Example: PCB

Although these issues have already been briefly noted, they deserve a few additional comments. For freely water-soluble substrates that have low volatility, there are few difficulties in carrying out the appropriate experiments described above. There is, however, increasing interest in xenobiotics such as polycyclic aromatic hydrocarbons (PAHs) and highly chlorinated compounds including, for example, PCBs, which have only low water solubility. In addition, attention has been focused on volatile chlorinated aliphatic compounds such as the chloroethenes, dichloromethane, and carbon tetrachloride. All of these substrates present experimental difficulties of greater or lesser severity. 

The highest total PCB concentrations recorded in terrestrial mammalian wildlife occurred in fat and liver tissues of species collected near urban areas di-ort/jo-congeners were the major contributors to PCB tissue burdens (Table 24.12). Atmospheric transport of PCBs governed uptake in terrestrial mammalian herbivores and predators. For example, PCB residues in tissues of voles and shrews in the Scandinavian peninsula directly correlated with fallout loadings (Larsson et al. 1990). An increase in atmospheric deposition of PCBs increased PCB burdens in plants, herbivores, and predators of the herbivores. But herbivores and predators differentially metabolized PCBs, raising concentrations of highly chlorinated congeners in predators and concentrations of the more easily metabolized low-chlorinated PCBs in herbivores (Larsson et al. 1990). 

Persistent compound Material that does not or only slowly biodegrades, for example, PCBs and DDT. 

Contamination of biota and sediment in the Rhine estuary with, for example, PCBs and organochlorine pesticides has remained essentially constant over the... 

Functional groups Chapter 3 uses the functional groups to introduce important properties of organic chemistry. Relevant examples—PCBs, vitamins, soap, and the cell membrane—illustrate basic solubility concepts. In this way, practical topics that are sometimes found in the last few chapters of an organic chemisuy text (and thus often omitted because instructors run out of time) are introduced early so that students can better grasp why they are studying the discipline. 

The health effect of endocrine disruptors is further complicated by the fact that an endocrine disrupter or a family of endocrine disruptors may have multiple mechanisms of actions. For example, PCBs may mimic estrogen, prevent binding of thyroid hormone to thyroid binding globulin, and accelerate the metabolism and excretion of several steroid hormones. 

Lindane is a less lipophilic substance in comparison with many other serious envirotoxicants but still is characterized by a high degree of bioavailability. Furthermore, the substance is rather persistent, volatile and subject to longdistance transport. It bioaccumulates to a pronounced degree in lower biota and fish, although mammals and birds biotransform and excrete the metabolites to a great extent. Thus, the levels of lindane in contrast to, for example, PCBs have... 

Once in the environment, PCBs do not readily break down and therefore may remain for very long periods of time. They can easily cycle between air, water, and soil. For example, PCBs can... 

Significant interspecies differences in the quantitative metabolism of PCBs contributes directly to the species differences in the relative persistence (biological half-life) of PCB congeners. For example, PCB 153 is often the most prevalent PCB detected in humans, due to exposure and the slow rate of biotransformation of this congener. 3-Hydroxy-2,4,5,2 ,4 ,5 -hexaCB was identified as the major metabolite of PCB 153 formed by human CYP2B6 (Ariyoshi et al. 1995). CYP2B6 is constitutively expressed in humans, but only accounts for a maximum of 1-2% of the total CYPs in human liver. 

Toxic effects of PCBs may also involve Ah-receptor independent mechanisms, or both Ah-receptor dependent and independent mechanisms (see Section 3.5.2). For example, PCBs can be metabolized to reactive arene oxide intermediates that may alkylate critical cellular macromolecules and result in injury (Gardner et al. 1973 Safe 1990). Clinical intervention to interfere with Ah-receptor independent mechanisms have not been developed. 

Recycling of PCBs, due to volatilization of PCBs from the water column and subsequent release of PCBs from the sediments, occurs when inputs from the atmosphere decrease (Achman et al. 1996 Sanders et al. 1996). The process of recycling tends to increase with higher PCB solubility (Sanders et al. 1996). There are several mechanisms by which PCBs can exchange between the sediment bed and the overlying water. For example, PCBs dissolved or associated with colloidal particles can exchange across the sediment-water interface by diffusive and/or advective processes (Berner 1980 Formica et al. 1988). 

Variables in sampling methods greatly influence results. For example, PCB levels in milk fat may decrease during lactation, with maternal age and weight, and with number of children born (Jensen... 

Endocrine disruptors are exogenous chemicals (for example, PCBs) that enter the body and act like hormones. They can dismpt physiology, sometimes with devastating consequences. Startling evidence from animal studies shows that male fish in detergent-contaminated water express female characteristics, turtles are sex-reversed by PCBs, male frogs exposed to a common herbicide form multiple ovaries, pseudohermaphroditic offspring are produced by polar bears found in contaminated waters, and seals in contaminated water have an excess of uterine fibroids.46... 

For example, PCB, mirex, and DDT, can be detected earlio and environmental damage held to an acceptable level if the characteristics of the substances can be inserted into a model which can predict ... 

The structural characteristics of individual PCB congeners influence their induction of various P450 activities. In mammals, PCB congeners have been characterized as 3-methylcholanthrene-type inducers, phenobarbital-type inducers, or mixed-type inducers of both. AHH and EROD activities (which are preferentially catalyzed by the P450IA gene subfamily) have been induced by planar PCBs in fish and mammals and by some mono- and di-ortho analogs of planar PCBs in mammals. The mechanism of toxic action of planar and mono-ortho planar PCBs is linked to an interaction with the 2,3,7,8-TCDD (or Ah) receptor protein. But this mechanism does not account for all observed PCB toxici-ties. Toxic responses uiuelated to Ah receptor effects have been reported of PCBs 4, 28, 31, 49, 52, 84, 95, 110, 136, and 153. For example, PCB 153 is less cytotoxic than PCB 169... 

The toxicity of PCBs depends not only on the degree of chlorination but also on the types of isomer. For example, PCBs devoid of orthosubstitution but heavily substituted at the meta and para positions can assume a planar conformation that can interact with the same receptor as TCDD. Examples of these isomers are 3,3, 4,4 -tetrachlorobiphenyl 3,3, 4,4, 5-pentachlorobiphenyl and 3,3, 4,4, 5,5 -hexachlorobiphenyl (NIOSH, 1977). 

There are a number of pigment products excluded from the TSCA PCB rules as excluded products and processes. EPA now indicates that these excluded products require TSCA Section 12(b) export notification. TSCA Section 12(b) notifications to the EPA are required whenever chemicals regulated under specific parts of the TSCA statute are exported from the United States. The notices only identify the regulated chemical, for example PCBs, not the product the PCBs may be contained in. The notices also contain the name of the receiving country and other basic identification information. EPA, in turn, notifies the receiving country once for each regulated chemical once per year. 

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