Dr Masatoshi Morita, National Institute for Environmental Studies, Japan

2,3,7,8-Tetrachlorodizenzodioxin(TCDD)and related compounds have been known to be extremely toxic, persistent in the environment, and bio-accumulative. The emission sources and exposure routes of these compounds have changed during the past 50 years and contamination levels in the human body must have reflected those changes.

The major source of Dioxins in the 1960's and early 1970's in Japan was from the production of specific organochlorine chemicals. PCDD is known to occur as an impurity of chlorophenols. For example, the most toxic 2,3,7,8-TCDD is known as a trace impurity of 2,4,5-trichrophenol and its derivates such as 2,4,5-trichlorophenoxy acetic and (2,4,5-T) and its related compounds.

Case reports of TCDD intoxication among workers in chlorophenol production plants are seen worldwide, and also in Japan. The 2,4,5-T formulation, used in the 1960's for forest herbicide, might contain c.a.10 ppm, and 2,3,7,8-TCDD coming from 2,4,5-T. This caused serious problems in Vietnam. Another herbicide, CNP, is known to contain a significant amount of PCDDs in the early product (Masunaga et al.).

Pentachlorophenol (PCP) was one of the high volume production chemicals and was used for a variety of applications, including herbicide for rice cultivation and fungicide for wood preservation. Major dioxin isomers of PCP are octachlorodioxin and heptachlorodioxins with a lesser extent of hexachlorodioxins. Na salt of PCP is known to convert to OCDD quantitatively at 360 ? (Langer et al. 1973).

Levels of Dioxin contamination in humans are reflected in human milk samples. The dioxin isomer that appears dominantly in human milk is OCDD. The large use of PCP during the 1960's and its discontinuation in the 1970's may be reflected by a high concentration of OCDD in the older Japanese population.

A major contribution to total toxicity equivalency (TEQ) is from 1,2,3,7,8-pentachlorodioxin, 1,2,3,6,7,8-hexachlorodioxin and 2,3,4,7,8-pentachlorodibenzodibenzofuran. The sum of these three isomers occupies more than 80% of total TEQ of PCDD/PCDFs. Dioxin levels in humans are slowly declining, probably reflecting the termination of use of organochlorine herbicides and PCBs.

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