|MadSci Network: Chemistry|
Unfortunately I don't know the specific chemical reactions involved in dioxin synthesis, although I have found a useful document regarding the sources of dioxins that are present. The United States Environmental Protection Agency (EPA) is in the process of preparing a report on sources of dioxin and related compounds in the US. Fortunately they have a draft available for review, although I must stress (since the EPA did) that this is not yet an official document, so it is subject to change. At any rate, in the US in 1995 there were >50 potential categories of dioxin sources listed. They range from the mundane, such as municipal waste incineration, to the technical, such as pentachlorophenol production by chemical manufacturing processes. By far the greatest source of dioxin was the production of pentachlorophenols, accounting for about 90% of the estimated total. Dioxin is a byproduct in this process. However, this source of dioxin has long been known, and very little of the dioxin produced in pentachlorophenol manufacture ever enters the environment in a form where it could be dangerous. The EPA was kind enough to list sources in four categories: air, water, land, and products. Dioxin from pentachlorophenol manufacture belongs to the last category. Aside from dioxin in other products, the largest amount produced enters the atmosphere through incineration. Municipal and hospital waste incineration produce about 1600 g/year of dioxin equivalents, while burning of fossil fuels and wood, brush and straw add another 400 or so grams per year. Inputs of dioxin to water and land are much less well known. The largest known source of dioxins directly to water is through bleaching of wood pulp and paper with chlorine. Again I should point out that these are estimates and only from the US for a specific year. The amounts and sources have changed over time, even over the last decade, although earlier estimates are in some cases even more error prone, since the sources are still being discovered. I hope this helps answer your question!
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