language-icon Old Web
English
Sign In

Polychlorinated dibenzodioxins

Polychlorinated dibenzodioxins (PCDDs), or simply dioxins, are a group of polyhalogenated organic compounds that are significant environmental pollutants. Polychlorinated dibenzodioxins (PCDDs), or simply dioxins, are a group of polyhalogenated organic compounds that are significant environmental pollutants. They are commonly but inaccurately referred to as dioxins for simplicity, because every PCDD molecule contains a dibenzo-1,4-dioxin skeletal structure, with 1,4-dioxin as the central ring. Members of the PCDD family bioaccumulate in humans and wildlife because of their lipophilic properties, and may cause developmental disturbances and cancer. Dioxins occur as by-products in the manufacture of some organochlorides, in the incineration of chlorine-containing substances such as polyvinyl chloride (PVC), in the chlorine bleaching of paper, and from natural sources such as volcanoes and forest fires. There have been many incidents of dioxin pollution resulting from industrial emissions and accidents; the earliest such incidents were in the mid 19th century during the Industrial Revolution. The word 'dioxins' may also refer to other similarly acting chlorinated compounds (see Dioxins and dioxin-like compounds). The structure of dibenzo-1,4-dioxin consists of two benzene rings joined by two oxygen bridges. This makes the compound an aromatic diether. The name dioxin formally refers to the central dioxygenated ring, which is stabilized by the two flanking benzene rings. In PCDDs, chlorine atoms are attached to this structure at any of 8 different places on the molecule, at positions 1–4 and 6–9. There are 75 different PCDD congeners (that is, related dioxin compounds). The toxicity of PCDDs depends on the number and positions of the chlorine atoms. Congeners that have chlorine in the 2, 3, 7, and 8 positions have been found to be significantly toxic. In fact, 7 congeners have chlorine atoms in the relevant positions which were considered toxic by the World Health Organization toxic equivalent (WHO-TEQ) scheme. Low concentrations of dioxins existed in nature prior to industrialization as a result of natural combustion and geological processes. Dioxins were first unintentionally produced as by-products from 1848 onwards as Leblanc process plants started operating in Germany. The first intentional synthesis of chlorinated dibenzodioxin was in 1872. Today, concentrations of dioxins are found in all humans, with higher levels commonly found in persons living in more industrialized countries. The most toxic dioxin, 2,3,7,8-tetrachlorodibenzodioxin (TCDD), became well known as a contaminant of Agent Orange, a herbicide used in the Malayan Emergency and the Vietnam War. Later, dioxins were found in Times Beach, Missouri and Love Canal, New York and Seveso, Italy. More recently, dioxins have been in the news with the poisoning of President Viktor Yushchenko of Ukraine in 2004, the Naples Mozzarella Crisis the 2008 Irish pork crisis, and the German feed incident of 2010. The United States Environmental Protection Agency inventory of sources of dioxin-like compounds is possibly the most comprehensive review of the sources and releases of dioxins, but other countries now have substantial research as well.

[ "Toxicity", "Polychlorinated dibenzofurans", "Contamination", "Ecology", "Environmental chemistry", "Polychlorinated Dibenzodioxin", "2,3,7,8-Tetrachlorodibenzodioxin", "Octachlorodibenzodioxin" ]
Parent Topic
Child Topic
    No Parent Topic