PCBs: Polychlorinated Biphenyls
Polychlorinated biphenyls are a family of man-made chemicals that contain two hundred and nine individual compounds with varying toxicity. Many different combinations are possible. In chemical terminology, ?phenyl? denotes a ring structure of six carbon atoms attached to something else; ?biphenyl? results when two such rings are attached to each other. Polychlorinated biphenyl (PCB) is any molecule having multiple chlorine atoms attached to the carbon atoms of a biphenyl nucleus. Chlorine atoms can be placed at any or all of ten available sites.
PCB’s have been used widely as coolants and lubricants in transformers, capacitors, and other electrical equipment. The manufacture of PCBs stopped in the United States in October 1977 because of evidence that PCBs accumulate in the environment and may cause health hazards for humans.
Although PCBs are no longer manufactured, human exposure still occurs. Many older transformers and capacitors still contain fluids that contain PCBs. The useful lifetime of these transformers can be thirty years or more. Prior to 1977, certain occupational settings had, and may still have, higher levels of human exposure. Animal experiments have shown that some PCB mixtures produce adverse health effects that include liver damage, skin irritations, reproductive and developmental effects, and cancer. With these observations, it is most considerably dangerous for humans. The U.S Department of Health and Human Services has determined that PCBs may reasonably be expected to be carcinogens. Human studies to date show that irritations, such as acne-like lesions and rashes, can occur in PCB-exposed workers. Other studies of people with occupational exposure suggest that PCBs might cause liver cancer. Reproductive and developmental effects may also be related to occupational exposure and eating of contaminated fish. While the role in producing cancer, reproductive and developmental effects can not be clearly determined, the suggestive evidence provides an additional basis for public health concerns about humans who might be exposed to PCBs. It has also been suggested that chemicals such as PCBs in the environment can mimic the body’s natural hormones. This could lead to infertility, certain types of cancer, and other hormone-related disorders.
In the past, discharges of PCB-filled wastes into rivers, streams and open landfills were considered acceptable, legal and hazard-free practices. PCBs were also sometimes intentionally released into the environment to reduce dust emissions from dirt roads, or as extenders in some agricultural pesticides formulations. These practices were inappropriate and potentially harmful. There have also been accidental releases of PCB and associated contaminants into the environment via leaking of sealed PCB fluid compartments during commercial use of transformers and capacitors, as a result of improper disposal of PCB-containing equipment or chemical products. The contamination of food for animal and human consumption has also occurred from PCBs leaking from malfunctioning heating coils into foods during manufacturing. PCBs can also enter the food chain through the ingestion of contaminated fish from PCB-contaminated water. Finally, transformer and capacitor fires have led to the environmental release of PCBs and thermal degradation products.
Concern about the presence of PCBs in our environment began around 1966 when research in Sweden revealed the presence of PCBs in environmental samples being screened for DDT. Further study confirmed suspicions that the rate of natural breakdown was very slow for some of the PCB congeners. Concern continued to rise due to
widespread human poisoning episodes in 1968 and 1978. PCBs are now regarded as environmental contaminants that build up in the food chain. They build up in the food chain due to their relative insolubility in water and high solubility in fats.
To date, there are tests to determine PCBs in blood, body fat and breast milk. These tests are not routine clinical tests, but they can detect PCBs in members of the general population as well as in workers with occupational exposure to PCBs. Although these tests indicate if one has been exposed to PCBs, they do not predict potential health effects. Blood tests are the easiest, safest and perhaps, the best method for detecting recent large exposures. As well as tests, remediation techniques and disposal technologies continue to evolve. Many approaches have been tested out but none has emerged as a technically and economically viable solution for the endless environmental situations encountered. A useful method for cleansing water will probably not be effective in treating soil. Highly chlorinated PCBs will not be as readily decomposed as less-chlorinated PCBs. These types of factors contribute to the challenge of effective and cost-efficient remediation of PCB contaminated sites. Bacterial build-up or breakdown of PCBs in soil and sediment is potentially useful technology that is receiving much attention. Introductory results on the breakdown of soil samples, including those taken from the Hudson River, show promise. A ?thermal blanket? approach, in which soil is heated from the surface downward and PCBs are removed under vacuum, is also investigated. Another approach to reduce PCB problems is finding alternatives to PCBs.
There are various chemical alternatives to PCBs (including silicon fluids, fluorocarbons, high molecular weight hydrocarbons, low molecular weight chlorinated hydrocarbons and high boiling oils and esters) which will work in electrical equipment; however, a transformer constructed for use with PCB fluid may not operate at the same power load with a substitute chemical. Such a transformer becomes less efficient, resulting in a higher operating cost as well as added risk of fire. Precautionary measures must be taken to ensure fire safety and proper disposal of waste. Clearly shifting from PCBs to alternatives has involved tradeoffs, but not elimination, of health and safety risks.
In conclusion, because of the ban on PCB production, decreasing use of PCBs and efforts to remediate contaminated sites, general population exposure to PCBs in fish and other foods has been significantly reduced, and PCB levels in human blood are also decreasing. Industry and government should focus on the development of novel and cost-effective remediation and disposal techniques, as well as on scientifically improved health risk analysis to reduce the uncertainty associated with PCB exposure and health effects in humans.
Work Cited Page
Internet, Polychlorinated Biphenyls, ?Yahoo’ 1992
Internet, Organochlorine and Metal Pollution, ?About’ 04/20/98
Wall, E. Byron, Science in Society, Wall & Emerson, Inc. Toronto, Ontario. 1989