1. INTRODUCTION 1-1 Contents

  • 1.1. BACKGROUND 1-1
  • 1.2. TEFS and TEQS 1-2
  • 1.3. OVERALL COMMENTS ON THE USE OF THE DIOXIN EXPOSURE DOCUMENT 1-6
  • 1.4. ORGANIZATION OF VOLUME II 1-7
1. INTRODUCTION

1.1. BACKGROUND

In May of 1991, the Environmental Protection Agency (EPA) announced a scientific reassessment of the human health and exposure issues concerning dioxin and dioxin-like compounds (56 FR 50903). This reassessment has resulted in two reports: a health reassessment document (EPA, 1994), and Estimating Exposure to Dioxin-Like Compounds [this three-volume report], which expands upon a 1988 draft exposure report titled, Estimating Exposure to 2,3,7,8-TCDD (EPA, 1988).

The health and exposure reassessment documents can be used together to assess potential health risks from exposure to dioxin-like compounds. In a related area, EPA has also discussed the data and methods for evaluating risks to aquatic life from 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) (EPA, 1993).

The purpose of the exposure portion of the dioxin reassessment is to describe the causes and magnitude of background exposures, and provide site-specific procedures for evaluating the incremental exposures due to specific sources of dioxin-like compounds.

In September of 1992, EPA convened workshops to review the first public drafts of the health (EPA, 1992a) and the exposure documents (EPA, 1992b). The current draft of the exposure document incorporates changes as a result of that workshop as well as other review comments.

The exposure document is presented in three volumes. Following is a summary of the material contained in each of the three volumes:

Volume I - Executive Summary

This volume includes summaries of findings from Volumes II and III. It also includes a unique section on research needs and recommendations for dioxin-like compounds.
Volume II - Properties, Sources, Environmental Levels, and Background Exposures

This volume presents and evaluates information on the physical-chemical properties, environmental fate, sources, environmental levels, and background human exposures to dioxin-like compounds.

It summarizes and evaluates relevant information obtained from published literature searches, EPA program offices and other Federal agencies, and published literature provided by peer reviewers of previous versions of this document. The data contained in this volume is current through 1993 with some new information published in early 1994.
Volume III - Site-Specific Assessment Procedures

This volume presents procedures for evaluating the incremental impact from sources of dioxin release into the environment. The sources covered include contaminated soils, stack emissions, and point discharges into surface water.

This volume includes sections on: exposure parameters and exposure scenario development; stack emissions and atmospheric transport modeling; aquatic and terrestrial soil, sediment, and food chain modeling; demonstration of methodologies; and uncertainty evaluations including exercises on sensitivity analysis and model validation, review of Monte Carlo assessments conducted for dioxin-like compounds, and other discussions. The data contained in this volume is current through 1993 with some new information published in early 1994.
1.2. TOXICITY EQUIVALENCY FACTORS
Dioxin-like compounds are defined to include those compounds with nonzero Toxicity Equivalency Factor (TEF) values as defined in a 1989 international scheme, I-TEFs/89.

This procedure was developed under the auspices of the North Atlantic Treaty Organization's Committee on Challenges of Modern Society (NATO-CCMS, 1988a; 1988b) to promote international consistency in addressing contamination involving CDDs and CDFs. EPA has adopted the I-TEFs/89 as an interim procedure for assessing the risks associated with exposures to complex mixtures of CDDs and CDFs (EPA, 1989).

As shown in Table 1-1, this TEF scheme assigns nonzero values to all chlorinated dibenzo-p-dioxins (CDDs) and chlorinated dibenzofurans (CDFs) with chlorines substituted in the 2,3,7,8 positions. Additionally, the analogous brominated compounds (BDDs and BDFs) and certain polychlorinated biphenyls (PCBs, see Table 1-2) have recently been identified as having dioxin-like toxicity (EPA, 1994) and thus are also included in the definition of dioxin-like compounds.

However, EPA has not assigned TEF values for BDDs, BDFs, and PCBs. In the case of PCBs, research on the applicability of the TEF approach is ongoing but there is not yet any formal EPA policy. The nomenclature adopted here for purposes of describing these compounds is summarized in Table 1-3.
table Table 1-1. Toxicity Equivalency Factors (TEF) for CDDs and CDFs. table Table 1-2. Dioxin-Like PCBs.
expand table table 3-1 expand table Table3-1ii
table Table 1-3. Nomenclature for dioxin-like compounds..

The procedure relates the toxicity of 210 structurally related individual CDD and CDF congeners and is based on a limited data base of [in vivo] and [in vitro] toxicity testing.

By relating the toxicity of the 209 CDDs and CDFs to the highly-studied 2,3,7,8-TCDD, the approach simplifies the assessment of risks involving exposures to mixtures of CDDs and CDFs (EPA, 1989).

In general, the assessment of the human health risk to a mixture of CDDs and CDFs, using the TEF procedure, involves the following steps (EPA, 1989):

expand table table 3-1

1. Analytical determination of the CDDs and CDFs in the sample.

2. Multiplication of congener concentrations in the sample by the TEFs in Table 1-1 to express the concentration in terms of 2,3,7,8-TCDD equivalents (TEQs).

3. Summation of the products in Step 2 to obtain the total TEQs in the sample.

4. Determination of human exposure to the mixture in question, expressed in terms of TEQs.

5. Combination of exposure from step 4 with toxicity information on 2,3,7,8-TCDD to estimate risks associated with the mixture.

Samples of this calculation for several environmental mixtures are provided in EPA (1989). Also, this procedure is demonstrated in Volume III of this assessment in the context of the demonstration of the stack emission source category. The seventeen dioxin-like congeners are individually modeled from stack to exposure site. TEQ concentrations are estimated given predictions of individual congener concentrations using Steps 2 and 3 above.

1.3. OVERALL COMMENTS ON THE USE OF THE DIOXIN EXPOSURE DOCUMENT
Users of the dioxin exposure document should recognize the following:

1. This document does not present detailed procedures for evaluating multiple sources of release.
However, it can be used in two ways to address this issue. Incremental impacts estimated with procedures in Volume III can be compared to background exposure estimates which are presented in Volume II. This would be a way of comparing the incremental impact of a specific source to an individual's total exposure. If the releases from multiple sources behave independently, it is possible to model them individually and then add the impacts. For example, if several stack emission sources are identified and their emissions quantified, and it is desired to evaluate the impact of all sources simultaneously, then it may be possible to model each stack emission source individually and then sum the concentrations and depositions at points of interest in the surrounding area.

2. The procedures and estimates presented in this three-volume exposure document best serve as general examples for evaluating exposures to dioxin-like compounds, rather than specific assessments.
This document was not generated for purposes of supporting any specific regulation. Rather, it is intended to be a general information source which Agency programs can adopt or modify as needed for their individual purposes. For example, the demonstration scenarios of Volume III were not crafted as Agency policy on "high end" or "central tendency" scenarios for evaluating land contamination, stack emissions, or effluent discharges. Rather, they were designed to illustrate the site-specific methodologies in Volume III.

3. The understanding of the exposure to dioxin-like compounds continues to expand.
Despite being one of the most studied groups of organic enivronmental contaminants, new information is generated almost daily about dioxin-like compounds. This document is considered to be current through 1993, with some information published early in 1994 included as well. Section IV of Volume I, Executive Summary, discusses research needs for dioxin exposure evaluation.

1.4. ORGANIZATION OF VOLUME II
The scope of the five remaining chapters and three appendices of this report are as follows:
  • Chapter 2, Physical and Chemical Properties and Fate, summarizes available information on the physical/chemical properties and environmental fate of the dioxin-like compounds.
  • Chapter 3, Sources, summarizes information about the known sources from which these compounds may enter the environment and provides estimates of annual releases to the environment from various sources.
  • Chapter 4, Environmental Levels, summarizes information on the concentrations of dioxin-like compounds found in various media (i.e., air, water, food, soil, and sediment).
  • Chapter 5, Background Exposures, provides estimates of the background exposures to dioxin-like compounds from environmental media and foods.
  • Chapter 6, Pharmacokinetics, summarizes information about uptake and distribution of dioxin-like compounds in the body and presents pharmacokinetic models to predict blood and tissues resulting from exposure. In addition, measured CDD/CDF concentrations in blood and tissues are used to back-calculate associated exposure levels.
  • Appendix A, Tables of Chemical Properties, presents congener-specific data for a variety of chemical properties.
  • Appendix B, Tables of Media Levels, summarizes congener-specific monitoring data for congeners in various media.
  • Appendix C, Bioavailability of Dioxins, summarizes data on the bioavailability of 2,3,7,8-TCDD from environmental matrices.
REFERENCES FOR CHAPTER 1
  • NATO/CCMS (North Atlantic Treaty Organization, Committee on the Challenges of Modern Society). (1988a) International toxicity equivalency factor (I-TEF) method of risk assessment for complex mixtures of dioxins and related compounds. Report No. 176.
  • NATO/CCMS (North Atlantic Treaty Organization, Committee on the Challenges of Modern Society). (1988b) Scientific basis for the development of international toxicity equivalency (I-TEF) factor method of risk assessment for complex mixtures of dioxins and related compounds. Report No. 178.
  • U.S. Environmental Protection Agency. (1988) Estimating exposure to 2,3,7,8-TCDD. U.S. Environmental Protection Agency, Office of Health and Environmental Assessment, Washington, DC; EPA/600/6-88/005A.
  • U.S. Environmental Protection Agency. (1989) Interim procedures for estimating risks associated with exposures to mixtures of chlorinated dibenzo-p-dioxins and -dibenzofurans (CDDs and CDFs) and 1989 update. U.S. Environmental Protection Agency, Risk Assessment Forum, Washington, DC; EPA/625/3-89/016.
  • U.S. Environmental Protection Agency. (1992a) Health reassessment of dioxin-like compounds, Chapters 1-8. U.S. Environmental Protection Agency, Office of Health and Environmental Assessment, Washington, DC. EPA/600/AP-92/001a through EPA/600/AP-92/001h. August 1992 Workshop Review Draft.
  • U.S. Environmental Protection Agency. (1992b) Estimating Exposure to Dioxin-Like Compounds. U.S. Environmental Protection Agency, Office of Health and Environmental Assessment, Washington, DC. EPA/600/6-88/005B. August 1992 Workshop Review Draft.
  • U.S. Environmental Protection Agency. (1993) Interim Report on Data and Methods for Assessment of 2,3,7,8-Tetrachlorodibenzo-p-dioxin Risks to Aquatic Life and Associated Wildlife. Environmental Research Laboratory, Duluth, MN, Office of Research and Development, U.S. Environmental Protection Agency. EPA/600/R-93/055. March, 1993.
  • U.S. Environmental Protection Agency. (1994) Health Assessment for 2,3,7,8-TCDD and Related Compounds. Public Review Draft. EPA/600/EP-92/001.