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The natural formation of polychlorinated dioxins and dibenzofurans
08/2008 The polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are two series of organic compounds that have been the subject of much concern during the last 30 years, due to their toxic effects. In all, there exist 210 members or "congeners" of the PCDD and PCDF family of compounds, but only 17 of these are considered to be highly toxic. The most toxic of all is 2,3,7,8-tetrachlorodibenzo-p-dioxin. During the 1970s the dioxin debate focused on the formation of PCDDs and PCDFs as unwanted by-products from the production of pesticides and other chemicals. In the 1980s, the debate shifted to the incineration of various types of wastes and the use of elemental chlorine for paper pulp bleaching. However, the improved analytical methods introduced in the late 1980s have proved that PCDDs and PCDFs can also be formed as a result of more natural reactions such as normal combustion processes and light-induced and biochemical reactions. It is now established that the normal burning of wood for heating purposes can generate PCDDs and PCDFs. This is not a totally natural reaction, but it occurs under conditions similar to those of forest fires. Due to sampling difficulties, only one study on the latter topic has been performed and this study confirmed that PCDDs and PCDFs can be formed in forest fires. It has been reported in a few studies that PCDDs and PCDFs can be formed from chlorophenols by light induced (photochemical) reactions. During the late 1980s, work in my laboratory proved that PCDDs and PCDFs could be formed by biochemical reactions from chlorinated phenols under laboratory conditions. Chlorinated phenols are both man-made and natural products. In the 1990s our work has focused on finding environmental situations where such reactions can take place. At the moment we have identified two situations: sewage sludge and compost. In the case of sewage sludge, analyses of a sample stored in the laboratory for one week showed a pronounced net formation of dioxins. We have also monitored the composition of organic matter undergoing composting. With grass cuttings and garden wastes we found an increase in concentrations of PCDDs and PCDFs, in one case by an order of magnitude. Another interesting observation is that sediments from remote and pristine lakes in southern USA are much more contaminated than sediments from highly industrialized areas like the Great Lakes. It has been assumed that the major portion of these dioxins are formed by natural processes. Similar processes can account for the dioxin contamination of a virgin clay additive which caused an unexpected contamination of catfish and chicken feed and subsequently catfish and chickens in Arkansas, USA. Presently, it is difficult to establish the quantitative importance of the natural formation of PCDDs and PCDFs. However, analyses of historical samples can give some indications. An 8,000-year old sediment from Japan was found to be contaminated by PCDDs. Archived soil and herbage samples, 100-150 years old, from the UK, have concentrations of PCDDs and PCDFs around one-third of levels found today. In these samples it is difficult to separate out the contribution due to truly natural processes, but the observed congener distribution pattern ("fingerprint") is not in accordance with that which would be expected from known human activities during that period, indicating the importance of natural formation. |
