ii 
iii 

FINAL REPORT: WORKSHOP TO 

DEVELOP A FRAMEWORK FOR 

ASSESSING RISKS TO CHILDREN 

FROM EXPOSURES TO 

ENVIRONMENTAL AGENTS 

ILSI Risk Science Institute 

One Thomas Circle, NW, Ninth Floor 

Washington, DC 20005-5802 

202-659-3306 

rsi@ilsi.org

CONTACT: Stephen S. Olin, Ph.D. 

 solin@ilsi.org
iv 
v 

Table of Contents

Page

ABOUT ILSI AND THE ILSI RISK SCIENCE INSTITUTE ..................................................... ix

ACKNOWLEDGMENTS .............................................................................................................. x

EXECUTIVE SUMMARY ........................................................................................................... xi

I. INTRODUCTION .............................................................................................................. 1 

• Preparation for the Workshop....................................................................................... 1 

• The Workshop............................................................................................................... 3 

• The Report .................................................................................................................... 6

II. A PROPOSED FRAMEWORK FOR ASSESSING RISKS TO CHILDREN FROM 

ENVIRONMENTAL EXPOSURES.................................................................................. 7 

• Problem Formulation .................................................................................................. 10 

• Analysis....................................................................................................................... 11 

• Risk Characterization.................................................................................................. 13

III. TOXICOKINETIC CONSIDERATIONS IN UNDERSTANDING CHILDREN'S 

HEALTH RISKS FROM EXPOSURE TO ENVIRONMENTAL AGENTS ................. 15 

• Introduction................................................................................................................. 15

• Problem Formulation for the TK Analysis ................................................................. 16 

• Generally Important TK Determinants ....................................................................... 18 

• Analysis of TK Data ................................................................................................... 21 

• Selection of Age Groups for Special Focus................................................................ 28 

• Selection of Analytical Approach............................................................................... 29 

• Engaging the Framework: Addressing Modeling and Data Needs............................ 31

IV. TOXICODYNAMIC CONSIDERATIONS IN UNDERSTANDING CHILDREN'S 

HEALTH RISKS FROM EXPOSURE TO ENVIRONMENTAL AGENTS ................. 35 

• Introduction................................................................................................................. 35

• Toxicodynamics in the Risk Assessment Framework ................................................ 35 

• Step 1: Toxicodynamic Considerations During Problem Formulation...................... 35 

• Steps 2 & 3: Toxicodynamic Considerations During Analysis and Risk 

Characterization .......................................................................................................... 37 

• Questions..................................................................................................................... 37 

• Critical Data Needs..................................................................................................... 57 

• Conclusions................................................................................................................. 58

V. RISK CHARACTERIZATION AND THE FRAMEWORK........................................... 61

REFERENCES ............................................................................................................................. 67 
vi 

APPENDIX 1 – Children’s Health and the Environment: Public Health Issues and Challenges for 

Risk Assessment ........................................................................................................................... 73 

• 1.0 Introduction........................................................................................................ 73 

• 2.0 The Historical Development of Child-Protective Risk Assessment .................. 74 

• 3.0 Children's Unique Vulnerability to Toxicants in the Environment ................... 76 

• 4.0 Changing Risks at Different Ages ..................................................................... 78 

• 5.0 Diseases in Children of Known or Suspected Environmental Origin................ 82 

• 6.0 A Framework for Assessing Environmental Health Risks to Children ............. 84 

• 7.0 Conclusion ......................................................................................................... 87

References to APPENDIX 1......................................................................................................... 87

APPENDIX 2 – Hazard Identification and Predictability of Children’s Health Risk from 

Animal Data...................................................................................................................... 99 

• Introduction................................................................................................................. 99

• Critical periods and important milestones in development......................................... 99 

• Relevance/predictability of extrapolation of animal data to children's risk 

assessment................................................................................................................. 103 

• Methods for assessing potential hazards to children................................................. 107 

• Summary................................................................................................................... 111 

References to APPENDIX 2....................................................................................................... 111

APPENDIX 3 – Incorporating Children’s Toxicokinetics into a Risk Framework*.................. 115 

• 1.0 Introduction - Children Can Be a Tough Group to Figure Out........................ 115 

1.1 Toxicokinetics as a Key Element in Children’s Risk Assessments........ 115 

1.2 Proposed Building Blocks for a Children’s TK Assessment.................. 116 

• 2.0 Phase I. TK Questions in Problem Formulation ............................................. 118 

• 3.0 Phase II: Data Analysis and Development of Risk Assessment Approach .... 120 

• 4.0 Phase III. Risk Characterization ..................................................................... 144 

References to APPENDIX 3....................................................................................................... 146 

APPENDIX 4 – FOCUS QUESTIONS...................................................................................... 159

APPENDIX 5 – WORKSHOP PARTICIPANTS AND OBSERVERS CONTACT 

INFORMATION......................................................................................................................... 163 

APPENDIX 6 – WORKSHOP PROGRAM............................................................................... 169 
vii 

LIST OF FIGURES

Figure II-1. Proposed Framework for Assessing Risks to Children from Exposure to 

Environmental Agents .................................................................................................................... 8 

Figure II-2. Stages in the Integrated Risk Assessment Process..................................................... 9 

Figure IV-1: Exposures and Endpoints Related to Developmental Toxicity Evaluations........... 39 

Figure IV-2: Initial Appearance of Organ Systems during Gestation in Humans and Rodents .. 42 

Figure IV-3: Respiratory System Development in Humans and Rodents ................................... 44 

Figure IV-4: Immune System Development in Humans and in Rodents .................................... 46 

Figure IV-5: Nervous System Development in Humans and Rodents ........................................ 48 

Figure IV-6: Patterns of Neuronal Proliferation in Specific Brain Regions................................ 52 

Figure IV-7: Levels of Mechanistic Inquiry for Assessing the Effects of a Toxicant on 

Development................................................................................................................................. 53 

Figure A2-1. Differentiation Increases with Age of Developing Organisms ............................ 100 

Figure A2-2. Time to Develop Adult Characteristics................................................................ 101 

Figure A2-3. Relationship Between Extent of Maturation and Birth in Rats and Humans....... 101 

Figure A3-1. Outline of Toxicokinetic (TK) Assessment Process for Children........................ 117 

Figure A3-2. Analysis of Children’s Pharmacokinetic Database Half-Life Results for CYP3A 

Substrates -- Alfentanil, Carbamazepine, Fentanyl, Lignocaine, Midazolam, Nifedipine, 

Quinidine, Triazolam .................................................................................................................. 128 

Figure A3-3. Analysis of Children’s Pharmacokinetic Database: Half-Life Results for Full 

Database – 40 Substrates .............................................................................................................129 

Figure A3-4a. Age-Related Variations of CYP2E1 Protein in the Human Liver...................... 130 

Figure A3-5. Age-Related Changes Over Seven Stages of Life in the Serum Dimethadione/ 

Trimethadione Ratio 4 h After Oral Administration of Trimethadione (4mg/kg).......................131 
viii 

LIST OF TABLES

Table III-1. Critical Data Needs for Children’s TK Framework ................................................. 32 

Table IV-1. How Does Developmental Dynamic Information Impact Risk Assessments for 

Children?....................................................................................................................................... 36 

Table IV-2. Some Questions Considered by the Dynamics Breakout Group.............................. 38 

Table IV-3. Assessment Questions to be Considered for Each Exposure Window .................... 56 

Table IV-4. Conclusions from the Dynamics Workgroup........................................................... 59 

Table A1-1. Major provisions of the Food Quality Protection Act of 1996................................ 76 

Table A2-1. Fetal Endpoints in Rat, Rabbit and Human ........................................................... 104 

Table A2-2. Endpoint (Humans and Laboratory Animals) ....................................................... 105 

Table A2-3. EPA Testing Guidelines ........................................................................................ 107 

Table A2-4. FDA Testing Guidelines........................................................................................ 108 

Table A3-1: Post-Natal Developmental Profile of Pharmacokinetic Functions........................ 124 

Legend for Table A3-1: .............................................................................................................. 126 

Table A3-2. Partial list of human placental xenobiotic and hormone metabolizing enzymes or 

isoenzymes.................................................................................................................................. 134 

Table A3-3. Some Cytochrome P-450 (CYP) Isozymes and Their Occurrence in 

Human Tissues............................................................................................................................ 137 
ix 

ABOUT ILSI AND THE ILSI RISK SCIENCE INSTITUTE

The International Life Sciences Institute (ILSI) is a nonprofit, worldwide foundation 

established in 1978 to advance the understanding of scientific issues relating to nutrition, food 

safety, toxicology, risk assessment, and the environment. By bringing together scientists from 

academia, government, industry, and the public sector, ILSI seeks a balanced approach to 

solving problems of common concern for the well-being of the general public. 

ILSI is affiliated with the World Health Organization as a nongovernmental organization 

and has specialized consultative status with the Food and Agriculture Organization of the United 

Nations. 

ILSI is headquartered in Washington, DC. ILSI branches include Argentina, Australasia, 

Brasil, Europe, India, Japan, Korea, Mexico, North Africa and Gulf Region, North America, 

North Andean, South Africa, South Andean, Southeast Asia, Thailand, the Focal Point in China, 

the ILSI Health and Environmental Sciences Institute, and a study group in Central America. 

ILSI receives financial support from industry, government, and foundations. 

Because science is the base of ILSI’s achievements, the ILSI Research Foundation was 

formed in 1984 to create a vehicle for ILSI to support basic scientific research in underfunded 

and unexplored areas. The ILSI Risk Science Institute (RSI), a component of the ILSI 

Research Foundation, seeks to advance and improve the scientific basis of risk assessment. ILSI 

RSI serves as a catalyst for consensus on complex scientific issues in risk assessment by 

facilitating discussion and cooperation among scientists from all sectors. 
x 

ACKNOWLEDGMENTS

This report is the product of two years’ effort by many individuals, who are identified in 

the Introduction. We note, in particular, the key roles played by the Workshop Planning 

Committee, the breakout group chairs and rapporteurs, and the authors of the background papers. 

We thank the 45 workshop participants and 9 observers for the enthusiasm and collegiality with 

which they accomplished their task of developing a framework for assessing children’s risks. 

Special thanks go to two observers, Dr. LaRonda Morford (Eli Lilly & Co.) who assisted with 

rapporteuring the Toxicodynamics breakout group and Dr. Nancy Beck (EPA/ORD/NCEA 

AAAS Fellow) who helped with the preparation of the Toxicodynamics chapter. 

This project was conducted under a cooperative agreement (CR 827308-01) with the U.S. 

Environmental Protection Agency, Office of Research and Development, National Center for 

Environmental Assessment, Washington, DC. Financial support from EPA (the National Center 

for Environmental Assessment/ORD and the Office of Children’s Health Protection), Health 

Canada, the American Chemistry Council, the American Crop Protection Association (now 

known as CropLife America), and the International Life Sciences Institute is gratefully 

acknowledged. 

ILSI Risk Science Institute staff support in the organization and conduct of the workshop 

was ably provided by Stephanie Carter, Diane Dalisera, and Nanette Thompson, and Audrey Ray 

and Rita Johnson assisted with word processing in preparation of the report. Finally, the 

encouragement, counsel, and active participation in this project of Dr. Penny Fenner-Crisp, ILSI 

RSI Executive Director, is also gratefully acknowledged. 

Dr. Stephen Olin, Workshop Chair Dr. Babasaheb Sonawane, Workshop Co-chair 

Deputy Director Chief, Effects Identification and 

ILSI Risk Science Institute Characterization Group, EPA/ORD/NCEA 

_________________________ 

The views expressed in this report are those of the individual authors and workshop participants and do not 

necessarily reflect the views of ILSI, EPA, or any other organization. Mention of trade names of commercial 

products does not constitute endorsement or recommendation for use.
xi 

EXECUTIVE SUMMARY

Under a cooperative agreement with the EPA/ORD National Center for Environmental 

Assessment, the ILSI Risk Science Institute organized and convened a workshop July 30 – 

August 2, 2001 to develop a framework for assessing risks to children from exposure to 

environmental agents. The 45 invited participants and 9 invited observers, drawn from 

government, academia, industry and the public health community, represented a wide diversity 

of fields of expertise, ranging from developmental biology/toxicology and pediatrics to 

pharmacokinetics and risk assessment. 

The conceptual framework created by the workshop (Figure II-1, p. 8) is based on the 

Problem Formulation → Analysis → Risk Characterization paradigm as applied to early life 

stage exposures. It recognizes the potential significance of the timing of exposures in relation to 

the susceptibility of the developing human, from the perspective of both toxicokinetics and 

toxicodynamics. It offers a systematic approach to the consideration of the factors that may 

influence risk during development, from conception through organ maturation (in adolescence). 

It acknowledges that the complexity and unique insights of a risk assessment focusing on early 

life stages will depend critically on the data available and the scope of the assessment. 

Among the conclusions from the workshop were the following: 

• There are distinct life stages during development with both known and 

hypothesized ‘windows of susceptibility’ in humans and experimental animal 

models. These developmental life stages are defined by differences in relevant 

kinetic and dynamic processes occurring at the molecular, cellular, organ, and 

physiological level. Interspecies comparisons must consider differences in life 

stages and kinetic and dynamic processes, including timing and dosimetry. 

• In addition to considerations of intrinsic sensitivity of the developing human, life 

stage-specific behaviors, activity patterns, functions, and intakes often can lead to 

dramatic differences in exposures. Life stage-linked exposure assessment is a 

critical component of any children’s environmental health risk assessment. 

• In Problem Formulation, in the context of the proposed framework, defining the 

overall scope and objectives of the risk assessment is important for the initial 

assessment of life stages, exposure scenarios, and toxic effects to be considered. 

• Problem Formulation produces a conceptual model of the likely key relationships 

between exposures and effects of the environmental agent(s) on ‘host’ (exposed) 

populations, informed by the initial identification of exposure scenarios, exposed 

life stages, and the known or anticipated biological effects of the environmental 

agent(s). The conceptual model for the risk assessment arises from and guides the 

collection of data in preparation for the Analysis phase. 
xii 

• Toxicokinetic considerations in Analysis include agent/chemical-specific factors 

and life stage/age-specific factors, both of which can include effects on absorption, 

distribution, metabolism and excretion. Examination of these factors may reveal 

one or more age groups of particular toxicokinetic concern. 

• Toxicodynamic considerations in Analysis include the identification of uniquely 

susceptible dynamic processes of concern and the functional consequences of 

altering these processes, and consideration of available data that may indicate 

differential toxicity from exposures during susceptible periods. 

• Analysis of the timing of development and exposures and of the dosimetrics of the 

agent (including both kinetic and dynamic factors) links the characterization of life 

stage-specific exposures with life stage-specific effects. 

• Risk Characterization for early life stage exposures may be qualitative (e.g., when 

quantitative data are lacking or a quantitative analysis is unnecessary) or 

quantitative (e.g., incorporating a PBTK or BBDR life stage-specific model) or 

some other semi-quantitative assessment. 

• The full spectrum of potential developmental effects cannot be predicted from data 

on exposed adults. A core data set from studies in developing organisms is 

essential. 

• The workshop cited a number of data resources to support the risk assessment 

approach outlined in the framework, including the papers developed for the 

workshop (Appendices 1-3), and identified critical research needs for improved 

assessments. 
1 

I. INTRODUCTION

“Children are not simply small adults but rather are a unique population for health risk 

assessment.” So begins the summary of the ILSI Risk Science Institute’s conference on 

Similarities and Differences Between Children and Adults: Implications for Risk Assessment, 

convened in Hunt Valley, Maryland in November 1990 (Guzelian et al, 1992). That conference 

summary also recognized the need for further work on the “specific application of the 

information presented at this conference to risk assessment methodologies,” thus setting the 

stage for the workshop that is the subject of this report. 

Nearly 11 years after the ‘Similarities and Differences’ conference, the ILSI Risk Science 

Institute (RSI) held a workshop in Stowe, Vermont, July 30-August 2, 2001 to develop a 

framework for assessing risks to children from exposure to environmental agents. The 45 invited 

participants and 9 invited observers, working in three breakout groups, focused on 

toxicokinetics, toxicodynamics, and risk characterization and drafted a structured approach to 

identifying and assessing potential risks from exposures occurring from conception through 

organ maturation (in adolescence). The approach is based on the problem formulation →

analysis → risk characterization paradigm that has been incorporated in many risk assessment 

frameworks over the past decade, and is depicted in a schematic diagram (Figure II-1). 

This report and its appendices comprise the primary work product from the workshop. A 

series of manuscripts presenting the technical content of the report and the complementary, 

authored background papers (included here as Appendices 1-3) also have been submitted for 

publication in the peer-reviewed journal Environmental Health Perspectives. 

Preparation for the Workshop

In 1999 ILSI RSI was awarded a cooperative agreement with the National Center for 

Environmental Assessment (NCEA), Office of Research and Development, U.S. Environmental 

Protection Agency, after responding to a competitive Request for Applications. ILSI RSI and 

NCEA agreed that a major focus of the cooperative agreement would be methodologies for 

assessing children’s risks, and in March 2000 ILSI RSI convened a scoping meeting to provide 

input on a possible workshop to develop a framework for children’s risk assessment. 

Participating in the meeting were Dr. Bob Sonawane, Dr. Bruce Rodan, and Dr. Carole Kimmel 

(EPA/ORD/NCEA), Dr. Michael Firestone (EPA Office of Children’s Health Protection), Dr. 

Michael Bolger (FDA CFSAN), Dr. Terri Damstra (WHO/ILO/UNEP International Programme 

on Chemical Safety), Dr. Joseph Scimeca (Pillsbury, and Chair, ILSI N.A. Food Toxicology and 

Safety Assessment Committee), and Dr. Stephen Olin and Dr. Isabel Walls (ILSI RSI). This 

committee agreed with the concept of a workshop to include both presentations and breakout 

groups, recognized the need for background papers prepared before the workshop, and noted the 

desirability of obtaining broad-based funding support. The committee also provided ILSI RSI 

with nominations for a Workshop Planning Committee. 
2 

In August 2000, ILSI RSI held the first meeting of the 16-member Workshop Planning 

Committee (see inset). Over the course of the next nine months, including a second meeting in 

January 2001 and many conference calls, the Planning Committee: 

• Developed an Outline for a Framework for Assessing Children's Risks, including a 

first draft of the schematic diagram 

• Prepared four background papers for the workshop on: 

− Children's Health and the Environment: Public Health Issues and Challenges 

for Risk Assessment 

WORKSHOP TO DEVELOP A FRAMEWORK FOR ASSESSING 

RISKS TO CHILDREN FROM EXPOSURE TO ENVIRONMENTAL AGENTS 

WORKSHOP PLANNING COMMITTEE

Dr. Sherlita Amler* ATSDR/CDC 

Dr. William Breslin Eli Lilly & Company 

Dr. Adolfo Correa Center for Environmental Health/CDC 

Dr. George Daston The Procter & Gamble Company 

Dr. Karen Davis-Bruno CDER/FDA 

Dr. Brenda Eskenazi University of California, Berkeley 

Dr. Elaine Faustman University of Washington 

Dr. Gary Ginsberg Connecticut Department of Public Health 

Dr. Daniel Goldstein Monsanto Company 

Dr. Susan Kess* ATSDR/CDC 

Dr. Carole Kimmel NCEA/ORD/EPA 

Dr. Philip Landrigan Mount Sinai School of Medicine 

Dr. Stephen Olin ILSI Risk Science Institute 

Dr. Bill Slikker, Jr. Nat’l Ctr. for Toxicological Research/FDA 

Dr. Ralph Smialowicz NHEERL/EPA 

Dr. Bob Sonawane NCEA/ORD/EPA 

Dr. Tom Trautman General Mills 

_______ 

* Dr. Amler replaced Dr. Kess in late spring, 2001 
3 

− Hazard Identification and Predictability of Children's Health Risk from 

Animal Data 

− Incorporating Children's Toxicokinetics into a Risk Framework 

− Risk Assessment Practices and Challenges for Protecting Children's Health 

• Generated focus questions for the workshop and for each of the breakout groups 

(see Appendix 4) 

• Nominated experts in the key scientific disciplines and assisted ILSI RSI with the 

assignment of participants to breakout groups 

• Worked with ILSI RSI on the program for the workshop. 

In addition, all members of the Workshop Planning Committee were invited to participate in the 

workshop, and most were able to do so. 

The Workshop

Workshop participants were selected principally for their scientific expertise and 

experience. Among the areas of expertise represented were developmental biology and 

toxicology (neuro, repro/developmental, immuno, pulmonary, general), pediatrics, genetics, 

epidemiology, pharmacokinetics, modeling, exposure assessment, and risk assessment. 

Participants were drawn from government, academia, industry and the public health community. 

Workshop participants and observers with their affiliations at the time of the workshop are listed 

in the inset (p. 4). Their current contact information is given in Appendix 5. 

The workshop began with dinner on July 30, and concluded mid-afternoon on August 2, 

2001. The workshop program is in Appendix 6. Dr. Olin (ILSI RSI) chaired the workshop, with 

Dr. Sonawane (EPA/ORD/NCEA) as co-chair. Dr. Landrigan set the stage with a presentation 

after dinner on the first evening. The next morning Dr. Knott summarized the conclusions from 

the EPA Risk Assessment Forum Technical Workshop on Issues Associated with Considering 

Developmental Changes in Behavior and Anatomy when Assessing Exposure to Children, held 

in July 2000 (U.S. EPA Risk Assessment Forum, 2000), and on the second morning of the 

meeting informal presentations of case studies were given by Dr. Dellarco, Dr. Clewell, and Dr. 

Davis-Bruno to encourage discussion of the applicability of the framework. Dr. Olin presented 

the charge to the workshop on the first morning, delineating the workshop scope and objectives. 

He noted that there were some topics that were specifically NOT included in the scope: 

regulatory policies, evaluation of individual chemicals and critique of prior risk assessments, and 

he also noted that a detailed consideration of exposure-related issues was not planned in view of 

the prior work of the EPA Risk Assessment Forum. 
4 

Most of the time in the workshop was devoted to the work of the breakout groups. 

Participant assignments to breakout groups are shown in the inset. Breakout group chairs and 

rapporteurs are also identified. Observers were free to sit in on any of the breakout groups. 

WORKSHOP TO DEVELOP A FRAMEWORK FOR ASSESSING 

RISKS TO CHILDREN FROM EXPOSURE TO ENVIRONMENTAL AGENTS 

July 30 – August 2, 2001 – Stowe, VT 

PARTICIPANTS 

 

Names Affiliations Names Affiliations

John Adgate 

Richard Albertini 

Sherlita Amler 

Hugh Barton 

Matthew Bogdanffy 

William Breslin 

James Bruckner 

Bob Chapin 

Harvey Clewell 

Elaine Cohen Hubal 

Adolfo Correa 

George Daston 

Karen Davis-Bruno 

Vicki Dellarco 

John DeSesso 

Rodney Dietert 

Joyce Donohue 

Brenda Eskenazi 

Elaine Faustman 

Penny Fenner-Crisp 

Gary Ginsberg 

Daniel Goldstein 

Jean Harry 

Univ. of Minnesota 

Univ. of Vermont 

ATSDR 

EPA/ORD/NHEERL 

DuPont Haskell Laboratory 

Eli Lilly & Co. 

Univ. of Georgia 

DuPont Pharmaceutical 

ICF Kaiser/KS Crump Group 

EPA/ORD/NERL 

CDC 

Procter & Gamble 

FDA/CDER 

EPA/OPP 

Mitretek Systems 

Cornell Univ. 

EPA/OW 

Univ. of California, Berkeley 

Univ. of Washington 

ILSI Risk Science Institute 

Conn. Dept. of Public Health 

Monsanto 

NIEHS 

Dale Hattis 

Robert Kavlock 

Carole Kimmel 

Gary Kimmel 

Dan Krewski 

Kannan Krishnan 

Phil Landrigan 

Bruce Lanphear 

Melanie Marty 

Bette Meek 

Stephen Olin*

Merle Paule 

Kent Pinkerton 

Jennifer Seed 

Larry Sheets 

Michael Shelby 

Wayne Snodgrass 

Diana Somers 

Bob Sonawane**

Tom Trautman 

Isabel Walls 

Tracey Zoetis 

Clark University 

EPA/ORD/NHEERL 

EPA/ORD/NCEA 

EPA/ORD/NCEA 

Univ. of Ottawa 

Univ. of Montreal 

Mt. Sinai School of Medicine 

Univ. of Cincinnati 

CalEPA/OEHHA 

Health Canada 

ILSI Risk Science Institute 

FDA/NCTR 

Univ. of California, Davis 

EPA/OPPT 

Bayer Corp. 

NIEHS 

Univ. of Texas Medical Ctr. 

PMRA/Canada 

EPA/ORD/NCEA 

General Mills 

ILSI Risk Science Institute 

Milestone Biomedical 

 Associates 

* Workshop Chair ** Workshop Co-Chair 

 

OBSERVERS 

 

Names Affiliations

Robert Amler 

Nancy Beck 

Richard Becker 

Terri Damstra 

Michael Firestone 

Steven Knott 

Ray McAllister 

LaRonda Morford 

Vanessa Vu 

ATSDR 

EPA/ORD/NCEA (AAAS Fellow) 

American Chemistry Council 

WHO/IPCS 

EPA/OCHP 

EPA/ORD/NCEA 

American Crop Protection Association 

Eli Lilly & Co. 

EPA/OPPTS/OSCP 
5 

WORKSHOP TO DEVELOP A FRAMEWORK 

FOR ASSESSING RISKS TO CHILDREN FROM EXPOSURE 

TO ENVIRONMENTAL AGENTS 

Toxicokinetics Toxicodynamics 

Risk 

Characterization 

Chair: 

Gary Ginsberg 

Chair: 

Elaine Faustman 

Chair: 

George Daston 

Rapporteur: 

James Bruckner 

Rapporteur: 

William Breslin 

Rapporteur: 

Penny Fenner-Crisp 

Breakout Group: 

• Hugh Barton 

• Matthew Bogdanffy 

• Harvey Clewell 

• Karen Davis-Bruno 

• Dale Hattis 

• Dan Krewski 

• Kannan Krishnan 

• Stephen Olin 

• Wayne Snodgrass 

• Bob Sonawane 

Breakout Group: 

• Richard Albertini 

• Adolfo Correa 

• John DeSesso 

• Rodney Dietert 

• Joyce Donohue 

• Jean Harry 

• Robert Kavlock 

• Gary Kimmel 

• Bruce Lanphear 

• Merle Paule 

• Kent Pinkerton 

• Jennifer Seed 

• Michael Shelby 

• Diana Somers 

• Tom Trautman 

• Isabel Walls 

Breakout Group: 

• John Adgate 

• Sherlita Amler 

• Bob Chapin 

• Vicki Dellarco 

• Brenda Eskenazi 

• Daniel Goldstein 

• Elaine Cohen Hubal 

• Carole Kimmel 

• Phil Landrigan 

• Melanie Marty 

• Bette Meek 

• Larry Sheets 

• Tracey Zoetis 

The breakout group chairs (Drs. Ginsberg, Faustman, and Daston) and their rapporteurs 

were the keys to the success of the workshop, as they moved their respective groups through 

their tasks. With their leadership and the active participation of the breakout group members, the 

framework drafted by the Workshop Planning Committee was discussed, further developed and 

modified, and adopted by the workshop participants; the focus questions were addressed; 
6 

concepts, insights, conclusions and recommendations developed in breakout groups were 

presented and discussed in plenary sessions and revised, as appropriate, by the breakout groups; 

and critical data needs for improving the assessment of children’s risks were identified. 

The Report

Following the workshop, the chairs and rapporteurs, in collaboration with their respective 

breakout groups, prepared the breakout group reports. Drafts were circulated by the chairs to the 

breakout group members for comment and input, and the overall workshop chair assembled and 

edited the report. The full report was then circulated to all workshop participants and observers 

for comment, comments were addressed by breakout group chairs and background paper authors, 

and this final workshop report was prepared. 

Chapters II and V of the report were drafted by the Risk Characterization breakout group. 

Chapter II presents the framework developed by the workshop participants, as informed by the 

extensive discussions in all three breakout groups and in plenary sessions during the workshop. 

Chapter V summarizes the Risk Characterization group’s discussions. Chapters III and IV are 

the products of the Toxicokinetics and Toxicodynamics breakout groups, respectively. 

Three of the four background papers for the workshop were further developed and 

modified by the authors after the workshop and are included in this report as Appendices 1-3. 

Although prepared specifically for the workshop and used and discussed extensively at the 

workshop, these papers have remained the work products of their respective authors. They are 

included as appendices to the report because they complement, support, and enhance the report 

and the framework for assessing children’s risks. 

This report with its appendices will be posted on the ILSI website at: 

http://www.ilsi.org/committees/rsi/childrensriskassessmentwkshpreport.pdf .
7 

II. A PROPOSED FRAMEWORK FOR ASSESSING RISKS TO CHILDREN FROM 

ENVIRONMENTAL EXPOSURES

Over the past decade there has been a dramatic increase in the recognition and concern for 

children as a potentially susceptible population for exposure to toxic environmental agents. The 

U.S. Federal government has developed and implemented many new policies and programs to 

assess and reduce environmental risks to children. As the body of knowledge on children’s 

health and risk factors expands rapidly, there is an increasing need for the systematic application 

of this knowledge on children in the risk assessment process. The evaluation of children’s health 

risks from environmental exposures should be structured, informed, and guided by the best 

available information on the many factors influencing children’s exposures (e.g., activity 

patterns, diet, physiology) and sensitivities (e.g., toxicokinetics and toxicodynamics). This kind 

of information needs to be organized and presented in a format that focuses on its application to 

risk assessment. 

The objective of the ILSI Risk Science Institute workshop was to develop a framework 

for assessing children’s health risks from exposure to environmental agents, focusing principally 

on hazard characterization (i.e., hazard identification and dose-response assessment) in the 

traditional risk assessment paradigm. Detailed consideration of issues related to exposure 

assessment are being addressed elsewhere (e.g., U.S. EPA Risk Assessment Forum 2000). In the 

workshop and in this report, the term ‘children’ was defined to include humans from conception 

through organ maturation (in adolescence). It was recognized that the effects of childhood 

exposures may present during childhood or later in life, and that the framework should 

incorporate this understanding. Use of a framework can reveal what already is known as well as 

what is not yet known, identifying critical data gaps and research needs as necessary. 

The Framework for Assessing Risks to Children from Exposure to Environmental 

Agents, as developed in this workshop, incorporates many of the principles and elements of other 

frameworks and risk assessment guidance developed by the U.S. Environmental Protection 

Agency over the past decade (e.g., U.S. EPA Risk Assessment Forum 1992; U.S. EPA 1997). It 

is responsive to, and consistent with, the directives articulated in the EPA Administrator’s 

children’s risk assessment policy guidance (U.S. EPA 1995a) and Executive Order 13045 (April, 

1997). 

The proposed Framework, presented in Figure II-1, is broadly analogous to frameworks 

previously established by U.S. EPA for use in its risk assessment/risk management process 

(RA/RM). It must be emphasized repeatedly that risk assessment is an iterative, not a linear, 

process. This concept is rigorously reinforced by the graphic inclusion of many arrows coursing 

back and forth, up and down, and around the Framework. 
8 

The proposed Framework retains the three major steps envisioned in the risk assessment 

phase of the RA/RM process - Problem Formulation, Analysis, and Risk Characterization - 

refining each to capture the areas of special emphasis for the life stages constituting “childhood” 

(i.e., conception through adolescence). As with these other frameworks, the proposed 

Framework for Assessing Risks to Children from Exposure to Environmental Agents visualizes 

its role within the larger context of an integrated process. This integrated process is illustrated in 

Figure II-2. The integrated process presumes that before any significant effort is made to 

conduct a risk assessment, a planning and scoping exercise has been carried out to assure an 

Figure II-1. Proposed Framework for Assessing Risks to Children from 

Exposure to Environmental Agents 

Children’s Health Risk Assessment

PROBLEM FORMULATION

Characterize 

LifestageSpecific 

Exposures

ANALYSIS 

RISK CHARACTERIZATION 

Develop 

Narrative 

Description 

Objective and Scope: 

The Dimensions -- 

Exposure ←→ Host Factors ←→ Biological Effects 

Evaluate Potential 

for LifestageSpecific 

Health Effects

(Hazard Characterization/ 

Dose Response Assessment)

• Characterize Risks 

 Across Lifestages 

• Perform Uncertainty 

and Variability 

Analyses 

Timing and 

Dosimetry

Develop Conceptual Model 

and Consider Assessment Options 

(Qualitative/Quantitative, 

Needs/Capabilities) 

Conduct Lifestagespecific Risk 

Characterization(s)

Manage Risk 
9 

Figure II-2. Stages in the Integrated Risk Assessment Process* 

*From U.S. EPA (1997) 

understanding of the purpose(s) for which the assessment is being done and what its scope 

should and/or can be, given available information. As the 1997 Cumulative Risk Assessment 

Guidance (U.S. EPA 1997) states: 

“The risk manager must explain clearly why the assessment is being performed and what 

questions need to be addressed. The manager must also advise the assessors, economists, 

engineers, and other contributing experts on the planning team of any interested party, 

affected party, or policy interests to be considered in the context of the risk issue. These 

factors may influence the risk management options, management goals, key participants, 

data sources, selection of assessment endpoints, or the schedule for developing the 

assessment. The manager and assessment planning team must discuss any regulatory 

basis for the risk assessment and what kind of information is required to satisfy such 

requirements.” 

The Guidance goes on to say: 

“Initially, the risk assessor and manager (and the planning team) need to evaluate and 

select the kind of risk information, exposure scenarios and assessment issues that need to 
10 

be covered. At this point, most EPA assessments focus on technical information related 

to the sources, effects, populations and the routes of exposure. Reasons to limit the 

technical scope of the assessment must be stated explicitly and must include details on 

limitations on resources, data, the impact of risk elements on the risk estimate, and 

methods available. In cases where an element of risk is likely to be important, but no 

valid data are available, the assessor must highlight this deficiency or use judgment or 

assumed values to approximate the missing data. Such judgments and approximations 

must be noted clearly and explained to the manager in the risk characterization.” 

Problem Formulation

The stage is now set to proceed to the first phase of risk assessment: Problem 

Formulation. Problem Formulation continues and expands the characterization of exposure and 

effects as well as the examination of the adequacy of scientific data and data needs, policy and 

public health issues, and specific factors to define the feasibility, scope, and objectives for the 

risk assessment. Problem Formulation provides an early identification of key factors to be 

considered to develop a scientifically sound risk assessment. It should include a statement of the 

key questions the risk assessment is seeking to answer, with a rationale for focusing the 

assessment on particular ages or toxic effects. 

The Workshop participants redefined, reduced in number and/or reassigned the 

components of the risk dimensions and elements presented in the 1997 Cumulative Risk 

Assessment Guidance. The Guidance presented six dimensions for inclusion in the development 

of an outline: Sources, Stressors, Pathways, Population, Endpoints and Time frames. The 

proposed Framework includes only three dimensions: Exposure, Host Factors and Biological 

Effects, and emphasizes, by inclusion of arrows pointing in both directions between the three, the 

reciprocal dependence of each dimension upon the others. The Framework merges Sources and 

Pathways into a single dimension: Exposure. Stressors are encompassed by Exposure and Host 

Factors, and Endpoints become Biological Effects. Because timing of exposure is considered as, 

if not more, critical than duration of exposure, Time Frames is dropped as a risk dimension, with 

the concept of criticality of Timing introduced in the second phase of risk assessment—Analysis. 

The risk dimension Population is dropped since the population (‘children’) has been 

predetermined. 

Problem Formulation is grounded in a clear articulation and understanding of several key 

elements: 

• Objective – Defining the purpose of the risk assessment. Why is it being done? 

How will it be used? What is the public health need? What is (are) the risk 

question(s) being asked? 

• Overall Scope –Determining the scope of the risk assessment, general or specific. 

Is the assessment to consider, for example, all developmental phases from in utero

through adolescence in the general population and all possible sources and routes of 

exposure (aggregate and cumulative), or is it confined to specific scenarios such as 

children living near a specific Superfund site potentially exposed via air, soil, and 

groundwater? 
11 

• Exposure Considerations: Preliminary Identification of Life Stages Potentially 

Affected – Identifying the life stages likely to be affected, given the properties of the 

environmental agent(s) and the defined scope of the assessment. Qualitatively 

characterize the sources, duration and pattern of exposures to women of childbearing age and/or children, as appropriate, including potential for dietary, drinking 

water, soil and air exposures, pharmaceutical use and other sources. Will all ages 

be at risk for exposure (e.g., from air toxicants, water contaminants), or are we only 

concerned with prenatal exposures, newborns (e.g., from nursing exposures) or 

older children (agents in diet or soil, or pediatric drugs)? This decision may be 

site-specific (e.g., only kids of a certain age are exposed, if it’s a day care center 

where 3-5 yr olds attend) or it may be less specific and thus dependent upon the 

exposure characteristics of several different life stages/age groups. 

• Biological Effects Considerations: Preliminary Identification of Toxic Effects and 

Kinetic and Dynamic Profiles – What do we know about the chemical being 

evaluated that may be important for considering age-specific risk? Does the 

chemical cause known organ-specific toxicity? What organs, and how are these 

organs potentially differentially susceptible during development? What should be 

the specific time periods of concern? Do we know of kinetic or dynamic 

considerations that might make the chemical differentially toxic during 

development? 

• Result of Problem Formulation – The outcome of this phase of risk assessment 

should be the accumulation of the information needed to develop a Conceptual 

Model, shown in the proposed Framework as a task linking the Problem 

Formulation phase to the Analysis phase. The Conceptual Model can be either a 

diagram/flow chart or a written description of the predicted key relationships 

between the Host Factors and the Biological Effects, informed by the initial 

identification of exposure scenarios, exposed life stage groups, and the chemical(s)’ 

identified characteristics and toxicological endpoints that may contribute to 

children’s risk. 

Analysis

The Analysis phase of risk assessment consists of an in-depth characterization of 

exposures and evaluation of the potential health effects (hazard characterization) on a life stagespecific basis. The hazard characterization should include both hazard identification and dose 

response assessment. The life stages for which the analysis is to be conducted will have been 

identified in the Conceptual Model. It is important to note that the proposed Framework 

incorporates the concepts of timing and dosimetry as unifying factors for both the exposure 

assessment and hazard assessment components of the analysis. 

• Characterization of Age-specific Exposures – Characterize exposures for all life 

stages of interest. Are quantitative exposure data available? Can exposures be 

estimated? Can life stages/age groups be ranked by exposure? Which life 

stages/age groups are most likely to be exposed more than adults? 
12 

• Evaluation of Potential for Life Stage-specific Health Effects – Consider data 

available for hazard identification and dose response assessments for specific life 

stages. [Access information on the capabilities of humans and animal models in the 

selected life stages to absorb, metabolize, and excrete xenobiotics and on the timing 

of developmental vulnerabilities in terms of organ and systems growth/maturation.] 

Evaluate the chemical’s toxicokinetic profile to understand major clearance 

pathways and mechanisms for activation and detoxification. Evaluate the 

chemical’s toxicodynamic effects ranging from cellular/molecular mechanisms of 

action to identifying the critical target organs and types of toxic effects. Consider 

how each potentially exposed age group might handle the chemical in terms of 

kinetic factors (which developmental life stages are likely to have greater internal 

dose (per unit of exposure) than adults based upon absorption, clearance, 

activation/detoxification?) and toxicodynamic/vulnerability factors (which are the 

critical periods of organ or systems development that can be affected by the 

chemical based upon its mechanism of action? What are the target organs and toxic 

effects of concern?). 

• Consider Need for Further Assessment – Determine the need for continuing with 

the assessment based upon the following three issues: 1) Unique Effects (are there 

any life stage/toxic effect combinations which represent novel toxicities that would 

not be seen in adult-only exposure scenarios?) 2) Quantitative Differences in Effect 

(are there any developmental life stages in which a greater effective exposure dose 

and/or greater adverse reaction is likely as compared to adults? If so, prioritize for 

further analysis.) 3) Lack of Adult Risk Assessment: If there is no pre-existing or 

relevant adult risk assessment, then continue forward with the children’s analysis. 

This may indicate that children’s exposure issues are unique in this scenario such 

that an adult assessment is unnecessary. In making this decision, one must remain 

mindful of legislative or other mandates which may direct what is or is not to be 

done. 

• Consider Assessment Options and their Feasibility and Appropriateness for 

Prioritized Life Stages – Based upon the public health needs, data available, and 

level of quantitative and qualitative assessments possible, an appropriate level of 

analysis for the risk characterization will be determined. Methodological options 

include: 1) Qualitative approaches in which the risk for one or more developmental 

life stage(s) is described as an additional uncertainty or source of inter-subject 

variability in the adult-based assessment. While such an approach may not drive 

the risk assessment, it could add weight to its overall conclusions. 2) Semiquantitative approaches in which uncertainty factors are modified as needed, or a 

specific children’s uncertainty/safety factor is considered. 3) Quantitative analysis 

of exposure differences in which standard exposure equations are modified to 

capture children’s behaviors and life stage exposure variables. 4) Quantitative 

analysis of kinetic differences, using life stage-specific physiologically-based 

toxicokinetic (PBTK) approaches, if feasible and appropriate; 5) Quantitative 

analysis of toxicodynamic differences based upon dose-response assessment of 

effects in developmental life stages relative to adults. 6) Quantitative approaches to 

describe inter-child variability within a given life stage group. This Analytic Phase 
13 

can determine that a single approach suffices, or that a combination of two or more 

of these options is needed, or that other approaches are required to address the 

children’s risk questions raised in Problem Formulation. 

Risk Characterization

The final phase of the risk assessment process is Risk Characterization. Risk 

characterization is the final integrative step of risk assessment for both ecological and human 

health assessment, for any life stage. EPA’s Risk Characterization Policy states that “risk 

characterization integrates information from the preceding components of the risk assessment 

and synthesizes an overall conclusion about risk that is complete, informative, and useful for 

decision makers” (U.S. EPA 1995b). 

Risk characterization employs the methods selected in the earlier phases to calculate or 

otherwise assess risks to life stage groups prioritized for detailed analysis. It results in a 

statement of the likelihood that children’s risks for specific effects will be higher or lower than 

adult risks, to what degree these groups may differ, and how this impacts overall risk conclusions 

regarding the scenarios analyzed. High quality risk characterizations also include analyses of 

uncertainty and variability and describe the impact(s) of these two factors on the integrity and 

accuracy of the assessment. 

• Conduct Life Stage-specific Risk Assessment(s) – This step is the natural 

culmination of the preceding phases and could range in level of complexity from a 

straightforward justification for use of a certain uncertainty factor to a highly 

refined quantitative analysis incorporating mode of action, dose-response analysis, 

and child-specific toxicokinetic and toxicodynamic data. 

• Characterize Risks for Children – Develop narrative description of the overall 

process of consideration of potential risks for children and the conclusions from the 

risk assessment, including characterization of variabilities and uncertainties and 

identification of critical assumptions, confidence in the database, data gaps, and 

research needs. This would also include a discussion of comparative risks for 

children versus adults. 
14 
15 

III. TOXICOKINETIC CONSIDERATIONS IN UNDERSTANDING CHILDREN'S 

HEALTH RISKS FROM EXPOSURE TO ENVIRONMENTAL AGENTS

Introduction

The charge to this breakout group was to evaluate how toxicokinetic (TK) analyses can 

be used to decrease the uncertainties and refine the analysis of children’s risks. In advance of the 

workshop, the Workshop Planning Committee had designed a draft framework for children’s risk 

assessments which involves a 3 step process: Problem Formulation, Analysis, and Risk 

Characterization. Our objective was to evaluate whether a children’s toxicokinetic analysis is 

best conducted within such a framework and to describe the types of data and considerations that 

are needed at each step in the process. While the workgroup’s focus was upon toxicokinetics, 

there was an awareness that this portion of the analysis would also rely upon exposure 

assessment and toxicodynamic (TD) considerations for input to risk characterization. The goal 

was not to provide detailed instructions or guidelines for how the TK analysis should be 

conducted. Instead, the focus was on creating a broad perspective which ensures that the 

relevant questions related to absorption, distribution, metabolism, and excretion of xenobiotics 

across the various developmental stages (in utero through adolescence) are addressed. The 

workgroup recognizes that every chemical and risk assessment will have unique considerations 

that will affect the way the assessment is conducted. The goal here is to develop a framework 

that is broad enough to be useful for a wide variety of children’s TK analyses. 

It is also important to note that risk assessments are often governed by the type of data 

available. It may be possible to approach children’s risk assessments by an extrapolation from 

animal data for in utero or juvenile life stages if such data are available. Alternatively (or 

perhaps, additionally), the database may be most amenable to an extrapolation from human 

adults to early life stages. While the latter type of extrapolation is the primary focus of this 

framework document, much of what is discussed and recommended is also relevant to direct 

extrapolation from animal toxicity studies. 

The workgroup’s overall approach to the task was to: (a) identify the key TK 

determinants that tend to govern internal dose in general; (b) summarize what is known 

regarding these determinants for the in utero period and for children; (c) describe how this 

information can be used to better refine internal dose estimates for these early life stages; (d) 

discuss how the TK approaches and methods fit into the overall children’s risk assessment draft 

framework; and (e) identify critical data needs. 

In working with the three-step framework process mentioned above, the workgroup 

found that it offers a logical, stepwise progression that can readily incorporate the questions, 

approaches and tools used in TK analyses. The workgroup also found that the TK analysis 

would be part of the Problem Formulation and Analysis phases, with its results feeding into Risk 

Characterization. Therefore, the following workgroup summary is organized around the 

activities that would occur in the first two steps of the three-step framework process. 
16 

Problem Formulation for the TK Analysis

The initial step of Problem Formulation is to state the purpose of the assessment, 

followed by definition of the problem/issue being addressed and identification of potential 

methods and datasets that may be applied. The workgroup began this process by defining the 

overall goal of TK assessment, a goal applicable to children’s risk assessments in general. 

Overall Goal

The broad goal of TK assessment is to improve the characterization of risks by 

developing more accurate internal dose estimates for specific life stages and between genders, 

species, dose routes and exposure patterns. TK assessment can remove some of the uncertainty 

in risk assessment by replacing inter-species scaling defaults with more precise estimates of 

internal dose. This allows the internal dose associated with toxicity in experimental animals to 

be related to the internal dose humans may experience via environmental exposures under 

various conditions of exposure. Further, TK assessments can take into account the range of 

inter-individual variability (where such distributions have been described) to show both the 

central tendency and upper bound estimates of internal dose. It is also important to recognize 

that TK assessment can help understand toxic mechanism by providing various estimates of 

internal dose whose relationships to adverse effect can be tested with regression or other 

correlational analyses. Those dose metrics (e.g., metabolites vs. parent compound) best 

correlated to toxicity are also most likely to be related to the toxic mechanism. These functions 

are equally relevant to risk assessments involving the in utero and post-natal periods. 

Moving beyond the general reasons for TK analysis, each assessment needs to consider 

the specific purpose and objectives of a TK analysis for the risk scenario being analyzed. This 

involves an understanding of scenario-specific factors that affect exposure and chemical-specific 

TD issues that affect target organ and key internal dose metrics (e.g., parent compound vs. 

metabolite). Problem Formulation also needs to take stock of the key TK factors that generally 

tend to govern internal dose, identify the types of chemical-specific and developmental data 

needed for a children’s TK analysis, and then develop a set of analytic options for conducting the 

analysis. Brief descriptions of these aspects of Problem Formulation follow. 

Exposure Inputs to TK Analysis 

One of the key inputs to TK analysis is exposure assessment. The manner in which 

children are exposed governs the route of uptake (g.i. absorption, dermal penetration, respiratory 

tract absorption), whether first pass effects will occur (e.g., hepatic extraction before systemic 

circulation) after oral exposure, and which contact sites will receive the largest applied dose. 

The manner of exposure will also affect the dose rate, whether sporadic (e.g., soil ingestion) or 

continuous (e.g., inhalation), and whether occurring as a single bolus (e.g., pica ingestion) or 

more evenly spread out (e.g., contaminants in drinking water or diet). The exposure point 

concentration in diet, water, air, or soil combined with the intensity and frequency of contact 

typically define the exposure dose in risk assessments. This dose and the pattern of exposure 

become the input for TK analysis, which goes on to consider the absorption, distribution, 

metabolism, and ultimate excretion of the dose. Thus, the exposure scenario has a large bearing 

on measures of internal dose: peak concentration in blood/tissues and the total area-under-the 
17 

concentration x time curve (AUC) dose. The peak level of exposure can be an important 

determinant of whether saturation of metabolism, binding, or clearance processes will occur. 

Peak concentrations may also be a critical determinant of toxicity for acute health effects or 

where chronic effects are dependent upon repeatedly attaining levels of toxicant that supercede 

cellular defenses. The AUC dose metric is typically most relevant for assessing cumulative risks 

and cancer. The chemical form of the toxicant in the exposure medium (e.g., dissolved in 

drinking water vs. sorbed to soil particles or air particulates) can affect the chemical’s 

bioavailability and so should be considered as part of exposure assessment and initial scoping of 

the TK analysis. 

Exposure assessment is a particularly critical input to children’s TK analysis given that, 

per body weight, children’s exposure patterns and rates differ considerably from adults. This not 

only applies to inhalation rate, and ingestion of food and water, but also to intake of 

contaminants at ground level (e.g., soil, house dust, carpet contaminants) with which children 

have a greater contact rate than adults. Of course, the potential for contaminant excretion in 

breast milk and subsequent nursing infant exposure can be a key pathway to consider. The 

behaviors and physiologic factors that lead to greater exposures during childhood were recently 

summarized (U.S. EPA Risk Assessment Forum 2000). The exposure analyst (not the kineticist) 

may attempt to prioritize age groups for more detailed analysis based upon which groups appear 

to have the greatest exposures. 

In summary, the following factors need to be extracted from exposure assessment for 

input to TK analysis: (a) ages at which exposure occurs and behaviors that lead to exposure; (b) 

route(s) of exposure; (c) chemical form of contaminant in exposure medium and estimates of 

bioavailability; (d) pattern of exposure (intensity – how much inhaled, ingested, contacted per 

event; frequency – how often; duration - over how many days, weeks, or years); (e) estimate of 

daily dose (external exposure dose as opposed to internal dose), which also considers body 

weight, breathing rate during rest and play activities, etc. Information describing these processes 

should be reviewed during Problem Formulation to help scope the overall assessment and ensure 

that exposure assessment provides sufficient information for the conduct of a TK analysis. This 

includes seeking data on the distribution of key exposure data across children’s age groups (e.g., 

soil ingestion data, dietary, drinking water and inhalation distributions) to prepare for a TK 

analysis that can represent the range of exposures and internal doses that are possible. 

TD Factors to Consider in TK Analysis

Toxicodynamics and mechanism of toxicant action have a direct bearing on how the TK 

analysis will be framed. The chemical’s toxicity profile often provides evidence of target organ 

specificity of action, with some chemicals (typically highly reactive) most toxic at initial contact 

sites, while others exert their greatest toxicity at systemic sites. To the extent possible, TK 

analyses attempt to define the dose at the target site as well as in the central (blood) 

compartment. Thus, the toxicity profile will help determine which compartments to include in 

the TK analysis. This decision is also affected by mechanism of action considerations which 

may show that metabolic activation at one site leads to toxicity in another. In this case, both the 

activating tissue and the target tissue would be included in the analysis. The mechanism of 

action also can determine which dose metric(s) need to be evaluated, both in terms of parent 

compound vs. metabolites and in terms of peak vs. AUC doses. This also becomes important in 
18 

choosing datasets for PBTK model calibration and validation. These datasets should reflect 

internal or excreted concentrations of the active toxicant or a surrogate (e.g., urinary metabolite) 

that represents throughput through the activation step. 

In the absence of data to the contrary, the starting assumption in TK analysis for children 

would be that the target sites and dose metrics relevant to toxicity in adults (or in test animals) 

are also relevant to children. There may be cases where additional target sites and dose metrics 

may be pertinent to children, as identified in toxicity and mechanistic studies in utero, in juvenile 

animals or in children. The TD information thus focuses the assessment on a particular target 

site(s) (e.g., contact site vs. internal organs, vs. fetus) and dose metric(s) (chemical form and 

method of dose integration). It also provides information about critical metabolic pathways of 

activation, detoxification, and clearance. During the Analysis phase, this information needs to be 

combined with knowledge about children’s functional capacity in these critical pathways to 

derive internal dose estimates specific to children. 

Generally Important TK Determinants

Regardless of whether animals or humans (adults or children) are being modeled, certain 

TK inputs are likely to be more influential than others in determining internal dose. Recognition 

of this early in the process ensures that the analyst will prioritize these inputs for special attention 

and thus decrease uncertainties (to the extent possible) in the areas that are most likely to drive 

the assessment. However, the goal of Problem Formulation is not to eliminate any TK factors 

from consideration; for certain chemicals or age groups additional factors may take on a more 

prominent role. The importance of such additional factors may only come to light after an initial 

analysis, and may be part of an iterative process. 

The workgroup considered the following areas to be of key importance in any TK 

assessment. The ways in which these inputs can be affected by developmental stage are 

described under Analysis. 

Absorption Factors: Absorption involves uptake of chemical from the point of contact – 

g.i. tract for oral exposure, upper respiratory tract and lungs for inhalation exposure, and skin for 

dermal exposure. While chemical absorption is one of the key determinants of internal 

(systemic) dose, it is also recognized that the portal of entry may be a critical target site. In those 

cases, another important factor is the amount of chemical per unit surface area of contact site per 

unit time. Children’s absorptive capacities may differ from adults in certain cases. Other issues, 

such as contact site doses to the respiratory tract, may be especially important in children given 

the high rates of childhood asthma. 

Distribution Factors: These factors describe the structure of a physiological model, 

involving compartments where absorbed chemical may reside for some period of time. It is 

essential to include tissues involved in chemical metabolism and elimination, tissues which 

represent potential storage sites (e.g., fat, muscle) and any additional tissues which are target 

sites for a chemical’s toxicity. Even if not constructing a physiologic model, the analyst must 

consider how the developmental life stage influences the size of and blood flow to these 

compartments. Of particular importance is the rate of blood flow to the liver, the primary site of 

metabolism for most xenobiotics. Some chemicals are metabolized rapidly by the liver (high 
19 

hepatic extraction), and so clearance by this organ can be limited by the delivery of chemical 

(hepatic blood flow) rather than maximal metabolic rate (Kedderis 1997). Another key 

compartment is the embryo/fetus if exposure is during gestation. In this case, consideration 

needs to be given to both maternal (including placenta) and embryo/fetal TK factors in 

understanding the potential for in utero exposure. Permeability factors may also be of 

importance to children’s assessments, especially with regards to the ability of chemicals to cross 

the placenta or the blood-brain barrier. Finally, the availability of chemical binding sites in 

blood (plasma proteins) may be quite different early in life and thus affect the availability of a 

chemical for transport across the placenta, uptake into tissues or excretion. 

Metabolism Factors: For many xenobiotics, whether drugs or environmental chemicals, 

the ability of liver enzymes to catalyze Phase I and Phase II metabolic reactions governs the rate 

of removal of parent compound and generation of metabolites. Given that some metabolites can 

be more toxic than the parent compound, hepatic metabolism has implications for both 

toxicokinetics and toxicodynamics. While the liver typically has the greatest metabolizing 

capacity, extra-hepatic metabolism can be a key factor in determining where a chemical’s 

toxicity is targeted. This is relevant to the case of in utero exposures where a number of Phase I 

and Phase II enzymes exist in the fetus although most of these systems are immature. This 

immaturity carries over into early post-natal life and this has been shown to affect the clearance 

of drugs in infants. 

Elimination Factors: A key clearance pathway for certain water-soluble xenobiotics is 

renal elimination, either as unchanged parent compound or as metabolites. Large molecular 

weight compounds and their conjugated metabolites tend to be excreted into bile, wherein 

hepatobiliary cycling can extend the half-life and perhaps action of the agent. Finally, highly 

volatile compounds may have exhalation as a primary excretion pathway. While fetal excretion 

of xenobiotics is mainly via return flow to the maternal circulation across the placenta, the 

newborn must rely upon immature renal and hepatobiliary systems for excretion. 

Hopefully it is clear from the above discussion that while certain TK factors are often 

important in determining internal dose, the particular pathways and factors involved for any one 

chemical will be specific to that chemical’s ADME properties. Therefore, a children’s TK 

analysis is highly dependent upon and must be tailored to the properties of the chemical being 

analyzed. 

Identification of Key Chemical-specific and Age-specific TK Datasets

During Problem Formulation the data needed for the TK analysis is acquired and 

organized according to type of information being provided, and an initial review is conducted to 

evaluate the type of analysis that may be possible (see analytic options below). The data needs 

and sources can be organized into the following areas: 

• Exposure Inputs: from the exposure assessment portion of the risk assessment 

• Chemical-specific TD Data: including information on toxic mechanism and key 

target organs. This information can be sought via computerized searches to identify 

primary and secondary literature sources, together with review of chemical 
20 

monographs such as the ATSDR Toxicological Profiles, IARC monographs, and 

documents available from U.S. EPA (e.g., toxicologic reviews in support of IRIS). 

• Chemical-specific TK Data: including data describing mechanisms of uptake by 

various routes of exposure, chemical properties that affect uptake and distribution 

(e.g., oil and water solubility, volatility, sorption to environmental media, partition 

coefficients), distribution information such as volume of distribution and data on 

localization of chemical in different body compartments/fluids, data describing sites 

and pathways of metabolism, major metabolites found that may be important for 

biomonitoring, any other metabolites that may be related to toxic mechanisms, and 

data on rate and pathways of excretion. Datasets describing parent compound and 

metabolite concentrations over time in body compartments should be specifically 

noted as these may be important in calibrating and validating PBTK models. Any 

data in children or juvenile animals, or that describe in utero exposure, should also 

be specifically noted as these data may be critical to understanding how internal 

dose during development can differ from that in adults. Literature searches to 

retrieve primary data sources are needed. If PBTK models already exist for the 

chemical, publications describing these models are usually an excellent source of 

information regarding analytic approaches and underlying data sources. The 

availability of a validated model for the chemical in animals, adult humans or 

during pregnancy will greatly expedite the ensuing TK analysis for children and the 

in utero period. 

• Child-specific TK Data: The ideal is to have chemical-specific TK data for the 

relevant age groups, but this will rarely be the case for environmental toxicants. 

This is due to the lack of sequential monitoring data after defined exposure events 

in children. Therefore, data for surrogate chemicals in children, typically 

therapeutic drugs, can help the assessor understand how children’s distribution, 

metabolism, and excretion can differ from adults. Based upon the clearance 

mechanisms of the drugs tested in children, it is possible to make generalizations 

about how active a given metabolism or excretory pathway is in children. 

Combining this information with in vitro studies of metabolic enzyme function at 

various life stages and with developmental physiology information that describes 

renal function, blood flows, compartment size, water and lipid contents, and 

permeability of barriers, one can develop a database of children’s TK development. 

Resources and citations useful in developing such a database are presented in the 

supporting background paper for this workgroup (Appendix 3). 

Listing of Analytic Options

The types of TK analyses possible for a children’s risk assessment can range from 

qualitative to semi-quantitative to quantitative. The least data-intensive option is a qualitative 

discussion of whether internal dose in children (per unit of external dose) may be different than 

in adults, in which direction these differences may run (e.g., more internal exposure to parent 

compound but less to metabolites in neonates; or less protein binding so greater potential for 

high peak exposures but shorter half-life in neonates). The qualitatitive analysis might discuss 

the risk implications of these child-specific TK factors relative to what is normally anticipated in 
21 

non-cancer risk assessments for inter-subject TK variability (one half log or 3.16 fold). The 

analysis may become semi-quantitative if it attempts to derive child-specific TK uncertainty or 

adjustment factors based upon the likely size of differences between adults and children in terms 

of internal dose. Such assessments might take on bounding assumptions in which only one or a 

few factors are adjusted for children; in this case, a simplified calculation rather than a complete 

model would attempt to relate internal dose across age groups. For example, if it is known that 

glucuronidation is a key elimination pathway for a toxic metabolite, the deficient glucuronidation 

capacity and higher blood concentrations seen with surrogate chemicals (e.g., lorazepam, 

morphine) in newborns might form the basis of describing how much more internal exposure 

there could be to the active metabolite in neonates. Quantitative analysis would rely upon 

PBTK models to predict internal doses of environmental toxicant (or key metabolites) in children 

of specific age groups. Such models need to be developed and validated in children to the extent 

possible. In this case, many parameters would need to be described for the age groups being 

evaluated so that the physiology and function of these groups can be adequately represented. 

The distributions of parameter values across a given age group need to also be described so that a 

range of values or the full distribution can be loaded into the model rather than a single 

parameter point estimate. Use of Monte Carlo simulation software can then enable the output of 

the distribution of internal doses, allowing the assessor to focus on a particular level of protection 

(e.g., the median or the 95th percentile) and to evaluate the degree of TK variability in a more 

refined way. 

Problem Formulation should list these options and the types of data needed for each. 

Unless there is an a priori reason to not conduct a full PBTK assessment (e.g., exposure pathway 

unlikely to be widespread or to be a significant risk driver; very little chemical-specific TK data 

in adult humans or animals), Problem Formulation should retrieve a broad TK database that 

could potentially be used in children’s PBTK modeling. In this way, the assessor can evaluate 

what types of analyses the data will support. The gathered data will be valuable regardless of 

which analytic option is ultimately chosen. 

Analysis of TK Data

This phase of the TK assessment reviews the chemical-specific TD and TK information 

described above, together with the child/age group-specific TK information. The major 

questions raised in this phase are: 1) How does TK affect the chemical’s toxic mechanism via 

activation or detoxification pathways? 2) What are the key dose metrics and target organs for the 

chemical being analyzed? 3) What are the major in utero or child-specific TK factors that may 

alter chemical fate? 4) Based upon internal dose considerations, which age groups should be 

prioritized for more detailed analysis? 5) Which TK analytical methods are best suited to 

evaluating children’s internal dose? 6) Once the analysis is conducted, what do the results tell us 

about how internal exposure can vary (per unit external dose) across developmental stages and 

between children and adults? The stepwise process outlined below is intended to address these 

questions. 

Analysis of Chemical-specific Data

The first task is to combine TD and TK information from animal models and, to the 

extent available, from adult humans; this type of data will usually be lacking in children or 
22 

juvenile animals. The combination of TD and TK data is used to understand the TK 

determinants of toxicity: whether metabolism represents detoxification or activation or both (i.e., 

some metabolic pathways are detoxifying while others create active products). This involves 

identification of the various enzymes that may be important in chemical activation and removal 

(e.g., Phase I enzymes - specific cytochrome P450s (CYPs), peroxidases, dehydrogenases; Phase 

II systems - glucuronidation, sulfation, glutathione (GSH) conjugation; Other enzymes - epoxide 

hydrolase, serum esterases). Since an enzyme can be part of an activation pathway for one 

chemical and yet be detoxifying for another (e.g., glutathione S-transferases, epoxide hydrolase, 

various CYPs), determining the role of a given enzyme is highly chemical-specific. Therefore, 

the toxicological significance of the presence or absence of an enzyme at a particular 

developmental stage is also chemical-specific. 

Once the TK determinants of toxicity and detoxification are understood, the next area to 

review is target sites, specifically with respect to whether the TK determinants of toxicity differ 

at different sites. For example, perchloroethylene and trichloroethylene are believed to cause 

toxicity and cancer via oxidative metabolism in the liver, but to cause renal toxicity and cancer 

via pathways involving GSH conjugates. Therefore, the key activation and detoxification steps 

in each target organ should be evaluated. Toxic metabolites should be identified so that internal 

dose metrics can be selected for each target organ. Since target sites can be different in the fetus 

or child than in adults, toxicity information for early life stages is particularly important in 

understanding the variety of toxic endpoints and target sites that should be considered. 

Other TK determinants of chemical fate should be evaluated so that the mechanisms and 

factors involved in chemical absorption (from scenario-relevant portals of entry), distribution 

(e.g., serum binding, partitioning), and excretion (e.g., renal, biliary, exhalation) are understood. 

This segment of the Analysis phase should provide the following chemical-specific 

information: identification of key metabolic pathways for activation and detoxification; target 

organs for which dosimetry information would be needed; key dose metrics which should be 

modeled or evaluated in other ways; TK factors that control chemical distribution and excretion. 

Analysis of Age Group-specific TK Factors

Numerous TK factors differ across life stages, particularly because of rapidly changing 

physiology and the immaturity of various systems in utero and in early life. However, the 

importance of any single factor in altering internal dosimetry depends upon the TK mechanisms 

involved in the chemical’s ADME, the life stage where exposure occurs, and the interplay of 

other factors which may tend to accentuate or offset the dosimetry change. For example, 

immature metabolism in early life via Phase I enzymes may have less influence on internal dose 

and long-term retention of highly lipid soluble organochlorines whose TK is most influenced by 

partitioning into lipids. An example of offsetting factors would be where immaturity of 

metabolism causes both the activation and detoxification steps to be slow relative to older age 

groups. In this case, formation of toxic metabolites may be low, but their removal may be 

sufficiently impeded (perhaps also by slow renal and biliary clearance) as to create similar or 

higher levels of ultimate toxicant at key target sites. 
23 

These considerations illustrate the need for this phase of the analysis to take into account 

the various ADME differences that are possible in early life. The following general discussion 

provides a framework into which chemical-specific information can be added to focus the TK 

analysis for children. 

Absorption Factors 

The exposure scenario (age groups involved, contaminated media, behaviors leading to 

exposure) will determine the route of uptake: oral, dermal, inhalation or a combination of 

several. Also, as stated above, the scenario will dictate the frequency and intensity of exposure 

which can differ substantially across age groups. Aside from differences in exposure, the 

amount of uptake can differ because the percent absorption from the g.i. tract, respiratory tract, 

and skin may be different in newborns and infants relative to older children and adults. 

Oral Absorption 

Greater oral absorption of lead (Pb) has been documented in infants relative to adults, 

with this attributed to greater pinocytic activity of intestinal epithelium prior to closure. This 

non-selective uptake mechanism may also increase the absorption of other metals and organic 

compounds, as suggested by data in juvenile rats (Kostial et al, 1978; NRC 1993). In animals, 

closure occurs around the time of weaning, although the time course for this in humans has not 

been defined. 

A variety of other factors may influence oral absorption in different age groups including: 

nutrient deficiencies (e.g., low iron or calcium intake increases the absorption of lead; low 

stomach pH up to 2 years of age; blood flow and surface area of g.i. tract absorptive regions; 

presence of milk in stomach) (NRC 1993). 

The possibility of higher g.i. uptake of ingested chemicals early in life should be 

evaluated within the context of the chemical’s behavior in the gut. If it is generally well 

absorbed in rodents and adult humans by the oral route (e.g., small organic molecules), then any 

increase in absorption during early life stages may not create a large difference in uptake. 

However, for chemicals which are poorly absorbed (e.g., a variety of metals), increased uptake in 

children may be an important factor in the exposure and risk assessment. For such chemicals, 

the mode of absorption should be investigated to determine whether these mechanisms may be 

enhanced in early life. Further, g.i. uptake data for these and analogous chemicals in children or 

juvenile animals should be sought. These efforts may allow age group-specific adjustment 

factors to be applied for g.i. absorption. 

Respiratory Dosimetry and Absorption 

Particles and Aerosols. Inhalation exposure during early life may lead to a different 

degree of exposure than at older ages because of the greater respiratory volume per surface area 

in young children. On average, this can lead to an approximately 2-fold increase in respiratory 

tract exposure (per unit surface area) of young as compared to adults (U.S. EPA 1999, 2000; 

NRC 1993). Preliminary modeling efforts for young children suggest that this differential can be 

larger when considering local deposition (Martonen et al, 2000). This exposure dose differential 
24 

for particles and aerosols may be of particular consequence to young children who are sensitive 

to respiratory irritants and allergens due to asthma or other conditions. Further, in asthma, the 

changes in breathing pattern and respiratory volume/resistance may create local exposure 

patterns that are different from those in healthy children or adults. Therefore, it is important to 

analyze respiratory deposition of particles and aerosols in children, both healthy and asthmatic. 

This can be aided by the development of regional deposited dose ratio (RDDR) models which 

take into account respiratory physiology at different life stages as well as a distribution of 

particle sizes. These models would be similar to those currently used in extrapolating from 

animal-to-human data for RfC development (U.S. EPA 1994; see also U.S. EPA Risk 

Assessment Forum 2002). Additional consideration should be given to whether there are life 

stages where mucociliary clearance and macrophage clearance of particles is substantially 

different than in adults. Until these models are available, the greater inhalation volume per 

respiratory surface area in young children should be considered for input into the analysis. 

Reactive Gases. RfC methodology uses the regional gas dose ratio (RGDR) to 

extrapolate from extra-thoracic dose in animals (where the toxicity data are obtained) to adult 

humans for gases with respiratory effects. This adjustment factor is based upon the difference in 

respiratory volume per surface area in the upper respiratory tract across species. This 

methodology should be extended to children of various ages by inputing upper respiratory tract 

surface area and inhalation volumes for these ages. Until that is available, the overall ~ 2 fold 

differential in inhalation volume per respiratory surface area described above may be assumed as 

a first approximation for reactive gases. 

Non-Reactive Gases. Uptake of this class of inhaled chemicals is currently modeled by 

estimating the difference in pulmonary absorption (net systemic uptake) between test animals 

and adult humans. Uptake across alveoli is driven by the blood:air partition coefficient, 

respiratory rate, cardiac output and systemic extraction (e.g., partitioning into lipid, removal 

from circulation via metabolism or excretion). However, for chemicals which rapidly reach 

steady state (e.g., perchloroethylene), the major (but not only) determinant of net uptake is the 

partition coefficient. In that case, increased respiration and cardiac output also leads to increased 

exhalation. This may be especially relevant for neonates and infants whose metabolic and renal 

capacities are immature and quite limited. The blood:air coefficient is determined by the 

presence of carriers (e.g., hemoglobin) or lipid in blood, which can vary across species and age 

groups. Since partition coefficients can be determined in vitro from blood samples, a database of 

children’s partition coefficients (across age and chemicals) may be possible to develop. Also, it 

may be possible to model uptake of these gases in children based upon datasets in adult animals 

and humans, with extrapolation to younger age groups based upon known differences in blood 

composition.

 In summary, in evaluating systemic absorption of non-reactive gases in children, 

estimates of children’s blood:air partition coefficients are important, especially at early life 

stages where non-exhalation clearance pathways are likely to be deficient. However, in older 

age groups where these systems are more functional, the blood:air coefficient together with the 

other physiologic and metabolic parameters need to be incorporated in models to understand how 

uptake may vary across age groups. For short-term uptake, simplified adjustments to absorbed 

dose based upon inhalation volume per unit body weight may provide a frame of reference in 

these age groups where modeling is not practical. While this simplified approach would not 
25 

predict internal concentrations, it would show how uptake can be a function of respiratory 

volume and body weight when other factors (metabolic and renal clearance) do not substantially 

differ between children and adults. 

Dermal Absorption 

The assessment should consider how child-specific factors may tend to increase 

absorption across the skin. A major consideration is whether dermal contact with the 

contaminated medium is greater in children of certain ages than adults because of behavioral 

factors (e.g., crawling; play activities leading to a high percentage of the body surface area 

becoming covered with soil) or physiological factors (higher skin surface area per body weight in 

young children) (NRC 1993). A second consideration is whether children’s skin is more 

permeable to chemicals than the skin used to derive uptake factors, typically adult animal or 

human skin. Since full term newborns have a well-developed stratum corneum, it is generally 

believed that the age of the child has little bearing on dermal permeability (U.S. EPA 1992). 

This has been shown in limited in vitro testing using skin from neonates and adults (U.S. EPA 

1992; Wester et. al. 1985). However, the skin of premature neonates can be substantially more 

permeable than that of full term neonates due to immaturity of the stratum corneum (U.S. EPA 

1992; Barker et al, 1987). This potential for increased dermal uptake in premature neonates may 

be an important factor in scenarios where these neonates are dermally exposed to contaminants 

present in bath water or chemicals in hygienic or diaper rash products. Dermal penetrability may 

also be enhanced when skin is damaged or highly hydrated (U.S. EPA 1992). These conditions 

are more prevalent in infants whose skin under a diaper is more likely to be excessively hydrated 

and possibly compromised by irritation and rash. Exposure scenarios involving this portion of 

an infant’s body (e.g., bathing in bath water containing contaminants) should consider this 

potential for increased penetration of chemicals. 

A final consideration is whether chemical sorption on the exposure matrix (e.g., soil) 

significantly impedes dermal penetration. While this factor affects adults as well as children, in 

certain cases binding to the exposure matrix may substantially decrease dermal uptake and thus 

reduce the importance of any child/adult differences in this route of exposure. 

Distribution Factors 

Distribution into systemic compartments depends upon a number of chemical-specific 

factors: lipid and water solubility as these determine partition coefficients, chemical size, ability 

to be carried by transporters across membranes, and affinity for plasma or tissue proteins. Age 

group-specific factors that affect chemical disposition include lipid and water content of body 

(generally more water and less lipid in neonates), quantity of plasma protein binding sites (fewer 

in neonates, and those that do exist may be less available for xenobiotic binding than at older 

ages), and less intact blood brain barrier in neonates. These factors tend to increase the volume 

of distribution (Vd) for many chemicals in early life, which is borne out by a recent compilation 

of PK data in children (Appendix 3; Ginsberg et al, 2002). Higher Vd can lead to lower blood 

concentrations and longer chemical half-lives as the chemical is less available to the central 

compartment for transfer to sites of metabolism (e.g., liver) and elimination (kidney, lung, bile). 

However, the interplay of distributional, metabolic, and elimination factors can be complex, thus 

defying use of a simple adjustment factor to compensate for differences in chemical distribution 
26 

in children. PBTK modeling for children holds the greatest potential to combine the absorption 

factors described above with distribution, metabolism, and elimination information to enable 

predictions of blood and tissue concentration estimates over time. Short of this approach, there 

are some simplified generalizations regarding distribution and age groups that may be useful in 

the assessment: greater permeability of the blood-brain barrier in early life can produce higher 

chemical concentrations in the CNS of children; lower lipid content in early life would cause less 

storage and retention of lipophilic chemicals; less plasma protein binding in early life might 

accenuate chemical toxicity due to greater percentage of free chemical in the circulation (as 

suggested for lidocaine, cisplatin and other drugs: Zemlickis et al, 1994; Kakiuchi, et al, 1999). 

The importance of such distributional differences across age groups may be described 

qualitatively for the chemical under assessment. However, PBTK modeling would be needed to 

quantitatively incorporate such factors. 

Two additional distributional phenomena critical to early life exposures are placental 

transport of chemicals from mother to fetus, and partitioning of chemicals from maternal blood 

into breast milk. The existing database suggests that most chemicals can cross the placenta, 

although the rates can vary depending upon molecular size, lipophilicity, and serum protein 

binding (Appendix 3; Ginsberg et al, 2002). This suggests that toxicant exposure in the mother 

will generally lead to toxicant exposure of the fetus, although maternal metabolism/clearance 

factors may lead to lower concentrations in the fetus as compared to mother. Thus, fetal 

exposure needs to be considered where maternal exposure occurs. Fortunately, there are a 

number of PBTK models to describe pregnancy and fetal exposure (Clewell et al,, 1999; 

Krishnan and Andersen 1998). Similarly, the partitioning of chemicals into breast milk has been 

evaluated for various types of chemicals and can be described via modeling approaches in cases 

where empirical data are missing (Byczkowski et al, 1994). 

Metabolism Factors 

The companion paper (Appendix 3) summarizes a variety of in vitro data (enzyme levels 

and function) and in vivo data (therapeutic drug PK studies) that show that young children, 

particularly in the first 2 months of life, are immature with respect to metabolic and renal 

clearance. This appears to be a consistent finding across a number of metabolic pathways 

including a variety of CYPs (including CYP1A2 and 2E1, two that are particularly important in 

toxicant activation), glucuronidation, serum esterases, epoxide hydrolase, and perhaps also 

GSTs. There are fetal forms of some enzymes (e.g., CYP 3A7, GST-pi), but these appear to be 

functionally distinct from the adult enzymes and so don’t compensate for their lack in early life. 

Renal function is also immature in the first weeks to months of life, leading to prolonged halflife of a variety of renally cleared drugs. This condition changes by 6 months of age such that 

some enzymatic functions (most notably CYP 1A2) appear to become somewhat more active 

than in adults (Appendix 3; Renwick et al, 2000; Dorne et al, 2001; Ginsberg et al, 2002). This 

may be a function of the higher liver mass per body weight (and assumed higher hepatic blood 

flow per body weight) that exists in children as compared to adults. This becomes normalized 

when scaling across ages on a surface area rather than body weight basis (Gibbs et al, 1997), 

which suggests that beyond 6 months of age a surface area correction may be a good first 

approximation of how metabolism changes with age once a system has reached functional 

maturity (approx. 6 months to 1 year for many systems). 
27 

The significance of these changes in metabolism with postnatal development depends 

upon whether chemical metabolism leads to activation or detoxification, which pathways are 

involved in activation and detoxification, and whether blood flow limitations to the metabolizing 

tissue (e.g., the liver) prevent the full expression of changes in enzymatic function (Kedderis 

1997). The importance of changes in metabolic function also depends upon whether other 

clearance pathways (e.g., renal, biliary, exhalation) can compensate for slow metabolism in early 

life. 

The interplay of distributional, metabolic, and renal factors is best understood and 

quantitatively evaluated via PBTK models. However, if these models do not exist and cannot be 

developed within the scope of the children’s risk assessment being performed, then some 

simplifying first approximations of chemical clearance and metabolic activation may be possible 

from the existing literature. In vitro and in vivo datasets that are available provide quantitative 

data on metabolic processing in children relative to adults for a wide variety of pathways and 

drugs (Appendix 3; Renwick et al, 2000; Ginsberg et al,, 2002; Hines and McCarver 2002; 

McCarver and Hines 2002). At a minimum, the slower clearance of many chemicals very early 

in life should be qualitatively discussed in terms of internal dosimetry and risk implications for 

children (e.g., more of parent compound but less of metabolites present in tissues; however, 

possibly also slower removal of metabolites). It should be noted that neonate/adult differences in 

half-life can be large relative to the default assumptions for inter-individual PK variability (3.16 

factor), especially when considering the full range of results from individual neonates (Ginsberg 

et al, 2002). This may warrant semi-quantitative approaches such as adjusting uncertainty 

factors to incorporate PK into risk assessments for newborns. However, this will take careful 

consideration of chemical mechanism of action (activation and detoxification pathways) and 

potential blood flow limitations. Ultimately, a PBTK model would provide the best assurance 

that all relevant PK factors have been accounted for when estimating internal doses. 

In utero, placental, fetal, and maternal factors can all play a role in chemical metabolism. 

A variety of placental enzymes exist and can be induced by maternal exposure to cigarette smoke 

and other types of drugs and toxicants (Juchau 1980). Metabolism by the fetus itself can in some 

cases outweigh the importance of maternal metabolism in terms of fetal toxicant exposure. This 

has been seen with fetal mice, whose risk of lung tumors from maternal exposure to 3-

methylcholanthrene was greatest when the fetal mice had induced levels of CYP1A1 and the 

mother was non-inducible (Anderson et al, 1989; Miller et al, 1990). Lower tumor incidence 

was seen in offspring when the mothers were inducible, demonstrating the protective role 

maternal metabolism can have even when that metabolism leads to more toxic moieties. Another 

issue is that fetal metabolism may create metabolites which are less able than parent compound 

to cross the placenta back to the maternal circulation (e.g., zidovudine – Garland et al, 1998; 

hormonal agents - Slikker et al, 1982). This could lead to an accumulation of metabolites in fetal 

tissues. However, the fetus may be subject to lower exposure to reactive metabolites due to the 

lack of activating metabolic pathways, as is recognized for CYP2E1 in the fetus (Cresteil 1998). 

The time course for the development of in utero metabolic capabilites may point out 

which gestational periods are most significant in terms of TK susceptibility via placental or fetal 

activation. These placental and fetal metabolism factors are best incorporated into a modeling 

framework to be useful in risk calculations. 
28 

Elimination Factors 

As mentioned above, renal elimination of drugs is generally reduced in newborns, which 

is consistent with developmental studies on the maturation of renal glomerular filtration and 

tubular secretory functions. Biliary excretion can also be diminished in newborns because 

glucuronidation capability and other hepatic functions are immature in the first months of life. 

However, enterohepatic circulation is functional in early life, which can lead to substantial 

reabsorption of chemicals excreted in bile (Suchy 1987). Exhalation of volatile chemicals may 

be enhanced in the young because of high ventilation rates per body weight and the fact that 

other clearance pathways are immature. While PBTK modeling of these elimination pathways in 

conjuction with other TK inputs specific to children is the ideal, one can assume that elimination 

via renal and biliary systems will be slower in newborns than adults. This can lead to potentially 

greater levels of parent compound or metabolites in newborns, depending upon their primary 

route of clearance. The children’s risk assessment should consider the implications of deficits in 

chemical elimination during this early life stage. 

Selection of Age Groups for Special Focus

Review of chemical-specific and age group-specific data may reveal a specific age group 

that is of particular concern from a TK perspective. These would be age groups where the TK 

mechanisms central to the chemical’s absorption, distribution, activation, detoxification, and 

elimination are expected to be most different from adults. In general, neonates (both premature 

and full-term) through the first several months of life are most different from older age groups 

and adults with respect to certain absorptive properties (e.g., g.i. tract closure), distributional 

properties (low protein binding, high permeability of blood brain-barrier), altered body 

composition (in terms of water and lipid content), and metabolism and elimination function 

(immaturity of many systems). Therefore, this age period should be carefully considered for the 

possibility of substantiative changes in internal dose relative to adults. Somewhat older age 

groups (6 months to 2 years) are also important from a TK perspective in that these groups 

generally have greater hepatic extraction and shorter drug half-lives due to larger liver size per 

body weight (Appendix 3; Gibbs et al, 1997; Ginsberg et al, 2002). In utero may also be a 

critical life stage from a TK perspective since most chemicals cross the placenta and placental or 

in utero enzymes may be sites of chemical metabolic activation. The other portions of the risk 

assessment (exposure assessment; toxicity assessment) may identify key life stages that need to 

be fully analyzed, regardless of whether unique TK considerations exist in those stages. 

Changes in TK function may be defined within specific age groupings as a way to 

compile and organize the data. It appears that the rapid maturational changes that occur within 

the first weeks and months of life warrant subdividing that period into several age groups. 

Beyond that age, broader age groupings are possible given that changes in metabolism and other 

factors may be possible to scale allometrically based upon body surface area. Alteratively, for 

these age groups a continuous physiological model based upon a set of equations that describe 

physiological development of organ systems and blood flows may be suitable (Pelekis et al, 

2001). The risk assessment may dictate that certain age groups be the prime focus on the basis 

of exposure issues or toxicodynamic issues. The TK portion of the analysis can accommodate 

this focus, providing some idea of how internal dose may be affected by the stage of 

development during these critical exposure or susceptibility periods. However, the risk 
29 

assessment should also take on as a focus those age groups which appear to have the most unique 

TK features relative to adults (particularly the first weeks and months of life as described above). 

Selection of Analytical Approach

The possible approaches for assessing TK factors as part of a children’s risk assessment 

are: PBTK modeling, semi-quantitative assessment of children’s internal exposure relative to 

adults, and qualitative description of the issues and uncertainties. Each approach has advantages 

and disadvantages as described below. 

Quantitative Approaches: PBTK Models

PBTK models have had great utility in refining risk assessments involving extrapolation 

of exposure and toxicity across species. The same will likely be true of PBTK modeling for 

children. However, models do not currently exist which take into account the numerous factors 

that can create TK differences between children and adults or that are calibrated against actual 

TK data in children. Several initial efforts (Haddad 1999; Pelekis 2001; Gentry et al, 2002) form 

useful building blocks and we can expect children’s PBTK models to evolve over the next few 

years. Therefore, while this analytical tool holds great promise for providing refined estimates of 

internal dose in children, it has the drawback of requiring a period of intensive model 

development. Other drawbacks include the fact that PBTK modeling requires a large amount of 

empirical data for model calibration and validation, and this type of data will not be available for 

environmental toxicants in children. Therefore, such modeling will be difficult to validate, and 

will often depend upon developing confidence in the model structure by simulating PK data in 

children who have been exposed to therapeutic drugs. While this will introduce uncertainties, it 

does not invalidate this approach. 

As mentioned above, PBTK modeling in children can represent an important advance in 

refining the risk assessment process. This is especially true when PBTK models are combined 

with Monte Carlo approaches which incorporate the distribution of children’s capacities in the 

various ADME areas. In this way, variability within a given age group of children can be 

explicitly examined and pre-determined percentiles of the distribution of internal dose (e.g., 50th

or 90th percentile) can be selected for inclusion in risk calculations. Alternatively, the full 

distribution can be used in combination with distributions for other risk inputs (exposure, doseresponse) for a complete probabilistic description of risk. In this way, the TK contribution to 

variability and uncertainty in the assessment can be explicit and readily expressed. 

While PBTK efforts are recommended for children’s risk assessments, not all types of 

assessments may warrant this level of effort, even when working children’s models are available. 

For example, risk assessments in which the exposure or dose-response inputs are associated with 

a high degree of uncertainty may not warrant extensive effort to refine the TK component. In 

such cases, screening level analyses may be the only realistic option. Additionally, in cases 

where a chemical’s TK properties have not been well studied in rodents and adult humans, 

children’s models become more uncertain and less worthwhile. Therefore, the choice of whether 

to utilize PBTK approaches for children depends upon whether refined estimates of internal dose 

are feasible for children and will be worthwhile relative to the level of analysis being conducted 

in other portions of the risk assessment. 
30 

Semi-Quantitative Approaches

When there is no need to conduct a detailed quantitative analysis, or when such an 

analysis is not feasible, the analyst can consider a semi-quantitative approach. In these cases, 

review of the underlying chemical-specific and age group-specific databases may give reason to 

suspect that there may be greater internal dose at certain early life stages than in adult humans or 

in the laboratory animals from which toxicity data are extrapolated. For example, if the chemical 

could be metabolically activated to toxic metabolites by CYP3A7 (e.g., aflatoxin B1; 2-amino-3-

imidazoquinoline – Kitada et al, 1990; Hashimoto et al, 1995), a form of cytochrome P-450 

prevalent in utero and just after birth, this may constitute a sufficient rationale to develop at least 

a semi-quantitative or screening level estimate of relative internal dose in this age group. This 

would especially be the case if the active metabolite formed from CYP3A7 metabolism is 

expected to be poorly detoxified and excreted in this age group. Of course, the semi-quantitative 

approach would only be used if a more detailed PBTK assessment was ruled out. This screening 

level approach can be seen as supplementing the existing set of uncertainty factors which already 

exist in non-cancer risk assessment – specifically the half-log (3.16 fold) uncertainty factor for 

inter-individual variability in TK (Renwick 1998). While that uncertainty factor is designed to 

address a large array of general inter-individual differences that might affect TK handling of 

xenobiotics (e.g., genetics, gender, disease states, other concomitant exposures, age), it may not 

always be adequate to address specific differences between subgroups of the population (e.g., 

children of certain age groups) who might have their own unique mean and standard deviation 

for internal dose. The semi-quantitative assessment could evaluate whether a sufficient 

difference between children and adults might exist in the direction of increased toxicant dose to 

warrant an age group-specific adjustment factor for TK. 

Since this would not be a comprehensive PBTK approach, the semi-quantitative 

assessment would focus upon one or several key TK factors/pathways. An assessment that does 

not incorporate all factors that may influence the estimate of internal dose has the disadvantage 

of greater analytic uncertainty. However, by taking stock of key factors with obvious 

implications for internal dose differences in children, the analysis can point out what types of 

concerns exist, how large the across-age differences may generally be, and whether more 

detailed PBTK assessment is ultimately needed to refine the dose estimate. 

The semi-quantitative assessment can evaluate how known differences in key TK 

pathways (Appendix 3; Ginsberg et al, 2002) may affect the absorption, metabolism and 

elimination of the chemical under analysis. The analysis would be semi-quantitative and 

comparative in that the size of functional differences between children and adults (e.g., 

child/adult ratio) would be used as an initial estimate of the change in internal dose in a 

particular age group. Other TK factors may increase or decrease the influence of changes in, 

say, a specific CYP pathway (e.g., differences in blood flow, protein binding, distribution to 

CNS, renal elimination changes, Phase II conjugation activity). These factors would need to be 

considered for their possibility to alter or negate the importance of the key child/adult difference 

the analysis is focused upon (e.g., specific CYP in children vs. adults). If these factors appear 

likely to negate the child/adult difference in CYP activity, then the analysis may be deemed too 

indeterminate for the application of a specific adjustment factor. However, if these additional 

factors appear to be of little consequence or to augment the age differential in CYP, then the 

child/adult ratio may be used as an adjustment factor directly, or with an additional uncertainty 
31 

factor. It is essential that this screening level approach be described as providing only a crude 

estimate of the differences in internal dose that may be possible, and that the various 

uncertainties be made explicit. 

Qualitative Approaches

When the review of chemical-specific and age group-specific TK data suggest that 

child/adult differences may not cause substantially more internal dose in children, or where this 

review indicates large areas of uncertainty, a purely qualitative approach may be warranted. This 

approach can summarize what is known about the chemical’s TK properties vis-à-vis the 

development of TK functions in utero and in children. This can lead to a discussion of how 

these various factors may interact to alter internal dosimetry relative to adults and if there are age 

groups where such alterations are more likely. If this is a considerable source of uncertainty, it 

may affect how much confidence is placed on the overall risk assessment regarding in utero and 

children’s exposures. 

Engaging the Framework: Addressing Modeling and Data Needs

The TK assessment framework described above involves a large array of parameters that 

need to be informed by empirical data for a variety of age groups. Given that there are very few 

TK data for environmental chemicals in children, a large number of datagaps will need to be 

filled for individual chemicals, either through new data acquisition (e.g., studies in juvenile 

animals) or by reliance on surrogate chemicals (e.g., drugs which have similar 

metabolism/clearance pathways and which have been tested in children). 

To engage the framework there is an overriding need for the development of well 

calibrated, and to the extent possible, validated PBTK models for children. These models can be 

extensions of adult models with appropriate adjustments for the physiologic and 

metabolism/elimination differences that exist for children at specific developmental periods. 

This modeling can progress in stages from initial descriptions of children’s growth and 

maturation (changes in body weight and water/lipid composition, body compartment sizes, tissue 

blood flows, ventilation rates, and serum protein binding capacity), to more complete, chemicalspecific PBTK models with the activity of metabolism and elimination pathways for the 

chemical being modeled reflective of what is known about these pathways in children. Model 

development would likely involve a number of case studies for specific chemicals, which would 

then lead to a flexible modeling framework that would describe the underlying physiology of 

children’s development and be adaptable to a variety of chemicals and age groups. A similar 

approach can be used for the in utero period, in which established models for specific chemicals 

can be adapted to new chemicals for which there is exposure during pregnancy. 

Table III-1 summarizes the general TK data and modeling needs to make this framework 

fully feasible. In addition to these general needs, there may be a variety of chemical-specific 

data needs for any individual analysis, depending upon the extent of TK evaluation the chemical 

has already undergone and the degree to which it has been modeled in test animals and humans. 
32 

Table III-1. Critical Data Needs for Children’s TK Framework 

Data Needed Type of Effort Rationale 

Accessible database of age-specific 

physiology inputs in 

animals/humans (cross-species 

ontogeny mapping) 

Literature search to compile. 

New studies may be needed (e.g., 

child blood:air partition coeffs.; 

plasma protein binding of toxicants) 

Needed for optimal PBTK modeling 

PBTK model development – 

age/life stage-specific (in utero, 

lactational, neonatal thru puberty) 

Model calibration and validation 

with therapeutic drugs or with 

toxicants in non-human primates or 

lower animals 

Key tool for children’s TK 

assessments 

TK modeling case studies: 5 chems 

x 3 scenarios x 3 ages 

Model predictions of internal dose 

in children relative to adults or 

animals 

Demonstrate how to adjust internal 

dose for range of cases 

Juvenile animal toxicology and TK 

data 

New lab data - acquisition of 

chemical-specific data 

Obtain actual data where critical; 

data useful for model assessment 

TK submodels : 

Respiratory uptake 

 - Particle deposition 

 - Reactive gases 

Oral absorption 

Respiratory deposition 

a) adapt RDDR/RGDR to early life 

stages 

b) adapt for asthmatic children 

Oral – develop absorption model 

responsive to children’s g.i. function 

RDDR – particle dosimetry likely 

different in children 

RGDR – gas dosimetry may differ 

– need to evaluate possible 

differences in resp. distribution of 

chemical dose 

Model development where critical 

need 

Lung clearance mechanisms 

(mucociliary, macrophage) 

New data for variety of particles at 

early ages 

Can affect lung AUC dose – 

prioritize certain age 

groups/conditions 

Dermal uptake Compile existing data for drugs and 

env. chems. in adults and children 

Need to evaluate signficance of agedependent absorption factors 

Metabolic systems 

(esp. GSTs, epoxide hydrolase, 

alcohol dehydrogenase) 

Continue analysis of in vivo/in vitro

data; new data may be required 

(e.g., Cyp3A7, GSTs); evaluate 

distributions and sources of 

variability 

Need improved maturational data 

for selected systems of 

environmental relevance, especially 

where only sketchy data exist or for 

enzymes high in fetus/neonate 
33 

Table III-1. Critical Data Needs for Children’s TK Framework, Cont’d 

Data Needed Type of Effort Rationale 

In utero dosimetry Compile maternal/fetal dosimetry 

estimates from existing case studies; 

extension of existing models; 

evaluation of governing principles 

Need evaluation of fetal dosimetry 

and modeling approaches 

Lactational factors Compile data and models of 

excretion into milk 

Important to identify which types of 

chemicals and what mechanisms 

govern milk transfer 
34 
35 

IV. TOXICODYNAMIC CONSIDERATIONS IN UNDERSTANDING CHILDREN'S 

HEALTH RISKS FROM EXPOSURE TO ENVIRONMENTAL AGENTS

Introduction

The charge to this breakout group was to evaluate how to use known dynamic differences 

in development to better understand children's susceptibility to environmental agents. Towards 

that end, this group focused on what, how and when such information could be used in an overall 

risk assessment framework for evaluating children’s health risks. 

The term developmental dynamics was used to describe the biochemical, molecular, 

cellular, organ and organism processes that change throughout development and which define 

and characterize the developing organism at each life stage. Toxicodynamics was defined as the 

response of these normal developmental processes to toxicant exposure. Such alterations need to 

be considered both in a temporal and dose-related context in order to understand the immediate 

and long-term consequences of such changes. 

Toxicodynamics in the Risk Assessment Framework

The dynamics breakout group reviewed the proposed framework (see Figure II-1) to 

ensure that dynamic considerations could contribute to and would fit within the framework. 

Table IV-1 describes for each phase of the risk assessment process (Problem Formulation, 

Analysis, and Risk Characterization) how developmental dynamic information could impact risk 

assessments for children. 

The dynamics group felt that the majority of their deliberations would be within the 

Analysis and Risk Characterization stages of the overall children’s risk assessment framework. 

Thus, the primary focus of this group’s report is targeted to a discussion of how dynamic 

considerations would inform those stages in the framework process. 

Step 1: Toxicodynamic Considerations During Problem Formulation

The Problem Formulation stage of the children’s risk assessment framework provides an 

opportunity for the assessor to understand the purpose and focus of the risk assessment. 

Obviously, a narrow focus for the assessment could refine life stage-specific considerations and 

thus could define initial toxicodynamic considerations relevant for that assessment scope. Hence, 

the Problem Formulation stage in the framework provides the context for the breadth of 

subsequent toxicodynamic considerations. 

The Problem Formulation stage also, by identifying chemicals or chemical classes to be 

evaluated, provides critical input into potential biological systems for consideration, i.e. do we 

already know that this class of chemicals is neurotoxic? If yes, then specific dynamic factors 

relevant for the developing nervous system should be considered in the analysis. Likewise, if 

this class of chemicals is known to affect particular cellular or molecular processes, i.e. 

chemicals that are known to affect apoptotic processes, then organ systems that are known to use 

these dynamic processes should be considered in the analysis. Also, because such apoptotic 

processes are known to occur at specific times in normal development, critical windows of 
36 

vulnerability could be identified. Thus, both critical time as well as target organ system could be 

identified for further evaluation using dynamic information. 

Table IV-1. How Does Developmental Dynamic Information Impact 

Risk Assessments for Children? 

Problem Formulation 

• Determination of risk assessment context and scope 

• Definition of scope provides context for risk assessment and leads to the identification of 

relevant life stages, systems or processes of interest for the risk assessment 

• Determination of relevant exposure pathways/scenarios will provide context for identifying 

relevant developmental life stages 

• Determination of chemical-specific factors will also provide context for the identification of 

potential life stages for evaluation as it will identify potential toxicological processes of 

interest and hence identify developmental systems for potential evaluation 

• Identify cross-species relevance of potential responses 

Analysis 

• Identification of uniquely susceptible dynamic processes 

• Identification of developmental milestones and/or endpoints for testing/assessment 

• Identification of functional consequences of processes if altered 

• Illustrate the inter-relatedness of dynamic developmental processes and thus identify impacts 

that could occur at later life stages and within other organ systems 

• Identification of immediate or delayed responses 

Risk Characterization 

• Define dose-response relationships, especially dose, time and response relationships 

• Characterize potential magnitude of effect, reversibility, repair, functional reserve, etc. of 

dynamic developmental processes 

During Problem Formulation, when thinking about toxicodynamics, the risk assessor 

needs to take into consideration the life stage of the population(s) of concern, and the specificity 

of the agent of concern. 
37 

Steps 2 & 3: Toxicodynamic Considerations During Analysis and Risk Characterization

The proposed children’s risk assessment framework emphasizes the need to connect 

toxicodynamic considerations with concurrent toxicokinetic considerations. Obviously, it is 

essential to understand if the parent compound or if a metabolite would be expected to reach 

developing tissues. Also, it would be important to know what dose levels and at what times such 

exposures would be expected. Hence, the dynamics group also discussed in detail the 

importance of timing and dosimety considerations within the Analysis phase of the framework. 

Dynamic considerations would also inform quantitative considerations in dose-response 

assessments and for risk characterization. 

During the Analysis and Risk Characterization stages, when thinking about 

toxicodynamics, the risk assessor needs to take into consideration whether the parent compound 

or its metabolite(s) is the toxic agent. Timing and dosimetry should also be considered. 

The dynamic breakout group first evaluated life stage considerations by developmental 

organ system. The group conducted a brief survey of some of the relevant resources and 

summarized some example organ systems with an aim at highlighting example dynamic 

processes occurring at various times within each system and the implication of those processes 

for identifying and characterizing “windows of susceptibility.” Some examples of why these 

processes were important for children’s health risk assessments were given. The final section in 

this report identifies “critical data needs.” 

Questions

Table IV-2 shows a list of questions posed to the dynamics breakout group in their 

workshop deliberations. These questions were designed to provide background and justification 

for the specific consideration of developmental dynamics in the risk assessment process. They 

provide some context for determining the magnitude and diversity of toxicokinetic 

considerations for children health risk assessments. 
38 

Table IV-2. Some Questions Considered by the Dynamics Breakout Group 

• What do we know about the dynamics of developmental processes that may make them 

uniquely sensitive (qualitatively and/or quantitatively) to environmental factors? Can we 

define the types of dynamic information that are useful for our Framework? 

• Are there common characteristics of these processes? 

• Are the animal tests robust enough to provide the types of dynamic information that we 

need? What are our qualitative and quantitative needs for these assessments? 

• Can the group provide some indications of level and type of impact that might result from 

altering these developmental processes? How would these compare with adult dynamic 

processes? 

• How can dynamic information support or refute proposed modes of action in animals or 

humans? 

1. Are there distinct life stages? 

The dynamics group identified a series of references and workshops that have reviewed 

the evidence for distinct life stages and critical windows of susceptibility in development 

(Adams et al, 2000; Adkins et al, 2000; Dietert et al, 2000; Lemasters et al, 2000; Pinkerton and 

Joad 2000; Pryor et al, 2000; Rice and Barone 2000; Selevan et al, 2000; Weiss 2000; Zoetis and 

Walls 2003). Figure IV-1 shows a summary of developmental life stages including those of 

interest for this workshop, which range from preconception through adolescence. The dynamics 

group felt that it was critical to the overall risk assessment process to be able to identify distinct 

life stages in order to more fully identify potential windows of susceptibility. The group also felt 

it was important to be able to compare life stages and particular endpoints and patterns of 

dynamic processes across species. For their deliberations, the group used a series of general, 

simplified life stage events (summarized in Figure IV-1) which included conception, embryonic, 

fetal, newborn, neonatal, pre-weaning, weaning, juvenile, puberty, adolescence, adulthood and 

old age. The group thus reviewed the structure of the children’s risk assessment framework to 

consider approaches for evaluating impacts of exposure of children at any or all these life stages. 

The workgroup noted that there are a number of different ways of viewing the period 

called “childhood.” In considering the toxicodynamic processes involved with the adverse 

effects of xenobiotic materials, it is helpful to be aware of the way different organizations and 

disciplines have viewed the developmental periods that occur during childhood. The workgroup 

felt it was not important for the framework to fully integrate all the various categorical views of 

childhood and major developmental periods. However, it can be important to understand how 

other disciplines have categorized the age periods that constitute childhood in their evaluation of 

data. 
39 

Figure IV-1: Exposures and Endpoints Related to Developmental Toxicity Evaluation

(Adapted from Kimmel 2001) 

Life Stages Preconception Emb/Fetal

NB/Preweaning Juvenile Adulthood 

Study Designs:

Prenatal Developmental Toxicity Study

Reproduction andFertility Study

ConceptionBirth Weaning Maturity Sen

Litter size, sex ratio, fetal survival, 

weight, gross structure

Developmental NeurotoxicityStudy

F1 sexual 

maturation

F1 estrous 

measures, fe

maintenance, 

weight, gross s

Adolescence

Puberty

P

F1

Litter size, sex ratio, F1 & F2 survival, weight, gross 

structure, brain, spleen, thymus weights

Subchronic/chronic(Kimmel et al, 2002.)

Embryonic death 

Dominant Lethal

Clinical observations, viability, 

body weight, food & water 

consumption

Neurotoxicity evaluations

Litter size, sex ratio, survival, weigh

structure, developmental landm

including sexual maturation

Functional observations,

motor activity, auditory startle

Clinical pathology (eve

ophthalmological exam, gros

pathology

Motor activity, auditory startle, 

learning & memory, 

neuropathology 

P estrous cycles, s

fertility, pregnanc

parturition, organ 

structure, 

Life Stages Preconception Emb/Fetal

NB/Preweaning Juvenile Adulthood 

Study Designs:

Prenatal Developmental Toxicity Study

Reproduction andFertility Study

ConceptionBirth Weaning Maturity Sen

Litter size, sex ratio, fetal survival, 

weight, gross structure

Developmental NeurotoxicityStudy

F1 sexual 

maturation

F1 estrous 

measures, fe

maintenance, 

weight, gross s

Adolescence

Puberty

P

F1

Litter size, sex ratio, F1 & F2 survival, weight, gross 

structure, brain, spleen, thymus weights

Subchronic/chronic(Kimmel et al, 2002.)

Embryonic death 

Dominant Lethal

Clinical observations, viability, 

body weight, food & water 

consumption

Neurotoxicity evaluations

Litter size, sex ratio, survival, weigh

structure, developmental landm

including sexual maturation

Functional observations,

motor activity, auditory startle

Clinical pathology (eve

ophthalmological exam, gros

pathology

Motor activity, auditory startle, 

learning & memory, 

neuropathology 

P estrous cycles, s

fertility, pregnanc

parturition, organ 

structure, 
40 

Categorization of life stages by dietary age divisions

As an example, the Institute of Medicine (IOM) of the National Academies of Science 

(NAS) is establishing updated dietary recommendations for nutrients. As has been the situation 

with prior guidelines, the nutrient recommendations are targeted towards a series of age ranges 

that correlate with the physiological demands of normal growth and development. The age 

ranges that are used for the present set of Dietary Reference Intakes (Institute of Medicine 1997) 

divide the childhood period into three major life stage categories, which are then subdivided into 

narrower age ranges. The three major developmental categories are: infancy, childhood and 

adolescence. 

The infancy period is subdivided into the first and second six months after birth. 

Lactation demands and the age of introduction of solid foods were considered in establishing this 

division. The childhood period is subdivided into two periods (1-3 years and 4-8 years). The 

adolescent period is divided into two periods (9-13 years and 14-18 years) based on the 

beginnings of puberty and growth demands. Males and females are treated differently during the 

adolescent period. 

The IOM dietary age divisions are important considerations in risk assessment because 

nutrition and food exposure pathways can have a strong impact on the evaluation of all 

environmental effects. Accordingly, it may become possible to link nutritional status in a 

particular age group with toxicant exposure and manifestation of effects. 

Categorization of life stages by behavior and exposure windows

Likewise, the dynamics group recognized that exposure assessors have identified a series 

of different age-specific exposure windows based on age-specific behaviors and physiological 

considerations. For example, toddler versus neonatal differences in exposure to carpet and floor 

and for hand-to-mouth exposure pathways as well as dietary differences are defined by both 

differences in activities as well as physiological development. Exposure considerations in 

categorizing life stages are reviewed in the EPA children’s exposure factors handbook (U.S. 

EPA 2000) and the EPA Risk Assessment Forum document on issues associated with 

considering developmental changes in behavior and anatomy when assessing exposure to 

children (U.S. EPA Risk Assessment Forum 2000). Child psychologists or childhood 

developmental specialists also define life stages using behavioral landmarks. Many other 

examples of discipline-based differences in childhood categorizations have been proposed. 

Recognizing categorical overlap between disciplines

The group recognized that although general developmental life stages did not always 

match the discipline-specific exposure windows, this difference would not inhibit the ability to 

look at dose-response relationships for critical developmental endpoints. These differences 

however did highlight the need for the framework to be robust enough to allow for iterative 

interactions between exposures and effect analysis both in problem formulation as well as data 

analysis. It also identifies a need for risk assessment methods that would allow risk assessors to 

relate output from the exposure assessments or from discipline-specific assessments using 

multiple exposure times during various life stages. Thus, the timing and dosimetry relationships 
41 

shown in Figure IV-1 become very important for the hazard/risk assessment equation and hence 

linkage with developmental hazard identification and dose-response assessments. 

Examples of the importance of life stage considerations

The dynamics group chose three specific organs or biological systems to evaluate and 

illustrate distinct life stage considerations. These three systems are the immune, respiratory, and 

nervous systems. These were chosen to illustrate the importance of toxicodynamic 

considerations for children’s risk and were not intended to be comprehensive. Many other organ 

systems in development would also be important in this context including, but not limited to, 

cardiovascular and endocrine development. 

Figure IV-2 shows the initial appearance of organ systems during gestation in humans 

and in rodents. Note that the relative temporal initial development of each organ system, as 

defined by the first appearance of cellular structure of each system, is evident and can vary 

greatly across species. 

Respiratory System

Figure IV-3 shows the temporal development of the respiratory system in humans 

and in rodents (Pinkerton and Joad 2000). The human respiratory system involves 

the formation of a highly ordered airway branching system with 25,000 distinct 

terminations giving rise to more than 300 million alveoli as well as the 

differentiation and proliferation of over 40 different cell types. The transition of the 

lungs from a simple protruding bud of tissue from the foregut into a highly 

organized, integrated, complex structure that is innervated, ventilated and 

vascularized is a multi-step process. Obvious from Figure IV-3 is the fact that 

although remarkable structural changes occur during the embryonic development 

such as pseudoglandular, canalicular and saccular stages of prenatal development, 

changes to the lungs continue into the postnatal developmental period. 

Approximately 80 percent of the alveoli present in the adult lung are formed 

following birth. Numerous metabolic and biochemical functions of the lungs 

undergo development and maturation during the postnatal time frame which 

includes the proliferative period of the alveolar phase of postnatal lung growth. 

Physiologic development of the lungs continues to increase in large measure during 

the period of alveolar expansion in the postnatal period. The alveolar period of 

growth also encompasses further development of the airways. Although branching 

morphogenesis of the bronchial tree is essentially complete at birth, the airways 

continue to undergo maturation, growth and expansion through early adulthood. 

 
42 

Figure IV-2: Initial Appearance of Organ Systems during Gestation in Humans and Rod
43 

A number of studies suggest that the processes of cellular differentiation, branching 

morphogenesis and overall lung growth can be affected by exposure to chemicals 

and particles. Both embryogenesis and fetal gestation represent critical periods of 

cellular differentiation and branching morphogenesis. The effects of exposure, 

however, are likely to be different for each period of development. For example, 

during embryogenesis and fetal development, cell number, type and function of the 

airways and alveoli may be significantly affected by exposure to a diverse number 

of substances and/or conditions. However, since cells continue to differentiate and 

divide during the postnatal period, chemical exposure during the postnatal period is 

also likely to affect the respiratory system, but in a different manner based on 

changes in the process of differentiation and morphogenesis (Smiley-Jewell et al, 

1998). Since growth is essentially complete by the end of adolescence, exposure to 

chemicals and other factors at this time are likely to have completely different 

consequences in the adult compared to those found in children (Smiley-Jewell et al, 

1998; Plopper et al, 1994; Fanucchi et al, 1997). 

Inhalation exposure to substances during critical windows of development may 

have profound effects that would not be seen if the same exposure were to occur in 

the adult. Since lung development occurs over the entire prenatal period, exposure 

effects can have significantly different consequences depending upon whether they 

occur during the pre- or postnatal period of life. Although our understanding of 

these changes at this time is extremely limited, one would expect that abnormal 

developmental changes, which occur in the prenatal period due to exposure to a 

variety of chemicals, may have long-term effects persisting into adult life. 

Examples of altered lung growth or functional deficits in respiration have been 

shown to result from exposure early in organogenesis to neonatal and adolescent 

developmental time periods (Pinkerton and Joad 2000). In contrast, structural 

abnormalities in the lung which result from exposure to environmental toxicants are 

believed to be manifested only while lung morphogenesis is still occurring, i.e. 

including the neonatal but not adolescent developmental periods. Very recent 

studies have suggested that the development of asthma and immune disorders of the 

respiratory system may result from exposures during organogenesis as well as 

throughout neonatal and adolescent development (Pinkerton and Joad 2000). 
44 

Figure IV-3: Respiratory System Development in Humans and Rodents 

(Adapted from Dietert et al, 2000; Pinkerton and Joad 2000.) 
45 

Immune system 

The immune system undergoes a number of key changes throughout embryonic, 

fetal, neonatal and juvenile development that would be expected to alter the 

potential risk from environmental exposure to toxicants. Figure IV-4 shows a 

comparison of some of these critical changes for immune system development in 

humans and in rodents. Five specific stages in the development of a mature 

immune system are illustrated and include: initiation of hematopoiesis, migration of 

stem cells and expansion of progenitor cells, colonization of bone marrow and 

thymus, maturation to immunocompetence, and establishment of immune memory. 

These specific stages were chosen as they represent discrete steps in the formation 

of the mature immune system and also represent periods in which differential 

vulnerabilities to immunotoxicants would be anticipated (Holladay and Smialowicz 

2000). Early embryonic issues concern the location and source of stem cells to seed 

the primary immune organs such as the thymus. The initiation of hematopoiesis is a 

benchmark that signals the appearance of cells necessary to sustain immune 

development. Obviously, exposures occurring before vs. after the beginning of 

hematopoiesis, the migration of stem cells and expansion of progenitor cells, and 

the emergence of the bone marrow as an important progenitor cell source, might 

lead to differences in the manifestation of impact or outcome. Other benchmarks 

include the formation and innervation of the thymus as well as the seeding of the 

thymus by waves of lymphoid cells. Exposures timed such that they target different 

waves of lymphoid cells important in thymic-dependent T lymphocyte maturation is 

another area for potential differential impact. Developmental changes involving 

peripheral lymphoid organs such as the spleen may also be important for 

consideration of the timing of exposures compared with risk of immunotoxicity. 

These events all are initiated prior to birth in rodents and humans. However, postnatal processes are also important for complete maturation. These immune 

developmental stages could also provide windows of differential immune sensitivity 

to toxicants when compared with exposure of the fully matured adult immune 

system. Among changes occurring largely during the post-natal period in rodents 

and humans are the maturation to complete immunocompetence and the 

establishment of immune memory. 

An example of differential immune system outcome based on the life stage in which 

exposure occurred is seen in studies evaluating lead. The heavy metal lead (Pb) is a 

known immunotoxicant capable of producing numerous immune changes including 

depression of cell-mediated immunity. A hallmark of Pb-induced immunotoxicity 

in adult rodents is suppression of the delayed type hypersensitivity (DTH) response 

(McCabe et al, 1999). This functional change is likely linked to the capacity of Pb 

to shift immune response capabilities away from Th1-dependent responses toward 

Th2-dependent responses (McCabe and Lawrence 1991; Heo et al, 1997). 

Exposure of rats throughout gestation to levels of Pb that do not alter maternal 

immune function can produce persistent depression of the DTH response in both 

juvenile and adult offspring (Chen et al, 1999; Bunn et al, 2001a). However, the 

timing of exposure appears to be an important factor. Pulsed exposure of dams to 
46 

Figure IV-4: Immune System Development in Humans and in Rodents 
47 

Pb during late gestation (days 15-21) results in offspring with depressed DTH 

response function (similar to the complete gestational exposure). But the same Pb 

exposure earlier in gestation (days 3-9) fails to alter DTH response function in the 

offspring (Bunn et al, 2001b). 

These findings of temporally dependent differential immunotoxic outcome have 

been extended to other species. For example, exposure of chickens in ovo to a 

single administration of Pb on embryonic day 12 causes depressed DTH function in 

juvenile chickens. However, exposure of embryos to the same level of Pb only 

three days earlier, producing identical blood Pb levels at hatching, fails to alter 

DTH function in the offspring (Lee et al, 2001). It has been hypothesized that the 

development of the thymus may be key to these differential effects following Pb 

exposure. This example suggests that comparable environmental exposures during 

different windows of development have the potential to produce qualitative 

differences in immune system outcomes. 

Nervous System

Figure IV-5 shows a comparison of nervous system development in humans and in 

rodents. Following neurogenesis each neuronal cell continues to mature through a 

process of migration, settling to a specific location and extending projections to a 

designated target site. In many cases such as for the external germinal layer, this 

process of migration continues well after birth and in the human can continue for 7 

months to 2 years following birth. Earliest synapses develop during the embryonic 

period and by 10 weeks immature synapses are present. Cortical synapses at birth 

are still immature and in the human the morphological characteristics of maturity 

are reached between 6 and 24 months following birth. The full functional 

maturation of synapses may be related to the elimination of unnecessary synapses. 

Myelination occurs first in the spinal cord by the end of the first trimester period in 

the human and proceeds in a caudocranial fashion. At birth the brain is immature in 

regard to the extent of myelination with prominent myelination present in the brain 

stem, cerebellar white matter, posterior limb of the internal capsule, thalamus, and 

the basal nuclei. In the human, the rate of myelin deposition is the greatest in the 

first 2 years following birth. In the rodent this is comparable to the first 35-40 days 

of life. Nervous system malformations can arise from alterations of neurogenesis, 

changes in the timing of migration, and perturbation of migratory mechanisms and 

synaptic development. 

Excellent examples exist that demonstrate concordance of functional changes in 

behavior across primates and humans when appropriate age-specific comparisons 

are conducted (Paule et al, 1988; Slikker et al, 2000). For example, performance of 

behavioral tasks in humans and primates which are designed to monitor learning, 

short-term memory, color and position discrimination, time perception and 

motivation are indistinguishable. Depending upon task and endpoint, the behavior 

of young adult monkeys is identical to that of children over the age range of 4-12 

years. Of significance is that performance on many of these tasks is highly 
48 

Figure IV-5: Nervous System Development in Humans and Rodents 
49 

correlated with IQ in children (Paule et al, 1999b). Interesting age-related effects 

have been seen in functional outcomes for the nervous system. For example, 

chronic marijuana smoke exposure in peri-pubescent male monkeys resulted in an 

“amotivational”-like syndrome similar to that reported for human subjects 

(teenagers or young adults only—never reported for mature adult humans) (Paule et 

al, 1999a; Schulze et al, 1988). 

Lessons Learned from Life stage Examples

The dynamics working group identified key considerations from these specific biological 

system examples. 

• First, for each biological system there were multiple windows of susceptibility. 

• Second, windows of susceptibility were frequently identified throughout childhood. 

In fact, for many cross-species comparisons, birth was a rather “arbitrary” milestone 

in the developmental process. 

• Third, the windows of susceptibility were defined upon the basis of distinct 

development processes and many of these processes were common across windows 

for different biological systems (i.e. apoptosis relevant for both neurological and 

immunological development). 

As mentioned, the workshop participants chose to discuss these three example systems. 

However even with the limited systems evaluated, large data gaps exist in the completeness of 

our understanding of these processes and in particular which developmental windows represent 

real or hypothesized windows of susceptibility. The workshop participants also noted that 

functional data was often lacking and that it did not allow for the translation of developmental 

impacts into public health relevant endpoints. Obviously, such lack of data complicates 

interpretation of animal-to-human comparisons. 

An Endpoint Example: Cancer

The workshop participants also felt it was important to evaluate not only windows of 

susceptibility based on organ system but also by common endpoint. Towards this goal, the group 

considered the available animal data on cancer susceptibility across life stages. In the 

monograph from a previous ILSI workshop, McConnell (1992) compared outcomes of cancer 

bioassays conducted at various exposure times throughout gestation. In general, conclusions 

from such reviews were that usually the timing of exposures did not affect tumor type (no 

qualitative differences), but quantitative differences in dose-response relationships and the 

magnitude of tumors were observed. However, a disclaimer was given that there were a limited 

number of studies, with few chemicals tested in a consistent manner at varying times across 

gestation and postnatal developmental periods. That observation was essentially reconfirmed by 

another ILSI Risk Science Institute working group focusing on research needs (ILSI RSI 1996). 

Thus, a comprehensive comparison from which to make a comparative evaluation is still lacking. 
50 

An important exception to this generalized statement is the example of acute Tlymphocytic leukemia. Somatic cell gene mutations that arise through an aberrant differentiation 

process are limited to cell or life stages where the process is normally operative. An example is 

the V(D)J recombination mechanism that normally functions to rearrange variable (V), diversity 

(D) and junctional (J) regions of immunoglobulin (Ig) and T-cell receptor (TCR) genes in B- and 

T-lymphocytes, respectively. B-cells differentiate in the bone marrow throughout life in 

humans. However, normal T-cell differentiation is limited to the thymus gland in fetuses and 

children, being complete by late adolescence. Aberrant functioning of the V(D)J recombinase 

mechanism may be induced by environmental agents such as passive exposure to tobacco smoke 

(Finette et al, 1997). This results in rearrangements of genetic segments other than those of the 

Ig or TCR genes in developing lymphocytes. Some of these aberrant rearrangements constitute 

the chromosome deletions and translocations that characterize lymphoid malignancies. A 

specific example is acute T-lymphocytic leukemia. A high percentage of these leukemias is due 

to a V(D)J recombinase mediated deletion (tald

) or translocation ( t-1:14) that can occur only in 

the fetus or in children (Finette et al, 1997). Thus, qualitative differences in cancer outcome are 

possible following age-specific alterations. 

2. Are there common unique developmental processes? 

To answer this question the workgroup participants addressed three points: 1) What 

processes?, 2) How/why might these particular processes be important for developmental 

outcomes? and 3) How would redundancy or repair affect the functional consequences of process 

perturbation? 

What are the processes?

Developing tissues and organs, especially during the pre-natal stages of life, participate in 

common complex interactions that permit, encourage and control cellular processes. These 

dynamic processes include: differentiation, proliferation, migration, secretion, and apoptosis. As 

an example, Figure IV-6 shows the temporal differences in one of these processes (proliferation) 

across various brain regions during nervous system development in the rat. A different temporal 

pattern would be seen if apoptosis were plotted against brain regions. In some regions it appears 

simultaneously with proliferation and in some brain regions it appears at a later developmental 

time. 

For the most part, those tissue populations that engage in developmentally important 

interactions share a set of characteristics, which may make them vulnerable to perturbations by 

outside influences. The set of characteristics includes the following: 

• populations of cells interact – as opposed to individual cells 

• the interacting populations of cells experience developmentally different histories; 

that is to say, they experience divergence in their differentiation pathways 

• the interacting populations are in proximity to each other 
51 

• one population of cells (e.g. the inducer) transmits a message of developmental 

importance (usually considered to be a signal molecule) during a finite period 

• the second (responding) population of cells must be capable of receiving and 

responding to the signal (i.e., must be competent); the state of competence is 

maintained for a finite period of time 

How/why might these particular processes be important for developmental outcomes?

The latter three characteristics involve one of the signal transduction pathways. A recent 

report by the National Resarch Council (2000) has identified 17 signal transduction pathways 

which are highly evolutionarily conserved across multiple phyla and which appear to be able to 

explain most if not all relevant signaling pathways in development. The developmental process 

may be derailed by: disruption of these steps by altering the length of the period for, or timing of, 

induction or competence so that they are not contemporaneous; diminishment of the amount of 

available developmental message; interference with reception of the message; or prevention of 

appropriate activity by the responding tissue. The importance of many of these pathways has 

been illustrated with genetically sensitized test organisms or transgenic animal models. Because 

developing tissues and organs rely on such complex, temporally orchestrated interactions (see 

this orchestration in Figure IV-6 for just one of these dynamic processes, proliferation), they are 

exquisitely sensitive to perturbations of their environment. Additionally, because normal 

development proceeds from a cascade of such orchestrations, developmental processes are far 

more vulnerable to environmental vicissitudes than are stable, mature tissues. Furthermore, as 

maturation proceeds, the impact of small environmental challenges becomes increasingly subtle. 

This contributes to the difficulty in recognizing the effects of environmental challenges on 

differentiation processes that occur after most gross morphological structures have been 

established. Impacts of modifications in histological architecture are often manifested as 

changes in function and, as such, are more difficult to detect than alterations that occurred early 

in development, which are often manifested as gross malformations. 

How would redundancy or repair affect the functional consequences of process perturbation?

It is noted that the process of development has an inherently dynamic nature and that 

developing systems possess and exercise multiple signaling pathways simultaneously. 

Furthermore, many of the pathways have been demonstrated to exert overlapping functions 

especially in mammalian species. As discussed in National Research Council (2000), this 

redundancy has contributed to both the plasticity of developing organisms to develop normally 

after challenges and also has been the reason for failure of some of the knock-out models. 

Consequently, it is important that the developmental consequences of perturbation of any of the 

signaling pathways be determined and that the changes not be viewed in isolation. In Figure IV7, we see a detailed diagram from the National Research Council (2000) report that shows that 

such cell signaling processes occur at the molecular, organelle and cellular levels but must be put 

into a broader context of organ, tissue and conceptual development as well as a kinetic and 

dynamic context to understand both dose-response relationships and ultimate impacts on 

developmental outcome. 
52 

Figure IV-6: Patterns of Neuronal Proliferation in Specific Brain Regions 

(Figure adapted from Rodier et al, 1977 with permission of John Wiley & Sons, Inc.) 
53 

Figure IV-7: Levels of Mechanistic Inquiry for Assessing the Effects of a Toxicant on Develo

(National Research Council 2000) 
54 

3. Does knowledge about dynamic developmental processes impact our evaluation of 

mode of action (MOA) information? 

To answer this question the dynamics breakout group addressed two issues: 1) Can we 

use knowledge from windows of susceptibility to support or refute proposed modes of action?, 

and 2) Are there different expectations for how developing systems respond to chemical impacts 

that would make us re-assess our proposed MOA or anticipate additional or different health 

impacts in children (Faustman et al, 1997, 2000)? Atrazine was used as an illustrative case study 

to show that knowledge of developmental dynamics did make a difference (U.S. EPA 2002). 

Atrazine is the most commonly detected pesticide in ground and surface water, given the 

volume of usage and tendency to persist and move with water. The major exposure pathway is 

through drinking water, and there are episodic peaks of exposure. Other pathways for exposure 

are through food (minimal) and residential (dermal/inhalation). Atrazine has been shown to 

cause mammary tumors in Sprague-Dawley rats. Given the endocrine target organ site in the 

rodent bioassay (i.e., mammary gland tumors), studies were undertaken to determine whether a 

neuroendocrine mode of action was involved. It was concluded that the mammary tumors in this 

strain are not relevant for humans. However, the finding of disruption of the neuroendocrine 

system raised concerns for potential effects on the development and maintenance of the 

reproductive system. Subsequent studies showed that the compound alters ovarian function 

(cyclicity), disrupts critical reproductive processes, including delaying puberty (males: PND 23-

53 & females: PND 22- 45), pregnancy loss (GD 6-10), decreased dam prolactin release and 

prostatitis in offspring (PND 1-4), and has effects on lactation (milk quality/production). 

Atrazine is thus a good an example of a compound whose mode of action in an animal model 

was useful in highlighting the need to examine specific potential target organs and life stages. 

Application of Life Stage-specific Toxicodynamics in Assessing Children’s Risks

The workgroup participants used the proposed framework to identify key ways in which 

life stage information would inform risk assessment during both the Analysis and the Risk 

Characterization stages. First, as was illustrated in the previous section, an understanding of the 

timing and cross-species comparison of the developmental processes occurring during various 

life stages would inform the hazard characterization processes by identifying potentially unique 

times and organ systems during development. This information could suggest specific organ 

systems and functional impacts that might occur if exposures were to occur during those life 

stages. This information would also suggest the need to evaluate the potential hazard in specific 

types of animal tests (see Figure IV-1) and would provide some cross-species context for hazard 

characterization. It could also provide some mechanistic basis for evaluating impacts of the test 

agent on isolated developmental processes such as apoptosis and differentiation. Of particular 

importance is that life stage-specific assessment of health effects would more easily allow the 

assessor to link and evaluate the potential for subsequent functional alterations. Understanding 

the temporal and physiological inter-relatedness of developmental processes would allow the 

evaluator to better anticipate health impacts in other biological systems and to better forecast or 

evaluate impacts at later life stages. 
55 

In addition, workshop participants felt that such information would provide quantitative 

information relevant for assessing dose-response, especially dose and time relationships. It could 

also begin to inform our assessments of acute versus chronic exposure impacts. It would also 

provide some context for linking potentially susceptible tissues with kinetic profiles to provide a 

basis for evaluating kinetic measurements of “target tissue” doses. For example, an 

understanding of the underlying temporal relationships for the dynamic processes occurring in 

development would inform kinetic measurements such as determining whether area under the 

curve (AUC) or peak in utero concentrations of the toxicant or metabolite are more important for 

our risk analysis. 

An example of where life stage-specific exposure information has been shown to have an 

impact on quantitative dose-response assessments is seen in cases of pre-, post- and neonatal 

carcinogen exposure. Anderson et al, (2000) has summarized published literature for 

transplacental and neonatal carcinogens by target tissue and time of exposure for chemical and 

radiation exposures. This paper methodically evaluates this published literature and discusses a 

number of factors hypothesized as determining susceptibility to carcinogenic insult at different 

developmental stages. These “susceptibility” defining factors include “a) numbers of target cells 

at risk, b) sensitivity to cell killing, c) effects of rate of cell division..., d) ability to repair DNA 

damage, e) expansion of clones of mutated cells..., f) presence of undifferentiated stem cells, g) 

development of differentiated characteristics, including ability to carry out metabolic activation, 

h) metabolic detoxification by placenta and/or maternal tissues and, i) metabolic detoxification 

by the perinate itself...” The paper cites experimental evidence for each of these factors. 

Increasing understanding of how all these factors can impact qualitative and quantitative tissues 

and species specificity is needed; however, compelling examples exist for quantitative 

differences. For example, “in patas monkeys transplacental ENU caused more tumors than the 

same dose given to juvenile monkeys, confirming the quantitatively higher sensitivity of the 

fetuses seen for this chemical in rodents” (Anderson et al, 2000; Rice et al, 1989). 

Since tumor incidence determines the slope of the dose-response curve and the Q* value, 

a Q* derived from an adult animal study will have a flatter slope than that which would be 

derived from a study that incorporated dosing of the neonatal animal where tumor incidence is 

higher. The practice of amortizing exposure to a carcinogenic compound has the effect of 

lowering the much higher dose that children may receive during the first years of life. Taken 

together, the use of a Q* value that does not accurately represent the slope of the dose-response 

curve for young animals and the effect of amortizing children’s doses may result in a cancer risk 

assessment that is not adequately protective of children. The current approach for cancer risk 

assessment for children and the impact of early life stage exposures needs to be re-examined to 

ensure adequate protection of children in cancer risk assessment. 

Table IV-3 lists a number of prompts that can be useful in defining the depth of the 

database that is available. The dynamics breakout group discussed these prompts and felt that 

these or similar prompts should be included in every assessment of the potential for childhood 

toxicity. 
56 

Table IV-3. Assessment Questions to be Considered for Each Exposure Window 

Exposure: questions to be developed in separate activities. 

Is the exposure interval (potentially) relevant to the outcome of concern? 

Is the relevant kinetic data (ADME and/or embryofetal) adequately defined? 

Does the relevant receptor exist during the exposure interval in question? 

Have the toxicodynamics been adequately defined? 

Are other relevant physiologic and metabolic parameters defined? 

Is this a critical period for the outcome of concern? 

Is a mechanism or MOA relevant to the exposure period understood? 

Does an animal model exist for the relevant effect/exposure interval? 

.......Is the model adequate? 

.......Is the model validated? 

.......Does the model cover relevant critical period(s) 

.......Is the model validated as to ontology relative to human ontology? 

.......Is the model validated as to PK/PD? 

Does a protocol currently exist which would allow evaluation of the outcome of concern? 

.......Is the protocol adequate? 

.......Does the protocol cover relevant critical periods of exposure? 

.......Is the protocol model validated as above? 

Is hazard characterization adequate for the interval in question? 

.......Is the outcome of exposure in this time interval the same or different from adults? 

.......If same, are dose-response parameters unchanged? 

.......If different, are dose-response parameters defined? 

.......Is uncertainty in hazard characterization quantified? 

Do effects in this interval violate classical dose-response models (cellular reprogramming, 

etc.)? 

Has variability in susceptibility been defined for this exposure interval? 

Are epidemiologic data available? 

.......Is this reproducible? 

.......How strong is the association? 

.......Is association biologically plausible? 

Have extrapolation issues been adequately addressed: 

.......across species? 

.......across routes of exposure? 

.......across dose ranges (high to low dose extrapolation)? 

Are environmental interactions relevant to this time period known? 

.......Disease factors 

.......Lifestyle factors 

.......Nutritional factors 
57 

Table IV-3. Assessment Questions to be Considered for Each Exposure Window, Cont’d 

OVERALL ASSESSMENT QUESTIONS:

Given above information, is the exposure interval relevant to the outcome of concern? 

Is knowledge defined above sufficient for appropriate risk assessment? 

If knowledge inadequate, is a default approach necessary? 

What further research is necessary to improve risk assessment for this exposure interval and 

outcome? 

The prompts are applicable in all phases of the proposed framework. Moreover, given 

that the framework allows for an iterative process between the various phases, it is likely that 

these prompts may be reevaluated at various stages of the process. It is important to recognize 

that not all of the prompts will be important for every risk assessment. The risk assessor must 

make initial judgements about what data are relevant for the specific assessment. This should 

include consideration of “critical” data needs, as well as “nice to have” data needs (Moore et al, 

1995). Also to be considered is the impact of the use of default values in the absence of that 

data. 

In the Problem Formulation phase, the prompts will provide an initial evaluation of the 

database available, and will function together with the risk management focus for the assessment 

in determining what will or can be carried out in the Analysis phase. 

In the Analysis phase, the prompts will provide the risk assessor with another opportunity 

to evaluate whether there are adequate data to conduct a risk assessment that will provide the risk 

manager with the information outlined in the Problem Formulation stage. As data are analyzed, 

the risk assessor should continue to evaluate the impact of the data on potential data needs. 

In the Risk Characterization phase, the prompts will serve as a final review of the 

database and be important to developing a complete narrative statement that captures for the risk 

manager both the impact of the data that are available and the limitations of the database where 

data are not available. These prompts would also serve to identify where both qualitative and 

quantitative considerations for children’s risk would be possible. 

Critical Data Needs

The dynamics workgroup recognized and identified data needs throughout its 

deliberations, and many are implicit in the preceding sections of the workgroup report. In 

addition, some specific data needs and questions were highlighted as being particularly critical to 

an improved application of toxicodynamic principles in assessing risks for the developing 

human. These critical data needs include: 

• An improved understanding of the meaning (significance) of “subtle effects” 

(biomarkers) and validation of their relevance for risk assessment 

• Ability to link assessments to more robust functional endpoints 
58 

− Development of more endpoints for assessing system function that can be 

used in both humans and animals. This is a major issue since the absence of 

these tools is a huge impediment to actually assessing the effects of 

exposure. Imaging techniques could be very valuable as well, especially 

FMRI. 

− Resources for animal to human correlation 

• Better understanding of the toxicodynamics of “public health” relevant endpoints, 

such as asthma and cardiovascular disease 

• Comparison of the toxicodynamic links between the effects of acute, subchronic 

and chronic exposures 

• Better characterization of the development of homeostatic “set points” for many 

physiological systems 

• Better understanding of repair, tolerance, hypersensitivity in animal and human 

responses 

• More and better “diagrams” across life stages 

− There needs to be a concerted effort in comparative biology/physiology to 

develop tables or other references for easily identifying analogous periods in 

development across species. 

• More epidemiology studies that encompass multiple life stages (including 

early/developmental periods) 

In addition, the workgroup noted the need for multidisciplinary training for work in 

children’s health at all levels (graduate students, post-docs, scientists in the field). The group 

also acknowledged the value of and need for multidisciplinary workshops and interactions, such 

as the present workshop. 

Conclusions 

The toxicodynamics workgroup conclusions are summarized in Table IV-4. The 

workgroup participants concluded that there were distinct life stages evident across development 

with both known and hypothesized “windows of susceptibility”. These various life stages were 

based on differences in development defined by differences in relevant dynamic processes 

occurring at the molecular, cellular, organ and physiological level, and these differences could 

define in what systems and at what magnitude an environmental impact would be manifested. 

They identified the potential for apparent species differences in response to environmental 

exposures if the dynamic processes were not compared at equivalent doses, timepoints and 

processes across species. 

Differences in developmental dynamic processes can impact all stages of the proposed 

children’s risk assessment framework, as well as all components of the traditional risk 
59 

assessment paradigm such as hazard characterization, dose-response and risk characterization. 

The workgroup identified and discussed some common dynamic processes that can impact 

susceptibility. For example, impacts on apoptosis versus migration would provide clues as to 

what biological systems and at what times these impacts would be identifiable. Given these 

conclusions, the group considered some of the implications for animal testing and briefly 

discussed some of the ways to improve our understanding of dynamic processes across species, 

dose and life stage. Finally, the group emphasized the need to improve our assessments of 

functional and public health relevant alterations in our current testing approaches. 

Table IV-4. Conclusions from the Dynamics Workgroup 

1. There are distinct life stages with both known and hypothesized “windows of susceptibility.” 

2. Dynamic differences that impart susceptibility can exist between organ systems, within 

organs and at the biochemical or molecular process level. 

3. There can be apparent species differences in response if these dynamic processes across time 

and process are not considered. 

4. These differences can impact all processes of risk assessment from hazard characterization 

and dose-response evaluation to risk characterization. 

5. There are common dynamic processes that can impact susceptibility. 

6. The group considered some of the animal testing protocols and identified research needs to 

improve our understanding of dynamic processes across species, dose and life stage. 

7. The group emphasized the need to improve our assessment of functional and public health 

relevant alterations in our current testing approaches. 
60 
61 

V. RISK CHARACTERIZATION AND THE FRAMEWORK

The Risk Characterization workgroup was charged with determining the data and 

procedures that are needed in order to ensure that risks to children are accounted for in the risk 

assessment process. Much of the effort of the group was devoted to refining the draft risk 

assessment framework that was prepared before the workshop. While risk characterization is the 

final step in the framework, it cannot succeed unless the inputs from the preceding steps are 

appropriately directed at the problems associated with assessing risks to developing individuals. 

Therefore, refining the early stages of the process, particularly the problem formulation step, was 

felt to be critical work for our breakout group. 

Our definition of the life stages encompassed in the term “childhood” is broader than a 

dictionary definition. It encompasses not only life after birth, but also embryonic and fetal 

development. Our definition is rooted in the concept that special risks to children are the result 

of actions of toxicants on developmental processes, leading to different mechanisms and/or 

manifestations of toxicity than in adults. These unique mechanisms and outcomes arise because 

the individual is developing; birth, while significant, does not mark the end of development or of 

the capacity for an agent to produce permanent, organizational effects on function. Therefore, 

from the context of developmental biology and toxicology, children’s risk assessment is really 

“developmental” risk assessment, and as such must include the developmental stages that take 

place before birth. 

In addition to considerations of intrinsic sensitivity, exposures to toxicants may be 

different during development because of life stage-specific behaviors (e.g., mouthing during 

infancy, breastfeeding) or functions (e.g., the presence of a placenta prenatally, high respiratory 

rate in early childhood). 

There were a number of focus questions that the breakout group was asked to address. 

Our responses are given below, but the important overall message is that we do consider it 

necessary to have developmental toxicity data and life stage-specific exposure assessments in 

order to adequately characterize children’s risk. Specifically: 

1. Have unique susceptibilities been identified associated with one or more stages of 

prenatal development or childhood? 

Yes. There is abundant literature spanning many decades demonstrating the unique 

susceptibility of the embryonic period to structural teratogens. The thalidomide tragedy 

of the late 1950s-early 1960s demonstrated to the world that an agent can have radically 

different effects in the embryo than in the adult, and that these effects may be permanent. 

Research on the developmental toxicity of lead or ethanol, to name two examples, 

demonstrates that the fetal and neonatal periods are also sensitive, with manifestations of 

toxicity being largely functional in nature with few obvious structural correlates. 

Epidemiological evidence indicates that early menarche increases the risk for breast 

cancer; there is the potential for agents with estrogenic activity to accelerate puberty and, 

presumably, the risk for later effects. These are just a few examples of the unique 

susceptibilities of developing life stages (c.f., Appendix 1). 
62 

2. Are there mechanisms of toxicity unique to children, or are just the outcomes 

different? 

There are clear examples in which the outcomes of exposure are radically different in 

developing life stages than in adults, so much so that the nature of the outcomes could not 

be predicted from observations in adults or experiments in mature animals. Whether 

these are attributable to different mechanisms of action is unanswerable at this point 

because we have too little information about toxic mechanisms, particularly during 

development. It was the suspicion of the group that there probably are mechanisms of 

action that are specific to development, while in other cases the mechanism may be the 

same as in adults, but with a different outcome. For example, it appears clear that the 

effects of retinoic acid are mediated through retinoic acid receptors in embryos and 

adults, but the teratogenic outcome of retinoic acid exposure in the embryo is not at all 

similar to adult intoxication. Because of this, the possibility of unique developmental 

outcomes makes the problem of children’s hazard identification and risk characterization 

an important one irrespective of whether the mechanisms of action of a toxicant are the 

same as in an adult. 

3. Are there behaviors that are peculiar to children that make exposure by certain 

routes or media more of a concern? 

Yes. A number of examples were provided in the workgroup discussions, including 

breastfeeding, the propensity of children of a certain age to place things in their mouths, 

diets consisting of a relatively narrow selection of foods, crawling or short stature, 

making the breathing zone much closer to the floor or ground, and many others. There 

are reviews available on the subject (e.g., Cohen Hubal et al, 2000). 

4. In the context of risk assessment, how should we address responses in children that 

are different from those in adults? For example, is an additional uncertainty factor 

warranted? If so, what is the nature of the uncertainty? Could there be multiple 

sources of child-specific uncertainty? Is the use of an uncertainty factor dependent 

on whether the difference between child and adult response is qualitative or 

quantitative? For qualitative differences in outcome, does it matter if these are 

mechanistically different? Does it matter whether there is likely to be a 

cumulative/chronic effect from exposure? What magnitude (or range) should the 

uncertainty factor (UF) be? What data would be necessary to alleviate the 

uncertainty represented by the UF? 

The most straightforward way to determine whether there are differences between adult 

and developmental responses to an agent is to test for developmental effects in 

appropriate models, and to acquire life stage-specific exposure information. The question 

of whether an additional uncertainty factor should be used is one that must be determined 

on a case-by-case basis, using a weight of evidence evaluation of existing data. If there 

are no data, or the database is deficient, then this uncertainty needs to be addressed, e.g., 

by generating more data or by using the uncertainty factor already in place at EPA for 

accommodating database deficiencies. The magnitude of the uncertainty factor(s) 

depends on a variety of factors associated with the database and should be assigned using 
63 

a weight of evidence approach. Chronic, cumulative, or irreversible effects tend to be of 

greater concern. 

Of course there can be multiple sources of child-specific uncertainty. These can include 

anything from the comprehensiveness of the exposure and toxicology databases to the 

appropriateness of the animal model used or the strength of epidemiological data. Only 

by learning more about human biology, including the potential range of responses, and 

more about the capacity (and limitations) of animal and other experimental models to 

predict effects in humans, can we expect to alleviate the uncertainty represented by the 

uncertainty factors used in risk assessment. It should be possible to develop minimum 

criteria to support a good risk assessment. Certainly, it needs to be acknowledged that 

additional data may increase or decrease the reference dose (RfD) for a compound. 

Finally, it was reiterated that there are already policies in place to accommodate database 

deficiencies. 

5. What assumptions can be made about childhood susceptibility or childhood 

exposure when the database is incomplete?

It was the strong consensus of the group that the full spectrum of potential developmental 

effects cannot be predicted from adult data; therefore, a core data set in developing 

organisms is needed. Adequacy of the data set to assess the potential for risk to children 

should be determined after existing data on exposure (both known and anticipated 

scenarios) and effects are described and summarized. 

The responses to the focus questions clearly indicate the need for explicit consideration 

of children in human health risk assessment, and provided guidance for development of the 

Framework depicted in Figure II-1. 

In this Framework, the Problem Formulation step focuses on the interrelationships among 

exposures, effects and host factors. These considerations are consistent with the way 

epidemiological data are collected and directly feed into the hazard characterization and 

exposure assessment steps of the classical risk assessment paradigm. Host considerations will 

include the life stages of concern in the assessment and also any factors that are specific to a 

given situation; e.g., genetic, nutritional or SES factors that may influence biological response or 

extent of exposure. 

The end result of Problem Formulation is the development of a conceptual model 

describing the problem and indicating the possible risk assessment options. This model then 

guides the Analysis phase. The Analysis phase consists of characterizing life stage-specific 

exposures and health effects; i.e., the content of exposure assessment and hazard 

characterization. The linkage between these two is a consideration of timing of development and 

exposure, and dosimetry. Toxicodynamics forms the basis for life stage-specific hazard 

characterization, and toxicokinetics is the underpinning for characterization of the timing of 

target tissue exposures and dosimetry. 

The purpose of the Analysis phase is to produce an adequate basis for Risk 

Characterization. It is possible for the Analysis phase to fail due to an inadequate conceptual 
64 

model, in which case the Framework allows for refinement of the model and re-entry into the 

Analysis phase. 

Risk Characterization consists of a life stage-specific consideration of risk combined with 

uncertainty and variability analyses, culminating in a narrative statement describing the nature of 

the risk, its likelihood under specific scenarios and the degree of uncertainty in and confidence in 

the assessment. Risk assessors need to be explicit in categorizing uncertainty and variability and 

evaluate the use of uncertainty factors vs. other methods for estimating and incorporating 

variability into the assessment. EPA has already identified a number of areas of uncertainty that 

should be accounted for in the assessment. These include extrapolation from an animal species 

to humans, the range of variability within the human population, extrapolating from less-thanchronic exposures to continuous exposure scenarios, extrapolating to a lowest-observed-adverseeffect level on the dose-response curve, and the absence or inadequacy of key data sets. The 

overall vision for risk characterization is a meaningful probabilistic calculation of risk. 

The workgroup developed a list of needs that if met would reduce, or at least allow us to 

characterize, the range of uncertainties for children’s risk assessment. 

For hazard characterization, these are: 

• Understanding the relevance of animal models for predicting outcomes in humans 

• Understanding the comparative developmental profiles in animal models and humans (are 

the life stages being studied in animal testing analogous to human life stages of concern?) 

• Availability of data for relevant life stages 

• Better methodology (e.g., in testing protocols), applied more often, on functional 

outcomes at relevant life stages in animal models and humans; the methods should be 

sensitive, specific, and account for variability in responses and norms 

• Understanding of modes of action underlying toxicity 

• Understanding the temporal relationships between exposure and outcomes, particularly 

for delayed outcomes 

• Understanding of host factors that contribute to susceptibility 

For toxicokinetics: 

• Descriptive kinetics for the appropriate life stage(s) 

• Exposure and dose-response relationships in animal models and children 
65 

For exposure assessment: 

• Life stage-appropriate exposure factor data in humans 

• Habits and practices (e.g., diet, behavior) of children 

• Sources and pathways (e.g., patterns of use) in children 

• Characterization of complex exposure scenarios, including simultaneous exposures to 

multiple chemicals through multiple pathways 

Of these, the group listed the following as the highest research priorities: 

• Understanding of critical windows of developmental susceptibility, and of comparative 

developmental schedules of animals and humans 

• Habits and practices data (food, behavior, location, etc.) for children 

• Better methodology (e.g., in testing protocols), applied more often, on functional 

outcomes at relevant life stages in animal models and humans; the methods should be 

sensitive, specific, and account for variability in responses and norms 

• Understanding the temporal relationships between exposure and outcomes, particularly 

for delayed outcomes 

• Understanding of host factors that contribute to susceptibility 

• Latent sequelae of early events 

• Monitoring of disease trends and exposures 
66 
67 

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APPENDIX 1

Children’s Health and the Environment: 

Public Health Issues and Challenges for Risk Assessment* 

Philip J. Landrigan, Mt. Sinai School of Medicine 

Carole A. Kimmel, EPA/ORD National Center for Environmental Assessment 

Adolfo Correa, CDC National Center on Birth Defects and Disabilities 

Brenda Eskenazi, University of California, Berkeley 

1.0 Introduction

The protection of human health against disease and injury caused by toxic chemicals in 

the environment is the ultimate goal of risk assessment and risk management. Historically, risk 

assessment focused on adult exposures and toxicities and gave little consideration to vulnerable 

life stages, such as fetal development and early childhood. An emphasis on adult cancer risk and 

the evolution of methodologies for estimating cancer risks that are different from the methods 

used to assess other health effects tended to further diminish the importance for risk assessment 

of children’s exposures and outcomes. In addition, the use of default factors based on the 

healthy young adult did not account adequately for the unique exposures and sensitivities of 

fetuses, infants and children (Landrigan and Carlson 1995). 

In the past decade, stimulated especially by the 1993 National Research Council report on 

Pesticides in the Diets of Infants and Children (National Academy of Sciences 1993), 

recognition has grown that children are a group within the population who have unique 

exposures and special vulnerabilities to chemical toxicants. It is now understood that children 

require an approach to risk assessment that considers their particular characteristics. The present 

report developed by the International Life Sciences Institute (ILSI) with support from the U.S. 

Environmental Protection Agency (EPA) is intended to consider the elements and structure of a 

child-protective approach to risk assessment. 

The purpose of this introductory chapter is to establish the context for a discussion on 

child-protective risk assessment by: 1) summarizing information on the vulnerability of children 

to chemicals in the environment; 2) presenting a rationale, based on considerations of public 

health and preventive medicine, for developing an approach to risk assessment that considers the 

unique exposures and special sensitivities of infants and children; and 3) outlining issues of 

importance for assessing environmental health risks to children. 

 The word “children” is used throughout this paper to include all stages of development 

(fetuses, infants and children) from conception to age 21 years. 

_________________________ 

* Manuscript submitted for publication in Environmental Health Perspectives November 20, 2002. 
74 

2.0 The Historical Development of Child-Protective Risk Assessment

The NRC Report on Pesticides in the Diets of Infants and Children

The publication in 1993 of the National Research Council’s (NRC) report on Pesticides 

in the Diets of Infants and Children (National Academy of Sciences 1993) was a critical event in 

raising awareness of the importance for risk assessment of children’s environmental health. This 

report elevated concern on a broad national level about children’s special vulnerabilities to 

environmental agents. It made clear that protection of the health of vulnerable populations 

would require a new approach to risk assessment. 

The NRC report recommended an approach to risk assessment that moved beyond 

consideration of “average” exposures based primarily on adult characteristics to one that 

accounted for the heterogeneity of exposures and for potential differential sensitivities at various 

life stages, particularly during prenatal development, infancy and childhood. The NRC report 

built on guidelines that EPA had published for developmental toxicity risk assessment in 1986 

and revised in 1991 (U.S. Environmental Protection Agency 1986, 1991). It built also on other 

published documents such as the International Life Sciences Institute’s (ILSI) report on 

Similarities and Differences between Children and Adults: Implications for Risk Assessment

(Spielberg 1992). 

Infants and children were identified in both the NRC and ILSI reports as groups within 

the population who require special consideration in risk assessment because of: 1) their unique 

patterns of exposures to environmental hazards, and 2) special vulnerabilities (the word 

“children” is used throughout this paper to include all stages of development (fetuses, infants and 

children) from conception to age 21 years). The NRC report noted that “children are not little 

adults.” It called for the development of new risk assessment methods that would incorporate 

better data on children’s exposures to chemicals, along with improved information on the 

potentially harmful effects of chemicals during fetal development, infancy and childhood 

(National Academy of Sciences 1993). 

To “provide a more complete characterization of risk”, the NRC Committee 

recommended use of exposure distributions rather than point estimates. It noted that levels of 

exposure could differ by several orders of magnitude between children and adults. The NRC 

report recommended also that exposure assessment methods be expanded to consider exposures 

to multiple chemicals with multiple routes of exposure (National Academy of Sciences 1993). 

To enhance characterization of the susceptibility of children, the NRC Committee 

recommended the development of physiologically based pharmacokinetic models that could 

describe the unique features of young, developing humans. To assess the long-term and delayed 

effects of early exposures, the Committee recommended that “it would be desirable to develop 

bioassay protocols that provide direct information on the relative contribution of exposures at 

different ages to lifetime risks.” The Committee called for further development of “appropriate 

toxicological tests for perinatal and childhood toxicity” to address issues not addressed in current 

testing guideline protocols (National Academy of Sciences 1993). 
75 

The NRC Committee concluded that “in the absence of data to the contrary, there should 

be a presumption of greater risk to infants and children. To validate this presumption, the 

Committee recommended that “the sensitivity of mature and immature individuals should be 

studied systematically to expand the current limited database as to relative sensitivity.” 

To provide enhanced protection to children during vulnerable periods of early 

development, the NRC Committee recommended that a child-protective uncertainty factor of up 

to 10-fold be considered in risk assessment “when there is evidence of developmental toxicity 

and when data from toxicity testing relative to children are incomplete.” The Committee noted 

that it had long been standard practice at EPA and FDA to divide the no-observed-effect-level 

(NOEL) obtained in animal test results by an uncertainty factor of 100-fold in establishing a 

reference dose (RfD) for toxic effects other than cancer or heritable mutation. The Committee 

noted that this 100-fold factor is comprised of two separate factors of 10-fold each: one allows 

for uncertainty in extrapolating data from animals to humans; the second accommodates 

variation within the human population. The Committee acknowledged that “this latter 

uncertainty factor generally provides adequate protection for infants and children.” 

Nevertheless, the Committee expressed concern that the standard 100-fold safety factor 

may not always be sufficient to account for unique susceptibilities at particularly sensitive stages 

of early development. The Committee was also concerned about the great gaps that currently 

exist in developmental toxicity testing data for many chemicals. It was for these reasons that the 

NRC Committee recommended consideration of a third, child-protective uncertainty factor in 

risk assessment. 

Food Quality Protection Act of 1996 

Following publication of the NRC report, the concept that children should be considered 

more vulnerable to pesticides than adults in the absence of evidence to the contrary was adopted 

by Congress. In 1996, by unanimous vote of both Houses, the concept was incorporated into the 

Food Quality Protection Act (FQPA) (Food Quality Protection Act of 1996), the principal federal 

statute governing the use of pesticides in agriculture (Table A1-1). 

FQPA incorporates most of the recommendations of the NRC report. It requires that 

standards for agricultural pesticides be set at levels sufficiently strict to protect the health of 

infants and children. It directs EPA to use an additional tenfold margin of safety in assessing the 

risks to infants and children to take into account the potential for pre- and postnatal toxicity, 

particularly when the toxicology and exposure databases are judged to be incomplete. The 

statute authorizes EPA to replace this default 10X “FQPA safety factor” with a different factor 

only if, based on reliable data, the resulting margin would be adequate to protect infants and 

children. The requirement for the FQPA safety factor was intended by the Congress to be a 

stimulus to the generation of data on developmental toxicology and on early life exposures.

Recently, the EPA published its final guidance on the application of the additional FQPA 

safety factor in risk assessment (U.S. Environmental Protection Agency 2002a). The Agency 

will apply the additional factor at the beginning of the risk assessment process, but recognizes 

that the intent of the FQPA safety factor overlaps with several uncertainty factors traditionally 

used to account for data gaps and concerns in the risk assessment process. These include default 
76 

10-fold factors to account for: 1) the lack of a NOAEL (LOAEL to NOAEL factor); 2) the lack 

of chronic data for setting the chronic Reference Doses (RfDs) and Reference Concentrations 

(RfCs) (subchronic to chronic factor); and 3) inadequacies or gaps in the database considered 

minimal for setting RfDs/RfCs (database factor). In most cases, EPA is of the opinion that these 

factors will be sufficient to account for the concerns related to children’s health. If, however, the 

adequacy and appropriateness of the toxicity assessment or the exposure assessment are judged 

to be insufficient, a child-protective factor will be applied to the RfD, and the resulting FQPAadjusted RfD is termed the population-adjusted dose (PAD). 

According to a report issued by Consumers Union in February 2000, EPA had applied 

(an) additional safety factor(s) for 104 out of 273 (38%) pesticides when evaluating acute and/or 

chronic exposure between August 1996 and early 2001. For the organophosphorous insecticides, 

one of EPA’s high priority categories for review, (an) additional safety factor(s) was/were 

applied for 26 of 49 substances in the evaluation of acute and/or chronic exposures (Consumers 

Union of the United States Inc. 2001). 

Table A1-1. Major provisions of the Food Quality Protection Act of 1996 

Requires that standards for pesticide residues in food be health-based. Standards must 

be set at levels that ensure a “reasonable certainty of no harm.” 

Exposure and vulnerabilities of infants and children must be specifically considered in 

establishing pesticide residue standards. 

When insufficient data exist to assess the special exposures and/or vulnerabilities of 

infants and children, an additional tenfold safety factor must be considered in setting 

standards. 

Consideration of the potential benefits of pesticides must be limited. 

All pesticide standards must be reviewed every ten years. 

Endocrine effects of pesticides must be systematically evaluated in toxicity testing. 

3.0 Children's Unique Vulnerability to Toxicants in the Environment

The detailed analysis of children’s exposures to environmental chemicals undertaken by 

the NRC established that children's heightened vulnerability to chemicals arises from four 

sources (National Academy of Sciences 1993): 
77 

Children have disproportionately heavy exposures to environmental agents 

Children drink more water, eat more food and breathe more air pound-for-pound of body 

weight compared to adults. For example, children in the first six months of life drink seven times 

as much water, while children ages one through five years eat three to four times more food on a 

body weight basis than the average adult. The air intake of a resting infant is twice that of an 

adult. The implication of these findings for health is that children will have substantially heavier 

exposures than adults to any environmental contaminants present in water, food and air (National 

Academy of Sciences 1993). Two additional characteristics of children further magnify their 

exposures: 1) their hand-to-mouth behavior; and 2) their play close to the ground. 

Children's metabolic pathways, especially in fetal life and in the first months after birth, are 

immature 

Children's ability to metabolize, detoxify, and excrete environmental agents is different 

from that of adults. In some instances, children are better able than adults to deal with 

environmental agents (National Academy of Sciences 1993; Spielberg 1992), because they 

cannot make the active metabolite required for toxicity. In other instances, children are less well 

able to deal with toxic chemicals and thus are more vulnerable to them (National Academy of 

Sciences 1993; Spielberg 1992). Differences in metabolism exist also between prenatal and 

post-natal life, and may vary over the course of pregnancy. An additional source of vulnerability 

in fetuses and young children is that the blood- brain barrier is not fully developed and therefore 

xenobiotics may be more easily able to enter the central nervous system (Rodier 1995).

Developmental processes are easily disrupted during rapid growth and development before and 

after birth 

Rapid growth and development occur during embryonic and fetal life as well as in the 

first years after birth. In the brain for example, billions of cells must form, move to their 

assigned positions, and establish precise connections with other cells (Rodier 1995). 

Development of the endocrine and reproductive organs is guided by a complex and precisely 

timed sequence of chemical messages. If cells in an infant's brain are destroyed by chemicals, if 

connections between neurons fail to form, or if false signals are sent to the developing 

reproductive organs by endocrine disruptors, there is the possibility that neurological or 

reproductive dysfunction will result (Bellinger et al, 1987; Needleman et al, 1990; Jacobson and 

Jacobson 1996). Because children have more future years of life than most adults, they have 

more time to develop chronic diseases that may be triggered by early exposures. Many diseases 

that are caused by toxic agents in the environment require decades to develop. 

Many of those diseases, including cancer and neurodegenerative diseases, are thought to 

arise through a series of changes within cells that require many years to evolve from earliest 

initiation to actual manifestation of illness. Exposures to environmental agents early in life, 

including prenatal exposures, appear more likely to produce chronic disease than similar 

exposures encountered later (Gray et al, 1991; Ekbom et al, 1997). Thus, there are likely to be 

critical windows of exposure even for these chronic diseases that need to be further explored. 
78 

4.0 Changing Risks at Different Ages

Children are exposed to toxic agents through the air they breathe, the water they drink, 

the foods they eat, the medications they consume, and through the wide variety of environments 

they inhabit, including their homes, day-care centers, schools, and motor vehicles. Children have 

unique routes of exposure that have no parallel among adults, and the routes of exposure and the 

resulting risks to health differ in different stages of childhood. Contact with toxic agents can 

occur in utero through trans-placental transfer of chemicals from mother to fetus; it can occur via 

breast milk in nursing infants; and it can occur in early childhood via hand-to-mouth transfer of 

toxic chemicals. Analysis of children’s varying patterns and pathways of exposure to 

environmental agents and the resulting health effects at various stages of development is an 

essential prerequisite to formulation of a child-protective approach to risk assessment. 

Examples of Unique Vulnerability in Pregnancy

In pregnancy, especially in the first trimester, maternal use of certain medications has 

been linked to a number of adverse effects in humans. The first of these outcomes to be 

recognized were anatomical birth defects. Examples of these unique risks include the following: 

• In 1961, a sudden increase in the frequency of limb reduction defects, phocomelia in 

particular, in West Germany and Australia was associated with maternal prenatal use of the 

sedative-hypnotic thalidomide (Lenz 1962; Taussig 1962; McBride 1961). Removal of the 

drug from the market led to a substantial decrease in the frequency of limb reduction defects. 

• In utero exposure to aminopterin, an antagonist of folic acid, has been associated with 

anencephaly, meningocele, hydrocephalus, and cleft lip and palate (Thiersch 1952; Warkany 

et al, 1959). 

• In utero exposure to the anticonvulsant diphenylhydantoin has been associated with a broad 

spectrum of abnormalities, including orofacial clefts, nail and digital hypoplasia, growth 

abnormalities, and mental deficiency (Speidel and Meadow 1972; Fedrick 1973; Monson et 

al, 1973). 

• In utero exposure to valproic acid, another anticonvulsant, has been associated with neural 

tube defects and heart, craniofacial, and limb anomalies (Kallen et al, 1989). 

• In utero exposure to the anticoagulant coumadin has been associated with hypoplasia of the 

nasal cartilage, chondrodysplasias, and atrophy of the optic nerves (Warkany 1976). 

• In utero exposure to isotretinoin (13-cis-retinoic acid), an analogue of vitamin A used to treat 

cystic acne, associated with a characteristic pattern of malformations, including abnormal ear 

development, a flat nasal bridge, mandibular hypoplasia, cleft palate, hydrocephalus, neural 

tube defects, and conotruncal heart defects (Lammer 1985). 

• Administration of diethylstilbestrol (DES) to pregnant women to prevent miscarriage has 

been linked to various genital abnormalities in their offspring (Bongiovanni et al, 1959; Gill 
79 

et al, 1959) as well as to the development of adenocarcinoma of the vagina of their daughters 

in their late teens and early twenties (Herbst et al, 1971; O’Brien et al, 1979). 

Many teratogenic agents were also found to cause neurological and other functional 

disorders in children that are not necessarily associated with gross structural alterations. For 

example, isotretinoin causes profound mental retardation in many children, and about half of 

those children do not have any major malformations (Adams and Lammer 1993, 1995). DES 

causes a variety of reproductive problems in addition to vaginal adenocarcinoma, including 

infertility and poor pregnancy outcomes (Kaufman et al, 2000). Most recently, exposures to 

thalidomide and valproic acid in early pregnancy have been linked to autism (Stroomland et al, 

1994; Moore et al, 2000). 

Examples of Unique Risks in Early Childhood

Heavy Metals. Despite a decline in exposures over the past two decades (Brody et al, 

1994; American Academy of Pediatrics Committee on Environmental Health 1998), resulting 

principally from removal of lead from gasoline, lead exposure continues to occur in utero and 

among preschool-aged children in the United States. A national survey (conducted from October 

1991 to September 1994) indicated that an estimated 940,000 preschool children had blood lead 

levels above the Centers for Disease Control and Prevention (CDC) intervention level of 10 

μg/dL; nearly 275,000 had blood lead levels greater than 15 μg/dL and nearly 85,000 had greater 

than twice the CDC intervention level (20 μg/dL) (Centers for Disease Control and Prevention 

1997). Today, lead-based paint in older homes is the most common source of lead exposure in 

children (Centers for Disease Control and Prevention 1997). Exposures can occur through 

ingestion of paint chips or dust from deteriorating surfaces, from chewing on painted cribs, or 

through inhalation of lead paint dust, as can occur during home renovation. Children exposed 

prenatally to blood lead levels as low as 10 μg/dl, and possibly even as low as 5 μg/dl, (Lanphear 

et al, 2000) may have delayed early mental development (Bellinger et al, 1987). Further, 

chronic low-level lead exposure during childhood may result in a decreased IQ, reading and 

learning disabilities, attention deficits, and persistent behavioral problems (Needleman et al, 

1990, 1996). The fact that toddlers are the age-group at highest risk of lead poisoning is a direct 

consequence of their unique hand-to-mouth behavior coupled with the fact that their brains are 

still undergoing growth, development, myelination and differentiation. 

Prenatal exposure to methyl mercury has been shown to have adverse effects on 

neurodevelopment. This is an age-specific, unique risk that results from transplacental transfer 

of mercury from maternal blood to the fetal brain. Mercury deposited from the atmosphere into 

lakes, streams and oceans can be converted into organic mercury compounds, which accumulate 

in fish and biomagnify as they move up the food chain to reach highest levels in top predator fish 

species and marine mammals. These compounds are lipid-soluble and are well absorbed from 

the gastrointestinal tract (Clarkson 1972). Methyl mercury crosses the placenta and is excreted 

in breast milk. Consumption of fish with high levels of methyl mercury by pregnant women in 

Minamata Bay, Japan, in the 1950s was associated with cerebral palsy in their offspring (Harada 

1968). When people in Iraq consumed grain treated with a methyl mercury fungicide between 

1959 and 1972, thousands were poisoned (Bakir et al, 1973). In both of those episodes, mothers 

who were asymptomatic or showed mild toxic effects gave birth to infants who appeared normal 

at birth, but in whom psychomotor retardation, blindness, deafness, and seizures developed over 
80 

time (Amin-Zaki et al, 1979). To further assess the susceptibility of the fetus and infants to the 

neurotoxic effects of methyl mercury, longitudinal studies are being conducted to evaluate the 

long-term subclinical effects among children whose mothers’ diets include large amounts of fish 

or marine mammals containing methyl mercury (Davidson et al, 1998; Grandjean et al, 1997, 

1997; Crump et al, 1998). A report from the National Academy of Sciences based on an analysis 

of three prospective studies of the children of fish-eating woman has concluded that low-level 

exposures to methyl mercury in utero can have adverse effects on neurobehavioral development 

(National Academy of Sciences 2000). EPA has issued a new risk assessment for methyl 

mercury, setting the chronic oral RfD at 0. 01 ug/kg/day (U.S. Environmental Protection Agency 

2001). This RfD is based on the finding of developmental neuropsychological impairments in 

children from the Faroe Island epidemiology study (Grandjean et al, 1997, 1997) and on 

supporting data from the New Zealand study (Crump et al, 1998). 

Environmental Tobacco Smoke

Environmental tobacco smoke (ETS) is a complex mixture of chemicals generated during 

the burning and smoking of tobacco products. Chemicals present in ETS include irritants and 

systemic toxicants such as hydrogen cyanide and sulfur dioxide, mutagens and carcinogens such 

as benzo(a)pyrene, formaldehyde and 4-aminobiphenyl, and the reproductive toxicants nicotine, 

cadmium, and carbon monoxide (Jenkins et al, 1992). Of children in the United States 11 years 

and younger, forty-three percent live in a home with at least one person who smokes (Pirkle et al, 

1996). Exposures of children to ETS produce a range of effects, some of which are unique to 

early life and others that are analogous to the effects produced by ETS in adults. 

Prenatal exposure to ETS affects fetal growth and is associated with a 20-40% elevated 

risk of low birth weight or “small for gestational age” (California Environmental Protection 

Agency 1997; Haddow et al, 1988; Eskenazi et al, 1995; World Health Organization 1999). The 

primary effect observed, reduction in mean low birth weight, is small in magnitude (25-50 

grams). However, if the distribution of birth weight in a population of babies is shifted 

downward by ETS exposure, infants who are already compromised may be pushed into higher 

risk categories. Since low birth weight is associated with increased infant morbidity and 

mortality, exposure to ETS is likely to augment such burden. Exposure to ETS during infancy 

has been associated with an increased risk of sudden infant death that is independent of low birth 

weight or prematurity (Taylor and Sanderson 1995; Klonoff-Cohen et al, 1995). 

In children, ETS exposure affects the upper and the lower respiratory tract. Infants and 

young children are at particular risk of exposure to ETS because of the small diameter of their 

airways, and because pound-for-pound they breathe more air than adults. Infants who are 

exposed to ETS in their home environment have a 1.5 to 3-fold increased risk of lower 

respiratory infection compared to unexposed children (Hall et al, 1984; McConnochie and 

Roghmann 1986; Ogston et al, 1987). The risk of lower respiratory infection associated with 

ETS is highest among infants under 3 months of age (Wright et al, 1991). Children whose 

parents smoke also are more likely to develop middle ear effusion as measured by tympanometry 

(Reed and Lutz 1988; Strachan et al, 1989) and chronic respiratory problems (cough, phlegm, or 

wheezing) (Mannino et al, 1996). Children of parents who smoke are more likely to develop 

asthma, and those with asthma are more likely to experience more severe disease (Weitzman et 

al, 1990; Martinez et al, 1992; Chilmonczyk et al, 1993). Childhood exposure to ETS affects 
81 

lung growth and development, as measured by small but significant decrements in pulmonary 

function tests (Lebowitz et al, 1992; Wang et al, 1994; Cullinan and Taylor 1994; Cunningham 

et al, 1994). Since early lung development is important in terms of future respiratory health, 

these results suggest that ETS may have adverse long-term effects on children’s respiratory 

health that warrant further investigation through longitudinal studies. 

Air Pollutants

Various indoor air pollutants are associated with respiratory disorders in children, and 

these pollutants include particles, gases, vapors, allergens and molds (Spengler 1991; U.S. 

Environmental Protection Agency 1994; Lambert and Samet 1996). In the home, common 

sources of air pollutants, other than tobacco smoking, include gas and wood stoves, and 

furnishings and construction materials that release organic gases and vapors, some of which 

contain formaldehyde. Combustion of natural gas results in the emission of nitrogen dioxide 

(NO2) and carbon monoxide (CO). Cooking or heating with wood results in the emission of 

several substances in the form of liquid (suspended droplets), solids (suspended particles), and 

gases such as NO2 and sulfur dioxide (SO2) (Lambert and Samet 1996). The aerosol mixture of 

very fine solid and liquid particles or “smoke” contains particles in the inhalable range (i.e., < 10 

μm in aerodynamic diameter, PM10). Exposure to CO disrupts oxygen transport, may result in 

symptoms that mimic influenza, including fatigue, headache, dizziness, nausea and vomiting, 

cognitive impairment, and tachycardia (Lambert and Samet 1996). Exposure to high levels of 

NO2 and SO2 may result in acute mucocutaneous irritation and respiratory effects, and chronic 

exposure to relatively lower levels has been linked to asthma and respiratory irritation (Speizer et 

al, 1980; Morrow 1984; Neas et al, 1991). Exposure to particles in wood smoke may result in 

irritation and inflammation of the respiratory tract, manifested as rhinitis, cough, wheezing, and 

worsening of asthma (Morris et al, 1990; Robin et al, 1996). Whether adults exposed to similar 

levels of wood smoke have a different probability of severe lower respiratory illnesses is unclear. 

Indoor environments also contain a number of aeroallergens that trigger asthma episodes 

in children, including allergens of house dust mites, cats, and cockroaches (Cullinan and Taylor 

1994). In urban dwellings, house dust mite and cockroach allergens are important in both onset 

and worsening of asthma symptoms (Cullinan and Taylor 1994). Cockroach allergens have been 

found to be common in the homes of inner city children with asthma (Call et al, 1992; 

Rosenstreich et al, 1997). Recent clinical and epidemiologic studies indicate that exposure to 

molds or dampness may increase the risk of respiratory symptoms among children (Delfino et al, 

1997; Dales et al, 1991; VerHoeff et al, 1995). Whether the increasing prevalence of asthma 

morbidity in children in recent years is due to an increased prevalence of exposure to 

aeroallergens, molds or dampness or to a combination of those indoor factors with changing 

patterns of ambient air pollution is unclear. 

Ambient air pollution levels have also been shown to be associated with asthma and other 

respiratory morbidity in children. Daily fluctuations in PM10 have been associated with 

increased emergency room visits for asthma (Schwartz et al, 1993; Rennick and Jarman 1992), 

hospital admissions for asthma (Montealegre et al, 1993), decrements in peak flow rates in 

normal children (Neas et al, 1995), increased respiratory symptoms (Forsbert et al, 1993), and 

increased medication use in children with asthma (Pope 1991). Ozone has been of particular 

concern since it provokes airway inflammation at very low levels (Aris et al, 1993). In addition, 
82 

ozone increases airway reactivity (Hortsman et al, 1990; Kreit et al, 1989), and there is evidence 

to suggest that ozone may potentate the effects of allergens (Molfino et al, 1991). Ozone levels 

have been related to increases in asthma admissions and emergency room visits among children 

in Atlanta (White et al, 1994), New Jersey (Cody et al, 1992), and Mexico City (Romieu et al, 

1995). A study of the effect of increases in ambient ozone levels on summer day camp children 

and workers in New Jersey suggests that children may be more sensitive than adults to increases 

in ambient ozone levels (Cody et al, 1992). In this study, increases in ozone concentrations above 

120 ppb were associated with an increase in respiratory symptoms and a decline in peak 

expiratory flow rate among children, but with no change in respiratory symptoms or pulmonary 

function among camp workers. Further studies are needed to determine whether the risk of 

respiratory effects from ambient air pollutants differs between children and adults and, if so, 

whether such differences reflect differences in exposure or in sensitivity. 

The potential impact of air pollution on asthma morbidity in a community is exemplified 

by a report from CDC that examined hospitalizations for asthma in the Atlanta metropolitan area 

during the summer of 1996 in the weeks before, during and after the summer Olympic Games. 

This analysis found that the rate of asthma hospitalization declined during the two weeks of the 

Games, when citizens of Atlanta heeded an advisory from the Mayor to improve air quality by 

not driving and by instead using public transportation. After the Games ended, motor vehicle 

traffic increased, air quality declined, and asthma hospitalization rates rebounded (Friedman et 

al, 2001). 

5.0 Diseases in Children of Known or Suspected Environmental Origin

Patterns of disease among children in the industrially developed nations today are quite 

different from those of generations past (Haggerty and Rothman 1975). The traditional infectious 

diseases have largely been controlled: smallpox is eradicated, polio is nearly gone, measles is 

under control, diphtheria and tetanus are rarities, and cholera has virtually disappeared. The 

expected life span of a baby born in the United States today is more than two decades longer than 

that of an infant born at the beginning of the twentieth century (Haggerty and Rothman 1975). 

Similar “epidemiologic transitions” from infectious to non-infectious diseases are occurring at 

various rates today in many nations around the world as those countries strengthen public health 

programs, control the classic infectious diseases and move towards industrial development. In 

other countries, sadly, the increasing incidence of HIV positivity and of AIDS threatens to undo 

those gains. 

Children today are at risk of disease caused by environmental hazards not encountered by 

previous generations. Over 85,000 synthetic chemical compounds, most of which have been 

developed since World War II, are now registered for commercial use in EPA’s Toxic 

Substances Control Act (TSCA) inventory. There are currently 2,800 chemicals produced in 

quantities of one million pounds or more per year (Goldman and Koduru 2000). These highproduction-volume (HPV) chemicals are those with the potential to be most widely used in foods 

and consumer products and to be most widely disseminated in the environment. Fewer than half 

of these HPV chemicals have been tested for their potential toxicity to humans, and fewer still 

for their toxicity to children (Goldman and Koduru 2000; National Academy of Sciences 1984). 
83 

Thus, their potential hazards to children's health and development are largely unknown (Schaefer 

1994). 

Diseases of great importance to children in America today that are thought, or at least 

suspected, to be caused or aggravated by chemicals in the environment include the following: 

Asthma 

A recent study by the National Center for Health for Health Statistics (NCHS) that 

surveyed data on self-reports of asthma, physician’s office visits for asthma, emergency room 

visits for asthma, and hospitalizations for asthma, provides evidence for increases in prevalence 

of asthma in the United States during the past 15 years, particularly among children (Centers for 

Disease Control and Prevention 1995a). Estimates of the average rates of asthma prevalence 

increased over time across all age groups. Asthma mortality also increased. Children 

experienced some of the higher rates of morbidity as measured by self-reports, office visits, 

emergency department visits and hospitalizations for asthma. These increases were particularly 

evident in urban localities. In New York and in other major cities, asthma has become the 

leading cause of admission of children to hospitals and the leading cause of school absenteeism 

(Centers for Disease Control and Prevention 1995b). The increasing prevalence of asthma and 

the higher asthma morbidity among children in the United States, albeit still unexplained, 

suggest that, compared to adults, children are more likely to develop asthma and asthma 

exacerbations and/or be exposed to environmental or other factors that cause or trigger asthma 

episodes. 

Childhood Cancer 

The reported incidence of childhood cancer has increased substantially in the United 

States in the past two decades (Devesa et al, 1995). Although death rates are down, as a 

consequence of early detection and vastly improved treatment, data from the National Cancer 

Institute show that the reported incidence of acute lymphoblastic leukemia (ALL) increased by 

27.4% from 1973 to 1990, from 2.8 cases per 100,000 children to 3.5 per 100,000. Since 1990, 

ALL incidence has declined in boys, but continues to rise in girls (Robison et al, 1995). From 

1973-1994, the incidence of brain cancer increased by 39.6%, with nearly equal increases in 

boys and girls (Schechter 1999). In young men, 20-39 years of age, the incidence of testicular 

cancer in the years 1973-1994 increased by 68% (Devesa et al, 1995; Robison 1995). 

Neurodevelopmental Disorders 

Neurodevelopmental disorders, including learning disabilities, dyslexia, mental 

retardation, attention deficit disorder and autism are widespread and affect 5-10% of the four 

million babies born in the United States each year. Some clinical investigators have reported that 

prevalence is increasing, but existing data are not of sufficient quality to either sustain or refute 

that position (American Academy of Pediatrics 2001). Causes are largely unknown, but in utero

and early life exposures to lead (Bellinger et al, 1987; Needleman et al, 1990), mercury (National 

Academy of Sciences 2000), PCBs (Jacobson and Jacobson 1996), certain pesticides (Eskenazi 

et al, 1999; Wiles and Campbell 1993; Whitney et al, 1995; Campbell et al, 1997) and other 

environmental neurotoxicants are known or thought to contribute (National Academy of 
84 

Sciences 1992). A recent report from the National Research Council concluded that 3% of 

developmental disabilities are the direct consequence of neurotoxic environmental exposures and 

that another 25% arise out of the interplay of environmental factors and individual genetic 

susceptibility (environment was defined broadly in this report and included diet, alcohol, tobacco 

and other “life-style” factors, as well as toxic chemicals) (National Research Council 2000). 

Endocrine Disruption 

Endocrine disruptors are chemicals that have the capacity to interfere with the body's 

hormonal signaling system (Harrison 2001; Longnecker et al, 1997). Effects of these chemicals, 

which have been hypothesized to include cancer (Kogevinas et al, 1997), decreased fertility 

(Longnecker et al, 2002), birth defects of the reproductive organs (Peterson et al, 1993; Paulozzi 

et al, 1997), altered sex ratios (Mocarelli et al, 2000), neurodevelopmental impairment (Jacobson 

and Jacobson 1996), thyroid dysfunction and diabetes (Longnecker and Daniels 2001) have been 

observed in cell systems in vitro (Birnbaum 1994), in experimental animals exposed to specific 

chemicals in the laboratory (Gary et al, 2000), and in wildlife populations in several broadly 

contaminated ecosystems such as the Great Lakes (Colborn et al, 1996) and Lake Apopka in 

Florida (Guillette et al, 1994). 

Evidence for the effects of endocrine disruptors on human health is less abundant than for 

evidence of effects in wildlife or in vitro, but data are accumulating. There are some data linking 

precocious puberty with PBB among girls exposed during gestation and breast-feeding (Blanck 

et al, 2000a, 2000b). Based on animal studies, the embryo, fetus, and neonate, and the prepubertal period would appear to be at particularly high risk of adverse consequences following 

exposure to endocrine disruptors (Selevan et al, 2000). Early exposures to these compounds 

have the potential to alter anatomic structures and may influence the subsequent course of 

endocrine functioning (Longnecker and Danels 2001), neurological development (Jacobson and 

Jacobson 1996) and sexual development (Longnecker and Daniels 2001; Birnbaum 1994; Gray 

et al, 2000; Colborn et al, 1996; Guillette et al, 1994; Blanck et al, 2000a, 2000b; Selevan et al, 

2000; Euling and Kimmel 2001). Through the Food Quality Protection Act of 1996, Congress 

has mandated more extensive screening of chemical compounds to assess their potential for 

endocrine disruption (Food Quality Protection Act of 1996). 

6.0 A Framework for Assessing Environmental Health Risks to Children

Protection of children’s health against the adverse effects of exposures to toxic chemicals 

will require a modified approach to risk assessment that moves beyond the use of “average” 

levels of exposure and risk and that goes beyond consideration of the 60- or 70-kilogram adult 

(U.S. Environmental Protection Agency 1999, 2002b). To account for the unique exposures and 

special vulnerabilities of infants and children (National Academy of Sciences 1993), this new 

approach will need to include the following elements: 
85 

1. Improved Exposure Assessment

Additional data are needed on children’s patterns and levels of exposures to chemicals in 

the environment. Because exposures vary by age, this information will need to be 

collected within different age groups. 

All sources of exposure need to be considered in evaluating the potential risks of 

environmental chemicals to infants and children (National Academy of Sciences 1993). 

Models need to be developed that can account for children’s simultaneous exposures to 

multiple chemicals of differing potency via multiple routes of exposure. These models 

also need to be able to assess the cumulative effects of chemicals that may have either 

synergistic or antagonistic actions. 

Exposure estimates need to be constructed differently depending on whether acute or 

chronic effects are of concern. The incorporation of biomarkers into data collection may 

be useful. 

Most importantly, it is essential to examine the full distribution of children’s exposures to 

chemicals in the environment. Point estimates of average are no longer sufficient. 

Appropriate mathematical models must be constructed, such as Monte Carlo models, that 

can permit the combining of various data sets and thus permit examination of full 

exposure distributions (National Academy of Sciences 1993). 

2. Enhanced Toxicity Testing

New, more sensitive approaches to chemical toxicity testing are needed that can reliably 

detect the unanticipated developmental consequences of exposures during critical 

windows of prenatal and postnatal vulnerability (Selevan et al, 2000) on systems that 

have not been adequately or thoroughly addressed in the past, e.g., the nervous, immune, 

respiratory reproductive, cardiovascular, and endocrine systems. 

Extensive past experience has demonstrated that infants and young children are uniquely 

vulnerable to certain chemicals that are relatively innocuous to adults. To detect 

unanticipated consequences of early exposures to chemicals, it will be necessary to 

develop new approaches to assay prenatal, perinatal and childhood toxicity (U.S. 

Environmental Protection Agency 1986, 1991) and to apply these approaches widely. 

For certain classes of chemicals it may be necessary to undertake studies in which 

chemicals are administered to experimental animals either in utero or shortly after birth 

and the subjects then followed over their entire lifespan. For other classes of compounds, 

it may be necessary to expose animals throughout the life span. The approach should 

attempt to replicate the human experience and may enhance detection of delayed effects 

(National Academy of Sciences 1993). 

Improved functional tests of neurobehavioral, immune, endocrine and reproductive 

toxicity are of great importance for detecting outcomes other than anatomic 

malformations (U.S. Environmental Protection Agency 1986, 1991). These functional 

assessments need to be applied on a more routine basis, especially when data from other 
86 

studies, e.g., adult target organ toxicity, or multigeneration studies, raise concerns about 

possible effects on functional systems in children that have not been thoroughly evaluated 

in more routine testing protocols. 

3. New Toxicodynamic and Toxicokinetic Models

The physiological and biochemical characteristics of children that influence metabolism 

and disposition of chemicals at different stages of development need to be considered in 

risk assessment. Physiological parameters such as tissue growth rates and biochemical 

parameters such as enzyme induction may differentially affect the responses of infants 

and children to environmental chemicals at different developmental stages (Ginsberg et 

al, 2002; Cresteil 1998). 

Pharmacokinetic models that account for the unique physiologic characteristics of 

infants, children and adolescents need to be developed. Physiologically-based 

pharmacokinetic models can be used to estimate the dose of toxic metabolites reaching 

target tissues at different developmental stages (O’Flaherty 1997; Welsch et al, 1995). 

4. An Outcomes-Oriented Approach to Hazard Assessment

The pathogenic mechanisms of environmentally-induced disease in children need to be 

elucidated at functional, organ, cellular, and molecular levels (Whitney et al, 1995; 

Campbell et al, 1997; Birnbaum 1994). These assessments could be undertaken in 

conjunction with toxicity testing of chemicals and also in the context of epidemiologic 

studies. Clinical and epidemiologic studies offer an excellent vehicle for studying 

etiologic associations between environmental exposures and pediatric disease (Bellinger et 

al, 1987; Needleman et al, 1990; Jacobson and Jacobson 1996). A strong case can be 

made for the need to establish a major multi-year prospective epidemiological study of 

children’s health in relation to environmental exposures as a means of identifying and 

characterizing the consequences of multiple, early, low-level exposures, as called for in 

the Children’s Health Act, 2000 (Berkowtiz et al, 2001). 

5. Application of Uncertainty and Safety Factors that Specifically Consider Children’s 

Risks

Children must be presumed to be more vulnerable to environmental toxic agents than 

adults in the absence of data to the contrary, as was specifically recommended by the 

NRC Committee on Pesticides in the Diets of Infants and Children (National Academy of 

Sciences 1993). That Committee called for the incorporation of an additional childprotective uncertainty factor into risk assessment to account for this greater vulnerability, 

particularly in the absence of data, and the Food Quality Protection Act mandated the 

application of an additional margin of safety for children’s risk in the case of pesticides 

(Food Quality Protection Act of 1996). Traditional approaches to risk assessment are 

now being modified to more carefully and explicitly account for risks to children (U.S. 

Environmental Protection Agency 2002a). However, a number of data gaps in exposure 

assessment and in developmental toxicity must be addressed through the development 

and implementation of additional testing guideline protocols (U.S. Environmental 
87 

Protection Agency 1999, 2002b), the acquisition of better information on children’s 

exposure patterns and sources, and the undertaking of basic research both on mechanisms 

of underlying development and on chemical interactions of environmental agents with 

developing organ systems (National Research Council 2000). 

7.0 Conclusion

The protection of children against toxic chemicals in the environment is a major 

challenge to modern society (Schaefer 1994). Children are not merely a “special” vulnerable 

group within our population, but rather they are the current inhabitants of a developmental stage 

through which all future generations must pass. Protection of the health of fetuses, infants and 

children is essential for sustainability of the human species. 

The current challenge to risk assessment stems from two facts: 1) that hundreds of new 

chemicals are developed every year, and released in varying quantities into the environment; and 

2) that the majority of these chemicals are not adequately evaluated prior to commercial 

introduction for their potential toxicity, for their potential effects on development, nor for their 

possible interactive effects with other chemicals (Goldman 2000; National Academy of Sciences 

1984). The need, in this context, is to design approaches to risk assessment that account for the 

unique exposures and sensitivities of children and that also will stimulate enhanced research in 

developmental toxicity. The ultimate goal is to formulate policies that will protect children 

against potential toxic agents and allow them to grow, develop, and reach maturity without 

incurring neurobehavioral impairment, immune dysfunction, reproductive damage, or increased 

risks of cancer as a consequence of environmental exposures in early life. 

The protection of children against toxic chemicals in the environment will require 

fundamental and far-reaching revisions of current approaches to surveillance, toxicity testing and 

risk assessment. No guidance document on risk assessment that fails to consider the unique 

exposures and special susceptibilities of fetuses, infants and children can today be considered 

adequate to protect human health. 

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98 
99 

APPENDIX 2

Hazard Identification and Predictability of Children’s Health Risk from Animal Data* 

L L Morford, Eli Lilly & Co. 

J M DeSesso, Mitretek Systems 

B Eskenazi, U. of California, Berkeley 

J W Henck, Eli Lilly & Co. 

M E McNerney, Pharmacia/Upjohn 

W J Breslin, Eli Lilly & Co. 

Introduction

Children and adults are different physiologically and behaviorally. Children eat and 

drink more (based on size), play and act differently (e.g. very young children engage in more 

hand-to-mouth activity), are still undergoing development, and may be less or more able to 

metabolize and excrete certain substances. Given these differences, children and adults may 

differ qualitatively and/or quantitatively in how they are affected by xenobiotic exposure. 

Effects of xenobiotics in children may be completely different from effects from the same 

exposure in adults (qualitative difference). On the other hand, the effect of xenobiotic exposure 

may be similar between children and adults but may occur to a greater or lesser extent in the 

child (quantitative difference). 

Children’s health risk assessment is the evaluation of the potential for xenobiotic 

exposures to cause any adverse developmental effect, including growth retardation, 

malformations, functional deficits, and lethality. Risk assessment includes evaluating available 

toxicity data (hazard identification) and exposure information (e.g. dose, route, duration, 

developmental stage of exposure) to determine if a xenobiotic causes potential adverse health 

effects in humans. The focus of this paper is on hazard identification with respect to children's 

health risk. 

Critical periods and important milestones in development

For the purposes of risk assessment, the human life span can be divided into a number of 

exposure periods: pre-conceptional (maternal and paternal), pre-implantation, post-implantation 

(organogenesis; 1st trimester), early and late fetal, premature infant, perinatal, neonatal (term), 

infant, toddler, pre-teen (pre-pubertal), and adult. Comparable periods for animal species are not 

as easily defined and are dependent on individual organs or systems. Furthermore, it is apparent 

from the work of Hoar and Monie (1981) and DeSesso (1997) that developmental events do not 

occur at the same chronological age across species. The developmental toxicology (Wilson 

1977) and developmental neurotoxicology (Vorhees 1986; Rodier 1980) literature contains many 

examples of the stage specificity of structural and functional damage in laboratory animal 

species, which depends on developmental age. Thus, developmental age of maturation is most 

relevant for interspecies comparison. 

_________________________ 

*Manuscript submitted for publication in Environmental Health Perspectives September 20, 2002. 
100 

Adverse developmental effects can occur during any period of the lifespan. While it can 

be argued that the entire lifetime of an individual comprises the period of development, the most 

dramatic manifestations of development occur during the period of maturation to adult status. 

This period of life includes both prenatal (preimplantation, embryonic, fetal periods) and 

postnatal (infancy, childhood, adolescence) development. For the purposes of the current effort, 

the entire period of pre- and postnatal development is considered childhood and is arbitrarily 

defined as the period of life encompassing conception to 18 years of age in humans and from 

conception to sexual maturity in experimental animals. Currently, more information is known 

about the details of prenatal development than those of the postnatal period. 

Organ system development 

During development, cells within an organism change from a state of pluripotency (i.e., 

ability to develop into a great number of different tissue types) to a state of differentiation (i.e., 

commitment to a particular structural and/or functional role within the body). This concept of 

increasing cellular/organ differentiation, or specialization, throughout development is depicted in 

Figure A2-1. The trend of increasing differentiation with maturation occurs in all species, but 

the chronological timetables can be very different from species to species. That is, the 

Figure A2-1. Differentiation Increases with Age of Developing Organisms 

developmental time course for species with prolonged gestation periods (e.g., humans) occurs 

over a greater period of time than that of species with shorter gestation periods (e.g., rats), as 

illustrated in Figure A2-2. Humans reach adulthood in 18-25 years (depending on the criteria 

used to measure adult status), with bursts of developmental activity during both early childhood 

and 
101 

Figure A2-2. Time to Develop Adult Characteristics 

puberty. In contrast, rats attain adult status very quickly. The lifespan of the two species can be 

scaled so that comparable stages of development are congruent, irrespective of chronological age 

(Figure A2-3) - this is scaling to physiological time. By doing so, it can be noted that, based on 

developmental stages, birth occurs much earlier in the rat than in the human. Birth is not a 

maturational landmark; it does not occur at the same developmental stage for each species. 

Rather, birth is a physiological occurrence that occurs at different developmental stages, 

depending on the species. 

Figure A2-3. Relationship Between Extent of Maturation and Birth in Rats and Humans 

In the 1820s, Karl van Baer made some observations about differentiation among species. 

Looking at prenatal development, he noted that the more general features of an embryo appear 

earlier than the more specialized features, and that as development progresses, different species 

%

Adult 

Status

Age (years)

0

100

0 20 5 10 15

Rat

Human

%

Adult 

Status

Age (years)

0

100

0 20 5 10 15

%

Adult 

Status

Age (years)

0

100

0 20 5 10 15

Rat

Human

Maturation

Physiologic Time

Conception

= Birth

Rat

Human

100% 

Adult 

Status

Maturation

Physiologic Time

Conception

= Birth

Rat

Human

100% 

Adult 

Status
102 

diverge morphologically. For example, forelimb buds appear early in development and look 

similar across species. Later in development, the forelimb buds of a fish become fins, and those 

of a bird become wings, while those of a human become arms. 

It is, in part, because of the morphological and presumed physiological similarities 

among species during the early stages of development that prenatal toxicity testing paradigms 

have been used so successfully. To date, researchers have generally focused on effects mediated 

during the period of organogenesis, when cells and organs are undergoing early differentiation 

but species are still relatively similar to one another. Early in development, cells and embryos of 

different species react similarly to a challenge (e.g., exposure to a xenobiotic) because at those 

developmental stages their cells have not differentiated greatly. Later in development, the cells 

of one organ may react differently to a challenge than the cells of another organ, and one species 

may react differently than another due to developmental divergence and specialization. This 

concept was captured in one of Wilson’s general principles of teratology: the response of a 

developing organism to a challenge depends on its stage of development (Wilson 1959, 1973). 

This principle applies to postnatal development as well. 

Postnatal development 

Prenatal developmental milestones have historically been morphological and welldefined (reviewed by DeSesso 1997). For the most part, especially during the embryonic and 

early fetal periods, milestones have been identified by the emergence/disappearance or change in 

form of particular structures. As development proceeds, the nascent organism takes on a 

progressively more mature form. Thus, the ability to discern milestones becomes more difficult, 

and tends towards histological or physiologically-based markers of development such as 

attainment of blood pressure, thermal homeostasis (resistance to cooling), closure of 

sutures/epiphyses, onset of glandular secretion, or appearance of cellular receptors. 

In postnatal life, some milestones in nonclinical species are denoted by the appearance of 

particular behaviors or neurological activities (e.g., acoustic startle response, exploratory 

behavior, rearing, mounting, etc.). There are few reliable, comparative compendia for many 

peri- and postnatal milestones in typically used experimental animals and humans. Without a 

database of normal milestone appearance, derived from many control animals, there is little 

information about the normal variation in these data. This, in turn, leads to obvious difficulties 

in interpreting such findings as a delay in the appearance of a particular milestone. Interpretation 

of some behaviors is confounded because they are related to olfaction in macrosmatic species, 

such as rodents and carnivores, and it is not clear that these behaviors have counterparts in 

microsmatic species like man. Thus, the relevance of some of these findings will be problematic 

for use in risk assessment. 

Because a generally accepted suite of postnatal developmental milestones in nonclinical 

species is not yet available, the critical periods for the developmental processes associated with 

those milestones have generally not yet been defined. Additionally, there may be more than one 

critical period of vulnerability and endpoint affected. For example, brain structures have 

different peak periods of growth. Therefore, depending on the particular time of exposure, 

compounds could differentially affect the structures undergoing peak development. In a review 

of animal studies and their clinical implications, Rodier (1980) found that exposures occurring at 
103 

different stages of brain development had different effects on brain and behavioral functions. 

For example, based on data from azacytidine-treated mice, hyperactivity seemed to result from 

insults that occurred during the middle part of neuron production (mid-prenatal insult) while 

hypo activity seemed to result from insults that coincided with cerebellum proliferation (early 

prenatal and early postnatal insults). 

Although a workshop on "Critical Windows of Exposure for Children's Health" was 

conducted to examine the data available on critical windows for exposure during the postnatal 

period, detailed periods of susceptibility were not identified (Selevan et al. 2000). For reviews of 

the development of the systems covered and the conclusions of the individual workgroups, the 

reader is directed to Environmental Health Perspectives Volume 108, Supplement 3, 2000. 

Understanding both the fundamental biology and the temporal schedule that underlies the 

development of whatever milestones are eventually selected is paramount for performing well 

grounded, scientifically based risk assessments. Therefore, it may be desirable to begin by 

defining important pediatric milestones for growth and development, both global and organsystem specific. These milestones should be readily assessed by clinicians, reflect normal 

maturational processes, be liable to disruption when normal maturation is perturbed, and 

demonstrate susceptibility to pharmacologic perturbation (i.e., display properties of doseresponse and time-action). Subsequently, nonclinical endpoints analogous to these pediatric 

milestones should be identified accordingly. 

Relevance/predictability of extrapolation of animal data to children's risk assessment

Animal/human concordance has been reasonably well characterized in two primary areas 

pertaining to children’s health: developmental toxicology and developmental neurotoxicology. 

Much has been written about the relevance of animal models to human risk assessment, and a 

few of these references are cited herein. Species comparisons of the development of physical 

structures and organ systems have been made by Hoar and Monie (1981) and DeSesso (1997). 

These comparisons enhance the predictability of the type of damage that may occur in humans 

following insult at various stages of development. Several studies have been conducted on the 

ability of animal species to identify known human teratogens. Schardein et al. (1985) found that 

with the exception of coumarin anticoagulants, every chemical or chemical group known to be 

teratogenic in humans is also teratogenic in one or more laboratory animal species. With regard 

to teratogenicity, animal models have been reported to demonstrate sensitivity in the range of 62-

75%, a positive predictive value of 75-100%, and a negative predictive value of 64-91%, which 

implies that studies in laboratory animals are capable of predicting human developmental 

toxicity (Jelovsek et al. 1989). 

Schardein and Keller (1989) developed a comparative table (Table A2-1) of fetal 

endpoints in the rat, rabbit, and human. Concordance with regard to growth and developmental 

milestones has not been as well established, primarily because it has not been established which 

milestones are the most appropriate to evaluate in humans or in laboratory animal species. 
104 

Table A2-1. Fetal Endpoints in Rat, Rabbit and Human 

Rat Rabbit Human 

Body weight at term (grams) 3.5 39.8 3587 

% Live births at term 93.1 90.2 86.7 

% Malformations 2.1 7.3 2-3 

Currently, the most abundant knowledge deals with effects of compounds administered 

over the course of embryofetal development in classic laboratory species. With regard to 

teratogenicity, the mouse and rat successfully modeled the human reaction approximately 70% 

of the time, while the rabbit was less likely than other species to give a false negative finding. 

Primates were considered to offer a high level of predictability. The ability of various species to 

correctly identify an agent as a human developmental toxicant with a positive response for 

growth retardation, death, or malformation was 98% for the rat, 91% for the mouse, 85% for the 

hamster, 82% for the monkey, and 77% for the rabbit (Schardein and Keller 1989). 

Developmental toxicology testing – encompassing the periods of embryonic and fetal 

development - routinely uses two species, generally rat and rabbit. Frankos (1985) reviewed 38 

known human teratogens and determined that 97% of these compounds were teratogenic in at 

least one animal species, 75% were positive in more than one species, and 21% were positive in 

all species tested. Mice identified a positive response 85% of the time, rats 80%, and rabbits 

60%. Schardein and Keller (1989) determined that concomitant use of the rabbit with either the 

mouse or the rat enhanced the predictive potential of these individual models. They also found 

that concordance was almost always additive. Evaluation of postnatal developmental landmarks 

in laboratory animals has routinely been conducted in one species, the rats. One of the major 

reasons for this is that this evaluation usually occurs in the labor-intensive pre- and postnatal 

development study. Given the developmental toxicity history on concordance, a future 

consideration should be evaluation of alternate species or more than one species for 

developmental landmarks. Naturally, this raises questions of which species would be appropriate 

and, once chosen, the extent of historical data needed to validate the use of this species. These 

are important issues for discussion in the developmental toxicology community. 

Table A2-2 indicates potential developmental landmarks that have been measured in 

humans and/or laboratory animals. This list is not all-inclusive, and requires input from 

individuals with pediatric expertise. Tests for acquisition of these landmarks may differ in 

humans and laboratory species, but the same basic endpoint is being measured. Representation 

on this list does not necessarily indicate that these tests are being conducted routinely, or that 

they are validated. Several areas such as hormone measurements, and evaluation of metabolism 

and pharmacokinetics/pharmacodynamics, are listed, but have not been well characterized in 

children and young animals. Evaluation of additional functional areas that develop postnatally, 

such as the immune and genitourinary systems, has not been extensively explored. Biomarkers 

for these areas are needed, and data should be evaluated for concordance between humans and 

laboratory animals. Surrogate markers in laboratory animals might include, for example, pinnae 

detachment and eye opening. What is the relevance to humans, other than a general indication 

that development is proceeding at a normal pace? Ability to taste and smell can be measured in 
105 

humans and laboratory animals, but is this highly relevant information to be included in a 

developmental landmarks test battery? 

Table A2-2. Endpoint (Humans and Laboratory Animals) 

Endpoint Humans Laboratory Animals 

Survival √ √

Growth and development 

 Body weight √ √

 Skeletal development (height/crown-rump) √ √

 Pinnae detachment √

 Tooth eruption √ √

 Eye opening √

Sensory development 

 Hearing (auditory startle) √ √

 Sight √

 Taste (taste aversion) 

 Touch (tactile startle) √

 Smell (odor threshold) √ √

Neuromuscular reflexes 

 Grip strength √ √

 Muscle coordination √ √

 Gait/movement √ √

Activity 

 Spontaneous activity (hypo- and hyperactivity) √ √

 Reactivity √ √

Sexual maturation 

 Vaginal opening √

 Testes descent √ √

 Preputial separation √

 Seconday sex characteristics √

 Hormone levels √ √

Learning and memory 

 Intelligence tests/various cognitive batteries √

 Mazes √ √

 Avoidance behavior √

 Operant behavior √ √

Language skills √

Social behavior √ √

Metabolic capability √ √

Pharmacokinetics/pharmacodynamics √ √
106 

A work group (Adams et al. 2000) specifically charged with addressing questions 

relevant to risk estimation in developmental neurotoxicology found that, within the context of 

methods used in regulatory testing batteries, extrapolation appears stronger with regard to effects 

on sensory and motor functioning than for cognitive or social functioning. To improve detection 

of learning and memory deficits, they suggested that the integrity of learning mechanisms should 

be further challenged through task complexity. Additionally, the workshop suggested the use of 

more contemporary and sensitive methods for evaluating behavior. These include adding 

prepulse inhibition to the startle paradigm, improving water maze learning tasks by adding 

reversal learning, and examining morphological and functional effects in young as well as aged 

animals. 

Mechanistic data 

The use of information regarding the locus of a compound’s mechanism of action 

(generally a receptor), coupled with the known distribution of this target, can provide important 

information in the course of predicting the pharmacology or toxicology of that entity. Further, 

known similarities or discrepancies between the test species and humans can be used to support 

or refute, respectively, the relevance of nonclinical effects for pediatric risk assessment. 

That said, traditional developmental milestones are considered to be the result of highly 

integrated processes and do not lend themselves to in vitro mechanistic evaluation. As with the 

assessment of developmental toxicology, the most expedient way to evaluate developmental 

milestones may be with a standardized screen to identify effects, leaving in vitro mechanistic 

evaluations in the realm of effect characterization on a case-by-case basis. 

Data gaps 

Data gaps, once identified, should be investigated by appropriate scientific 

experimentation. Despite many systems undergoing significant development during this time, 

one period with considerable data gaps is the peripubertal/adolescent period. For example, 

although prominent remodeling and maturation events occur in the brain during adolescence, 

little investigation in either humans or animals has occurred. Another area with limited data 

concerns the evaluation of functional deficits in relation to discrete windows of vulnerability. 

Rarely have early gestational versus late gestational versus lactational exposures been examined. 

There may be a role for scientific groups (e.g. ILSI and PhRMA), as well as 

governmental and regulatory agencies (e.g. EPA, FDA, NTP), in funding landmark assessments 

for development, especially postnatal development in which there are fewer validated assays and 

endpoints. Alternatively, nonclinical endpoints selected empirically could be monitored 

prospectively for clinical relevance. Finally, needed information about a given species might be 

ascertained through the use of additional concurrent control groups in nonclinical postnatal 

testing. 
107 

Methods for assessing potential hazards to children

Nonclinical studies 

Nonclinical toxicity assessments include prenatal developmental toxicity studies, fertility 

and reproduction studies, developmental neurotoxicity studies, and/or peri- and postnatal studies. 

Tables A2-3 and A2-4 are meant to summarize the overall designs of these protocols; for 

detailed descriptions the reader is referred to the original guidelines (EPA 1991, 1996, 1998; 

FDA 1994). Additionally, several reviews discuss and compare the EPA, FDA, and OECD 

guidelines (Collins et al. 1998; Kimmel and Makris 2001). 

Generally, these protocols include extended periods of dosing to simulate long-term 

human exposure and development periods. For example, dosing extends the entire period of 

gestation (implantation to term) in the prenatal developmental study; in the developmental 

neurotoxicity study, dosing usually begins at implantation and continues throughout prenatal 

development until midway through or to completion of the preweaning period to cover major 

periods of nervous system development. 

Table A2-3. EPA Testing Guidelines 

Prenatal Development 

Study 

(OPPTS 870.3700) 

Reproduction and 

Fertility Effects (2-

generation) 

(OPPTS 870.3800) 

Developmental 

Neurotoxicity Study 

(OPPTS 870.6300) 

Dosing Period GD 6-20 (rat) 

GD 6-29 (rabbit) 

P: 10 weeks prior to 

mating through weaning 

F: weaning through 

mating, pregnancy and 

lactation 

GD 6- PND 10 

Number of Animals 20 pregnant females 20 pregnant females 20 litters 

Typical Endpoints 

Evaluated 

• Maternal toxicity 

• Number of 

implantations and 

corpora lutea 

• Embryo/fetal 

mortality 

• Fetal weight and sex 

• Fetal morphology 

(external, visceral, 

skeletal) 

• Estrous cyclicity 

• Semen quality 

• Mating indices 

• Fertility indices 

• Parturition 

• Offspring growth 

and viability 

• Reproductive 

landmark 

development 

(vaginal opening and 

preputial separation) 

• Reproductive organ 

weights and 

histopathology 

• Offspring growth 

and viablity 

• Offspring toxicity 

• Developmental 

landmarks 

• Motor activity 

• Auditory startle 

habitation 

• Learning and 

memory 

• Neuropathology 

including 

morphometric 

analysis 
108 

During the past four decades, considerable effort has been expended and much 

experience had been gained in the area of prenatal developmental toxicology. As a result, 

scientific protocols to assess the potential for prenatal developmental toxicity have been designed 

and refined. These protocols have served us well in identifying potential hazards to developing 

embryos/fetuses. The rationale for these protocols was based (at least in part) on the strong 

morphological (and presumed physiological) similarity of mammalian offspring across species, 

especially at the most susceptible early stages of development, when comparative developmental 

stages are most similar. This period encompasses differentiation and organogenesis. As 

development proceeds, and especially as it is manifested during the later stages of prenatal 

development, differences between species with regard to tissue organization and phenotype 

appear and concordance of developmental schedules dissipates. 

Table A2-4. FDA Testing Guidelines 

Fertility and Early 

Embryonic Development 

Pre- and Post-Natal 

Development, Including 

Maternal Function 

Embryo-Fetal 

Development 

Dosing Period 2 weeks (females) and 4 

weeks (males) prior to 

mating through GD 6 

GD 6-PND 20 GD 6-17 (rat) 

GD 7-19 (rabbit) 

Typical Endpoints 

Evaluated 

• Maternal toxicity 

• Mating indices 

• Fertility indices 

• Number of 

implantations and 

corpora lutea 

• Embryo mortality 

• Growth and viability 

• Maturation and 

fertility indices 

• Sensory functions 

and reflexes 

• Behavior (motor 

activity; learning and 

memory) 

• Maternal toxicity 

• Number of 

implantations and 

corpora lutea 

• Embryo/fetal 

mortality 

• Fetal weight and sex 

• Fetal morphology 

(external, visceral, 

skeletal) 

For the most part, assessment of postnatal toxicity has relied on the multigenerational 

test, which treats the mother 10 weeks prior to mating through lactation and the pups themselves 

after weaning to sexual maturity. The test measures the effect of treatment on the pups’ survival, 

growth and maturation and reproductive ability. Occasionally, behavioral testing is also 

performed on the offspring, but functional deficits (e.g., endocrine, immune, cardiovascular, 

renal, hepatic, etc.) are generally not assessed in the multigenerational test. Importantly, 

however, multigenerational tests are conducted on relatively few environmental chemicals and 

are not routinely conducted in the course of nonclinical safety testing of drug products. For 

pharmaceuticals, the multigeneration study has been replaced by shorter-term studies designed to 

evaluate specific developmental stages and reproductive processes. For later stage 

developmental evaluations of pharmaceuticals, dams are treated from gestation day 6 through 

postnatal day 21 (the ICH Pre and Postnatal development paradigm), resulting in indirect 

exposure to the offspring. Therefore, there are gaps in the developmental intervals assessed and 

functional areas evaluated in conventional repeat-dose postnatal tests. 

One design flaw regarding exposure to the offspring is apparent with the 

multigenerational test and the pre- and postnatal test, which limits their use in determining the 
109 

potential postnatal toxicity of a compound. Pups are assumed to be exposed to the compound 

through the mother’s milk until weaning. However, exposure to the compound during the 

lactational period is usually not verified or measured in these studies. As a result, exposure to 

the offspring is uncertain and not quantified. The compound could, in fact, be excreted or 

sequestered by the mother, such that the offspring are not exposed to the extent assumed; 

alternatively, the compound could be hyper excreted in the milk, such that the offspring receive a 

much higher dose than expected. 

Another weakness associated with the multigenerational and the pre- and postnatal 

testing paradigms is that differences between the metabolic capacity of adults versus offspring 

and differences among species in terms of organ system development are often not well 

characterized (and frequently are completely unknown). Neither of these factors should be 

overlooked, and as discussed below, both should play an important role in choosing an 

appropriate animal model for postnatal toxicity testing. Furthermore, understanding the 

developmental differences in organ systems across species is critical to study design. If exposure 

does not occur at the right time (i.e. during critical period(s) for exposure), then potential adverse 

outcomes will be missed. Additionally, if the evaluation of the developmental milestone/process 

does not occur at the right time (i.e. during the critical period(s) of expression/assessment), then 

again potential adverse outcomes will be missed. 

Attributes of a successful animal model 

It is difficult - if not impossible - to make an a priori selection of an animal model for 

large-scale/routine postnatal toxicity testing. Rather, one must determine the best animal model 

for each chemical entity/class. No one model will be appropriate for all chemicals and testing 

needs. 

A successful (i.e., predictive) animal model must possess four important attributes. First, 

the model must be relevant; that is, it must accurately relate to the effects associated with 

chemical exposure in humans. As a prerequisite, it is necessary to evaluate the “correct” 

endpoints (i.e., those associated with chemical exposure). Second, the model must be sensitive, 

that is to say it should give clear-cut results and clearly show a dose-responsive relationship. 

Third, the model must produce reproducible (and thereby confirmable) results so that it can be 

used in multiple studies conducted by different investigators in various locations. And fourth, 

the model must be practical; this means it should be relatively inexpensive and not overly workintensive. 

The above attributes are required of any successful animal model. When one is studying 

the effect of a compound in laboratory animals with the goal of predicting effects in children, 

however, an additional condition must be met: The target organ of the animal model must be in 

the same developmental stage as that of the humans of concern. In order to meet this 

requirement, one must know the developmental stage of children being exposed, their metabolic 

capabilities, which organ systems might be targets of the study compound, and the 

developmental schedules of those organ systems in humans and in potential test species. Armed 

with such information, a researcher can choose an animal model that will be most appropriate for 

the case at hand. 
110 

One of the difficulties in choosing a successful animal model is the compressed 

developmental schedule, which occurs in animals, primarily rodents. For example, regional 

development of the brain proceeds in days in rodents but in weeks to months in humans. 

However, the sequence of events is comparable among species (Rice and Barone 2000). Another 

example is in relation to the reproductive system. The interval between birth and the initiation of 

gametogenesis differs between rodents (only a few days in absolute terms) and humans (years). 

This short interval in rodents limits interpretation of studies on chemicals that are thought to 

bioaccumulate in children and makes it difficult to use rodents to address questions of aggregate 

or intermittent exposures during childhood in humans. The appropriateness of other animal 

models (rabbits and primates) should then be explored for such studies. 

Another important consideration is that some developmental events occur postnatally in 

rodents but occur prenatally in humans; therefore, differences in route of exposures may occur. 

For example, rodents have considerable postnatal development of their nervous systems while 

humans have more prenatal maturation; therefore, the exposures during the same period of 

maturation would be different (i.e. lactational transfer during the first postnatal week in rodents 

and transplacental transfer during the third trimester in humans). 

Clinical and epidemiological studies 

Traditionally, a number of approaches have been utilized to evaluate human 

developmental toxicity. These include case studies, randomized controlled trials, cohort studies, 

and case-control studies. 

Case-control studies start with children with particular diseases and usually compare 

them to those without disease. In most cases, a case-control design would be utilized when the 

disease is rare such as for childhood cancer. These studies may ascertain exposures by 

questionnaire or by biologic or ambient samples. Inherent in the design is the assumption that 

the exposure preceded the disease; however, it is often difficult to ascertain this. This is 

particularly difficult to ascertain if the substance has a short biologic half-life or the disease has a 

long latency, and the inception of the disease process is unknown. 

Cohort studies typically are very expensive to conduct. They are best if the exposure is 

more rare than the disease, or if the temporality of the exposure is difficult to define otherwise. 

An advantage of a cohort study is that in a single study you can examine many disease endpoints 

in relation to the exposure. Multiple exposures can be ascertained in both case-control and 

cohort studies. 

It is important that all studies assess information on other covariates, i.e., variables that 

could confound or alter the results when not controlled for. Uncontrolled confounding can result 

in either an over- or an under-estimate of the effect. It is also important that information be 

obtained on variables that could modify the relationship between variables so that susceptible 

sub populations can be determined, e.g. children exposed only in utero may be at higher risk than 

those exposed only postnatally. 

Successful protocols include studies that have adequate sample size, sensitive and 

accurate exposure measurements, control for confounding and effect modification, and sensitive 
111 

and accurate measures of disease. Many of the most successful studies investigating the 

relationship of exposure and disease have been birth cohort studies where exposure is ascertained 

during pregnancy as well as during childhood; the children are followed for a number of years. 

This design has been used to study lead (Needleman and Bellinger 1991), polychlorinated 

biphenyls (Jacobson and Jacobson 1997), environmental tobacco smoke (Eskenazi and Castorina 

1999) and currently pesticides (Eskenazi et al. 1999). These studies tend to be small due to their 

expense, and therefore cannot examine rare disease outcomes such as birth defects or childhood 

cancer. 

Rare diseases such as cancer or birth defects are best studied in case-control studies. 

Cases are often ascertained from registries such as tumor registries or birth defect monitoring 

programs (e.g., California and metropolitan Atlanta), and controls are selected from the 

neighborhood, friends, or randomly such as through random digit dialing. If exposure is 

ascertained by questionnaire, there are concerns about recall bias and accuracy. Recall bias and 

accuracy is of concern given the time period for recall and the potential differences in recall 

depending on the health of the child. If exposure is ascertained by biologic or ambient samples it 

is difficult to determine if current exposure measures accurately reflect the critical period for 

disease development. Nevertheless, these studies have indicated associations with environmental 

tobacco smoke and with home pesticide use and certain forms of childhood cancers. 

Summary

Children are not adults. They differ by activities and stages of development. These 

differences, in turn, can affect how and when exposures to chemicals occur and the resulting 

responses. Historically, evaluation of developmental toxicity has focused on gestational 

exposures and morphological changes resulting from this exposure. Current processes for 

evaluating growth, survival, and morphological change due to gestational exposure are adequate. 

However, functional consequences of gestational exposure and postnatal exposure are not as well 

studied. Difficulties with our experience and knowledge base for postnatal toxicity evaluations 

include divergent differentiation of structure, function and physiology across species, lack of 

understanding of species differences in functional ontogeny, and lack of common endpoints and 

milestones across species. Ultimately, we need to identify relevant landmarks in children, 

correlate these with a relevant battery of landmarks in animals, and ensure that we have enough 

historical data to make sense of subsequent nonclinical testing. 

References to APPENDIX 2

Adams J, Barone S Jr, LaMantia A, Philen R, Rice DC, Spear L, et al. 2000. Workshop to 

identify critical windows of exposure for children's health: neurobehavioral work group 

summary. Environ Heath Perspect 108:535-544. 

Collins TF, Sprando RL, Hansen DL, Shackelford ME, Welsh JJ. 1998. Testing guidelines for 

evaluation of reproductive and developmental toxicity of food additives in females. Int J Toxicol 

17:299-325. 
112 

DeSesso JM. 1997. Comparative embryology. In: CRC Handbook of Developmental Toxicology 

(Hood RD, ed.). New York:CRC Press, 111-1170. 

Eskenazi B, Bradman A, Castorina R. 1999. Exposures of children to organophosphate pesticides 

and their potential adverse health effects. Environ Health Perspect 107:409-419. 

Eskenazi B, Castorina R. 1999. Association of prenatal maternal or postnatal child 

environmental tobacco smoke exposure and neurodevelopmental and behavioral problems in 

children. Environ Health Perspect 107:991-1000. 

Frankos V H. 1985. FDA perspectives on the use of teratology data for human risk assessment. 

Fundam Appl Toxicol 5:615-622. 

Hoar RM, Monie IW. 1981. Comparative development of specific organ systems. In: 

Developmental Toxicology (Kimmel CA, Buelke-Sam J, eds.). New York:Raven Press, 13-33. 

Jacobson JL, Jacobson SW. 1997. Evidence for PCBs as neurodevelopmental toxicants in 

humans. Neurotoxicology 18:415-424. 

Jelovsek FR, Mattison DR, Chen JJ. 1989. Prediction of risk for human developmental toxicity: 

How important are animal studies for hazard identification? Obstet Gynecol 74:624-636. 

Kimmel CA, Makris SL. 2001. Recent developments in regulatory requirements for 

developmental toxicology. Toxicol Lett 120:73-82. 

Needleman HL, Bellinger D. 1991. The health effects of low level exposure to lead. Annu Rev 

Public Health 12:111-140. 

Rice D, Barone G. 2000. Critical periods of vulnerability for the developing nervous system: 

evidence from humans and animal models. Environ Health Perspect 108:511-533. 

Rodier PM. 1980. Chronology of neuron development: animal studies and their clinical 

implications. Dev Med Child Neurol 22:525-545. 

Schardein JL, Keller KA. 1989. Potential human developmental toxicants and the role of animal 

testing in their identification and characterization. CRC Crit Rev Toxicol 19:251-339. 

Schardein JL, Schwetz BA, Kenel MF. 1985. Species sensitivities and prediction of teratogenic 

potential. Environ Health Perspect 61:55-67. 

Selevan SG, Kimmel CA, Mendola P. 2000. Identifying critical windows of exposure for 

children’s health. Environ Health Perspect 108:451-455. 

U.S. EPA. 1991. Guidelines for Developmental Toxicity Risk Assessment. Fed Reg 56:63798-

63826. 
113 

U.S. EPA. 1996. Guidelines for Reproductive Toxicity Risk Assessment. Fed Reg 61:56274-

56322. 

U.S. EPA. 1998. Guidelines for Neurotoxicity Risk Assessment. Fed Reg 63:26926-26954. 

U.S. FDA. 1994. International Conference on Harmonisation: Guideline on Detection of 

Toxicity to Reproduction for Medicinal Products. Fed Reg 59:48746-48752. 

Vorhees CV. 1986. Handbook of Behavioral Teratology (Riley EP, Vorhees CV, eds.). New 

York:Plenum Press, 3-48. 

Wilson JG. 1959. Experimental studies on congenital malformations. J Chron Dis 10:111-130. 

Wilson JG. 1965. Embryological considerations in teratology. In: Teratology: Principles and 

Techniques (Wilson JG, Warkany J, eds.). Illinois:University of Chicago Press, 251-261. 

Wilson JG. 1973. Environment and Birth Defects (Wilson JG, ed.). New York:Academic Press, 

305. 

Wilson JG. 1977. Handbook of Teratology (Wilson JG, Fraser FC, eds.). New York:Plenum 

Press, 47-74. 
114 
115 

APPENDIX 3

Incorporating Children’s Toxicokinetics into a Risk Framework* 

Gary Ginsberg, Connecticut Department of Public Health 

William Slikker, Jr., FDA National Center for Toxicological Research 

James Bruckner, University of Georgia

Babasaheb Sonawane, EPA/ORD National Center for Environmental Assessment 

1.0 Introduction - Children Can Be a Tough Group to Figure Out

Perhaps the risk assessment community has not embraced the arena of children’s risk 

because of the imposing task. Children are difficult to study for a number of reasons, not the 

least of which is a practical one: it is not ethically feasible to introduce chemicals of 

environmental concern, even at trace amounts, into infants and children. Thus, there is very little 

toxicokinetic (TK) data (that is pharmacokinetic data for environmental toxicants) in this age 

group. One can gather data from “natural experiments” in which children are exposed 

inadvertantly to a pesticide or airborne chemical. The resulting biomarker data may be 

somewhat useful in assessing the extent of exposure, but these are not the well-controlled TK 

data one needs to start building a predictive children’s model. Another equally daunting problem 

is that children are highly diverse, stretching in definition (for the purposes of this framework) 

from fetal through adolescent stages and beyond. Generalizations and defaults are not possible 

for such a sweeping range of development. Even within a narrow age range, there can be 

considerable variability given the rapid and variable rate of development in early life. Thus, 

improving children’s risk assessments through toxicokinetics is a worthwhile endeavor, but one 

that will be hampered by data gaps and uncertainties. 

The goal of this issues paper is to outline the TK questions that need to be addressed if 

this area is to contribute to a children’s risk assessment. We also hope to provide the reader with 

some background and resources that can help shed light on these questions, and then begin 

framing an approach which will facilitate answering these questions on a chemical-by-chemical 

basis. It is recognized that children’s risk assessments also need to address critical exposure and 

toxicodynamic issues, and that in some ways these areas overlap with toxicokinetics. This paper 

will point out those interfaces but for this discussion will keep toxicokinetics as a separate 

consideration in the children’s risk assessment process. 

1.1 Toxicokinetics as a Key Element in Children’s Risk Assessments 

The importance of toxicokinetics in risk assessment has been increasing as we have 

learned more about how toxicokinetic processes (especially chemical metabolism) are involved 

in mechanisms of toxicity, and that these processes can differ markedly across species. In 

numerous cases, toxicokinetic analyses have allowed replacement of the tradtional dose metric: 

applied dose per body weight per day, by a more relevant internal dose metric that facilitates 

_________________________ 

* Manuscript submitted for publication in Environmental Health Perspectives September 20, 2002. 
116 

extrapolation of animal dose-response data to humans (Andersen et al, 1987; Bois et al, 1990; 

Rao and Ginsberg 1997; Hattis et al, 1993). Most commonly this is accomplished with 

physiologically-based pharmacokinetic (PBPK) models which simulate chemical uptake, 

distribution, metabolism, and excretion in both animals and humans. Through these models, 

adjustments are made for cross-species differences in toxicokinetics so that the animal internal 

dose-response profile can be applied to humans. This allows estimation of the exposure dose 

needed to produce toxic levels of chemicals or key metabolites in humans, the key input into risk 

assessments. 

While progress in toxicokinetics has removed some of the uncertainty in cross-species 

extrapolations in risk assessment for adults, these principles have yet to be applied in any 

systematic manner to the risk assessment of children. This is a critical need given that 

children’s toxicokinetics differ from adults in a number of ways: smaller body size; different 

ratios of fat, muscle, and water; higher breathing and metabolic rates per body weight; and 

immaturity of clearance systems and enzymatic reactions (Kearns and Reed 1989; Renwick 

1998; Anderson et al, 1997; Besunder et al, 1988; Ginsberg et al, 2002). Another obvious 

difference from adults is that children are more diverse, undergoing a developmental program of 

growth and maturation that continuously alters the way in which chemicals are processed and 

cleared. Thus, incorporating children’s toxicokinetics into risk assessment is complicated by the 

need to consider many developmental stages, ranging from in utero to adolescence, and by the 

extensive variability that can occur even within a each age group. 

Pharmacokinetic differences between children and adults with respect to the clearance of 

therapeutic drugs have been recognized for years. These differences have spawned numerous 

clinical pharmacokinetic studies for the purpose of better titrating drug dosage to a particular age 

or body size (Morselli 1989; Anderson et al, 1997; Renwick et al, 2000; Ginsberg et al, 2002). 

The focus on children as a pharmacological receptor has not been matched by a similar focus on 

children as a toxicant receptor. In large part this is due to the lack of pediatric toxicokinetic 

studies as mentioned above. This leaves us with a relatively rich pharmacokinetic (PK) database 

for children but a nearly empty toxicokinetic (TK) database. One of the main challenges facing 

the children’s risk assessment framework is how to combine basic information on children’s 

physiological development with what can be learned from children’s PK datasets, into an 

analytic process that allows us to compare internal doses between adults and children for 

environmental toxicants. 

1.2 Proposed Building Blocks for a Children’s TK Assessment 

In crafting a general framework for conducting children’s risk assessments, a committee 

organized by ILSI has suggested several phases: 

1) Problem Formulation: a) scoping of the risk assessment issue/problem statement, b) 

presentation of the major exposure routes to children with an initial assessment of 

the ages in which exposures may be greatest, c) evaluation of data needs (chemicalspecific, child-specific) and resources available for further analysis. 

2) Data Analysis and Development of Risk Assessment Approach: a) determine which 

TK and TD (toxic mechanism) pathways are of most importance for the 
117 

chemical(s) being analyzed; b) determine which age groups (if any) should be 

prioritized for further analysis based upon the exposure, TK (internal dose) or TD 

(susceptibility) factors that differ most between childen and adults; c) decide which 

analytic approaches are warranted and feasible. 

3) Risk Characterization: a) evaluate risks by specific age groups with comparison to 

adult exposures and risks; b) consider intra-subject variability and uncertainty in 

the exposure and risk estimates in describing degree of confidence; c) describe how 

the children’s risk assessment has or has not altered the conclusions of the overall 

risk assessment. 

These draft framework areas are a useful point of departure for considering TK issues in 

children’s risk assessment. The questions raised under each phase will help in analyzing 

adult/children TK issues in a stepwise manner. An overview of this approach is provided in 

Figure A3-1 and described in more detail. 

Figure A3-1. Outline of Toxicokinetic (TK) Assessment Process for Children 

Phase I – Problem Formulation 

• What type of risk assessment is being conducted? 

− non-cancer in which TK variability (typically 3.2x factor) is already considered? 

− cancer risk assessment in which no such uncertainty factor exists? 

• Might children be of special concern due to TK factors such as immaturity of metabolic systems and 

clearance pathways? 

• What data, resources and analytical approaches are available to determine whether: 

− TK factors might affect the degree of exposure (dose) in children? 

− TK factors might cause children to have greater internal exposure per unit of external dose than 

adults? 

♦ novel pathways of metabolism and disposition are possible in children such that children’s 

responses cannot readily be predicted from adult data? 
118 

Figure A3-1. Outline of Toxicokinetic (TK) Assessment Process for Children, Cont’d 

Phase II – Data Analysis and Development of Analytic Approach

• Chemical-Specific Analyis: determine which TK mechanisms govern chemical fate (activation & 

detoxification) 

• Child-Specific Analysis: determine which age groups (if any) should be prioritized base upon size of 

TK differences relative to adults 

− Resources for Children’s TK/PK Data: 

a) in vivo clearance data from therapeutic drug literature 

b) in vitro enzyme data from liver bank and serum studies 

c) developmental literature describing changes in body composition and functions in pre- and 

post-natal periods 

d) animal TK data for the chemical being analyzed 

• Determine which TK approaches are most useful and feasible 

Phase III – Risk Characterization 

• Determine likelihood that children will receive greater internal doses of parent compound or toxic 

metabolite 

• Utilize variability/uncertainty analysis to evaluate the distribution of internal exposures possible in 

children 

Evaluate whether adjustments of the adult risk assessment are needed to account for children’s TK factors 

(e.g., adjustment of uncertainty factors; more quantitative approaches) 

2.0 Phase I. TK Questions in Problem Formulation

A children’s risk assessment will likely start with a number of questions regarding age 

groups and exposures, TK handling of the chemical(s), and whether there are susceptible periods 

or unique toxic effects (TD issues). These questions and the preliminary answers developed in 

Problem Formulation help to scope the remainder of the analysis. A major TK question likely to 

arise in Problem Formulation is: 

1) Can early life stages be considered as part of the overall human variability 

distribution and thus be within the 3.2x TK uncertainty factor? This broad question 

can be broken into the following more defined areas that should help the risk 

assessor better scope the issues of children’s TK: 

What types of data, resources and analytic approaches are needed to determine whether: 

• 1a) children of certain ages experience higher exposures due to higher ventilation 

rates, cardiac outputs, or greater absorption of chemical(s)? 
119 

• 1b) children of certain ages experience a higher internal dose of parent compound 

or active metabolite (per unit of administered dose) than do adults? 

• 1c) children of certain ages process the chemical(s) by novel TK pathways so as to 

generate metabolite profiles that are qualitatively different from adults? 

These questions might be raised in Problem Formulation with the hope that by the end of 

Risk Characterization, the TK issues can be addressed and be factored into whatever risk 

conclusions are reached for children. 

The broad, scoping TK questions raised above are now discussed to provide background 

information and to help us to consider what kinds of resources and analyses will be needed for 

their resolution. 

2) Does children’s TK warrant an uncertainty factor that differs from the current 

practice of allowing a ~3.2x factor for intersubject variability in TK? 

Non-cancer risk assessments concern themselves with threshold effects, with the public 

health exposure level set well below the observed threshold seen in animal or high exposure 

human studies (U.S. EPA 1989). A series of uncertainty factors are used to attempt to 

conservatively cover what we don’t know about risks; the uncertainty factors lower the allowable 

exposure level from the animal or human no observable adverse effect level (NOAEL) or 

benchmark dose. Prominent among these is the interindividual variability factor, typically a 10- 

fold factor. This factor results from a general recognition that the animal species or worker 

cohort from which the NOAEL comes, rarely if ever represents the range of diversity present in 

the human population at large. This uncertainty factor is meant to encompass variability from a 

wide array of sources: inherited traits and genetic polymorphisms, gender and hormonal 

differences, lifestyle factors such as amount of stress, exercise, tobacco and alcohol use, obesity, 

nutritional status, and finally disease conditions, some of which can alter the handling of 

xenobiotics. Related to this last factor is the ingestion of prescription and over-the-counter 

medications, many of which can alter the pharmacokinetics of other chemicals. Another 

circumstance that this inter-subject uncertainty factor is asked to cover is age, ranging from the 

fetal period through geriatrics. 

As mentioned above, the non-cancer uncertainty factor is 10, which can be understood to 

consist of a half log factor (3.16x) for toxicokinetic variability and a similar half log factor for 

pharmacodynamic variability (Renwick 1998). Thus the inter-individual variability in TK 

created by the various genetic, lifestyle, physiologic state and age factors needs to fit within a 

3.2-fold factor for the default to be adequate. It becomes the job of the children’s risk assessor to 

determine whether this is so, and if not, whether a special children’s uncertainty factor or other 

more quantitative approaches are called for. 

At the heart of this question is whether children (or certain age groups of children or the 

in utero period) represent a sufficiently unique subpopulation in terms of the way xenobiotics are 

handled to be considered outside the variability bounds of the adult human population. This 

question must not only consider the central tendencies for key TK endpoints, but also the extent 

of variability in children. This is needed to determine whether the upper or lower ends of the 
120 

children’s distribution (e.g., low-end metabolizers) are captured within the bounds created by the 

adult mean value and a 3.2x uncertainty factor. 

It is important to point out that a similar uncertainty factor does not exist for cancer risk 

assessment. These assessments are generally considered conservative due to the nature of the 

linear low dose modeling often used, and because interspecies scaling of the unit risk factor 

employs a generic body weight (Garbis and Peters 1987; Hytten and Leitch 1971) function that 

increases human potency relative to rodents (Andersen et al, 1995; U.S. EPA 1992). However, 

the lack of any special consideration of children’s risk in cancer assessments leaves open the 

question of whether the derived unit risks are appropriate for all age groups. In fact, one recent 

cancer risk assessment did adjust the unit risk for child-specific factors, but this adjustment was 

not based upon toxicokinetics (U.S. EPA 2000). 

Thus, in the absence of precedents for using a children’s toxicokinetic analysis to adjust 

cancer or non-cancer assessments, a framework is needed for how this could be done in a 

rationale and consistent manner. The hope is that such a process will foster an understanding of 

the size of adult/child differences in key TK pathways relative to the amount of inter-individual 

variability we have come to expect in non-cancer assessments. 

To go beyond this large, overarching question, Problem Formulation needs to identify 

key databases, resources, and analytic approaches available for Areas 1a, 1b, and 1c as described 

above (TK influences on how much chemical is absorbed; TK determinants of internal dose per 

unit of external dose; Novel or shifting TK pathways during development). The overview of 

Phase II below describes resources and approaches which can be initially identified during 

Problem Formulation. The use of these resources will be determined in Phase II by chemicalspecific factors (e.g., pathways of metabolism and clearance, toxic mechanism and identification 

of key dose metric), child-specific factors (e.g., in utero and post-natal developmental profile of 

key TK pathways for target chemical), and scenario-specific factors (which age groups most 

likely to be most exposed; which exposure routes most likely to be substantial). 

3.0 Phase II: Data Analysis and Development of Risk Assessment Approach

In this phase, chemical-specific data regarding the fate of the chemical(s) being analyzed 

are combined with children’s data relating to the developmental profile of TK systems (from in 

utero periods through adolescence). This will hopefully foster an understanding of how children 

of various ages are likely to handle the chemical(s). Obviously, the best case is to have 

metabolism and disposition data for the chemical(s) in children. However, it will be extremely 

rare to find such TK data. Some approaches that can be considered for filling these data gaps are 

the following: 

3.1 Use of TK data for the chemical(s) under analysis to characterize the major fate and 

transport mechanisms. 

Typically, animal data, and in some cases, also adult human data, will be available for 

characterizing the chemical’s fate and disposition. Once the major TK factors governing 
121 

chemical fate are known, then the approaches which follow can be evaluated for utility and 

feasibility. 

3.2 Use of surrogate chemicals (typically therapeutic agents) for which PK data in children 

exist. 

Even if the target chemical(s) has not been evaluated in children, a chemical that is 

similarly processed may have been tested. Information for surrogate chemicals can help 

delineate the maturation of key pathways and how the target chemical will be handled at certain 

developmental stages. This section describes the kinds of datasets and resources available for 

this part of the Phase II assessment. 

Children’s PK databases have been developed over the past several years in which 

therapeutic drugs tested in both children and adults are identified and key PK parameters 

compared across ages (Renwick 1998; Renwick et al, 2000; Ginsberg et al, 2002). These 

databases are especially useful because the metabolism and clearance pathways of many of the 

drugs are known, making them useful indicators for particular pathways. For example, 

dextromethorphan and debrisoquine are known substrates for a particular cytochrome P-450 

enzyme (CYP 2D6), trimethadione, chlorzoxazone, and halothane are markers for CYP2E1 

activity, morphine is predominantly processed by glucuronidation, and a host of antibiotics are 

not extensively metabolized but are mostly excreted unchanged by the kidneys (Dollery 2000; 

Bertz and Granneman 1997; Kurata et al, 1998; Tanaka 1998). 

A well-known example of child-adult differences in metabolic processing is caffeine, in 

which an initial N-demethylation reaction is catalyzed by CYP1A2, and a secondary Nacetylation step is catalyzed by N-acetyltransferase (Welfare et al, 2000). The CYP1A2 reaction 

is the primary factor governing half-life of this drug. Caffeine’s half-life in newborns is 14-fold 

higher than in adults, which is likely the result of the immaturity of CYP1A2 at this age (Parsons 

1976; Parsons and Neims 1978; Ginsberg et al, 2002; Dorne et al, 2001). The metabolic 

clearance of caffeine and another CYP1A2 substrate, theophylline, become more like the adult 

rate by 2-6 months of age, after which time the half-life becomes significantly shorter than in 

adults (Ginsberg et al, 2002; Dorne et al, 2001). This suggests that once CYP1A2 function 

approaches adult levels, the greater size and blood flow to the liver (per body weight) in young 

children can lead to greater enzymatic clearance of CYP substrates (Gibbs et al, 1997). 

However, this also means that chemicals activated to toxicants by this pathway (e.g., aromatic 

amines, polycyclic aromatic hydrocarbons, aflatoxin B1) may produce more active metabolite in 

this age group as compared to adults. Unfortunately, there are no data that can be used to 

directly evaluate this possibility. 

Following up on the caffeine example further, this drug has been used to show that the Nacetyltransferase step is also deficient in early life. Once caffeine is demethylated by CYP1A2 it 

can be N-acetylated to form AFMU (5-acetylamino-6-formylamino-3-methyluracil) or excreted 

in urine in a non-acetylated form (1x). The ratio of AFMU to 1X has been used to phenotype the 

N-acetyltransferase trait in adults and children. In the case of children, the ratio of acetylated to 

non-acetylated metabolite is low in newborns thru 2 months of age (12% of adult ratio) but then 

rises to 65% of the adult ratio during the 2-6 month interval (Pariente-Khayat et al, 1991). This 

agrees with data showing that most newborns are slow acetylators, but that beyond 100 days, the 
122 

underlying genetic polymorphism (fast vs. slow) becomes evident (Pons et al, 1989; Szorady et 

al, 1987). Such data with caffeine are exemplary of the information that can be obtained from 

pharmacokinetic data for therapeutic agents. 

Table A3-1 compiles information obtained from in vivo PK analyses of drugs with in 

vitro analyses of enzyme levels from blood or liver samples. The combination of the two types 

of information for a given clearance pathway can provide a strong indication of how the 

pathway’s function develops in the postnatal period. In vivo PK data have been analyzed across 

chemicals that share a common mode of elimination to develop a more complete evaluation of 

the function of specific pathways (Ginsberg et al, 2002; Renwick et al, 2000). Table A3-1 

shows data for 6 CYP enzymes, several Phase II conjugation pathways, renal and biliary 

clearance, other metabolic functions (e.g., epoxide hydrolase, alcohol dehydrogenase), and two 

esterases involved in the detoxification of organophosphate pesticides. This compilation of PK 

data by pathway and age group indicates a fairly consistent pattern, i.e., premature neonates, fullterm neonates, and infants up to 6 months of age tend to have less metabolic and clearance 

capacity than adults. The two exceptions in the chart are for enzymes that are expressed 

primarily in the fetal and early post-natal period: CYP3A7 and glutathione transferase (GST) pi. 

These fetal forms are replaced during the first year of life by corresponding (but not 

enzymatically equivalent) adult forms. Beyond 6 months, many CYP enzymes are sufficiently 

active that clearance in vivo is actually greater than in adults. This appears to be due to the 

greater liver size and blood flow in children as compared to adults (Gibbs et al, 1997). Table 

A3-1 provides further evidence for this phenomenon in that the in vivo data indicate greater 

clearance capacity than suggested by the in vitro protein levels or enzyme activities for CYP1A2, 

2E1, 2C9/19, 2D6, and 3A4, particularly at 6 months of age. 

These overall patterns are illustrated for a specific CYP, CYP3A4 as evidenced by 

therapeutic drugs that depend upon this CYP for clearance (Figure A3-2) (Ginsberg et al, 2002). 

CYP3A4 is the major CYP in adult human liver but its function is evidently deficient in early 

life. This pattern appears to be widely applicable in that an analysis across 40 therapeutic drugs 

involving 11 different metabolic/excretory mechanisms shows a similar shift from immaturity in 

the earliest age categories to evidence of drug half-lives shorter than adults beyond 6 months of 

age (Figure A3-3). 

The period of shorter half-lives in children relative to adults may represent a time in 

which there is faster removal of parent compound but greater formation of metabolites, which 

can be a concern if chemical metabolism leads to more toxic moieties. It should be pointed out 

that despite the possibility of more rapid activation of some compounds, lower toxicity may be 

experienced by children due to more rapid systemic clearnance of metabolites. Thus, predicting 

the health implications of shifts in xenobiotic metabolism will be chemical-specific and needs to 

consider both parent compound and toxic metabolites. 

The period of accelerated clearance is in contrast to the earlier periods in which hepatic 

metabolism was slower than in adults leading to the potential for prolonged retention and higher 

levels of parent compound. This situation may be compounded by the slower renal and biliary 

function at these times. In terms of detoxification systems, the chart shows that epoxide 

hydrolase is active at birth but apparently only at 50% of adult function. Although the data are 

very limited, it appears that two forms of GST may be deficient (40-60% of adult levels) in early 
123 

life. These data may have risk implications given that GST mu is critical to epoxide conjugation 

with GSH and that epoxide hydrolase also appears to be slow in this age group. Thus, the 

newborn and very young infant may have a relative deficit in detoxifying epoxides, although 

compensating mechanisms (e.g., other GSTs) may exist. It should be noted that further research 

into the developmental profile of these detoxification systems is an important research need. 
124 

Table A3-1: Post-Natal Developmental Profile of Pharmacokinetic Functions 

(Pathway Function Shown as % Adult; Abbreviations: PL – Protein Level; EA-Enzyme Activity; for oth

PK Pathway Premature 

Neonates 

Full-Term 

Neonates 

1 Week– 2 Month 

2 - 6 Months 6 Months – 1 

Yr 

1-2 Y

Renal Clearance 35% - in vivo t1⁄2

for 7 drugsa

GFR ∼20% adultb

28% - in vivo t1⁄2

for 7 drugsa

88% - in vivo t1⁄2

for 7 drugsa 

GFR ∼100% 

adultb

137% - in vivo t1⁄2

for 7 drugsa

137% - ifor 7 dru

CYP 1A1 --c -- c -- c -- c -- c -CYP 1A2 2% - PLe

2% - in vitro EAf

11% - in vivo t1⁄2

for 2 drugsd 

4% - PLe 

3% - in vitro EAf

4% - in vitro EAg 

23% - in vivo t1⁄2

for 2 drugsd

16% - PLe 

9% - in vitro EAf

13% - in vitro

EAg 

81% - in vivo t1⁄2

for 2 drugsd

25% - PLe 

15% - in vitro

EAf 13% - in 

vitro EAg 

175% - in vivo t1⁄2

for 2 drugsd

175% - ifor 2 dru

CYP 2A6 20%- nicotine 

t1⁄2ff

CYP 2E1 13% - PLh 

27% -in vitro

EAi

27%-serum DMO/TMOj

22% - PLh 

39% - in vitro

EAI 

27%- in vivo

serumDMO/TMOj

30% - PLh 

47% - in vitro

EAi

36% - PLh 

41% - in vitro

EAi 

97%-serum DMO/TMOj

92%-in v

serum DMO/TM

CYP 2C9/19 21% - PLk 

33%-in vitroEAl

30% - in vivo t1⁄2

for 1 drugm

29% - PLk 

30% - in vitro

EAl

38% - PLk 

45% - in vitro

EAl

36% - PLk 

83% - in vitro

EAl 

182% - in vivo t1⁄2

for 1 drugm

36% - PL

83% - in 182% - ifor 1 dru

CYP 2D6 13% - PLn 22% - PLn 34% - PLn 45% - PLn CYP 3A4 19% - in vivo t1⁄2

for 8 drugso

17% - in vitro

EAp 

50% - in vivo t1⁄2

for 8 drugso

29% - in vitro

EAp 

55% - in vivo t1⁄2

for 8 drugso

37% - in vitro

EAp

46% - in vitro

EAp 

200% - in vivo t1⁄2

for 8 drugso

110%-in vitroEAp 

200% - ifor 8 dru
125 

Table 1. Post-Natal Developmental Profile of Pharmacokinetic Functions (Cont’d) 

Post-Natal Developmental Profile of Pharmacokinetic Functions (Pathway Function Shown as % 

PK Pathway Premature 

Neonates 

Full-Term Neonates 

1 Week– 2 Month 

2 - 6 Months 6 Months – 1 

Yr 

1-2 Y

CYP 3A7 1100%-in vitro

EAq

600%-in vitro

EAq

300%-in vitro

EAq

200%-in vitro

EAq

Epoxide Hydrolase 

 42% - PLr

50% - CBZE/CBZs

50% - CBZE/CBZs 50% - CBZ- E/CBZs 50% - CBZ- E/CBZs 50% - C

E/CBZs 

Glucuronidation 23% - in vivo t1⁄2 for 6 drugst 34% - in vivo t1⁄2 for 6 drugst 47% - in vivo t1⁄2 for 6 drugst 102% - in vivo t1⁄2 for 6 drugst 84% - in vivo t1⁄2 for 6 drugst 84% - in 

for 6 dru

Sulfation/Gluc. 84%-APAP 

clearanceu

 149%-APAP 

clearanceu

 

Acetylation 83% slow 

phenotypev

12%- in vivo

caffeine Nacetylationw

65%- in vivo

caffeine Nacetylationw

 

GSH Transferase GSTαB1 ∼ 100% -

PLx

GSTαB2 ∼ 40% - 

PLx 

GSTmu ∼ 60% - 

PLx 

GSTpi >2100% - PLx

GSTαB1 ∼ 100% -

PLx

GSTαB2 ∼ 40% - 

PLx 

GSTmu ∼ 60% - 

PLx 

GSTpi >2100% - PLx

GSTαB1 ∼ 100% -

PLx

GSTαB2 ∼ 40% - 

PLx 

GSTmu ∼ 60% - 

PLx 

GSTpi >2100% - PLx

GSTαB1 112% -

PLx

GSTαB2 62% - 

PLx 

GSTmu 93% - PLx GSTpi 2100% - PLx

Biliary Exc. 57% - BSP clearz 70% - BSP clearz 90% - BSP clearzSerum Protein few binding 

sitesaa

few binding sitesaa

few binding sitesaa

Increasing # sitesaa

increasing # sitesaa

adult lev

Serum Cholinesterase 

50% - EAbb 57% - EAbb 101% - EAbb 91% - EAbb 104% - EAbb

Serum Arylesterase 

 28% - EAcc 34% - EAcc 52% - EAcc 84% - EAcc 80% - E

Alcohol Dehydrogenase 

 55%-in vivo

EtOH clearancedd

15%- in vitroEAee

 32%- in vitro

EAee

 
126 

Legend for Table A3-1:aGinsberg et al. (2002) analysis of renally cleared drugs: ampicillin, cimetadine, furosemide, piperacillin, ticarcillin, tobramycin, vancomycin. bGlomerular filtration rate (GFR) in ml/min/kg (Morselli, 1989; Besunder, et al., 1988). cCYP1A1 protein or enzyme activity not detectable in microsomes from liver bank samples at any age indicating very low constitutive levels (SdGinsberg et al. (2002) analysis of drugs cleared primarily via CYP1A2: caffeine, theophylline.

eSonnier and Cresteil (1998) measured CYP1A2 protein levels via immunochemical methods in microsomes from liver bank samples (N=6 to 23 

fSonnier and Cresteil (1998) measured CYP1A2 activity in microsomes from liver bank samples using methoxyresorufin as substrate (N=6 to 23 gCazeneuve et al. (1994) measured CYP1A2 activity in microsomes from liver bank samples via caffeine N-demethylation. 

hVieira, et al. (1996) measured CYP2E1 protein levels in liver bank microsomes by immunochemical means (N=4 - 23 per age group).

iVieira, et al. (1996) measured CYP2E1 activity levels in liver bank microsomes using chlorzoxazone as substrate (N=2 for 1-10 yr old group; ojTanaka (1998) measured the ratio of oxidized metabolite dimethadione (DMO) to parent drug trimethadione (TMO) in serum of children dosed 

(N=5); 1 month to 1 yr (N=5); 1-10 yrs – N=21; adult – N=20). CYP2E1 converts TMO to DMO. 

kTreluyer et al. (1996) measured CYP2C protein levels in microsomal preparations from liver bank samples with immunochemical techniques. lTreluyer et al. (1996) measured CYP2C19 enzyme activity in microsomal preparations from liver bank samples using diazepam as substrate. mGinsberg et al. (2002) analysis of tolbutamide, a drug cleared primarily via CYP2C.

nTreluyer, et al. (1991) measured CYP2D6 protein levels in microsomal preparations from liver bank samples with immunochemical techniques

oGinsberg, et al. (2002) analysis of drugs cleared primarily by CYP3A: alfentanil, carbamazepine, fentanyl, lidocaine, midazolam, nifedipine, qupLaCroix, et al. (1997) measured CYP3A4 enzyme activity (EA) in microsomes from liver bank samples using testosterone as substrate (n=12 

@ 1-3 months, 11 @ 3-12 months, 2 > 1 yr, 11 adults). 

qLaCroix, et al. (1997) measured CYP3A7 enzyme activity (EA) in microsomes from liver bank samples using dehydroepiandrosterone as s ubs

month, 20 @ 1-3 months, 14 @ 3-12 months, 0 > 1 yr, 12 adults). 

rRatanasavanh, et al. (1991) measured epoxide hydrolase protein levels by Western blot on liver bank microsomal preparations (N=5 for 1 day t
127 

sEpoxide hydrolase levels estimated by carbamazepine-epoxide (CBZ-E) to CBZ ratio in blood in different age groups at steady state after repea

patients. Data pooled across 2 studies (Korinthenberg, et al., 1994; Pyonnonen, et al., 1977). Higher CBZ-E/CBZ ratios in children is indicative E formation and CBZ clearance is slower in young children (Kuhnz, 1983; Eichelbaum, 1975, 1985; Ohmori, et al., 1998). 

tGinsberg et al. (2002) analysis of drugs cleared directly via glucuronidation : lorazepam, morphine, oxazepam, trichloroethanol, valproic acid, z

uGinsberg et al. (2002) compilation of kinetic data for acetaminophen (APAP) across 5 studies; N=7 to 24 per age group. APAP is a substrate fo

with sulphation predominating in early life (Levy, 1975; Besunder, et al., 1988). 

vPariente-Khayat, et al. (1991) measured the ratio of acetylated to non-acetylated metabolite in urine after caffeine administration in children (N

compared against an older age group (6 month-2 yrs) since adult data not available and since onset of rapid phenotype occurs by this age. 

wSzorady, et al. (1987) phenotyped 100 newborns 2-3 days old as well as 100 or more subjects in older age groups using acetylation of sulfadim

xStrange, et al. (1989) measured GST protein levels by radioimmunoassay in liver bank tissue cytosols. GSTαB1 levels were 70% of adult in ute

months post-natal. Therefore, approximate that newborn thru 6 months levels would be approx. equal to adult. GSTαB2 protein levels were 25% 

62% of adult @ 5-10 months post-natal. Therefore, estimate is that newborn thru 6 months are 40% of adult. GSTmu is 22% of adult during in 

10 months. Therefore, estimate is that newborn thru 6 months are 60% of adults. GSTpi is 5300% of adult during in utero period and 2100% of 

yGibbs, et al. (1997) measured busulfan half-life in a group of 14 children (avg. age=3) relative to adults. Busulfan is metabolized predominatelzJusko et al. (1972) measured clearance of BSP (bromsulphphalein) in groups of 5 or more children; values in young children compared against 

since adult data not available. 

aaSerum protein binding sites (albumin, α1-acid glycoprotein) low in newborns but increases to adult levels by 1 year (Besunder, et al., 1988). bbData pooled across 4 studies (Lehmann, 1957; Augustinsson, 1963, Ecobichon and Stephens, 1973; Zsigmond, 1971) in which serum cholines

acetylcholine, benzoylcholine, butyrylcholine or procaine as substrate. 

ccAugustinnson et al. (1963) and Ecobichon and Stephens (1973) measured serum arylesterase activity with phenyl acetate as substrate. ddIdanpaan-Heikkila et al. (1972) followed the removal of ethanol from mother and newborn blood (N=6 for each age); newborns were exposed 

equal blood alcohol conc. as mother. 

eePikkarainen, et al. (1967) measured alcohol dehydrogenase activity in liver samples from newborns 1 wk to 2 months (N=2), infants 6 month-2 

(N=3), and adults (N=3). Data were 9 fetuses were approx. 25% of the newborn levels. 

ffDempsey, et al. (2000) measured nicotine elimination from newborns (N=5) exposed transplacentally 
128 

Figure A3-2. Analysis of Children’s Pharmacokinetic Database Half-Life Results for 

CYP3A Substrates -- Alfentanil, Carbamazepine, Fentanyl, Lignocaine, Midazolam, 

Nifedipine, Quinidine, Triazolam (Ginsberg et. al 2002) 

Figure reprinted with permission from Oxford University Press. 

Another potentially important deficit in children is glucuronidation during the first 2 

months of life. Given the relative deficiency of this enzyme and high bilirubin levels (an 

endogenous glucuronidation substrate), it is possible that there will be greater internal exposure 

early in life to xenobiotics (or their metabolites) that require glucuronide conjugation. This is the 

case with the antibacterial drug chloramphenicol, in which a relative lack of glucuronidation 

contributed to its accumulation and toxicity (anemia) in infants (Vest 1965; Mulhall et al, 1983). 

In contrast, some xenobiotics can undergo conjugation with alternative cofactors which allows a 

shift in metabolic profile if the primary pathway is compromised or immature. This is known to 

occur with acetaminophen, in which case sulfation predominates early in life until 

glucuronidation takes over (Levy et al, 1975). 

The findings described above are consistent with the pediatric PK summaries provided by 

Renwick’s group (1998, 2000), Dorne et al. (2001) in which they demonstrate that there are a 

variety of drugs and age groups for which clearance in childhood is slower than in adults. There 

are also cases in which clearance is more rapid, particularly when the pediatric group was at least 

several months of age (Renwick 1998). Further, the relative size of the neonate/adult differences 

shown in Table A3-1 and Figures A3-2 and A3-3 are similar to that seen in a composite analysis 

of 36 drugs with pediatric data (Renwick et al, 2000). These overall trends may be useful in 

making generalizations about the development of PK functions in pediatric populations. The 

type of age-specific and pathway-specific PK information available for children should prove 

0

2

4

6

8

10

12

Children’s t1/2 relative to adults 

1.0 (adults)

Premature 

neonate 

Full term 

neonate 

1 wk – 2 mo 2 – 6 mo 6 mo – 2 yr 2 – 12 yr 12 - 18 yr 

*p<0.1, **p<0.05 

**
129 

useful in predicting how children (particularly neonates) may differ from adults with respect to 

internal dosimetry and ultimate risk. 

Figure A3-3. Analysis of Children’s Pharmacokinetic Database: Half-Life Results for Full 

Database – 40 Substrates (Ginsberg et. al 2002) 

Figure reprinted with permission from Oxford University Press. 

Simply knowing about the function of particular pathways may not be enough 

information to predict in vivo handling of a xenobiotic at a particular age. There are numerous 

TK factors involved in chemical processing and clearance including partitioning into body 

compartments, protein binding, respiration, and organ flows. Therefore, a more comprehensive 

analysis, such as PBPK modeling, may be needed to integrate the various factors at work and 

predict xenobiotic fate in children. The information provided in Table A3-1 combined with 

basic physiologic information may make PBPK modeling of children more feasible. Where this 

is pursued, the uncertainty and data gaps surrounding input parameters for children need to be 

recognized and made transparent. 

While characterization of pathway function based upon indicator drugs is a useful 

approach, it carries the uncertainty that a drug may be processed by several clearance pathways. 

If one pathway is deficient at a certain age, but another is more functional, then the overall halflife or clearance rate may not be affected; instead parent compound may be shunted from the less 

active to the more active pathway, leading to a shift in metabolite profile. The assignment of 

chemicals to particular pathways is based upon the fate of the majority (generally 60% or more) 

of the administered dose in adult humans, as ascertained from the literature. Shifts in metabolic 

processing in early life would tend to obscure child vs. adult clearance differences based upon 

overall half-life or blood clearance. Thus, in some cases, the data in Table A3-1 and Figures A3-

0.00

2.00

4.00

6.00

Premature

neonate

Full term 

neonate

1 wk - 2 mo 2 - 6 mo 6 mo - 2 yr 2 -12 yr 12 - 18 yr

*** p<0.0001

Children's t1/2 relative to adults

***

ADULTS 1.0

 ** ***
130 

2 and A3-3 may be an underestimate of the degree of child-adult difference that might actually 

exist for a given pathway. 

Another caveat for some of the in vivo datasets cited in Table A3-1 and Figures A3-2 and 

A3-3 is that they represent clinical PK trials in children who are not in full health. The datasets 

summarized in Ginsberg et al. (2002) were screened not only for sufficient numbers of subjects 

per age group, but also for the health of the subjects on test. Data for children who were 

critically ill or severely comprimised, especially with regards to hepatic or renal conditions that 

would affect xenobiotic handling, were excluded from the database. However, it is possible that 

the clinical state of the children on test may have affected factors such as volume of distribution 

or other PK factors in certain datasets. The overall concordance between in vitro and in vivo data 

shown in Table A3-1 suggests that this factor is not a systematic issue here. 

3.3 Use of in vitro data to ascertain how TK pathways change during development. 

The liver bank data describing CYP protein levels and enzyme activites, as well as the 

serum esterase data shown in Table A3-1, can be used as indicators of pathway function at 

particular ages. These data are made more reliable for risk assessment when there are 

corroborating in vivo half-life or clearance data for indicator chemicals. An example is CYP 

2E1 function in which in vitro liver bank studies (Figures A3-4a and 4b) generally agree with in 

vivo drug metabolism data for the CYP 2E1 substrate, trimethadione (Figure A3-5) (Vieira et al, 

1996; Tanaka 1998). These developmental profiles can be essential for estimating the degree to 

which 2E1-mediated metabolic activation may occur for a wide variety of environmental 

toxicants (e.g., benzene, chlorinated solvents, ethanol). 

Figure A3-4a. Age-Related Variations of CYP2E1 Protein in the Human Liver 

(Figure from Vieira et al. (1996); reprinted with permission from Blackwell Publishing) 

Microsomal proteins (60 μg) were 

separated on a 9% SDS/PAGE and 

transferred to nylon membranes. 

CYP2E1 was immunochemically 

detected with a polyclonal 

antibody raised against rat 

orthologue CYP2E1: the antigenantibody complex was visualized 

after addition of peroxidaseconjugated anti-(rabbit IgG) 

antibody using 4-chloro-1-

naphthol as the dye, and quantified 

by scanning with an image 

analysis system. Results are 

expressed as the mean ± SE of 

densitometric values of 

immunochemically detected 

CYP2E1/mg protein: OD units 

are an arbitrary measure of the 

density. Groups are defined in 

Methods and the number of 

samples in each group is indicated 

between brackets (Vieira et al, 

1996). 

Foetus 

<30w >30w 

(19) (8) 

<24h 

(4) 

1-7d 

(8) 

8-28d 

(8) 

1-3m 

(23) 

3-12m 

(15) 

1-10y 

(4) 

Adults

(14) 

 Newborns 

7

6

5

4

3

2

1

0

CYP2E1 (OD units · mg protein-1) 
131 

Foetus

<30w >30w

(15) (4)

<24h

(3)

1-7d

(8)

8-28d

(9)

1-3m

(21)

3-12m

(13)

1-10y

(2)

Adults

(14)

Newborns

1.2

1

0.8

0.6

0.4

0.2

0

6-hydroxychlorzoxazone formation

(nmol • min-1 • mg protein-1)

Foetus

<30w >30w

(15) (4)

<24h

(3)

1-7d

(8)

8-28d

(9)

1-3m

(21)

3-12m

(13)

1-10y

(2)

Adults

(14)

Newborns

1.2

1

0.8

0.6

0.4

0.2

0

6-hydroxychlorzoxazone formation

(nmol • min-1 • mg protein-1)

Neonates Infants Children Adolescents Young Mature Elderly

Adults Adults Adults

Stage

0.8

0.6

0.4

0.2

0.0

Serum DMO/TMO ratio

Neonates Infants Children Adolescents Young Mature Elderly

Adults Adults Adults

Stage

0.8

0.6

0.4

0.2

0.0

Serum DMO/TMO ratio

Figure A3-4b. Age-Related Variations of Chlorzoxazone Hydroxylation in the Human 

Liver (Figure from Vieira et al, (1996); reprinted with permission from Blackwell Publishing) 

Microsomal samples (0.3 nmol 

Ρ-450) were incubated with 500 

μM chlorzoxazone and a 

NADPH-generating system. The 

formation of the 6-hydroxylated 

metabolite was monitored at 287 

nm after separation by HPLC. 

Results are expressed as the 

mean ± SE of activity measured 

as rate of formation of 6-

hydroxychlorzoxazone/mg 

microsomal protein. Groups are 

defined in Methods and the 

number of samples in each group 

is indicated between brackets 

(Vieira et al, 1996). 

Figure A3-5. Age-Related Changes Over Seven Stages of Life in the Serum Dimethadione/ 

Trimethadione Ratio 4 h After Oral Administration of Trimethadione (4mg/kg) 

(Figure from Tanaka (1998); reprinted with permission from Blackwell Publishing) 

The seven stages were: 

neonates (<4 weeks; n=5), 

infants (<12 months; n=12), 

children (<10 years; n=21), 

adolescents (<20 years; n=3), 

young adults (<40 years; 

n=20), mature adults (<65 

years; n=20), and elderly 

adults (>65 years; n=40). 

Mean ± SE (Tanaka 1998). 

3.4 Use of developmental information to evaluate how TK can affect susceptibility during 

the in utero period. 

The maternal-fetal environment presents unique TK considerations. The developing 

organism is exposed as part of the maternal system via placental transport. While maternal 
132 

factors (distribution, metabolism, excretion) govern fetal exposure, exposure to the fetus can be 

quantitatively or qualitatively different than in the mother. This may result from chemical 

accumulation in the fetal compartment (NRC 2000), or from in situ metabolism that causes fetal 

exposure to metabolites different from that experienced in the maternal system. Another factor 

is that metabolites formed in the fetus may be slower to cross the placenta than parent 

compounds due to their decreased lipid solubility, leading to a relative concentration of 

metabolites in the fetus. 

The following summarizes the key TK factors during in utero development. 

3.4.1 Placental Transport 

Between the site of maternal exposure and the conceptus is a specialized set of 

membranes that chemicals must cross. The placenta has its origins in the conceptus, with its 

unique anatomical and functional features influencing chemical transfer to the developing 

conceptus. This set of membranes undergoes considerable change with development and also 

exhibits substantial species differences. As pregnancy progresses, the syncytiotrophoblast layer 

(in direct contact with maternal blood and relatively thick in early pregnancy) becomes 

progressively thinner (Garbis and Peters 1987). The cytotrophoblast layer becomes more 

discontinuous and the endothelium of the embryo/fetal vessels within the villi becomes thinner. 

Thus, as pregnancy progresses, there is closer contact between the fetal blood and the 

syncytiotrophoblast, that placental cell layer most important to placental function and maternalembryo/fetal exchange. This leads to greater exchange of nutrients and chemicals across the 

placenta as gestation proceeds. On the macroscopic level, the effects of gestational age can be 

exemplified by comparing the tremendous change in the ratio of placental/fetal weight (ratio 

equals 4 at 10 weeks and 0.2 at 40 weeks) (Hytten and Leitch 1971). 

Neither the function nor the anatomical thickness of the placenta, however, is consistently 

related to the number of layers separating the conceptus from the mother. Importantly, any 

substance in the maternal circulation can, to some extent, be transferred across the placenta 

unless it is metabolized or detoxified before or during its placental passage (Slikker Jr 1987; 

Garbis and Peters 1987). 

A useful method for characterizing placental transfer across chemicals and species is the 

dually-perfused placenta. This is an in vitro preparation in which placental transfer is defined as 

a clearance ratio. Studies of nonmetabolized model compounds indicate decreases in placental 

transfer as the molecular weight of the model compounds increase (Illsley et al, 1985). Under 

these very controlled conditions without maternal or fetal involvement, differences in placental 

transfer between species are evident, with sheep being more different from human than guinea 

pig. These types of functional assessments help to predict placental transfer and fetal exposure 

after maternal dosing. 

Additional factors which influence the placental transfer of chemicals are 

uterine/placental blood flow, placental permeability, and placental metabolism (Mirkin and 

Singh 1976; Miller et al, 1976; Waddell and Marlowe 1981; Mihaly and Morgan 1983; Juchau 

1980a). These factors are not static during pregnancy but may change as gestation progresses. 
133 

Chemical delivery to the developing conceptus relies primarily on blood flow to the 

placenta. Although chemicals may transfer from mother to fetus via the amniotic fluid after 

crossing the amnion, the majority of agents gain access to the conceptus via placental passage 

(Nau and Liddiard 1978). In addition to the changes in placental blood flow that occur during 

gestation, changes in blood flow as a result of chemical exposure must also be considered. 

While it is known that experimentally induced changes in placental blood flow can alter normal 

development of the conceptus (Greiss and Gobble 1967; Barr and Brent 1978; Millicovsky and 

DeSesso 1980), the effect of such perturbations on placental transport of xenobiotics needs to be 

evaluated. 

Placental permeability to a chemical is influenced both by placental characteristics (e.g., 

thickness, surface area, carrier systems and lipid/protein content of the membranes) and also by 

characteristics of the chemical agent (e.g., degree of ionization, lipid solubility, protein binding, 

and molecular weight) (Mirkin and Singh 1976; Mihaly and Morgan 1983; Welsch 1982). The 

rate of placental transfer is rapid for nonionized, lipid-soluble chemicals of low molecular weight 

(less than 1000) and is largely controlled by placental blood flow (Mirkin and Singh 1976; 

Mihaly and Morgan 1983). However, charged molecules such as tubocurarine have also been 

shown to enter the fetus (Kivolo and Saarikoski 1972; Kivolo and Saarikoski 1976). Likewise, 

chemicals that are highly ionized at normal blood pH, such as the salicylates, readily cross the 

placenta (Wilson et al, 1977). The question is thus not whether a compound crosses the placenta, 

but rather at what rate. 

3.4.2 Maternal Considerations 

A chemical must pass through and may interact with several anatomical compartments on 

its journey from the site of maternal exposure to the organ systems of the developing conceptus. 

Maternal factors act to either enhance or diminish the concentration of an active chemical in the 

conceptus. Maternal detoxification decreases the amount of parent compound available for 

placental transport, whereas maternal bioactivation may make more active chemical available. 

The entire set of maternal TK factors (e.g., absorption, distribution, serum binding, and 

elimination) also influence the concentration of active agent at the target site. Due to the 

physiological changes that occur during pregnancy, the influence of these maternal factors on 

chemical delivery may also change during gestation (Noschel et al, 1980; Cummings 1983; 

Bogaert and Thiery 1983; Juchau and Faustman-Watts 1983; Juchau 1995). These issues have 

been reviewed (Juchau and Faustman-Watts 1983; Levy 1981; Brock-Utne et al, 1980; Krauer et 

al, 1980; Slikker Jr and Miller 1994).

For chemicals which bind avidly to plasma proteins, this factor can retard placental 

transfer. In general, only the free drug crosses the membranes of the placenta (Krauer et al, 

1980; Welsch 1982). Protein binding is usually reversible and there is a finite number of binding 

sites; thus, binding is saturable and equilibrium may be described by the law of mass action 

(Krauer et al, 1980; Miller et al, 1976). As long as binding is reversible, it does not prevent the 

chemical from crossing membranes but only slows the rate at which the transfer occurs (Levine 

1973). 
134 

Table A3-2. Partial list of human placental xenobiotic and hormone metabolizing enzymes or is

Phase Type Reaction (gene) Substrate Constitutive Inducer 

I MFO O-de-ethylase (CYP1A1) 7-

Ethoxycoumarin 

(+) Cigarette 

smoke 

A

I MFO Aryl-hydrocarbon 

hydroxylase (CYP1A1) 

PAH (?) Cigarette 

smoke 

An

I MFO Hydroxylase (CYP3A7) Cortisol + _ _ I MFO Aromatase (CYP19) Androgens (+) _ AI MFO Cholesterol side chain 

cleavage (CYP11A) 

Cholesterol (+) _ A

I MFO Estrogen catechol formation, 

2-hydroxylation (CYP 1A1) 4-hydroxylation (CYP 1B1) 

Estrogens (+) Cigarette 

smoke _ 

_ 

I MFO 25-hydroxycholecalciferol 

hydroxylase 

25-hydroxycholecalciferol _ _ 
135 

Table A3-2. Partial list of human placental xenobiotic and hormone metabolizing enzymes or isoenz

Phase Type Reaction (gene) Substrate Constitutive Inducer 

I Oxidoreductase 17β-Hydroxydehydrogenase Type 1 

Type 2 

Estrone to Estradiol Estradiol to Estrone 

(+) _ 

I Oxidoreductase 11β-Hydroxydehydrogenase Cortisol/cortisone (+) _ I Oxidation Dehydrogenase Alcohol/acetaldehyde (+) _ I Oxidation Monoamine Norepinephrine (+) _ 

II Sulfatase Sulfate cleavage Steroid sulfates (+) _ II Conjugation Glutathione-S-transferase Epoxides (+) _ II Conjugation Catechol-O-methyltransferase Catecholamines, catechol estrogens (+) _ 

• 

Modified from Slikker and Miller (Slikker Jr and Miller 1994; Juchau 1995; Arcuri et al, 1999; Rasheed et al, 1997; Zusterzeel et a
136 

3.4.3 Placental Biotransformation 

This factor may be the most critical in influencing the delivery of chemicals to the 

developing conceptus. Placental biotransformation of a chemical prior to fetal delivery may 

dramatically alter the chemical profile in the conceptus from that in the mother (Slikker Jr et al, 

1982b). Equilibrium factors, which influence the rate of placental transfer, can result in 

quantitative differences of exposure; placental metabolism, however, can qualitatively alter the 

exposure of the conceptus to potentially toxic chemicals. Placental metabolism is less well 

characterized than hepatic metabolism, but existing data suggest that the placenta has 

considerably less metabolic capacity than adult liver (Mirkin and Singh 1976; Juchau 1980b). A 

listing of some of the human placental xenobiotic and hormone metabolizing enzymes or 

isoenzymes is presented in Table A3-2. 

3.4.4 Embryo/Fetal Considerations 

As with most organ systems, the various chemical-metabolizing systems undergo 

quantitative if not qualitative changes during development. Chemical biotransformation by the 

developing conceptus has been extensively reviewed (Slikker Jr 1987; Juchau and FaustmanWatts 1983; Slikker Jr and Miller 1994; Leakey 1983; Dutton and Leakey 1981; Eltom et al, 

1993; Rane and Tomson 1980; Neims et al, 1976; Pelkonen 1977; Dutton 1978; Nau and 

Neubert 1978; Pelkonen 1980; Dvorchik 1981; Slikker Jr 1994). Despite the fact that data have 

been collected using a variety of techniques and some "data gaps" exist because of technical or 

ethical reasons, several general conclusions may be drawn from the literature: (1) during 

prenatal development, the activities of most enzymes which catalyze Phase I and II reactions are 

lower than in adults; (2) as in the adult, the conceptus exhibits substrate specificity in its ability 

to metabolize chemicals, suggesting the existence of several sets of enzymes or isozymes which 

may or may not be the same as in the adult; (3) these enzyme systems may be inhibited or 

induced by maternal pretreatment with a variety of chemicals; (4) enzyme activity generally 

increases with gestational age; (5) the ontogeny of each enzyme may be different and the 

controlling mechanisms of maturation of enzyme activity are incompletely understood; (6) 

prenatal human and nonhuman primates exhibit higher levels of many metabolizing enzymes 

(especially P-450s) than do other commonly used laboratory species; (7) as in the adult, the liver 

of the conceptus appears to have the greatest capacity for chemical metabolism. The fetal 

adrenal, kidney, lung and brain also exhibit metabolic capabilities. Table A3-3 summarizes 

recent data concerning some cytochrome P-450 (CYP) isozymes and their occurrence in human 

fetal tissues. 

3.4.5 Fetal Distribution and Elimination 

The majority of chemicals entering the fetal circulation do so via the umbilical vein after 

passage through the placenta. A portion of the blood flow entering the liver of the conceptus is 

shunted via the ductus venosus directly to the inferior vena cava and to the heart for total body 

distribution (Rudolph and Heymann 1967). The remaining umbilical flow enters hepatic tissue 

and exits to the vena cava via the portal vein (Dawes 1968). Therefore, there is the potential for 

a sigificant first pass effect from chemical passing through the fetal liver prior to other sites; 

however, a substantial fraction passes directly to the remaining tissues (Power and Longo 1975). 
137 

As in the adult, approximately 16% of the fetal cardiac output is directed toward the fetal brain 

(Behrman et al, 1970). 

Just as placental transfer of chemicals is the predominate pathway from the maternal 

system to the conceptus, placental transfer is also the predominate route for embryo/fetal 

elimination of chemicals. The same toxicokinetic rules apply for fetal-to-maternal transport 

across the placenta: chemicals which are nonionized and lipid soluble will diffuse across the 

placenta according to the concentration gradient from conceptus to mother. If, however, a 

chemical has been conjugated by the fetus (e.g., glucuronidation, sulfation, etc.) or otherwise 

metabolized to a more polar form, the rate of return to the maternal circulation will be slower 

than that for the parent compound (Dancis et al, 1958; Levitz et al, 1960; Goebelsmann et al, 

1968; Goebelsmann et al, 1972). Physiologically-based pharmacokinetic models to predict the 

fate of toxicants in fetal tissues as a function of development are emerging (Sandberg et al, 1996; 

Kim et al, 1996). 

Table A3-3. Some Cytochrome P-450 (CYP) Isozymes and Their Occurrence in Human 

Tissues (Hakkola et al, 1994, 1997, 1998; Carpenter et al, 1997; Chen et al, 1999; Farin and Omiecinski 1993; 

Lacroix et al, 1997; Maenpaa et al, 1993; Murray et al, 1992; Ravindranath and Boyd 1995; 

Slikker Jr 1997; Yang et al, 1995) 

CYP Adult Liver Adult Brain Fetal Liver Fetal Brain 

1A1 + + + 

1A2 

1B1 

+++ 

+ 

- - 

+ + 

2A6 

2A7 

++ 

+ 

 - 

- 

2B1/2B2 + + 

2B6/2B7 + - 

2C 

2C8-19 

+++ 

+ 

 +

+ + 

2D6 + + + 

2E1 + + + 

2F1 - - 

3A4 

3A5 

+++ 

++ 

+ +

+ 

3A7 + +++ 

4B1 - - 

+ = possibly present in small quantities or mRNA observed but no protein confirmation; 

- = not detected; + = present in low levels; ++ = present in moderate levels; 

+++ = present in high levels 
138 

3.5 Use of animal TK information to evaluate how development can affect internal 

dosimetry 

Laboratory animals can be useful in a number of respects in providing toxicokinetic data 

and related information for children’s risk assessments of environmental pollutants. The 

discussion here will be limited primarily to mice and rats, as most toxicokinetic and toxicology 

studies employ these two rodent species. Some of the major advantages of using immature 

animals as models, or surrogates for infants and children will be described, as will some of the 

inherent disadvantages. In several instances relevant examples will be briefly described. 

Diversity is one of the more daunting problems anticipated when studying children due to 

the numerous variability factors described earlier. Intersubject variability is substantially lower 

in homogenous rodent populations provided by major animal suppliers. Uniform groups of 

animals of the same established genetic and husbandry backgrounds can be maintained under 

defined and carefully-controlled conditions. Thereby it is possible with animal studies to control 

more variables and to better focus on age-dependent differences in chemical metabolism and 

disposition. 

Animals can be utilized to examine the maturation of specific physiological and 

biochemical systems, and their influence on toxicokinetics. Human data may be sparse or 

difficult to obtain, but findings in animals can pinpoint specific processes and/or critical 

developmental periods which significantly impact disposition of particular toxicants. Attention 

can then be focused on assessing their toxicological relevance in humans (Harroff 1997). 

3.5.1 Animal TK and susceptibility to xenobiotics 

Series of studies of organophosphates (OPs) in rats have clearly shown that the relatively 

low detoxification capacity of weanlings places them at increased risk of acute, high-dose OP 

poisoning. Benke and Murphy (1975) concluded that the lower acute toxicity of parathion and 

methyl parathion in adult rats was due to the adults’ greater detoxification capacity. More recent 

studies have confirmed and extended these findings. The maturational profiles of 

P450-catalyzed desulfuration (activation) and dearylation (inactivation), as well as 

carboxylesterase and arylesterase (inactivation) activities have been characterized in the liver 

and/or blood of rats (Atterberry et al, 1997; Moser et al, 1998). Despite very low 

carboxylesterase and arylesterase activities, rat fetuses and sucklings appear to be protected from 

chlorpyrifos, due to limited transfer of the compound from the mother (Mattsson et al, 2000). 

However, this protective effect would not be expected if neonates were exposed directly (i.e., not 

via the mother) to environment OPs. Based upon somewhat anecdotal information of groups of 

people poisoned by these pesticides, the suggestion is that young children (0 - 4 years) are at 

increased risk (Diggory et al, 1977; Zwiener and Ginsburg 1988). However, the exposure 

information is not detailed enough in these cases to confirm the sensitivity of any particular 

person or age group. Ecobichon and Stephens (1973), reported that blood arylesterase activity is 

relatively low in young children (1-2 yrs old), with this group having pronounced intersubject 

variability... No more recent information could be located on the time-course of maturation of 

arylesterase or related enzymes in human blood or liver. 
139 

Comparative studies show that neonatal rodents are frequently more susceptible to 

toxicants than adult animals, but such findings should be interpreted with caution when 

extrapolating to humans. Done (1964) and Goldenthal (1971) compiled the results of LD50

studies of several hundred chemicals in neonatal and mature rodents. The neonatal animals were 

more sensitive to many, but not all the compounds. Almost all the age-dependent differences in 

LD50s were less than an order of magnitude, indeed most varied no more than 2- to 3-fold. 

More pronounced interage differences were seen for a few drugs, some (e.g., chloramphenicol, 

diazepam) of which are known to be poorly metabolized and to accumulate to toxic levels in 

human newborns. It is logical that the greatest toxicokinetic and toxicity differences between 

infants/children and adults should be manifest in the most immature individuals (i.e., premature 

newborns). As full-term human newborns are more mature than their rodent counterparts with 

respect to liver metabolism, interage differences might be less pronounced in humans. However, 

maturation is much more rapid in rodents, such that even a few days of growth can result in 

marked disparity in chemical metabolism, disposition and effects. Results from the Done (1964) 

acute toxicity compilation (Figure A3-6) suggest that there are dramatic differences in sensitivity 

in early postnatal life. These types of examples from the animal literature may be excellent 

research opportunities for exploring the relative importance of TK vs. toxicodynamic (TD) 

mechanisms in determining susceptibility in early life. While the animal research may be able to 

point out mechanisms of TK susceptibility, extrapolation of temporal trends across species is 

made difficult because organs and their associated functions mature at different rates in different 

species. Therefore, when the variable of interspecies maturation patterns is introduced, the 

choice of an appropriate animal model for infants or children becomes complex. This makes 

cross-species extrapolations more uncertain than what we are accustomed to in adult risk 

assessments (NAS 1993; Bruckner 2000). 

The use of immature animals is obviously necessary where serial blood or tissue 

sampling is required, and when potentially toxic or carcinogenic chemicals are to be studied. 

Ethical questions are raised if it is proposed to give even trace doses of environmental 

contaminants to children. Despite the dissimilarities in maturation mentioned above, rodent 

studies can provide valuable information on mechanisms and on specific immaturities that may 

be broadly applicable to infants and children. Heavy metals are a good case in point. It is 

widely recognized that dietary antigens, maternal antibodies and other macromolecules may be 

absorbed through the immature gastrointestinal (GI) mucosa. Increased levels of relatively polar 

molecules in the cerebrospinal fluid of infants suggest that such compounds penetrate the 

blood-brain, or blood-cerebral spinal fluid barrier more readily in infants than in children and 

adults (Adinolfi 1985; Dziegielewska et al, 2000). Therefore, it has been hypothesized that 

equivalent exposures of infants and adults to heavy metals such as mercury and lead will result in 

greater oral absorption and in greater CNS deposition (and toxicity) in the infants. Such a theory 

can be tested directly in animal models, and related to more limited observations in humans. 

Kostial et al. (1978) demonstrated substantially greater GI absorption and whole-body retention 

of lead, mercury and cadmium by suckling rats than by adults. A similar phenomenon has been 

reported for lead in juvenile monkeys (Pounds et al, 1978) and human infants (Ziegler et al, 

1978). Retention of i.v-injected lead was found to be 8 times higher in the brain of suckling rats 

than in the brain of adults (Kostial et al, 1978). Brain retention of injected mercury was 19 times 

higher during the perinatal period. Rodents have been very useful in delineation of the 

toxicokinetic basis of age-dependent differences in heavy metal toxicity, but sub-human primates 

are generally the animals of choice to examine toxicodynamic factors (Davis et al, 1990). 
140 

Toxicokinetic and metabolic data are lacking in both laboratory animals and humans for 

many chemicals (Renwick 1998). This is particularly true for immature populations. This data 

gap is partially compensated for by the extensive in vivo database for therapeutic agents in 

children and in vitro human liver bank studies as described above. Relatively few in vivo data 

are available from kinetics studies of drugs or other chemicals in pre-adolescent animals, due to 

technical difficulties in working with such small subjects, as well as prior lack of interest in 

immature populations. Similarly, there is a paucity of information on the maturation of many 

physiological processes in small animals. The maturation of hepatic xenobiotic metabolism in 

rats, in contrast, has been relatively well characterized (Imaoka et al, 1991; Watanabe et al, 1993; 

Renwick 1998). While rat liver is immature at birth with respect to many metabolic functions, 

certain CYP functions, epoxide hydrolase, and glucuronidation function reach adult levels with 

the first week to 10 days. Other functions, such as glutathione transferase and aryl hydrocarbon 

hydroxylase, take longer to develop (Renwick 1998). 

Figure A3-6. Changes in Chemical Susceptibility with Age (Adapted from Done AK (1964)) 

In spite of these known metabolic differences during development, intrigueing findings of 

increased susceptibility in juvenile rodents have not been followed up to determine whether TK 

mechanisms underly these susceptibility differences. For example, Yoo et al. (1987) observed 

that P450 2E1-catalyzed metabolic activation of N-nitrosodimethylamine to a mutagen was 

substantially greater by hepatic microsomes of weanlings (25 days old) than by adult rats. 
141 

Carbon tetrachloride, a chemical that also undergoes P450 2E1-catalyzed metabolic activation, 

was found to be more hepatotoxic in 15- than in 60-day-old male rats (Jahn et al, 1993). 

However, no accounts were located of animal studies relating susceptibility to injury by these 

2E1 substrates or other chemicals to the time-course of maturational changes in the chemical’s 

metabolism. 

3.5.2 Data gaps and challenges in applying animal data to children’s TK assessments 

It is possible, under certain well-defined conditions, to make reasonable predictions of the 

disposition of drugs in adult humans on the basis of animal studies (Lin 1995). This author 

points out that oral absorption of quite lipophilic compounds is relatively species-independent, 

but that dissimilarities in diet, gastic and intestinal transit, GI blood flow and pH can result in 

differences in absorption of ionizable agents. The extent of plasma protein binding of drugs 

differs considerably among mammals, though the volume of distribution of the unbound fraction 

is often less variable (Fichtl et al, 1991). Extrapolation of results from xenobiotic metabolism 

studies in rodents to humans if often difficult. There are considerable differences between rats 

and humans in the expression and catalytic activities of a number of cytochrome P450 isozymes 

(Nedelcheva and Gut 1994). Stevens et al. (1993) observed higher hepatic P450 levels and 

greater in vitro metabolism of 7 out of 8 phase I and II substrates by rhesus monkeys than by 

humans. Nevertheless, metabolic clearance of low doses of flow-limited drugs should be less 

dependent on species-inherent rates of metabolism than on relative liver blood flow rates (Lin 

1995). Similarly, reasonable estimates of renal excretion of filtered drugs in people can be made 

by use of glomerular filtration rate ratios between humans and animals. Thus, data from animal 

studies can often be quite useful in forecasting toxicokinetics in humans when its limitations are 

taken into account. However, this conclusion is based upon comparison of adult animals to 

human adults. The development of toxicokinetic functions in early life is sufficiently different in 

rodents as compared to humans (see above) to make direct extrapolation from juvenile animal 

studies difficult. 

One of the most promising approaches to quantitative estimation of internal doses of 

chemicals in children is physiologically-based pharmacokinetic (PBPK) modeling. PBPK 

modeling has been successfully applied to risk assessments of a number of environmental 

contaminants in adults, but its use has been extremely limited in protection of infants and 

children. Models have been developed to describe the disposition of a number of chemicals in 

pregnant rodents and their fetuses, as well as the lactating rat and nursing pup (Krishnan and 

Andersen 1998). Luecke et al. (1994) and Welsch et al. (1995) have adapted such models to 

human pregnancy to forecast potential teratogenic events. O’Flaherty developed PBPK models 

that accurately predicted time-courses of lead in the blood and its deposition in bones of 

developing rats (1991) and children (1995). These models incorporated age-dependent changes 

in body weight, tissue volumes and blood flows, and bone formation and resorption rates. 

Pelekis et al. (2001) have made assumptions about distributions of physiological and 

biochemical parameters in children, and estimated their effects on the disposition of VOCs. No 

other accounts of PBPK models for post-lactational exposures of immature humans or animals to 

drugs or other chemicals were found in the peer-reviewed literature. 

Knowledge gained from PBPK modeling efforts with animals can be of significant 

benefit in understanding the kinetic behavior of chemicals in humans and in developing PBPK 
142 

models for humans. Toxicokinetic studies in animals can yield a number of important 

parameters, including in vivo and in vitro partition coefficients, metabolic rate constants, and 

identity and stoichimetric yield of metabolites. Such chemical- and species-specific data are 

needed to construct an appropriate model. The model may be used to generate simulations of the 

time-course of parent compound and/or metabolite(s) in blood, tissues and urine. This 

information is often useful in design of the in vivo kinetics experiments needed to assess the 

accuracy of the model’s predictions. Sensitivity analyses can be carried out with the animal 

model, in order to determine which parameters are major determinants of the toxicokinetics of a 

chemical. These parameters would receive special attention in human kinetics studies and 

modeling efforts. The parameters could be scaled allometrically, or adjusted by an optimization 

routine to fit human plasma time-course data or urinary or pulmonary elimination data. 

Development of PBPK models for children faces at least two major hurdles: 

determination of accurate physiological and biochemical input parameters for different age 

groups; and validation of models’ predictions of internal dosimetry. Anderson et al. (1997), for 

example, point out that there are no human data on age-dependent changes in cardiac output. 

There is reasonably good information for some physiological indices, but data obtained with 

state-of-the-art methods from healthy subjects are limited. Non-invasive techniques (e.g., 

Doppler ultrasound, MRI, 3-D CT scans) are now available for measuring many key indices 

(e.g., cardiac output, tissue blood flows, organ volumes), though the procedures are expensive 

and generally of limited benefit to healthy children. Characterization of physiological and 

biochemical maturation in rodents seems to be of limited utility for extrapolation to humans, due 

to laboratory animals’ disparate, species-dependent maturation patterns. Such an effort in 

sub-human primates would appear more worthwhile, due to their availability for research and 

their similarity to growing children (Conrad et al, 1995). Research projects with monkeys are 

expensive, but they offer scientists the opportunity to conduct kinetic studies of toxicants and to 

use the data to develop PBPK models and to validate their predictions. 

3.6 Use of basic physiologic differences across age groups to ascertain how factors such as 

renal clearance, protein binding, and lipid partitioning may vary with age. 

Renal clearance and protein binding capacity are immature at early post-natal stages and 

develop over the course of the first 6 months to year of life (Table A3-1). Glomerular filtration 

rate as well as transporter (secretory) systems in the proximal convoluted tubule are deficient at 

birth (Morselli 1989; Kearns and Reed 1989), leading to relatively slow clearance of a wide array 

of antibiotics and other renally cleared drugs and metabolites. Compounding the chances for 

greater chemical effect in the first months of life is a deficient protein binding status due to lower 

levels of albumin (binds weakly acidic chemicals) and alph-1-glycoprotein (binds basic 

chemicals). Another issue is the relative deficiency to conjugate and excrete bilirubin early in 

life, which leads to greater occupation of serum binding sites by bilirubin. These factors may 

contribute to a greater ratio of free as compared to bound chemical, leading to a higher potential 

for uptake into tissues and toxic action. Diminished protein binding can also lead to shorter 

duration of action due to greater availability of chemical for metabolism and elimination. These 

factors would likely only be important for chemicals with that exhibit a high degree of plasma 

protein binding (e.g., trichloroacetic acid). 
143 

3.7 Summary of Phase II TK Analysis 

As the available information in the areas described above is gathered and processed, 

judgements can be made regarding which age group(s) appear to be most exposed from an 

internal dose perspective. Age groups which appear to differ most from adults can be prioritzed 

for more detailed analysis as the process proceeds. To assist in this prioritization, the following 

questions may be helpful: 

3.7.1 Are there natural age break points where the PK profile shifts? 

An early step in analyzing children’s TK data is deciding whether and how to create bins 

across ages. The rapid development of children requires that the population be broken into age 

groups that are relatively small, particularly in the early postnatal period. If larger bins are used, 

the data may become highly variable with age specificity becoming a casuality of the desire to 

simplify the assessment. Thus, bins must be constrained to reflect key developmental stages. 

Nevertheless, it is useful to make them as large as possible since this maximizes the “n” (number 

of subjects per group) and enhances the power of cross-age comparisons. It should be noted that 

similar age “binning” may occur in the exposure and TD areas to reflect critical changes in 

behavior or windows of heightened vulnerability. At some point the children’s kineticist may be 

asked to adjust his bins to match those created in these other areas for the sake of harmony and to 

allow risk calculations to be done for neatly defined age groups. Age bins were developed to 

organize the data shown in Table A3-1 and Figures A3-2 and A3-3. These bins are somewhat 

arbitrary, based upon the overall availabilty of data for a variety of pathways and chemicals. For 

specific chemicals and pathways, alternative bins can certainly be considered. 

3.7.2 Which PK parameters are most important in determining internal dose? 

Although PBPK models aim for the simplest approach that yields predictive results, there 

are inevitably many parameters that need to be estimated to run the model. These parameters 

include compartment size, partition coefficients, cardiac output, organ blood flows, metabolic 

rates, lung ventilation rates, and urinary elimination rates. Given that these models contain 

multiple tissue compartments and that we may be dealing with several different age groups, there 

are a potentially large number of parameter values that would require elucidation. However, the 

model may not be equally sensitive to all the inputs, making those parameters that have the 

greatest influence on model performance the most important to calibrate properly. Efforts to 

simplify PK modeling in both adults and children have relied upon a series of algebraic 

equations that highlight key PK parameters while not including others which may be modelinsensitive (Pelekis et al, 2001). Such approaches point out the importance of identifying those 

PK parameters which drive internal dose. Which PK factors are most influential will depend 

upon the properties of the chemical under investigation. The clearance of chemicals with high 

hepatic extraction (rapid liver metabolism) will depend primarily upon hepatic blood flow, while 

chemicals with low hepatic extraction depend most on the rate of liver metabolism relative to 

other competing rates (urinary elimination or exhalation of parent compound). To determine 

how to focus a children’s TK analysis, it is important to first analyze which TK parameters are 

most important in a validated adult model. Then, extra effort can be put into identifying how 

these parameters change as different child age groups are considered. Ultimately, the fate of any 
144 

chemical will be determined by the interaction of all PK factors acting simulataneously. This 

kind of sensitivity analysis will ensure that those parameters with the greatest influence are given 

the attention they warrant. 

3.7.3 How do these parameter values change during development? 

Regardless of whether one is conducting a full PBPK assessment across ages or a less 

rigorous approach, it is important to know how key parameters change with age. As discussed 

above, various data resources and analytic approaches may be used to determine how key 

parameter values differ between children and adults. Similar to Table A3-1, it may be helpful to 

develop a table of key physiologic and metabolic parameters that affect the fate of the chemical 

in question, and then list what is known about those pathways for the in utero and post-natal 

periods. If the parameter values are not materially different between children and adults, or if it 

appears that certain factors negate one another, then one might conclude that it is not important 

to conduct any further TK assessment for that age groups(s). However, for those chemicals and 

age groups where children appear to receive a greater internal dose of active toxicant, further 

analytic steps may be considered as follows: 

3.7.3.1 Semi-quantitative approaches 

Utilize information on the predominant disposition pathways to conclude how much 

different children are vs. adults. It may be possible to base conclusions on one or more 

particularly useful surrogates from the drug literature. 

3.7.3.2 Quantitative approaches - PBPK analysis 

This involves constructing a child age group-specific model and then incorporating 

chemical-specific parameter values to simulate how children will handle the chemical. It is 

essential to carefully consider data needs for such an analysis and whether sufficient data exist or 

can be generated/estimated. 

Thus, Phase II of the TK children’s risk assessment will hopefully result in the following: 

• An understanding of the TK mechanisms that govern the fate of the target 

chemical(s). 

• Data resources that describe how these mechanisms change during development. 

• An initial prioritization of age groups for more detailed analysis based upon 

likelihood of internal dosimetry differences relative to adults. 

• An analytic strategy (semi-quantitative or quantitative techniques) for incorporating 

these data into the risk assessment process. 

4.0 Phase III. Risk Characterization

The analytic approaches developed in Phase II would be followed through so that Phase 

III can attempt to answer the following questions: 
145 

• How similar/dissimilar are in utero and childhood stages to adults in terms of 

internal dose received per unit of administered dose? 

• How variable are young age groups versus adults? 

• Do all age groups fall within the default interindividual variability factor of 3.2? 

• Do child:adult differences warrant the development of chemical- and age-specific 

uncertainty factors; 

• Can internal dosimetry estimates for different age groups be developed and used in 

quantitative risk assessmnt? 

In describing the analytical results, the risk characterization will usually try to describe 

the degree of uncertainty and variability in the analysis. The following questions may be 

particularly pertinent to such a discussion. 

Are children more variable in their PK responses than adults? How do we assess and 

apply this information? The role of variability in the risk assessment process can range from 

purely qualitatitive (e.g., people are different from one another so we are not as certain as we’d 

like to be about the degree of risk), to semi-quantitative (e.g., people vary considerably in 

metabolizing chemical X so it seems prudent to use a half-log TK uncertainty factor), to fully 

quantitative (e.g., we have enough individual data to plot the population distribution of effective 

internal doses per unit of external dose, so let’s show the risk manager the cleanup options that 

protect the 50th, 75th, 90th and 95th percentiles). The fully quantitative mode of variability 

analysis is used in lead uptake/biokinetic modeling to enable risk assessors to calculate the 

amount of lead reduction in water, soil, air, or diet needed to bring 95% of the population of 

exposed children below the blood lead target of 10ug/dl (U.S. EPA 1994). It is logical that 

children are more toxicokinetically variable than adults due to their variable growth and 

maturation rates, as well as the genetic, nutritional, disease, body composition, and prescription 

(and other) drug factors that create TK variability in adults. Children also present the problem of 

variable growth rates, which can make even small age bins relatively heterogeneous. Greater 

variability can affect risk conclusions especially if one is concerned about protecting the tails 

(e.g., 90th percentile) of the distribution. With greater variability, it is also more likely that a 

substantial fraction of a certain age group will lie outside the half-log TK variability range we 

normally allocate to the adult defaults. Thus, it is critical that a children’s TK assessment 

characterize in some way the degree of variability present in each age group’s dataset, and 

determine whether that variability is greater than in the adult case. 

By addressing these questions, the risk characterization will endeavor to determine 

whether TK differences are likely between children and adults and whether these differences will 

be important to the overall risk assessment conclusions for this chemical/scenario. If so, then the 

risk characterization can describe the advantages and disadvantages of applying qualitative (e.g., 

professional judgement), semi-quantitative (e.g., modified uncertainty factors) or quantitative 

approaches (PBPK modeling with distributional analyses). The major caveats and data gaps 

should be elaborated so that the limitations of the assessment are clear and critical research needs 
146 

identified. Finally, the characterization should describe which TK factors appear most influential 

in creating child/adult differences so that the key risk drivers for children are explained. 

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159 

APPENDIX 4: FOCUS QUESTIONS

WORKSHOP TO DEVELOP A FRAMEWORK FOR ASSESSING 

RISKS TO CHILDREN FROM EXPOSURE TO ENVIRONMENTAL AGENTS 

 The Workshop Planning Committee has suggested a number of questions that may help 

to focus your preparation for the workshop and to guide our discussions during the workshop. 

They are grouped below as (1) questions to be addressed by all breakout groups, focusing 

primarily on the Framework, (2) generic questions that raise some broad issues, and (3) potential 

questions for the individual breakout groups. 

All Breakout Groups

 The Planning Committee has suggested that the Framework should lead the risk assessor 

through a process that addresses questions like: 

• What kinds of generic information do we have related to children’s risks? 

• What kinds of chemical-specific information do we have/need? 

• How do we link and interpret this information to assess risks at critical life stages from 

conception through adolescence? 

The Committee proposed that, given the complexity of the matrix of developmental life stages 

and potential adverse effects, the Framework should help the risk assessor focus on the most 

important (critical) effects for the chemical/exposures in question, identifying the resources and 

data available to make this determination. It was agreed that the unique exposure patterns during 

development are key factors that need to be integrated somehow into the risk assessment process 

and recognized in the Framework (building on the EPA/RAF July 2000 workshop). It was also 

suggested that the Framework might point to options that may be considered in the absence of 

chemical-specific data. 

 The Framework is not intended to be as detailed or prescriptive as Guidelines, but EPA 

(and perhaps other organizations) intends to use the Framework that we are developing as a 

starting point for the preparation of a guidance document for assessing children’s risks. 

 Questions for all of the workshop breakout groups, therefore, are: 

1. Does the draft Framework capture the major components of risk assessment for 

effects resulting from exposures occurring from conception through adolescence? 

2. What modifications to the Framework and the associated Framework Diagram does 

your breakout group recommend? Can more detail be added to the Diagram? 

3. How can the Framework be modified/expanded to incorporate concepts from your 

breakout group? How can your responses to the focus questions be reflected in the 

Framework? 
160 

4. How do the conclusions/recommendations from the EPA Risk Assessment Forum 

Workshop (July 2000) affect the Framework and your breakout group’s responses to 

the focus questions? 

5. How could the Framework be tested, following the workshop? Are there specific 

case studies that you could recommend? 

6. What are the critical research needs and data gaps for the assessment of children’s 

risks from exposure to environmental agents? 

Generic Questions

1. What are the unique life stages from conception through adolescence? 

2. Why/how are they important in risk assessment? 

3. What outcomes/endpoints are likely to be the most important for each life stage? 

4. How can animal models/studies be used to predict potential health consequences for 

humans? 

Kinetics Breakout Group 

1. What are the critical PK determinants of internal dose (e.g., those to which PBPK 

models are most sensitive)? 

2. How do these PK determinants differ across the age spectrum from in utero through 

adolescence, and how do they compare in adults? 

3. Which analytical methods and risk assessment practices can be applied to child/adult 

PK differences? Under what conditions would you use each method? Which 

methods are currently feasible and what new data are needed to improve the use of 

these techniques for children? 

4. Is it appropriate to create age groupings that represent specific PK developmental 

phases? If so, what might these look like? 

5. Are children more or less variable than adults in terms of PK handling of xenobiotics? 

6. Is it possible to lump together groups of xenobiotics handled via similar PK 

mechanisms so that generalizations regarding groups of chemicals can be made for 

various developmental stages? 

7. How can animal data from in utero and early life stage studies be used to inform 

children’s PK assessments? 
161 

Dynamics Breakout Group 

1. What do we know about the dynamics of developmental processes that may make 

them uniquely sensitive (qualitatively and/or quantitatively) to environmental 

responses? 

2. Are there common characteristics of these processes that can help us in identifying or 

focusing our assessments? 

i.e., Does the presence of active proliferation automatically target those times, tissues 

for further investigation for unique sensitivity? What other process might we "target" 

for consideration? 

3. Define the types of dynamic information that we need for our Framework. 

4. Can we use the overall developmental process timeline to target key tissues, systems 

and processes for evaluation of developmental dynamics for unique sensitivity at 

specific times in development? As was asked for Kinetics, is it appropriate to create 

age groupings that represent specific developmental phases for dynamics? If so, what 

might these look like? 

5. Are the animal tests robust enough to provide the types of dynamic information that 

we need? Again, what are our qualitative and quantitative needs for these 

assessments? 

6. Presuming that we can answer some of the above questions, can the breakout group 

provide some indications of level and type of impact that might result from altering 

these developmental processes? How would these compare with adult dynamic 

processes? How would we go about answering this question? 

For example, if we use the revised WHO/Renwick approach for evaluating kinetic 

and dynamic factors in reference dose calculations, do we have support for specific 

values for the dynamic component? How might we evaluate proposed factors for 

evaluating dynamic processes? 

7. How can dynamic information support or refute proposed modes of action? 

8. How might this differ from support for modes of action for that chemical in adults? 

Risk Characterization Breakout Group 

What are the critical components to consider in the context of a comprehensive risk 

assessment that explicitly addresses risks to children? In particular: 

1. Have unique susceptibilities been identified associated with one or more stages of 

prenatal development or childhood? 
162 

2. Are there mechanisms of toxicity unique to children, or are just the outcomes 

different? 

3. Are there behaviors that are peculiar to children that make exposure by certain routes 

or media more of concern? 

4. In the context of risk assessment, how should we address responses in children that 

are different from those in adults? For example, is an additional uncertainty factor 

warranted? If so, what is the nature of the uncertainty? Could there be multiple 

sources of child-specific uncertainty? Is the use of an uncertainty factor dependent on 

whether the difference between child and adult response is qualitative or 

quantitative? For qualitative differences in outcome, does it matter if these are 

mechanistically different? Does it matter whether there is likely to be a 

cumulative/chronic effect from exposure? What magnitude (or range) should the 

uncertainty factor be? What data would be necessary to alleviate the uncertainty 

represented by the UF? 

5. What assumptions can we make about childhood susceptibility or childhood exposure 

when the data set is incomplete? 
163 

APPENDIX 5: WORKSHOP PARTICIPANTS AND OBSERVERS CONTACT 

INFORMATION

ILSI RISK SCIENCE INSTITUTE

WORKSHOP TO DEVELOP A FRAMEWORK FOR ASSESSING RISKS 

TO CHILDREN FROM EXPOSURE TO ENVIRONMENTAL AGENTS 

July 30 – August 2, 2001 

Topnotch at Stowe Resort and Conference Center 

Stowe, Vermont 

Dr. John Adgate 

Department of Environmental and 

Occupational Health 

University of Minnesota 

MMC 807 

420 Delaware Street, SE 

Minneapolis, MN 55455 

T: 612-624-2601 

F: 612-626-0650 

E: jadgate@umn.edu

Dr. Richard Albertini 

Genetic Toxicology Laboratory 

University of Vermont 

32 North Prospect Street 

Burlington, VT 05401 

T: 802-656-8346 

F: 802-656-8333 

E: ralberti@zoo.uvm.edu

Dr. Robert W. Amler 

Chief Medical Officer 

Agency for Toxic Substances & Disease 

Registry 

1600 Clifton Road, NE (MS E28) 

Atlanta, GA 30333 

T: 404-498-0115 

F: 404-498-0083 

E: ramler@cdc.gov

Dr. Sherlita Amler 

Agency for Toxic Substances & Disease 

 Registry – Division of Toxicology 

1600 Clifton Road, NE (MS E29) 

Atlanta, GA 30333 

T: 404-498-0160 

F: 404-498-0094 

E: sgr6@cdc.gov

Dr. Hugh A. Barton 

NHEERL 

U.S. Environmental Protection Agency 

Mail Code: B143-05 

Research Triangle Park, NC 27711 

T: 919-541-1995 

F: 919-541-4284 

E: barton.hugh@epa.gov

Dr. Nancy B. Beck 

Toxicologist/Risk Assessor 

Office of Information and Regulatory Affairs 

Office of Management and Budget 

New Executive Office Building, Room 10201 

Washington DC 20503 

T: 202-395-3258 

F: 202-395-7245 

E: Nancy_Beck@omb.eop.gov

Dr. Richard Becker 

American Chemistry Council 

1300 Wilson Blvd. 

Arlington, VA 22209 

T: 703-741-5210 

F: 703-741-6056 

E: rick_becker@americanchemistry.com

Dr. Matthew Bogdanffy 

DuPont Haskell Laboratory 

P.O. Box 50, Elkton Road 

Newark, DE 19714 

T: 302-366-5574 

F: 302-366-5003 

E: matthew.s.bogdanffy@usa.dupont.com
164 

Dr. William Breslin 

Lilly Research Laboratories 

2001 West Main Street 

Drop Code: GL46 

Indianapolis, IN 46140 

T: 317-433-3601 

F: 317-277-5002 

E: wjbreslin@lilly.com

Dr. James V. Bruckner 

College of Pharmacy 

University of Georgia 

D.W. Brooks Drive 

Athens, GA 30602-2352 

T: 706-542-5405 

F: 706-542-3398 

E: bruckner@rx.uga.edu

Dr. Robert E. Chapin 

Global Research and Development 

Pfizer, Inc. 

Eastern Point Road, MS 8274-1336 

Building 274 

Groton, CT 06340 

T: 860-441-0571 

F: 860-715-3577 

E: robert_e_chapin@groton.pfizer.com

Dr. Harvey Clewell 

Principal 

ENVIRON International Corporation 

602 East Georgia Avenue 

Ruston, LA 71270 

T: 318-251-6983 

F: 318-255-2040 

E: HClewell@environcorp.com

Dr. Elaine Cohen Hubal 

National Exposure Research Laboratory 

U.S. EPA (MD E205-01) 

Research Triangle Park, NC 27711 

T: 919-541-4077 

F: 919-541-0905 

E: hubal.elaine@epa.gov

Dr. Adolfo Correa 

Centers for Disease Control and Prevention 

4770 Buford Highway NE (MS F-45) 

Atlanta, GA 30341-3724 

T: 770-488-7164 

F: 770-488-7197 

E: acorrea@cdc.gov

Dr. Terri Damstra 

Interregional Research Unit 

International Programme on Chemical Safety 

World Health Organization 

4401 Research Commons Bldg. 

79 T.W. Alexander Drive, Suite 3110 

Research Triangle Park, NC 27709 

T: 919-541-7537 

F: 919-541-3276 

E: damstra@niehs.nih.gov

Dr. George Daston 

Miami Valley Laboratories 

The Procter & Gamble Company 

11810 East Miami River Road 

Cincinnati, OH 45252 

T: 513-627-2886 

F: 513-627-0323 

E: daston.gp@pg.com

Dr. Karen Davis-Bruno 

Supervisory Pharmacologist 

Div. of Metabolic and Endocrine Drug Products 

USFDA/CDER (HFD-510) 

5600 Fishers Lane 

Rockville, MD 20857 

T: 301-827-6430 

F: 301-443-9282 

E: davisbrunok@cder.fda.gov

Dr. Vicki Dellarco 

Health Effects Division (7509C) 

Office of Pesticide Programs 

U.S. Environmental Protection Agency 

1200 Pennsylvania Ave., NW 

Washington, DC 20460 

T: 703-305-1803 

F: 703-305-5147 

E: dellarco.vicki@epamail.epa.gov

Dr. John DeSesso 

Mitretek Systems 

3150 Fairview Park Drive 

McLean, VA 22042-4519 

T: 703-610-2130 

F: 703-610-1702 

E: jdesesso@mitretek.org
165 

Dr. Rodney R. Dietert 

Dept. of Microbiology and Immunology 

College of Veterinary Medicine 

Cornell University 

C5135 Veterinary Medical Center 

Tower Road 

Ithaca, NY 14853-6401 

T: 607-253-4015 

F: 607-253-3384 

E: rrd1@cornell.edu

Dr. Joyce Donohue 

U.S. Environmental Protection Agency 

Ariel Rios Building (Mail Code 4304T) 

1200 Pennsylvania Avenue, NW 

Washington, DC 20460 

T: 202-566-1098 

F: 202-566-1139 

E: donohue.joyce@epamail.epa.gov

Dr. Brenda Eskenazi 

School of Public Health 

University of California, Berkeley 

140 Warren Hall 

Berkeley, CA 94720-7360 

T: 510-642-3496 

F: 510-642-9083 

E: eskenazi@uclink4.berkeley.edu

Dr. Elaine Faustman 

Professor and Director 

Institute for Risk Analysis and Risk 

 Communication 

Department of Environmental Health 

4225 Roosevelt Way NE, Suite 100 

Box 354695 

Seattle, WA 98105-6099 

P: 206-685-2269 

F: 206-685-4696 

E: faustman@u.washington.edu

Dr. Penny Fenner-Crisp 

Executive Director, Risk Science Institute 

International Life Sciences Institute (ILSI) 

One Thomas Circle, NW, Ninth Floor 

Washington, DC 20005 

T: 202-659-3306 

F: 202-659-3617 

E: pfennercrisp@ilsi.org

Dr. Michael P. Firestone 

Office of Children’s Health Protection 

U.S. Environmental Protection Agency 

1200 Pennsylvania Ave., NW (1107A) 

Washington, DC 20460 

T: 202-564-2199 

F: 202-564-2733 

E: firestone.michael@epa.gov

Dr. Gary Ginsberg 

Dept. of Environmental Epidemiology & 

 Occupational Health (EEOH) 

Connecticut Dept. of Public Health 

410 Capitol Avenue [MS-11CHA] 

Hartford, CT 06134-0308 

T: 860-509-7750 

F: 860-509-7785 

E: gary.ginsberg@po.state.ct.us

Dr. Daniel Goldstein 

Monsanto Company 

800 N. Lindbergh Blvd. (C2SE) 

St. Louis, MO 63167 

T: 314-694-6469 

F: 314-694-4028 

E: daniel.a.goldstein@monsanto.com

Dr. Jean Harry 

Laboratory of Toxicology 

NIEHS 

111 T.W. Alexander Drive (MD C1-04) 

Research Triangle Park, NC 27709 

T: 919-541-0927 

F: 919-541-0870 

E: harry@niehs.nih.gov

Dr. Dale Hattis 

Clark University-CENTED 

950 Main Street 

Worcester, MA 01610 

T: 781-641-0305 

F: 508-751-4603 

E: dhattis@aol.com
166 

Dr. Robert Kavlock 

Reproductive Toxicology Division 

NHEERL (MD 72) 

Office of Research and Development 

U.S. Environmental Protection Agency 

Research Triangle Park, NC 27711 

T: 919-541-2326 

F: 919-541-1499 

E: kavlock.robert@epa.gov

Dr. Carole Kimmel 

National Center for Environmental Assessment 

U.S. Environmental Protection Agency 

Ariel Rios Bldg (8623D) 

1200 Pennsylvania Ave., NW 

Washington, DC 20460 

T: 202-564-3307 

F: 202-565-0078 

E: kimmel.carole@epa.gov

Dr. Gary Kimmel 

National Center for Environmental Assessment 

U.S. Environmental Protection Agency 

Ariel Rios Bldg (8623D) 

1200 Pennsylvania Ave., NW 

Washington, DC 20460 

T: 202-564-3308 

F: 202-565-0078 

E: kimmel.gary@epa.gov

Dr. Steven Knott 

Risk Assessment Forum 

U.S. Environmental Protection Agency 

Ariel Rios Building (8601D) 

1200 Pennsylvania Ave., NW 

Washington, DC 20460 

T: 202-564-3359 

F: 202-565-0062 

E: knott.steven@epa.gov

Dr. Daniel Krewski 

Department of Epidemiology and 

 Community Medicine 

University of Ottawa 

Room 3229C, 451 Smyth Road 

Ottawa, Ontario, K1H 8M5 

CANADA 

T: 613-562-5660 

F: 613-562-5659 

E: dkrewski@uottawa.ca

Dr. Kannan Krishnan 

University of Montreal 

2375 Cote Ste. Catherine, Room 4105 

Montreal, PQ, Canada H3C 3J9 

T: 514-343-6581 

F: 514-343-2200 

E: kannan.krishnan@umontreal.ca

Dr. Phil Landrigan 

Community and Preventive Medicine 

Mount Sinai School of Medicine 

1 Gustave Levy Place, P.O. Box 1057 

New York, NY 10029 

T: 212-241-4804 

F: 212-996-0407 

E: phil.landrigan@mssm.edu

Dr. Bruce Lanphear 

General and Community Pediatrics 

 Research Center 

Children’s Hospital Medical Center 

3333 Burnet Avenue, ML 7035 

Cincinnati, OH 45229-3039 

T: 513-636-3778 

F: 513-636-4402 

E: bruce.lanphear@chmcc.org

Dr. Melanie Marty 

Office of Environmental Health Hazard 

 Assessment 

California Environmental Protection Agency 

1515 Clay Street, 16th Floor 

Oakland, CA 94612 

T: 510-622-3154 

F: 510-622-3210 

E: mmarty@oehha.ca.gov

Dr. Ray McAllister 

CropLife America 

1156 15th Street, NW, Suite 400 

Washington, DC 20005 

T: 202-872-3874 

F: 202-463-0474 

E: rmcallister@croplifeamerica.org
167 

Ms. M.E. (Bette) Meek 

Existing Substances Division 

Health Canada 

EHC Building, Room 145 

Tunney’s Pasture, AL: 0801C2 

Ottawa, Ontario, K1A OL2 

CANADA 

T: 613-957-3129 

F: 613-954-2486 

E: bette_meek@hc-sc.gc.ca

Dr. LaRonda Morford 

Eli Lilly & Company 

2001 West Main Street 

P.O. Box 708 

Greenfield, IN 46140 

T: 317-433-1780 

F: 317-277-5002 

E: lmorford@lilly.com

Dr. Stephen S. Olin 

Deputy Director, Risk Science Institute 

International Life Sciences Institute (ILSI) 

One Thomas Circle, NW, Ninth Floor 

Washington, DC 20005 

T: 202-659-3306 

F: 202-659-3617 

E: solin@ilsi.org

Dr. Merle Paule 

Division of Neurotoxicology (HFT-132) 

National Center for Toxicological Research 

U.S. FDA 

3900 NCTR Road 

Jefferson, AR 72079-9502 

T: 870-543-7147/7203 

F: 870-543-7745/7720 

E: mpaule@nctr.fda.gov

Dr. Kent Pinkerton 

Center for Health and the Environment 

University of California, Davis 

1 Shields Avenue 

Davis, CA 95616 

T: 530-752-8334 

F: 530-752-5300 

E: kepinkerton@ucdavis.edu

Dr. Jennifer Seed 

Risk Assessment Division (7403M) 

Office of Pollution Prevention and Toxics 

U.S. Environmental Protection Agency 

1200 Pennsylvania Ave., NW 

Washington, DC 20460 

T: 202-564-7634 

F: TBD 

E: seed.jennifer@epa.gov

Dr. Larry Sheets 

Toxicology Department 

Bayer CropScience 

17745 South Metcalf Avenue 

Stilwell, KS 66085-9104 

T: 913-433-5269 

F: 913-433-5125 

E: larry.sheets@bayercropscience.com

Dr. Michael Shelby 

Center for Evaluation of Risks to Human 

 Reproduction 

National Toxicology Program 

NIEHS 

P.O. Box 12233 (MD EC-32) 

Research Triangle Park, NC 27709-2233 

T: 919-541-3455 

F: 919-316-4511 

E: shelby@niehs.nih.gov

Dr. Wayne Snodgrass 

Clinical Pharmacology-Toxicology 

University of Texas Medical Center 

301 University Blvd 

Galveston, TX 77550-0631 

T: 409-772-9612 

F: 409-747-5205 

E: wsnodgra@utmb.edu

Dr. Diana Somers 

Pest Management Regulatory Agency 

Health Canada 

2720 Riverside Drive 

Ottawa, Ontario K1A 0K9 

CANADA 

T: 613-736-3510 

F: 613-736-3489 

E: diana_somers@hc-sc.gc.ca
168 

Dr. Babasaheb R. Sonawane 

NCEA/ORD 

U.S. Environmental Protection Agency 

Ariel Rios Building, Mail Drop 8623-D 

1200 Pennsylvania Ave., NW 

Washington, DC 20460 

T: 202-564-3292 

F: 202-565-0078 

E: sonawane.bob@epa.gov

Dr. Thomas Trautman 

General Mills, Inc. 

One General Mills Boulevard, ICS-3 

Minneapolis, MN 55426 

T: 763-764-7584 

F: 763-764-4242 

E: tom.trautman@genmills.com

Dr. Vanessa Vu 

Science Advisory Board 

U.S. Environmental Protection Agency 

Ariel Rios Building (MC-1400A) 

1200 Pennsylvania Ave., NW 

Washington, DC 20460 

T: 202-564-4533 

F: 202-501-0323 

E: vu.vanessa@epa.gov

Dr. Isabel Walls 

Senior Scientist, Risk Science Institute 

International Life Sciences Institute (ILSI) 

One Thomas Circle, NW, Ninth Floor 

Washington, DC 20005 

T: 202-659-3306 

F: 202-659-3617 

E: iwalls@ilsi.org

Ms. Tracey Zoetis 

Milestone Biomedical Associates 

15 Worman’s Mill Court, Suite I 

Frederick, MD 21701 

T: 301-624-2950 

T: 703-430-0603 

F: 301-663-4738 

E: tzoetis@milestone-pai.com
169 

APPENDIX 6: WORKSHOP PROGRAM

WORKSHOP TO DEVELOP A FRAMEWORK FOR ASSESSING RISKS TO 

CHILDREN FROM EXPOSURE TO ENVIRONMENTAL AGENTS 

July 30 – August 2, 2001 

Topnotch at Stowe Resort and Conference Center 

Stowe, VT 

Monday, July 30

DINNER 

 6:30-9:00 p.m. 

 “Child-Protective Risk Assessment: Challenge and Opportunity Phil Landrigan

Tuesday, July 31

 8:30-10:30 

• Welcome Steve Olin & Bob Sonawane

• Setting the Stage Steve Olin

- Workshop Objective and Scope 

- Process and Procedures 

• Background Presentations (+ Q&A) 

- Conclusions from EPA/RAF July 2000 Workshop Steven Knott

- Brief overviews of the draft framework and Lead Authors

 the background papers 

BREAK 

 10:45-11:00 

• Charge to the Breakout Groups Steve Olin

11:00-12:00 

• Breakout Group Session I – Kinetics, Dynamics, Risk Characterization 

LUNCH 

 1:00-3:00 

• Breakout Group Session I (Cont’d) 

BREAK 

 3:30-5:00 

• Breakout Group Session I (Cont’d) – Risk Characterization BOG meets 

briefly with Kinetics and Dynamics BOGs to present its questions for 

 these BOGs 

 5:00-5:30 

• Coordination Meeting: Chairs & Rapporteurs of the Breakout Groups 

6:00 DINNER 
170 

Wednesday, August 1

 8:00-8:30 

• Status Reports from Breakout Groups Chairs/Rapporteurs

8:30-10:00 

• Case Studies: Presentation and discussion of examples of risk assessments 

 in which differential internal dose or sensitivity of the developing human 

 was specifically considered and impacted the risk characterization 

- Atrazine Vicki Dellarco 

- Vinyl chloride Harvey Clewell 

- Pediatric drugs Karen Davis-Bruno 

BREAK 

10:30-12:00 

• Breakout Group Session II 

LUNCH 

 1:00-3:30 

• Breakout Group Session II (Cont’d) 

BREAK 

4:00-5:30 

• Reports from Breakout Groups Chairs/Rapporteurs

5:30-6:00 

• Coordination Meeting: Chairs & Rapporteurs of the Breakout Groups 

6:30 DINNER 

Thursday, August 2

 8:00-10:30 

• Breakout Group Session III (internal review and further work on draft reports) 

BREAK 

11:00- 12:30 

• Presentation and Discussion of Drafts Chairs/Rapporteurs

LUNCH 

 1:30-3:00 

• Presentation and Discussion of Critical Chairs/Rapporteurs

Data Gaps and Research Needs 

3:00 ADJOURN