ENVIRONMENTAL CHEMISTRY 



The PCDDs consist of 75 isomers that differ in the number and position of 

 attached chlorine atoms; each isomer has its own unique identity and 

 toxicological properties. The most toxic of the chlorinated dioxin isomers is 

 2,3,7,8-TCDD (Figure 1). It is one of 22 possible congeners of 

 tetrachlorodibenzo- £- dioxin. There is general agreement that PCDDs, 

 including 2,3,7,8-TCDD, are (or were, until recently) found in chlorophenols, 

 especially trichlorophenol and pentachlorophenol (Table 1), in certain phenoxy 

 pesticides (2,4,5-T; 2,4-D; Fenoprop; Silvex; Ronnel ; Erbon; Agent Orange), in 

 hexachlorophene, and in polychlorinated biphenyls (used in electrical 

 transformers and capacitors, and contaminated with trichlorobenzenes). PCDDs 

 enter the environment through accidental release during chlorophenol 

 production, through aerial application of some phenoxy herbicides, and through 

 improper disposal of wastes into terrestrial and aquatic ecosystems (Ramel 

 1978; NRCC 1981; Ogilvie 1981: Choudhary et al . 1983; Josephson 1983; 

 Stolzenburg and Sullivan 1983; NIOSH 1984; Rappe 1984; Kamrin and Rodgers 

 1985). The PCDD content of technical products varies between manufacturers, 

 between lots and grades, and between various formulations of pesticidal 

 chemicals (NRCC 1981). More recently, PCDDs have been identified in effluents 

 from combustion products of municipal and industrial incinerators, including 

 fly ash and flue gas (Czuczwa and Hites 1984). These PCDDs may be associated 

 with small particles which have long residence times in the atmosphere and can 

 become distributed over large areas. For example, in the Great Lakes 

 atmospheric transport of combustion products is the major source of PCDDs 

 (mostly octa-, hepta-, and hexa-CDDs) in sediments (Czuczwa et al • 1984). 

 High-temperature combustion of bituminous coal in an oxidized and chlorinated 

 atmosphere (produced experimentally) yielded chlorodioxins, mostly octa-, 

 hepta-, and hexa-CDDs and measurable quantities of tetra-CDDs (Mahle and 

 Whiting 1980). Other potential sources of PCDDs include fossil fuel power 

 plants, internal combustion engines, home fireplaces, and cigarette smoke 

 (Kociba and Schwetz 1982a, b), but they require verification. 



In general, PCDDs exhibit a relative inertness to acids, bases, 

 oxidation, reduction, and heat. With increasing halogen content, they become 

 more environmentally and chemically stabile (NRCC 1981). PCDDs are usually 

 destroyed at temperatures greater than 1,000 C. They are resistant to 

 biological breakdown, concentrated in fat, not readily excreted, extremely 

 toxic to some animals, and the cumulative effects of small doses to both 

 animals and humans are a source of increasing concern (Stolzenburg and 

 Sullivan 1983). Most PCDDs are relatively insoluble in water, sparingly 



