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Environmental impact

PCBs have been released to the environment solely by human activity.
Because PCBs are no longer manufactured in developed countries, significant releases of newly manufactured or imported materials to theenvironment do not occur. Rather, PCBs predominantly are redistributed from one environmental compartment to another (e.g., soil to water, waterto air, air to water, sediments to water). Thus, for example, the majority of PCBs in air result from volatilization of PCBs from soil andwater. Some PCBs may be released to the atmosphere from uncontrolled landfills and hazardous waste sites, incineration of PCBcontaining wastes,leakage from older electrical equipment in use, and improper disposal or spills.
PCBs may be released to water from accidental spillage of PCBcontaining hydraulic fluids, improper disposal, combined sewer overflows (CSOs) orstorm water runoff, and from runoff and leachate from PCBcontaminated sewage sludge applied to farmland. PCBs may be released to soil fromaccidental leaks and spills, releases from contaminated soils in landfills and hazardous waste sites, deposition of vehicular emissions nearroadway soil, and land application of sewage sludges containing PCBs. PCBs are globally circulated and are present in all environmental media.Atmospheric transport is the most important mechanism for global dispersion of PCBs. Biphenyls with 01 chlorine atom remain in the atmosphere,those with 14 chlorines gradually migrate toward polar latitudes in a series of volatilization/deposition cycles, those with 48 chlorinesremain in midlatitudes, and those with 89 chlorines remain close to the source of contamination. PCBs enter the atmosphere from volatilizationfrom both soil and water surfaces. Once in the atmosphere, PCBs are present both in the vapour phase and sorbed to particles. Wet and drydeposition remove PCBs from the atmosphere. The dominant source of PCBs to surface waters is atmospheric deposition. However, redissolution ofsedimentbound PCBs also accounts for water concentrations. PCBs in water are transported by diffusion and currents. PCBs are removed from thewater column by sorption to suspended solids and sediments as well as by volatilization from water surfaces. Higher chlorinated congeners aremore likely to sorb, while lower chlorinated congeners are more likely to volatilize. PCBs also leave the water column by concentrating inbiota. PCBs accumulate most in higher trophic levels through the consumption of contaminated food. PCBs in soil are unlikely to migrate togroundwater because of strong binding to soil.
Volatilization from soil appears to be an important loss mechanism, it is more important for the lower chlorinated congeners than for the higherchlorinated congeners. Vapourphase PCBs accumulate in the aerial parts of terrestrial vegetation and food crops by vapourtoplant transfer.
The ability of PCBs to be degraded or transformed in the environment depends on the degree of chlorination of the biphenyl molecule as well ason the isomeric substitution pattern. The vapourphase reaction of PCBs with hydroxyl radicals is the dominant transformation process in theatmosphere, while photolysis appears to be the only viable chemical degradation process in water. Biodegradation has been demonstrated underboth aerobic and anaerobic conditions and is the major degradation process for PCBs in soil and sediment.
Monitoring studies conducted over the years have shown that atmospheric concentrations of PCBs have decreased since the late 1970s. Watermonitoring studies indicate that PCB concentrations are generally higher near sites of anthropogenic input and in inshore waters.
The detection of PCBs in blood, adipose tissue, breast milk, and other tissue samples from the general population indicates widespread exposureto PCBs from environmental sources.

 


Structure formula of 2,2',3,4,5,6'-Hexachlorobiphenyl


3D structure of 2,2',3,4,5,6'-Hexachlorobiphenyl