9.1. INTRODUCTION

Chlorinated dibenzo-p-dioxins and related compounds (commonly known simply as dioxins) are contaminants present in a variety of environmental media. This class of compounds has caused great concern in the general public as well as intense interest in the scientific community. Much of the public concern revolves around the characterization of these compounds as among the most potent "man-made" toxicants ever studied. Indeed, these compounds are extremely potent in producing a variety of effects in experimental animals based on traditional toxicology studies at levels hundreds or thousands of times lower than most chemicals of environmental interest. In addition, human studies demonstrate that exposure to dioxin and related compounds is associated with subtle biochemical and biological changes whose clinical significance is as yet unknown and with chloracne, a serious skin condition associated with these and similar organic chemicals. Laboratory studies suggest the probability that exposure to dioxin-like compounds may be associated with other serious health effects including cancer. Human data, while often limited in their ability to answer questions of hazard and risk, are generally consistent with the observations in animals. Whether the adverse effects noted above are expressed in humans, or are detectable in human population studies, is dependent on the dose absorbed and the intrinsic sensitivity of humans to these compounds. Recent laboratory studies have provided new insights into the mechanisms involved in the impact of dioxins on various cells and tissues and, ultimately, on toxicity. Dioxins have been demonstrated to be potent modulators of cellular growth and differentiation, particularly in epithelial tissues. These data, together with the collective body of information from animal and human studies, when coupled with assumptions and inferences regarding extrapolation from experimental animals to humans and from high doses to low doses, allow a characterization of dioxin hazards.

This chapter presents a risk characterization for dioxin and related compounds. In the risk characterization, key findings pertinent to understanding the hazards and risks of dioxin and related compounds are described and integrated. All of the available information is considered in proposing hypotheses or in reaching conclusions. The risk characterization is not meant to be an executive summary of the extensive data base that has been analyzed in detail in preceding chapters and in the Exposure Document. Risk characterization requires a discussion of likely routes, patterns, and levels of exposure as well as aspects of hazard and dose response. Information contained in the document titled Estimating Exposure to Dioxin-like Compounds (U.S. EPA, 1994), hereafter referred to as the Exposure Document, will be integrated with the health effects information on this class of compounds found in previous chapters of this assessment. The risk characterization articulates the strengths and weaknesses of the available evidence and clearly presents assumptions made and inferences used. Risk is characterized in both qualitative and quantitative terms, as appropriate. Finally, overall conclusions regarding the health risks of dioxin and related compounds are presented.

The process for developing this risk characterization of dioxin and related compounds has been an open and participatory one. The Health Assessment and Exposure Documents that provide the basis for this characterization have been developed in collaboration with scientists from within and from outside of the Federal Government. Each of these has undergone extensive internal and external review, including review at a meeting of experts after a first draft was completed. Additional input to this characterization comes from comments on those draft chapters as well as from the panel of experts that met in September 1992. Panel members were asked to provide their perspective on themes to be carried into the characterization and their contributions are reflected here. Finally, the characterization, as presented here, reflects review and comment by both those Federal scientists involved in developing the health assessment and exposure chapters as well as representatives of other Federal agencies. However, the views expressed in this characterization are those of the collective authors and, as a draft undergoing public comment and further external review, no Agency-level endorsement should be inferred at this time.

Once fully peer reviewed and revised accordingly, this risk characterization is meant to provide a balanced picture of the scientific findings of the health and exposure assessments for use by risk managers in selecting risk management options, based on this and other information. As an integrated analysis of a complex data base, it is meant to answer key questions concerning the science behind concerns for dioxins and should be useful in developing strategies for risk communication.