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| Let thy food be thy medicine,
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ToxicAlert |
Helping people take charge of their health |
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The 1994 EPA
Dioxin Reassessment Health Assessment, Volume III: Risk Characterization 9.3. ENVIRONMENTAL FATE Despite a growing body of literature from laboratory, field, and monitoring studies examining the environmental fate and environmental distribution of CDDs and CDFs, the fate of these environmentally ubiquitous compounds is not yet fully understood, and the following represents our best understanding, based on available data. In soil, sediment, the water column, and probably air, CDDs/CDFs are primarily associated with particulate and organic matter because of their high lipophilicity and low water solubility. They exhibit little potential for significant leaching or volatilization once sorbed to particulate matter. The available evidence indicates that CDDs and CDFs, particularly the tetra- and higher chlorinated congeners, are extremely stable compounds under most environmental conditions, with environmental persistence measured in decades. The only environmentally significant transformation process for these congeners is believed to be photodegradation of chemicals not bound to particles in the gaseous phase or at the soil- or water-air interface. Brominated congeners are significantly more readily transformed by photodegradation. CDDs/CDFs entering the atmosphere are removed either by photodegradation or by dry or wet deposition. Although some volatilization of dioxin-like compounds on soil does occur, the predominant fate of CDDs/CDFs sorbed to soil is to remain in place near the surface of undisturbed soil or to move to water bodies with erosion of soil. CDDs/CDFs entering the water column primarily undergo sedimentation and burial. The ultimate environmental sink of these CDDs/CDFs is believed to be aquatic sediments. Little specific information exists on the environmental transport and fate of the dioxin-like PCBs. However, the available information on the physical/chemical properties of dioxin-like PCBs, coupled with the body of information available on the widespread occurrence and persistence of PCBs in the environment, indicates that these PCBs are likely to be associated primarily with soils and sediments and to be thermally and chemically stable. Soil erosion and sediment transport in water bodies and emissions to the air (via volatilization, dust resuspension, or point source emissions) followed by atmospheric transport and deposition are believed to be the dominant transport mechanisms responsible for the widespread environmental occurrence of PCBs. Photodegradation to less chlorinated congeners followed by slow anaerobic and/or aerobic biodegradation is believed to be the principal path for destruction of PCBs. Similar situations exist for the polybrominated biphenyls (PBBs). Little information is available on the occurrence and fate of biphenyl congeners containing both chlorine and bromine, but their contribution to dioxin-like activity in the environment is thought to be small. |
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