Redistribution of PCBs

The polychlorinated biphenyls (PCBs) have been used commercially since 1929 as dielectric and heat exchange fluids and in a variety of other applications. The presence of PCBs in human and wildlife tissues was first recognized in 1966. Investigations in many parts of the world have since revealed widespread distribution of PCBs in the environment, including remote areas with no PCB production or use. There is evidence that the major source of PCB exposure in the general environment is the redistribution of PCBs previously introduced into the environment. It is believed that large bodies of water, such as the Baltic Sea and the Canadian Great Lakes, may... 

Pig. 1. The hypothetical redistribution of PCBs from liver to brain during a seasonal starvation cycle in fish. 

For decades, many countries and intergovernmental organizations have taken measures to prevent the formation and release of PCDD/PCDFs, and have also banned or severely restricted the production, use, handling, transport and disposal of PCBs. As a consequence, release of these substances into the environment has decreased in many developed countries. Nevertheless, analysis of food and breast-milk show that they are still present, although in levels lower than those measured in the 1960s and 1970s. At present, the major source of PCB exposure in the general environment appears to be the redistribution of previously introduced PCBs. 

Because PCBs are no longer manufactured or imported in large quantities, significant releases of newly manufactured or imported materials to the environment do not occur. Rather, PCBs predominantly are redistributed from one environmental compartment to another (e.g., soil to water, water to air, sediments to water) (Eisenreich et al. 1992 Larsson 1985 Larsson and Okla 1989 Lin and ( ue Hee 1987 Mackay 1989 Murphy et al. 1985, 1987 Swackhamer and Armstrong 1986). Thus, for example, the majority of PCBs in air result from volatilization of PCBs from soil and water. Some PCBs may be released to the... 

The major source of PCB release to the atmosphere (2 million pounds/year) is the redistribution of the compounds that are already present in soil and water (Eisenreich et al. 1992 Murphy et al. 1985). 

Irvine KN, Loganathan BG. 1998. Localized enrichment of PCB levels in street dust due to redistribution by wind. Water Air Soil Pollut 105 603-615. 

TCDD (57). Any photochemical or chemical treatment process which would substantially enrich a PCB deposit in these structures, regardless of its destruction efficiency, would be undesirable. PBB photoproduct mixtures are of increased toxicity due to this sort of isomer redistribution (60). In the case of PCB the preferential photochemical reaction of o ibO halogens may lead to intermediate mixtures of enhanced toxicity and this aspect of PCB reactivity warrants further study. As noted above the product distribution ratio changes markedly on going from direct to sensitized photolysis (IJ,) and thus it may be possible to control the nature of the photolysate residue. 

PCBs have been implicated in the decline of certain populations of fish-eating birds, for example, in the Great Lakes of North America. Although their use is now banned in most countries and very little is released into the environment as a consequence of human activity, considerable quantities remain in sinks (e.g., contaminated sediments and landfill sites), from which they are slowly redistributed to other compartments of the environment. There continues to be evidence that PCB residues are still having environmental effects, for example, on birds and fish. 

On a worldwide basis, toxic concentrations of the heavy metals have thus far been limited to industrialized harbors. The only metals that appear to have accumulated to toxic levels on a regional scale are mercury, cadmium, and lead in the Arctic Ocean. This concentration of mercury and lead has been fecilitated by a natural process, called the grasshopper effect, which acts to transport volatile compoimds poleward. This transport plays a major role in redistributing the volatile organic pollutants, such as the PCBs, and, hence, is discussed at further length in Chapter 26.7. The process responsible for the cadmium enrichment in the Arctic appears to involve low-altitude transport of the fine particles that compose Arctic haze. 

As with the structurally related PCBs (ATSDR 2000), PBBs are rapidly (minutes to hours) cleared from the blood and initially accumulate mainly in the liver, lungs, and muscle (Domino et al. 1982 Matthews et al. 1977). Due to their high affinity for lipid-rich tissues, PBBs are subsequently redistributed to adipose and skin for storage or metabolism in the liver, and a dynamic equilibrium of PBB concentrations is established among all tissues for each PBB homolog (Tuey and Matthews 1980). 

An in-lagoon slurry bioremediation at the French Limited Lagoon (Texas) was conducted at both pilot and eventually full scale and successfully bioremediated many of the aromatic chemicals found at that site, including BTEX and PAHs. This site also contained approximately 100 ppm PCBs however, it is not clear from the results released to date whether the decrease observed in PCB concentration during the field work can be attributed to biodegradation or some abiotic redistribution or loss (Unterman, 1991 Biotreatment News, October, November, December, 1992 Shannon Unterman, 1993). 

There have been accidental releases of pollutants that have cause effects on humans and wildlife, such as Yucheng in Taiwan, Yusho in Japan (Ikeda, 1996), PCB contamination of poultry in Belgium (Bernard et al., 1999), Seveso in Italy (Mocarelli, 2001), and dioxin contamination by redistribution and burial of contaminated soils at Love Canal in the USA (Vianna, 1983). However, despite intensive and rapid industrialization and urbanization, such massive POPs related cases have not occurred in South Korea. Since the 1990s, monitoring South Korea has rarely found POP concentrations as great as parts per million (ppm). 

Analytical results showed relatively uniform distribution of HCHs along with the latitudes, and the ratio (S/W) showed the highest degree of positive correlation with latitude. This phenomenon indicates that HCH isomers pose higher potency for atmospheric transport and global redistribution. Other compounds such as DDTs, CHLs and PCBs with lower correlations and gradients exhibited less efficiency for atmospheric transport. 

After being deposited in the environment, PCBs may remain resident in a given area for a period of time or be subject to redistribution and alteration by biogeochemical processes. 

The conventional PCB constructions today cannot handle any grid arrays with pitches below 0.020 in, while some flip-chip ball grid arrays go below 0.010-in pitch. In cases when grid distances of the area terminations are below 0.50 mm (0.040 in), special redistribution layers are frequently used, which distribute signals to the conventionally made PTHs in supporting MLB. 

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