harmful to rats, based on the results of a 3-generation study (McNulty 1977; 

 Murray et a1. 1979). Unacceptable dietary levels of 50 ppt (equivalent to 1.7 

 ng/kg body weight daily) are recorded for monkeys (Ramel 1978; Barsotti et al. 

 1979; NRCC 1981). 



In the past, the major source of 2,3,7,8-TCDD in the environment was as a 

 contaminant in phenoxy herbicides (such as 2,4,5-T; Silvex; 2,4-D; and Agent 

 Orange), in hexachl orophene , and in other chlorophenol -type compounds. 

 Concentrations of 2,3,7,8-TCDD in some of these products exceeded 60,000 ppb. 

 However, this situation has been largely corrected by new manufacturing 

 processes and by increasingly stringent Federal regulations (NRCC 1981; 

 Choudhary et al. 1983; Stolzenburg and Sullivan 1983; NIOSH 1984; Rappe 

 1984). For example, 2,3,7,8-TCDD level in 2,4,5-T has decreased from 60,000 

 ppb in 1957 to 2,000 ppb in 1965 as a result of new manufacturing processes, 

 and it was limited to 500 ppb in 1970 by the Canadian Federal Government. In 

 1970, the U.S. Department of Defense halted the spraying of Agent Orange. In 

 1972, the U.S. Food and Drug Administration banned the use of hexachlorophene 

 in nonprescription soaps and deodorants. In 1978, 7 of 14 major producers of 

 2,4,5-T no longer manufactured this product, and the remainder claimed that 

 their products contained less than 100 ppb of 2,3,7,8-TCDD. In 1979, 

 production of 2,4,5-T and Silvex ceased in the United States, although 

 stockpiles of both are still being distributed and permitted for use on rice 

 fields, sugarcane fields, orchards, fence rows, vacant lots, and lumber 

 yards. In 1982, the EPA required some industries to certify that 

 chlorophenol -type compounds were no longer used as slime control agents. On 

 October 18, 1983, EPA published its intent to cancel the registration of 

 pesticide products containing 2,4,5-T and Silvex, and to prohibit the 

 transfer, distribution, sale, or importation of any unregistered product 

 containing 2,4,5-T, Silvex, or their derivitives (NIOSH 1984). 



At present, burning or heating of commercial and purified chlorophenates , 

 and pyrolysis of polychlori nated biphenyls contaminated with trichlorobenzenes 

 can result in the production of 2,3,7,8-TCDD and other PCDDs (NIOSH 1984). 

 These sources together with discharges from various municipal and industrial 

 incinerators of chlorinated compounds probably constitute the largest source 

 of PCDDs in the environment today. In 1983, the U.S. Environmental Protection 

 Agency proposed to monitor 2,3,7,8-TCDD in the environment (Stolzenburg and 

 Sullivan 1983). Specific goals of the monitoring program include: 

 determination of 2,3,7,8-TCDD concentrations in soils and biota, with emphasis 

 on geographic areas where PCDDs may have been manufactured, used, or 

 stored--and where concentrations may be in excess of 1,000 ppt; monitoring of 

 industrial and municipal incinerators for TCDD emissions; and establishment of 

 background levels for PCDDs in areas where these compounds are not expected to 

 occur in high levels. It seems that information is also needed on the 

 toxicological interactions of groups of polychlorinated chemicals (such as 

 certain biphenyls, biphenylenes , and dibenzofurans) known to be isosteric with 



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