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To help understand how serious the train derailment in Ohio farmland is. 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 case of dioxin-like PCBs and PBBs, unwanted minor components of intentionally produced mixtures.[1][2]Some of them are highly toxic, but the toxicity among them varies 30,000-fold. They are grouped together because their mechanism of action is 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:[1][3][4]Polychlorinated dibenzo-p-dioxins (PCDDs), or simply dioxins. PCDDs are derivatives of dibenzo-p-dioxin. There are 75 PCDD congeners, differing in the number and location of chlorine atoms, and 7 of them are specifically toxic, the most toxic being 2,3,7,8-tetrachlorodibenzodioxin (TCDD).Polychlorinated dibenzofurans (PCDFs), or furans. PCDFs are derivatives of dibenzofuran. There are 135 isomers; 10 have dioxin-like properties.Polychlorinated biphenyls (PCBs), derived from biphenyl, of which 12 are "dioxin-like". 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.