Dioxins and PCBs

Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (generally known as PCDDs and PCDFs) are ubiquitous environmental contaminants. The known sources of PCDDs and PCDFs are many, including the following  

The wide range of potential sources and their environmental persistence may well explain why PCDDs and PCDFs are ubiquitous. It is believed that there are two main routes by which PCDDs and PCDFs are released into the environment via the atmosphere leading to deposition on soil, water and plants and via solid or liquid waste with subsequent contamination of land (e.g. sewage sludge). These findings should help to reduce the already extensive effort needed to trace back the contamination to its sources, although the persistence of PCDDs and PCDFs in the environment for many years means that historical as well as current sources need to be taken into account. 

Concern over dioxins originally arose over one particular dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). This was found to produce clinical effects (chloracne) in workers exposed to it through industrial accidents.35 It is also toxic to some species of laboratory animals. A few PCBs have been shown in experimental systems to exert a number of toxic responses similar to those observed for TCDD. In consequence, the total concentrations of both dioxins and PCBs in environmental media, including food, are now commonly determined as part of the same chemical analysis, and each cannot be discussed now without reference to the other. 

There are 75 possible isomers of polychlorinated-p-dibenzodioxin (PCDD) and 135 such isomers of polychlorinated dibenzofuran (PCDF). These related compounds are known as congeners. In addition, PCBs form a similar group of 209 related congeners that differ only in the number and pattern of chlorine atoms attached to the parent biphenyl molecule. 

The initial system that was widely adopted was that devised by the North Atlantic Treaty Organisation, Committee on the Challenges of Modem Society (NATO-CCMS) of International Toxic Equivalency Factors (I-TEF).36 Similar 

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Brouwer, A. (1996). Biomarkers for exposure and effect assessment of dioxins and PCBs. In lEH Report on The use of Biomarkers in Environmental Exposure Assessment Institute Environmental Health, Leicester, U.K. 51-58. 

Boyer, I.J., CJ. Kokoski, and P.M. Bolger. 1991. Role of FDA in establishing tolerable levels for dioxin and PCBs in aquatic organisms. Jour. Toxicol. Environ. Health 33 93-101. 

PCDD/PCDFs are today found in almost all compartments of the global ecosystem in at least trace amounts. They are ubiquitous in soil, sediments and air. Excluding occupational or accidental exposures, most human background exposure to dioxins and PCBs occurs through the diet, with food of animal origin being the major source, as they are persistent in the environment and accumulate in animal fat. 

Maximum residue levels in certain foods are set for the following contaminants nitrate, my cotoxins (aflatoxins, ochratoxin A, pamlin, deoxynivalenol, zearalenone, fumonisins, T -2 and HT-2-toxin), metals (lead, cadmium, mercury, inorganic tin), 3-MCPD (3-monochloro-propane-l,2-diol), dioxins and PCBs, and polycyclic aromatic hydrocarbons (benzo(a)pyrene) (EC 2007c). 

Until recently most people and nearly all corporations accepted the release of at least some level of hazardous wastes into the environment as an unpleasant, but necessary, consequence of the huge success of modern chemical technology. Certainly no one is happy about the presence of dioxins (and PCBs and PAHs and other hazardous chemicals) in the environment. They undoubtedly result in some number of health problems and deaths around the world each year. But that is a small price to pay, some would argue, for having such a diverse and rich supply of pesticides, drugs, perfumes, synthetic foods, medicines, and other chemical products. 

The quest for funding can lead to some unholy alliances between scientists and regulators and the creation of cash cows for researchers. If, for example, the dioxin or the PCB issues were defused, several laboratories that for decades have specialized in dioxin and PCB-related research, successfully milking agency and other sources for support, could lose their cash cows. 

Several other dioxin and PCB congeners have been tested and were shown to give a response that reflects the differences in the TEF values (Fig. 5.3). However, congeners with a low TEF value showed a relatively low response in the test. This is similary true for 1,2,3,7,8-PeCDD which TEF value was recently adjusted from 0.5 to 1, and which is often a relatively important contributor to the total dioxin content. As a result the test may underestimate the total TEQ content, if calculations were based on the calibration curve for TCDD. However, in general it is evident that the bioassay obeys the TEQ principle and that the result will reflect the total TEQ content of the sample. 

The converted figure of 0.9 pg I-TEQ/kg bodyweight for PCBs alone may be compared with 1.8 pg I-TEQ/kg bodyweight in Norway,46 2.2 I-TEQ/kg bodyweight for 1995 in Spain47 and another low value of 0.2 pg I-TEQ/kg bodyweight for New Zealand.48 In addition to the above studies of population average exposure, there have been surveys for dioxins and PCBs in individual foods. One example is discussed below. 

A survey of fish for PCDDs, PCDFs and PCBs in samples of various UK-landed and imported marine fish species, salmon and fish fingers collected in 1995/96 found concentrations of dioxins and PCBs in the range 0.9-140 ng WHO-TEQ/kg fat.49 Further details are given in Table 8.3. Exposure to dioxins and PCBs from the consumption of fish in combination with the rest of the diet was estimated to be 2.6 pg WHO-TEQ/kg bodyweight/day for an average UK adult consumer and 5.6 pg WHO-TEQ/kg bodyweight/day for a high-level adult consumer. 

Fish type PCDDs and PCDFs Mean Range PCBs Mean Range Dioxins and PCBs Mean Range ... 

FOOD STANDARDS agency Dioxins and PCBs in the UK diet 1997 Total Diet samples , Food Surveillance Information Sheet No. 4/00, 2000. 

Under the Law concerning Reporting of Releases to the Environment of Specific Chemical Substances and Promoting Improvements in Their Management (Law No. 86 of 1999, hereafter referred to as the Chemical Substances Release Reporting and Management Promotion Law), dioxins and PCBs are subject to the requirements of the Pollutants Release and Transfer Register (PRTR) system. 

Seegal RF, Schantz SL (1994) Neurochemical and behavioral sequelae of exposure to dioxins and PCBs. In Schecter A (ed) Dioxins and health. Plenum, New York, N.Y,... 

However some chemicals are not metabolized, such as the environmental contaminants dioxin and PCBs. Because these chemicals are soluble in fat, they can be absorbed into the body where they will remain for long periods of time. 

PCDFs are very similar to dioxins and, like them, are very toxic, especially 2,4,7,8-tetrachlorodibenzofuran, although this is not quite as toxic as TCDD. They have been found to be contaminants of PCBs and in the Yusho and Yu-Cheng poisoning incidents were more abundant in the PCBs than was usual. As with dioxins and PCBs, there are many different isomers and congeners. 

The toxic potential of pure dioxins and PCBs is indicated in toxic equivalence factors (TEF) giving the relative toxicity of the relevant dioxin or PCB relative to the toxic effect of 2,3,7,8-TCDD. TEF for different dioxins and furans vary from 0.01 to 1, whereas TEF for the different PCB congeners is much lower (0.0001-0.01) (26, 27). (Figure 8). 

Food contamination with dioxins and PCB seems to be comparable in terms of TEQ and is usually below 5 ppt-TEQ (= 5 picogram-TEQ/g fat) (26, 27). A higher degree of contamination (>10 ppt-TEQ) is commonly detected in fish and fish products. 

Controlled studies have shown some birds to be susceptible to dioxins, showing reduced egg production, embryotoxicity, and cardiovascular and brain malformations. Mink eating contaminated fish show listlessness, anorexia, reduced red blood cell counts, and enlarged spleens, livers, and lungs. Again, the contamination by PCBs often makes the discrimination between dioxins and PCBs difficult. 

Dioxin and PCBs are hypothesized to be toxic because of three modes of action (McKinney and Waller 1994). First, these compounds are toxic due to their irreversible chemical reactivity in binding to a variety of macromolecules such as DNA. Second, these compounds are highly lipid soluble and may accumulate in lipid-rich cellular components. Third, these compounds can reversibly react to specific sites in receptors and enzymes. Overall toxicity is certainly due to a combination of these items although we will concentrate on the third mode of action. 

Stacking by 2,3,7,8 dioxin and PCBs. Note that the 2.2, 4,4, 6,6 HCB does not effectively interact with the Porphine ring system. 

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