Part I Volume I.

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:

VolumeI- 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.

VolumeII - 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.

VolumeIII- Site-Specific Assessment Procedures

This volume presents procedures for evaluating the incremental impact from sources of dioxin released 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.


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 I-1, this TEF scheme assigns nonzero values to all chlorinated dibenzo-p-dioxins (CDDs) and chlorinated dibenzofurans (CDFs) with chlorine substituted in the 2,3,7,8 positions.

Additionally, the analogous brominated compounds (BDDs and BDFs) and certain polychlorinated biphenyls (PCBs, see Table I-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 I-3.

table Table I-1. Toxicity Equivalency Factors (TEF) for CDDs and CDFs. table Table I-2. Dioxin-Like PCBs.
expand table table 3-1 expand table Table3-1ii
table Table I-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.

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

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

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

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.


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 it 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 an information source for evaluating exposures to dioxin-like compounds.
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 environmental 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.

  • 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.
Part I Volume II.
Polychlorinated dibenzodioxins (CDDs), polychlorinated dibenzofurans (CDFs), and polychlorinated biphenyls (PCBs) are chemically classified as halogenated aromatic hydrocarbons.

The chlorinated and brominated dibenzodioxins and dibenzofurans are tricyclic aromatic compounds with similar physical and chemical properties, and both classes are quite similar structurally.

There are 75 possible different positional congeners of CDDs and 135 different CDF congeners. Only 7 of the 75 possible CDD congeners, and 10 of the 135 possible CDF congeners, those with chlorine substitution in the 2,3,7,8 positions, are thought to have dioxin-like toxicity. Likewise, there are 75 possible different positional congeners of BDDs and 135 different congeners of BDFs (see Table II-1). The basic structure and numbering of each chemical class is shown in Figure II-1.
table Figure II-1. Structure of Dioxins and Furans. table Table II-1. Possible number of positional CDD (or BDD) and CDF (or BDF) congeners.
expand table table 3-1 expand table Table3-1ii

There are 209 possible PCB congeners, only 11 of which are thought to have dioxin-like toxicity. These dioxin-like congeners have four or more chlorine atoms with no more than one substitution in the ortho positions (positions designated 2, 2', 6 or 6' in Figure II-2). Dioxin-like PCBs are listed in Table I-2. These compounds are sometimes referred to as coplanar PCBs, since the rings can rotate into the same plane if not blocked from rotation by ortho-substituted chlorine atoms. The physical/chemicalproperties of each congener vary according to the degree and position of chlorine substitution.

The basic structure and numbering of each chemical class is shown in Figure II-2.In general, these compounds have very low water solubility, high octanol-water partition coefficients, low vapor pressure and tend to bioaccumulate. Volume II presents congener-specific values for water solubility, vapor pressure, partition coefficients and photo quantum yields.

table Figure II-2 Structure of dioxin-like PCBs.
Despite a growing body of literature from laboratory, field, and monitoring studies examining the environmental fate and environmental distribution of CDDs and CDFs, the fate of these environmentally ubiquitous compounds is not yet well understood. In soil, sediment, and the water column, CDDs/CDFs are primarily associated with particulate and organic matter because of their high lipophilicity and low water solubility.

In a detailed evaluation of ambient air monitoring studies in which researchers evaluated the partitioning of dioxin-like compounds between the vapor and particle phases, a principal conclusion was that the higher chlorinated congeners, the hexa through hepta congeners, were principally sorbed to airborne particulates, whereas the tetra and penta congeners significantly, if not predominantly, partition to the vapor phase.

expand table table 3-1

This finding is consistent with vapor/particle partitioning as theoretically modeled in Bidleman (1988). Dioxin-like compounds exhibit little potential for significant leaching or volatilization once sorbed to particulate matter. The available evidence indicates that CDDs and CDFs, particularly the tetra- and higher chlorinated congeners, are extremely stable compounds under most environmental conditions. The only environmentally significant transformation process for these congeners is believed to be photodegradation of nonsorbed species in the gaseous phase, at the soil-air or water-air interface, or in association with organic cosolvents. CDDs/CDFs entering the atmosphere are removed either by photodegradation or by deposition. Burial in-place, resuspension back into the air, or erosion of soil to water bodies appears to be the predominant fate of CDDs/CDFs sorbed to soil. CDDs/CDFs entering the water column primarily undergo sedimentation and burial. The ultimate environmental sink of CDDs/CDFs is believed to be aquatic sediments.

Little specific information exists on the environmental transport and fate of the 11 coplanar PCBs. However, the available information on the physical/chemical properties of coplanar PCBs coupled with the body of information available on the widespread occurrence and persistence of PCBs in the environment indicates that these coplanar PCBs are likely to be associated primarily with soils and sediments, and to be thermally and chemically stable. PCBs volatilize from the surfaces of soils and water bodies and are dispersed via air movement. Subsequently they can be deposited back into soil or water. In water bodies, they can be spread via sediment transport. Though not rapid processes, these mechanisms account for the widespread environmental occurrence of PCBs. Photodegradation to less chlorinated congeners followed by slow anaerobic and/or aerobic biodegradation is believed to be the principal path for destruction of PCBs.


Ancient human tissue sampling shows much lower CDD/F levels than found today (Ligon et al., 1989). Studies of sediment cores in lakes near industrial centers of the United States have shown that dioxins and furans were quite low until about 1920 (Czuczwa, et al., 1984; Czuczwa and Hites, 1985; Smith, et al., 1992). These studies show increases in CDD/F concentrations beginning in the 1920s and continuing until about 1970.

Declining concentrations have been measured since this time. These trends cannot be explained by changes in natural processes and have been shown to correspond to chlorophenol production trends (Czuczwa and Hites, 1984). On this basis, it appears that the presence of dioxin-like compounds in the environment occurs primarily as a result of anthropogenic practices. This section will review the theories of formation and emission of these compounds, and then discuss the possible sources which can release them to the environment.

II.2.1.Theories of Formation During Combustion

The emission of CDDs and CDFs into the environment from combustion processes can be explained by three principal theories, which should not be regarded as being mutually exclusive:

(1) contaminated feedstock,
(2) formation from precursors, and
(3) formation de novo.

In general, the primary theories can be summarized as follows:

(1) The feed material to the combustor contains CDDs and CDFs and some portion survives the thermal stress imposed by the heat of the incineration or combustion process, and is subsequently emitted from the stack. While this explanation is not thought to be the principal explanation for dioxin and furan emissions from combustor sources (explanations 2 and 3 below are thought to be the predominant cause of these emissions), in fact it is the single theory best thought to explain the release of the dioxin-like, coplanar PCBs.

(2) CDDs/CDFs are ultimately formed from the thermal breakdown and molecular rearrangement of precursor compounds. Precursor compounds are chlorinated aromatic hydrocarbons having a structural resemblance to the CDD/CDF molecule. Among the precursors that have been identified are polychlorinated biphenyls (PCBs), chlorinated phenols (CPs), and chlorinated benzenes (CBs). The formation of CDDs/CDFs is believed to occur after the precursor has condensed and adsorbed onto the binding sites on the surface of fly ash particles.

The active sites of the surface of fly ash particles promote the chemical reactions forming CDDs/CDFs. These reactions have been observed to be catalyzed by the presence of inorganic chlorides sorbed to the particulate. Temperature in a range of 250-450 C has been identified as a necessary condition for these reactions to occur, with either lower or higher temperatures inhibiting the process.

Therefore, the precursor theory focuses on the region of the combustor that is downstream and away from the high temperature zone of the furnace or combustion chamber. This is a location where the gases and smoke derived from combustion of the organic materials have cooled during conduction through flue ducts, heat exchanger and boiler tubes, air pollution control equipment or the stack.

(3) CDDs/CDFs are synthesized de novo in the same region of the combustion process as described in (2), e.g. the so-called cool zone. In this theory, CDDs/CDFs are formed from moieties bearing little resemblance to the molecular structure of CDDs and CDFs. In broad terms, these are non-precursors and include such diverse substances as petroleum products, chlorinated plastics (PVC), non-chlorinated plastics (polystyrene), cellulose, lignin, coke, coal, particulate carbon, and hydrogen chloride gas. Formation of CDDs/CDFs requires the presence of a chlorine donor (a molecule that provides a chlorine atom to the pre-dioxin molecule) and the formation and chlorination of a chemical intermediate that is a precursor.

The primary distinction between theories (2) and (3) is that theory (2) requires the presence of precursor compounds in the feed material whereas theory (3) begins with the combustion of diverse substances that are not defined as precursors, which eventually react to form precursors and eventually, dioxin-like molecules.