Octa-CDD

Data on the bioavailability of PCDDs are limited. It is known that PCDDs incorporated into wood as a result of chlorophenol (preservative) treatment are bioavailable. Swine and poultry using chlorophenol-treated wooden pens or litter have been found to be contaminated with PCDDs (NRCC 1981). Toxicities of individual PCDD isomers can vary by a factor of 1000 to 10,000 for isomers as closely related as 2,3,7,8-TCDD and 1,2,3,8-TCDD, or 1,2,3,7,8-penta-CDD and 1,2,4,7,8-penta-CDD (Rappe 1984). Isomers with the highest biological activity and acute toxicity have four to six chlorine atoms, and all lateral (i.e., 2,3,7, and 8) positions substituted with chlorine. On this basis, the most toxic PCDD isomers are 2,3,7,8-TCDD, 1,2,3,7,8-penta-CDD, 1,2,3,6,7,8-hexa-CDD, 1,2,3,7,8,9-hexa-CDD, and 1,2,3,4,7,8-hexa-CDD (Rappe 1984). Ishizuka et al. (1998) have assigned toxic equivalencies for various PCDDs, with 2,3,7,8-TCDD given a value of 1 (highest biological activity), followed by a value of 0.5 for 1,2,3,7,8-penta-CDD a value of 0.1 for three PCDD isomers (1,2,3,4,7,8-hexa-CDD, 1,2,3,4,7,8-hexa-CDD, 1,2,3,7,8,9-hexa-CDD), a value of 0.01 for 1,2,3,4,6,7,8-hepta-CDD and a value of 0.001 for 1,2,3,4,6,7,8,9-octa-CDD. 

Mutagenic responses were produced in Escherichia coli and certain strains of Salmonella typhimurium bacteria by 2,3,7,8-TCDD, but not by octa-CDD (Vos 1978). Further, chromosomal aberrations were induced in at least one species of higher plant and mammal (Ramel 1978). It must be concluded at this time that 2,3,7,8-TCDD is mutagenic or has mutagenic potential. 

The environmental burden of waterways with polychlorinated dibenzo-p-dioxins (PCDD) has been at the forefront of public and regulatory concern, because of the toxicity associated with particularly the 2,3,7,8-(laterally) substituted congeners, which have a tendency to bioaccumulate throughout the trophic food chain. Contamination of aquatic sediments by dioxins includes both non-point (e.g., atmospheric deposition) and point sources (e.g., industrial effluents, combined sewage overflows), and is generally characterized by a dominance of hepta- and octa-CDD, with minor contributions of hexa- to tetra-CDD [429]. Elevated concentrations of the 2,3,7,8-TCDD isomer tend to be associated with direct discharge from sources such as 2,4,5-trichlorophenol production [54,430]. 

Based on information by Fu et al. [219], other reactions include trans-chlorinations (migration of chlorine from PCDD to organic matter) and polymerizations, which have not been quantified. In spiked sediments from the Hudson and Passaic Rivers and sediment microorganisms, lesser-chlorinated products accounted for 10-15% of the decrease in octa-CDD. Hepta-, tetra-, tri-, and 2-mono-CDD congeners tend to dominate the dechlorination pattern [432, 433]. The microbial dechlorination sequence of octa-CDD is provided in Fig. 29 [432], which distinguishes a pathway via... 

Fig. 29. Dechlorination pathway of octa-CDD in the presence of microbial cells [432]...

From a chemical point of view TCDD is considered to be a stable compound, but due to its extreme toxicity, its chemistry has not been fully evaluated. However, it undergoes substitution reactions like chlorination to octa-CDD (11 as well as photochemical dechlorination (12 ). Thermally, TCDD is quite stable, and rapid decomposition occurs only at temperatures above 750°C... 

The results are rationalized in terms that the metabolism of the PCDDs occurs mainly via 2,3-epoxides. In the octa-CDD as in 2,3,7 8-t etra-CDD these positions are blocked, consequently the reaction is less likely to take place or takes place at a highly reduced rate (23). 

Buser and Bosshardt reported on the results of a survey on the PCDD and PCDF contents of pentachlorophenol (PCP) and PCP-Na from commercial sources in Switzerland (UO). From the results, the grouping of the samples into two series can be observed a first series with generally low levels (hexa-CDD < 1 ppm) and a second series with much higher levels (hexa-CDD > 1 ppm) of PCDDs and PCDFs. Samples of high PCDD contents had also high PCDF contents. For most samples, the contents of these contaminants were in the order tetra- penta < hexa < hepta octa--CDD/CDF. The ranges of the combined levels of PCDDs and PCDFs were 2-l6 and 1-26 ppm, respectively, for the first series of samples, and 120-500 and 85 570 ppm, respectively, for the second series of samples. The levels of octa-CDD and octa-CDF were as high as 370 and 300 ppm, respectively (UP). 

Commercial and technical products. During the 1960 s and 19T0 s most of the interest was focused on the PCDD and PCDF contamination in technical products like 2,U,5-T and other chlorophenoxy acids, pentachlorophenol and other chlorinated phenols, and PCB. The levels were found to vary from batch to batch, but the analyses were mostly non-isomer specific. The major contaminant in 2,U,5-T was 2,3,T 8--tetra-CDD, normally in the lower ppm-range or less (l). In general, technical pentachlorophenols were found to be the most contaminated products, levels were reported in the range of 1000 ppm, mainly for octa-CDD, but also a multitude of lower chlorinated congeners (3.). 

Air and particulate. Due to sampling and analytical problems very few data are available on the levels of PCDDs and PCDFs in normal urban air. Samples of air particulates from Washington, D.C., and St. Louis, MO, USA have been analyzed by Crummett et al. ik) and Czuczwa and Hites (15). Crummett et al. ik) report on isomer specific analyses for tetra-CDDs and hexa-CDDs while Czuczwa and Hites (15) only report a congener profile and do not discuss any specific isomers. In both studies octa-CDD was found to be a major constituent, reported in levels of 150-200 ppb. The level of 2,3,T 8-tetra--CDD was 5 50 ppt and of 2,3,T,8-tetra-CDF was IOO-38O ppt (lU). 

Residues of PCDD and PCDF were detected in highest concentration in the fruit composite with the predominent residues being Penta-CDF, (0.28 ug/kg), Hex-CDF (2.0 ug/kg), Hex-CDD (0.45 ug/kg), Hep-CDF (6.6 ug/kg), Hep-CDD (1.2 ug/kg) Octa-CDF (1.2 ug/kg) and Octa CDD (1.2 ug/kg). Residues in milk were the second highest with the predominant congeners being Tetra-CDF, Tetra-CDD, Pe CDF, Hep-CDF, Hep-CDD, Octa-CDF, and Octa CDD all found between 0.01 and 0.1 ug/kg. Root vege tables and the meat/eggs composite were contaminated at lower concentrations. 

After each soil sample was blended, air dried, and screened (2 mm sieve opening) triplicate aliquots were taken and analyzed for 2,3,7,8-TCDD, CDD and CDF congeners, and 2,4-D and 2,4,5-T. The three prepared soils had 2,3,7,8-TCDD levels greater than 100 ng/g and 2,4-D/2,4,5-T levels of about 1000 yg/g. The JI soil had significant concentrations of hepta and octa CDD compared with the other two samples. In addition, selected physical and chemical properties presented in Table I, were measured (6). The EPA test program (5) had indicated that soil properties had only a minor influence on removal efficiencies for 2,3,7,8-TCDD. 

Buser (1979) has shown that pyrolysis of chlorobenzenes at 600 C in the presence of excess air yielded about 1% yield of tetra- to octa-CDFs and tetra- to octa-CDDs. It has also been shown (Buser and Rappe, 1979) that the pyrolysis of PCBs occurs intramolecularly by four alternative reaction routes to yield different isomeric PCDF products. 

The TEF-value for 2,3,7,8-tetra-CDD is set = 1 by definition, the same value is valid for 2,3,7,8-tetra-CDF. The factors of the other dioxins vary from 0.001 for octa-CDDs... 

Figure 4.120 Lowest level calibrations peaks (Cal 1) for tetra- to octa-CDD/F congeners, top traces PCDDs, bottom PCDFs with Injected amounts.

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