For decades, PCBs have been recognized as important and potentially harmful environmental contaminants. The intrinsic properties of PCBs, such as high environmental persistence, resistance to metabolism in organisms, and tendency to accumulate in lipids have contributed to their ubiquity in environmental media and have induced concern for their toxic effects after prolonged exposure.
PCBs are bioaccumulated mainly by aquatic and terrestrial organisms and thus enter the food web. Humans and wildlife that consume contaminated organisms can also accumulate PCBs in their tissues. Such accumulation is of concern, because it may lead to body burdens of PCBs that could have adverse health effects in humans and wildlife. PCBs may affect not only individual organisms but ultimately whole ecosystems.
Moreover, PCBs are slower to biodegrade in the environment than are many other organic chemicals. The low water solubility and the low vapor pressure of PCBs, coupled with air, water, and sediment transport processes, means that they are readily transported from local or regional sites of contamination to remote areas. In the atmosphere , PCBs that are present in the atmosphere tend to react with ozone and water under the effect of sunlight. The reactions results amongst other things in chlorine atoms being removed.
The more chlorine atoms there are the longer this reaction takes. The time it takes for half of the amount of PCBs initially present to be broken down ranges from 3. In water , PCBs are essentially broken down by the effect of sunlight photolysis.
In shallow water, in summer sunlight, it takes 17 to days for half of the amount initially present to be broken down for molecules with 1 to 4 chlorine atoms. Breakdown by sunlight is slower during winter.
PCB congeners with a greater number of chlorine atoms seven or more absorb more sunlight and are thus broken down more easily. In soil and sediment , PCBs are mostly broken down by microorganisms.
How quickly PCBs are broken down depends on several factors, including the number and location of chlorine atoms, PCB concentration, the type of microorganisms present, available nutrients, and temperature. Breakdown of PCBs by microorganisms, though slow, can occur whether or not oxygen is present in soil and sediments, and can also occur to some extent in water. In outdoor air , average PCB concentration varies between urban and rural locations, and other locations distant from PCB sources.
Since the early s, there has been a slight but continuous decrease in the levels of PCBs in air observed in urban, rural, and coastal areas. In indoor air PCB concentrations in the early s were typically at least ten times higher than in the surrounding outdoor air.
This may be due to the fact that PCBs are emitted by certain electrical appliances and devices such as fluorescent lighting ballasts and building materials such as elastic sealants.
In seawater , in industrial areas, PCB levels were observed to be at least times higher than further off-shore, based on samples of water taken from the upper few millimeters of the surface. Particularly high concentrations were reported in the North Sea 0. From the s until their ban, an estimated 1.
PCBs were also sprayed on dirt roads to keep the dust down prior to knowing some of the unintended consequences from widespread use. Prior to the ban in , PCBs entered the air, water, and soil during manufacture and use.
Wastes from the manufacturing process that contained PCBs were often placed in dump sites or landfills. Occasionally, accidental spills and leaks from these facilities or transformer fires could result in PCBs entering the environment. PCBs can be found worldwide.
In the s, when initial research results were released, traces of PCBs could be detected in people and animals around the world — not only in heavily populated areas such as New York City, but also in remote areas as far as the Arctic. These findings of such widespread and persistent contamination contributed to the banning of the chemical in
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