 PCDDs, 10 PCDFs, and 12 dl-PCBs. Only chlorinated dioxins and furans (PCDD/F) are regulated by EU for emissions of persistent organic pollutants (POPs) from waste incinerators. Dioxin-like polychlorinated biphenyls, brominated and mixed halogenated dioxins, all substances with dioxin-like properties, are (still) not regulated in the EU.

Dioxins, furans and dioxin-like PCBs
Why using eggs for biomonitoring dioxins?

When chickens are free to forage on natural uncovered soil in the open air without roofing, they are in optimal contact with the environment. Eggs can reflect the chemical situation of soil biota related to the atmospheric deposition of hazardous chemical particles from industrial emissions, such as car shredding, metallurgy, coal-fired power plants, foundries, the PVC industry, cement kilns, the paper industry, and waste incineration. Chickens forage on and in the soil, eating insects, invertebrates, vegetation even grass. As a result, persistent organic pollutants (POPs) like dioxins (PCDD/F/dl-PCB)can be found in the fatty egg yolk and act as a biomarker for the environment. The chicken excretes toxic compounds like dioxins into the fatty yolk when producing the eggs (dioxins are fat related). The older the chicken is, the more toxic compounds can be collected in the body, a process called bioaccumulation. Biotransformation refers to the capability of an organism to break down certain substances. Xenobiotic metabolism refers to the metabolism or breakdown of foreign substances not belonging to the substances of an organism of an ecological system. 

Biomonitoring eggs of backyard chicken
EU regulations of emissions waste incineration
Short-term versus long-term sampling of flue gases
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