2,3,7,8-tetra-CDD

There is a pronounced difference in biological and toxicological effects between different PCDD and PCDF isomers which is contradictory to the chemical and physical properties of these compounds discussed above. The isomers with the highest acute toxicity are 2,3,7,8-tetra-CDD, 1,2,3,7,8-penta-CDD, 1,2,3,, 7 8-, 1,2,3,6,7,8- and 1,2,3,7,8,9-hexa-CDD, 2,3,7,8-tetra-CDF, 1,2,3,7 8 and 2,3,, 7 8-penta-CDF and 2,3,U,6,7,8-hexa-CDF, see Figure 2. All these isomers have their four lateral positions substituted for chlorine, and they all have LD,-0 values in the range 1-100 yg/kg for the most sensitive animal species (15-17) The same isomers have been reported to have the highest biological potency (l8). 

A. Phenoxy Acids (2,U,5 T). In 1971 Courtney and Moore (29) observed teratogenic effects for the phenoxy acid 2,U,5-T. They related these effects to the presence of 30 yg/g of 2,3,7 8 -tetra-CDD found in the particular sample used in that study. 

The levels of 2,3,7 8 tetra-CDD in drums of Herbicide Orange placed in storage in the USA and in the Pacific before 1970 have been found to vary between 0.1 and U7 yg/g (3M. Since Herbicide Orange was formulated as a 1 1 mixture of the butyl esters of 2,U-D and 2,U,5-T, the levels of 2,3,7,8 tetra-CDD in individual 2,U,5 T preparations used in the 1960 s could be as high as 100 yg/g. As a result of governmental regulations, efforts were made during the 1970 s to control and to minimize the formation of 2,3,7>8 tetra-CDD, and now all producers claim that their products contain less than 0.1 yg/g of 2,3,7 8 tetra-CDD. 

In analyses using high-resolution GC/MS and MS confirmation, Rappe et al. (35.1 and Norstrom et al. (30) have reported that in other samples of Herbicide Orange, as well as in European and US 2,U,5-T formulations from the 1950 s and 1960 s, 2,3,7,8--tetra-CDD was the dominating compound of this group. Only minor amounts of other PCDDs and PCDFs could be found, primarily lower chlorinated PCDDs in samples of Herbicide Orange, see ref 35. 

Table II. Levels of 2,3,7,8-tetra-CDD in 2,U,5-T acid and 2,U,5-T ester formulations (30, 35).

The toxicity of the isomers varies considerably among the different PCDD s and PCDFs. Only about 17 out of the 210 dioxin and fiiran congeners are toxic. The greatest toxicity possesses the 2,3,7,8-tetra-CDD (the so-called Seveso poison ), and the 2,3,7,8-tetra-CDF. By convention, the toxicity of a mixture of PCDD s and PCDF s is determined as Total Toxic Equivalent = TEQ by multiplying the concentration of the different congeners with a specific Toxic Equivalent Factor = TEF ... 

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... 

The results from each of the 13 sites stack tested under Tier 4 are presented in Table 3. Data presented in this table represent concentrations of emissions measured in the stack gases and are not representative of ground-level concentrations where people reside In addition, the specific 2,3,7,8-tetra isomers for both CDD and CDF are presented in the respective tables. 

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]. 

TCDD (perz-dechlorination), from one, which does not produce this tetra-CDD isomer (perz-lateral dechlorination). The relative contribution of each pathway (i.e., the ratio of 2,3,7,8- to other tetra-substituted congeners) observed is dependent on the system tested, whereby the presence of phenolic compounds appears to shift the pathway toward peri-dechlorination and enhances the total yield of lesser chlorinated products [433]. 

Wood duck, Aixsponsa] Arkansas 1988-90 eggs PCDD-contaminated site vs. reference site 2,3,7,8-TCDD Tetra-CDDs... 

Chlorinated dioxins (CDDs) are a family of compounds that includes some extremely toxic and potent congeners. The two most toxic of the CDDs in mammals are 2,3,7,8-TCDD and 1,2,3,7,8-PeCDD (Buser 1987 Poland and Knutson 1982 Safe 1986 WHO 1997). In general, the more toxic congeners to mammals appear to be the 2,3,7,8-substituted tetra-, penta-, and hexachloro- compounds, (e.g., 2,3,7,8-TCDD, 1,2,3,7,8-PeCDD, 1,2,3,4,7,8-HxCDD, 1,2,3,6,7,8-HxCDD, and 1,2,3,7,8,9-HxCDD)... 

Hockel J, Hagenmaier H (1995), Organohalogene Compounds 23 139-140.. .Selective separation of 2,3,7,8 - Substituted tetra to hexa CDD/CDF from other PCDD/PCDF by fractionation on Alumina. B Super I for Dioxin Analysis", Eds. Droxin 95 Secretariat, Edmonton, Canada, ISBN 3-928379-13-5... 

TCDD. The total quantified tetra-hexa congeners were less than the treatment goal of 1 ng/g for the first three tests, which were performed at the lower feed rates. Test 4, made at the highest feed rate, nearly met this value, whereas the much lower soil temperature used for the final test resulted in almost 3 ng/g combined residual CDD and CDF. A longer residence time could have improved this performance. Residual 2,4-D and 2,4,5-T concentrations were less than 1 yg/g for all but the final test. This reduction represents greater than 99.97 percent removal efficiency for these primary HO constituents. 

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