i

 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

 WASHINGTON D.C. 20460

OFFICE OF THE ADMINISTRATOR

 SCIENCE ADVISORY BOARD

March 17, 2011

EPA-SAB-11-004

The Honorable Lisa P. Jackson

Administrator

U.S. Environmental Protection Agency

1200 Pennsylvania Avenue, N.W.

Washington, D.C. 20460

Subject: SAB Review of EPA’s “Development of a Relative Potency Factor (RPF) 

Approach for Polycyclic Aromatic Hydrocarbon (PAH) Mixtures (February 

2010 Draft)”

Dear Administrator Jackson:

EPA’s current approach to assessing cancer risk for polycyclic aromatic hydrocarbon 

(PAH) mixtures uses the relative potency factor (RPF) approach, which estimates the cancer risk 

of individual PAHs relative to benzo[a]pyrene (BaP). In 1993, EPA published RPF values for 6 

PAHs. EPA’s Office of Research and Development (ORD) has updated the RPF values for these 

6 PAHs and developed new RPF values for 18 additional PAHs, using recent studies from the 

published literature, as described in Development of a Relative Potency Factor (RPF) Approach 

for Polycyclic Aromatic Hydrocarbon (PAH) Mixtures (February 2010 Draft). 

ORD requested the Science Advisory Board (SAB) to peer review the PAH Mixtures 

document, focusing on: rationale for recommending an RPF approach, discussion of previously 

published RPF approaches, evaluation of the carcinogenicity of individual PAHs, methods for 

dose-response assessment and RPF calculation, selection of PAHs for inclusion in the RPF 

approach, derivation of RPFs for selected PAHs, and uncertainties and limitations associated 

with the RPF approach. The SAB convened the PAH Mixtures Review Panel to provide advice 

to the Agency. The key points and recommendations of the Panel are detailed in the report. 

Below is a brief highlight of the major comments and recommendations.

Overall, the SAB finds the document to be logical, clear, and concise. The SAB

recognizes the pragmatic need for the RPF approach. Based upon the currently available data, 

the SAB supports EPA’s use of the RPF approach for assessing carcinogenic risk from PAH 

mixtures. The SAB, however, provides recommendations to strengthen the scientific rationale 

for the RPF approach, the selection of studies, methods for dose-response modeling, and 

calculations of final RPFs. 
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Although the SAB supports the use of benzo[a]pyrene (BaP) as the index compound for 

the RPF approach, the cancer slope factor for BaP is outdated and it is essential that EPA 

expeditiously update the cancer slope factor for BaP.

The SAB also recommends that EPA consider developing a whole mixtures approach for 

PAHs. This approach could validate the RPF approach and in the future, could replace the RPF 

approach. The Agency should set this as a strategic initiative, with a specific timeline and 

benchmarks, that lays the foundation for an underlying concerted research program. The SAB 

recommends that the Agency seek support from the National Toxicology Program (NTP) and/or 

other entities to conduct testing of an appropriate portfolio of different complex PAH mixtures. 

These complex PAH mixtures should represent a diverse array of mixtures, but also represent the 

most important PAH mixture classes of concern to EPA. 

We appreciate the opportunity to provide EPA with advice. We look forward to 

receiving the Agency’s response.

Sincerely,

/Signed/ /Signed/

Dr. Deborah L. Swackhamer, Chair Dr. Nancy K. Kim, Chair

EPA Science Advisory Board SAB PAH Mixtures Review Panel
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NOTICE

This report has been written as part of the activities of the EPA Science Advisory Board, 

a public advisory committee providing extramural scientific information and advice to the 

Administrator and other officials of the Environmental Protection Agency. The Board is 

structured to provide balanced, expert assessment of scientific matters related to problems facing 

the Agency. This report has not been reviewed for approval by the Agency and, hence, the 

contents of this report do not necessarily represent the views and policies of the Environmental 

Protection Agency, nor of other agencies in the Executive Branch of the Federal government, nor 

does mention of trade names or commercial products constitute a recommendation for use. 

Reports of the EPA Science Advisory Board are posted on the EPA Web site at: 

http://www.epa.gov/sab.
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U.S. Environmental Protection Agency

Science Advisory Board

Polycylic Aromatic Hydrocarbon (PAH) Mixtures Review Panel

CHAIR

Dr. Nancy K. Kim, Senior Executive, Health Research, Inc., Troy, NY

MEMBERS

Dr. Shantu Amin, Professor, Department of Pharmacology, Penn State Hershey Cancer 

Institute , Penn State College of Medicine, Hershey, PA

Dr. Frederick A. Beland, Director, Division of Biochemical Toxicology, National Center for 

Toxicological Research, U.S. Food and Drug Administration, Jefferson, AR

Dr. James Chen, Senior Biomedical Research Service/Senior Mathematical Statistician, 

National Center for Toxicological Research, U.S. Food and Drug Administration, Jefferson, AR

Dr. John DiGiovanni, Professor and Coulter R. Sublett Chair in Pharmacy, Division of 

Pharmacology and Toxicology and Department of Nutritional Sciences, Dell Pediatric Research 

Institute, The University of Texas at Austin, Austin, TX

Dr. Marilie Gammon, Professor, Epidemiology, Gillings School of Global Public Health, 

University of North Carolina at Chapel Hill, Chapel Hill, NC

Dr. David Gaylor, President, Gaylor and Associates, LLC, Eureka Springs, AR

Dr. Nicholas Geacintov, Professor, Chemistry, New York University, New York, NY

Dr. Chris Gennings, Professor, Department of Biostatistics, Virginia Commonwealth 

University, Richmond, VA

Dr. Joshua Hamilton, Chief Academic and Scientific Officer; Senior Scientist, Bay Paul Center 

for Comparative Molecular Biology and Evolution, Marine Biological Laboratory (MBL), 

Woods Hole, MA

Dr. Edmond LaVoie, Professor and Chair, Department of Pharmaceutical Chemistry, College of 

Pharmacy, Rutgers, The State University of New Jersey, Piscataway, NJ

Dr. Aramandla Ramesh, Assistant Professor, Biochemistry and Cancer Biology, School of 

Medicine, Meharry Medical College, Nashville, TN

Dr. Benjamin Rybicki, Senior Scientist, Department of Research Epidemiology and 

Biostatistics, Henry Ford Hospital, Detroit, MI
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Dr. Paul Strickland, Professor, Environmental Health Sciences, Bloomberg School of Public 

Health, Johns Hopkins University, Baltimore, MD

Dr. Emanuela Taioli, Professor, Department of Epidemiology and Biostatistics, School of 

Public Health, State University of New York (SUNY) Downstate Medical Center, Brooklyn, NY

SCIENCE ADVISORY BOARD STAFF

Mr. Aaron Yeow, Designated Federal Officer, U.S. Environmental Protection Agency, 

Washington, DC 
vi

U.S. Environmental Protection Agency

Science Advisory Board

BOARD

CHAIR

Dr. Deborah L. Swackhamer, Professor and Charles M. Denny, Jr., Chair in Science, 

Technology and Public Policy and Co-Director of the Water Resources Center, Hubert H. 

Humphrey School of Public Affairs, University of Minnesota, St. Paul, MN

SAB MEMBERS

Dr. David T. Allen, Professor, Department of Chemical Engineering, University of Texas, 

Austin, TX

Dr. Claudia Benitez-Nelson, Full Professor and 

Director of the Marine Science Program, Department of Earth and Ocean Sciences , University 

of South Carolina, Columbia, SC

Dr. Timothy Buckley, Associate Professor and Chair, Division of Environmental Health 

Sciences, College of Public Health, The Ohio State University, Columbus, OH

Dr. Patricia Buffler, Professor of Epidemiology and Dean Emerita, Department of 

Epidemiology, School of Public Health, University of California, Berkeley, CA

Dr. Ingrid Burke, Director, Haub School and Ruckelshaus Institute of Environment and Natural 

Resources, University of Wyoming, Laramie, WY

Dr. Thomas Burke, Professor, Department of Health Policy and Management, Johns Hopkins 

Bloomberg School of Public Health, Johns Hopkins University, Baltimore, MD

Dr. Terry Daniel, Professor of Psychology and Natural Resources, Department of Psychology, 

School of Natural Resources, University of Arizona, Tucson, AZ

Dr. George Daston, Victor Mills Society Research Fellow, Product Safety and Regulatory 

Affairs, Procter & Gamble, Cincinnati, OH

Dr. Costel Denson, Managing Member, Costech Technologies, LLC, Newark, DE

Dr. Otto C. Doering III, Professor, Department of Agricultural Economics, Purdue University, 

W. Lafayette, IN
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Dr. David A. Dzombak, Walter J. Blenko Sr. Professor of Environmental Engineering , 

Department of Civil and Environmental Engineering, College of Engineering, Carnegie Mellon 

University, Pittsburgh, PA

Dr. T. Taylor Eighmy, Vice President for Research, Office of the Vice President for Research, 

Texas Tech University, Lubbock, TX

Dr. Elaine Faustman, Professor, Department of Environmental and Occupational Health 

Sciences, School of Public Health and Community Medicine, University of Washington, Seattle, 

WA

Dr. John P. Giesy, Professor and Canada Research Chair, Veterinary Biomedical Sciences and 

Toxicology Centre, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

Dr. Jeffrey Griffiths, Associate Professor, Department of Public Health and Community 

Medicine, School of Medicine, Tufts University, Boston, MA

Dr. James K. Hammitt, Professor, Center for Risk Analysis, Harvard University, Boston, MA

Dr. Bernd Kahn, Professor Emeritus and Associate Director, Environmental Radiation Center, 

Georgia Institute of Technology, Atlanta, GA

Dr. Agnes Kane, Professor and Chair, Department of Pathology and Laboratory Medicine, 

Brown University, Providence, RI

Dr. Madhu Khanna, Professor, Department of Agricultural and Consumer Economics, 

University of Illinois at Urbana-Champaign, Urbana, IL

Dr. Nancy K. Kim, Senior Executive, Health Research, Inc., Troy, NY

Dr. Catherine Kling, Professor, Department of Economics, Iowa State University, Ames, IA

Dr. Kai Lee, Program Officer, Conservation and Science Program, David & Lucile Packard 

Foundation, Los Altos, CA (affiliation listed for identification purposes only)

Dr. Cecil Lue-Hing, President, Cecil Lue-Hing & Assoc. Inc., Burr Ridge, IL

Dr. Floyd Malveaux, Executive Director, Merck Childhood Asthma Network, Inc., Washington, 

DC

Dr. Lee D. McMullen, Water Resources Practice Leader, Snyder & Associates, Inc., Ankeny, 

IA

Dr. Judith L. Meyer, Professor Emeritus, Odum School of Ecology, University of Georgia, 

Lopez Island, WA
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Dr. James R. Mihelcic, Professor, Civil and Environmental Engineering, State of Florida 21st 

Century World Class Scholar, University of South Florida, Tampa, FL

Dr. Jana Milford, Professor, Department of Mechanical Engineering, University of Colorado, 

Boulder, CO

Dr. Christine Moe, Eugene J. Gangarosa Professor, Hubert Department of Global Health, 

Rollins School of Public Health, Emory University, Atlanta, GA

Dr. Horace Moo-Young, Dean and Professor, College of Engineering, Computer Science, and 

Technology, California State University, Los Angeles, CA

Dr. Eileen Murphy, Grants Facilitator, Ernest Mario School of Pharmacy, Rutgers University, 

Piscataway, NJ

Dr. Duncan Patten, Research Professor, Hydroecology Research Program , Department of Land 

Resources and Environmental Sciences, Montana State University, Bozeman, MT

Dr. Stephen Polasky, Fesler-Lampert Professor of Ecological/Environmental Economics, 

Department of Applied Economics, University of Minnesota, St. Paul, MN

Dr. Arden Pope, Professor, Department of Economics, Brigham Young University , Provo, UT

Dr. Stephen M. Roberts, Professor, Department of Physiological Sciences, Director, Center for 

Environmental and Human Toxicology, University of Florida, Gainesville, FL

Dr. Amanda Rodewald, Professor of Wildlife Ecology, School of Environment and Natural 

Resources, The Ohio State University, Columbus, OH

Dr. Jonathan M. Samet, Professor and Flora L. Thornton Chair, Department of Preventive 

Medicine, University of Southern California, Los Angeles, CA

Dr. James Sanders, Director and Professor, Skidaway Institute of Oceanography, Savannah, 

GA

Dr. Jerald Schnoor, Allen S. Henry Chair Professor, Department of Civil and Environmental 

Engineering, Co-Director, Center for Global and Regional Environmental Research, University 

of Iowa, Iowa City, IA

Dr. Kathleen Segerson, Philip E. Austin Professor of Economics , Department of Economics, 

University of Connecticut, Storrs, CT

Dr. Herman Taylor, Director, Principal Investigator, Jackson Heart Study, University of 

Mississippi Medical Center, Jackson, MS
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Dr. Barton H. (Buzz) Thompson, Jr., Robert E. Paradise Professor of Natural Resources Law 

at the Stanford Law School and Perry L. McCarty Director, Woods Institute for the Environment, 

Stanford University, Stanford, CA

Dr. Paige Tolbert, Professor and Chair, Department of Environmental Health, Rollins School of 

Public Health, Emory University, Atlanta, GA

Dr. John Vena, Professor and Department Head, Department of Epidemiology and Biostatistics, 

College of Public Health, University of Georgia, Athens, GA

Dr. Thomas S. Wallsten, Professor and Chair, Department of Psychology, University of 

Maryland, College Park, MD

Dr. Robert Watts, Professor of Mechanical Engineering Emeritus, Tulane University, 

Annapolis, MD

Dr. R. Thomas Zoeller, Professor, Department of Biology, University of Massachusetts, 

Amherst, MA

SCIENCE ADVISORY BOARD STAFF

Dr. Angela Nugent, Designated Federal Officer, U.S. Environmental Protection Agency, 

Washington, DC

Dr. Thomas Armitage, Designated Federal Officer, U.S. Environmental Protection Agency, 

Washington, DC
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ACRONYMS

BaP Benzo[a]pyrene

BMD Benchmark Dose

BMDL Benchmark Dose (Lower Confidence Limit)

BMR Benchmark Response

CSF Cancer Slope Factor

EPA Environmental Protection Agency

IARC International Agency for Research on Cancer

IRIS Integrated Risk Information System

NCEA EPA’s National Center for Environmental Assessment

ORD EPA’s Office of Research Development

PAH Polycyclic Aromatic Hydrocarbon

RPF Relative Potency Factor

SAB Science Advisory Board
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TABLE OF CONTENTS

1. EXECUTIVE SUMMARY .................................................................................................. 1

2. INTRODUCTION................................................................................................................. 5

3. RESPONSE TO EPA CHARGE QUESTIONS................................................................. 6

3.1. Charge Question 1 – Overall Scientific Soundness of the RPF Approach ............................. 6

3.2. Charge Question 2 – Rationale for Recommending an RPF Approach.................................. 6

3.3. Charge Question 3 – Discussion of Previously Published RPF Approaches........................ 10

3.4. Charge Question 4 – Evaluation of the Carcinogenicity of Individual PAHs...................... 11

3.5. Charge Question 5 – Methods for Dose-Response Assessment and RPF Calculation......... 12

3.6. Charge Question 6 – Selection of PAHs for Inclusion in the Relative Potency Approach .. 20

3.7. Charge Question 7 – Derivation of RPFs for Selected PAHs............................................... 23

3.8. Charge Question 8 – Uncertainties and Limitations Associated with the RPF Approach.... 27

3.9. Charge Question 9 – Adequacy of Appendices for Independent Verification ..................... 29

4. REFERENCES.................................................................................................................... 30

APPENDIX – CHARGE QUESTIONS.................................................................................. A-1
1

1. EXECUTIVE SUMMARY

In 1993, EPA developed the document Provisional Guidance for Quantitative Risk 

Assessment of PAH that recommends a Relative Potency Factor (RPF) approach for assessing 

PAH mixtures. EPA’s Office of Research and Development (ORD) has developed a draft 

technical document, Development of a Relative Potency Factor (RPF) Approach for Polycyclic 

Aromatic Hydrocarbon (PAH) Mixtures, hereafter called “PAH Mixtures document”, to update 

the 1993 document by expanding the number of PAHs assessed and including recent studies 

from the published literature. 

EPA’s Office of Research and Development (ORD) requested that the Science Advisory 

Board (SAB) review the PAH Mixtures document. There were nine charge questions, which 

focused on the overall scientific soundness of the approach, on the specific chapters of the 

document, and the adequacy of the appendices to allow for independent verification. These 

charge questions are included in the Appendix and the responses to the charge questions are 

detailed in the report. The recommendations from the Panel for the major charge questions are 

highlighted below. Detailed responses to all the charge questions are presented in the body of 

the report. 

General Comments

Overall, the Panel finds the document to be logical, clear, and concise. The Panel 

recognizes the pragmatic need for the RPF approach, and based upon the currently available data, 

recommends that EPA continue to use the RPF approach for assessing cancer risk for PAH 

mixtures. The Panel agrees with EPA’s decision to update the 1993 approach by increasing the 

number of compounds in the approach, and including more recent data in calculating and 

expanding the RPF values for PAHs. The Panel recommends that the Agency finalize the 

document based upon the Panel’s comments and recommendations.

Rationale for Recommending an RPF Approach

EPA’s document presents the scientific rationale for recommending an RPF approach for 

PAH mixtures. The Panel has several recommendations for strengthening the rationale for using 

the RPF approach. Additional historical perspective should be added, since it is an important 

component in, and justification for the agency’s practical decision to continue with the RPF 

method. EPA indicated at the meeting that they had previously considered implementing a 

whole mixtures approach, but decided against it due to significant data gaps in available 

information. The Panel recommends including a discussion of these previous considerations and 

evaluation of data gaps, which would add to the rationale to continue with the RPF approach. 

The Panel recommends strengthening the rationale by discussing that the RPF approach 

relies on a direct comparison between dose-response curves from actual cancer bioassay data 

between BaP as the index compound and the target PAH. The Panel finds that the choice of BaP 

as the index chemical is well justified and is appropriately described for this RPF approach. The 

Panel is aware that a revised Integrated Risk Information System (IRIS) assessment for BaP is 

under concurrent development, and urges the Agency to quickly finalize that assessment. 
2

The Panel finds that EPA’s assumption that interactions among PAH mixture 

components do not occur at low levels of environmental exposure is not well justified in the 

document; however, in the absence of data that support a specific interaction (additive, sub- or 

super-additive, etc.), a default assumption of additivity is reasonable for the purposes of the RPF 

analysis.

Concurrent with the continued use of the RPF approach, the Panel recommends that EPA 

pursue developing a whole mixtures approach for PAHs to potentially validate the RPF approach 

and to serve as a possible replacement for the RPF approach in the future. 

Discussion of Previously Published RPF Values

EPA presents a background on how RPFs have been derived in the past and a qualitative 

comparison between the previous RPF approaches and studies testing whole mixtures of PAHs. 

The Panel believes that the document adequately summarizes the previous RPF approaches, but

could be improved by providing more quantitative information on the comparison between 

cancer risk estimates derived from the previous RPF approaches and those estimates derived 

from the whole mixtures approach. The Panel also recommends editing Table 3-1 to use a 

standardized approach for reporting values (same significant figures, scale, etc.). 

Evaluation of the Carcinogenicity of Individual PAHs

EPA discusses the development of a database of primary literature and the criteria used to 

include or exclude studies. Based upon the initial literature search, a list of 74 PAHs was 

evaluated. The Panel finds that the list of 74 PAHs is reasonable and that the database of 

primary literature appears adequate, but recommends that a recently published IARC Monograph 

on PAHs, Volume 92, be added to the database as an additional resource (IARC, 2010).

One of EPA’s study selection criteria is the stipulation that BaP must be tested 

concurrently with the target PAH being considered. This restriction raises the concern that 

animal bioassay data from studies of high quality may be dismissed. The Panel recommends that 

EPA consider exploring an approach where a target PAH that was tested with BaP could serve as 

a surrogate for BaP in studies where BaP was not tested concurrently. This may allow for 

additional data from studies of high quality to be included. However, in considering this 

alternative approach, EPA should also take into account factors that could potentially outweigh 

the benefits in the establishment of a RPF for a specific PAH, such as cross-study and crosslaboratory comparability issues. 

The Panel believes that a quality assessment should be done for each individual study. 

The Panel recommends including information such as sample size, dosing, mortality (prior to 

tumor development), test compound purity, and whether or not the data used are derived from 

tumor incidence or multiplicity data. 
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Methods for Dose-Response Assessment and RPF Calculation 

EPA presents the selection of dose-response data and methods for dose-response 

assessment and RPF calculation. For quantal data (i.e., tumor incidence), EPA used the multistage cancer model. The Panel agrees with EPA’s use of the multi-stage cancer model for 

quantal data, but has specific recommendations on the parameterization of the model. The Panel 

also recommends that EPA provide further detail on the assumptions regarding the distribution 

of data and further detail on the parameterization of the model. 

For continuous data (i.e. tumor counts), EPA used a linear model to calculate the 

benchmark dose (BMD). The Panel finds that the justification for using a linear model for multidose continuous data is insufficient and recommends that EPA provide further justification on 

the use of a linear model. In addition, the Panel recommends that the modeling strategy for 

continuous data include polynomial models or nonlinear models (e.g., the Hill model) that are 

flexible enough to fit the data and would also adequately approximate a linear relationship. 

Selection of PAHs for Inclusion in the Relative Potency Approach 

EPA describes the selection of PAHs for inclusion in the RPF approach. The Panel finds 

that the method for selecting the PAHs appears to be scientifically justified, but several issues 

such as individual study quality and study design variability across studies are incompletely 

considered. The Panel recommends that a list of quality criteria be defined, articulated, and 

applied a priori, prior to the weight of the evidence evaluation. Only studies of sufficient quality

should be considered in the weight-of-evidence evaluation. The Panel recommends that once a 

study is considered to have sufficient quality, the variability in study design characteristics 

among studies be carefully considered prior to inclusion in the RPF calculation. Differences 

among studies in some of these design characteristics may significantly affect the dose-response 

within each study, which in turn, will affect the RPF calculation.

Derivation of RPFs for Selected PAHs

EPA describes various methods (e.g., prioritization of studies) and different averaging 

approaches for deriving final RPFs. The Panel has several reservations regarding the RPF 

calculation approach. The Panel is concerned about calculating RPFs based upon a single 

experiment as well as calculating RPFs using studies with only a single-dose level of BaP and/or 

the target PAH, particularly if it was a high dose or if, only single doses of both the target PAH 

and BaP are available. The Panel does not make a recommendation on whether or not to 

calculate RPFs for PAHs with these data characteristics, recognizing that the Agency will need 

to apply professional judgment based on analyzing the actual available data. However, 

calculating an RPF from only a single dose of BaP is reasonable if the BaP tumor incidence is in 

the low-dose range and when adequate dose response data are available for the target PAH. 

The Agency is encouraged to continue evaluating other methods for combining RPFs 

across studies, such as using a geometric mean instead of an arithmetic mean. Using a geometric 

mean would give less weight to outlier values.
4

The Panel strongly believes that use of cancer bioassay data is essential for determining 

the RPF for a given PAH. Cancer-related endpoint data are useful as supporting data but the 

Panel does not recommend the use of only cancer-related endpoint data for determining the RPF. 

Therefore, the Panel does not recommend calculating an RPF for dibenz[a,c]anthracene and 

recommends that it be removed from Table 7.2 until further bioassay data become available.

Uncertainties and Limitations Associated with the RPF Approach

EPA discusses the uncertainties and limitations associated with using the RPF approach 

for PAH mixtures risk assessment. The Panel finds that the uncertainties in the methodology of 

deriving RPFs are well described. The major methodological uncertainties are clearly defined 

and discussed so that little doubt remains about the methods that were used and the limitations of 

the final RPF values. The Panel has the following recommendations to strengthen this section of 

the document: 

• Include comparisons of cancer risk estimates of complex mixtures using the RPF 

approach and bioassay data. 

• Include a discussion on bioavailability.

• Include a discussion of the uncertainty that arises from the difficulty and limitation of 

completely characterizing mixtures.

Adequacy of Appendices for Independent Verification

The appendices in the document include dose-response data for potency calculations, 

benchmark dose modeling outputs, and calculation of RPFs to allow independent verification of 

the calculated RPFs. The Panel finds the appendices to be generally useful for verifying the 

calculations of the RPFs, but has the following recommendations:

• Reorganize the appendices by chemical (with each identified in the Table of 

Contents). This would include the corresponding BaP data for each study within each 

chemical section which may be repeated across PAHs. 

• Revise the plots from the BMD software output to be based on BMDs instead of the 

lower confidence limits of the BMDs (BMDLs).
5

2. INTRODUCTION

In 1993, EPA developed the document Provisional Guidance for Quantitative Risk 

Assessment of PAH that recommends a Relative Potency Factor (RPF) approach for assessing 

PAH mixtures. EPA’s Office of Research and Development (ORD) has developed a draft 

technical document, Development of a Relative Potency Factor (RPF) Approach for Polycyclic 

Aromatic Hydrocarbon (PAH) Mixtures (February 2010 Draft), hereafter called “PAH Mixtures 

document”, to update the 1993 document by expanding the number of PAHs assessed and 

including recent studies from the published literature. 

PAHs are a class of chemicals that have variously been defined to include organic 

compounds containing either two or more, or three or more, fused rings made up of hydrogen 

and carbon atoms (WHO, 1998). The number of chemicals that comprise the PAH class is not 

known, but hundreds of PAHs are thought to be present in complex mixtures (WHO, 1998). 

PAHs do not occur in the environment as isolated entities; they primarily occur in complex 

mixtures generated from the incomplete combustion or pyrolysis of substances containing 

hydrocarbons. Some of the complex mixtures containing PAHs that are typically found in the 

environment include coal tar, manufactured gas plant (MGP) residues, coke oven emissions, 

diesel and gasoline exhaust, and coal plant emissions. Many PAHs are demonstrated to be 

carcinogenic in animal bioassays. 

EPA’s PAH Mixtures document presents a component-based approach to assessing the 

cancer risk of PAH mixtures by summing doses of component PAHs after scaling the doses 

relative to the potency of the selected index PAH, benzo[a]pyrene (BaP). The cancer risk is then 

estimated using the dose-response curve for the index PAH.

The PAH Mixtures document is limited in focus to analyzing only unsubstituted PAHs 

with three or more fused aromatic hydrocarbon rings. The analysis evaluated 74 PAHs, and final 

RPFs were calculated for 24 of the PAHs. Six of these PAHs have updated RPFs from the 1993 

guidance, and 18 of these PAHs have new RPF values. Additionally, 3 PAHs were assigned an 

RPF of zero.

ORD has requested that the Science Advisory Board (SAB) conduct a review of the 

document. In response to ORD’s request, the SAB Staff Office solicited nominations of experts 

and formed the SAB PAH Mixtures Review Panel. The Panel held a public teleconference on 

June 8, 2010, and a public meeting on June 21-23, 2010, to review EPA's Development of a 

Relative Potency Factor (RPF) Approach for Polycyclic Aromatic Hydrocarbon (PAH) Mixtures

document and to deliberate over the charge questions. The Panel discussed its draft report during

a subsequent conference call on September 30, 2010. The Panel’s draft report was approved by 

the Chartered SAB on December 16, 2010, on a public teleconference call. There were nine 

charge questions, which focused on an overview of the document, on the specific chapters of the 

document, and the appendices. These charge questions are included in the Appendix and the 

responses to the charge questions are presented below.
6

3. RESPONSE TO EPA CHARGE QUESTIONS

The charge questions are presented below in italics, followed by the responses from the 

Panel in normal text.

3.1. Charge Question 1 – General Charge Questions

1a. Please comment on whether the report is logical, clear and concise. Please comment on 

whether EPA has clearly synthesized the scientific evidence for the derivation of relative potency 

factors for individual PAHs.

Overall the Panel finds the PAH Mixtures document to be logical, clear, and concise.

EPA has clearly synthesized the scientific evidence for the derivation of relative potency factors 

for individual PAHs. The Panel recognizes the pragmatic need for the RPF approach. Based 

upon the currently available data, the Panel recommends that EPA continue to use the RPF 

approach for PAH mixtures. The Panel agrees with the Agency that in order to continue with the 

RPF method, it is important to expand the number of compounds in the 1993 guidance, and for 

the most part the candidate compounds for this expanded list are appropriate (see Chapter 4 

discussion). The Panel also agrees that it is important to include more recent data for these 

compounds (since 1993) in calculating and expanding the RPF values for PAHs, since many of 

the values used in the current RPF method are based on older data. 

1b. Please comment on whether the report provides adequate context for how the proposed RPF 

approach could be used in a PAH mixtures risk assessment.

The Panel finds that the PAH Mixtures document does not provide an adequate context 

for how the proposed RPF approach could be used in a PAH mixtures risk assessment. The 

Panel recommends that more discussion is needed to provide this context, including moving 

relevant portions from Chapter 7 (sections 7.3 and 7.4) into earlier sections of the document and 

providing an example (also see response to Charge Question 7e). 

3.2. Charge Question 2 – Rationale for Recommending an RPF Approach

Chapter 2 presents the rationale for recommending an RPF approach. In an RPF approach, 

doses of component chemicals that act in a toxicologically similar manner are added together, 

after scaling the doses relative to the potency of an index chemical. Benzo[a]pyrene (B[a]P) is 

selected as the index compound for this RPF approach. The RPF approach involves two key 

assumptions related to the application of a dose-additivity model: (1) PAH components in the 

mixture act in a similar toxicological manner; and (2) interactions among PAH mixture 

components do not occur at low levels of exposure typically encountered in the environment.
7

2a. Please comment on whether the report provides adequate justification for using an RPF 

approach as a scientifically defensible method to assess the cancer risk associated with exposure 

to PAH mixtures.

At the face-to-face meeting, the Panel discussed this issue in considerable detail, and 

concluded that this charge question actually represents two distinct questions: first whether, 

based on available literature, there is adequate justification for use of the single-agent relative 

potency factor (RPF) approach, particularly with respect to the two core assumptions of this 

rationale that were proposed in the PAH Mixtures document; and second, whether there is a 

reasonable practical consideration in using the RPF approach at this time, independent of the 

justification and underlying assumptions. The rationale for this dichotomy is outlined below.

With regard to the first question, the Panel concludes that the rationale for the proposed 

RPF approach is not well justified in the current document. There are two basic assumptions that 

are proposed in the document as the basis for considering the RPF approach specifically for

PAHs: first, that the chemicals of comparison are all assumed to act in a similar toxicological 

manner as the reference compound (i.e., benzo[a]pyrene - BaP); and second, that their effects are 

additive by assuming no significant interactions at low, environmentally relevant doses. 

The Panel considered the PAH Mixtures document, the studies cited within, as well as 

other data. The document discusses studies that call into question both of the underlying core 

assumptions and further elaborates on a number of other uncertainties, some of which cannot 

currently be validated or dismissed, that further undermine the logical basis for the assumptions 

on which the RPF method is based. These are discussed in more detail in response to charge 

questions 2c and 2d below, but are briefly summarized here. It is not clear that the first 

assumption – i.e., that the other PAHs under consideration act in a similar toxicological manner 

as BaP - is required as a foundation for the RPF method, since for these particular PAHs the 

method is based on the outcomes of cancer bioassays, rather than the underlying toxicological 

action. There are also results, some of which are discussed in the document (page 23, lines 11-

19; page 39, lines 3-12 and Table 2-2), that call into question the second assumption – i.e., that 

there are no significant low-level interactions of PAHs in a mixture beyond simple additivity, 

and therefore that the effects (cancer risks) of a mixture of agents are the simple sum of the 

individual risks. 

Despite these concerns about the underlying justification for the RPF method and the 

logic of the two core assumptions, the Panel concludes that there is adequate practical

justification for continuing to use this approach in the near term to assess cancer risk of PAH

mixtures in the absence of a good alternative. In particular, although this Panel and previous 

expert panels have strongly suggested that the EPA move toward a whole mixtures-based 

approach, the fact remains that the regulatory and scientific communities do not have sufficient 

information to adopt a whole mixtures approach for risk assessment at this time. Therefore, the 

Panel recommends the continued use of the component-based RPF approach as the most 

practical choice but recommends that this should proceed in parallel with continued development 

of one or more whole mixtures-based approaches that could potentially validate the RPF

approach or possibly replace it.
8

Given these conclusions, the Panel has several recommendations for revising the 

document and moving forward with the RPF approach. First, additional historical perspective 

should be included in the revised document, since it is an important component in, and 

justification for the agency’s practical decision to continue with the RPF method. EPA indicated 

that they had previously considered implementing a whole mixtures approach, but decided 

against it due to data gaps in available information. The Panel recommends including a 

discussion of these previous considerations and evaluation of the data gaps. This would help 

with the rationale for continuing with the RPF approach. 

In parallel with the continued use of the RPF approach, the Panel recommends that EPA 

begin developing a whole mixtures approach to achieve two goals: (1) to potentially validate the 

RPF approach, and (2) to explore as a possible replacement for the RPF approach in the near 

future. The Panel recommends that the Agency set these goals as strategic initiatives, with 

specific timelines and benchmarks. This would lay the foundation for an underlying concerted 

research program to achieve these goals.

The Agency should seek support from the National Toxicology Program (NTP) or other 

entities to test a portfolio of 12-15 different complex mixtures in animal studies. These mixtures 

should represent a diverse array of mixtures but also represent the most important mixture

classes of concern to EPA (based on the level of health concerns and/or extent of exposure) such 

as coal tar, manufactured gas plant (MGP) residues, coke oven emissions, diesel and gasoline 

exhaust, coal plant emissions, etc. With these data in hand, one could then potentially validate 

the RPF approach and also compare an environmental mixture to this portfolio of standardized 

mixtures and be able to adequately estimate risk. 

These mixtures could also be compared to a surrogate mixture (e.g., a mixture 

representing the ca. two dozen compounds being assessed in the RPF method) as well as BaP as 

a single agent. This would provide a direct validation of the RPF method and link these results 

to previous data on environmental samples for which RPF compound values are known.

2b. Please comment on whether the choice of benzo[a]pyrene as the index compound is 

scientifically justified and appropriately described. Please identify and provide the rationale for 

any alternative index compound(s) that should be considered.

The choice of BaP as the index compound is well justified and is appropriately described 

for this RPF approach. It is the best studied PAH and meets the criteria for an index compound 

for an RPF assessment. The Agency noted that a revised IRIS assessment of BaP is undergoing 

a parallel review that will likely lead to a revised cancer slope factor (CSF) as well as separate 

values for oral, dermal and inhalation BaP exposures. An up-to-date estimate of the CSF for BaP 

is central to the validity of the RPF method since this is the index compound, and the Panel urges 

the Agency to quickly finalize the BaP assessment.
9

2c. Please comment on whether the weight of evidence indicating that PAHs, as a chemical class, 

have a similar mode of carcinogenic action has been adequately described and is scientifically 

justified.

There is some evidence that a subset of closely related PAHs have a similar mode of 

action in inducing cancer as described in the document. This is not unexpected since the 

compounds in question have already been defined in large part by their comparison to BaP. 

However, although these compounds have similar modes of action, available data indicate that 

they each act via different precise mechanisms when examined at a more detailed level. For 

example, even though many PAHs are metabolized to reactive intermediates that then form DNA 

adducts at guanine residues, their potency for conversion of DNA adducts to mutations varies 

among compounds. Moreover, the pattern of guanine mutations within specific DNA sequences 

varies among these adducts. By definition, these adducts are therefore acting by slightly 

different mechanisms. Since cancer risk can be related to mutation rate and to specific mutations 

within certain DNA sequences, this will result in different risks even though these compounds 

share a similar mode of action.

Additionally, there are hundreds of other PAHs and PACs that may not act by these 

modes of action and that likely, particularly in complex mixtures, contribute in positive or 

negative ways to the overall carcinogenicity of the mixture. These compounds should also be 

considered in the RPF method if adequate cancer bioassay data are available. 

Also of importance, other PAHs in a mixture may alter the risk for known PAHs in that 

mixture in more complicated ways. For example, through mass action a complex mixture may 

contain total PAHs that collectively overwhelm the levels of an individual PAH such as BaP, 

perhaps by 1000:1 or greater. These may collectively interfere with the overall metabolism of 

BaP, or ratios of specific metabolites, or the capacity to repair DNA adducts from BaP, etc., such 

that one cannot predict the cancer risk from BaP solely from its concentration in the mixture. 

Therefore, this assumption is not well justified. 

In addition, there is a question as to whether similar modes of action are sufficient to 

predict in vivo carcinogenicity. As discussed in the PAH Mixtures document (e.g., page 35, 

section 2.6), mutagenicity, genotoxicity and similar short-term assays are relatively poor 

predictors of in vivo carcinogenesis. Yet a basic assumption of the document is that this 

information is sufficient to predict their relative carcinogenicity. There are PAHs that are 

positive in short-term in vitro assays but negative or weak in animal bioassays, and vice versa, 

further undercutting this basic assumption. 

The document also discusses the role of the Ah receptor (AhR) in detail as another 

potential unifying mode of action for carcinogenic PAHs. However, elsewhere in the document, 

EPA also acknowledges that interaction with and activation of the AhR is not a good indicator of 

promotion or in vivo tumorigenesis for PAHs (as opposed to dioxins). The Panel agrees with 

this latter assessment.

Taken together, these points argue that there are weaknesses in using the assumption of 

PAHs having a similar mode of action as a rationale for using the RPF approach. The Panel 
10

recommends strengthening the rationale by including a discussion about the RPF approach 

relying on a direct comparison between dose-response curves from actual cancer bioassay data 

between BaP and the target PAH.

2d. Please comment on whether the assumption that interactions among PAH mixture 

components do not occur at low levels of exposure typically encountered in the environment has 

been adequately described and is scientifically justified.

The assumption that there are not significant interactions among PAHs in complex 

mixtures at low doses is not well justified in the document. As discussed in the document (page 

23, lines 11-19) coal tar behaved differently in in vivo carcinogenesis assays than would be 

predicted from studies with BaP as a single agent, or what would likely be predicted from a RPF 

approach based on BaP as the index chemical. Likewise, as discussed in the document (page 39, 

lines 3-12 and Table 2-2), the complex and unpredictable results to date of simple binary 

combinations of PAHs that do not follow simple additivity also undercuts the assumption of 

interactions between PAHs not occurring at low environmental levels. However, in the absence 

of consistent data that support a specific type of interaction (additive, sub- or super-additive, etc.) 

that could be used for a variety of PAH mixtures, a default assumption of additivity is a 

reasonable assumption for the purposes of the RPF analysis. 

It should be noted, however, that complex mixtures such as coal tar, MGP residues, 

creosote, diesel exhaust and other PAH mixtures contain hundreds of other compounds, not 

included in this RPF assessment, that likely contribute to the overall biological effects of the 

mixtures. Other contributing mechanisms may include: induction or suppression of specific 

metabolic pathways; competition for metabolism through mass action at active sites; epigenetic 

effects; promotion and progression effects; endocrine disruption, and neurological and 

immunological effects that contribute to cancer risk. Other classes of potentially potent 

carcinogens including substituted PAHs, volatile organic compounds (VOCs), metals, as well as 

other compounds may also be present. Collectively, these mechanisms and compounds may 

contribute in complicated ways to the overall cancer risk of a complex mixture. This uncertainty 

can be reduced by directly testing mixtures in cancer bioassays. As discussed above the Panel 

recommends the Agency test 12-15 complex mixtures of concern to EPA.

3.3. Charge Question 3 – Discussion of Previously Published RPF Approaches

This chapter presents a discussion of previously published RPF approaches. Due to the 

evolution of the state of the science and an increased understanding of PAH toxicology, EPA is 

reevaluating the RPF approach for PAHs in this analysis.

3. Please comment on whether the discussion provides a meaningful background on how RPFs 

have been derived in the past, and the advantages and disadvantages of previous methods.

This chapter provides a summary of previous RPF approaches and provides a qualitative 

discussion and comparison of cancer risk estimates based on RPF approaches with estimates 

obtained from testing whole mixtures. The Panel finds that this chapter adequately summarizes
11

the previous RPF approaches, but could be improved by providing more quantitative information

on the comparison between the previous RPF approaches and the whole mixtures approach. The 

Panel also recommends editing Table 3-1 to use a standardized approach for reporting values 

(same significant figures, scale, etc.).

3.4. Charge Question 4 – Evaluation of the Carcinogenicity of Individual PAHs

This chapter discusses the development of a database of primary literature on PAH 

carcinogenicity and cancer-related endpoints and the criteria used to include or exclude studies 

from the database. 

4a. Please comment on whether the list of 74 PAHs (Table 2-1) included in the initial literature 

search is complete. Please comment on whether the rationale for the choice of PAHs included in 

the literature search has been appropriately described. Please identify other databases or 

resources that should be included.

Chapter 4 of the PAH Mixtures document details the basis for the selection criteria that 

was used to develop the database related to PAH carcinogenicity and cancer-related endpoints. 

The list of 74 PAHs provided in Table 2-1 is believed by the Panel to be reasonable in view of 

the criteria of having three or more fused rings and not containing heteroatoms, alkyl or nitro 

substituents. The development of the database of primary literature on PAH carcinogenicity and 

cancer-related endpoints and the criteria used to include or exclude studies from the database are

described in detail within this chapter. The database appears adequate, with the recommendation 

that a recently published IARC Monograph on PAHs, Volume 92, be included as an additional 

resource (IARC, 2010).

4b. Chapter 4 includes a description of how studies were selected for use in dose-response 

assessment. Please comment on whether the choices and assumptions in making the selection 

have been adequately described. Please comment on whether the information in Tables 4-1 

through 4-14 provides adequate information to inform how decisions were made. Please 

comment on whether studies were rejected or included appropriately. Please comment on 

whether positive and nonpositive studies have been considered appropriately.

The basis for selecting which studies were used in dose-response assessment is clearly 

delineated. The information in Tables 4-1 through 4-14 provides adequate information related to 

whether certain studies were rejected or included in this document. However, the criteria for 

including or rejecting a study should be revised to include only studies that are deemed to be of 

sufficient quality using a priori standards as described in the response to charge question 6a. 

Given this revision of including only studies of high quality, the EPA approach inappropriately 

discards data that do not achieve statistical significance. Please see the response to charge 

question 6a for further detail.
12

4c. The methodology for the choice of studies to use in the derivation of RPFs includes studies 

where at least one PAH was tested at the same time as B[a]P. Studies where individual PAHs 

were tested without concurrent testing of B[a]P were not included in the quantification of RPFs. 

Please comment on the scientific rationale for this approach. Please comment on whether the 

advantages and disadvantages of excluding certain data from the derivation of RPFs have been 

adequately described.

Chapter 4 of the document stipulates that BaP had to be tested concurrently to include a 

study on the carcinogenicity or other cancer-related endpoints of one or more of these 74 PAHs. 

This restriction raises a concern that data from carcinogenicity studies of high quality might be 

dismissed. The Panel recommends that EPA consider whether a PAH other than BaP, with a 

RPF that has a comparatively narrow range, might be able to serve as the surrogate for the BaP

index compound in those instances where BaP was not included in a bioassay. This approach 

offers the possibility that additional data from studies of high quality could be included in the 

development of a RPF for a given PAH. The Panel recommends that this be examined especially 

in those instances where limited animal bioassay data were used to establish a RPF value. 

However, in considering this alternative approach, EPA should also take into account factors that 

could potentially outweigh the benefits in the establishment of a RPF for a specific PAH, such as 

cross-study and cross-laboratory comparability issues. 

The Panel has a few recommendations that relate to the evaluation of the carcinogenicity 

studies for individual PAHs. These recommendations include providing some quality 

assessment to individual studies, such as a tabulation for various studies, including information 

on: 1) sample size, 2) dosing, 3) mortality (prior to tumor development), 4) defined test 

compound purity, and 5) whether or not the data used are tumor incidence or multiplicity data. 

In addition, the Panel recommends incorporating or reiterating some of the discussion 

about alternatives for ranking RPFs provided in Appendix G into the discussions on individual 

PAHs in Chapter 4 as well as in Chapter 6. For example, the Panel considers the discussion 

about the influence of the route of administration on the RPF calculations to be particularly 

informative.

3.5. Charge Question 5 – Methods for Dose-Response Assessment and RPF Calculation

This chapter describes the selection of dose-response data and methods for dose-response 

assessment and RPF calculation from the selected datasets. The methodology for estimation of 

the RPFs varied depending on the characteristics of the datasets, however, the general equation 

was the ratio of the slope of the dose-response curve for the subject PAH to the slope of the doseresponse curve for B[a]P.
13

5a. Please comment on whether the scientific rationale for the dose-response modeling 

approaches used in the derivation of RPFs is adequately described. Please comment on whether 

there are other appropriate modeling approaches for estimating the relative potencies of PAHs. 

Please describe alternative approaches (e.g., other model forms) that could be considered.

The Panel finds that the scientific rationale for the dose-modeling approaches is 

adequately described. The panel does have recommendations on additional modeling approaches 

that could be considered. The modeling approaches described in Chapter 5 of this document for 

multi-dose studies are based on whether the data are quantal (binary) or continuous. The quantal 

endpoints considered in this document include tumor incidence or incidence of cancer-related 

endpoints, including frequency of mutations per number of cells interrogated. The continuous 

endpoints include tumor counts (number of tumors per animal) or cancer-related endpoints of a 

continuous-variable nature (e.g., number of sister chromatid exchanges, number of 

morphologically transformed colonies). 

When modeling quantal data, the mean model is for the probability of response (e.g., 

tumor incidence) and is generally assumed to follow a sigmoid-shape. Commonly used models 

that could be used include the logistic, probit, multi-stage, and Gompertz models. Since the 

multi-stage cancer model has a biological basis, it is the standard model used for cancer 

incidence and is considered sufficiently flexible to accommodate the dose-response data for these 

PAHs. Specifically, the multi-stage cancer model for the probability of a tumor is parameterized 

based on the number of dose groups (g) with the polynomial assumed to equal g-2: 

2 2

0 0 12 2 (1 ) 1 exp( ... gg μβ β β β β xx x −

− = +− − + ++     .

It should be noted that a model for data with g dose groups will exactly track the sample 

means (here, sample proportions) if the degree of the polynomial is g-1. However, a variation of 

this general model, typically used in risk assessment, assumes a monotonic relationship and 

constrains all parameters to be non-negative. The Benchmark Dose (BMD) Software used in the 

document makes such an assumption as the default analysis. With quantal data, assumed to be 

independent across and within-dose groups, it is generally assumed that the data are binomially 

distributed with binomial variance (i.e., with n subjects evaluated at a dose group, the variance in 

the number “responding” is assumed to be nμ μ (1 ) − ). Alternatively, the data may follow hyperor hypo-binomial variability, i.e., greater than or less than binomial variability. These 

assumptions are not specified in the document and should be. The BMD Software used to 

estimate unknown model parameters uses a maximum likelihood estimation criterion and 

standard iterative algorithms for estimation. However, these distributional assumptions and the 

parameterization of the multi-stage cancer model should be clearly stated in the document. It is 

not clear whether the assumption of binomial variability was verified; the assumption of 

binomial variability should be verified and the document should include information about the 

verification. Instead, the model checking was based on the goodness-of-fit of the mean model 

and did not assess the assumptions regarding variability.

For continuous endpoints, a nonlinear dose-response shape may be expected from the 

data. However, the analysis plan for continuous endpoints is to use a linear model (i.e., a linear 

function). The justification for using the linear model for the multi-dose continuous data is 
14

insufficient and additional justification should be added. Although the linear model is the 

simplest model, there are other models such as the Hill model or polynomial model that are 

commonly used. EPA’s justification for using the linear model is that there are a small number 

of dose groups. This is an inadequate explanation. 

The modeling strategy for the continuous endpoints should include polynomial models or 

nonlinear models (e.g., the Hill model) that are flexible enough to fit the data and would also 

adequately approximate a linear relationship. In some cases, the variance in response is assumed 

to be constant over the dose range of observed data. A least-squares (or nonlinear least squares) 

criterion is used to estimate unknown model parameters. In contrast, the sample variance may 

change with the mean. For example, the responses in the low-dose region may have lower 

variance than that observed as the dose (and response) increase. Such data may be estimated 

using a quasi-likelihood estimation criterion. 

For the continuous data included in this document, the assumption about whether the 

variance changes across the dose groups is not addressed and the potential for a nonlinear shape 

is not allowed. Only a linear model was used to estimate the mean response. A goodness-of-fit 

criterion was used; if the model did not provide adequate fit, high-dose groups were sequentially 

eliminated in an effort to achieve adequate fit. This strategy is arbitrary and should be avoided. 

A more flexible model should be used instead that accommodates the nonlinearity of the data.

Selection of Benchmark Response (BMR)

Since the RPFs are going to be used to estimate cancer risks at generally low 

environmental exposures, the calculation of RPFs should be applicable to the low-dose range, 

preferably excess risks ≤0.10 for quantal data. Similarly for continuous data, the calculation of 

RPFs should preferably be based on changes in the mean of less than or equal to one standard 

deviation (and certainly less than two standard deviations) in order to remain in the low-dose 

region of interest. For normally-distributed data, a change in the mean from the control mean of 

two standard deviations will result in approximately 50% of the subjects in the abnormal range. 

The RPF can increase or decrease substantially as dose (incidence or response) changes.

The analysis strategy described in Chapter 5 (with the suggested changes included) 

should be specifically followed. Deviations from the planned analysis strategy should be clearly 

explained. 

To illustrate the use of a nonlinear model, the in vitro clastogenicity dose-response data 

of Tong et al (1981) (Table C-19, page C-85 of PAH Mixtures document) is reanalyzed. For 

convenience, the data table is reproduced in Table 1. The data clearly follow a nonlinear 

relationship, which is particularly evident, considering the two highest concentrations of 

benz[a]anthracene (BaA) that have similar responses with a log change in concentration.
15

Instead of fitting a linear model to these data, a 3-parameter Hill model is selected, which 

can accommodate an asymptotic response for large concentrations, i.e., 

x

x

γ μ α θ

= +

+ ,

where x is the concentration of the PAH, α is the response for the control group, γ is the range of 

response such that α+γ is the asymptote for large x. Since only sample means and standard 

deviations are available at each concentration level, a weighted analysis is imposed, with weights 

set to the inverse of the sample standard deviation at each concentration. Unknown parameters 

are estimated using a weighted least squares criterion in a Gauss-Newton iterative algorithm 

using PROC NLIN in SAS (version 9.2). The resulting predicted models for BaP and BaA are 

provided in Figure 1. Using all of the data, a Hill model adequately fits the observed sample 

means for both PAHs.

Table 1: Data from Tong et al, 1981, for sister chromatid exchange summary data 

(Record number: 21710; Table C-19, page C-85). The BMR was set to the control 

mean from the predicted Hill model + SD of the control group. The BMDs are 

estimated from the Hill model using the specified BMR.

PAH Concentration

(Molar)

Mean Sister 

Chromatid 

Exchange/cell

Standard 

Deviation 

(SD)

Benchmark 

Response 

(BMR)

Benchmark 

Dose 

(BMD)

(Molar)

Control 0 11.15 3.81 13.7

4×10

BaP 10 -7

-6 16.15 3.83

BaP 10-5 59.75 16.96

BaP 10-4 103.3 22.75

Control 0 15.75 5.18 20.9

7×10 BaA 10 -6

-5 21.2 9.59

BaA 10-4 29.15 9.93

BaA 10-3 26.2 6.96
16

The specified BMR for continuous data is one standard deviation (SD) above the control 

mean as predicted from the Hill model (shown in Table 1). For BaP, the estimated BMD1SD is 

4×10-7 and for BaA, the estimate is 7×10-6. However, in Table E-14 (page E-31), the BMR and 

BMD values are blank and the point estimate responses are 92 and 13 for BaP and BaA, 

respectively; and the point estimate dose is 1x10-4 for both compounds. It is not clear how the 

point estimate responses were calculated. This is an example where the described analysis plan 

does not seem to be followed without any explanation of why it was not followed.

5b. For each individual dataset considered in the assessment, the B[a]P dose-response was 

calculated from the study-specific data. Please comment on whether this approach has been 

appropriately described. If there are additional approaches using the available data that should 

be considered, please describe how the approach could lead to a better estimate of cancer risk.

The strategy of using study-specific data for the BaP dose-response with PAH doseresponse is advantageous since downstream calculations are intra-study and avoid comparisons 

that would require accounting for possible study-specific effects. This strategy has been 

appropriately described and the Panel does not have other approaches to suggest.

It should be noted that the estimates of BaP slope across studies with different 

characteristics are very different. The range of the estimates can be more than 1,000 fold. This 

supports the idea of using study-specific estimates for calculating the RPFs. 

Figure 1: Observed sample means and predicted response from a Hill model. Sample means are 

denoted with dots and +/- one standard deviation from each mean is denoted by the error bars.

A B
17

5c. The point of departure for slope estimation that has been used for the derivation of RPFs is 

the benchmark dose (BMD) estimate rather than the lower confidence limit on the benchmark 

dose (BMDL). Please comment on whether this approach is scientifically justified and 

adequately described. Please comment on whether alternative approaches should be considered.

It is correct to base the derivation of the RPFs on the estimate derived from the BMD, 

rather than the lower confidence limit on the benchmark dose (BMDL), in order to obtain an 

estimate of the total exposure for a mixture (expressed as the total BaP equivalent dose). Due to 

chance experimental variation, some of the RPFs will be overestimated and some will be 

underestimated. These biases will tend to cancel each other for the total exposure of a mixture. 

On the other hand, when the study sizes are similar, the BMDLs between the BaP and PAH may 

be stable. But when the two studies have different precision, the ratio of BMDLs is tenuous. 

Therefore, the ratio of BMDs is advisable. The Panel does not believe that any alternative 

approaches are necessary.

5d. Please comment on the methodology used for the RPF calculations for multidose and single 

dose datasets. Please comment on whether the process for calculating RPFs from the various 

datasets is scientifically justified and adequately described. Please comment on the utilization of 

high response levels in some instances as the point of comparison. Please describe alternative 

approaches that could lead to a better estimate of cancer risk that should be considered using 

the available data. Please comment on whether the considerations for RPF calculation as 

outlined in Sections 5.6 and 5.7 are scientifically justified and adequately 

described.

When multiple doses are available for dose-response modeling, all of the data should be 

used with a sufficiently flexible model, e.g., the multi-stage cancer model or a polynomial model 

for continuous endpoints. An example of such an analysis strategy is given in 5a above. In the 

Appendix, there are cases where single-dose data were used when multiple doses were available; 

this should be explained. 

Generally, the Panel is concerned about using high-BMR values to calculate the RPFs in 

single-dose studies. If the dose-response curves were parallel across PAHs, then the choice of 

BMR would not impact the estimation of a relative potency factor. However, as discussed in 

earlier chapters, it is generally assumed that the chemicals are not dilutions of one another, so 

their dose-response curves will generally not be parallel. Thus, the choice of the BMR should be 

in the low dose-region. However, in some special cases, the RPF calculation is not dependent on 

the response level. For example, consider the data from a BaP single-dose study and multi-dose 

comparison PAH for benzo[k]fluoranthene (BkF) (LaVoie et al, 1982). For convenience, the 

data from Table C-1, page C-4 of the PAH Mixtures document are reproduced below in Table 2.
18

Suppose a one-stage model is used for analysis of the single-point BaP study, i.e.,

μβ β β = +− − − 00 1 (1 ) 1 exp( ) [ x ]

where β 0 =0, x is the dose of BaP, and β1 is the unknown parameter associated with the slope. 

Assuming a zero background response rate (i.e., β 0 =0), the BMD(10) is estimated as 

1 BMD(10) log(0.9)/ = − β and the BMD(85) is estimated as 1 BMD(85) log(0.15)/ = − β . Since 

there are four dose groups for BkF, a multi-stage model is used, parameterized with linear and 

quadratic terms (i.e., g-2= 2 for a second-degree model):

2

0 0 1 2 μβ β β β = +− − − − (1 ) 1 exp( )   x x  

where x is the dose of BkF and again we assume β 0 =0. However, in the EPA document, β 2 was 

set to zero and the one-stage model was used due to convergence problems. Therefore the same 

parameterization is used for both BaP and BkF. The fitted dose-response curves are provided in 

Figure 2. Notice the predicted response from the single-dose study is the sample mean (here, 

observed sample proportion).

Table 2: Data from LaVoie et al, 1982 for dermal bioassay data 

(Record number: 630; Table C-1, page C-4) – primarily squamous cell 

papilloma in female mice. The data include a single-dose study for BaP 

and multiple-dose study for the PAH, BkF.

PAH Dose

(μg/mouse)

Number of 

Animals in 

Group

Number of 

Animals 

with 

Tumors

% 

Tumorbearing 

animals

Control 0 20 0 0

BaP 30 20 17 85

BkF 30 20 1 5

BkF 100 20 5 25

BkF 1000 20 15 75
19

When the one-stage model is used for both chemicals, the choice of BMR is not relevant 

in the calculation of the RPF. Consider the following algebraic manipulations to demonstrate for 

a general BMR=μ0 and for a general jth PAH:

0 0 0

00 0

0

0

/ () ( )

/ () ()

log(1 )/

log(1 )/

j BaP

BaP j

BaP

j

j

BaP

BMD BMD RPF

BMD BMD

μ μ μ

μμ μ

μ β

μ β

β

β

= =

− − = − −

=

Thus, the RPF is not a function of the BMR when a one-stage model is used for both the BaP and 

comparison PAH. To illustrate from the LaVoie (1982) data, the results for a BMR of 10% and 

85% (the observed response from BaP) are given in Table 3. The resulting RPFs are identical.

Figure 2: Single-dose data for BaP (red) and multidose data for BkF (LaVoie et al, 1982).
20

This illustration demonstrates that in a single-dose study, a one-stage model can be fit,

which will exactly predict the observed mean response. In this case, the ratio of slopes for 

calculating the RPF is not dependent on the BMR. However, with the single-dose studies, there 

is no way to verify the prediction where data are not available. Therefore the result is based on a 

lack of information rather than evidence that both the BaP and PAH dose-response data are 

adequately approximated with one-stage models.

Although the use of single-dose study data may be helpful in informing the risk 

assessment, these studies are clearly less informative than multi-dose studies. When single-dose 

studies are used to calculate the RPF, the Panel recommends describing the impact on the RPF 

calculation. For example, in Table 7-1, the Panel recommends including the number of studies 

per RPF calculation based on a one-dose study.

For section 5.7 of the document, the Panel recommends the use of a (g-1)-degree 

polynomial in the multi-stage model (page 111, lines 31-36) instead of reducing the degree of the 

polynomial. This model will exactly track the observed sample means.

3.6. Charge Question 6 – Selection of PAHs for Inclusion in the Relative Potency 

Approach

This chapter describes the selection of PAHs for inclusion in the RPF approach. The evaluation 

focuses on whether the available data were adequate to assess the carcinogenic potential of each 

compound. If the data were not considered adequate, then the PAH was excluded. 

6a. Please comment on whether the rationale for the weight-of-evidence evaluation is 

scientifically justified and adequately described. Please comment on whether the approach 

adequately considers the available information. Please comment on whether other information 

(e.g., additional structure-activity) could contribute further to the weight-of-evidence evaluation 

and how this information could be utilized in the analysis. 

The Panel believes that the method for selecting the PAH appears to be scientifically 

justified, but has recommendations about several issues that are incompletely considered. These

Table 3: Illustration with BaP single dose study and multi-dose comparison PAH, here BkF from 

LaVoie et al, 1982.

LaVoie et al 1982 data BMD10 

estimates

(μg)

Slope = 

0.1/BMD10

BMD85 

estimates

(μg)

Slope = 

0.85/BMD85

BaP 1.7 0.060 30 0.028

BkF 64.6 0.0015 1163 0.0007

RPF = slope PAH

slope BaP 0.025 0.025
21

issues include: (1) the quality of the individual studies considered and (2) the variability of other 

design characteristics among studies, and how this may weigh on their evaluation prior to 

inclusion in the weight-of-evidence evaluation or calculation of an RPF. 

Regarding the quality of individual studies being considered, the Panel recommends that a 

list of quality criteria should be defined, articulated, and applied a priori (e.g., methodologically 

robust, such as inclusion of an adequate control group, sample size, dose level, number of doses,

number of PAHs measured, purity of the compounds considered, etc.) prior to the weight-ofevidence evaluation. This information should be illustrated in the form of tables or individual 

graphs. Only studies of sufficient quality, as defined a priori, should be considered in the 

weight-of-evidence evaluation.

The Panel recommends that once a study is considered to have sufficient quality, the 

variability of other study characteristics among studies should be carefully considered prior to 

their use in the calculation of the RPF. Some of these study characteristics include: species, 

strain and sex of animal model, route of exposure, form of exposure (injection, implantation, 

etc.), frequency of administration, exposure duration, location of tumors, types of tumors 

(papillomas, adenomas, carcinomas etc.), and stage of tumors (benign, malignant). Differences 

among studies in some of these characteristics may significantly affect the dose-response within 

each study, which in turn, will affect the RPF calculation. For example, for a given PAH one 

may have one study that used skin tumorigenesis and another that used implantation of solid 

material intratracheally. The latter study, if positive, might add weight to the overall 

determination that the PAH is tumorigenic in animals, but may be a poor study from which to 

calculate dose-response or relative potency. Or one could be comparing one study that has a 

physiological exposure route such as inhalation, ingestion, or dermal application, with one with a 

non-physiological exposure such as an intraperitoneal or implantation study. Likewise, there are 

tumor models, such as the A/J mouse, where tumor multiplicity counting is required due to the 

high penetrance of tumor response. It is not a simple task to reconcile these studies with other 

studies of tumor incidence for the purposes of quantitatively assessing dose-response. 

There is no simple formulaic method for determining, a priori, how to include or exclude 

such studies or how to weight them, since this will vary depending on the individual PAH and 

also depending on what studies are available; it also requires a measure of expert scientific 

judgment. Instead, the Agency should clearly articulate the quality criteria (e.g., expand and 

articulate the characteristics listed in Table 7.1), and then only use studies with adequate quality 

to calculate RPFs. Weighting factors may be required for inclusion of some studies for RPF 

calculations, or they may only be valuable as a qualitative, weight-of-evidence assessment of 

carcinogenicity rather than for quantitative RPF calculations. The criteria for how such decisions 

are made for each study and each PAH should be clearly defined and described by the Agency as 

part of its assessment. 

The EPA approach inappropriately discards data that do not achieve statistical 

significance. Lack of statistical significance does not necessarily mean that an effect is zero. It 

could be that there is an effect with biological relevance, but the sample sizes were too small to 

achieve statistical significance. Using a cutoff P-value of 0.05 for inclusion of data in the 

weight-of-evidence assessment is arbitrary. It can create a scenario where there are two studies 
22

of equally high quality and one study is included because it has a P-value = 0.04 and the other 

study is not included because it has a P-value = 0.06. A study of high quality that produces a 

low statistically non-significant RPF is relevant and must be included in calculating the best 

(weighted average) estimate for an RPF. Discarding values in the lower tail of a statistical 

distribution, solely due to lack of statistical significance, results in a biased estimate of the effect. 

6b. The weight-of-evidence analysis does not include data related to Ah-receptor binding, 

cytotoxicity or tumor promotion. Please comment on whether the scientific rationale for this 

decision is appropriate. If these data should be considered in the derivation of RPFs, please 

describe how they should be incorporated into the analysis. 

The Panel finds that the rationale for omission of Ah-receptor data is well justified. 

Additional information is not necessary. The Panel also agrees with EPA’s decision that once 

information demonstrating tumor formation is obtained, additional information on cytotoxicity 

and tumor promotion is not necessary. However, the document should clearly state the reasons 

for the omission of these data. 

6c. The analysis uses an RPF detection limit as a means of comparing positive and nonpositive 

(or negative) bioassays. Please comment on whether this method is scientifically justified and 

adequately described. 

EPA employed the use of an “RPF detection limit” to evaluate the results of positive and 

nonpositive results in the same test system. The “RPF detection limit” was defined as the RPF 

determined by the lowest response that would have been statistically significant for the subject 

PAH and the actual benzo[a]pyrene response. The Panel did not find this definition to be clear 

nor did the Panel find the description of the use of “RPF detection limits” to be clear. The Panel 

does not recommend utilizing statistical significance as a means to determine which studies to 

include or exclude. As discussed above, the Panel recommends assessing study quality to 

determine which studies to include or exclude from the weight-of-evidence evaluation. It is 

scientifically incorrect to discard data of sufficient high quality that do not achieve statistical 

significance and therefore, the Panel does not recommend using “RPF detection limits” for that 

purpose.

6d. Graphic arrays of the calculated RPFs (Figures 6-2 through 6-35) are presented as a means 

of representing the variability in RPFs from different data sources, the weight-of-evidence for 

carcinogenic potential, and the basis for the selected RPF. Please comment on whether the 

figures are informative and adequately described. Please comment on whether there is other 

information that should be included in the figures. Please comment on whether the narratives 

are informative and complete.

The Panel finds that Figures 6-2 through 6-35 provide a good summary of the individual 

studies considered and the variability of individual RPF estimates across studies. However, the 

Panel recommends clearly indicating which studies were used to estimate the final RPF. This 

would make the figures much more informative.
23

With respect to the presentation of RPFs for individual studies, the Panel proposes that 

rather than graphically displaying the RPF for each individual study as a bar, it can be shown as 

a point estimate coupled with information on variability (e.g., standard error, standard deviation, 

confidence interval, and range). The information on variability in the study is viewed as key, to 

help the reader interpret the study findings. 

The Panel recommends that, for ease of reading and to ensure completeness, the 

narratives be presented in a consistent structure and format, both in terms of the information 

presented, as well as the order in which they are presented. The Panel also recommends

integrating information provided in Appendix G into the narratives that correspond to Figures 6-

2 through 6-35.

3.7. Charge Question 7 – Derivation of RPFs for Selected PAHs

This chapter describes various methods (e.g. prioritization of studies) and different approaches 

for deriving final RPFs (e.g., arithmetic mean). Final RPFs were derived by averaging the 

individual study RPFs (across all exposure routes) calculated from bioassay data for PAHs that 

had at least one RPF based on a bioassay. The exception was dibenz[a,c]anthracene, where the 

RPF was calculated from cancer-related endpoint data. 

7a. Please comment on the scientific justification for the approach for deriving the final RPFs 

and the discussion of alternative options for the estimation of the final RPFs. Please comment 

on the reporting of the range of RPFs as a measure of variability instead of a confidence interval. 

Please comment on whether the data are adequate to support more (or less) precision in 

deriving the RPFs. 

The Panel believes that presenting the range instead of a confidence interval is 

appropriate. The Panel does have reservations regarding several aspects of the RPF calculation 

approach. First, the Panel has concerns regarding calculating RPFs based upon a single 

experiment (e.g., 11H-benz[b,c]aceanthrylene, benzo[g,h,i]perylene, benzo[e]aceanthrylene, 

benz[j]aceanthrylene, dibenzo[a,h]pyrene, indeno[1,2,3-c,d]pyrene and naphtho[2,3-e]pyrene). 

Second, there is concern regarding the use of data for calculating RPF values in which there was 

only a single-dose level of BaP and/or the target PAH being evaluated (e.g., benz[a]anthracene, 

11H-benz[b,c]aceanthrylene, benzo[e]aceanthrylene, naphtho[2,3-e]pyrene and fluoranthene), 

particularly if it was a high dose, and/or particularly if only single doses of both the target PAH 

and BaP are available. An RPF can be calculated from only a single dose of BaP when dose 

response data are available for the PAH. Since the RPF varies with the level of the tumor 

incidence, RPFs should only be calculated from a single dose of BaP if the tumor incidence at 

the dose of BaP is in the low-dose range; certainly, only if the BaP tumor incidence is less than 

50%. Finally, there is concern about calculating the arithmetic mean for PAHs that have 

markedly divergent individual RPFs (e.g., benzo[c]fluorene). The Panel does not make a 

recommendation on whether or not to calculate RPFs for PAHs with these data characteristics, 

recognizing that the Agency will need to apply professional judgment based on analyzing the 

actual available data. 
24

The Agency is encouraged to continue evaluating other methods, such as using a 

geometric mean instead of an arithmetic mean. Where sufficient data are available, the use of a 

geometric mean would give less weight to outlier values. Further, examination of the variability 

for estimates of RPFs in several of the figures indicates that a log-normal distribution may be 

appropriate to describe the variability of RPFs. Hence, the geometric mean would be a better 

estimate of central tendency. Also, the best central estimate would be a weighted geometric 

mean where the weights are inversely proportional to the square of the standard errors. That is, 

RPFs with large standard errors would receive less weight. The Panel believes that calculating 

RPFs to one significant figure is appropriate. 

7b. Please comment on whether the scientific rationale for consideration of bioassay data 

versus cancer-related endpoint data has been adequately described. Please comment on 

whether the cancer-related endpoint data could be used in a more quantitative manner. Please 

comment on the justification of the final RPF derived for dibenz[a,c]anthracene. Please 

comment on the use of tumor multiplicity data in the weight-of-evidence evaluations and for the 

determination of the RPFs. 

The Panel believes that the scientific rationale for considering bioassay data versus 

cancer-related endpoint data has been adequately described. The Panel strongly believes that the 

use of cancer bioassay data is essential for determining the RPF for a given PAH. Cancer-related 

endpoint data are useful as supporting data, but the Panel does not recommend the use of only 

cancer-related endpoint data for determining the RPF. As such, the Panel does not have 

recommendations on how to use cancer-related endpoint data in a more quantitative manner. 

The Panel does not recommend calculating an RPF for dibenz[a,c]anthracene and recommends 

that it be removed from Table 7.2 until further bioassay data become available.

The Panel recommends that additional information and justification be provided for the 

inclusion or exclusion of cancer bioassay data for PAHs that did not give significant tumor 

responses in well-designed studies. One suggestion is to include the IARC classification for 

those PAHs where a classification exists in Table 7.1 or perhaps in Table 7.3. The Panel 

believes that there is a need for some additional measure of the quality of individual studies used 

in determining the final RPF values. This is important in addition to the confidence ratings 

provided in Table 7.3 (see also further discussion below). The Panel also strongly believes that 

more cancer bioassay data with mixtures would be extremely helpful in further validating the 

RPF approach.

Tumor multiplicity (continuous data; average number of tumors per mouse) and tumor 

incidence (quantal data; percentage of mice with tumors) represent different measures of 

tumorgenicity/carcinogenicity. In the document, RPFs calculated from tumor multiplicity data 

are combined with other RPFs calculated from tumor incidence data to calculate final RPFs. An 

example of the problem is benzo[c]fluorene. The divergent RPFs used to calculate the final RPF 

value for benzo[c]fluorene in Table 7.1 come from averaging a study where multiplicity data 

were used (RPF of 50) and one where incidence data were used (RPF of 1). It is recommended 

that RPF values not be averaged from these two different measures without sufficient 

justification for using the multiplicity data. In this regard, accurate assessment of differences in 

potency using tumor multiplicity requires adequate dose-response data. For accurate 
25

comparisons, at least 3 doses of each PAH should be available for comparison. In addition, these 

doses should be distributed across the dose-response range and not be clustered on the high or 

low end of the dose response range. In lieu of adequate dose-response data for tumor 

multiplicity, the Panel recommends that only tumor incidence data be used to calculate final 

RPFs. Additionally, if calculated RPFs for a given PAH still remain divergent across multiple 

well-designed studies due to multiple factors (e.g., combining incidence and multiplicity, 

combining data from different organs, combining data from different routes of exposure, etc) the 

Agency may wish to consider use of the geometric mean in place of the arithmetic mean as 

discussed above.

7c. Please comment on whether the recommendation to apply the proposed RPFs across all 

routes of exposure is adequately described. Please comment on whether there is additional 

scientific information that would inform this recommendation. Please comment on whether the 

available data are adequate to recommend exposure route-or target organ-specific RPFs. 

The Panel finds that the recommendation to apply the proposed RPFs across all routes of 

exposure is adequately described. The Panel does not believe that there would be much value in 

providing route- or target organ-specific RPFs at the present time because a significant 

proportion of the studies used to calculate the final RPFs involved dermal 

application/carcinogenesis (approximately 60% of the studies involve dermal application to mice 

and >90% of the studies were conducted in mice). Additional studies and data using different 

routes of exposure and tumor data from other organ sites would be necessary to calculate such 

RPFs. Although the Panel agrees with the decision to not calculate separate RPF values for 

different routes of exposure, the route of exposure may be an issue of concern for generating 

RPF values for compounds where the available data are only via non-physiological routes (e.g., 

benzo[g,h,i]perylene, lung implantation in rat only; benzo[j]aceanthrylene, intra-peritoneal only; 

fluoranthrene, intra-peritoneal only; indeno[1,2,3-e]pyrene, lung inplantation in rat only). 

Additional dermal or oral tumor studies may be warranted in these cases since the route of 

exposure can play an important role in bioavailability and toxicokinetics that may alter the 

relative potency of the test compound as compared to BaP, when tested via a more standard route 

of exposure. A sensitivity analysis should be performed to determine, in those cases where there 

are data from several routes of exposure, whether these alternative routes cause a particular bias 

or greater variability in the RPF values. It is interesting to note in this regard, that some 

compounds, such as benzo[c]fluorene, demonstrate widely divergent RPFs in studies using 

different routes of exposure (in this case, oral versus interperitoneal, with values of 1 and 50) 

(see also dibenz[a,h]anthracene and dibenzo[a,l]pyrene). Without additional supporting data or 

justification, the Panel does not recommend developing RPFs for compounds with data only

from studies using non-physiological routes of exposure. 
26

7d. Please comment on whether the scientific rationale for the assignment of an RPF of zero for 

some PAHs is adequately described. Please comment on whether there are other data that 

should be considered to assess whether an RPF of zero is appropriate. Please comment on 

whether the scientific rationale for assigning no RPF based on inadequate data for some PAHs 

is adequately described. Please comment on whether there are alternative methods for assigning 

RPFs to these PAHs. Please comment on whether the text provides adequate distinction between 

PAHs with RPFs of zero and PAHs with no selected RPF and whether this distinction is useful 

for describing uncertainty in determining the cancer risk associated with PAH exposure. 

The Panel generally believes that the scientific rationale presented in the document for 

assignment of an RPF of zero, the assignment of no RPF, and the distinction between them is 

adequately described. However, the Panel does have concern regarding the quality of the data 

used to assign an RPF of zero for some studies and also regarding the inconsistent use of studies 

with RPFs of zero in calculating the final RPFs. The Panel recommends that a consistent 

approach be adopted for using RPFs of zero for all compounds for which final RPFs are 

calculated. In addition, the Panel recommends that the Agency continue to evaluate how RPFs 

of zero are calculated as well as the rationale for assigning no selected RPF values. In addition, 

the Panel recommends the Agency discontinue assigning a value of zero to studies of high 

quality that have non-statistically significant results. See the response to charge question 6a for 

further details. The Panel did not identify alternative methods for assigning RPFs to PAHs that 

had inadequate data.

7e. The final RPFs are characterized with confidence ratings. Please comment on whether the 

rationale for the confidence ratings is appropriately described. Please comment on whether 

there are other approaches for describing confidence using the available data that could be 

applied in either a qualitative or quantitative manner that would be more useful for risk 

assessment.

In general, the Panel believes that characterizing the final RPFs with confidence ratings is

a good idea and finds that the rationale for confidence ratings is appropriately described. 

However, the confidence ratings do not appear to give any indication of the overall quality of the 

data being assessed and used for the RPF calculation. Based on the information provided in 

Table 7.3, confidence ratings appear to be related to the number of studies used, data from more 

than one route of exposure, presence of non-cancer endpoint supporting data to calculate the 

RPFs, etc. The Panel strongly believes that there needs to be some measure of the quality of the 

individual studies used to generate the RPFs. In this context, quality refers to study 

characteristics, such as sample size and statistical power, presence or absence of non-lethal 

toxicity, unusual mortality, and other potential confounding factors. Also, the Panel’s comments 

and recommendations in the response to Charge Question 7c may be useful in developing 

confidence ratings. 

Chapter 7 also includes a description of how the RPF method is used to calculate relative 

cancer risk from exposure to PAH mixtures (section 7.3). In addition, there is a section (section 

7.4) dealing with the use of age-dependent adjustment factors (ADAFs) to adjust for differences 

in susceptibility during early life (i.e., <16 years of age). The Panel believes that these two 

sections are extremely important to the overall presentation of the document and are somehow 

lost by inclusion at the end of Chapter 7. It is strongly recommended that the information on 
27

cancer risk assessment (sections 7.3 and 7.4) be moved to the beginning of the document as a 

separate section.

3.8. Charge Question 8 – Uncertainties and Limitations Associated with the RPF 

Approach

This chapter discusses the uncertainties and limitations associated with using the RPF approach 

for PAH mixtures risk assessment. Many of the general uncertainties related to chemicalspecific risk assessment are also applicable to the proposed RPF approach for PAHs. In 

addition, uncertainties exist regarding the selection of data and dose-response assessment 

methodology, the selection of PAHs for inclusion in the analysis, the derivation of the final RPF, 

the assumption of a common mode of action and dose additivity, and the extrapolation of RPFs 

across exposure routes. 

8. Please comment on whether, overall, the document describes the uncertainties and limitations 

in the methodology used to derive RPFs in a transparent manner. Please comment on whether

the most important uncertainties and limitations are identified. Please comment on whether 

there is existing information that could be used to evaluate the accuracy or validity of the RPF 

values to predict the cancer risk associated with exposure to PAH mixtures.

The uncertainties in the methodology of deriving RPFs are described quite well in the 

PAH Mixtures document. The major methodological uncertainties are clearly defined and 

discussed so that there is little doubt about the methods that were used and the limitations of the 

final RPF values reported. 

More data dealing with the comparisons of the RPF approach and estimates of cancer risk 

derived from complex mixtures are needed, which would reduce some of the uncertainties 

associated with the RPF approach described in the document. The feasibility of directly studying 

complex mixtures is illustrated by the limited pair of existing data sets. Chronic bioassays in 

mice for two synthesized coal tar mixtures were conducted at the National Center for 

Toxicological Research, Food and Drug Administration (Culp et al., 1998). The RPF approach 

applied to these data were reported in the Electric Power Research Institute (EPRI) public 

comments (Rohr, 2010). Comparisons of cancer risk observed in the chronic animal bioassays 

for the two coal tar mixtures were within a factor of two to four (lower) of the cancer risks based 

on the RPF approach. This is an encouraging result for use of the RPF approach, albeit for only 

two mixtures. Additional comparisons, such as those submitted by EPRI, should be added to the 

document as it provides very useful information about the RPF approach. Statistical variation of 

cancer risk estimates between chronic animal bioassays on the order of three to four is expected 

(Gaylor et al., 2000). More data dealing with the comparisons of the RPF approach and 

estimates of cancer from tested mixtures are needed.

Additional mixtures of PAHs need to be studied in chronic animal bioassays in order to 

compare the observed cancer risk of a mixture with the risk estimated from the RPF approach. 

Section 3.1 of the PAH Mixtures document discusses the availability of several studies on 

mixtures that provide data for comparing cancer risk estimates using the RPF approach with 
28

direct estimates of risk from the mixtures. Unfortunately, no quantitative information was 

presented in the document to indicate the potential size of uncertainty for the RPF approach. 

This quantitative information needs to be added to the document in order to evaluate the 

accuracy and precision of the RPF approach from existing examples.

The cancer slope factor for BaP is multiplied by the RPFs in order to obtain cancer unit 

risk factors for each of the PAHs. Hence, the cancer unit risk factor for BaP is critical to the 

calculation of the cancer risk estimate for a mixture using the RPF approach. Based on old 

studies, the upper limit of the cancer unit risk factor for lifetime oral exposure to BaP is 

7.3 x 10-3 per μg/ kg per day listed in the EPA Integrated Risk Information System (IRIS), 1994. 

Based on a Good Laboratory Practice (GLP) study the upper limit of the cancer unit risk factor 

for BaP is 1.2 x 10-3 per μg/kg per day (Gaylor et al., 2000). Because of the relatively large 

uncertainty in the cancer unit risk factor for BaP, this value needs to be updated before reliable 

estimates of cancer risk can be derived for mixtures of PAHs.

Extending the classes of PAH should be considered by incorporating other PAH 

derivatives, e.g., PACs that occur in mixtures, particularly where bioassays exist such as for 

nitro-aromatics and alkylated PAHs.

 The relevance of high doses in animal studies to the much lower doses experienced by 

humans is not discussed in the document. The Panel recommends that additional information or 

discussion of the uncertainty that arises from extrapolating from high animal doses to low human 

doses be added to the document. 

The state of a single PAH administered to animals in bioassays may be different from the 

state of the same PAHs in mixtures where they may not be easily desorbed from solid particles. 

The bioavailability to humans for PAHs in a mixture needs to be compared to the bioavailability 

in animal bioassay experiments that utilize purified PAH compounds. Cancer risk estimates 

based on the RPF values and the total concentration of PAH in mixtures may be overestimated.

Using measured concentrations of PAHs in mixtures, sensitivity analyses can indicate 

which uncertainties in individual RPFs have a significant impact on the total BaP equivalents for 

a mixture. EPA should consider adding this to the document, perhaps by using the mixtures 

discussed in the EPRI comments. 

The composition for each individual mixture must be adequately determined, otherwise 

uncertainty is added to the RPF approach. Completely characterizing mixtures is difficult, and 

this limitation and uncertainty should be discussed. For example, different PAHs may have 

different effects on the induction phase I and/or phase II enzymes that might affect the metabolic 

activation or deactivation of other potentially highly tumorigenic PAHs, i.e., a non-additive 

effect as mentioned in the PAH Mixtures document. Various PAHs may inhibit each other. 

Mixtures may or may not contain substances that act as promoters of tumorigenesis rather than 

as genotoxic initiators. Without adequately characterizing mixtures, these effects may not be 

considered. 
29

3.9. Charge Question 9 – Adequacy of Appendices for Independent Verification

9. Please comment on whether the information in the Appendices is adequate to allow 

independent verification of the calculated RPFs. If not, please comment on what additional 

information would be useful.

There are 7 appendices in the document and the information contained in them include: a 

bibliography of secondary sources reviewed for identification of primary literature, a 

bibliography of studies without BaP as a reference compound, dose-response data for potency 

calculations, benchmark dose modeling outputs, calculation of RPFs, an example calculation of 

an RPF detection limit, and evaluation of alternatives for ranking RPFs.

The appendices are generally useful for verifying the calculations of the RPFs. However, 

the Panel recommends reorganizing the appendices by chemical (with each identified in the 

Table of Contents). This would include the corresponding BaP data for each study within each 

chemical section which may be repeated across PAHs. 

The plots from the Benchmark Dose Software output are useful but it should be noted 

that the linear extrapolation to the origin is based on BMDLs instead of BMDs. The calculation 

of the multi-stage cancer slope factor is also given based on the BMDL instead of the BMD. The 

Panel recommends that the slope factors be added to these appendices based on the BMD –

which is the approach taken in the document.
30

4. REFERENCES

Culp SJ, Gaylor DW, Sheldon WG, Goldstein LS, and Beland FA. 1998. A comparison of the 

tumors induced by coal tar and benzo[a]pyrene in a 2-year bioassay. Carcinogenesis, 19: 117-

124.

Gaylor DW, Culp SJ, Goldstein LS, and Beland FA. 2000. Cancer risk estimation for mixtures 

of coal tars and benzo[a[pyrene. Risk Analysis, 20: 81-85.

IARC (International Agency for Research on Cancer). 2010. World Health Organization. IARC

Monographs on the Evaluation of Carcinogenic Risks to Humans. Volume 92. Some Nonheterocyclic Polycyclic Aromatic Hydrocarbons and Some Related Exposures. Available at:

http://monographs.iarc.fr/ENG/Monographs/vol92/index.php.

LaVoie EJ,Amin S, Hecht SS, Furuya K, Hoffmann D. 1982. Tumour initiating activity of 

dihydrodiols of benzo[b]fluoranthene, benzo[j]fluoranthene, and benzo[k]fluoranthene. 

Carcinogenesis, 3:49–52.

Rohr AC. 2010. Comments Prepared for Consideration by Science Advisory Board PAH

Mixtures Review Panel. Electric Power Research Institute (EPRI). Available at: 

http://yosemite.epa.gov/sab/sabproduct.nsf/E10147F622E092188525773E0069A262/$File/EPRI

+Comments-SAB+PAH+Mixtures+Review+Panel.pdf

Tong C, Laspia MF, Telang S, Williams GM. 1981. The use of adult rat liver cultures in the 

detection of the genotoxicity of various polycyclic aromatic hydrocarbons. Environmental

Mutagenesis, 3:477–487.

WHO (World Health Organization). 1998. Selected non-heterocyclic polycyclic aromatic 

hydrocarbons Environmental health criteria. Vol. 202. International Programme on Chemical 

Safety, Geneva, Switzerland.
A-1

APPENDIX – EPA CHARGE QUESTIONS

National Center of Environmental Assessment Charge to External Reviewers for the 

Development of a Relative Potency Factor (RPF) Approach for Polycyclic Aromatic 

Hydrocarbon (PAH) Mixtures for the IRIS Program

February 2010

U.S. EPA’s IRIS Program is seeking an external peer review of the scientific basis supporting 

the document titled Relative Potency Factor (RPF) Approach for Polycyclic Aromatic 

Hydrocarbon (PAH) Mixtures that will appear on the Agency’s online database, the Integrated 

Risk Information System (IRIS). IRIS is a human health assessment program that evaluates 

quantitative and qualitative risk information on effects that may result from exposure to specific 

chemical substances found in the environment. Through the IRIS Program, EPA provides 

quality science-based human health assessments to support the Agency’s regulatory activities. 

Combined with specific exposure information, government and private entities use IRIS to help 

characterize public health risks of chemical substances in site-specific situations in support of 

risk management decisions.

PAHs do not occur in the environment as isolated entities; they primarily occur in complex 

mixtures generated from the incomplete combustion or pyrolysis of substances containing carbon 

and hydrogen. Many PAHs are demonstrated tumorigenic agents in animal bioassays and are 

active in cancer–related in vivo or in vitro tests. In addition, PAHs exhibit noncancer effects that 

may be of concern to public health. The analysis presented in the document under review 

represents an RPF approach for estimating cancer risk and is characterized as one approach to 

assessing cancer risk from exposure to PAH mixtures. 

In concordance with U.S. EPA (2000, 1986) guidance for health risk assessment of chemical 

mixtures, assessment of the cancer risk from human exposure to a particular PAH mixture would 

best be conducted with quantitative information on the dose-response relationship for the mixture 

of concern. When data for the mixture of concern are not available, the recommendation is to 

use toxicity data on a sufficiently similar mixture. However, quantitative cancer dose-response 

information exists only for a few complex PAH-containing mixtures. Component-based 

approaches, involving an analysis of the toxicity of components of the mixture, are 

recommended when appropriate toxicity data on a complex mixture of concern, or on a 

sufficiently similar mixture, are unavailable. The RPF analysis under review is not a 

reassessment of individual PAH carcinogenicity, but rather provides an approach for estimating 

cancer risk for PAH mixtures by summing doses of component PAHs after scaling the doses 

(with RPFs) relative to the potency of an index PAH (i.e., benzo[a]pyrene). The cancer risk is 

then estimated using the dose-response curve for the index PAH. 

Below is a set of charge questions that address general and scientific issues in the document. 

Please provide detailed explanations for responses to the charge questions. 
A-2

General Charge Questions

1a. Please comment on whether the report is logical, clear and concise. Please comment on 

whether EPA has clearly synthesized the scientific evidence for the derivation of relative 

potency factors for individual PAHs. 

1b. Please comment on whether the report provides adequate context for how the proposed RPF 

approach could be used in a PAH mixtures risk assessment.

Chapter 2. Rationale for Recommending an RPF Approach

Chapter 2 presents the rationale for recommending an RPF approach. In an RPF approach, doses 

of component chemicals that act in a toxicologically similar manner are added together, after 

scaling the doses relative to the potency of an index chemical. Benzo[a]pyrene (B[a]P) is 

selected as the index compound for this RPF approach. The RPF approach involves two key 

assumptions related to the application of a dose-additivity model: (1) PAH components in the 

mixture act in a similar toxicological manner; and (2) interactions among PAH mixture 

components do not occur at low levels of exposure typically encountered in the environment. 

2a. Please comment on whether the report provides adequate justification for using an RPF 

approach as a scientifically defensible method to assess the cancer risk associated with 

exposure to PAH mixtures. 

2b Please comment on whether the choice of benzo[a]pyrene as the index compound is 

scientifically justified and appropriately described. Please identify and provide the rationale 

for any alternative index compound(s) that should be considered.

2c.Please comment on whether the weight of evidence indicating that PAHs, as a chemical class, 

have a similar mode of carcinogenic action has been adequately described and is 

scientifically justified. 

2d. Please comment on whether the assumption that interactions among PAH mixture 

components do not occur at low levels of exposure typically encountered in the environment 

has been adequately described and is scientifically justified.

Chapter 3. Discussion of Previously Published RPF Approaches

This chapter presents a discussion of previously published RPF approaches. Due to the 

evolution of the state of the science and an increased understanding of PAH toxicology, EPA is 

reevaluating the RPF approach for PAHs in this analysis.

3. Please comment on whether the discussion provides a meaningful background on how RPFs 

have been derived in the past, and the advantages and disadvantages of previous methods.

Chapter 4. Evaluation of the Carcinogenicity of Individual PAHs

This chapter discusses the development of a database of primary literature on PAH 

carcinogenicity and cancer-related endpoints and the criteria used to include or exclude studies 

from the database.
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4a. Please comment on whether the list of 74 PAHs (Table 2-1) included in the initial literature 

search is complete. Please comment on whether the rationale for the choice of PAHs 

included in the literature search has been appropriately described. Please identify other 

databases or resources that should be included.

4b. Chapter 4 includes a description of how studies were selected for use in dose-response 

assessment. Please comment on whether the choices and assumptions in making the selection 

have been adequately described. Please comment on whether the information in Tables 4-1 

through 4-14 provides adequate information to inform how decisions were made. Please 

comment on whether studies were rejected or included appropriately. Please comment on 

whether positive and nonpositive studies have been considered appropriately. 

4c. The methodology for the choice of studies to use in the derivation of RPFs includes studies 

where at least one PAH was tested at the same time as B[a]P. Studies where individual 

PAHs were tested without concurrent testing of B[a]P were not included in the quantification 

of RPFs. Please comment on the scientific rationale for this approach. Please comment on 

whether the advantages and disadvantages of excluding certain data from the derivation of 

RPFs have been adequately described.

Chapter 5: Methods for Dose Response Assessment and RPF Calculation

This chapter describes the selection of dose-response data and methods for dose-response 

assessment and RPF calculation from the selected datasets. The methodology for estimation of 

the RPFs varied depending on the characteristics of the datasets, however, the general equation 

was the ratio of the slope of the dose-response curve for the subject PAH to the slope of the 

dose-response curve for B[a]P.

5a. Please comment on whether the scientific rationale for the dose-response modeling 

approaches used in the derivation of RPFs is adequately described. Please comment on 

whether there are other appropriate modeling approaches for estimating the relative potencies 

of PAHs. Please describe alternative approaches (e.g., other model forms) that could be 

considered.

5b. For each individual dataset considered in the assessment, the B[a]P dose-response was 

calculated from the study-specific data. Please comment on whether this approach has been 

appropriately described. If there are additional approaches using the available data that 

should be considered, please describe how the approach could lead to a better estimate of 

cancer risk.

5c. The point of departure for slope estimation that has been used for the derivation of RPFs is 

the benchmark dose (BMD) estimate rather than the lower confidence limit on the 

benchmark dose (BMDL). Please comment on whether this approach is scientifically 

justified and adequately described. Please comment on whether alternative approaches 

should be considered.

5d. Please comment on the methodology used for the RPF calculations for multidose and single 

dose datasets. Please comment on whether the process for calculating RPFs from the various 
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datasets is scientifically justified and adequately described. Please comment on the 

utilization of high response levels in some instances as the point of comparison. Please 

describe alternative approaches that could lead to a better estimate of cancer risk that should 

be considered using the available data. Please comment on whether the considerations for 

RPF calculation as outlined in Sections 5.6 and 5.7 are scientifically justified and adequately 

described. 

Chapter 6: Selection of PAHs for Inclusion in the Relative Potency Approach

This chapter describes the selection of PAHs for inclusion in the RPF approach. The evaluation 

focuses on whether the available data were adequate to assess the carcinogenic potential of each 

compound. If the data were not considered adequate, then the PAH was excluded.

6a. Please comment on whether the rationale for the weight-of-evidence evaluation is 

scientifically justified and adequately described. Please comment on whether the approach 

adequately considers the available information. Please comment on whether other 

information (e.g., additional structure-activity) could contribute further to the weight-ofevidence evaluation and how this information could be utilized in the analysis.

6b. The weight-of-evidence analysis does not include data related to Ah-receptor binding, 

cytotoxicity or tumor promotion. Please comment on whether the scientific rationale for this 

decision is appropriate. If these data should be considered in the derivation of RPFs, please 

describe how they should be incorporated into the analysis. 

6c. The analysis uses an RPF detection limit as a means of comparing positive and nonpositive 

(or negative) bioassays. Please comment on whether this method is scientifically justified 

and adequately described. 

6d. Graphic arrays of the calculated RPFs (Figures 6-2 through 6-35) are presented as a means of 

representing the variability in RPFs from different data sources, the weight-of-evidence for 

carcinogenic potential, and the basis for the selected RPF. Please comment on whether the 

figures are informative and adequately described. Please comment on whether there is other 

information that should be included in the figures. Please comment on whether the narratives 

are informative and complete.

Chapter 7: Derivation of RPFs for Selected PAHs

This chapter describes various methods (e.g. prioritization of studies) and different approaches 

for deriving final RPFs (e.g., arithmetic mean). Final RPFs were derived by averaging the 

individual study RPFs (across all exposure routes) calculated from bioassay data for PAHs that 

had at least one RPF based on a bioassay. The exception was dibenz[a,c]anthracene, where the 

RPF was calculated from cancer-related endpoint data.

7a. Please comment on the scientific justification for the approach for deriving the final RPFs 

and the discussion of alternative options for the estimation of the final RPFs. Please 

comment on the reporting of the range of RPFs as a measure of variability instead of a 

confidence interval. Please comment on whether the data are adequate to support more (or 

less) precision in deriving the RPFs.
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7b. Please comment on whether the scientific rationale for consideration of bioassay data versus 

cancer-related endpoint data has been adequately described. Please comment on whether the 

cancer-related endpoint data could be used in a more quantitative manner. Please comment 

on the justification of the final RPF derived for dibenz[a,c]anthracene. Please comment on 

the use of tumor multiplicity data in the weight-of-evidence evaluations and for the 

determination of the RPFs. 

7c. Please comment on whether the recommendation to apply the proposed RPFs across all 

routes of exposure is adequately described. Please comment on whether there is additional 

scientific information that would inform this recommendation. Please comment on whether 

the available data are adequate to recommend exposure route- or target organ-specific RPFs. 

7d. Please comment on whether the scientific rationale for the assignment of an RPF of zero for 

some PAHs is adequately described. Please comment on whether there are other data that 

should be considered to assess whether an RPF of zero is appropriate. Please comment on 

whether the scientific rationale for assigning no RPF based on inadequate data for some 

PAHs is adequately described. Please comment on whether there are alternative methods for 

assigning RPFs to these PAHs. Please comment on whether the text provides adequate 

distinction between PAHs with RPFs of zero and PAHs with no selected RPF and whether 

this distinction is useful for describing uncertainty in determining the cancer risk associated 

with PAH exposure.

7e. The final RPFs are characterized with confidence ratings. Please comment on whether the 

rationale for the confidence ratings is appropriately described. Please comment on whether 

there are other approaches for describing confidence using the available data that could be 

applied in either a qualitative or quantitative manner that would be more useful for risk 

assessment.

Chapter 8. Uncertainties and Limitations Associated with the RPF Approach

This chapter discusses the uncertainties and limitations associated with using the RPF approach 

for PAH mixtures risk assessment. Many of the general uncertainties related to chemicalspecific risk assessment are also applicable to the proposed RPF approach for PAHs. In addition, 

uncertainties exist regarding the selection of data and dose-response assessment methodology, 

the selection of PAHs for inclusion in the analysis, the derivation of the final RPF, the 

assumption of a common mode of action and dose additivity, and the extrapolation of RPFs 

across exposure routes.

8. Please comment on whether, overall, the document describes the uncertainties and limitations 

in the methodology used to derive RPFs in a transparent manner. Please comment on 

whether the most important uncertainties and limitations are identified. Please comment on 

whether there is existing information that could be used to evaluate the accuracy or validity 

of the RPF values to predict the cancer risk associated with exposure to PAH mixtures. 
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Appendices

9. Please comment on whether the information in the Appendices is adequate to allow 

independent verification of the calculated RPFs. If not, please comment on what additional 

information would be useful.