ORIGINS OF POLYCHLORODIBENZO-P-DIOXINS (PCDD) AND POLYCHLORODIBENZOFURANS (PCDF) IN THE ENVIRONMENT.

Contamination of the environment by polychlorinated dibenzo-p-dioxins (PCDDs) originates from three major sources: the use of certain herbicides, chemical waste and incineration. In addition, all forms of fire are also thought to be sources of PCDDs. Polychlorinated dibenzofurans (PCDF), currently known to be more widespread than PCDD, enter the environment from two main sources: as a contaminant in PCB and as a byproduct of incineration. The dibenzofurans are also known to be formed in accidental PCB fires and possibly in all types of combustion. Herbicides containing esters of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) have been used in a variety of ways in both the United States and abroad. These phenoxy herbicides are still being used on rice and sugar grown in certain parts of the USA. The trichlorophenoxy herbicides contain trace amounts of tetrachlorodibenzo-p-dioxins (TCDDs) as impurities resulting during the synthesis of precursors. In this review, an attempt has been made to provide some perspective of the various sources of PCDD and PCDF in the environment. Much of the research cited herein has been conducted at the University of Nebraska-Lincoln with several collaborators, and the review should not be considered an exhaustive survey. Refs.