In 1991, the EPA began a comprehensive Reassessment of 2,3,7,8-TCDD (dioxin) and Related Compounds. In 1985 and in 1988, the EPA had issued assessments and re-assessments of these super-toxic chemical by-products, and each time had been challenged to study,. verify, and refine its initial reports using more data. In 1994, the Dioxin Reassessment was issued in draft form for review, and was supposed to be issued to the public within six months. Unfortunately, because of pressure from the chemical and other industries, the Science Advisory Board has not released this assessment to the general public, nor can the government formulate policy decisions based on it.

This document is the most important chapter of the Reassessment - the Risk Characterization - which is a summary of the Health Assessment and Exposure sections of the Reassessment and conclusions about human exposure and toxic effects.

TABLE OF CONTENTS - OVERVIEW

9.1. INTRODUCTION 9-1

9.2. CHEMICAL STRUCTURE AND PROPERTIES 9-3

9.3. ENVIRONMENTAL FATE 9-7

9.4. SOURCES 9-8

9.5. DISPOSITION AND PHARMACOKINETICS 9-23

9.6. MECHANISMS OF DIOXIN ACTION 9-30

9.7. TOXIC EFFECTS OF DIOXIN 9-36

9.8. DOSE-RESPONSE CONSIDERATIONS 9-53

9.9. USE OF TOXICITY EQUIVALENCE 9-69

9.10. KEY ASSUMPTIONS AND INFERENCES 9-70

9.11. OVERALL CONCLUSIONS REGARDING THE IMPACT OF DIOXIN AND RELATED COMPOUNDS ON HUMAN HEALTH 9-74

REFERENCES FOR CHAPTER 9 9-89