EPA/600/R-09/027 

May 2009 

Summary of U.S. EPA 

Dioxin Workshop 

February 18−20, 2009 

Cincinnati, Ohio 

National Center for Environmental Assessment 

Office of Research and Development 

U.S. Environmental Protection Agency 

Cincinnati, OH 45268 
DISCLAIMER 

This document summarizes the discussions presented at the Dioxin Workshop in 

February 2009, in Cincinnati, OH, as documented by the Session Co-Chairs. This document is 

not all inclusive or binding. Conclusions and recommendations to the U.S. EPA may not 

represent full consensus. The views expressed in this document are those of the Dioxin 

Workshop Panelists and do not necessarily reflect the views and policies of the U.S. 

Environmental Protection Agency. Mention of trade names or commercial products does not 

constitute endorsement or recommendation for use. 

Preferred Citation: 

U.S. Environmental Protection Agency (U.S. EPA). (2009) Summary of U.S. EPA Dioxin Workshop: 

February 18−20, 2009. U.S. Environmental Protection Agency, National Center for Environmental Assessment, 

Cincinnati, OH. EPA/600/R-09/027. 

ii
TABLE OF CONTENTS 

DIOXIN WORKSHOP TEAM ..................................................................................................... iv

ACKNOWLEDGMENTS ............................................................................................................. iv 

INTRODUCTION .......................................................................................................................... 1 

REFERENCES ................................................................................................................... 2 

SCIENTIFIC WORKSHOP TO INFORM THE TECHNICAL WORK PLAN FOR 

U.S. EPA’S RESPONSE TO NAS COMMENTS ON THE HEALTH EFFECTS 

OF DIOXIN PRESENTED IN U.S. EPA’S DIOXIN REASSESSMENT .............................. 3 

SESSION 1: QUANTITATIVE DOSE-RESPONSE MODELING ISSUES.................... 3 

SESSION 2: IMMUNOTOXICITY ................................................................................... 6 

SESSION 3A: DOSE-RESPONSE FOR NEUROTOXICITY AND 

NONREPRODUCTIVE ENDOCRINE EFFECTS................................................... 8 

SESSION 3B: DOSE-RESPONSE FOR CARDIOVASCULAR TOXICITY AND 

HEPATOTOXICITY............................................................................................... 11 

SESSION 4A: DOSE-RESPONSE FOR CANCER ........................................................ 13 

SESSION 4B: DOSE-RESPONSE FOR REPRODUCTIVE/DEVELOPMENTAL 

TOXICITY............................................................................................................... 16 

SESSION 5: QUANTITATIVE UNCERTAINTY ANALYSIS OF DOSERESPONSE.............................................................................................................. 20 

APPENDIX A: 2009 U.S. EPA DIOXIN WORKSHOP AGENDA............................................ 24 

APPENDIX B: 2009 U.S. EPA DIOXIN WORKSHOP QUESTIONS TO GUIDE 

PANEL DISCUSSIONS......................................................................................................... 31 

APPENDIX C: 2009 U.S. EPA DIOXIN WORKSHOP DRAFT SELECTION 

CRITERIA TO IDENTIFY KEY IN VIVO MAMMALIAN STUDIES THAT 

INFORM DOSE-RESPONSE MODELING FOR 

2,3,7,8-TETRACHLORODIBENZO-p-DIOXIN (TCDD) ............................................... 34 

iii
DIOXIN WORKSHOP TEAM 

The Dioxin Workshop Team, under the leadership of Peter W. Preuss, Director, NCEA, 

comprised the following members: 

National Center for Environmental Assessment, Office of Research and Development, 

U.S. Environmental Protection Agency, Cincinnati, OH 45268 

 Belinda S. Hawkins 

 Janet Hess-Wilson 

 Glenn Rice 

 Jeff Swartout 

 Linda K. Teuschler 

 Bette Zwayer 

Argonne National Laboratory, Argonne, IL 60439 

 Maryka H. Bhattacharyya 

 Andrew Davidson 

 Mary E. Finster 

 Margaret M. MacDonell 

 David P. Peterson 

ACKNOWLEDGMENTS 

The Track Group, Alexandria, VA 22312 

 Kara Hennigan 

 Alan Minton 

 Brandy Quinn 

ECFlex, Inc., Fairborn, OH 45324 

 Dan Heing 

 Heidi Glick 

 Amy Prues 

 Lana Wood 

IntelliTech Systems, Inc., Fairborn, OH 45324 

 Cris Broyles 

 Luella Kessler 

 Stacey Lewis 

 Linda Tackett 

iv
INTRODUCTION 

This document provides a summary of the Scientific Workshop to Inform EPA’s 

Response to National Academy of Science Comments on the Health Effects of Dioxin in EPA’s 

2003 Dioxin Reassessment. The U.S. Environmental Protection Agency (U.S. EPA) and 

Argonne National Laboratories (ANL), through an inter-Agency agreement with the U.S. 

Department of Energy, convened this scientific workshop (“Dioxin Workshop”) on February 

18−20, 2009, in Cincinnati, Ohio. The goals of the Dioxin Workshop were to identify and 

address issues related to the dose-response assessment of 2,3,7,8-tetrachlorodibenzo-p-dioxin 

(TCDD). This report summarizes the discussions and conclusions from this workshop. 

Previously, at the request of the U.S. EPA, the National Academy of Sciences (NAS) prepared a 

report, Health Risks from Dioxin and Related Compounds: Evaluation of the EPA Reassessment

(NAS, 2006), which made a number of recommendations to improve the U.S. EPA’s risk 

assessment for TCDD (U.S. EPA, 2003). The 3-day Dioxin Workshop was convened 

specifically to ensure that the U.S. EPA’s response to the NAS recommendations focuses on the 

key issues and reflects the most meaningful science. 

The Dioxin Workshop included seven scientific sessions: 

(1) Session 1: Quantitative Dose-Response Modeling Issues 

(2) Session 2: Immunotoxicity 

(3) Session 3A: Dose-Response for Neurotoxicity and Nonreproductive Endocrine Effects 

(4) Session 3B: Dose-Response for Cardiovascular Toxicity and Hepatotoxicity 

(5) Session 4A: Dose-Response for Cancer 

(6) Session 4B: Dose-Response for Reproductive/Developmental Toxicity 

(7) Session 5: Quantitative Uncertainty Analysis of Dose-Response 

During each session, the U.S. EPA asked a panel of expert scientists to: 

• identify and discuss the technical challenges involved in addressing the key NAS 

comments on the TCDD dose-response assessment in the U.S. EPA Reassessment 

(U.S. EPA, 2003); 

• discuss approaches for addressing the key NAS comments; and 

• identify important published, independently peer-reviewed literature, particularly studies 

describing epidemiologic and in vivo mammalian bioassays, which are expected to be 

most useful for informing the U.S. EPA’s response. 

The sessions were followed by open comment periods during which members of the 

audience were invited to address the Panels. At the conclusion of the open comment periods, the 

Panel Co-Chairs were asked to summarize and present the results of the panel discussions. The 

summaries could include minority opinions stated by panelists. The main points derived from 

the session summaries were used to prepare this document. Additionally, this document includes 

a list of the session panelists and their affiliations and three appendices. Appendix A presents 

the Dioxin Workshop Agenda. Appendix B identifies the charge questions presented to the 

Panel. Appendix C describes draft study selection criteria proposed by the Dioxin Workshop 

Team for consideration by the workshop panelists. 

1
REFERENCES 

NAS (National Academy of Sciences). 2006. Health Risks from Dioxin and Related 

Compounds: Evaluation of the EPA Reassessment. National Academies Press, Washington, DC 

(July). Available at http://www.nap.edu/catalog.php?record_id=11688. 

U.S. EPA (U.S. Environmental Protection Agency). 2003. Exposure and Human Health 

Reassessment of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds. NAS 

review draft, Volumes 1−3 (EPA/600/P-00/001Cb, Volume 1). U.S. Environmental Protection 

Agency, National Center for Environmental Assessment, Washington, DC (December). 

Available at http://www.epa.gov/nceawww1/pdfs/dioxin/nas-review/. 

2
SCIENTIFIC WORKSHOP TO INFORM THE TECHNICAL WORK PLAN FOR U.S. 

EPA’S RESPONSE TO NAS COMMENTS ON THE HEALTH EFFECTS OF DIOXIN 

PRESENTED IN U.S. EPA’S DIOXIN REASSESSMENT 

Dioxin Workshop Co-Chairs: Peter W. Preuss and Glenn Rice 

The Dioxin Workshop session summaries were prepared by the session panel Co-Chairs 

with input from the panelists, as requested by the U.S. EPA prior to the workshop. The CoChairs subsequently presented these summaries to all of the workshop participants during 

designated periods at the workshop. In these summaries, the U.S. EPA asked that the Co-Chairs 

summarize the key issues from the panel discussions. Because the sessions were not designed to 

achieve consensus among the panelists, the summaries do not necessarily represent consensus 

opinions; rather, they reflect the essence of the panel discussions. Some of the specific points 

may represent the views of multiple panelists, while others only the views of a single panelist. 

Prior to the summarizations, there were opportunities for public comments on the discussion 

topics. Some Co-Chairs met with their sessions’ panelists after their sessions ended to develop 

these summaries, while others developed reports based on their personal notes. Because Session 

5 was the last session of the workshop—with little time provided to develop the summary—the 

Co-Chairs circulated a draft for comment by the Session 5 panelists after the workshop, prior to 

finalizing the session summary. The U.S. EPA collected the session summaries and then 

prepared this document. A draft of this document was distributed to all of the session Co-Chairs 

to provide them with a final opportunity to comment and make revisions. Finally, it should be 

noted that U.S. EPA was not prescriptive to the session Co-Chairs with respect to the format of 

the presentation materials and provided no specific instructions, resulting in unique formats 

among the session summaries. 

SESSION 1: QUANTITATIVE DOSE-RESPONSE MODELING ISSUES 

This session discussed the general dose-response modeling issues related to TCDD. 

Many of these issues were highlighted by NAS (2006). There was a general introductory 

presentation on TCDD kinetics, including information and uncertainties pertaining to the 

conversion of administered doses in animals to human body burden (BB) and additivity to 

background issues. This presentation was followed by a Panel discussion on the state of the 

science regarding dioxin dose-response modeling issues. 

Session 1 Panelists (Session Co-Chairs are identified by asterisk) 

• Bruce Allen, Bruce Allen Consulting 

• Lesa Aylward, Summit Toxicology 

• Roger Cooke, Resources for the Future 

• Kenny Crump, Louisiana Tech University 

• Mike DeVito, U.S. EPA 

• Dale Hattis, Clark University 

• Rick Hertzberg, Biomath Consulting 

• Rob McDowell, U.S. Department of Agriculture 

• Jim Olson, State University of New York, University at Buffalo 

3
• *Lorenz Rhomberg, Gradient 

• Woody Setzer, U.S. EPA 

• *Jeff Swartout, U.S. EPA 

Please note that the use of the term “concluded” or “recommended” in this summary does not mean that a consensus 

was reached. Session Summaries were written from the material prepared by the non-EPA/ANL Co-Chair and 

represent a synopsis of the panel discussions. 

Key Study Selection Criteria 

 The Panel discussed the advantages and disadvantages of using key study criteria 

(Appendix C). They concluded that a priori criteria foster transparency and consistency, and 

could deflect a posteriori criticism. However, the Panel also acknowledged that having a priori

criteria could introduce the potential for excluding useful data. Although the key study criteria 

provided by the U.S. EPA listed studies using TCDD only as a criterion, the Panel posed the 

possibility of using closely related dioxin-like compounds (DLCs) as surrogates for TCDD. The 

criterion for use of data from mammalian studies only was one criterion that received generalized 

support due to the lack of extrapolation protocols for nonmammalian species. The Panel also 

discussed the specific exposure-duration criterion and asked if there should be a preference for 

longer-term rather than acute studies. The Panel made three suggestions to modify U.S. EPA’s 

key study selection criteria: 

(1) Define more relevant exposure-level (i.e., dose) cut points using tissue concentrations. 

(2) Reword statistical criteria to include do-it-yourself analysis. 

(3) Reword the response criteria to clarify “outside of normal range.” 

Dose Metrics 

 The Panel discussed the relative merits of various measures of dose for modeling TCDD 

dose response. One general conclusion was that tissue concentration (TC) is the preferred 

metric, especially lipid-adjusted TC, because this measure more closely approximates exposures 

close to the target tissue when compared to administered doses. However, the Panel 

acknowledged that these data are often unavailable. They further noted that BB, which is 

defined as the concentration of TCDD in the body (ng/kg body weight) (U.S. EPA, 2003), might 

be useful as a surrogate for TC provided the two measures were proportional. 

The Panel suggested that a linear approach to BB estimation, which was utilized by 

U.S. EPA (2003), is too simplistic because this approach does not take into account toxicokinetic 

issues related to TCDD—e.g., sequestration in the liver and fat, age-dependent elimination, and 

changing elimination rates over time. The Panel recommended the use of kinetic/mechanistic 

modeling to the extent possible to quantify tissue-based metrics. 

The Panel raised the issue of whether the preferred dose metric would be different for 

different endpoints and exposure durations. This led to the Panel’s comment that the peak 

exposure might be a more important metric than average BB for variable exposure scenarios. 

Given this discussion about different exposure durations being relevant to a specific endpoint, 

the Panel suggested that the U.S. EPA also consider peak measures in dose-response modeling. 

4
The last point raised in this part of the discussion centered on the possibility of dose 

errors in experimental studies. The Panel highlighted the need for the U.S. EPA to consider dose 

error (i.e., uncertainty in the x-axis of the dose-response curve) when using dose surrogates. 

Dose-Response Modeling of Mammalian Bioassays 

 The Panel considered several issues related to dose-response modeling of mammalian 

bioassay data for TCDD: supralinearity and incomplete response data (“anchoring”), defining the 

benchmark response (BMR) level with respect to establishing the point of departure (POD), and 

the use of threshold modeling—as further explained below. 

 The Panel discussed the specific issues of supralinearity and anchoring raised by the 

U.S. EPA with respect to modeling noncancer endpoints. The panel recognized that, for many of 

the most sensitive endpoints, the response at the lowest dose is high (e.g., quantal responses 

above 25% and continuous endpoints differ substantially from the mean, often implying 100% 

incidence in the treated animals). This lack of response anchoring at the low end of the doseresponse curve (near the BMR) results in the higher responses determining the shape of the 

curve. 

 The Panel asked whether new tools might be needed or whether the current tools could be 

applied differently. In the context of developing new tools, the Panel emphasized the need for 

collaboration between biologists and mathematicians. When discussing application, the Panel 

suggested that the problem with supralinearity might be overcome by simply dropping the 

requirement for using the lower bound on the Benchmark Dose. In addition, the Panel posed 

several more approaches for further consideration in dose-response modeling by the U.S. EPA: 

(1) Combine similar data sets to fill in data gaps. 

(2) Use mechanistic approaches to model the data gaps. 

(3) Dichotomize continuous data. 

Finally, the Panel acknowledged that, in certain situations, there simply may not be enough 

information to provide meaningful answers. 

 The Panel discussed the BMR level for establishing a POD in the context of deriving a 

Reference Dose (RfD). The Panel generally agreed that, while the effective dose level (ED01) 

used in the 2003 Reassessment may be useful for comparative analysis across endpoints, the 

ED01 estimates developed for all endpoints considered in the Reassessment were not appropriate 

for deriving an RfD because they were not based on the effect’s adversity. The panel noted that 

ED01 also is much lower than typical EPA BMR levels. The Panel recommended that the U.S. 

EPA work to define endpoint-specific BMRs based on the consideration of adversity. Given that 

the same uncertainty factor framework is applied to all PODs, the Panel emphasized the need for 

consistency in BMRs; numerical consistency is needed for quantal BMRs and consistency in the 

choice of biological relevance should be applied for continuous BMRs. 

 The Panel generally discouraged threshold modeling by stating that thresholds are very 

difficult to pin down and suggested that the lower bound may always be zero. 

5
Dose-Response Modeling of Epidemiological Studies 

 The Panel noted that many studies have been published with measured concentrations of 

TCDD that could be used for dose reconstruction. In this discussion, the Panel acknowledged 

that use of these data would entail dealing with toxicity equivalence (TEQ) issues and 

pharmacokinetic (PK) modeling. Pertaining to the use of these data for quantitative risk 

assessment by the U.S. EPA, the Panel posed the question, “At what point does indirect or 

confounded human data supersede controlled animal bioassay data?”, or alternatively, “How 

much human data uncertainty can we tolerate?” The Panel suggested, at the least, that the 

epidemiologic data could be used to “ground-truth” the animal bioassay modeling results. 

Supporting Information 

The Panel acknowledged that Ah receptor (AhR) binding affinities are not necessarily 

tied to endpoint sensitivity, but they reiterated the need to consider mechanistic modeling to aid 

in developing appropriate dose metrics or filling in data gaps in the existing dose-response data. 

References 

NAS (National Academy of Sciences). 2006. Health Risks from Dioxin and Related 

Compounds: Evaluation of the EPA Reassessment. National Academies Press, Washington, DC 

(July). Available at http://www.nap.edu/catalog.php?record_id=11688. 

U.S. EPA (U.S. Environmental Protection Agency). 2003. Exposure and Human Health 

Reassessment of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds. NAS 

Review Draft (EPA/600/P-00/001Cb). U.S. Environmental Protection Agency, National Center 

for Environmental Assessment, Washington, DC. Available at 

http://www.epa.gov/nceawww1/pdfs/dioxin/nas-review/. 

SESSION 2: IMMUNOTOXICITY 

The U.S. EPA plans to consider development of a quantitative dose-response assessment 

for the immunologic effects associated with TCDD exposure. Such an assessment would be 

based on information in U.S. EPA (2003), NAS (2006) and key studies identified in this 

workshop. The purpose of this session was to identify and discuss key issues pertaining to doseresponse assessment for dioxin-induced immunologic effects. 

Session 2 Panelists (Session Co-Chairs are identified by asterisk) 

• Roger Cooke, Resources for the Future 

• Rob Goble, Clark University 

• *Belinda Hawkins, U.S. EPA 

• Nancy Kerkvliet, Oregon State University 

• Manolis Kogevinas, Centre for Research in Environmental Epidemiology 

• Robert Luebke, U.S. EPA 

• Paolo Mocarelli, University of Milan 

• *Allen Silverstone, State University of New York, Upstate Medical University 

6
• Courtney Sulentic, Wright State University 

• Nigel Walker, National Institute of Environmental Health Sciences 

Please note that the use of the term “concluded” or “recommended” in this summary does not mean that a consensus 

was reached. Session Summaries were written from the material prepared by the non-EPA/ANL Co-Chair and 

represent a synopsis of the panel discussions. 

Key Study Selection Criteria 

The Panel first addressed the Key Study Selection Criteria proposed by the U.S. EPA 

(Appendix C). The Panel raised the issue that the key study criteria do not apply to most studies 

designed to investigate immunotoxicity, including those used to calculate ED01s (U.S. EPA, 

2003). The Panel observed that most dioxin immunotoxicity studies are relatively high dose 

(>200 ng/kg-d) acute studies and/or use parenteral rather than oral administration. 

The Panel discussed several studies often considered important for assessing the 

immunotoxic effects of TCDD exposure. The Oughton et al. (1995) mouse bioassay was 

discussed and, although the study does meet the proposed criteria, it could not be considered a 

key study; specifically, the Panel contended that since there were no functional alterations 

observed or measured in this bioassay, the changes in cellular phenotypes are only “suggestive” 

of immune alterations and cannot be regarded as having immunopathologic significance. 

 The Panel discussed two additional studies for further consideration by the U.S. EPA: 

• Baccarelli et al. (2002). The Panel discussed this as a potentially key human 

epidemiological study that should be reviewed and considered further by the U.S. EPA. 

It measured the level of IgG, demonstrating a significant decline relative to dioxin body 

burdens. 

• Smialowicz et al. (2008). The Panel noted that this study identified the antibody response 

to sheep red blood cells (SRBCs) as the critical effect, labeling this protocol as a 

functional assay. The Panel stated that if modeled, the U.S. EPA could calculate the 

BMR for this endpoint as 1 standard deviation from the control mean. 

References 

Baccarelli, A., P. Mocarelli, D.G. Patterson et al. 2002. Immunologic effects of dioxin: New 

results from Seveso and comparison with other studies. Environ. Health Perspect. 

110(12):1169-1173. 

NAS (National Academy of Sciences). 2006. Health Risks from Dioxin and Related 

Compounds: Evaluation of the EPA Reassessment. National Academies Press, Washington, DC 

(July). Available at http://www.nap.edu/catalog.php?record_id=11688. 

Oughton, J.A., C.B. Pereira, G.K. Dekrey, J.M. Collier, A.A. Frank and N.I. Kerkvliet. 1995. 

Phenotypic analysis of spleen, thymus, and peripheral blood cells in aged C57BI/6 mice 

following long-term exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin. Toxicol. Sci. 25(1):60-69. 

7
Smialowicz, R.J., M.J. DeVito, W.C. Williams and L.S. Birnbaum. 2008. Relative potency 

based on hepatic enzyme induction predicts immunosuppressive effects of a mixture of 

PCDDS/PCDFS and PCBS. Toxicol. Appl. Pharmacol. 227(3):477-484. 

U.S. EPA (U.S. Environmental Protection Agency). 2003. Exposure and Human Health 

Reassessment of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds. NAS 

Review Draft (EPA/600/P-00/001Cb). U.S. Environmental Protection Agency, National Center 

for Environmental Assessment, Washington, DC. Available at 

http://www.epa.gov/nceawww1/pdfs/dioxin/nas-review/. 

SESSION 3A: DOSE-RESPONSE FOR NEUROTOXICITY AND NONREPRODUCTIVE 

ENDOCRINE EFFECTS 

The U.S. EPA plans to consider development of a quantitative dose-response assessment 

for neurological and/or nonreproductive endocrine effects associated with TCDD exposure. 

Such an assessment would be based on information in U.S. EPA (2003), NAS (2006) and key 

studies identified in this workshop. The purpose of this session was to identify and discuss key 

issues pertaining to dose-response assessment for dioxin-induced neurological and/or 

nonreproductive endocrine effects. 

 

Session 3A Panelists (Session Co-Chairs are identified by asterisk) 

• *Maryka Bhattacharyya, Argonne National Laboratory 

• Mike DeVito, U.S. EPA 

• Mary Gilbert, U.S. EPA 

• Rob Goble, Clark University 

• Nancy Kerkvliet, Oregon State University 

• Fumio Matsumura, University of California-Davis 

• Paolo Mocarelli, University of Milan 

• Chris Portier, National Institute of Environmental Health Sciences 

• Lorenz Rhomberg, Gradient 

• Allen Silverstone, State University of New York, Upstate Medical University 

• Marie Sweeney, National Institute of Occupational Safety and Health 

• *Bernie Weiss, University of Rochester 

Please note that the use of the term “concluded” or “recommended” in this summary does not mean that a consensus 

was reached. Session Summaries were written from the material prepared by the non-EPA/ANL Co-Chair and 

represent a synopsis of the panel discussions. 

What Are the Key Questions Regarding These Endpoints? 

The Panel used the following question to initiate discussion: “Are there identifiable 

indices of neurotoxicity and nonreproductive endocrine effects in animal studies and human 

populations?” Under this discussion topic, the Panel discussed three endpoints: neurotoxicity 

(with focus on developmental exposures), thyroid dysfunction (e.g., thyroid hormone deficits), 

and diabetes. The Panel also addressed the relevance of windows of vulnerability to each 

8
endpoint. The Panel acknowledged that, in some cases, the window of exposure may precede the 

window of expression of toxicity. 

Epidemiological Study Selection 

Developmental Neurotoxicity 

The Panel recognized that an unusual feature for this endpoint is that there are sufficient 

human data for dose-response modeling (e.g., Dutch children [Huisman et al., 1995; Patandin et 

al., 1999] and U.S. children [Jacobson and Jacobson, 1996]) and there is an internal dose metric 

(serum concentrations). Additionally, the Panel discussed recent studies that address this 

endpoint in humans (from Japan [reference not provided] and Holland [e.g., Koopman-Esseboom 

et al., 1996; Vreugdenhil et al., 2002]). For continued investigation into this endpoint, the Panel 

raised two issues to the U.S. EPA: 

• Conduct an evaluation of whether a modeled effect can be attributed to TCDD and not 

some other persistent organic pollutant (POP), although the Panel recognized that it is 

unlikely U.S. EPA will be able to distinguish among these exposures because other POPs 

are intrinsic confounders in the Dutch study. 

• Allow animal data to inform the dose-response modeling of epidemiological data.

Thyroid Dysfunction 

The Panel identified the availability of human data for this endpoint (e.g., Calvert et al., 

1999; Koopman-Esseboomet al., 1994). Much of the thyroid dysfunction literature has been 

published since the 2003 Reassessment (e.g., Wang et al., 2005; Baccarelli et al., 2008). The 

Panel also noted the availability of an internal dose metric (serum concentrations). Additionally, 

the Panel discussed the mechanistic studies in animals that link TCDD to thyroid dysfunction. 

For continued investigation into this endpoint, the Panel raised three issues for the U.S. EPA to 

consider: 

• Consider the newly available human data since the Reassessment. 

• Investigate and clarify of the role of TCDD-induced thyroid dysfunction in 

developmental neurotoxicity. 

• Evaluate and determine whether an effect can be attributed to TCDD or other 

contaminants. 

Diabetes 

The Panel discussed that data suggest that diabetes incidence in those under 55 years old 

may be associated with exposure to PCBs. They acknowledged that whether this is a dioxin-like 

compound (DLC) mediated effect or whether other POPs are responsible is still undetermined. 

The Panel also acknowledged that no animal model exists for the investigation of xenobioticinduced diabetes, and that separating the injury dose level from the current body burdens would 

depend on good pharmacokinetics in humans. For continued investigation into this endpoint, the 

Panel listed two issues for the U.S. EPA to consider: 

• Results from the Anniston study and the Great Lakes Fishermen study (references not 

provided) should be examined for dose metrics (both studies examine human PCB 

exposures). 

9
• Changes of adipose tissue status need to be considered, given that dieting can cause 

release of lipid-soluble contaminants. 

References 

Baccarelli, A., S.M. Giacomini, C. Corbetta et al. 2008. Neonatal thyroid function in Seveso 25 

years after maternal exposure to dDioxin. PLoS Med. 5(7):e161.

doi:10.1371/journal.pmed.0050161. 

Calvert, G.M., M.H. Sweeney, J. Deddens and D.K. Wall. 1999. Evaluation of diabetes 

mellitus, serum glucose, and thyroid function among United States workers exposed to 

2,3,7,8-tetrachlorodibenzo-p-dioxin. Occ. Env. Med. 56:270-276. 

Huisman, M., C. Koopman-Esseboom, V. Fidler et al. 1995. Perinatal exposure to 

polychlorinated biphenyls and dioxins and its effect on neonatal neurological development. 

Early Hum. Devel. 41(2):111-127. 

Jacobson, J.L. and S.W. Jacobson. 1996. Intellectual impairment in children exposed to 

polychlorinated biphenyls in utero. N. Engl. J. Med. 335:783–789. 

Koopman-Esseboom, C., N. Weisglas-Kuperus, M.A.J. de Ridder, C.G. Van der Paauw, 

L.G.M.Th. Tuinstra and P.J.J. Sauer. 1996. Effects of polychlorinated biphenyl/dioxin exposure 

and feeding type on infants’ mental and psychomotor development. J. Pediatr. 97(5):700-706. 

Koopman-Esseboom, C., D.-C. Morse, N. Weisglas-Kuperus et al. 1994. Effects of dioxins and 

polychlorinated biphenyls on thyroid hormone status of pregnant women and their infants. 

Pediatr. Res. 36:468–473. 

Patandin, S., C.I. Lanting, P.G.H. Mulder, E.R. Boersma, P.J.J. Sauer and N. Weisglas-Kuperus. 

1999. Effects of environmental exposure to polychlorinated biphenyls and dioxins on cognitive 

abilities in Dutch children at 42 months of age. J. Pediatr. 134:33–41. 

NAS (National Academy of Sciences). 2006. Health Risks from Dioxin and Related 

Compounds: Evaluation of the EPA Reassessment. National Academies Press, Washington, DC 

(July). Available at http://www.nap.edu/catalog.php?record_id=11688. 

U.S. EPA (U.S. Environmental Protection Agency). 2003. Exposure and Human Health 

Reassessment of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds. NAS 

Review Draft (EPA/600/P-00/001Cb). U.S. Environmental Protection Agency, National Center 

for Environmental Assessment, Washington, DC. Available at 

http://www.epa.gov/nceawww1/pdfs/dioxin/nas-review/. 

Vreugdenhil, H.J., C.I. Lanting, P.G. Mulder, E.R. Boersma and N. Weisglas-Kuperus. 2002. 

Effects of prenatal PCB and dioxin background exposure on cognitive and motor abilities in 

Dutch children at school age. J. Pediatr. 140:48–56. 

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Wang S.L., P.H. Su, S.B. Jong, Y.L. Guo, W.L. Chou and O. Päpke. 2005. In utero exposure to 

dioxins and polychlorinated biphenyls and its relations to thyroid function and growth hormone 

in newborns. Environ. Health Perspect. 113:1645–1650. 

SESSION 3B: DOSE-RESPONSE FOR CARDIOVASCULAR TOXICITY AND 

HEPATOTOXICITY 

The U.S. EPA plans to consider development of a quantitative dose-response assessment 

for cardiovascular and/or hepatic effects associated with TCDD exposure. Such an assessment 

would be based on information in U.S. EPA (2003), NAS (2006) and key studies identified in 

this workshop. The purpose of this session was to identify and discuss key issues pertaining to 

dose-response assessment for dioxin-induced cardiovascular and/or hepatic effects. 

Session 3B Panelists (Session Co-Chairs are identified by asterisk) 

• Bob Budinksy, Dow Chemical 

• Manolis Kogevinas, Centre for Research in Environmental Epidemiology 

• Rob McDowell, U.S. Department of Agriculture 

• Jim Olson, State University of New York, University at Buffalo 

• Marian Pavuk, Agency for Toxic Substances and Disease Registry 

• *Jeff Swartout, U.S. EPA 

• *Mary Walker, University of New Mexico 

• Nigel Walker, National Institute of Environmental Health Sciences 

Please note that the use of the term “concluded” or “recommended” in this summary does not mean that a consensus 

was reached. Session Summaries were written from the material prepared by the non-EPA/ANL Co-chair and 

represents a synopsis of the panel discussions. 

Key Study Selection Criteria 

 The Panel initially focused on the draft key study selection criteria offered by the 

U.S. EPA (Appendix C). The panel recommended that for cardiovascular effects, which are not 

usually observed in rodents, the use of knockout mouse models (ApoE KO and LDLR KO) be 

moved to the “primary” column because only these studies establish the cardiovascular toxicity 

model in mice. 

The panel also was concerned that the gavage procedure can increase mouse blood 

pressure. Consequently, the panel recommended that gavage studies not be used for the blood 

pressure endpoint (i.e., only dietary dosing studies should be considered). 

Human Health Endpoints 

 In relation to the hepatic endpoint, the Panel acknowledged the large body of dose 

response information on hepatic effects in rodents and that enzyme (mostly CYP1A1) induction 

was a sensitive effect. However, the Panel cited the lack of linkage of CYP1A1 to downstream 

events, which complicates the toxicological interpretation of this endpoint, and concluded that 

11
the more important liver effects in rodents are probably on the “road to cancer.” The Panel noted 

that hepatic effects were not seen in the epidemiological studies, but acknowledged that these 

studies were not designed to detect them. 

In relation to the cardiovascular endpoint, the Panel identified hypertension and ischemic 

heart disease (IHD) as two key endpoints from the epidemiological studies. The Panel 

recommended that the U.S. EPA perform a meta-analysis of these data. The Panel also 

commented that recent animal studies support the observations linking TCDD exposure to IHD 

and hypertension. In particular, the National Toxicology Program (NTP) study shows 

inflammatory and structural effects on resistant vascular arterioles (NTP, 2006). Additional 

evidence from the study suggests that the vascular effects may be CYP1A1-dependent. The 

Panel suggested that the NTP study data might be used as a surrogate for dose-response 

modeling of hypertension and that such an approach would be supported by data on the role of 

AhR in vascular function and remodeling. 

POD Issues 

The Panel was not supportive of 1% of maximal response (ED01), which was utilized in 

the 2003 Reassessment. The Panel concluded that the POD should depend on the specific 

endpoint and recommended the following to the U.S. EPA: 

• For continuous measures, base the BMR on difference from control. Consider the 

adversity level—at what point does the endpoint become adverse? 

• For incidence data, set the BMR to a fixed-risk level. 

Supporting Information 

The Panel posed several suggestions to the U.S. EPA for reducing uncertainty and 

improving the knowledge base for TCDD toxicity. 

• Use in vitro data to define uncertainties, such as the relative sensitivity between rodents 

and humans and around the definition of a POD. 

• Consider studies on dioxin-like compounds (DLCs). 

• Use PK modeling to define the dose metric for hepatic effects. 

• Use body burden or serum concentrations for cardiovascular endpoints. 

Finally, the Panel recommended that U.S. EPA finish the reassessment quickly and establish a 

definitive plan to review and incorporate new data as they become available. 

References 

NAS (National Academy of Sciences). 2006. Health Risks from Dioxin and Related 

Compounds: Evaluation of the EPA Reassessment. National Academies Press, Washington, DC 

(July). Available at http://www.nap.edu/catalog.php?record_id=11688. 

12
NTP (National Toxicology Program). 2006. Toxicology and Carcinogenesis Studies of 

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) (CAS No. 1746-01-6) in Female Harlan SpragueDawley Rats (Gavage Studies). U.S. Department of Health and Human Services. NTP TR 521. 

Research Triangle Park, NC (April). 

U.S. EPA (U.S. Environmental Protection Agency). 2003. Exposure and Human Health 

Reassessment of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds. NAS 

Review Draft (EPA/600/P-00/001Cb). U.S. Environmental Protection Agency, National Center 

for Environmental Assessment, Washington, DC. Available at 

http://www.epa.gov/nceawww1/pdfs/dioxin/nas-review/. 

SESSION 4A: DOSE-RESPONSE FOR CANCER 

The U.S. EPA plans to consider development of a quantitative dose-response assessment 

for cancer associated with TCDD exposure. Such an assessment would be based on information 

in U.S. EPA (2003), NAS (2006) and key studies identified in this workshop. The purpose of 

this session was to identify and discuss key issues pertaining to dose-response assessment for 

dioxin-induced cancer. 

Session 4A Panelists (Session Co-Chairs are identified by asterisk) 

• Lesa Aylward, Summit Toxicology 

• Kenny Crump, Louisiana Tech University 

• Dale Hattis, Clark University 

• *Janet Hess-Wilson, U.S. EPA 

• Karen Hogan, U.S. EPA 

• Manolis Kogevinas, Centre for Research in Environmental Epidemiology 

• Marian Pavuk, Agency for Toxic Substances and Disease Registry 

• Chris Portier, National Institute of Environmental Health Sciences 

• Lorenz Rhomberg, Gradient 

• Jay Silkworth, General Electric 

• *Nigel Walker, National Institute of Environmental Health Sciences 

Please note that the use of the term “concluded” or “recommended” in this summary does not mean that a consensus 

was reached. Session Summaries were written from the material prepared by the non-EPA/ANL Co-chair and 

represent a synopsis of the panel discussions. 

Key Study Selection 

 The Panel discussed both human and rodent studies. In reviewing the epidemiological 

data, the Panel agreed the EPA should focus on four cohort studies (Dutch cohort, NIOSH 

cohort, BASF accident cohort, and Hamburg cohort) and pointed out that there are numerous 

updates and reevaluations of data now in the literature and others will be published soon. The 

Panel stated that it is appropriate for the U.S. EPA to consider the increase in total cancers for 

modeling human cancer data, however, Non-Hodgkin's lymphoma, and lung tumors are the main 

TCDD-related cancer types seen in humans exposed to TCDD. The Panel suggested the U.S. 

EPA focus the quantitative dose-response modeling on the human data. 

13
In reviewing the rat data, the Panel identified four new NTP rodent cancer bioassays with 

liver and lungs as the main target organs. However, they suggested that dose-response modeling 

efforts should model “all cancers” from these NTP data sets as well and use tumor incidence—

not individual rats as measures. 

Key Study Selection Criteria 

The Panel discussed whether data for TCDD only should be used or if PCB126 could be 

used to develop a dose-response curve. From this discussion, the Panel reached a general 

agreement that limiting the dose-response modeling and cancer assessment to TCDD only would 

be the best approach. 

Regarding the oral dosing regimens, the Panel discussed the differences in results from 

different bioassays. They concluded that there were insufficient data to pick between oral feed 

(Kociba et al., 1978) and oral gavage (NTP, 2006) studies, but stated “If all aspects of studies 

were equal, an oral feed study is preferred.” However, given that current data sets are not equal, 

they agreed that U.S. EPA should consider both feed and gavage studies. 

The Panel put forth the recommendation that studies that include initiation-promotion 

model data and TgAC transgenic model data from oral exposure studies should be excluded from 

the primary category in the key study selection criteria (Appendix C lists the draft study selection 

criteria distributed prior to the meeting). Studies from both classifications should be moved to 

the second tier. 

The Panel was also unsupportive of the “response magnitude outside the range of normal 

variability” criterion, as they did not believe it was applicable to a cancer endpoint. 

Critical Endpoints to Consider 

 The Panel recognized that the MOA for TCDD includes cell growth/differentiation 

dysregulation, that different endpoints (tumor types) across species may be expected, and that 

there are differences in tumor sites across species. The Panel further acknowledged that there is 

insufficient information to determine if rodent tumor types observed are relevant to humans. 

Thus, the Panel suggests the following: 

• U.S. EPA should consider all the observed cancer endpoints in its evaluation. 

Nonlinear (aka threshold) Versus Linear Dose-Response Modeling 

 The Panel agreed that NTP bioassays appear to demonstrate nonlinear dose response, but 

they expressed concern about using animal data to infer slope and dose response for humans. 

The Panel pointed out that there are differences in slopes across different bioassays, and 

specifically, that some appear linear while others appear nonlinear. Given the observation of 

both nonlinear vs. linear, the Panel concluded that neither could be ruled out for extrapolation 

below the POD simply based on the available data. One panelist noted that U.S. EPA Cancer 

Guidelines (U.S. EPA, 2005) state that only if one can demonstrate that the MOA has a threshold 

dose-response shape, and can exclude all other potential linear MOAs, can one use a nonlinear 

model. Lastly, the Panel noted that there are data and rationales to support use of both linear and 

14
nonlinear response below POD. From this discussion, the Panel raised one possibility to the U.S. 

EPA: 

• Both linear and nonlinear model functions should be considered in the dose-response 

analysis. 

Dose Metrics 

 In considering human data, the Panel expressed a preference for lipid-adjusted serum 

levels over body burden (BB), and they expressed concerns over the assumptions used in the 

back calculation of the BB in the epidemiologic cohorts. In considering the rat data, the Panel 

supported the use of BB—especially lipid-adjusted BB. The Panel, however, did express 

concern over the sequestering of TCDD in liver and then the use of liver levels in BB 

calculations. 

Supporting Information—Biologically-Based Dose-Response (BBDR) Models and MOA 

The Panel discussed BBDR. Though once considered an attractive proposition, BBDR 

models may mask uncertainty within the models, necessitating them to be used with greater 

caution. The Panel suggested two issues for the U.S. EPA to consider: 

• If there is a published model, use it if it is valid—do not generate a new model. 

• Focus on the actual experimental data to drive the analysis. 

References 

Kociba, R.J., D.G. Keyes, J.E. Beyer et al. 1978. Results of a two-year chronic toxicity and 

oncogenicity study of 2,3,7,8-tetrachlorodibenzo-p-dioxin in rats. Toxicol. Appl. Pharmacol. 

46:279-303. 

NAS (National Academy of Sciences). 2006. Health Risks from Dioxin and Related 

Compounds: Evaluation of the EPA Reassessment. National Academies Press, Washington, DC 

(July). Available at http://www.nap.edu/catalog.php?record_id=11688. 

NTP (National Toxicology Program). 2006. Toxicology and Carcinogenesis Studies of 

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) (CAS No. 1746-01-6) in Female Harlan SpragueDawley Rats (Gavage Studies). U.S. Department of Health and Human Services. NTP TR 521. 

Research Triangle Park, NC (April). 

U.S. EPA (U.S. Environmental Protection Agency). 2003. Exposure and Human Health 

Reassessment of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds. NAS 

Review Draft (EPA/600/P-00/001Cb). U.S. Environmental Protection Agency, National Center 

for Environmental Assessment, Washington, DC. Available at 

http://www.epa.gov/nceawww1/pdfs/dioxin/nas-review/. 

US EPA (U.S. Environmental Protection Agency). 2005. Guidelines for Carcinogen Risk 

Assessment. U.S. Environmental Protection Agency Risk Assessment Forum. 

EPA/630/P-03/001F. 

15
SESSION 4B: DOSE-RESPONSE FOR REPRODUCTIVE/DEVELOPMENTAL 

TOXICITY 

The U.S. EPA plans to consider development of a quantitative dose-response assessment 

for reproductive and developmental effects associated with TCDD exposure. Such an 

assessment would be based on information in U.S. EPA (2003), NAS (2006) and key studies 

identified in this workshop. The purpose of this session was to identify and discuss key issues 

pertaining to dose-response assessment for dioxin-induced reproductive and developmental 

effects. 

Session 4B Panelists (Session Co-Chairs are identified by asterisk) 

• Barbara Abbott, U.S. EPA 

• Bruce Allen, Bruce Allen Consulting 

• Roger Cooke, Resources for the Future 

• George Daston, Procter & Gamble 

• Mike DeVito, U.S. EPA 

• Rob Goble, Clark University 

• *Fumio Matsumura, University of California-Davis 

• Paolo Mocarelli, University of Milan 

• Brian Petroff, University of Kansas 

• *Glenn Rice, U.S. EPA 

• Marie Sweeney, National Institute of Occupational Safety and Health 

• Mary Walker, University of New Mexico 

• Bernie Weiss, University of Rochester 

Please note that the use of the term “concluded” or “recommended” in this summary does not mean that a consensus 

was reached. Session Summaries were written from the material prepared by the non-EPA/ANL Co-Chair and 

represent a synopsis of the panel discussions. 

A Major Question Posed During this Workshop Session was “Are Human Embryos and 

Infants Less Sensitive to Dioxin Exposures Than Some Experimental Animals?” 

The Panel recognized that animal data show a wide range of species sensitivity to dioxin 

for a given developmental or reproductive endpoint. Presently, there are data for some endpoints 

that show that human sensitivity is comparable to experimental animals (e.g., semen quality), 

and for other endpoints the data demonstrate that humans are insensitive compared to other 

species (e.g., cleft palate). Lastly, the Panel recognized that there are some endpoints for which 

relative human sensitivity remains uncertain. 

Key Study Selection 

 The Panel reviewed the charge questions (Appendix B), discussed them, and listed two 

issues for the U.S. EPA to consider: 

• Concerning key study determination, use a stepwise approach that is dependent upon the 

information available and needed to address the question. 

16
• Concerning the key studies informing the POD and the POD endpoint choice, use the 

POD to depart from what is certain and use a high-confidence study that has found 

effects at a low enough level at which other effects are protected. 

The Panel also developed Table 1, based on the information presented in this session. Table 1 

identifies specific reproductive and developmental effects of concern, listing whether an effect 

has been observed in test animals and epidemiologic cohorts. It also identifies the ED10

estimated by the U.S. EPA (2003) for health effects observed in rodent bioassays. If the U.S. 

EPA did not report an ED10 for an effect, the table identifies a study where the effect was 

reported and the lowest study dose where the effect was observed. Table 1 also identifies the 

epidemiologic cohort where the specific reproductive and developmental effects were observed. 

Epidemiological Study Utility 

The Panel reviewed the charge questions (Appendix B), discussed them, and made two 

suggestions to the U.S. EPA: 

• Concerning the ability of epidemiological studies to inform critical effects, start with 

concordance across species (including humans) for the spectrum of effects. 

• Concerning the ability of epidemiological studies to inform dose-response modeling, start 

with the epidemiology and then go to animal data if the dose response has not been well 

characterized for an endpoint of interest and compare to animal data as a reality check. 

Animal Model Utility 

The Panel reviewed and discussed the charge questions (Appendix B). Table 1, which 

identifies the effects that occur in animals and also have relevance to humans, summarizes much 

of this discussion. Regarding the influence of mode of action (MOA) on animal model choice, 

the Panel concluded that by evaluating concordance among health effects reported in 

epidemiologic and animal bioassay data, the U.S. EPA could identify a set of plausible 

reproductive and developmental effects to consider. Actual animal and human MOA 

information is helpful in that it creates comfort with the animal models and in defining the 

boundaries of possible effects. 

17
TABLE 1 

Reproductive/Developmental Effects of Concern for Human Health 

Endpoint Rodent 

(ED10 ng/kg-d) Human Notes 

Sperm Count/Motility Yes (6.2−28; 

66−200) 

Yes ED10 bases Mabley et al. (1992a,b) caudal 

sperm count and daily sperm production 

range from 6.2−28; Gray et al. (1997) 

epididymal sperm count and total testis sperm 

counts range from 66−200. 

Sex Ratio No Yes, Seveso 

Delayed Puberty Males Yes (94) Yu-cheng ED10 basis rat male puberty delay Gray et al. 

(1997). Need to qualify epidemiology data 

because of cohort PCDD/PCDFs exposures. 

Delayed Puberty in Females Yes No in Seveso Gray and Ostby (2002) report delayed 

puberty in female offspring of pregnant rats 

receiving a single dose of 1 μg TCDD/kg on 

GD 15. 

Cleft Palate Yes (6300−6400) No ED10 basis Birnbaum et al. (1989). 

Premature Senescence Yes No, Seveso Franczac et al. (2006) report that rats 

prematurely entered reproductive senescence, 

after receiving cumulative TCDD doses as 

low as 1.7 μg TCDD/kg. They considered 

first occurrence of prolonged interestrous 

interval (>6 d) as evidence of onset of 

reproductive senescence. 

Hormones E2 Yes Yes, Males—

Seveso 

Li et al (1995) report serum estradiol-17β

(E2) concentrations induced by equine 

Chorionic Gonadotropin injection were 

significantly elevated in female rats orally 

administered 10 μg/kg TCDD on PND 22. 

While E2 decreased dramatically in control 

animals during the preovulatory LH surge, it 

did not in TCDD-treated rats. 

Low Birth Weight Yes (190) Suggestive 

effect in Seveso 

in first 8 years 

after exposure 

ED10 basis Gray et al. (1997). 

Reproductive Cycling 

(prolongation) 

Yes Yes, Seveso 

Prepubertal 

exposure 

Franczac et al (2006) report loss of normal 

cyclicity in female rats at 8 months of age

following a cumulative dose of 1.7 μg 

TCDD/kg.

18
Supporting Information 

 The Panel reviewed the charge questions (Appendix B), discussed them, and made two 

suggestions to the U.S. EPA: 

• Concerning deviation from default approaches for noncancer endpoints, there needs to be 

a careful assessment of the POD and the application of uncertainty factors in light of 

PK/pharmacodynamics (PD), population characteristics and variability, and MOA 

information. 

• Concerning the MOA’s ability to clarify endpoint and the incorporation of a cascade of 

cellular event into dose-response for noncancer endpoint, any study that helps inform the 

dose response should be considered—including studies not specific to dioxins. 

Complicated mechanistic models need not be developed. Standard dose-response models 

can be applied. One can look at the cascade of events in a stepwise, simple way. 

References 

Birnbaum, L.S., M.W. Harris, L.M. Stocking et al. 1989. Retinoic acid and 2,3,7,8-

tetrachlorodibenzo-p-dioxin selectively enhance teratogenesis in C57BL/6N mice. Toxicol. 

Appl. Pharmacol. 98:487-500. 

Franczak, A., A. Nynca, K.E. Valdez, K.M. Mizinga and B.K. Petroff. 2006. Effects of acute 

and chronic exposure to the aryl hydrocarbon receptor agonist 2,3,7,8-tetrachlorodibenzop-dioxin on the transition to reproductive senescence in female Sprague-Dawley rats. Biol. 

Reprod. 74:125-130. 

Gray, L.E. and J.S. Ostby. 2002. In utero 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alters 

reproductive morphology and function in female rat offspring. Toxicol. Appl. Pharmacol. 

133(2):285-294. 

Gray, L.E., J.S. Ostby and W.R. Kelce. 1997. A dose-response analysis of the reproductive 

effects of a single gestational dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin in male Long Evans 

Hooded rat offspring. Toxicol. Appl. Pharmacol. 146:11-20. 

Li, X., D.C. Johnson and K.K. Rozman. 1995. Reproductive effects of 2,3,7,8-

tetrachlorodibenzo-p-dioxin (TCDD) in female rats: ovulation, hormonal regulation, and possible 

mechanism(s). Toxicol. Appl. Pharmacol. 133:321-327. 

Mably, T.A., D.L. Bjerke, R.W. Moore et al. 1992a. In utero and lactational exposure of male 

rats to 2,3,7,8-tetrachlorodibenzo-p-dioxin. 3. Effects on spermatogenesis and reproductive 

capability. Toxicol. Appl. Pharmacol. 114:118-126. 

Mably, T.A., R.W. Moore, R.W. Goy et al. 1992b. In utero and lactational exposure of male 

rats to 2,3,7,8-tetrachlorodibenzo-p-dioxin. 2. Effects on sexual behavior and the regulation of 

luteinizing hormone secretion in adulthood. Toxicol. Appl. Pharmacol. 114:108-117. 

19
NAS (National Academy of Sciences). 2006. Health Risks from Dioxin and Related 

Compounds: Evaluation of the EPA Reassessment. National Academies Press, Washington, DC 

(July). Available at http://www.nap.edu/catalog.php?record_id=11688. 

U.S. EPA (U.S. Environmental Protection Agency). 2003. Exposure and Human Health 

Reassessment of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds. NAS 

Review Draft (EPA/600/P-00/001Cb). U.S. Environmental Protection Agency, National Center 

for Environmental Assessment, Washington, DC. Available at 

http://www.epa.gov/nceawww1/pdfs/dioxin/nas-review/. 

SESSION 5: QUANTITATIVE UNCERTAINTY ANALYSIS OF DOSE-RESPONSE 

This session addressed the uncertainty analysis to be considered for the dose-response 

assessments. The session opened with a presentation on current estimates of dioxin exposure 

levels. Then it focused on the factors to include in the scope of an uncertainty analysis including 

dioxin kinetics. 

Session 5 Panelists (Session Co-Chairs are identified by asterisk) 

• Bruce Allen, Bruce Allen Consulting 

• Lesa Aylward, Summit Toxicology 

• Roger Cooke, Resources for the Future 

• Kenny Crump, Louisiana Tech University 

• Mike DeVito, U.S. EPA 

• Dale Hattis, Clark University 

• *Rick Hertzberg, Biomath Consulting 

• Nancy Kerkvliet, Oregon State University 

• Leonid Kopylev, U.S. EPA 

• Rob McDowell, U.S. Department of Agriculture 

• Lorenz Rhomberg, Gradient 

• Woody Setzer, U.S. EPA 

• Marie Sweeney, National Institute of Occupational Safety and Health 

• *Linda Teuschler, U.S. EPA 

Please note that the use of the term “concluded” or “recommended” in this summary does not mean that a consensus 

was reached. Session Summaries were written from the material prepared by the non-EPA/ANL Co-Chair and 

represent a synopsis of the panel discussions. 

The Panel summarized the NAS comments regarding uncertainty. Areas for improvement 

include: 

• Ensure “transparency, thoroughness, and clarity in quantitative uncertainty analysis.” 

• Describe and define (quantitatively to the extent possible) the variability and uncertainty 

for key assumptions used for each key endpoint-specific risk assessment, including 

choices of data set, point of departure, dose-response model, and dose metric. 

• Incorporate probabilistic models to represent the range of plausible values. 

20
• Assess goodness-of-fit of dose-response models. 

• Provide upper and lower bounds on central tendency estimates for all statistical estimates. 

• When quantification is not possible, clearly state it, and explain what would be required 

to achieve quantification. 

Identification of Important Uncertainties 

 The Panel reviewed the charge questions (Appendix B), discussed them, and listed eight 

issues for consideration by the U.S. EPA: 

• Concerning species and strain differences in the U.S. EPA’s Response to NAS, current 

U.S. EPA procedures do not take this into account when selecting one data set for risk 

assessment. Issues include “Where are humans in the distribution of potencies that can 

be generated? How likely is it that human response is similar to the selected data? Can 

we infer inter-individual variability from these differences?” 

• Concerning the use of animal data for cross species extrapolation to humans (PK and PD 

uncertainties), issues to consider include differences in distribution and responses 

following bolus doses from those of subchronic and chronic protocols; uncertainty in 

liver doses due to sequestration; differences in receptor binding affinity among 

congeners; and age factors (e.g., assumption of a lifetime constant daily dose for a cancer 

extrapolation). 

• Concerning the description of AhR response, biochemical changes occur at lower doses 

than toxicological changes. There should be an effort to identify the biochemical changes 

that would mark Ah receptor binding to inform the BMR, and, thus, prevent toxicity. 

• Concerning model uncertainty, the mathematical model choice depends on endpoint. 

There should be an effort towards determining what is the most sensitive endpoint(s) for 

humans and conducting animal studies to model that endpoint(s). 

• Concerning exposure and dose response in human studies, ensure enough similarity to 

current human exposure profiles (mixture composition) so that a dose-response 

assessment can be done. Incorporate new epidemiological studies. Evaluate 

concordance with animal data and consistency across studies. Panel-acknowledged 

uncertainties include exposure estimates from person to person, shape of human doseresponse curve, healthy worker effect, and age dependence. 

• Concerning POD determination, uncertainty factors are inherently mathematically 

inconsistent and that should be conveyed in the discussion of uncertainties when 

interpreting the POD. 

• Concerning dose metric, tissue concentration is preferred. It should be evaluated against 

a background of variability in AhR-binding expression. There is uncertainty in what 

level of binding should be considered, in different cell types, tissues, life stage 

(development). The relationship between dose metric and causation of adverse effects 

should be examined. 

21
Low-Dose Extrapolation 

 The Panel reviewed the charge questions and discussed them (Appendix B). The Panel 

concluded that curve-fitting uncertainty (for a given dataset, dose metric, and model) can be 

characterized and is useful, but, by itself, it is an incomplete characterization of uncertainty. The 

Panel acknowledged the difficulty of fully characterizing uncertainty, especially quantitatively. 

Some panelists argued that the problem is insurmountable and that no meaningful uncertainty 

analysis is likely to be performable. Other panelists contended that, the difficulties 

notwithstanding, “good-faith” efforts to do something practical and forthright to characterize 

uncertainty in low-dose extrapolation would be useful and important. The Panel clarified “good 

faith” as meaning a characterization that is useful and not misleading to decision makers and is 

inclusive of approaches that have meaningful support in the scientific community as a whole. 

Being in “good faith” is more important than being complete (i.e., addressing every uncertain 

element), especially since completeness is not a realistic goal. From this discussion, the Panel 

listed four issues for consideration by the U.S. EPA: 

• Review alternative data sets, dose metrics, and models to see where consequential 

uncertainties and impacts on low-dose implications arise. 

• Consider the impacts of choices among plausible alternative data sets, dose metrics, 

models, and other more qualitative choices—issues include how much difference the 

choices make and also how much relative credence should be put to each alternative as a 

way of gauging and describing the landscape of imperfect knowledge 

regarding possibilities for the true dose-response. 

 Hard to do quantitatively, since the factors are not readily expressed as statistical 

distributions, but can describe the rationale for believing/doubting each alternative in 

terms of available supporting evidence, contrary evidence, and needed assumptions. 

 Expert judgment methods may be helpful in characterizing the relative weights of 

scientific credibility among alternatives. The expert judgment process, when 

conducted systematically, can be thought of as adding data to the assessment of 

credibility of alternatives, rather than as just an opinion poll. 

 Information on plausibility of alternative low-dose extrapolation approaches can 

come from external considerations of mode of action, and not just from statistical 

success at fitting particular (high-dose) data sets. 

• Characterizing uncertainty through a variety of approaches could be tried, and their 

relative merits and shortcomings discussed, as a way forward. 

• Consider the sources of potential error, particularly in epidemiological data (e.g., TEF 

uncertainty and variation in congener mixtures) and if possible quantify their impact on 

the dose-response assessment. 

Considerations for Conducting Uncertainty Analysis 

Overall, the Panel was split on whether U.S. EPA should do quantitative uncertainty 

analyses. The Panel noted that if done on only some of the uncertainties, then results would be 

misleading and could be misused. Ultimately, the Panel listed seven issues for consideration by 

the U.S. EPA: 

22
• The Panel recapped what some consider as being the first integrated risk assessment, with 

structured expert judgment and uncertainty analysis, i.e., the Rasmussen Report 

(WASH-1400; U.S. Nuclear Regulatory Commission, 1975). In their discussion of the 

report, the Panel noted that in addition to standard event tree/fault tree modeling, this 

report also tackled difficult model uncertainty issues involved in accident progression, 

dispersion of released pollutants in the atmosphere, environmental transport, exposure, 

health, and economic impacts. And though the Panel also recognized that this method 

was no longer state-of-the-art, the Panel contended that it represents a good example of a 

structured approach and methodology that could be built upon. 

• The Panel also discussed TEQs used in epidemiological studies, based on intake, and 

recognized that the key uncertainty in what was measured was not just intake but also 

involved PK/PD issues. The Panel acknowledged that the TEQ system is regularly used 

on a concentration basis, but they expressed concern that the qualification becomes lost. 

TEQs ignore pharmacokinetics and the common practice of rounding to orders of 

magnitude introduces more error. 

• Structure the risk assessment along MOA steps—identify key biochemical measures 

(~5−10) common across toxic endpoints and identify the degree of meaningful change in 

effect or effect variance. Make a table with all options for data set, model, etc.; make 

best estimates/choices and determine which of these choices matter the most to the 

answer. 

• Use expert panels—expert judgment can be collected scientifically (procedures are 

published). But there are known biases; central tendency estimates work much better 

than extremes. 

• Use supporting studies to fill in critical data gaps—Info filling methods do exist (e.g., PK 

modeling). Put short-term studies into the “supporting info” category (unless, of course, 

the risk assessment is for acute exposures, such as chemical spills). 

• Be creative in the analysis of uncertainty. Intermediate steps between AhR binding and 

the end processes can be hypothesized based on data, experiences, and analogies related 

to other chemicals. 

• The 2003 Reassessment presented potency estimates on wide variety of 

endpoints/models; needed to be more transparent in that discussion. Statistical graphics 

can be used to convey uncertainties. 

Reference 

U.S. Nuclear Regulatory Commission. 1975. Reactor Safety Study: An Assessment of Accident 

Risks in U.S. Commercial Nuclear Power Plants. WASH-1400 (NUREG-75-014). Washington, 

DC. 

23
APPENDIX A: 2009 U.S. EPA DIOXIN WORKSHOP AGENDA 

SCIENTIFIC WORKSHOP 

TO INFORM THE TECHNICAL WORK PLAN FOR U.S. EPA’S RESPONSE TO 

NAS COMMENTS ON THE HEALTH EFFECTS OF DIOXIN 

PRESENTED IN U.S. EPA’S DIOXIN REASSESSMENT 

Cincinnati, OH 

Date: February 18−20, 2009 

BACKGROUND/WORKSHOP OBJECTIVE 

At the request of the U.S. Environmental Protection Agency (U.S. EPA), the National 

Academy of Sciences (NAS) prepared a report, Health Risks from Dioxin and Related 

Compounds: Evaluation of the EPA Reassessment (NAS, 2006), that made a number of 

recommendations to improve the U.S. EPA’s risk assessment for 2,3,7,8-tetrachlorodibenzop-dioxin (TCDD). In response, the U.S. EPA will prepare a technical report that addresses key 

comments on the dose-response assessment for TCDD. The U.S. EPA intends to develop its 

response through a transparent process that provides multiple opportunities for input. 

To assist in this effort, a Workshop will be held to inform the U.S. EPA’s evaluation of 

the NAS recommendations. The Workshop will be open to the public. At the Workshop, the 

U.S. EPA will solicit input from expert scientists and the public. 

The goal of the Workshop is to ensure that the U.S. EPA’s response to the NAS 

comments focuses on the key issues and reflects the most meaningful science. The three main 

objectives of the Workshop are to (1) identify and discuss the technical challenges involved in 

addressing the NAS key comments on the TCDD dose-response assessment in the U.S. EPA 

Reassessment (U.S. EPA, 2003), (2) discuss approaches for addressing these comments, and 

(3) identify key published, independently peer-reviewed literature, particularly studies describing 

epidemiologic and in vivo mammalian bioassays, which are expected to be most useful for 

informing the U.S. EPA response. 

Workshop participants will be encouraged to think broadly about the body of scientific 

information that can be used to inform the U.S. EPA’s response and to participate in open 

dialogue regarding ways in which the science can best be used to address the key dose-response 

issues. This Workshop is similar to scientific workshops being conducted under the new review 

process for the National Ambient Air Quality Standards (NAAQS)1

 that assess health-related 

information for criteria pollutants. 

 

1

 Please see http://www.epa.gov/ttn/naaqs/ for more information on the new NAAQS review process. 

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 The Workshop discussions are expected to build upon two prior publications: 

1. Exposure and Human Health Reassessment of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin 

(TCDD) and Related Compounds (U.S. EPA, 2003). This external review draft 

provides a comprehensive reassessment of dioxin exposure and human health effects. 

This “dioxin reassessment” was submitted in October 2004 to the National Academy 

of Sciences (NAS) for review. 

2. Health Risks from Dioxin and Related Compounds: Evaluation of the EPA 

Reassessment (NAS, 2006). 

 Workshop participants are encouraged to review both of these documents and other 

relevant materials (e.g., the National Toxicology Program report on TCDD [NTP, 2006]) before 

the meeting because they provide important insights into the key questions and challenges. 

There are a number of open comment periods that are intended to facilitate a broad discussion of 

the issues. 

Scientists with significant expertise and experience relevant to the health effects of 

TCDD or dioxin-like compounds and associated topics will be asked to serve on “expert panels” 

for discussions throughout the Workshop. Workshop panelists will include a wide range of 

experts representing many scientific areas needed to assess TCDD dose-response (e.g., 

epidemiology, human and animal toxicology, nuclear receptor biology, dose-response modeling, 

risk assessment, and uncertainty analysis). The Workshop panelists will be asked to highlight 

significant and emerging research and to make recommendations to the U.S. EPA regarding the 

design and scope of the technical response to NAS comments on the dose-response analysis for 

TCDD—including, but not limited to, recommendations for evaluating associated uncertainty. 

Open comment periods will follow each panel discussion session. Public participation will be 

encouraged by way of these designated open comment periods and, also, by participation in the 

scientific poster session planned for the second evening (February 19). 

U.S. EPA will use the input received during this Workshop as the foundation for its 

development of a technical work plan for responding to the NAS comments on the TCDD doseresponse analysis. The work plan will outline the schedule, process, and approaches for 

evaluating the relevant scientific information and addressing the key issues. The work plan also 

will identify the key literature to be utilized in U.S. EPA’s response. 

As a follow-on activity to this Workshop, a panel is being established under the Federal 

Advisory Committee Act (FACA) to guide and review the U.S. EPA’s response to NAS 

comments. The FACA panel will be asked to conduct a consultation with the Agency on the 

draft technical work plan. At the same time, the public will also have the opportunity to provide 

comments to the FACA panel on the work plan. The final technical work plan will guide the 

development of the technical report that will constitute the U.S. EPA’s response to NAS 

comments. During the development of this response, the U.S. EPA will seek advice from the 

FACA panel and the public several times. Finally, the FACA panel will be asked to review the 

technical report in a public forum. 

The preliminary Agenda presented on the following pages may be revised prior to the 

Workshop following review by the session Co-Chairs; the dates and general timing of the 

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sessions, however, will not change. A final Agenda and a set of charge questions, intended to 

provide general direction for the Workshop discussions, will be posted on the Workshop Internet 

site (http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=199923) prior to the meeting. 

A poster session will be held on the evening of the second day (February 19). The 

purpose of this poster session is to provide a forum for scientists to present recent studies 

relevant to TCDD dose-response assessment and to encourage open discussion about these 

presentations. 

REFERENCES 

NAS (National Academy of Sciences). 2006. Health Risks from Dioxin and Related 

Compounds: Evaluation of the EPA Reassessment. National Academies Press, Washington, DC 

(July). Available at http://www.nap.edu/catalog.php?record_id=11688. 

NTP (National Toxicology Program). 2006. Toxicology and Carcinogenesis Studies of 

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) (CAS No. 1746-01-6) in Female Harlan SpragueDawley Rats (Gavage Studies). U.S. Department of Health and Human Services. NTP TR 521. 

Research Triangle Park, NC (April). 

U.S. EPA (U.S. Environmental Protection Agency). 2003. Exposure and Human Health 

Reassessment of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds, NAS 

review draft, Volumes 1-3 (EPA/600/P-00/001Cb, Volume 1). U.S. Environmental Protection 

Agency, National Center for Environmental Assessment, Washington, DC (December). 

Available at http://www.epa.gov/nceawww1/pdfs/dioxin/nas-review/. 

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WORKSHOP AGENDA 

Day 1

8:00–9:00 Registration 

9:00–9:30 Welcome/Purpose of Meeting/Document Development Process 

9:30–9:45 Panel Comments/Questions on Charge

9:45–2:45 Session 1: Quantitative Dose-Response Modeling Issues 

(Hall of Mirrors)

 9:45–10:10 Background/Introductory Remarks 

 10:10–10:35 TCDD Kinetics: Converting Administered Doses in Animals to 

Human Body Burdens 

 Presenter: Michael Devito 

 10:35–11:30 Panel Discussion

 11:30–1:00 Lunch

 1:00–2:00 Panel Discussion cont.

 2:00–2:45 Open Comment Period 

2:45–3:05 Break 

3:05–5:15 Session 2: Immunotoxicity (Hall of Mirrors)

 3:05–3:15 Background/Introductory Remarks 

 

 3:15–4:45 Panel Discussion

 4:45–5:15 Open Comment Period 

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Day 2

8:00–8:30 Report-Outs for Sessions 1 and 2 (Hall of Mirrors)

 8:00–8:15 Report-Out for 1: Quantitative Dose-Response Modeling Issues 

 8:15–8:30 Report-Out for 2: Immunotoxicity 

8:30–11:30 Sessions 3A and 3B (concurrent sessions)

8:30–11:30 Session 3A: Dose-Response for Neurotoxicity and 

Nonreproductive Endocrine Effects (Hall of Mirrors)

 

 8:30–8:45 Background/Introductory Remarks 

 

 8:45–11:00 Panel Discussion

 11:00–11:30 Open Comment Period

8:30–11:30 Session 3B: Dose-Response for Cardiovascular Toxicity and 

Hepatotoxicity (Rookwood Room)

 8:30–8:45 Background/Introductory Remarks 

 

 8:45–11:00 Panel Discussion 

 11:00–11:30 Open Comment Period

11:30–1:00 Lunch 

1:00–2:00 Report-Outs for Sessions 3A and 3B (Hall of Mirrors)

The structure of the session report-outs will include the following: 

 Summary of session presentation including minority opinion 

 Public comments 

 Discussion 

 1:00–1:15 Report-Out for 3A: Dose-Response for Neurotoxicity and 

Nonreproductive Endocrine Effects 

 1:15–1:30 Open Comment Period 

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 1:30–1:45 Report-Out for 3B: Dose-Response for Cardiovascular Toxicity and 

Hepatotoxicity 

 1:45–2:00 Open Comment Period 

2:00–5:15 Sessions 4A and 4B (concurrent sessions)

2:00–5:15 Session 4A: Dose-Response for Cancer (Hall of Mirrors)

 2:00–2:15 Background/Introductory Remarks 

 

 2:15–4:45 Panel Discussion

 4:45–5:15 Open Comment Period 

2:00–5:15 Session 4B: Dose-Response for 

Reproductive/Developmental Toxicity (Rookwood Room)

 2:00–2:15 Background/Introductory Remarks 

 

 2:15–4:45 Panel Discussion

 4:45–5:15 Open Comment Period 

6:45–8:15 Poster Session (Rosewood Room)

Day 3

8:30–9:30 Report-Outs for Sessions 4A and 4B (Hall of Mirrors)

 8:30–8:45 Report-Out for 4A: Dose-Response for Cancer 

 8:45–9:00 Open Comment Period 

 9:00–9:15 Report-Out for 4B: Dose-Response for Reproductive/Developmental 

Toxicity 

 9:15–9:30 Open Comment Period

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9:30–3:30 Session 5: Quantitative Uncertainty Analysis of DoseResponse (Hall of Mirrors)

 9:30–9:40 Background/Introductory Remarks

 9:40–10:10 Evidence of a Decline in Background Dioxin Exposures in Americans 

Between the 1990s and 2000s

 Presenter: Matt Lorber 

10:10–10:30 Break 

 10:30–11:30 Panel Discussion 

 11:30–1:00 Lunch 

 1:00–2:15 Panel Discussion cont.

 2:15−2:30 Break 

 2:30–3:00 Open Comment Period 

 3:00–3:15 Report-Out for 5: Quantitative Uncertainty Analysis of DoseResponse 

 3:15–3:30 Closing Remarks 

3:30 Adjourn 

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APPENDIX B: 2009 U.S. EPA DIOXIN WORKSHOP 

QUESTIONS TO GUIDE PANEL DISCUSSIONS 

SESSION 1 

Dose Metric 

Considering all of the endpoints or target tissues, and species that U.S. Environmental Protection 

Agency (U.S. EPA)’s dose-response modeling might evaluate, what are the best measures of 

dose (e.g., ingested, tissue concentrations, body burden, receptor occupancy, other surrogate) and 

why? 

Developing Dose-Response Models from Mammalian Bioassays 

How best can the point of departure (POD) be determined when the response range is 

incompletely characterized (i.e., high response at the lowest dose or low response at the highest 

dose; observed in several key 2,3,7,8-Tetrachlorodibenzo-p-Dioxin [TCDD] studies)? 

If considered to be biologically plausible, how can a threshold be incorporated into a doseresponse function (e.g., for TCDD cancer data)? 

How can nonmonotonic responses be incorporated into the dose-response function? 

Developing Dose-Response Models from Epidemiological Studies 

How can the epidemiological data be utilized best to inform the TCDD exposure-response 

modeling? Which epidemiological studies are most relevant? 

Supporting Information 

For those toxicological endpoints that are Ah receptor-mediated, how would the receptor kinetics 

influence the shape of the dose-response curve? How would downstream cellular events affect 

the shape of the dose-response curve? How can this cascade of cellular events be incorporated 

into a quantitative model of dose-response? 

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SESSIONS 2, 3A, 3B, 4A, AND 4B 

Key Study Selection 

For this endpoint, what refinements should be made to the draft criteria for selection of key 

studies? 

What are the specific effects of concern for human health for this endpoint? 

Based on the draft criteria for the selection of key studies, what are the key studies informing the 

shape of the dose-response curve above the POD and the choice of the POD for this endpoint? 

Epidemiological Study Utility 

How and to what extent do the epidemiological data inform the choice of critical effect? 

How can the epidemiological data inform the quantitative dose-response modeling? 

Animal Model Utility 

Are there types of effects observed in animal models that are more relevant to humans than 

others? To what extent does information on mode of action (MOA) influence the choice of 

animal model (species, strain, sex)? 

Supporting Information 

Are there studies that establish a sufficient justification for departure from the default procedures 

that address the shape of the dose-response curve below the POD under the cancer guidelines? 

Are there studies that establish a sufficient justification for departing from U.S. EPA’s default 

approaches for noncancer endpoints? 

To what extent can MOA information clarify the identification of endpoints of concern and doseresponse metric for this endpoint? How can the cascade of cellular events for this endpoint be 

incorporated into a quantitative model of dose response? 

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SESSION 5 

For cancer and noncancer TCDD dose-response assessments, U.S. EPA is interested in 

developing a quantitative uncertainty analysis addressing both parameter and model uncertainty, 

if feasible. Uncertainties will include, among others, choice of endpoint; underlying study 

uncertainties; choice of dose metric; interspecies extrapolations such as kinetic uncertainties; and 

choice of dose-response model, including threshold models. The U.S. EPA is currently 

examining techniques and tools for uncertainty analysis—including Bayesian and frequentist 

approaches. 

Identification of Important Uncertainties 

What are the major uncertainties pertaining to modeling the animal data? 

Consider the dose metric (species or tissue specificity), vehicle of administration, 

exposure frequency, exposure duration, and POD determination (e.g., benchmark 

response selection or no-observed-adverse-effect level/lowest-observed-adverse-effect 

level identification). 

What are the major uncertainties pertaining to dose-response modeling below the POD? 

Consider how receptor kinetics and downstream cellular event information might be used 

to bound the uncertainties associated with dose-response modeling below the POD. 

What are the major uncertainties in cross-species extrapolation (e.g., half-lives, tissue 

distribution, and toxicodynamics)? 

Consider the primary species dosed with TCDD: mice, hamsters, rats, guinea pigs, and 

monkeys. 

What are the major uncertainties pertaining to intrahuman variability? 

Consider what data sets would be useful to represent sensitive subpopulations. 

What are other significant sources of uncertainty for the cancer and noncancer assessments? 

Considerations for Conducting Uncertainty Analysis 

What data sets could be used to quantify uncertainties in cancer and noncancer TCDD doseresponse assessments? 

 Consider dioxin-like compound dose-response data. 

 Consider MOA information. 

What are the appropriate techniques for the TCDD dose-response uncertainty analysis, and what 

are their respective strengths and weaknesses of these approaches as applied to TCDD? 
3 February 2009

APPENDIX C: 2009 U.S. EPA DIOXIN WORKSHOP DRAFT SELECTION CRITERIA TO IDENT

MAMMALIAN STUDIES THAT INFORM DOSE-RESPONSE MODELING FOR 2,3,7,8-TETRAC

p-DIOXIN (TCDD)a 

Study Feature Selection Rationale

Primaryb Secondaryc Cu

Chemical, purity, matrix/medium 

TCDD-only doses included, purity specified, matrix in which TCDD is administered is identified 

TCDD purity or matrix not clearly identified Studies of 

(DLCs) or 

Peer review Independently peer-reviewed, publicly available Supplementary materials accompanying 

peer-reviewed publication 

Not formanot public

Study design, execution, and reporting 

Clearly documented and consistent with standard toxicological principles, testing protocols, and practice (i.e., endpoint-appropriate, particularly for negative findings) 

Testing protocol provides incomplete coverage of relevant endpoint-specific measures, particularly for negative findings 

Studies noprinciples 

Study subject: species, strain, and sensitivity for given endpoint; litter; life stage; gender 

Mammalian species Strain and gender identified Animal age at beginning of treatment identified Litter confounders (within/between) accounted for 

Mammalian species, in vivo, but only 

studying an artificially sensitive subject (e.g., knockout mouse) 

Non-mam

Exposure route Oral Parenteral (e.g., intravenous, intramuscular, 

intraperitoneal, subcutaneous) 

Inhalation

Dose level Lowest dose ≤200 ng/kg-d for noncancer 

 endpoints and ≤1 μg/kg-d for cancer 

Lowest dose >200 ng/kg-d for noncancer endpoints, or >1.0 μg/kg-d for cancer 

Exposure frequency, duration, and timing 

Dosing regimen characterized and explained Character

or cannot 

Controls Appropriate and well characterized Effect reported, but with no negative controlResponse Effect relevant to human health 

Magnitude outside range of normal variability 

Precursor effects, or adaptive responses potentially relevant to human health 

Lethality 

Statistical evaluation Clearly described and appropriate to the endpoint 

and study design (e.g., per error variance, magnitude of effect) 

Limited statistical context 

a NAS (2006) commented that the selection of data sets for quantitative dose-response modeling needed to be more transparent. 

offered for consideration at the kickoff workshop. These criteria would be used to identify candidate studies of non-human madefine the point-of-departure (POD). These criteria are not designed for hazard identification or weight-of-evidence determinatdata other than direct TCDD dose-response in mammals (including toxicokinetic data on absorption, distribution, metabolism, on physiologically-based pharmacokinetic [PBPK] modeling, and mode of action data) will be evaluated separately. b Presents preliminary draft criteria for evaluating a study being considered for estimating a POD in a TCDD dose-response mod

c Presents preliminary draft criteria that could qualify a study as primary with support from other lines of evidence (e.g., PBPK mo

an endpoint meets the “primary” criteria.

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