United States Solid Waste and Emergency Response EPA530-R-05-020 

Environmental Protection (5305W) June 2005 

Agency www.epa.gov/osw 

____________________________________________________________________________________________________

 

 

 

U.S. EPA Response to Comments on the 

Human Health Risk Assessment Protocol for 

Hazardous Waste Combustion Facilities 

(EPA530-D-98-001A, July 1998) 

Final 

 June 30, 2005 
United States Solid Waste and Emergency Response EPA530-R-05-020 

Environmental Protection (5305W) June 2005 

Agency www.epa.gov/osw 

____________________________________________________________________________________________________

 

 

 

 

 DISCLAIMER 

This document provides U.S. EPA responses to peer and public comments received on the Peer Review 

Draft of the Human Health Risk Assessment Protocol for Hazardous Waste Combustion Facilities 

(HHRAP, July 1988). This Response to Comments Document includes summaries of public comments 

received on the HHRAP and EPA responses to the summary comments. Since the comments have been 

summarized, some individual comments may be not be explicitly visible and should not be taken as being 

intentionally ignored by EPA. However, all individual public and peer comments received were 

considered in finalizing the HHRAP. 
Response to Public Comments on the Human Health Risk Assessment Protocol June 2005 ________________________________________________________________________________________________________

________________________________________________________________________________________________________

U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 1 

CONTENTS

Sections Page

 

CONTENTS ..........................................................................................................................................1 

ACRONYMS ..............................................................................................................................................4 

VARIABLES ..............................................................................................................................................7 

1.0 INTRODUCTION ......................................................................................................................................8 

1.1 BACKGROUND ............................................................................................................................9 

1.2 PUBLIC COMMENTS .................................................................................................................9 

1.3 REPORT ORGANIZATION ........................................................................................................10 

2.0 SUMMARY OF PUBLIC AND INDIVIDUAL PEER COMMENTS .....................................................12 

2.1 POLICY, OBJECTIVES PURPOSE.........................................................................................12 

2.1.1 Policy, Objectives and Purpose - General Policy Issues ..............................................................12 

2.1.2 Policy, Objectives and Purpose - HHRAP Objectives ..........................................................16 

2.1.3 Policy, Objectives and Purpose - Model and Chemical Data Validation .....................................21 

2.1.4 Policy, Objectives and Purpose - Clarification/Editorial .......................................................23 

2.2 IDENTIFYING EMISSION SOURCRES AND COMPOUNDS OF POTENTIAL CONCERN ...............23 

 2.2.1 Emission Sources and COPCs - Estimating Stack Emissions ..................................................24 

 2.2.2 Emission Sources and COPCs -Estimating RCRA/CKD Fugitive Emissions ...............................29 

 2.2.3 Emission Sources and COPCs -Identifying Compounds of Potential Concern (COPCs) .................30 

 2.2.4 Emission Sources and COPCs -Chemical-Specific Issues ......................................................33 

 2.2.5 Emission Sources and COPCs -Clarifications/Editorial .........................................................45 

 2.3 AIR DISPERSION AND DEPOSITION MODELLING .....................................................................49 

 2.3.1 Air Dispersion & Deposition Modeling - General Issues ......................................................49 

 2.3.2 Air Dispersion & Deposition Modeling - Determining Spatial Locations ...................................52 

 2.3.3 Air Dispersion & Deposition Modeling - Surface Roughness Height (Length) .............................53 

 2.3.4 Air Dispersion & Deposition Modeling - Facility Building Characteristics .................................54 

 2.3.5 Air Dispersion & Deposition Modeling - Chemical Partitioning of Emissions .............................55 

 2.3.6 Air Dispersion & Deposition Modeling - Estimating Dry Depositions of Vapor 

 And Vapor Emission ............................................................................................58 

 2.3.7 Air Dispersion & Deposition Modeling - Estimating Wet Deposition ..........................................61 

 2.3.8 Air Dispersion & Deposition Modeling - Stack Temperature ..................................................61 

 2.3.9 Air Dispersion & Deposition Modeling - Clarifications/Editorial ............................................62 

 2.4 EXPOSURE SCENARIO IDENTIFICATION .............................................................................63 

2.4.1 Exposure Scenario Identification - General Issues .................................................................64 

 2.4.2 Exposure Scenario Identification - Foodstuff Ingestion .........................................................65 

 2.4.3 Exposure Scenario Identification - Acute Risk Assessment ...................................................66 

 2.4.4 Exposure Scenario Identification - Selecting Exposure Scenario Locations .................................69 

 2.4.5 Exposure Scenario Identification - Special Subpopulations ...................................................71 

 2.4.6 Exposure Scenario Identification - Land Use ........... .........................................................73 

 2.4.7 Exposure Scenario Identification - Clarifications/Editorial ....................................................74 

 
Response to Public Comments on the Human Health Risk Assessment Protocol June 2005 ________________________________________________________________________________________________________ 

 

________________________________________________________________________________________________________

U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 2 

 2.5 ESTIMATING MEDIA CONCENTRATIONS ..........................................................................75 

 2.5.1 Estimating Media - General Modeling Issues .....................................................................75 

2.5.2 Estimating Media - Recommended Input Values for Equation Parameters ............................... 80 

 2.5.3 Estimating Media - Equations for Estimating COPC Levels ...................................................84 

 2.5.4 Estimating Media - Conservation of Mass ........................................................................89 

 2.5.5 Estimating Media - Screening of VOCs ...........................................................................92 

 2.5.6 Estimating Media - Clarification / Editorial .......................................................................93 

 2.6 QUANTIFYING EXPOSURE ................................................................................................95 

 2.6.1 Quantifying Exposure - General ...................................................................................95 

 2.6.2 Quantifying Exposure - Exposure Duration .....................................................................96 

 2.6.3 Quantifying Exposure - Exposure Pathways .....................................................................97 

 2.6.4 Quantifying Exposure - Clarifications/Editorial ...............................................................100 

 2.7 RISK HAZARD CHARACTERIZATION ................................................................................101 

 2.7.1 Risk and Hazard Characterization - General ...................................................................101 

 2.7.2 Risk and Hazard Characterization - Risk and Hazard Toxicity Factor Determination ...................105 

 2.7.3 Risk and Hazard Characterization - Route-to-route Extrapolation of Toxicity Benchmarks .............108 

 2.7.4 Risk and Hazard Characterization - Inhalation Toxicity Benchmarks .......................................110 

 2.7.5 Risk and Hazard Characterization - Non-cancer Effects Estimation ........................................111

2.7.6 Risk and Hazard Characterization - Clarification / Editorial .................................................111

 2.8 UNCERTAINTY INTERPRETATION FOR HUMAN HEALTH RISK ASSESSMENT ......................112 

 2.8.1 Uncertainty Interpretation - General Uncertainty Comments ................................................112 

 2.8.2 Uncertainty Interpretation - Qualitative Uncertainty ................................. .............. .......... 113 

 2.8.3 Uncertainty Interpretation - Quantitative Uncertainty ..........................................................114 

 2.8.4 Uncertainty Interpretation - Clarification/ Editorial ................................................... .......115 

2.9 COMPLETION OF RISK ASSESSMENT AND FOLLOW-ON ACTIVITIES.............. ....................115 

2.10 CHEMICAL SPECIFIC DATA [Appendix A] ..........................................................................116 

2.11 MEDIA CONCENTRATION EQUATIONS [Appendix B] ..........................................................121 

2.12 RISK CHARACTRIZATION EQUATIONS [Appendix C] ..........................................................122 

3.0 PEER REVIEW COMMENTS ON MAJOR ISSUES RELATED TO DRAFT HHRAP ..............................123 

 3.1 GENERAL ISSUES .............................................................................................................123 

 3.1.1 General Issue 1: Organization and Documentation ............................................................123 

 3.1.2 General Issue 2: Alignment of Purpose and Methods .........................................................124 

 3.1.3 General Issue 3: Scientific Soundness ...........................................................................124 

 3.1.4 General Issue 4: Credibility of Results ...........................................................................125 

 3.1.5 General Issue 5: Major Data Gaps and Limitations ............................................................125 

 3.1.6 General Issue 6: Long-Term Research ...........................................................................126 

 3.2 COMBUSTION ENGINEERING [draft HHRAP Chapter 2]...........................................................128 

 3.2.1 Combustion Engineering: Process Upsets ......................................................................129 

 3.2.2 Combustion Engineering: Selecting COPCs ...................................................................129 

 3.2.3 Combustion Engineering: Unspeciated TOE ...................................................................132 

 3.2.4 Combustion Engineering: 95th Percentile Emission Rate .....................................................133 
Response to Public Comments on the Human Health Risk Assessment Protocol June 2005 ________________________________________________________________________________________________________ 

 

________________________________________________________________________________________________________

U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 3 

 3.2.5 Combustion Engineering: Quantifying Non-Detect Compounds ............................................134 

 3.3 AIR DISPERSION AND DEPOSITION MODELING [draft HHRAP Chapter 3] ..............................135 

 3.3.1 Air Dispersion And Deposition Modeling: Modeling Vapor Phase, Particle Phase, 

 and Particle-bound Phase Emissions .......................................................................135 

 3.3.2 Air Dispersion And Deposition Modeling: Estimating Fugitive Emissions and Acute 

 Exposures ......................................................................................................136 

 3.3.3 Air Dispersion And Deposition Modeling: Particle and Vapor Phase Emission Partitioning ......... 137 

 3.3.4 Air Dispersion And Deposition Modeling: Air Modeling Input Default Values ...........................137 

 3.3.5 Air Dispersion And Deposition Modeling: Dry Vapor Phase Deposition Modeling .....................138 

 3.3.6 Air Dispersion And Deposition Modeling: Particle Size Distributions .....................................140 

 3.4 EXPOSURE ASSESSMENT [draft HHRAP Chapters 4 & 6, Appendix C] ........................................140 

 3.4.1 Exposure Assessment: Acute Inhalation Exposure Criteria Values and Acute Toxicity 

 Evaluation .......................................................................................................141 

 3.4.2 Exposure Assessment: Route-to-Route Extrapolations .......................................................143 

 3.4.3 Exposure Assessment: Atmospheric Degradation of Contaminants .........................................144 

 3.4.4 Exposure Assessment: Exposure Scenario Locations .........................................................144 

 3.4.5 Exposure Assessment: Non-Carcinogenic Risk of Dioxins Benchmark .....................................145 

 3.4.6 Exposure Assessment: Using TEFs for Coplanar PCBs .......................................................145 

 3.4.7 Exposure Assessment: Dioxin in Breast Milk ..................................................................146 

 3.4.8 Exposure Assessment: Inhalation Rate Value ..................................................................147 

 3.5 ENVIRONMENTAL FATE AND TRANSPORT [draft HHRAP Chapter 5, Appendices A&B] ..............147 

 

 3.5.1 Environmental Fate And Transport: Water Ingestion by Cows Pathway ...................................147 

 3.5.2 Environmental Fate And Transport: Speciating and Modeling Mercury ...................................148 

 3.5.3 Environmental Fate And Transport: Biotransfer Values ......................................................149 

 3.5.4 Environmental Fate And Transport: Organic Compounds and Conservation of Mass ...................151 

 3.5.5 Environmental Fate And Transport: Losses to Soil due to Erosion ..........................................152 

 3.5.6 Environmental Fate And Transport: Benchmarks Based on Mild Adverse Health Effects ...............153 

 

4.0 REFERENCES .......................................................................................................................154 

APPENDIX A ....................................................................................................................................158

 A1. Internal U.S. EPA and State Reviewers .......................................................................................158 

 A2. Public Commenters ..............................................................................................................162 

 A3. External Peer Reviewers .........................................................................................................165 
Response to Public Comments on the Human Health Risk Assessment Protocol June 2005 ________________________________________________________________________________________________________ 

 

________________________________________________________________________________________________________

U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 4 

ACRONYMS

ACGIH American Conference of Governmental Industrial Hygienists 

ADI Average Daily Intake 

AEFA Average Emission Factor Approach 

AEGL Acute Exposure Guideline Level 

AIEC Acute inhalation exposure criteria 

ATEL Acute Toxicity Exposure Levels 

BAF Bioaccumulation Factor 

BaP Benzo(a)pyrene 

BEHP Bis (2-ethylhexyl) phthalate 

BIF Boiler and industrial furnace 

BSAF Biota Sediment Accumulation Factor 

BTEX Benzene, Toluene, Ethyl benzene, Xylene 

CAA Clean Air Act 

CARB California Air Resources Board 

CCC Chlorine Chemistry Council 

CCSE Center of Combustion Science and Engineering 

CFR Code of Federal Regulations 

CKD Cement kiln dust 

COPC Compounds of Potential Concern 

CRQL/CRDL Contract required quantitation limit / Contract required detection limit 

CRWI Coalition for Responsible Waste Incineration 

CSF Cancer Slope Factor 

DNOP Di (n) octyl phthalate 

DOE U.S. Department of Energy 

DRE Destruction and removal efficiency 

EDL Estimated Detection Level 

EMPC Estimated Maximum Possible Concentration 

EPA Environmental protection agency 

EPACA US EPA Correlation Approach 

ER Sediment Enrichment Ratio 

ERPG Emergency Response Planning Guide 

GAQM Guide for Air Quality Management 

GC Gas Chromatograph 

GRAV Unspeciated gravimetric compounds 

HCL/CL Hydrogen Chloride and Chlorine Gas 

HEAST Health Effects Assessment Summary Tables 

HEPA High Energy Particulate Air 

HHRAP Human Health Risk Assessment Protocol 

HQ Hazard Quotient 

HWIR Hazardous Waste Identification Rule 

IDL Instrument Detection Limit 
Response to Public Comments on the Human Health Risk Assessment Protocol June 2005 ________________________________________________________________________________________________________ 

 

________________________________________________________________________________________________________

U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 5 

IEUBK Integrated Exposure Uptake Biokinetic Model 

ISCST3 Industrial Source Complex Short-Term Model 

LDAR Leak Detection and Repair 

MACT Maximum Available Control Technology 

MACT Maximum Available Control Technology 

MDL Method Detection Limit 

MF Metabolism Factor 

MPRM Meteorological processor for regulatory models 

MS Mass Spectrometry 

NAAQS National Ambient Air Quality Standards 

NCEA National Center for Environmental Assessment 

ND Non-Detect 

NESHAPS National Emissions Standards for Hazardous Waste Air Pollutants 

NIOSH National Institute of Occupational Safety and Health 

OAQPS Office of Air Quality Planning and Standards 

OCDD Octa Chloro Dibenzo Dioxin 

OEHHA Office of Environmental Health Hazard Assessments 

ORD Office of Research and Development 

OSHA U.S. Occupational Safety and Health Administration 

OSW Office of Solid Waste Management 

PAH Polynuclear aromatic hydrocarbon 

PCBs Polychlorinated Biphenyls 

PCDD Polychlorinated dibenzo(p)dioxin 

PCDF Polychlorinated dibenzofuran 

PCRAMMET Personal computer version of the meteorological preprocessor for the old RAM program 

PIC Product of Incomplete Combustion 

POHC Principal organic hazardous constituent 

PSD Particle Size Distribution 

QC Quality Control 

RC (this) Response to Comments (document) 

RCRA Resource Conservation and Recovery Act 

RDL Reliable Detection Level 

REL California EPA Air Toxics Hot Spots Program acute reference exposure levels 

RLIS Runoff Loading from Impervious Surfaces 

RME Reasonable maximum exposure 

RP Relative potency 

RPF Relative Potency Factor 

SAB EPA’s Science Advisory Board 

SCAPA Subcommittee on Consequence Assessment and Protective Actions 

SCDM Superfund Chemical Data Matrix 

SQL Sample quantitation limit 

SRA Screening Ranges Approach 

TB Trial Burn 

TCDD Tetrachlorodibenzo(p)dioxin 
Response to Public Comments on the Human Health Risk Assessment Protocol June 2005 ________________________________________________________________________________________________________ 

 

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U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 6 

TEEL Temporary Emergency Exposure Limit 

TEFs Toxicity Equivalency Factors 

TEQ Toxicity equivalent quotient 

TIC Tentatively identified compound 

TNRCC Texas Natural Resource Conservation Commission (now TCEQ; Texas Commission of 

 Environmental Quality) 

TOE Total Organic Emission 

TRIM Total Risk Integrated Methodology 

TSS Total Suspended Solids 

UCL Upper Confidence Level 

URF Unit Risk Factors 

USLE Universal Soil Loss Equation 

USGS United States Geological Survey 

UTM Universal transverse mercator 

VOC Volatile organic compound 

WHO World Health Organization 

WTI Waste Technologies Industries 
Response to Public Comments on the Human Health Risk Assessment Protocol June 2005 ________________________________________________________________________________________________________ 

 

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U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 7 

VARIABLES

B Biotransfer factor 

Ba Bioavailability factor 

Ca Air Concentration 

Cyv Unitized yearly average concentration from vapor phase 

Ds Deposition term 

Dydw Unitized yearly average wet deposition (s/m2 -yr) 

Dydp Unitized yearly wet deposition from particle phase (s/m2 -yr) 

Dywwv Unitized yearly (water body and watershed) average wet deposition from vapor phase 

(s/m2

-yr) 

Dytwp Unitized yearly (water body and watershed) average total (wet and dry) deposition from 

vapor phase (s/m2

-yr) 

Fv Fraction in vapor phase 

kb Benthic burial rate constant 

Kow Octanol – water partition coefficient (mg COPC/L Octanol) 

kp Plant surface loss coefficient (yr-1) 

ks Soil loss constant 

kse COPC loss constant due to soil erosion (yr-1) 

ksg value COPC loss constant due to biotic and abiotic degradation (yr-1) 

kwt water body dissipation rate 

Ldif diffusion load to water body 

Pd Direct Deposition 

Q COPC emission rate (g/s) 

RfC Reference Concentration 

RfD Reference dose 

Rp Relative potency 

TOCVOC Stack concentration of volatile TOC, including speciated and unspeciated compounds 

(mg/m3 ) 

VGag Empirical correction factor for aboveground produce (forage and silage) (unitless) 

VGrootveg Empirical correction factor for root vegetable (unitless)

Vp Vapor Pressure 

Xe Unit soil loss (kg/m2

 - yr) 
Response to Public Comments on the Human Health Risk Assessment Protocol June 2005 ________________________________________________________________________________________________________ 

 

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U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 8 

 

1.0 INTRODUCTION

In the September 30, 1999, preamble to the proposed Maximum Achievable Control Technology 

(MACT) rule, the U.S. Environmental Protection Agency (EPA) recommended that site-specific risk 

assessments be conducted as part of the Resource Conservation and Recovery Act (RCRA) permitting 

process for hazardous waste combustion facilities as necessary to protect human health and the 

environment. Often, the determination of whether or not a permit is sufficiently protective can be based 

on its conformance to the applicable technical standards specified in the regulations. However, since the 

time the regulations for hazardous waste incinerators (1981, 40 CFR Part 264, Subpart O) and 

boilers/industrial furnaces (1991, 40 CFR Part 266, Subpart H) were issued, information has become 

available to suggest that these performance standards may not fully address potentially significant risks. 

In many cases, risks from indirect exposures may constitute the majority of the risk from a hazardous 

waste combustor. This key portion of the risk from hazardous waste combustion emissions was not 

appropriately taken into account when the hazardous waste combustion standards were developed. In 

addition, uncertainty remains regarding the types and quantities of non-dioxin products of incomplete 

combustion (PICs) emitted from combustion units and the risks posed by these compounds. 

The RCRA “omnibus” authority of §3005(c)(3) of RCRA, 42 U.S.C. §6925(c)(3) and 40 

C.F.R.§270.32(b)(2) gives the U.S. EPA both the authority and the responsibility to establish permit 

conditions on a case-by-case basis as necessary to protect human health and the environment. 

Performance of a site-specific risk assessment can provide the information necessary to determine what, if 

any, additional permit conditions are necessary for each situation to ensure that operation of the 

combustion unit is protective of human health and the environment. Under 40 C.F.R. §270.10(k), the 

RCRA permitting authority may require a permit applicant to submit additional information (e.g., waste 

feed analysis, emissions data, operating parameters) that U.S. EPA needs to establish permit conditions 

under the omnibus authority. In certain cases, U.S. EPA may also seek additional testing or data under 

the authority of RCRA §3013 (where the presence or release of a hazardous waste “may present a 

substantial hazard to human health or the environment”) and may issue an order requiring the facility to 

conduct monitoring, testing, analysis, and reporting. Any decision to add permit conditions based on a 

site-specific risk assessment under this authority must be justified in the administrative record for each 

facility, and the implementing agency should explain the basis for the conditions. 

We developed the “Human Health Risk Assessment Protocol (HHRAP) for Hazardous Waste 

Combustion Facilities” to explain how we recommend performing risk assessments of hazardous waste 

combustion facilities. The HHRAP is intended as the guidance for personnel conducting risk 

assessments, and an information resource for permit writers, risk managers, and community relations 

personnel. The HHRAP supports the U.S. EPA Office of Solid Waste (OSW) mission to provide 

permitting authorities for hazardous waste combustion facilities with state-of-the-art methodology in 

predicting the risk associated with human exposure to compounds found in the air, soil, and water. The 

document also provides guidance to risk assessors dealing with direct and indirect exposure from 

contaminants emitted from stationary combustion sources. The HHRAP has been developed as national 
Response to Public Comments on the Human Health Risk Assessment Protocol June 2005 ________________________________________________________________________________________________________ 

 

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U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 9 

guidance to consolidate information presented in other risk assessment guidance and methodology 

documents previously prepared by U.S. EPA and state environmental agencies. In addition, the HHRAP 

addresses issues that have been identified while conducting risk assessments for existing hazardous waste 

combustion units. This U.S. EPA Response to Comments Document contains Agency responses to public 

and expert peer reviewer comments received on the Peer Review Draft of the HHRAP (U.S. EPA, 1998a, 

EPA530-D-98-001). 

1.1 BACKGROUND 

U.S. EPA Region 6 developed the Peer Review Draft HHRAP (U.S. EPA, 1998a, EPA530-D-98-001) 

document for the Center of Combustion Science and Engineering (CCSE) in coordination with the Office 

of Solid Waste. The foundation for the procedures and methodologies outlined in the HHRAP were first 

developed by the Office of Research and Development (ORD) and the Office of Solid Waste in previous 

versions of combustion risk assessment guidance. The State of North Carolina’s combustion risk 

assessment methodology was also evaluated in preparation of the HHRAP. The interim final draft 

version of the HHRAP, published in July 1998, was originally initiated in response to the desire of the 

Region 6 Multimedia Planning and Permitting Division to implement an up-to-date and technically sound 

hazardous waste combustion permitting program. The decision to incorporate guidance on a full range of 

national combustion risk assessment issues into the HHRAP was encouraged and supported by the 

Director of the Office of Solid Waste. 

1.2 PUBLIC COMMENT AND PEER REVIEW 

Draft versions of the HHRAP underwent extensive internal peer review by U.S. EPA and state personnel. 

A list of these reviewers can be found in Appendix A. An open public comment period followed the 

internal review. In October 1998, EPA’s Office of Solid Waste issued a Federal Register Notice of 

Availability and Request for Comment on the HHRAP (FR 10/30/1998, Volume 63, Number 210). 

Twenty-eight commenters, including individual businesses, federal and state agencies, universities, 

environmental and public interest groups, and trade associations, submitted about 600 individual 

comments on the HHRAP. A list of the public commenters can be found in Appendix A. All public 

comments were organized by topic and then summarized into individual issues for U.S. EPA to address. 

These issues and associated U.S. EPA responses are contained in Section 2 of this Response to Comments 

document. 

U.S. EPA determined that the HHRAP is a “major scientific and technical work product” and thus, the 

Office of Solid Waste requested the HHRAP undergo an independent, external peer review. The peer 

review was conducted according to U.S. EPA’s Peer Review Guidance document (1998b, EPA 100-B-98-

001). Based on public comments received, U.S. EPA identified 30 major issues raised by the public that 

required technical input and opinion from external peer reviewers to address. A total of nine scientists 

familiar with issues associated with multi-source, multi-pathway risk assessments for hazardous waste 

combustion facilities were selected to address the major issues. A list of these peer reviewers can be 

found in Appendix A. The 30 major issues were provided to the external peer review panel members, 
Response to Public Comments on the Human Health Risk Assessment Protocol June 2005 ________________________________________________________________________________________________________ 

 

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U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 10 

according to their five areas of expertise. These areas of expertise included combustion engineering, 

atmospheric modeling, human health toxicology, human health exposure, and chemical fate and transport. 

The peer review panel first responded to the 30 major issues in writing. The written comments 

demonstrated a consensus among the panel members on 13 of the major issues. The peer review panel 

members’ responses to these 13 major issues have been summarized in Section 3.1 of this Response to 

Comments document. The remaining 17 issues required further discussion among the panel members to 

reach consensus. Therefore, a peer review workshop, which included limited public participation, was 

held to address these 17 major issues. The workshop was held in Dallas, TX on May 24 and 25, 2000. 

Summarized in Sections 3.2 through 3.5 of this Response to Comments document are the consensus 

opinions of the peer reviewers on the 17 major issues discussed during the workshop. 

In addition to addressing the 30 major issues given to the external peer review panel, the panel members 

were also invited by EPA to provide any other comments they wished to make regarding any part of the 

Peer Review Draft HHRAP (1998a, EPA530-D-98-001), hereafter referred to as the draft HHRAP. Each 

of the panel members provided individual comments directed at specific sections of the draft HHRAP. 

These individual panel member comments have been summarized and incorporated with the public 

comments under the appropriate topical areas in Section 2 of this Response to Comments document. 

Based on input from the public and external peer reviewers, U.S. EPA has revised and finalized the 

HHRAP. 

1.3 REPORT ORGANIZATION

This Response to Comments document has been organized into four major sections: 

• Section 1 is an introduction with a brief overview of the comment/response process that U.S. EPA 

followed in finalizing the HHRAP. 

• Section 2 contains summaries of written public comments received on the draft HHRAP. Section 

2 also incorporates summaries of written comments, specific to particular sections of the draft 

HHRAP, received from individual external peer reviewers. 

• Section 3 contains summaries of the external peer reviewers’ responses to the 30 major issues 

raised by the public and requiring expert technical input. These summaries derived from the 

written comments and the peer review workshop discussion. 

• Section 4 contains references cited in this Response to Comments document. 

Appendix A contains the names of the reviewers of the draft HHRAP, representing federal and state 

agencies, the public, and the external peer review panel. 
Response to Public Comments on the Human Health Risk Assessment Protocol June 2005 ________________________________________________________________________________________________________ 

 

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U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 11 

 

2.0 SUMMARY OF PUBLIC AND INDIVIDUAL PEER COMMENTS

Throughout Section 2 of this Response to Comments (RC) document, we have mapped the public 

comments to the applicable sections of the draft HHRAP. Where appropriate, we have also mapped the 

public comments to one or more of the 30 major issues given to the external peer review panel to address. 

In Section 2, we have organized summaries of public comments into subsections according to Chapters 1 

through 9 and Appendices A, B, and C of the draft HHRAP. Specific sections of the draft HHRAP 

chapters with which the public comments are generally associated are provided [in brackets]. The reader 

should note that these referenced sections from the draft HHRAP may not coincide with the same section 

numbers in the Final HHRAP due to revisions made to the draft HHRAP. 

Under each subsection in Section 2, we have organized the public comment summaries by topical issues. 

Some of these public comments are specifically related to the 30 major issues given to the external peer 

review panel to address. For these particular public comments, we have provided (in parentheses) a 

reference to the location within Section 3 of this RC document where the reader can find that specific 

major issue addressed by the external peer reviewers. 

In addition to summarizing the public comments in Section 2, we have also incorporated into Section 2 

summaries of specific comments related to the draft HHRAP that were provided by individual peer 

reviewers, in addition to their responses to the 30 major issues they were given. 

2.1 POLICY, OBJECTIVES, AND PURPOSE 

Policy includes EPA’s approach to the broad issues generally applicable to the HHRAP and 

implementation of the risk assessment process as a whole. A wide range of comments on broad issues 

was received. These issues were categorized as: 

• General Policy;

• HHRAP Objectives; and

• Model and Chemical Data Validation.

2.1.1 Policy, Objectives, and Purpose - General Policy Issues

General policy issues raised by commenters included the: 

• Application of the HHRAP as guidance, not as regulation; 

• Purpose of the HHRAP for use by the regulated community; 

• Focus of HHRAP; 

• Authority of EPA to require risk assessments; 
Response to Public Comments on the Human Health Risk Assessment Protocol June 2005 ________________________________________________________________________________________________________ 

 

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U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 12 

• Impacts of implementing the Maximum Achievable Control Technology (MACT) rule; 

• Criteria to determine when risk assessments are necessary; 

• EPA NCEA Guidance; 

• Data reporting schedules; 

• Use of Confidential Business Information; 

• Application of risk offsets; and 

• Inclusion of off-site sources in the risk assessment. 

Issues: General Policy - Guidance versus Regulation 

It was recommended that EPA should make it clear on a continuing basis suggestions in guidance 

documents cannot be ‘rigidly’ followed as if they are regulations. 

EPA Response: 

EPA’s Office of Government Council has recommended changes that we’ve incorporated into HHRAP 

Chapter 1 to further enunciate that the HHRAP is guidance, not a regulation. We also reviewed the tone 

throughout the protocol. We modified the language where appropriate, to stress the function of the 

HHRAP as guidance, not regulation. 

Issues: General Policy - Science Policy Decisions

The draft HHRAP reflects numerous policy decisions that were made in developing the risk assessment 

methodologies. It is difficult, if not impossible, to conduct a full technical review of the HHRAP without 

a consideration of the science policy decisions that have guided its development. Therefore, policy 

considerations and other risk management factors that affect the HHRAP methodologies should be 

explicitly identified and their relevance discussed. 

EPA Response: 

The HHRAP methodology is guidance and contains recommendations, not policy. Every effort was taken 

to document recommended methodology such that a guidance user could identify and modify where 

appropriate recommended methods. 

Issue: General Policy - Purpose of the HHRAP for Use by the Regulated Community 

It was recommended that the stated purpose of the HHRAP should explicitly include use by the regulated 

community because the document will be used by many facilities. Inclusion of the regulated community 

gives EPA an opportunity to explain and justify conservative assumptions and reduce the perception of an 

overly conservative risk assessment process, which could well have severe repercussions to that group. 

Additionally, at several points in the draft HHRAP, the phrase "Agency consensus" was used. This entire 

process would work more effectively if the consensus could be widened to include the regulated 

community. 

It was recommended that HHRAP Chapter 1 Introduction discuss the applicability of the HHRAP to other 

types of combustion facilities and clearly spell out the caveats about the relevance of using the HHRAP 

guidance for risk assessments of other combustion facilities. The draft HHRAP clearly states it is 

intended for use in evaluating hazardous waste combustors. However, it is obvious that the methodology 
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can and probably will be used at other facilities, such as medical waste incinerators, waste-to-energy 

plants, etc. 

 

EPA Response: 

Comment noted. The stated target audience for the HHRAP includes risk assessors and risk managers, 

without specifying the various groups to which these individuals are affiliated. Risk assessors might be 

employed by the permitting authority, but might also work for the facility, or a contractor hired by the 

facility or interested community organization. The External Peer Review process is implemented to 

widen consensus to include the regulated community. The words “Agency consensus” are no longer used 

in the HHRAP. 

It is gratifying that both the regulating and regulated communities consider the HHRAP methodology so 

valuable as to apply it beyond its intended purpose. However, discussing the use of the HHRAP beyond 

its stated purpose is beyond the scope of this document. Possible expansion of the scope will have to wait 

for future additions of the Protocol. 

Issue: General Policy - Focus of the HHRAP 

It was suggested that in the draft HHRAP Chapter 1, Introduction, the first sentence presents a misleading 

message about what a risk assessment is and what it does. While risk assessment is a type of science, it is 

a "tool" used to evaluate health impact. It is not, as stated, "a science used to evaluate..." Also, the 

sentence erroneously implies that a risk assessment is only used to evaluate health impacts from exposure 

to emissions from hazardous waste combustion facilities. It was recommended that the sentence should 

clarify the issues indicated above. 

It was also noted that on page 1-1, sentence 5 of the Introduction makes an important distinction between 

the relative focus of the HHRAP on direct inhalation risk versus risk through indirect pathways. This 

sentence should be clarified and expanded because this is an important consideration. For example, why 

is the primary focus on indirect pathways? 

EPA Response: 

We’ve removed the referenced sentences. The definition for risk assessment in the HHRAP is adopted 

directly from the National Academy of Sciences, “Risk Assessment in the Federal Government” (1983). 

A section has been added to Chapter 1 to more clearly state the use of the document. 

Issue: General Policy - EPA and Permitting Authority 

Commenters believed that EPA's authority to require risk assessments is limited by RCRA's omnibus 

provisions. EPA cannot make legally binding decisions based on the use of indirect exposure assessment. 

Also, the decision regarding whether population risk estimates should be considered is not one that rests 

with the permitting authority, but one that should be made by the applicant (draft HHRAP Chapter 7, 

page 7.2). 

EPA Response: 

A facility is not required by the guidance or by regulation to conduct a risk assessment. It is the 

permitting authority that is required to write operating permits that are protective of human health and 
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the environment. A risk assessment is one tool used by permit writers on a site-specific basis to ensure 

that permit conditions are protective of human health and the environment under the omnibus authority 

40 CFR Section 270.32(b)(2). We are unaware of a method other than risk assessment to evaluate 

protectiveness. If site-specific conditions warrant evaluation of population risks to develop unit-specific 

permit conditions protective of human health and the environment, a risk assessment that considers 

population effects could be conducted. The permitting authority does not have the authority to require a 

facility to conduct such an assessment, but may conduct or contract out such a risk assessment if the 

facility chooses not to conduct its own. 

Issue: General Policy - MACT Rule 

It was suggested that it would be helpful if the HHRAP and the related EPA “Guidance on Collection of 

Emissions Data to Support Site-Specific Risk Assessments at Hazardous Waste Combustion Facilities” 

(U.S. EPA, 1998c) could address potential changes to trial burns, risk burns, and risk estimation activities 

that would allow performance testing required by the forthcoming Maximum Achievable Control 

Technology (MACT) rule for hazardous waste combustors to be coordinated with the collection of data 

for use in risk assessments. In particular, these changes could relate to the potential effects of installing 

carbon adsorption emissions control systems for mercury and dioxin/furan emissions and systems for 

improved particulate emissions control. 

It was also recommended that decisions to require risk assessments should generally be deferred until 

after implementation of the MACT rule because with the lower emissions following MACT compliance, 

risk assessments should not be routinely required. 

EPA Response: 

The suggestion has been forwarded to the authors of the emissions data guidance. With regard to the 

recommendation, MACT standards are technology based and not risk-based. RCRA requires that permits 

for hazardous waste burning combustion units be protective of human health and the environment. We 

are unaware of a method other than risk assessment to evaluate protectiveness. The national risk 

assessment performed to initially evaluate the MACT standards did not conclude that the standards, when 

implemented, would be protective of human health and the environment for all units.

Issue: General Policy - Criteria for Implementing Risk Assessments 

It was recommended that EPA provide further guidance on the circumstances under which a risk 

assessment does or does not need to be performed. Commenters did not agree that EPA's omnibus 

authority should be used to require risk assessments at all facilities, regardless of the demonstration of 

need. They recommended that EPA include on the list of criteria on draft HHRAP page. 1-3: 

• Objective criteria only and not the level of public interest or community involvement. 

• Whether the incremental emissions from burning hazardous waste are anticipated to increase risk 

from an industrial facility when compared to its baseline emissions from non-hazardous waste 

combustion. 

• Risk assessments already completed should be used to identify the exceptional cases where risk 

assessments may be warranted. Specifically, there is a strong body of evidence that demonstrates 
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hazardous waste combustion cement kilns do not pose unacceptable risks to human health and the 

environment. 

• Risk that is already controlled through stringent regulations limiting operations and/or emissions. 

• A change in the wording of the second bullet: “several factors” to “whether the facility is known to 

exceed any final technical standards.” Sometimes it may be difficult to determine “whether the 

facility is exceeding any final technical standards” unless the risk analysis is performed. The 

suggested wording more precisely indicates the intended purpose of this statement. 

EPA Response: 

Chapter 1 now states that the HHRAP does not determine when a site-specific risk assessment should be 

performed. U.S. EPA’s most recent recommendations for when, or if, a site-specific risk assessment 

should be considered can be found in documentation of the MACT rule, published on September 30, 1999 

Federal Register (U.S. EPA, 1999a). 

Many pre-existing risk assessments were performed to support permitting under the BIF Rule. These risk 

assessments only addressed the inhalation exposure pathway. Since then, information has become 

available which suggests that evaluating the inhalation pathway alone would not necessarily lead to 

permit conditions protective of human health and the environment. 

Issue: General Policy - EPA NCEA Guidance 

It was recommended that discussion be added in the HHRAP about what connection there is between the 

HHRAP and the U.S. EPA NCEA guidance, Methodology for Assessing Health Risks Associated with 

Multiple Pathways of Exposure to Combustion Emissions, mentioned in Section 1.3. Should the NCEA 

methods and data be integrated into or coordinated with the approaches in the HHRAP? Section 1.3 

should also include the “Risk Assessment for the Waste Technologies Industries (WTI) Hazardous Waste 

Incineration Facility (East Liverpool, Ohio)” (U.S. EPA, 1997a, EPA905-R97-002) as one of the 

reference documents used to prepare the HHRAP because it appears that much of the methodology was 

developed and/or applied in the WTI study. 

In many places, the draft HHRAP states that the methodology derives from the U.S. EPA NCEA 

guidance document, but this document is not available for review. It was felt that it is inappropriate for 

EPA to refer to unpublished supporting documentation. 

EPA Response: 

The “Methodology for Assessing Health Risks Associated with Multiple Pathways of Exposure to 

Combustor Emissions” (U.S. EPA, 1998d, EPA/600/R-98/137) also known as the “MPE” document, was 

published after the release of the draft HHRAP. The final version of the protocol discusses the 

relationship between the protocol and the MPE, as well as the WTI study. 

Issue: General Policy - Reporting Schedules

It was noted that the draft HHRAP states "Monthly, quarterly, or annual reports which document longterm operations will likely be required of the facility." Since the permit conditions will likely be based on 

annual averages (e.g., mass feed limitations, temperature, or gas flow rates), it was suggested that 

reporting requirements should be no more frequent than the averaging time outlined in the permit. 
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EPA Response: 

Comment noted. This should be worked out with the permitting authority. 

Issue: General Policy - Confidential Business Information 

It was suggested that language should be inserted into the text stating that only information that is not 

confidential business and that is relevant to the risk assessment should be included in the facility 

description. 

EPA Response: 

The HHRAP does not ask for confidential business information or data irrelevant to the risk assessment 

for compiling basic facility information. 

Issue: General Policy - Risk Offsets 

It was suggested that risk offsets should be recognized for displaced fuel combustion. 

EPA Response: 

The topic of risk offsets is beyond the scope of the HHRAP. 

Issue: General Policy - Inclusion of Off-Site Sources 

It was noted that the draft HHRAP describes the need to include all RCRA combustion and other RCRA 

units in the risk assessment, whether or not a particular unit is being permitted. It was recommended that 

the risk assessment conducted for a given facility should not be required to characterize risk from off-site 

sources (draft HHRAP Chapter 2, page 2-1). 

EPA Response: 

The HHRAP recommends that the permitting authority and the facility agree upon the units to be 

evaluated in the risk assessment. There is no requirement in the HHRAP to characterize risk from off-site 

sources. However, this might be considered on a site-specific basis by the permitting authority. 

2.1.2 Policy, Objectives, and Purpose - HHRAP Objectives 

Issues raised with regard to objectives stated in the HHRAP included: 

• Meeting the Guidance Objectives; 

• Figure 1.1; 

• Flexibility; 

• Completeness; 

• Reasonableness; 

• Efficient Use of Time and Resources; 

• Implementation Practices; and 

• Clarification / Editorial. 
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Issue: HHRAP Objectives - Meeting Guidance Objectives 

Commenters believed that the draft HHRAP fails to meet its own primary objectives. 

EPA Response: 

We disagree. The HHRAP has been successfully implemented for numerous risk assessments. Although 

any process which requires integration of several technical disciplines (i.e. combustion engineering, air 

modeling, terrestrial fate and transport modeling, exposure assessment, toxicology) may be viewed as 

cumbersome and unreasonable, regional and state personnel have been implementing the guidance since 

the draft was released and have not found the guidance, with the exceptions brought out in the external 

peer review, particularly burdensome to use. Specifics of exceptions are addressed in our responses to 

technical comments. 

Issue: HHRAP Objectives – Figure 1.1 

With regard to draft HHRAP Figure 1.1, commenters: 

• Thought it would be helpful for the uninitiated if there were a narrative description accompanying 

the flow diagram depicted in Figure 1-1 that would convey the type of information that flows 

between boxes and particularly the products of the Risk and Hazard Characterization step. 

• Recommended adding a sentence to the end of the first full paragraph on draft HHRAP page 1-4, 

Section 1.1 to indicate “The organization of this document follows closely the process outlined in 

Figure 1-1.” 

• Suggested that sometimes the Exposure Scenario Selection may be based on the Estimation of 

Media Concentrations, especially considering site-specific characteristics. Would it be more 

appropriate to have the "Exposure Scenario Selection" box interweaved below that for "Estimation 

of Media Concentrations,” or at least have data from "Estimation of Media Concentrations" feeding 

into "Exposure Scenario Selection”? There might be some concern that the Scenario Selection 

would exclusively drive the Media selections. This should be a two-way process. 

EPA Response: 

Figure 1.1 is meant as a graphical representation of the risk assessment process documented in the 

HHRAP. Each box in Figure 1.1 contains a reference to the sections in the HHRAP in which a detailed 

narrative can be found. 

Issue: HHRAP Objectives - Flexibility 

One commenter recommended more guidance be provided in draft HHRAP Section 1.1 to help in 

deciding if and how modifications to the protocol should be made. While he was generally in favor of 

retaining flexibility in the HHRAP for site-specific modifications, the flip side is that for a guidance 

document ("a set of user-friendly procedures for performing risk assessments”), much is left for the risk 

assessor to figure out. 

Commenters were of the opinion that the HHRAP is portrayed as risk assessment guidance even though 

most of its information and algorithms reflect methodology once considered by EPA as only screeninglevel guidance. Moreover, the new guidance is structured differently than risk assessment guidance 

typically issued by EPA. Specifically: 

• A tiered approach is not included. 
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• There is very limited allowance for site-specific information. 

• Specified receptors are required regardless of whether they exist at a particular location. 

• The methodology relies on correlation algorithms that are very poorly validated for the range of 

chemicals included in the guidance. 

It was specifically recommended that EPA use a three-tiered risk assessment approach that includes a 

screening risk assessment to determine which sources are of potential concern, a second tier that is more 

detailed with site-specific data and is only required when the first tier indicates potential problems. The 

third tier would allow the use of probabilistic methods, such as Monte Carlo analysis, if the permittee 

desires to reduce the degree of conservatism to a more defined and reasonable level. 

EPA Response: 

The guidance was written to be flexible and can be applied to a continuum of tiers. There is full flexibility 

in the allowance for site-specific data. Nevertheless, we further modified the HHRAP to stress that sitespecific data should always be preferred over default values provided in the guidance. For example, if 

detailed site-specific information is available up front for mercury fate and transport modeling, there is 

nothing to prevent this information from being used in the risk assessment. However, if site-specific 

information changes the end result from a hazard quotient of 0.001 to 0.000001, it may not make sense to 

spend the time and money to obtain this site-specific information. Specified receptors are recommended 

in the risk assessment evaluation, they are not required. It is our position that specified receptors should 

be assumed to be present until proven otherwise, rather than the other way around. We agree that some 

of the correlation algorithms are poorly validated for a few chemicals, and we are currently updating 

these correlations. However, algorithms in the guidance are based on the best available science 

currently available and except for a few instances, provide a reasonable estimate of exposure. 

Issue: HHRAP Objectives - Reasonableness 

It was suggested that the scope of the risk assessment should be limited to those pathways, which are 

significant, demonstrated, and scientifically defensible. Commenters believed that conducting risk 

assessments according to the HHRAP guidance would result in overly conservative estimates of risk. 

Some of the specific issues raised that contribute to this overly conservative approach include: 

• Using default values instead of site-specific data in the risk assessment models; 

• Applying worst-case values to default parameters used in the risk assessment models; 

• The ineffectiveness of a screening-level tool that finds unacceptable risk based on emissions 

 quantified at non-detect and laboratory contamination levels; and 

• The compounding effect of using a combination of trial burn data, non-detects, a TOE adjustment, 

 and RDLs, which will unnecessarily overestimate emission rates and associated risks. 

The risk assessment approach of winnowing insignificant pathways has recently become the de facto 

scientific paradigm in recent EPA risk assessment guidance. For example, before initiating a risk 

assessment, most facilities first screen the site to identify high-risk chemicals of concern (COCs) and 

exposure pathways. The Presidential/Congressional Commission on Risk Assessment and Risk 

Management’s reports, Framework for Environmental Health Risk Management and Risk Assessment 

(1997a) and Risk Management in Regulatory Decision-Making (1997b), strongly recommend this 
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approach to wisely use valuable resources, time, and effort to conduct a risk assessment. To conduct a 

risk assessment for every conceivable permutation of exposure just because computer spreadsheet 

software is available may be a waste of valuable resources and divert attention from truly protecting the 

public health. 

A major problem with EPA's overall approach was cited to be the confusion about whether the purpose of 

this HHRAP is to produce a screening level or a realistic, site-specific risk assessment. Specifically, 

confusion arises from the apparent contradiction of statements on pages 1-6 and 5-89 in the draft 

HHRAP. Page 1-6 states that "the use of existing and site-specific information early in, and throughout, 

the risk assessment process is encouraged..." while page 5-89 states that "after completing a risk 

assessment based on the default parameter values recommended in this guidance, risk assessors may 

choose to investigate the use of site-specific parameter values...” 

EPA Response: 

Default values are presented in the guidance as a starting point. The use of site-specific data is always 

preferred over default assumptions. However, it is time consuming and expensive to obtain site-specific 

values for all parameters in the complicated fate and transport models. In addition, many default inputs 

can vary by several orders of magnitude and have little to no effect on the risk assessment results. We 

will continue to recommend use of site-specific inputs where practical and where these inputs could 

significantly affect results of the risk assessment. As with many parameters and procedures provided in 

the HHRAP, the scientific basis and reference are provided. A facility is then able to evaluate whether to 

propose and justify to the permitting authority a site-specific option or other alternative to that 

recommended in the HHRAP. Although any process which requires integration of several technical 

disciplines (i.e. combustion engineering, air modeling, terrestrial fate and transport modeling, exposure 

assessment, toxicology) may be viewed as cumbersome and unreasonable, regional and state personnel 

have been implementing the guidance since the draft was released and have not found the guidance, with 

the exceptions brought out in the external peer review, particularly burdensome to use. Specifics of 

exceptions are addressed in our responses to technical comments. The statement in Section 5.8 has been 

changed to be consistent with the recommendation in Chapter 1.

Issue: HHRAP Objectives - Efficient Use of Time and Resources 

Commenters believed that conducting a risk assessment according to the HHRAP guidance would result 

in an inefficient use of time and resources. Some of the issues they raised in this regard include: 

• First requiring completion of calculations using the recommended default input parameters, then 

supplementing with site-specific information. 

• The requirement for evaluation of all COPCs and pathways and inclusion of insignificant emission 

sources, such as fugitives, emergency stack openings, and waste feed cutoffs. The focus should be 

on areas that have real risks and a significant role in risk assessment calculations, rather than 

including the many areas that have minimal to no impact or only perceived risks. 

• The level of assessment being described is difficult to justify with respect to actual risks. It is 

important to note that even the most extensive risk assessments performed are showing that there 

are very few constituents driving the risk assessment, even using worst case assumptions. Many 
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issues described in the draft HHRAP and recommended to be performed are costly, but are not 

relevant when put in perspective with the actual risks. 

• The recommendation that risk be continually evaluated. The process, as described on draft HHRAP 

pages 1-7, 6-3, and 9-2, is seemingly iterative and never ending and filled with policy statements 

that suggest setting additional permit limits which overarch all other limits set by actual 

environmental regulations. The implication is that facility permits will become a moving target 

based on perceived risks, and facilities will be subjected to a resource intensive and costly risk 

assessment process. 

EPA Response: 

As guidance, the HHRAP requires nothing. Rather, it makes recommendations on how to perform and 

report a transparent, scientifically defensible human health risk assessment. Because the HHRAP 

provides default parameters, it is neither inefficient nor costly to evaluate them in the risk assessment. In 

fact, we believe that evaluating default values in the risk assessment will help focus where it makes sense 

to expend efforts to collect site-specific data and refine the risk assessment. Chemical-specific data are 

provided in the guidance such that, if applied through a computer program, it takes an insignificant 

amount of time to evaluate all chemicals (that satisfy the 6-step approach detailed in HHRAP Chapter 

two) rather than writing up detailed justifications of why some compounds should not be evaluated. If a 

chemical is not of concern, it is far easier to let its own fate, transport, exposure, and toxicity eliminate it 

from concern than it is to justify why it shouldn’t even be evaluated. A facility is not required by the 

guidance or by regulation to conduct a risk assessment. It is the charge of the permitting authority to 

issue a permit protective of human health and the environment. EPA is unaware of a methodology other 

than risk assessment with which to evaluate protectiveness. Therefore, the permitting authority may 

conduct or contract out a risk assessment. Should a facility choose to conduct its own risk assessment, 

we recommend that they work with their permitting authority to minimize the number of iterations 

necessary to complete the assessment.

Issue: Protocol Objectives - Implementation Practices 

Because the HHRAP calls for using conservative default criteria and assumptions, with actual data being 

used only if the estimated risks are deemed unacceptable, some commenters feared that implementing 

agencies will feel compelled to insist on use of default assumptions and parameters, resulting in the 

overestimation of risks, and use of results from the worst case scenario, not the more realistic one. This 

may also lead to incorrect information landing in the public domain, which can unduly raise third-party 

concerns and cloud already highly technical issues. 

It was stated that furthermore, some EPA Regions and state agencies are increasingly requiring facilities 

to include worst-case parameter values for emissions and risk assessments. The implementation has 

evolved to the point that a facility's limits are being set based on performance of the trial burn, plugging 

the results into the risk assessment, and accepting limits more stringent than BIF based on hypothetical 

worst case assumptions. The commenters believed that some of the impetus for these overly conservative 

requirements by regulators is misleading broad statements in EPA guidance documents suggesting 

concern of risks and making far-reaching recommendations. For example, the discussion in the draft 

HHRAP Section 2.2.1, (p. 2-4, last paragraph) cautions about the "highly toxic" nature of hazardous 
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waste combustion emissions. Also, paragraph 2 on page 2-7 suggests several additional criteria that can 

be required along with risk assessment. 

EPA Response: 

The HHRAP sets out our recommendations (including the reasoning behind those recommendations) for 

conducting and reporting a transparent and scientifically defensible risk assessment. Whenever guidance 

is issued (as opposed to regulation requiring strict adherence), there is always a variation in the 

implementation across regional and state permitting authorities. To help bring consistency to 

implementation efforts, we’ve offered training to regional and state personnel. Also note that neither the 

guidance nor the regulations require industry to conduct a risk assessment. Lastly, limits required under 

the BIF rules were developed considering only the inhalation exposure pathway. Since then, scientific 

literature, as described in the HHRAP, does not support that evaluation of the inhalation exposure 

pathway alone would lead to permit conditions protective of human health and the environment.

2.1.3 Policy, Objectives, and Purpose - Model and Chemical Data Validation

General issues that were raised with regard to validation of HHRAP risk assessment models and chemical 

data include: 

• Application to Real Sources; 

• Setting Permit Limits; 

• Peer Review; 

• Model Validation; and 

• Chemical Parameters. 

Issue: Model and Chemical Data Validation - Application to Real Sources 

It was recommended that the HHRAP guidance be applied to real sources and its reasonableness 

evaluated prior to issuing the final guidance document. 

EPA Response: 

We agree. EPA and state permitting authorities have applied the guidance to numerous sources, and 

many of the revisions to the final guidance are the result of actual application. See response to technical 

issues for details on specific revisions. We agree that additional validation of the model would be good, 

but this is beyond the scope of the HHRAP. 

Issue: Model and Chemical Data Validation - Setting Permit Limits 

It was believed that the risk assessment process should not be used to set practical limits on operating 

parameters because the process has not been validated to the point where it can be used to set these limits. 

EPA Response: 

We are unaware of a process other than risk assessment to evaluate the protectiveness of permit limits to 

human health and the environment. 
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Issue: Model and Chemical Data Validation - Peer Review 

It was recommended that the HHRAP be quality assured before it is released or allowed to be used for 

regulatory purposes. EPA should conduct an independent peer review of the algorithms and numerical 

parameters used in the HHRAP. Chemical-specific parameter values and recommendations should be 

critically reviewed. 

EPA Response: 

The draft HHRAP was put forth as the best available guidance that could be offered at the time of its 

release. The HHRAP went through an extensive internal EPA and state review prior to release as a draft 

document. Public comments were received and an external peer panel reviewed the draft HHRAP. The 

public and peer panel comments have been summarized in this response to comments document, and the 

HHRAP has been revised accordingly. Appendix A parameters have been reviewed in their entirety and 

have been corrected or updated where appropriate. The HHRAP embodies our considered 

recommendations based on the best available science at the time the document was written. If better 

science can be supported, modifications to recommendations in the HHRAP are encouraged. 

Issue: Model and Chemical Data Validation - Model Validation 

It was believed that while parts of the model (the air dispersion portion) have been in use for a number of 

years and continually get refined, EPA has yet to rigorously validate any of the models in this guidance. 

It was recommended that equilibrium-partitioning methods be thoroughly validated. 

EPA Response: 

We strongly disagree that the model has not been validated for air concentrations (See studies done by 

EPA to support the Cumulative Exposure Project (USEPA, 1999b), the National Air Toxics Assessment 

(USEPA, 2001a) and the Model Evaluation Results for AERMOD (Paine et al., 1998)). Validation of the 

deposition portion of the model is less documented, but has generally been found to provide results within 

one order of magnitude. As with many parameters and procedures provided in the HHRAP, the scientific 

basis and reference are provided. A facility is then able to evaluate whether to propose and justify to the 

permitting authority a site-specific option or other alternative to that recommended in the HHRAP. 

Issue: Model and Chemical Data Validation - Chemical Parameters 

EPA was urged to develop a single Agency-wide chemical database of physicochemical and toxicological 

parameters. Many of the toxicological and physicochemical parameters relied on in the draft HHRAP are 

inconsistent with other EPA values and need thorough review and revision. 

It was recommended that TEFs should be considered an interim method and complemented by long-range 

planning to develop better toxicity data. EPA's comments regarding the resources needed for toxicity 

evaluations (p. 2-45) are valid justification in the short run for using relative potency factors of PAHs, 

coplanar PCBs, and polychlorinated dioxins and furans, but this does not mean that EPA should refrain 

from long-range planning to conduct detailed toxicity and bioassay studies. 

EPA Response: 

We agree with this premise, however, it was not within the scope of development of the HHRAP. 

Appendix A parameters have been reviewed in their entirety and have been corrected or updated where 
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appropriate. The majority of parameters are taken directly from the Superfund Chemical Data Matrix 

(SCDM) and all parameters are referenced such that other values for each parameter may be evaluated 

and considered in a risk assessment. 

2.1.4 Policy, Objectives, and Purpose - Clarification / Editorial

Issue: Clarification – Oral Dose 

Clarification was requested about the meaning of “one oral dose” on draft HHRAP page 1.1. 

EPA Response: 

The definition for the term “dose” was adopted directly from the National Academy of Sciences, “Risk 

Assessment in the Federal Government” (1983). 

Issue: Clarification – Trial Burn Issues Placement 

It was recommended that the discussion on page 1-7, Section 1.2 Related Trial Burn Issues be better left 

for Chapter 2 Facility Characterization because it is a very specific issue that seems to be out of place in 

the introductory chapter. 

EPA Response: 

We’ve removed the section on “Related Trial Burn Issues” from Chapter One. 

2.2 IDENTIFYING EMISSION SOURCES AND COMPOUNDS OF POTENTIAL 

CONCERN [draft HHRAP Section 2] 

Burning hazardous waste typically emits combustion by-products from a stack. Additional types of 

emissions that may be of concern include (1) process upset emissions, (2) accidental releases, (3) general 

RCRA fugitive emissions, and if the facility is a cement kiln (4) cement kiln dust (CKD) fugitive 

emissions. Each of these emission source types can result in generating compounds of potential concern 

(COPCs), which need to be evaluated during risk assessment. The COPCs to be addressed can vary from 

one combustion unit to the next, depending on a number of factors. 

Comments regarding emission sources and compounds of potential concern (COPCs) are categorized into 

the following topical issues: 

• Estimating Stack Emissions; 

• Estimating RCRA / CKD Fugitive Emissions; 

• Identifying Compounds of Potential Concern (COPCs); 

• Chemical-Specific Issues; and 

• Clarification / Editorial. 
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Center for Combustion Science and Engineering 24 

2.2.1 Emission Sources and COPCs - Estimating Stack Emissions [draft HHRAP Section 2.2] 

Issues with regard to estimating hazardous waste combustion facility stack emissions include: 

• Data Collection; 

• Trial and Risk Burns; 

• Total Organic Emission (TOE) Rate; 

• Multiple Stacks; 

• Previously Operated Facilities; and 

• Process Upsets. 

Issue: Estimating Stack Emissions – Data Collection (See RC Sections 3.2.2, 3.2.4 and 3.2.5) 

Issues were raised with regard to what statistical measures, detection limits, and types of data are most 

appropriate for estimating emission values for COPCs. These issues included: 

• Clarifying the definition of the “95th percentile emission rate.” 

• Whether the method detection limit (MDL), reliable detection limit (RDL), or zero should be used 

for the concentration of non-detected compounds. 

• To require all current and historical stack sampling information for use in the risk assessment, as 

suggested in the Draft HHRAP, is inappropriate and an inefficient use of resources. 

• Several terms in Draft HHRAP Section 2.5 need to be better defined and clarified with regard to 

various types of sample blanks, such as method blank, reagent blank, trip blank, field blank, and 

instrument blank. 

• The Draft HHRAP should be modified to permit use of statistical comparisons of blank results and 

sampling data. 

• Agreement with the Draft HHRAP approach to exclusion of chemicals typically considered 

laboratory contaminants, especially for phthalates. It was suggested that chemicals should be 

excluded as COPCs if they are present in all samples at concentrations less than five times that 

found in the blanks. 

It was felt the HHRAP appears to have an undue emphasis on being able to say with legally defensible 

certainty that actual stack emissions from a given facility are less than those used in the facility's risk 

assessments. To meet this objective, the HHRAP requires that RDL values be used for non-detect COPC 

stack compounds even when an unbiased estimate of actual concentration (in the form of J-values) is 

available. It appears that in an effort to have bulletproof COPC stack concentration numbers, the best 

information available is ignored and replaced by frequently much higher RDL values. Why disregard this 

information, especially when it may jeopardize the usefulness of the overall process, i.e., facilities with 

few, if any, hits could fail the risk assessment? The best possible estimate of actual risk should be 

produced, and conservative assumptions should be used in the face of uncertainty. EPA should 

implement the risk assessment process in a manner that is and appears to be outcome-independent. 

EPA Response: 

The issue of average versus maximum emission rates was submitted to the external peer review panel, 

who recommended against using the 95% UCL due to lack of data to generate an appropriate statistic for 

this parameter. The issue of detection limits, their definitions, and use were also addressed during the 
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external peer review. Recommendations by the peer reviewers were used to modify the HHRAP 

accordingly. See section 3.2.2 of this Response to Comments document for further discussion of the peer 

reviewer response. 

 

We revised the HHRAP to recommend using all relevant data in the risk assessment. 

We have made editorial changes to the HHRAP, as appropriate, with regard to information about sample 

blanks. We do not think it is appropriate to make statistical comparisons between blank results and 

sampling data due to the limited blank sample size. 

The only outcome that the HHRAP encourages is the consistent production of transparent, scientifically 

defensible risk assessments. 

Issue: Estimating Stack Emissions – Trial and Risk Burns [draft HHRAP Section 2.2.1] 

Issues were raised regarding the use of “maximum” or “normal” operating conditions, presumably 

generated during a trial burn or risk burn, to perform a risk assessment. Issues included: 

• The questionable appropriateness of using trial burn data for estimating long-term risks because 

they are gathered under worst case operating conditions. Using average emissions and normal 

operating conditions may yield better long-term risk estimates. 

• The requirement for the use of the "worst case" waste for the risk burn contradicts other HHRAP 

statements that the risk assessment would use reasonable approaches. The text should be revised to 

include consideration of the volume of the waste stream and the frequency with which each waste 

stream is burned. 

• The use of hazardous waste fuels in the cement process should not be represented the same as 

facilities that incinerate ""highly toxic"" wastes. 

• The permitting authority may require that trial burn data be used over risk burn data, given the 

variability of waste streams or the presence of highly toxic constituents. The decision to use trial 

burn /risk burn data should be made in the pre-trial/pre-risk burn phase to avoid unnecessary waste 

of resources. 

• Limiting use of risk burn to facilities with entirely homogenous waste feed streams is overly 

restrictive. 

• Using a combination of trial burn data, non-detects, a TOE adjustment, and RDL’s may result in 

overestimating emission rates and associated risks. 

• Limiting use of “data-in-lieu” for only on-site units is overly restrictive. 

• The HHRAP should clearly note that the BIF adjusted Tier 1 approach is an allowable option. 

• Organic emission samples collected during periods of POHC spiking are not representative of 

emissions without 'spiking'. Comparing emission estimates from risk and trial burns is not 

reasonable and will obscure the true risk. 

• The HHRAP should reflect that a burn at “normal” conditions should result in a minimum of 

additional permit conditions. Such conditions should be long-term and minimize the reporting 

burden. 
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It was noted that on Page 1-3 the HHRAP states: "Any decision to add permit conditions based on a sitespecific risk assessment under this authority must be justified . . . and the implementing Agency should 

explain the basis for this condition." However, on Page 2-5, the HHRAP states: "If a facility desires to 

receive a permit with no limits other than those traditionally based . . . on a trial burn (TB), then risk 

testing should be conducted at trial burn or worst case conditions." Elsewhere on the page and elsewhere 

in the HHRAP it is stated that TB risk data are expected to have higher emissions than would occur at 

normal conditions. Furthermore, no consideration is given in the statement about added permit conditions 

as to the level of risk shown in that facility's risk assessment. For example, if a facility conducted a risk 

burn (not a trial burn) showing a risk of 1.0 x 10-7, why would permit conditions be needed since a risk 

<10-5 would seem to be protective without additional limits? 

EPA Response: 

As stated in Section 2.2.1.2 of the draft HHRAP, the risk burn does not replace the trial burn for 

conducting the destruction and removal efficiency (DRE) testing. We consider the risk burn as an 

additional operating condition of the trial burn. In some cases, due to highly variable waste, it is not 

practical to determine normal operating conditions and worst-case waste. Under these situations, the 

HHRAP recommends using trial burn data. Testing conditions of a risk or trial burn are left up to a 

facility and should take into account the flexibility desired in the operating permit. 

Worst-case wastes are used to evaluate potential risks, not necessarily to set permit limits. This allows a 

facility the flexibility in its operating parameters. The recommendation to use worst-case waste in the 

risk burn is employed to provide a facility the maximum flexibility in its operating permit and to ensure 

that all waste types are evaluated at a facility. Worst-case waste under normal operating conditions 

allows for a reasonable approach that also ensures that one can evaluate protectiveness of operating 

conditions in reference to human health and the environment. 

The HHRAP is guidance and contains no requirements. As with many parameters and equations 

provided in the HHRAP, the scientific basis and reference are provided. A facility is then able to evaluate 

whether to propose and justify to the permitting authority an alternative they consider more appropriate 

than that recommended in the HHRAP. 

It was never our intention to suggest that only data from on-site units could be used to generate data-inlieu of information. We’ve further clarified this issue in the HHRAP. 

The comment regarding the BIF adjusted Tier 1 approach is more appropriate to the trial/risk burn 

guidance. Tier numbers are only used in the risk assessment with respect to starting points at which to 

develop risk-based limits. 

An example was provided in the draft HHRAP to evaluate whether or not worst-case waste has been 

selected for use in the risk burn. No mention of spiking is provided in the example. 

The permitting issue is not a risk assessment issue. EPA Region 6 developed a permitting manual, which 

may be helpful in addressing this issue. 
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We’ve revised the referenced text in Draft HHRAP Chapter 2 to clarify the relationship between trial 

burns and risk burns, and to remove recommendations regarding the facility’s strategy for conducting 

testing as related to permit conditions because this is really facility- and permitting authority-specific. 

Issue: Estimating Stack Emissions - Total Organic Emission (TOE) Rate 

[draft HHRAP Section 2.2.1] (See RC Section 3.2.3) 

The Draft HHRAP states that EPA developed the total organic emissions (TOE) test to account for 

unidentified organic compounds. The Commenters raised several potential problems with using TOE 

data to evaluate the potential risks from the unidentified fraction of organic compounds in the stack gas, 

including the assumption of similar toxicity and chemical properties between the known and unknown 

compounds. Recommendations included limiting evaluation of the unknown organics to the uncertainty 

section of the risk assessment and for EPA providing more discussion on: 

• The uncertainties or potential problems that may arise when using TOvoc and TOsvoc portions of the 

test; 

• The basis for using the reliable detection limit (RDL) and estimated detection limit (EDL) or if 

there is flexibility to apply more reasonable assumptions; and 

• Whether it is acceptable to use either the SW-846 EDL or the laboratory EDL. 

It was also recommended that EPA not require determination of the TCO and GRAV fractions of its TOE 

method because of major deficiencies in the GRAV determination and redundancy of TCO when Method 

8270 TICs are determined. 

EPA Response: 

Use of the TOE method is a policy decision made to encourage facilities to conduct analyses that identify 

as much of the mass being emitted from units as practically possible. The issue of TOE was presented to 

the external peer review panel. Recommendations of the panel were used to revise the HHRAP 

accordingly. See section 3.2.1 of this Response to Comments document for more discussion of the peer 

reviewer response.

Issue: Estimating Stack Emissions - Multiple Stacks [draft HHRAP Section 2.2.2]

Commenters point out that risks from facilities that have multiple stacks are not necessarily cumulative. 

It depends on the placement of the receptor for each stack. Because the potential receptor location will 

vary due to the height of the stack, operating parameters, etc., it is unlikely that the same potential 

receptor will be selected for all stacks. 

The HHRAP does not clearly indicate when emissions from other RCRA treatment facilities should be 

included in the risk evaluation, and it appears at odds with the guidance document addendum. It was also 

suggested that inclusion of other emissions should be made on case-specific basis. 

EPA Response: 

When choosing exposure scenario locations, the HHRAP recommends identifying the receptor locations 

with the highest air concentrations & deposition rates for each source (e.g., stack) separately AND for all 

sources combined. 
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We agree that the receptor location of maximum exposure for one stack is not necessarily the receptor 

location of maximum exposure for another stack, even if both stacks are at the same facility. That is one 

reason it is valuable to assess each source separately. As discussed further in the HHRAP, this may lead 

to assessing risk at multiple locations. 

However, any one location is impacted, to a greater or lesser extent, by ALL sources. It is therefore 

appropriate to also assess locations by summing the air concentration and deposition rate contributions 

of all sources to each location. Because concentration and deposition are defined with multiple 

parameters (e.g., wet vapor deposition, dry vapor deposition), each with its own point of highest impact, 

this may lead to assessing multiple receptor locations. Please see the HHRAP for further discussion. 

The addendum described in the comment is not part of the HHRAP, but is EPA Region 6 guidance. The 

unit to be evaluated in the risk assessment is a site-specific decision and should be agreed upon between 

the facility and regulatory authority. 

Issue: Estimating Stack Emissions - Previously Operated Facilities [draft HHRAP Section 2.2.4] 

Clarification was requested regarding estimating stack emissions from previously operated facilities. For 

example, is EPA interested in recovering information about units in the past that were operating but now 

closed along with evaluation of new and/or existing units? For an existing unit that has been operating 

for a number of years, should all the prior years be evaluated? How would all the necessary information 

be obtained for the risk assessment? Clarification was also requested about whether sampling data can be 

used in lieu of or in addition to modeling and fate and transport data to determine risk from previous 

operations. 

EPA Response: 

This is a site-specific decision to be made with the permitting authority. Guidance is provided in Draft 

HHRAP Section 2.2.4 for how this information might be used. Additional information is beyond the scope 

of the HHRAP. Facilities should provide historic operating information as requested on a site-specific 

basis to be negotiated with the permitting authority. Further information on data-in-lieu is available in 

Draft HHRAP Section 2.2.1.1.

Issue: Estimating Stack Emissions - Process Upsets 

 [draft HHRAP Section 2.2.5] (See RC Section 3.2.1) 

The HHRAP suggests using process upset default values of 2.8 for organics and 1.45 for metals if no sitespecific data are available, based on a California Air Resources Board (1990) document, to adjust 

emission rates in the risk assessment. These factors were calculated based on the assumption that 

emission rates are 10 times higher during upset conditions than during normal operations, and the facility 

operates in the upset mode for 5% to 20% of the time. Commenters questioned the validity of these 

assumptions and values of the derived process upset factors, as well as some of the operating conditions 

included in the term upset. It was also suggested that any use of upset conditions should be confined to 

the uncertainty section of the risk analysis until data show otherwise and that the use of upset factors 

should be dropped until data can be produced to prove that such emission scale-up factors are appropriate. 
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EPA Response: 

The use of process upset factors was addressed by the external peer review panel. For further discussion, 

see section 3.2.4 of this Response to Comments document. Upset factors are generally changed based on 

historical information provided by a facility, as noted in Section 2.2.5 of the Draft HHRAP. The HHRAP 

recommends generating site-specific upset factors whenever possible. The HHRAP is guidance and will 

continue to contain no requirements. As with many parameters and equations provided in the HHRAP, 

the scientific basis and reference are provided. A facility is then able to evaluate whether to propose and 

justify to the permitting authority an alternative they consider more appropriate than that recommended 

in the HHRAP. 

2.2.2 Emission Sources and COPCs - Estimating RCRA / CKD Fugitive Emissions 

[draft HHRAP Sections 2.2.6, 2.2.7 and 2.2.8] (See RC Section 3.3.2) 

Commenters differed in their opinions about whether RCRA fugitive emissions should be included in the 

risk assessment. Some recommended that cement kiln dust (CKD) fugitive emissions should be 

quantitatively evaluated because of the metals loading of CKD which may present a risk through both 

direct and indirect exposure pathways. Others questioned whether fugitive emissions estimates would be 

meaningful to making risk assessment decisions, especially when balanced against the time and cost of 

obtaining the estimate. It was also suggested that: 

Fugitive emission estimates could lead to misleading interpretations of risk if conservative default 

approaches are used to estimate the fugitive emissions; 

• Fugitive emissions from sources already controlled under 40 CFR 264 Subparts AA, BB, and CC or 

the Clean Air Act (CAA) should not be incorporated into the risk assessment; 

• The risk from a RCRA facility fugitive emissions are already addressed in the development of the 

leak detection and repair (LDAR) regulations; 

• EPA has not demonstrated a need to consider fugitive emissions impacts based on any real impact 

or equivalent risk; 

• Inclusion of CKD fugitive emissions in the risk assessment is not warranted; and 

• EPA should acknowledge the distinct differences in cement kilns with regard to fugitive emissions, 

so that cement kiln facilities are not required to address issues that have previously been resolved 

and do not apply to cement kilns. 

EPA mentions 4 techniques for fugitive emissions from process equipment in RCRA service: Average 

Emission Factor Approach (AEFA), Screening Ranges Approach (SRA), US EPA Correlation Approach 

(EPACA) and Unit Specific Correlation Approach (USCA). Three of these techniques, the SRA, 

EPACA, and USCA were questioned for several reasons. One commenter offered examples of how it 

estimates RCRA fugitive emissions at its facilities. 

More information was requested to address data requirements and calculation techniques needed to 

perform an analysis for cases in which process equipment in close proximity are grouped into a combined 

source and fugitive emissions are estimated when components within the single group handle wastes of 

different composition. 
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EPA Response: 

We’ve edited the HHRAP to recommend considering the CKD samples collected during maximum waste 

metal feed rate conditions from the trial burn, risk burn and/or certification of compliance tests when 

evaluating CKD fugitive emissions. 

We’ve evaluated fugitive emissions from tanks and other RCRA equipment and have not always found the 

risk to be low. Neither the CAA nor current RCRA programs evaluate these emission sources on a sitespecific basis. Therefore, we’ll continue to recommend considering units regulated under 40 CFR 264 

Subparts AA, BB, and CC in the risk assessment. The state and federal LDR programs are not set up 

based on site-specific risk. Leak detection programs do serve as a means to adjust emission factors, thus 

allowing credit in the risk assessment. Including fugitive emissions in a risk assessment is not an 

extensive exercise and is routinely performed for large facilities in a day. Most if not all facilities should 

already be collecting information to model fugitives under the TRI, NEI, or other state program. The 

HHRAP allows for an up-front qualitative assessment for CKD via comparison of site-specific data to 

those used in the national risk assessment. If site-specific information is similar to those used in the 

national assessment, no further risk analysis is recommended. The HHRAP has been designed such that 

permits can be written that are protective of human health and the environment. If data exist under other 

regulations that assure the protectiveness of the permit, they should be considered. 

As with many parameters and equations provided in the HHRAP, the scientific basis and reference are 

provided. A facility is then able to evaluate whether to propose and justify to the permitting authority an 

alternative they consider more appropriate than that recommended in the HHRAP. 

Sources may be grouped to allow for a reduction in air modeling runs. When grouping sources, a single 

emission rate is calculated by summing total fugitive emissions from individual unit. Therefore, 

individual waste streams are accounted for in the grouping methodology. The grouped sources are then 

modeled as an area source as noted in Chapter 3. 

2.2.3 Emission Sources and COPCs - Identifying Compounds of Potential Concern (COPCs) 

[draft HHRAP Sections 2.3 and 2.4]

The guidance outlines a series of steps to be followed in developing a list of COPCs to be evaluated in the 

risk assessment. Commenters raised issues, including: 

• How to handle non-detected compounds that are contained in the waste feed; 

• What factors should indicate COPC elimination early in the risk assessment process; and 

• How to determine surrogate toxicity values for some tentatively identified compounds (TICs). 

Issue: Identifying COPCs - Non-Detects (See RC Section 3.2.5) 

Commenters recommended that EPA should provide a discussion on how to handle COPCs that are 

initially present in the hazardous waste feed stream but cannot be sampled and/or analyzed in stack 

emissions by known methods. With regard to handling COPCs present in the waste feed but not detected 

in the stack emissions, commenters offered the following: 
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• A discussion should be included in the HHRAP on whether it is appropriate to calculate wastebased emission rates and if so, how waste-based emission rates should be calculated. 

• A reality check should be used to compare a calculated value of a potentially emitted chemical, 

based on the quantity in the waste feed and engineering operations, against the detection limit used 

resulting in non-detects. If the detection limit is appropriately low enough, then non-detected 

chemicals should be eliminated from the list of COPCs. 

• Constituents not detected under trial burn conditions should not be carried any further in the 

quantitative risk estimates beyond hazard identification simply because of the type of waste that is 

incinerated. 

• Setting an arbitrary number (e.g., 30) of the largest tentatively identified compounds (TICs) may 

force identification of peaks that are so small they can not be easily separated from the background. 

Furthermore this arbitrary number fails to consider potency. The largest peak may not be the 

chemical of greatest concern while the smallest peak could be the chemical of greatest concern. It 

was suggested that peaks less than 10% of the nearest internal standard should be ignored. 

• It was suggested that chemical class evaluations be used as part of the determination of potential 

TIC emissions. 

• Non-detects from emissions data sets should only be discussed in the uncertainty section of the risk 

assessment. The contribution of non-detects to the estimated risk should be featured in the 

conclusion of the risk assessment report to help decision makers decipher the breakdown of the risk 

estimate. 

• COPCs that are never detected in any stack sampling run should be distinguished from those 

detected in at least one run. 

• EPA should determine the minimum detection limits for compounds, such as dioxins, furans, and 

PAHs using standard sampling and analysis protocols. 

• It was noted that detection limits can and do drive risks, even at the extremely low detection levels 

currently achieved in laboratories. Furthermore, the science and technology of risk assessments has 

evolved to the point that constituent levels substantially below detection levels are driving the risk 

assessment. 

EPA Response: 

The comment regarding how waste-based emission rates should be calculated is beyond the scope of the 

HHRAP. We’ve forwarded this comment to EPA’s Office of Research and Development. 

The “reality check” and other suggested approaches may be acceptable on a site-specific basis. 

However, we don’t recommend such approaches in the HHRAP because we believe it’s preferable to 

carry a compound through the risk assessment and allow factors, such as quantity and fate and transport 

characteristics, to show if the risk falls below levels of concern. 

The distinction between COPCs never detected and those detected in at least one stack emissions 

sampling run is provided in the HHRAP following Step one of the COPC selection process. The external 

peer review panel addressed the issues of detection limits, their definitions, and use. Recommendations 

of the peer reviewers were used to revise the HHRAP. See section 3.2.3 of this Response to Comments 

document. 
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Detection limits become a concern for two PAHs, one phthalate, and mercury. The uncertainty in risk 

estimates for the two PAHs and one phthalate are high due to: 1) the lack of data supporting biotransfer 

up the food chain for compounds with high Kow values, and 2) the lack of information concerning 

metabolism of these compounds. Biotransfer equations have been updated for the beef and milk exposure 

pathways and other biotransfer equations have been capped at the high and low end of the Kow range 

used to generated those equations. As with any risk assessment method, uncertainty exists and will need 

to be dealt with through advances in the state of the science or risk management decisions. We’ve 

provided additional information in the HHRAP for all of these compounds to help understand the 

potential risk associated with these cases. We’ve modified mercury fate and transport equations based on 

external peer review comments, alleviating detection issues for this chemical. See section 3.5.1 of this 

Response to Comments document for more discussion about mercury. 

Issue: Identifying COPCs - Elimination Factors 

It was suggested that COPCs recommended for evaluation via the indirect food-chain pathway be limited 

to those expected to persist or bio-accumulate. Environmental degradation of pollutants should be 

considered and used as a tool to reduce the number of COPCs. Commenters contend that addressing a 

long list of COPCs is time-consuming and burdensome, and is not an effective use of resources, 

especially when one understands that the risk contribution of almost all of the constituents is minimal. 

EPA Response: 

The HHRAP is guidance and contains no requirements. Given that the fate, transport, and toxicity 

information is provided in the HHRAP, we believe it to be much more efficient to run the selected 

COPC’s through the risk assessment process, rather than go through the arduous process of justifying 

their elimination from the assessment. 

Issue: Identifying COPCs - Surrogate Toxicity Values 

[draft HHRAP Section 2.4.] (See RC Section 3.4.6) 

Although the use of toxicity equivalency factors (TEFs) is supported by some commenters, some 

suggested that TEFs should be considered only as an interim measure. EPA was urged to conduct longrange planning to conduct detailed toxicity and bioassay studies. 

Given EPA’s experience, the Agency was asked if there are typical compounds or categories of 

compounds likely to be found as tentatively identified compounds (TICs), and what advice can EPA offer 

on the choice of surrogate compounds for chemicals and classes of chemicals that frequently are found in 

TIC studies. Clarification was requested regarding how EPA determines a compound is “similar” with 

regard to obtaining surrogate toxicity values. Additionally, commenters questioned how surrogate 

toxicity values should be used qualitatively and how this should be used in risk management decision 

making with regard to permit applications. Finally, commenters requested EPA to clearly indicate how to 

qualitatively evaluate COPCs with no toxicological data in the uncertainty section of the risk assessment 

report. 

A definition was requested to better define what is meant by “compounds that have no toxicological 

data.” It was also suggested that additional guidance would be helpful regarding the use of preliminary 

toxicity data (data that has not undergone full peer review and obtained consensus approval, and is 
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therefore not incorporated into EPA’s Integrated Risk Information System (IRIS) or the Health Effects 

Assessment Summary Tables (HEAST) in the uncertainty portion of the risk assessment. For example, 

would coordination with EPA’s Environmental Criteria and Assessment Office be suggested? 

EPA Response: 

We note the comment regarding long-range planning for future studies. 

The HHRAP presents the state-of-the-science. Currently TIC analysis in risk assessment is highly 

uncertain. Additional guidance on selecting surrogate toxicity information and risk management 

implementation is beyond the scope of the HHRAP. 

A hierarchy of recommended sources of toxicity information is presented in HHRAP Appendix A. 

2.2.4 Emission Sources and COPCs - Chemical-Specific Issues [draft HHRAP Section 2.3]

This section includes chemical-specific public comments that are not generally summarized under other 

issues in this Response to Comments document. The chemicals discussed in this section include: 

• Dioxins and Furans (PCDD/PCDF); 

• Polynuclear Aromatic Hydrocarbons (PAHs); 

• Polychlorinated Biphenyls (PCBs); 

• Hexachlorobenzene and Pentachlorophenol; 

• Volatile Organic Compounds (VOCs); 

• Metals; 

• Hydrogen Chloride and Chlorine Gas (HCl/Cl); 

• Phthalates; 

• Criteria Pollutants; and 

• Radionuclides 

 

Issue: Chemical Specific - Dioxins and Furans [draft HHRAP Section 2.3.1] 

Issues related to quantifying dioxin and dioxin-related exposures include: 

• Fat and dioxin content in human mother’s milk; 

• Benchmark values for dioxins; 

• Contact fraction for exposure assessment; 

• Dioxin/Furan Formation; and 

• Fluorine, Bromine, and Sulfur PCDD/PCDF Analogs 

Human Milk (See RC Section 3.4.7) 

• The HHRAP overestimates the amount of fat in mother's milk by 25% to 30%. It was suggested 

that the milk fat intake should be reduced from 32 grams per day to 18.2 grams per day. 

• The HHRAP overestimates the average one-year concentration of dioxin TEQs in mother’s milk by 

35%. A correction for the natural degradation in the concentration of dioxin in mother's milk 
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during nursing would reduce the concentration of dioxin in mother's milk by an average of 35% 

over the first year of nursing. 

• EPA should consider reducing the 2,3,7,8-TCDD TEQ concentrations using more recent data. 

• EPA should provide guidance on how results from the margin of exposure analysis should be 

interpreted. 

• EPA should provide a strategy for evaluating background dioxin content in human milk and a 

recommended approach for nursing infants. 

• EPA was urged to make it explicitly clear in the HHRAP that breast feeding is safe and should be 

encouraged, and if a mother's average daily dose of PCDD/PCDF is not associated with an 

unacceptable cancer risk, and therefore safe, the nursing infant's dose is also safe. As currently 

drafted, the HHRAP could give readers the mistaken impression that breastfeeding is not safe. 

EPA Response: Human Milk 

In response to comments regarding the fat and dioxin content in milk, as with many parameters and 

equations provided in the HHRAP, the scientific basis and reference are provided. A facility is then able 

to evaluate whether to propose and justify to the permitting authority an alternative they consider more 

appropriate than that recommended in the HHRAP. 

As for the updating of the toxicity equivalency factors (TEFs), we have adopted the World Health 

Organization TEFs (World Health Organization, 1997) in the HHRAP. 

Evaluation of how breast milk margin of exposure data should be interpreted is considered risk 

management and beyond the scope of the HHRAP. 

For the evaluation of non-cancer effects in nursing infants, we continue to recommend that exposure be 

compared with the national average background level of 60 pg/Kg/day for nursing infants. 

The purpose of the HHRAP is to provide methodologies for evaluating whether breast-feeding is safe in 

site-specific situations. 

Benchmark Values (See RC Section 3.4.5) 

For the non-carcinogenic risk assessment of dioxin congeners, the HHRAP requires that the annual daily 

dose for all receptors be compared to a value of 1 pg TEQ/kg/day for adults, which is stated to be the 

national background exposure level. While such a benchmark may be appropriate for non-carcinogenic 

risk assessment of average residents, it was believed that it is not appropriate for non-carcinogenic risk 

assessment of the following receptors: adult subsistence farmers, child subsistence farmers, and infants 

consuming mother's milk. It was recommended that the non-carcinogenic benchmarks for dioxin 

congeners be modified from 1 pg/kg/day, which may be appropriate for an adult resident, to 2 pg/kg/day 

for an adult subsistence farmer and to 3 pg/kg/day for a child subsistence farmer. 

It was suggested that EPA should defer to the final Dioxin Reassessment for national average background 

exposure levels and for review of dioxin/furan sources and trends in the environment. Earlier references 

should be deleted from Draft HHRAP Section 2.3.1. Additionally, EPA was urged to exclude from the 
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HHRAP any non-cancer evaluation of dioxin because the draft Dioxin Reassessment has not been 

finalized and EPA has not yet responded to the 1995 Science Advisory Board’s extensive critique of the 

reassessment, particularly with regard to non-cancer effects. 

A change was suggested in the wording in Draft HHRAP Section 2.3.1.2, first paragraph to “If exposure 

due to the facility’s emissions during the exposure duration of concern are low compared to background 

exposures, then the emissions are not expected to cause an increase in non-cancer effects.” This would be 

a change to the current language in the Draft HHRAP, which says that the emissions will not cause health 

effects. 

EPA Response: Benchmark Values 

This issue was addressed during the external peer review. The peer panel agreed that it is reasonable to 

use 1 pg/kg/day. See Section 3.4.4 of this Response to Comments document for further details. As with 

many parameters and equations provided in the HHRAP, the scientific basis and reference are provided. 

A facility is then able to evaluate whether to propose and justify to the permitting authority an alternative 

they consider more appropriate than that recommended in the HHRAP. 

Authors of the Dioxin Reassessment (EPA/600/P-00/001B) have reviewed the Draft HHRAP. The only 

non-cancer assessment recommended in the Draft HHRAP is a margin of exposure comparison of dioxin 

in breast milk and background levels noted in the U.S. 

We agree with the suggested language change and have revised the HHRAP to reflect that emissions 

causing low exposure are not expected to increase non-cancer effects. 

Contact Fraction 

It was suggested that the contact fraction for subsistence farmers should be changed from 100% to 40% 

for milk and 44% for beef, as determined by U.S. Department of Agriculture (USDA) and as cited in the 

draft Dioxin Reassessment. Issues of conservancy are already covered by the choice of the subsistence 

farmer and other factors. 

 

EPA Response: Contact Fraction

As with many parameters and equations provided in the HHRAP, the scientific basis and reference are 

provided. A facility is then able to evaluate whether to propose and justify to the permitting authority an 

alternative they consider more appropriate than that recommended in the HHRAP. 

Dioxin/Furan Formation

It was noted that facilities have seen an increase in requests for testing for dioxins/furans during trial 

burns under many different conditions because the formation inhibition mechanisms for dioxins/furans 

are not completely understood. The result is that trial burns are being used like a research project to test 

various hypotheses, and this is an inappropriate use of resources. To include the many theories in the 

uncertainty section of the risk assessment report is also beyond what was believed to be reasonable. 

Additionally, more discussion was recommended about the relationship between chlorine feeds to 

combustors and dioxin emissions. 
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EPA Response: Dioxin/Furan Formation

We agree, and we have changed the HHRAP to recommend a site-specific decision made in consultation 

with the permitting authority. More information about the relationship between chlorine feeds and dioxin 

emissions can be found in the references cited in the HHRAP. Expanding the existing discussion in the 

Draft HHRAP is beyond the scope of the HHRAP. 

PCDD/PCDF Analogs

The inclusion of bromine, fluorine, and sulfur PCDD/PCDF analogs in the risk assessment was 

questioned. EPA has no approved method for sampling or analysis of these constituents. Without an 

approved, verified analytical method, no evidence of formation in combustion emissions, and no 

information regarding potential toxicity, commenters seriously question the intent, value, or usefulness of 

attempts to collect the proposed analytical data. They contend that this effort would be an ineffective use 

of resources. 

EPA Response: PCDD/PCDF Analogs

There is nothing in the HHRAP that suggests an analytical method will need to be developed by 

individual applicants. However, we‘ve revised the HHRAP to reflect that this is a site-specific decision to 

be made in consultation with the permitting authority, that includes careful consideration of the state-ofthe-science at the time.

Background Level Impacts

It was noted that with regard to section 2.3.1.2 PCDD/PCDF Non-cancer Hazards, the inherent 

assumption is that background exposure levels do not cause significant non-cancer effects in humans. 

Although, there are suggestive data to indicate that this may not be correct, there is certainly no 

conclusive evidence to indicate that in fact background exposure levels have any non-cancer effects in 

humans. Nevertheless, terminology that indicates this one way or the other should be avoided. This is a 

significant issue that should be clarified especially since (1) an assumption, that is not conservative, is 

made that background levels have no toxicity, and (2) the “background” levels in the area of concern are 

not likely to be known and there is no specific directive to determine these. It should also be clearly 

indicated whether using 1 pg/kg/day as a benchmark inherently assumes that this levels of exposure 

assumes no level of risk or an acceptable level of risk. In either case, the basis of this assumption needs 

to be clearly stated. 

EPA Response: Background Level Impacts 

The Agency does not have a reference dose with which to estimate the likelihood of non-cancer health 

effects from exposure to dioxin congeners. The language in the referenced section has been updated to 

lessen the implication that background levels of dioxin are safe. However, we do believe that comparison 

of incremental exposure to a national background helps put potential risk from facilities emissions into 

context and may be useful in a risk management decision. 
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Issue: Chemical Specific - Polynuclear Aromatic Hydrocarbons (PAHs) 

[draft HHRAP Section 2.3.2] (See RC Section 3.2.2) 

Several issues specific to PAHs were raised. These included: 

• To avoid confusion, chemical-specific inputs for benzo(a)pyrene (BaP) only should be provided in 

Appendix A (i.e., data for the other six carcinogenic PAHs should be removed). The chemicalspecific inputs in Appendix A, Table A-3-20 were questioned.

• It was recommended that emission rates be calculated for each of the individual carcinogenic PAHs 

and that their specific fate and transport factors used to model movement through the various 

media. This is critical because each PAH will be distributed differently based on their different 

chemical characteristics. These individual PAH values would then be summed to obtain an 

equivalent total concentration of BaP, used to evaluate risk.

• It was noted that one source of modeling uncertainty lies in the selection of chemical-specific 

model parameters. Specifically, although PAHs are known to be metabolized by animals and fish, 

EPA estimates biotransfer factors using Kow-based correlations that assume no such metabolism 

which, in turn, overestimates the concentrations of PAHs in animal tissues and food products. 

Some of these types of errors can be identified from the literature using empirical knowledge of 

exposure potential for various chemicals. EPA has used this technique with some success in the 

dioxin reassessment, and it was recommended that similar empirical checking of model estimates 

be applied to as many COPCs as possible, especially those frequently predicted to be risk drivers.

• It was noted that there are 7 PAHs included in the EPA TEF methodology, and the Draft HHRAP 

requires inclusion of PAHs in quantitative risk estimates even if these compounds are not detected 

in trial burn emissions. Commenters believed that in cases when all 7 PAHs and most other 

aromatic hydrocarbons are not detected, inclusion of PAHs in quantitative risk estimates is 

scientifically indefensible because there is a low probability that all PAHs are present in the 

emission stream and all are below detection limits. It was recommended that in such cases, the 

potential PAH formation should be discussed in the uncertainty section of the risk assessment. 

• It was pointed out that organic emissions, such as PAHs, monitored from the main stack in cement 

kilns typically originate from organics in the raw materials (e.g., limestone, shale, and clay) used in 

the cement manufacturing process and not from fuels. Because there are statements in the HHRAP 

indicating that baseline operations do not emit organic constituents, it was recommended that EPA 

should describe the known phenomena of organic emissions derived from naturally occurring raw 

materials, differentiate the process emissions from those derived from hazardous waste combustion 

and hazardous waste constituents.

• It was recommended that the discussion in Section 2.3.2 on carcinogenic versus non-carcinogenic 

PAHs is confusing and needs clarification. Paragraph 1 states that all PAHs are considered 

carcinogenic but paragraph 7 refers to non-carcinogenic PAHs. One suggestion was to delete 

“...may be emitted” from paragraph 7, last sentence.

• It was noted that if the terminology in the HHRAP is vague, the chemicals will be largely ignored 

unless specific guidance is given. For instance, in Section 2.3.2 Polynuclear Aromatic Compounds, 

page 2-46, line 13, “...emissions data indicate that ...,” it has not been clarified what a “significant” 

amount of non-carcinogenic PAH is.
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EPA Response:

Chemical-specific parameters have been reviewed in their entirety and have been corrected or updated 

where appropriate. We disagree with the comment suggesting that fate and transport characteristics of 

only BaP should be made available. Fate and transport characteristics of different PAH congeners result 

in different distribution characteristics in the environment. It is important to model each congener 

separately before normalizing them to BaP equivalents. 

We agree with the comment regarding calculation and modeling of individual carcinogenic PAHs, and 

have made this change in the HHRAP. 

We’ve endeavored to do similar empirical checking of the model. Several examples include PAH 

metabolism, phthalate metabolism, dioxin biotransfer, etc. We’ve made some changes to the HHRAP in 

response to these efforts. The HHRAP, however, is only guidance and is meant to help provide a welldocumented, consistent approach to risk assessment. As with many parameters and equations provided 

in the HHRAP, the scientific basis and reference is provided. A facility is then able to evaluate whether 

to propose and justify to the permitting authority an alternative they consider more appropriate than that 

recommended in the HHRAP. 

The HHRAP is guidance and contains no requirements. Given that the fate, transport, and toxicity 

information is provided in the guidance, we believe it to be much more efficient to run the selected 

COPC’s through the risk assessment process, rather than go through the arduous process of justifying 

their elimination from the assessment.

The recommendation to describe organic emissions from naturally occurring materials is beyond the 

scope of the HHRAP. 

With regard to Draft HHRAP Section 2.3.2, paragraph one indicates the commonly detected PAH’s listed 

in that paragraph are considered to be carcinogenic. Paragraph 7 focuses on potential non- carcinogenic 

effects of the same PAHs, rather than suggesting the existence of separate PAHs. 

Issue: Chemical Specific - Polychlorinated Biphenyls (PCBs) 

[draft HHRAP Section 2.3.3] (See RC Section 3.2.2, 3.4.6) 

Issues raised about PCBs include: 

• It was suggested that EPA replace the existing language in the Draft HHRAP on Toxicity 

Equivalent Factors (TEFs) for the coplanar PCBs with the most recent World Health Organization 

TEFs that were made available at Dioxin '97, 17th International Symposium on Chlorinated 

Dioxins and Related Compounds, Indianapolis, Indiana, USA, August 25-29, 1997. 

• The values for chemical-specific inputs for Aroclor 1016 and 1254 provided in Appendix A, Table 

A-3-13) were questioned. 

• It was recommended that EPA provide an explanation about why it is not necessary to include 

PCBs in the risk assessment when hazardous waste with low chlorine content is burned. Concern 

was expressed about limiting automatic inclusion of PCBs as COPCs to only three situations: (1) 

combustion units that burn PCB-contaminated wastes or waste oils; (2) highly variable waste 
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streams, such as municipal and commercial wastes for which PCB contamination is reasonable; and 

(3) highly chlorinated waste streams. 

• The narrative and table on page 2-50 of the Draft HHRAP was judged to be inconsistent and 

confusing. It was recommended that the HHRAP clearly state that the SF = 0.4 be used for the 

water ingestion pathway given that only the lower chlorinated PCB congeners are soluble in water. 

The more highly chlorinated PCBs will adhere to soil and sediments and the SF = 2 should be 

applied to evaluate risks through the food chain, sediment, or soil exposures unless at least 99.5 

percent of the mass of the released PCB mixture has fewer than four chlorine atoms per molecule. 

• It was noted that neither toxicity nor environmental fate of PCBs can be accurately predicted on the 

basis of chlorine content alone. 

• Commenters supported limiting which sources should consider PCBs as a COPC, but they 

disagreed that PCBs are COPCs for sources burning highly chlorinated wastes. 

• Cement kiln industry commenters state they do not believe that the low levels of PCBs found in 

cement kiln waste feeds warrant inclusion in testing or the risk assessment. Furthermore, the 

combustion characteristics of cement kilns would readily destroy 99.999+% of any organic fuel 

component, as demonstrated repeatedly by DRE testing at cement kilns. 

• It was noted that the risk assessment approach requiring that coplanar PCB congeners be analyzed 

in combustor emissions and evaluated in the risk assessment with dioxin toxicity equivalent factors 

(TEFs) is highly controversial and not generally accepted by the toxicological community. 

• It was noted that it is inappropriate to evaluate potential non-carcinogenic effects of PCBs by using 

the oral reference dose (RfD) for Aroclor 1254 or Aroclor 1016. 

• It was suggested that EPA should assess the risk of coplanar PCB emissions as PCBs rather than as 

dioxins. 

EPA Response:

The issue of using TEFs for coplanar PCBs was addressed during the external peer review. Accordingly, 

we’ve revised the HHRAP according to the latest World Health Organization information. See Section 

3.4.5 for more discussion. Appendix A parameters have been reviewed in their entirety and have been 

corrected or updated where appropriate. 

With regard to hazardous waste feed with low chlorine content, this is a site-specific decision to be made 

by the permitting authority. We’ve made recommendations in the HHRAP where the science is clear and 

left the decision open to a site-specific determination in other cases. 

With regard to the remaining comments about PCBs, the HHRAP is guidance and does not require 

anything. As with many parameters and equations provided in the HHRAP, the scientific basis and 

reference is provided. A facility is then able to evaluate whether to propose and justify to the permitting 

authority an alternative they consider more appropriate than that recommended in the HHRAP. 

Issue: Chemical Specific - Phthalates [draft HHRAP Section 2.3.5] 

EPA was commended for its improved approach to the inclusion of phthalates but aspects of that 

approach were questioned. Specifically: 
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• Does the metabolism factor (MF) of 0.01 recommended for bis (2-ethylhexyl) phthalate (BEHP) in 

the beef and milk pathway apply to any other phthalate identified as a COPC, such as di-(n)-

octylphthalate? Previously, none of the phthalates, if listed as COPCs, were included in the beef 

and milk pathway because of their rapid metabolisms and low bioaccumulation potential. 

• The Kow value for di-n-octyl phthalate (DNOP) in Table A-3-64 is theoretically calculated and is 

not substantiated as a measured value. The use of the theoretical Kow for DNOP in the calculation 

of the air-to-plant transfer constant (Bvag) results in an unrealistically high value for Bvag. It is more 

appropriate to set the Kow for DNOP equal to that of bis (2-ethylhexyl) phthalate, which is 

chemically similar and has a measured Kow value. 

EPA Response: 

No, the MF for BEHP is not also recommended for DNOP. The HHRAP recommends one MF for BEHP 

and another for “all other COPCs.” DNOP would fall within the latter category. We’ve added 

additional discussion regarding metabolism of DNOP. As with many parameters and equations provided 

in the HHRAP, the scientific basis and reference is provided. A facility is then able to evaluate whether 

to propose and justify to the permitting authority an alternative they consider more appropriate than that 

recommended in the HHRAP. 

 

Draft HHRAP chemical-specific parameters have been reviewed in their entirety and have been corrected 

or updated where appropriate. 

Issue: Chemical Specific - Hexachlorobenzene and Pentachlorophenol [draft HHRAP Section 2.3.6] 

Commenter issues regarding hexachlorobenzene and pentachlorophenol include: 

• Identification in the Draft HHRAP of hexachlorobenzene as a risk driver may be the result of faulty 

fate and transport modeling. The assumptions of previous risk assessments of hexachlorobenzene 

based on EPA guidance were questioned and it was suggested that those risk assessments most 

likely overestimate risks at receptor locations near facilities, perhaps by several orders of 

magnitude. 

• Clarification is needed in Draft HHRAP Section 2.3.6, which provides recommendations on when 

to include or exclude hexachlorobenzene and pentachlorophenol as COPCs in the risk assessment. 

EPA Response:

As with many parameters and equations provided in the HHRAP, we’ve included the scientific basis and 

references underlying our discussion and recommendations regarding hexachlorobenzene and 

pentachlorophenol. A facility is then able to evaluate whether to propose and justify to the permitting 

authority an alternative (e.g. a different fate & transport model) they consider more appropriate than that 

recommended in the HHRAP. 

The guidance recommends measuring the stack gas concentrations of hexachlorobenzene and 

pentachlorophenol. Including these compounds in the risk evaluation is left up to the permitting 

authority. 
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Issue: Chemical Specific - Volatile Organic Compounds (VOCs) [draft HHRAP Section 2.3.7] 

Commenters contend that evaluation of VOCs through the indirect food chain pathways is not technically 

defensible. VOCs are not hazards via indirect exposure pathways, and should be included as COPCs for 

only the direct inhalation pathway. According to EPA Region 6, "One argument for excluding these 

COPCs (VOCs) from evaluation is that there is no empirical evidence that VOC emissions pose a hazard 

via indirect pathways. U.S. EPA OSW agrees that it is not aware of such evidence; however, OSW is 

similarly unaware of a lack of evidence to the contrary.” 

For chemicals that do not appreciably deposit from the air, or are chemically degraded when they do 

deposit, the use of EPA’s fate and transport models is more likely to produce misleading results than to 

identify potentially important exposure pathways. For example, conservative default values for VOCs are 

given in Draft HHRAP Appendix B without appropriate justification. Contrary to EPA’s opinion, 

inclusion of VOCs does represent a significant amount of work for little return. 

It was noted that based on the discussion of VOCs in Section 2.3.7 Volatile Organic Compounds, it is not 

exactly clear what information, if any, is necessary for the risk assessment report. A clear statement 

should be made. 

EPA Response:

Given that the fate, transport, and toxicity information is provided in the guidance, we believe it is more 

efficient to run the subject COPCs through the risk assessment process, rather than go through the 

arduous process of justifying their elimination from portions of the assessment. In addition, if chemicals 

do not transport up various exposure routes, the fate and transport properties of these chemicals will 

cause them to result in insignificant risk. 

The same information is needed for the subject COPCs as is for all other COPCs: chemical/physical and 

toxicological parameter values sufficient to complete the calculations using the recommended equations. 

We’ve modified the HHRAP text to further clarify this. 

Issue: Chemical Specific - Metals 

General: It was suggested that expansion of the metals list beyond the 10 BIF metals should only be done 

on a site-specific basis. It was also suggested that zinc is not justified for consideration in the risk 

assessment. 

EPA Response: General 

We agree. See the text box just prior to Draft HHRAP Section 2.2.1.2. Similarly, consideration of zinc 

should be made on a site-specific basis in consultation with the permitting authority.

Chemical-specific comments were received for the following metals: 

• Chromium; 

• Lead; 

• Mercury; and 

• Nickel 
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Chromium [draft HHRAP Section 2.3.8.1] 

The section on chromium valence was considered by some to be confusing, particularly with regard to the 

recalculation of risk estimates based on the trivalent assumption if an unacceptable risk is derived using 

the hexavalent assumption. Some commenters disagreed with EPA's default recommendation that 100 

percent of chromium should be assumed to be in the hexavalent state, in the absence of facility-specific 

speciation data. Commenters believed that this assumption is conservative, and they asked if it is 

supported by data from facilities that have tested for both valences of chromium. It was suggested that 

EPA use medium-specific chromium speciation information that is readily available in the open literature 

in place of the overly conservative assumptions recommended in the Draft HHRAP. Commenters also 

stated that metals speciation is only valid if substantiated by fate and transport characteristics, and EPA 

should provide a consistent, uniform approach to metals speciation. 

EPA Response: Chromium

EPA considers it best to use measured, speciated emissions data in the risk assessment. If site-specific 

speciated emissions data are unavailable, a default speciation may be generated. Percentages of 

chromium assumed to be hexavalent were assessed during the peer review process. We’ve evaluated data 

available in the MACT database and derived a method to generate a default speciation. We’ve 

incorporated the new method into the HHRAP, including a detailed write-up on how we derived the 

method (Appendix D of the Final HHRAP). 

As with many parameters and equations provided in the HHRAP, the scientific basis and reference is 

provided. A facility is then able to evaluate whether to propose and justify to the permitting authority an 

alternative they consider more appropriate than that recommended in the HHRAP.

Lead [draft HHRAP Section 2.3.8.2] 

It was recommended that EPA must emphatically state that lead must always be fully evaluated as a 

COPC in the risk assessment because there is a significant level of risk from lead via indirect exposure 

pathways. CAA permits typically do not address exposure from indirect pathways, and they generally 

consider only inhalation pathways using protocols that are not as rigorous as EPA’s guidance for 

hazardous waste combustion risk assessments.

It was noted that EPA suggests the use of the IEUBK model for evaluating potential exposure to lead. 

The routine use of the IEUBK model was seriously questioned with regard to addressing lead impacts 

from incinerator emissions. Issues included: 

• Multi-pathway risk assessment predicts lead concentrations in many different environmental media, 

but these media concentrations do not directly correspond to the inputs appropriate to the IEUBK 

model. Commenters recommended that EPA should offer guidance about how the IEUBK model is 

to be run with regard to changes in exposure parameters and model default assumptions developed 

within the rest of the risk assessment. They suggested that a detailed section should be provided in 

the HHRAP in one or both of the chapters on exposure and risk characterization.

• Often calculated concentrations in soils from such emissions are quite low and the current version 

of the IEUBK model may not adequately handle such low impacts. Commenters suggested that, as 

specified in USEPA 1994, a guidance value of 400 mg/kg should be retained for soil impacts, 
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where calculated soil levels of 400 mg/kg or less would not require further evaluation and levels 

exceeding 400 mg/kg would be subjected to the IEUBK modeling. 

• It was noted that the requirement to evaluate lead by inputting the estimated soil concentration to 

the IEUBK model ignores other exposures to emitted lead, such as exposures through the food 

chain. However, the IEUBK model has the ability to include lead exposures via inhalation, 

drinking water consumption, and ingestion of foodstuffs. Commenters suggested that the model 

users could input site-specific levels of lead estimated in air, drinking water, and foodstuffs to 

estimate a blood lead level in children due to multiple media. 

EPA Response: Lead 

The comment is noted regarding the need to always evaluate lead as a COPC in the risk assessment. 

Detailed recommendations on what assumptions to use in the IEUBK model are site-specific. A 

screening method for addressing this is presented in the risk management addendum developed by EPA 

Region 6. Development of guidance about how the IEUBK model is to be run with regard to changes in 

exposure parameters and model default assumptions is beyond the scope of the HHRAP.

Mercury [draft HHRAP Section 2.3.8.3] (See RC Section 3.5.2)

There is agreement with EPA Region 6 that mercury should be evaluated in accordance with the 

information provided in the Mercury Study Report to Congress (U.S. EPA, 1997b). However, differences 

of opinion with U.S. EPA Region 6 were raised with regard to the implementation of this guidance in the 

equations used to calculate mercury concentrations in fish and plant media. Specifically, commenters did 

not agree with the technique used to calculate total load to the water body for the fish pathway, and they 

did not understand the rationale behind the root uptake equation for plant matter pathways. 

The discussion on modeling methods and partitioning for mercury was considered somewhat confusing. 

It was suggested that a brief summary might help clarify what is being modeled in the risk assessment. 

The approach outlined in the HHRAP was considered generally consistent with exposure and risk 

assessment methods in other guidance and regulatory reports evaluating mercury risks. However, 

commenters believed there are a number of difficulties with the default values EPA would have the risk 

assessor use for various parameters as well as with certain details of the methods. 

EPA Response: Mercury

Mercury fate and transport were addressed during the external peer review. See Section 3.5.1 of this 

Response to Comments document for more discussion. We have made several mercury-related changes 

to the HHRAP, including: 

• Including dry deposition of vapor in the air model; 

• Changing where the 85/15 speciation split is made; and 

• The recommended accumulation factor applied to mercuric chloride. 

Further discussions and examples of mercury modeling can be found in proceedings of the IT3 

conference (Kaleri, C.J. 2000. “Implementation Issues: Mercury Transport & Fate, Combustion Risk 
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Assessments in Region 6.” Proceeding of the 12th International Conference on Incineration and Thermal 

Treatment Technologies. May 14-18). This reference has been added to the HHRAP. 

As with many parameters and equations provided in the HHRAP, the scientific basis and reference is 

provided. This allows a facility to evaluate whether they come up with a more appropriate alternate or 

site-specific value other than that recommended in the protocol. 

Nickel [draft HHRAP Section 2.3.8.4] 

It was suggested that EPA should substantiate the policy change whereby nickel in stack emissions is now 

to be treated as carcinogenic. It was also noted that nickel and selenium requirements were not achieved 

through rulemaking.

EPA Response: Nickel 

We note these comments. As with many parameters and equations provided in the HHRAP, the scientific 

basis and reference is provided. A facility is then able to evaluate whether to propose and justify to the 

permitting authority an alternative they consider more appropriate than that recommended in the 

HHRAP. 

Expansion of the metals list beyond the 10 BIF metals is considered on a site-specific basis. See the Draft 

HHRAP text box just prior to Section 2.2.1.2 

Issue: Chemical Specific - Hydrogen Chloride and Chlorine Gas [draft HHRAP Section 2.3.10] 

It was noted that although there is discussion in the Draft HHRAP about some of the general toxicology 

of hydrogen chloride and chlorine gas, EPA does not come to any conclusion about how the risk 

assessment process should evaluate these emissions. This is in contrast to other similar sections (for 

example, the preceding section 2.3.9 dealing with particulate matter). It was suggested that the HHRAP 

should state specifically what combustion facilities need to address in their risk assessments regarding the 

measured level of emissions of hydrogen chloride and chlorine gas. 

EPA Response:

The HHRAP includes a discussion of hydrogen chloride and chlorine gas in order to educate the facility 

and permitting authority. The decision to include these chemicals in the risk assessment is a site-specific 

determination to be made by the permitting authority and is beyond the scope of the HHRAP. If the sitespecific decision is made to quantitatively assess them, chemical-specific parameter values for both 

hydrogen chloride and chlorine gas are available in the HHRAP companion database. 

Issue Chemical Specific - Criteria Air Pollutants [draft HHRAP Section 2.3.11] 

Issues that were raised regarding the inclusion of Clean Air Act (CAA) criteria air pollutants in the 

quantitative risk assessment include: 

• The decision about whether to include criteria air pollutants in the risk assessment should not be left 

up to the individual permitting authority. Typically, any area listed by a guidance document 

becomes another issue that the facility has to prove and negotiate to omit it from the risk 

assessment. 
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• It was suggested that because neither Table A-3 and Table A-4 of the Draft HHRAP, IRIS, nor 

HEAST lists toxicity values for sulfur dioxide, nitrogen dioxide, ozone, and carbon monoxide, it is 

impossible to include these compounds in the quantitative risk assessment. 

EPA Response: 

The HHRAP is guidance and not regulation. Regulatory implementation structures are such that, in the 

case of RCRA, the decision about what to evaluate in the risk assessment is always left up to the 

permitting authority. 

We’ve further clarified our recommended process for deciding which COPCs to quantitatively evaluate, 

which to qualitatively evaluate, and which to drop from consideration. 

Issue: Chemical Specific - Radionuclides [draft HHRAP Section 2.3.13] 

Given that the radionuclide dose calculations are currently being accepted by regional EPA and state 

regulators for the National Emissions Standards for Hazardous Waste Air Pollutants (NESHAPS) process, 

it was suggested that it would be helpful if flexibility could be added to the HHRAP to also allow risk to 

be estimated using accepted dose-to-risk conversion factors. This would reduce the unnecessary 

duplication of effort and would help improve consistency between the two EPA-required assessments, 

i.e., the NESHAPs radiological dose assessment and health risk assessment for mixed waste combustors 

(both which are made available for public review). Commenters noted that while the slope factor 

approach would result in more realistic risk estimates, using dose-to-risk conversion factors will provide 

comparable results within the error bounds of the overall assessment. 

EPA Response:

The HHRAP is consistent with the guidance used in the Superfund program and not NESHAPS. The 

HHRAP only provides a brief overview of radionuclides. A separate guidance would need to be 

developed to conduct a risk assessment of a facility emitting these constituents. 

2.2.5 Emission Sources and COPCs - Clarification /Editorial

Issue: Clarification - Wildlife

It was asked if wildlife is also of concern with reference to page 2-33, line 8 from the bottom. Although 

this is clearly a “human health” risk assessment protocol, it might be indicated that either another 

document deals with this issue or there is too little data to approach such a protocol. 

 

EPA Response:

Comment noted. As stated in the comment, the HHRAP is clearly a protocol for assessing potential 

“human health” impacts.

Issue: Clarification - 95th Percentile 

It was recommended to delete or modify the sentence “The lesser of the 95th percentile or maximum 

stack gas concentration from the three trial burn runs should be used to develop the emission rate estimate 
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used in the risk assessment” (page 2-5, second paragraph) because the 95th percentile cannot be higher 

than the maximum concentration. 

EPA Response:

We’ve deleted the subject sentence. 

Issue: Clarification – Non-Cancer Effects

It was recommended that line 4 on page 2-43 read “...not expected to result in a significant increase in 

non-cancer effects above that already observed in U.S. population.” The reader should realize that there 

is a significant background in non-cancer effects in the U.S. population that may be due to differing 

genetics, exposure to chemicals etc. Actually, it seems inappropriate not to have an RfD for these 

chemicals. On the other hand, if an RfD is not available (from any source) and there is reasonable 

evidence to indicate that the analyses for risk using the cancer slope factor is conservative enough to 

protect against non-cancer endpoints, then this should be clearly stated. 

EPA Response:

We’ve modified the subject sentence. We’ve noted the balance of the comment. 

Issue: Clarification - PCBs

It was recommended, with regard to section 2.3.3 Polychlorinated Biphenyls: page 2-47, lines 1-3 from 

bottom, eliminating “that” in “...to confirm that the absence....” to make the sentence clearer. Also, it 

should be more definitive whether or not the permitting authority should confirm the absence of the 

PCBs. The way the sentence is written, the permitting authority has the option to confirm. Either 

indicate which should be done, or provide some guidance into the process by which the decision is made. 

The commenter suggested that given the potential toxicity of the PCBs the recommendation should be 

that the absence should be confirmed. 

EPA Response:

We’ve modified the subject sentence. We’ve also further clarified our recommendation regarding 

confirming the absence of these compounds from stack emissions. It is only a recommendation, however, 

and the permitting authority may or may not abide by it. 

Issue: Clarification – Non-Coplanar PCBs

In Draft HHRAP Section 2.3.3.1 Carcinogenic Risks, the last paragraph on page 2-49 and first two 

paragraphs on page 2-50 were considered confusing because it is not clear if they refer specifically to the 

non-coplanar PCBs. Part of this could be clarified if there was a separate section for the non-coplanar 

PCBs. In addition, in most cases “non-coplanar” PCBs should be specified. For example, in the “Criteria 

for Use” Table on page 2-49, the slope factor 2 is indicated for early-life exposure by all routes to all PCB 

mixtures. Does this really mean all PCB mixtures or just the non-coplanar PCBs? Likewise, on page 2-

50 in lines 4, 5, and 8, is it correct as is, or is non-coplanar PCBs meant? Section 2.3.3.2 Potential NonCancer Effects is somewhat clearer because only mixtures are discussed. However, since section 2.3.1.2 

specifically designates guidance for PCDD/PCDF non-cancer hazards, it would seem appropriate that the 

coplanar PCBs be assessed for non-cancer hazards as described in 2.3.1.2. If this is done, then 2.3.3.2 

should be changed to indicate how the non-cancer hazards should be approached for non-coplanar PCBs. 
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EPA Response:

We’ve further clarified our recommendations. 

Issue: Clarification – Trial Burn Estimates

It was recommended that with regard to Draft HHRAP Section 2.2.1.1 Estimates from Trial Burns: 

• Page 2-4, lines 7-8 from bottom, “Trial burn tests are designed to produce emission rates higher 

than anticipated under normal operating conditions,” something should be added here to indicate 

why this should be the case. If it is because of the conditions required for these tests, a simple 

phrase should be added to indicate this. Also somewhere in this section it should be indicated that 

waste feeds used during trial burns should be similar to those used during normal operations. 

• Page 2-5, 2nd full paragraph, “High POHC...” should state some conclusion. It begins by stating 

 one aspect of the trial burn, then goes on to indicate something opposite by the PCDD and PCDF 

 example. 

EPA Response:

We address the subject further in the paragraphs below the referenced sentence. We’ve modified the 

structure of the section for clarity. 

Issue: Clarification – Process Upsets

It was recommended that clarification or addition be made to Draft HHRAP Section 2.2.5 Emissions from 

Process Upsets, page 2-14, lines 3-5 from bottom, “U.S. EPA (1994i) indicates that upsets are not 

generally expected to significantly increase stack emissions over the lifetime of the facility.” because it 

sounds like EPA is not concerned about acute risks. 

 

EPA Response:

Comment noted. It wasn’t our intention to suggest any opinion regarding acute risks in this portion of the 

HHRAP. 

Issue: Clarification – Appendix A Values

It was noted that in Draft HHRAP Section 2.3 IDENTIFYING COMPOUNDS OF POTENTIAL 

CONCERN, pages 2-36 and 2-37, many of the table numbers as indicated in Appendix A are incorrect 

here. 

 

EPA Response:

Comment noted. We’ve done an editorial review of the HHRAP, modifying where appropriate. 

Issue: Clarification - Estimated Maximum Possible Concentration (EMPC) 

Regarding the use of EMPC for contaminants analyzed by isotope dilution gas chromatography / mass 

spectrometry (GC/MS), it was suggested that use of alternative quantitation ions should be permitted if 

the signal-to-noise ratio of the ion signal is at least 2.5 and if the tune data indicate that the mass 

spectrometer is operating within specification. Such actions to reduce the EMPC would be far more cost 

effective than the suggested additional sample cleanup and/or reanalysis. 

EPA Response: 

We agree and have added language to the HHRAP to this effect. 
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Issue: Clarification – COPC Elimination Steps 

The HHRAP text and Appendix A do not appear to agree with the flowchart in Draft HHRAP section 2.3. 

The instructions on COPC deletion in step 3 may be eliminated without first asking whether these COPCs 

are potential PICs, which is inconsistent with the flowchart. 

EPA Response: 

We agree and have modified both the text and the figure to more clearly state our recommended process. 

Issue: Clarification - Radionuclides 

In the second sentence of the second paragraph of Draft HHRAP Section 2.3.13, it could be noted that air 

submersion is a significant exposure pathway only for certain radionuclides that emit high-energy beta 

particles, such as krypton-85. In the first sentence page 75, it may be helpful to clarify that Federal 

Guidance Report #11 provides dose conversion factors for internal pathways (ingestion and inhalation). 

External dose conversion factors are given in Federal Guidance Report #12., page 6-2, Equation 6-1. It. 

would be helpful to note that the denominator in this equation (BW x AT) does not apply for 

radionuclides. 

EPA Response:

The HHRAP only provides a brief overview of radionuclides. A separate guidance would need to be 

developed to conduct a risk assessment of a facility emitting these constituents. 

Issue: Editorial 

• Section 2.2.1.3 - First full paragraph: "phthalates" is misspelled. 

• Section 2.2.1.3 - In the definition of Fraction 2, one must assume that the temperature range given 

"between 100C and 300C" is inclusive. Recommendation: The subject sentence should read 

"...between 100C and 300C (inclusive)." 

• Section 2.2.4 - Paragraph 1, line 5: It is unclear when it is appropriate to add previous years of 

operation to the anticipated time period. 

• Section 2.2.7 - Step 1: Determining and Empirical Emission Factor Comment: The empirical 

emission factor is misstated the second time as "0.107" rather than "1.07." 

• Section 2.3.1 - Paragraph 4, line 3 (Polychlorinated Dibenzo(p)dioxins and Dibenzofurans): It is 

unclear to what "their" refers. 

EPA Response: 

Comments noted. Editorial errors have been reviewed and corrected where applicable. 

2.3 AIR DISPERSION AND DEPOSITION MODELING [draft HHRAP Section 3] 

Air dispersion models are mathematical constructs coded into computer programs that approximate the 

physical processes in the atmosphere directly affecting the dispersion of gaseous and particulate 

emissions from the stack of a combustion unit. The calculated air concentrations and deposition rates of 

the emitted constituents are, in turn, used to estimate potential human health risks associated with the 
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facility releases. In Chapter 3 of the draft HHRAP, we provide guidance on the use of the standard U.S. 

EPA Industrial Source Complex Short-Term (ISCST3) air dispersion model the Agency expects to use in 

most situations, as well as several preprocessing computer programs that prepare and organize data for 

use in the ISCST3 model. 

An extensive range of comments was received regarding air dispersion and deposition modeling. These 

comments were categorized into general modeling issues and eight other specific modeling topical issues, 

including: 

• Determining Spatial Locations; 

• Surface Roughness Height (Length); 

• Building Facility Characteristics 

• Chemical Partitioning of Emissions; 

• Estimating Dry Deposition of Vapor and Vapor Emission; 

• Estimating Wet Deposition; 

• Stack Temperature; and 

• Clarification / Editorial. 

2.3.1 General Air Dispersion and Deposition Modeling Issues

General comment issues include: 

• Model Limitations; 

• Model Validation; 

• Selecting Parameter Values; 

• Default vs. Site-Specific Parameters; 

• Modeling Conservation of Mass; 

• Number of Modeling Runs; 

• Acute Scenario Modeling; and 

• Air Dispersion and Deposition Model History. 

Issue: General - Model Limitations 

It was suggested that EPA emphasize the limitations of available air dispersion and deposition algorithms, 

and recognize that EPA models are frequently applied to situations for which they are not designed, 

leading to possibly highly erroneous predictions. EPA dispersion models for multi-pathway risk 

assessment are at best screening algorithms and should be described as such. Furthermore, EPA should 

suggest alternative air dispersion models that may also be appropriate for use. 

EPA Response: 

We agree that the HHRAP should suggest alternative air dispersion models. We’ve added a discussion of 

air models other than ISCST3 to the HHRAP, with a description of when they may be appropriate models 

of choice. Chapter 8 in the HHRAP is devoted to discussing uncertainty in the risk assessment that 

includes uncertainty in the air model. 
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Issue: General - Model Validation 

It was noted that although EPA has continued to refine the air dispersion and deposition model and make 

it more sophisticated, EPA has made no efforts to validate or improve the accuracy of the model, and 

therefore the model should not be used extensively for a regulatory purpose (i.e., permitting activities). 

Additionally, commenters note specifically: 

• There are insufficient field data to identify suitable inputs for these algorithms. 

• The Plume Depletion algorithm has not been adequately tested. 

• The draft HHRAP states that executable file "ISCST3.EXE" must be replaced with the latest 

version provided on the EPA bulletin. The version of ISCST3 provided with the ExInter 1.0 is not 

the latest available version and its results will not be consistent with current EPA requirements. 

EPA Response: 

We strongly disagree that the model has not been validated for air concentrations (See studies done by 

EPA to support the Cumulative Exposure Project (1999b), the National Air Toxics Assessment (2001a) 

and the Model Evaluation Results for AERMOD (Paine et al., 1998). Validation of the deposition portion 

of the model is less documented, but has generally been found to provide results within an order of 

magnitude. With regard to any specific algorithm provided in the HHRAP, the scientific basis and 

reference are provided. A facility is then able to evaluate whether to propose and justify to the permitting 

authority an alternative they consider more appropriate than that recommended in the HHRAP. The 

HHRAP has been updated with the latest version of ISCST3 and the discussion of ExInter1.0 has been 

deleted. 

Issue: General - Selecting Parameter Values 

It was recommended that EPA discuss how to choose appropriate stack conditions (low temperature and 

flow rates) and receptor spacing, how to collocate multiple source exposures, the methodology for 

addressing multiple source upsets, and the benefits of creating a database to find the actual point of 

maximum exposure. 

EPA Response: 

The HHRAP presents methodologies flexible enough to adapt to site-specific situations. A facility is then 

able to evaluate whether to propose and justify to the permitting authority a site-specific or other 

alternative more appropriate than that recommended in the HHRAP.

Issue: General - Default vs. Site-Specific Parameters 

It was noted that the draft HHRAP seems to rely heavily on default assumptions, although EPA generally 

requires the use of site-specific data whenever possible. The HHRAP should specifically provide for the 

option of using a more refined approach as required and more sophisticated models for portions of the 

assessments. 

EPA Response: 

The HHRAP provides default values for ease of use. As emphasized throughout the HHRAP, we 

encourage the use of site-specific information early in, and throughout the risk assessment process. 
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Conservative assumptions should be made only when needed to ensure that emissions from combustion 

units do not pose unacceptable risks. (See HHRAP page 1-6). 

Issue: General - Modeling Conservation of Mass (See RC Sections 3.3.1, 3.5.4 and 3.5.5) 

It was recommended that provisions be included in the HHRAP to assess conservation of mass in areas, 

such as dispersion and deposition modeling and exposure assessment. They recommend multi-pathway 

analysis be revised because it mixes steady state and non-steady state modeling which can result in 

violations of conservation of mass. 

EPA Response: 

We’ve made an extensive effort to provide the most accurate yet implementable guidance for conducting 

risk assessments. Although there is a mixing of steady state with non-steady state assumptions within the 

HHRAP, every effort has been made to bound estimates and preserve conservation of mass. The question 

of mass balance was submitted to the external peer review panel (For more details see EPA response to 

the external peer reviewers), and conservation of mass checks have been incorporated throughout the 

HHRAP. We’ve revised our recommendations regarding dry deposition of vapor based on comments 

received regarding the conservation of mass. The external peer review panel found conservation of mass 

to be not affected by the soil erosion loss equal to zero except at the tops of hills. Conservation of mass 

may be violated in food items, but are within the error bars of biotransfer parameters. We continue to 

refine biotransfer equations to reduce error and minimize violation of the conservation of mass. As with 

many equations and parameters provided in the HHRAP, the scientific basis and reference are provided. 

A facility is then able to evaluate whether to propose and justify to the permitting authority using a sitespecific option, or other alternative to that recommended in the HHRAP. 

Issue: General - Number of Modeling Runs [draft HHRAP Sections 3.7 and 3.10] 

It was recommended that EPA clarify the discussion in draft HHRAP Section 3.7 regarding the number of 

modeling runs needed per stack source and per fugitive emission source. 

EPA Response: 

We agree that the discussion about the number of modeling runs on the bottom of page 3-64 is confusing, 

and have modified it. We recommend that stack sources be modeled using vapor, particle, and particlebound air modeling runs, while fugitive sources be modeled using only vapor runs in the example 

presented. 

Issue: General - Acute Scenario Modeling [draft HHRAP Section 3.11] 

It was suggested that modeling of acute risks adds no additional complexity to the air modeling because 

the required output is obtained during the same runs for determining the average annual vapor and particle 

concentrations and depositions. However, it was noted that there is little guidance provided in draft 

HHRAP Section 3.11 on performing acute scenario modeling. It was recommended that EPA suggest an 

acute scenario modeling strategy, such as a tiered approach to isolate the maximum air concentration both 

onsite and offsite. 

EPA Response: 

The HHRAP was developed to be flexible to site-specific situations. The modeling approach presented in 
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the HHRAP allows the risk assessor to use the existing ISCST3 air model output to screen for acute 

effects. The ISCST3 one-hour emissions, for the worst-case metrological conditions, are used to estimate 

the maximum air concentration. The air modeling approach is also flexible enough to account sitespecific meteorological conditions and temporal variations in emissions. 

Issue: General - Air Dispersion and Deposition Model History [draft HHRAP Section 3.1.1]

It was noted that EPA mistakenly asserts that models were not available for multi-pathway risk 

assessment prior to 1990 (draft HHRAP, page 3-3), and does not adequately discuss in the draft HHRAP 

the status of specific EPA model use after 1990. 

EPA Response: 

The HHRAP states that before 1990, several air dispersion models were used by U.S. EPA and the 

regulated community. These models were of limited utility in risk assessments because they considered 

only concentration, and not the deposition of contaminants to land. The HHRAP has been updated to 

include the latest version of the ISCST3 and a discussion of other air models that may be more 

appropriately used on a site-specific basis. 

2.3.2 Determining Spatial Locations [draft HHRAP Sections 3.2 and 3.7]

Issue: Determining Spatial Locations 

It was suggested that modelers should be made aware that data used for analysis or as direct input to the 

model runs that have spatial attributes, such as spatial coordinates or association with specific locations, 

must be spatially consistent to be of any value. Specifically: 

• Using a single data set by itself poses no inconsistencies and any consistent coordinate system (i.e., 

State Plane Feet) should be allowed for air modeling. 

• When multiple data sets are compared, EPA recommends using Universal Transverse Mercator 

(UTM) coordinates for all spatial locations to properly align the data. Commenters recommend the 

HHRAP makes explicit that UTM is a map coordinate projection of geographic coordinates and the 

horizontal datum of the geographic coordinates from which the UTM coordinates were projected 

must be known. Since it is most likely that modelers will need to translate data to a single 

reference, modelers should be made aware that there are many computer programs available, some 

free and some at cost, that will, with varying degrees of accuracy, project coordinates from 

geographic coordinates to UTM coordinates. The U.S. Geographic Survey and US Army Corps of 

Engineers have reputable and widely used programs (NADCON and CORPSCON) with user 

guides and source computer code available. 

 EPA Response: 

The coordinate system recommended in the HHRAP was suggested to provide consistency. Other 

coordinate systems can be implemented provided they are done so consistently. 
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2.3.3 Surface Roughness Height (Length) [draft HHRAP Sections 3.2 and 3.6]

Several comments were received with regard to air modeling using surface roughness height. 

Commenters expressed a broad range of opinion, ranging from suggesting establishment of a simplified 

default method for determining surface roughness heights using a reference table containing average 

values, based on land use type, to the assertion that the draft HHRAP unrealistically simplifies the 

treatment of surface roughness height and should be revised to allow use of well established methods to 

compute this input. Following are comments addressing both sides of the issue. 

Issue: Surface Roughness Height - Simplified Default Modeling Approach 

Commenters advocating a more simple approach to modeling surface roughness height assert that: 

• Inclusion of the terrain elevations significantly increases both the level of effort to set up the 

modeling and the run time of the ISCST3 model without appreciably influencing the outcome of the 

air modeling results or having a significant impact on risk assessment. It was suggested that 

inclusion of terrain elevations should be made on a site-by-site basis. 

• The required level of detail is questionable in the methodology for determining the surface 

roughness height at the application site, as specified in the draft HHRAP, starting on page 3-11. 

Based upon EPA's sensitivity analysis (Secrest, 1997) only some variation, generally within 20%, is 

shown in results when varying the surface roughness height. Moreover, these sensitivities were 

modeled at extreme points (0.001 meter representing all water area and 1.3 meters representing all 

forest area) in comparison to the EPA default value of 0.1 meter. 

EPA Response: 

We do not recommend a simpler, reference table approach to determining surface roughness height 

because sensitivity analysis has shown surface roughness to be a rather sensitive parameter, and we do 

not feel there is sufficient difficultly in the site-specific surface roughness calculation to warrant this 

simpler approach. However, sensitivity analysis (see U. S. EPA Region 6 web page) also shows that 

although surface roughness might have a substantial effect on air modeling output, the effect on a risk 

assessment is generally less than a factor of two, which in most cases is not considered severe. Studies 

completed to date suggest the air model provides reasonable estimates of deposition, but we agree that 

more work could be done in validating the deposition portion of the air model. 

Issue: Surface Roughness Height – Modeling with Site-Specific Parameter Values 

Commenters claiming the draft HHRAP oversimplifies the treatment of surface roughness height raised 

the following issues: 

• The implications of using average values for surface roughness height and other parameters, such 

as albedo, Bowen ration, anthropogenic heat flux, and fraction of net radiation absorbed at the 

ground, are difficult to gauge. It was recommended that EPA engage in a study of model sensitivity 

to explore this matter. 

• How representative is the EPA recommended default value of 0.1 meter for surface roughness 

height? 
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• How can the default value of 0.1 meter be considered without approval when the deposition 

algorithms are quite sensitive to the surface roughness height, which can vary over several orders of 

magnitude and have large effects on the modeled dry deposition flux value? 

• EPA recommends using a calculated weighted-average value for surface roughness height based on 

land use characteristics in different directions from the modeled facility. It was recommended that 

the prediction of particle deposition consider the local surface roughness height at the receptor 

location. 

• Depending on the topography of an area, the 1:250,000 digital elevation model (DEM0) 90-meter 

spacing data, which have an accuracy of 8 to 15 meters, may not be accurate enough as the sole 

source of data for receptors within 10 km of the source. It was recommended that potential 

problems with digital elevation data be addressed by incorporating more accurate terrain data if the 

90-meter spacing DEM data are not adequate for the evaluation. 

EPA Response: 

The HHRAP is guidance and there is no requirement to determine surface roughness height or any other 

parameter by the methodology in the guidance. As with many parameters and procedures provided in the 

HHRAP, the scientific basis and reference are provided. A facility is then able to evaluate whether to 

propose and justify to the permitting authority a site-specific option, or other alternative to that 

recommended in the HHRAP. In most cases we would not question the default value of 0.1 meters for 

surface roughness height. However, where appropriate, a site-specific value may be developed in 

consultation with the permitting authority. We agree that more accurate terrain data should be 

incorporated if the 90-meter spacing DEM data are not adequate for the evaluation. 

Issue: Surface Roughness Height - (Length) Definition 

It was recommended that the definition for surface roughness length contained in the PCRAMMET User 

Guide be used instead of a definition from some other reference. 

EPA Response: 

We no longer recommend using PCRAMMET, instead recommending the use of MPRM to generate the 

input data ISCST3 needs to run properly. Therefore, definitions in the HHRAP, including surface 

roughness height, appropriate to MPRM have replaced those appropriate for PCRAMMET. 

2.3.4 Facility Building Characteristics [draft HHRAP Sections 3.2 and 3.7]

Issue: Facility Building Characteristics - Modeling Building Contours 

It was recommended that EPA building wake algorithms be revised because they do not reasonably 

support open or rounded aerodynamic structures and have overestimated impacts of a number of common 

industrial structures. 
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EPA Response: 

As with many parameters and equations provided in the HHRAP, the scientific basis and reference for the 

building wake algorithms are provided. A facility is then able to evaluate whether to propose and justify 

to the permitting authority a site-specific option, or other alternative to that recommended in the HHRAP. 

Issue: Facility Building Characteristics - Modeling Stack Height 

It was recommended that stacks exceeding 60 meters in height be modeled at their actual stack height and 

not limited to 60 meters as stated in the Guide for Air Quality Models (U.S. EPA, 1996). The purpose of 

the GAQM is to address the National Ambient Air Quality Standards (NAAQS) on a regional basis and 

independent of location, whereas the draft HHRAP addresses risk at specific locations. There is no 

inconsistency between modeling a stack taller than 60 meters at its actual height for the HHRAP and 

modeling the same stack at 60 meters for NAAQS analysis. 

EPA Response: 

The HHRAP recommends using actual stack height. Section 3.6.2.2 recommends using the height above 

the plant base elevation. We cannot find any point where the HHRAP recommends using a stack height 

of 60 meters for stacks actually taller than 60 meters. 

2.3.5 Chemical Partitioning of Emissions [draft HHRAP Sections 3.4, 3.7, and 3.9] 

Issues raised under chemical partitioning of emissions include: 

• Particle Phase Partitioning; 

• Determining Particle Size Distribution; 

• Determining Particle Density; and 

• Cost of Collecting Particle Density Data. 

Issue: Emission Chemical Partitioning - Particle Phase Partitioning 

 (See RC Sections 3.3.1, 3.3.3, 3.3.4 and 3.3.6) 

Particle deposition should consider the most appropriate particle size distribution, which may not in fact 

be that of stack emissions. For some chemicals that might partition in the air, it is debatable whether the 

particle distribution in the stack or in ambient air is of greater relevance. More specifically: 

• It was recommended that EPA should consider the matter and offer recommendations regarding the 

conditions, if any, where it is appropriate to consider a general particle size distribution for ambient 

air within deposition modeling, and develop a method that estimates site specific partitioning 

fractions based on both the background particles and the particles actually emitted. 

• Partitioning of chemicals between the particulate and vapor phases is scientifically invalid because 

it is based on ambient particles rather than site specific particle emission rates. The partitioning 

fraction (Fv) between the vapor and particle phases for each chemical is assumed constant, 

regardless of the mass, rate, and size of the particles emitted from the source. The Fv values are 

based on equilibrium between the vapor phase and the surface area of background particles in the 

air, not the actual particles being emitted. Since the Fv values are scientifically invalid, the COPC 

concentrations in the vapor and particle phases and deposition rates that are derived from them are 
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also incorrect. Because this approach omits the particles emitted from the source, it will 

overestimate the concentrations of chemicals in the vapor phase and underestimate the amounts in 

the particle phases for most facilities. 

EPA Response: 

Part one of the comment requests conditions where a general particle size distribution could be used For 

facilities with stack test results which indicate mass amounts lower than the detectable limit (or the filter 

weight is less after sampling than before), the HHRAP recommends using a single mean particle size 

diameter of 1.0 microns to represent all mass (e.g., particle diameter of 1.0 microns or a particle mass 

fraction of 1.0) in the particle and particle-bound model runs. Part two of the comment addresses the 

scientific validity of the partitioning fraction (Fv). We believe the method for chemical partitioning 

recommended in the HHRAP is the most appropriate existing method. A detailed discussion of the 

scientific basis for the recommended method can be found in Appendix A of the HHRAP. A facility can 

propose and justify to the permitting authority an alternative method for use on a case-by-case basis.

Issue: Emission Chemical Partitioning - Determining Particle Size Distribution 

 (See RC Sections 3.3.1 and 3.3.6) 

Comments were received requesting that EPA provide acceptable approaches for measuring or 

determining default values for particle size distribution. Values for particle size distribution are required 

as model inputs to calculate dry deposition. The related EPA trial/risk burn guidance document, 

“Guidance on Collection of Emissions Data to Support Site-Specific Risk Assessments at Hazardous 

Waste Combustion Facilities” (U.S. EPA, 1998c) did not provide any clear direction on measuring 

particle size distribution, stating “widely accepted methods for determining particle size-size distribution 

is not yet available.” Also, a number of facilities have attempted to measure particle size distribution but 

with limited success. In lieu of measured values, the draft HHRAP recommends default values for 

particle size distribution. Commenters raised various issues about the use of default values, including: 

• For low emitting sources, the draft HHRAP recommends a default of one particle size of 1 μm be 

used for the particle size distribution because, EPA reasons, there is particulate mass in the stream 

but it is all small and in the range of 1 μm and smaller. It was questioned whether the default value 

of 1 μm is valid for a low emitting source without emission testing. These commenters have taken 

samples from a low-emitting source over a long sampling time, and found very small 

concentrations of detectable particles much larger than 1 μm. These results illustrate the dilemma 

of choosing between a default value that might not be representative and costly long-period 

sampling of extremely low emissions that may be of little significance in the risk assessment. 

• It was recommended that EPA provide acceptable approaches in the HHRAP and trial/risk burn 

guidance for measuring particle size distribution and particle density for HEPA systems that have 

highly variable waste feed. One suggested alternative would be to assume a default particle size 

distribution for uncontrolled emissions at a generic incinerator (available in California Air 

Resources Board (1990) reports, EPA’s AP-42 (1995a), and the scientific literature), and make 

modifications via spreadsheets based on the penetration fraction through the specific air pollution 

control devices at the given facility. Including an example of this type of spreadsheet calculation in 

the guidance would be helpful. 
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• Given the information from the sensitivity analysis and the difficulty of developing accurate 

particle size distribution data with existing methods, it was strongly recommended that the default 

particle size distribution be changed to reflect the fact that best measurements to date indicate that 

80% of the particles are less than 2 microns. This will more accurately reflect the state of the art for 

coarse particle removal and will create more accurate deposition estimates. Measuring particle size 

distribution will become even more inconsequential when facilities install equipment to meet the 

new MACT PM standard, at which time; it is likely that virtually all particles emitted from all 

facilities will be below 1 micron. 

• The draft HHRAP states that “a minimum of three particle size categories (>10 μm, 2-10 μm, and 

<2 μm) detected during stack testing are required for air modeling.” Commenters ask whether the 

ranges stated, i.e., >10 μm, 2-10 μm, and <2 μm, are a requirement as well. The range requirement 

may not be practical because there may be instances when the particles in the sample train do not fit 

within each of the three ranges. 

• A sensitivity analysis of the ISCST3 model to the number of particle size categories (Secrest, 1997) 

shows that the model is not sensitive to the number of particle size categories as long as the same 

range of sizes is included. Stated another way, it did not matter whether there were 3 or 9 particle 

size categories as long as the range of categories did not change from the base case (0 to 15 micron 

diameters). Therefore, commenters question the draft HHRAP recommendation that 8 to 10 

particle size distribution values produced from the stack results be used during the air modeling 

(though it does state a minimum of 3 are required for air modeling). Computer run time is 

increased for every particle size added. 

EPA Response: 

The distribution of particulate by particle diameter will differ from one combustion process to another, 

and is greatly dependent on the: 

• Type of furnace; 

• Design of the combustion chamber; 

• Composition of the feed fuel; 

• Particulate removal efficiency; 

• Design of the APCS; 

• Amount of air in excess of stoichiometric amounts that are used to sustain combustion; and 

• Temperature of combustion. 

 

However, based on these variables, the particle size distribution cannot be calculated, but only directly 

measured or inferred from prior data. Unfortunately, few studies have been performed to directly 

measure particle size distributions from a variety of stationary combustion sources. We therefore 

recommend that existing facilities perform stack tests to identify a site-specific particle size distribution.

The “Guidance on Collection of Emissions Data to Support Site-Specific Risk Assessments at Hazardous 

Waste Combustion Facilities” (U.S. EPA, 1998e, EPA/600/6-90/003) has been updated (i.e., “Risk Burn 

Guidance for Hazardous Waste Combustion Facilities” (U.S. EPA 2001b, EPA530-R-01-001)), and now 

contains detailed and tested methods for measuring particle size distribution. Data can always be 

submitted in lieu of actual measurement of particle size distribution, along with documentation to justify 
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why it is acceptable for use in the air model. The example particle size distribution we use in the HHRAP 

has been clearly labeled as an example and not a default particle size distribution. 

The HHRAP is a guidance document and clearly recommends that a minimum of 3 particle size 

distributions are necessary. The eight to 10 particle size categories mentioned in the comment are what 

is typically measured by the cascade impactor methodology, and if they are measured, there appears to 

be no reason not to use them in the air model. We disagree that computer run time is a critical factor in 

choosing the particle size distribution values. The number of partitions can be negotiated with the 

permitting authority. 

Issue: Emission Chemical Partitioning - Determining Particle Density 

Guidance was requested on appropriate methods for measuring or determining default values for particle 

density. Values for particle density are required as model inputs to calculate dry deposition, but there 

does not appear to be any effective stack sampling methods to measure particle density. Commenters 

raised issues about measuring particle density and using alternative default values for particle density in 

lieu of measured values. These issues include: 

• The draft HHRAP assumes the particle density is the same for all sampling cuts. How does EPA 

propose the particle bound mass fractions be apportioned if a particle density can be determined for 

each sample cut, and the density varies from sample cut to sample cut? 

• Should the default value of 1 g/cm3

 to be used as representative in all cases since the mass of 

samples is typically on the order of milligrams, and the dry deposition flux is approximately 

linearly proportional to particle density? 

• It was recommended that EPA acknowledge in the HHRAP that particles emitted from cement kiln 

stacks are not similar to those of other combustion devices. A more appropriate default density 

value for the cement kiln industry would be 3.0 g/cm3

. 

 

EPA Response: 

We recommend that site-specific measured data on particle density be determined for all existing sources 

when possible. Determining particle density is part of the trial/risk burn guidance. With regard to any 

specific parameter provided in the HHRAP, the scientific basis and reference are provided. We 

acknowledge the variation in particles emitted from each type of facility. A facility can evaluate whether 

to propose and justify to the permitting authority an alternative to that recommended in the HHRAP. 

2.3.6 Estimating Dry Deposition of Vapor and Vapor Emission 

[draft HHRAP Sections 3.4, 3.6, 3.7 and 3.9]

Issue: Estimating Dry Deposition - Dry Deposition Vapor Velocity 

Several comments were received questioning the values and methods for determining the dry deposition 

vapor velocity, as contained in the draft HHRAP. Issues include: 
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• The approach for computing deposition velocities is too subjective and should be revised to 

reference an established method that will promote consistency. 

• EPA’s recommended single deposition velocity of 3 cm/s used to calculate the deposition of all 

vapors derives from measurements involving nitric acid, a highly soluble compound that readily 

deposits from the atmosphere. This assumption likely overestimates the vapor-phase deposition of 

hexachlorobenzene and other gases. 

• EPA should develop a generic but representative range of parameters for use with draft algorithms 

for gaseous deposition built into a test version of the ISCST3 code to estimate some typical or 

average deposition velocities for categories of vapors that can be distinguished by values of Henry's 

Law constants (the most critical gas-specific parameter), and use the results of the most appropriate 

run for each chemical of interest. 

• EPA has not considered the direct effect of temperature on dry deposition vapor phase diffusion. 

For instance, how can direct vapor transfer occur at the same level for a frozen or liquid surface? In 

the northern part of the country, many surface water bodies will only reach the assumed 

temperature of 25o

 C (about 780 F) for a brief portion of the year. 

EPA Response: 

Dry deposition of vapor was re-evaluated during the external peer review of the HHRAP, and alternative 

default dry depositions of vapor velocities have been provided. Furthermore, dry deposition of vapor has 

been included in the ISCST3 model (as opposed to a post-process mechanism), along with new 

recommended default values and literature citations upon which they are based. As with many default 

values provided in the HHRAP, the scientific basis and reference are provided. This HHRAP is guidance 

and allows a facility to evaluate whether to propose and justify to the permitting authority using sitespecific values, or other alternatives to those recommended in the HHRAP. 

Issue: Estimating Dry Deposition - Dry Deposition and Vapor Emission from Soils 

 (See RC Sections 3.3.5 and 3.5.4) 

Comments were received about the EPA’s recommended application of dry vapor phase deposition and 

diffusion in equations for estimating soil concentrations, as contained in the draft HHRAP. These 

include: 

• The HHRAP should more explicitly state in the text that the dry vapor phase diffusion load to soil 

(Ldif) parameter contained in draft HHRAP Equation 5-lA for calculating soil concentrations is no 

longer accounted for in the recommended equations for calculating soil concentration (i.e., 

equations 5-1C through 5-lE) because the Ldif term contributes negligibly to the soil concentration 

and can be reasonably excluded from the soil concentration calculation. 

• To be conservative in the face of uncertainty, EPA recommends a value of 0 for the soil COPC loss 

constant due to volatilization (ksv). This assumption, in combination with the recommended dry 

deposition velocity of 3 cm/s for all volatile organic compounds, simulates a one-way flux of vapor 

from the air into soils. This is not consistent with known fate and transport properties of many 

vapors, and implies that the soil acts as a sponge to remove vapors from the atmosphere. For 

instance, equilibrium-like transport behavior of environmentally persistent chemicals such as 

polychlorinated biphenyls (PCBs) demonstrates the inconsistency of EPA's approach. 
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EPA Response: 

The dry deposition of vapor was addressed during the external peer review of the HHRAP. Changes have 

been made according to recommendations of the external peer review panel. Both the ksv and dry 

deposition of vapor velocity sections of the HHRAP have been revised.

Issue: Estimating Dry Deposition - Dry Deposition and Vapor Emission from Water 

 (See RC Section 3.3.5) 

It was argued that dry deposition of vapors to the water body is modeled incorrectly. In the draft 

HHRAP, the equation to estimate dry deposition of vapors Ldif to the water body considers a one-way 

transfer of the pollutant from the air into the water body. However, mass transfer from the air into the 

water body should be modeled as the product of a mass transfer coefficient and the difference between 

two concentrations, that in the air and the effective concentration in the water, which serves as a 

resistance to mass transfer across the air-water interface. Given the way that EPA's model is constructed, 

loadings into the water body from soil erosion and runoff could be large enough to serve as a source of 

emissions to the air from the water body. By not allowing for the outward flux of vapors from the water 

to the air, EPA's model in some cases could incorrectly predict the trapping of a volatile chemical within 

the surface water body. 

EPA Response: 

This issue was reviewed during the external peer review of the HHRAP. Assuming that volatile chemicals 

are trapped in the water body (as suggested in the comment) would result in a conservative estimation of 

drinking water and fish concentrations. If this tended to result in risk/hazard levels above the benchmark, 

a more detailed site-specific fate and transport analysis could be conducted. However, we are unaware 

of any case where this has actually occurred. As with many parameters and equations provided in the 

HHRAP, the scientific basis and reference are provided. A facility is then able to evaluate whether to 

recommend and justify to the permitting authority a site-specific value or other alternative to that 

recommended in the HHRAP. 

Issue: Estimating Dry Deposition - Meteorological Inputs to Dry Deposition of Vapor Modeling 

Comments were received addressing modeling of meteorological parameters required to execute dry 

deposition of vapor algorithms included in the latest ISCST3 model. These include: 

• The basis for the intended update of the dry deposition algorithm by using hourly solar radiation 

data and leaf area index is not explained and should not be implemented without substantiation and 

review. 

• EPA states that only the Meteorological Processor for Regulatory Models (MPRM) program is 

scheduled to be updated to include the necessary solar radiation and leaf area index parameters 

required in calculating dry vapor deposition for use in risk assessment. The HHRAP should clarify 

that the facility "might be required to collect onsite meteorological data” if the MPRM is the only 

option for using dry deposition of vapor in future risk assessments. 

1) It was recommended that the HHRAP contain a discussion explaining the necessity of processing 

each year of meteorological data for each season and combining the results into a season-specific 

meteorological file. Because of seasonal variations, an annual average surface roughness or other 

site-specific parameter, such as wind speed, precipitation amounts, and temperature should not be 

used to process meteorological data. Using an annual average file would overestimate depositions 
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during the winter months and underestimate depositions during the summer months as site specific 

parameters change dramatically for each season. 

EPA Response: 

We’ve clarified the use of the MPRM. Concentrations and exposure in the HHRAP are based on longterm equilibrium assumptions. Variations in results based on seasonal effects are accounted for with 

site-specific inputs to the air and terrestrial fate and transport models. The HHRAP now recommends 

using MPRM as opposed to PCRAMMET to accommodate more currently available meteorological data 

for use within ISCST3 to calculate dry deposition of vapor. As with many parameters and equations 

provided in the HHRAP, the scientific basis and reference are provided. A facility is then able to evaluate 

whether to propose and justify to the permitting authority a site-specific option, or other alternative to 

that recommended in the HHRAP. 

2.3.7 Estimating Wet Deposition [draft HHRAP Section 3.7] (See RC Section 3.3.5)

Issue: Estimating Wet Deposition

Commenters acknowledge the wet deposition model has been improved by allowing use of more accurate 

precipitation data. However, the EPA needs to consider new information on wet deposition before 

finalizing the draft HHRAP. The model used for wet deposition of vapors needs to be revised to consider 

water solubility and other factors. There is only a very weak scientific basis for the scavenging 

coefficients used to compute wet deposition rates. The assumed relationship between wet deposition and 

precipitation rate should be verified or revised to take non-linearities into account. 

EPA Response:

In most situations, we recommend the latest version of the ISCST3 air model, which has been revised 

since the draft HHRAP was released. However, as air modeling advances, one should always take into 

account the most recent state-of-the-science. The comment regarding consideration of water solubility 

will be forwarded to OAQPS for further refinements in their air models. This issue was reviewed during 

the external peer review of the HHRAP, and the expert peer review panel did not believe that wet 

precipitation rates are linearly proportional. However, the panel could not recommend another 

methodology. As with many parameters provided in the HHRAP, the scientific basis and reference are 

provided. A facility is then able to evaluate whether to propose and justify to the permitting authority a 

site-specific option, or other alternative to that recommended in the HHRAP. 

2.3.8 Stack Temperature [draft HHRAP Sections 3.5 and 3.7]

Issue: Stack Temperature:

Several comments were received about modeling stack temperature. The issues raised include: 

• The statement in the draft HHRAP, that the stack exit temperature is the most critical parameter for 

influencing the concentration and deposition rates, is misleading because there are many parameters 

that can have a significant impact on the calculation of concentrations and deposition fluxes. 
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Further, the stack temperature will not play a dominant role in the plume rise if the plume rise is 

due to momentum flux and not buoyancy flux. 

• Because, in theory, risk burn results should not be significantly different from run to run, it was 

proposed that the arithmetic mean of the temperatures and velocities from the multiple risk burn 

runs be used as inputs to the model. 

• Stack temperature will remain constant in the modeling. Only the ambient temperature will vary 

from hour to hour. It was recommended that the EPA discuss the choice of stack temperature for 

the analysis in the HHRAP. To be conservative, it would be appropriate to choose the lowest 

operational stack temperature to decrease buoyancy induced plume rise, increasing air 

concentrations and depositions. Consideration may want to be made when choosing a 

meteorological data period with either average or above average annual temperatures for the area to 

also reduce plume rise. 

EPA Response: 

The relative importance of stack temperature is a matter of opinion and has no effect on results. Stack 

temperatures in the field have not been found to vary much among trial/risk burn runs. In most cases, 

the average velocity and temperature are used in the source parameters. If these numbers differ 

significantly during the trial or risk burn, a site-specific decision would need to be made. EPA’s GAQM 

recommends using 5 years of meteorological data to simulate long-term average meteorological 

conditions.

2.3.9 Clarification / Editorial

Issue: Clarification - Meteorological Data 

• Draft HHRAP Section 3.5.1.5 - Is pressure adjusted to surface elevation? If so, it would appear to 

be important for high-altitude locations. 

• Draft HHRAP Section 3.5.1.6 - ISC addendum 99155 incorporates a dry gas deposition algorithm. 

• Draft HHRAP Section 3.5.2 - “Upper air data” and “mixing height data” are NOT equivalent terms. 

EPA Response: 

Surface pressure referred to in the Draft HHRAP is the same as station pressure. The statement should 

more accurately read the "MPRM preprocessor for ISCST3 requires station pressure" since MPRM uses 

station pressure to compute Monin-Obukhov Length and Friction Velocity, then passes these two 

boundary layer parameters into ISCST3 to perform dry particle deposition. Station pressure is not an 

explicit meteorological input into ISCST3. The requirement for station pressure is to compute the air 

density at station pressure. Air density is used by MPRM to compute Monin-Obukhov Length and 

Friction Velocity, which in turn are used in ISCST3 for computing dry particle deposition velocity due to 

gravitational settling. At higher station elevations, air density is reduced from sea level. Conceptually, 

less dense air will have less resistance to deposition and result in higher deposition velocities at higher 

elevation. However, in theory and application, variations in deposition velocity are negligible except for 

very small particles between 0.1 and 1.0 micron diameter. For the 1-micron particle size, nominal effects 

on dry particle deposition as a consequence of variations in station pressure are expected. The latest 
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version of ISC has been incorporated into the HHRAP. The reference of upper air data as mixing height 

data has been corrected. 

Issue: Clarification - PCRAMMET model issues 

• Draft HHRAP Section 3.6.1.1 - This discussion is confusing. “L” in urban areas for nighttime 

stable conditions should have a (positive) maximum value that is smaller for more built-up cities 

than for less built-up cities (i.e. the former have less stable nighttime conditions). 

• Draft HHRAP Section 3.6.1.3 - The discussion of aerodynamic surface roughness and the 

sensitivity of atmospheric concentrations to this term is confusing and misleading. 

• Draft HHRAP Section 3.6.1.6 - Bowen ratio is not a “measure of surface moisture;” rather it is 

defined as the ratio of the sensible heat flux to the latent heat flux. The latter is sensitive to soil 

moisture. 

• Draft HHRAP Section 3.6.1.8 - This discussion is misleading. M-O length is sensitive to net 

radiation. The “fraction of net radiation absorbed” is a misnomer. 

EPA Response: 

The current version of ISCST3 utilizes MPRM rather than PCRAMMET. Comments were incorporated 

into the updates of the MPRM input parameters. 

Issue: Editorial 

• Draft HHRAP Section 3.7.5 – EPA’s recommended use of terrain grid files is not necessary. 

• Draft HHRAP Section 3.8 - The discussion of run times for different computers is out of date. 

• Draft HHRAP Section 3.9.1 - The reference to “theory” in the 3rd sentence might be better stated 

as “rationale” or some such term. 

• Draft HHRAP Section 3.9.1.2 - First line: Equation 3-1 should read 3-2. 

• Draft HHRAP Section 3.9.3.1 - The reference to “five percent” is unclear. 

• Draft HHRAP Section 3.10 - The height of the fugitive source is defined as “one-half ...” 

• The heading of Draft HHRAP Table 3-1 and the text on page3-20 are inconsistent (Chapter 3, page 

3-19). 

• There is a typo on Draft HHRAP page 3-21. “If particle density is held constant (such as 1 g/m3

)” 

should read “1 g/cm3.” 

EPA Response: 

Editorial changes where made as suggested above. 

2.4 EXPOSURE SCENARIO IDENTIFICATION [draft HHRAP Section 4] 

To conduct a risk assessment, the appropriate exposure scenarios must be identified. An exposure 

scenario is defined as a collection of exposure pathways to which a single, hypothetical receptor may be 

subjected. Common human exposure pathways for hazardous waste combustion facilities include 

inhalation of contaminants directly from the air and ingestion of contaminated media, such as water, soil, 

or food. 
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Comments regarding exposure scenario identification were categorized into: 

• General Issues; 

• Foodstuff Ingestion; 

• Acute Risk Assessment; 

• Selecting Exposure Scenario Locations; 

• Special Populations; 

• Land Use; and 

• Clarification / Editorial. 

2.4.1 Exposure Scenario Identification – General Issues

General issues include: 

• Identification of realistic receptors; and 

• Inclusion of all potentially exposed populations. 

Issue: General - Identification of Realistic Receptors (See RC Section 3.4.4) 

Commenters suggested that it is unrealistic to require a new or existing combustor to quantitatively 

evaluate in the risk assessment the subsistence farmer, the subsistence farmer child, the resident, the 

resident child, the subsistence fisher, the subsistence fisher child, and the acute risk scenario. A more 

realistic approach might be to characterize risk for actual or hypothetical rural residents or farmers who 

consume some home-produced food, but who are not actually subsistence farmers. A more realistic risk 

assessment can identify the most affected locations and the types of people most likely to be affected at 

those locations. 

EPA Response: 

Review of this issue during the external peer review concluded that the subsistence terminology is not 

consistent with the actual mass per day amounts of ingested food items evaluated in the recommended 

exposure pathways. The daily consumption amounts associated with the farmer scenario, which help to 

define the scenario, are more comparable to reasonable versus subsistence amounts. The HHRAP has 

been revised to remove references to "subsistence” farming. 

Issue: General - Inclusion of All Potentially Exposed Populations 

Additional guidance and details were requested regarding: 

• Information that should be gathered during the evaluation of the receptors of concern. It was 

recommended that there be specific requirements for the risk assessment to recognize and evaluate 

all potentially exposed populations, including sensitive populations. 

• When and how the subsistence fisher and subsistence fisher child scenarios should be evaluated, 

what documentation should be relied on for the presence of subsistence fishers, and how the results 

must be assessed. 

• Ensuring comprehensiveness by acknowledging the importance of including scenarios that may be 

unique to specific populations, such as Native Americans. 
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• The draft HHRAP statement that workers from the facility under direct evaluation in the risk 

assessment are excluded in most cases because there are other guidance and regulations for 

occupational exposures to hazardous waste and hazardous waste combustion emissions within the 

facility boundary (e.g., OSHA). The statement in the draft HHRAP (Section 4.1, page 4-3, 1st 

paragraph, last sentence) that “there might be some instances (e.g., acute risk) where worker 

exposure at nearby facilities or commercial areas within the assessment area are considered within 

the risk assessment” might imply that a requirement could exist to directly evaluate workers in the 

risk assessment in some undefined cases. It would be helpful to define these cases or crossreference to another section that does this, and to define when workers are explicitly excluded 

because they are covered by other guidance and regulations. 

EPA Response: 

The HHRAP is guidance and contains no requirements. Whether or not the fisher pathway should be 

evaluated in a risk assessment is a risk management decision. Site-specific data, such as fish 

consumption rates for Native American populations, is one of many considerations, which may need to be 

addressed. Sections 4.1.3 and 4.2 have been written to allow and promote this kind of evaluation. We’ve 

further clarified our recommendations regarding when to include or not include a worker scenario in a 

risk assessment. 

2.4.2 Exposure Scenario Identification - Foodstuff Ingestion

Issues pertaining to foodstuff ingestion include: 

• Amount and kinds of food ingested; and 

• Calculation of methyl mercury intake. 

Issue: Foodstuff Ingestion - Amount and Kinds of Food Ingested 

It was recommended that risk assessors be allowed to prepare a realistic, site-specific risk assessment that 

does not include hypothetical worst-case receptors. For example, the draft HHRAP provides examples of 

food consumption factors, but these values are actually higher than those that can be found in the EPA 

Exposure Factors Handbook (U.S. EPA., 1997c). If farmers are living within the vicinity of the 

incinerator, the farmers do not necessarily obtain all of their food from the farm. 

EPA Response: 

We’ve further clarified our recommended process for estimating consumption rates. These rates are 

average, rather than worst case rates. Nevertheless, the HHRAP is guidance and contains no 

requirements. As with many parameters and equations provided in the HHRAP, the scientific basis and 

reference are provided. A facility is then able to evaluate whether to propose and justify to the permitting 

authority an alternative they consider more appropriate than that recommended in the HHRAP. Sitespecific data, along with supporting documentation, can always be submitted to the permitting authority 

for further consideration. 
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Issue: Foodstuff Ingestion - Calculation of Methyl mercury Intake (See RC Section 3.5.2) 

The approach to calculating methylmercury ingestion was questioned. The draft HHRAP requires that a 

risk assessment assume that 2% of soil-deposited divalent Hg is methylated and that 15% of the steadystate Hg concentration estimated for surface water bodies is methylated. A bioaccumulation factor (BAF) 

of 6,800,000 must be used for methylmercury to estimate Hg concentrations in fish. Uptake into fish of 

elemental mercury or divalent mercury is not considered. Exposure through fish consumption is then 

evaluated for a hypothetical subsistence fisher. It was suggested that the default values are too high, the 

directions for use of the BAF are incorrect, and the fish consumption pattern is incorrect. 

EPA Response: 

We re-evaluated the environmental fate & transport of mercury, including methylation of divalent 

mercury. The external peer review also addressed these subjects, and the HHRAP was modified per peer 

comments. However, the particular items mentioned in the comment were not changed. As with many 

parameters and equations provided in the HHRAP, the scientific basis and reference are provided. A 

facility is then able to evaluate whether to propose and justify to the permitting authority an alternative 

they consider more appropriate than that recommended in the HHRAP. 

2.4.3 Exposure Scenario Identification - Acute Risk Assessment

Issues pertaining to assessment of acute risks include: 

• Including acute risk exposure assessment; 

• Availability and selection of criteria for acute exposure; 

• Calculating acute and long-term fugitive emissions; 

• Health effects from short-term exposures; 

• Particulate Matter Inhalation; 

• Cumulative risk calculation; and 

• Site workers. 

Issue: Acute Risk Assessment - Including Acute Risk Exposure Assessment (See RC Section 3.4.1) 

One commenter lauded including assessment of non-cancer health risks posed by brief (e.g., one-hour) 

inhalation exposures to airborne chemicals. Another commenter suggested that a careful, independent 

approach to acute health risk assessment should be developed. 

Other commenters questioned the utility of estimating acute risks for the following reasons: 

• Since the acute risk scenario evaluates the effects of direct inhalation of vapors and particles from 

short-term exposures, it is most likely to result in an extremely small lifetime exposure. 

• Acute standards are not available for all chemicals, and many of the available acute standards are 

controversial. 

• Acute toxicity standards are generally much higher than the corresponding chronic RfDs or RfCs. 

• Since it is unclear how this information would be used in permit decision-making, evaluating this 

scenario would increase the level of effort while making only a questionable contribution to the 

overall evaluation of risks at any facility. 
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• Acute exposures can be substantially overestimated, based on the methods shown. The guidance 

should also allow alternative methods. 

EPA Response: 

Comments noted. The issue of evaluating acute toxicity was addressed during the external peer review, 

and we’ve revised the HHRAP, taking into account the most recent science in the area of acute risk 

assessment. As with many parameters and equations provided in the HHRAP, the scientific basis and 

reference is provided. A facility is then able to evaluate whether to propose and justify to the permitting 

authority an alternative they consider more appropriate than that recommended in the HHRAP. 

Issue: Acute Risk Assessment - Availability and Selection of Criteria for Acute Exposure 

(See RC Section 3.4.1) 

The draft HHRAP states that "no one single organization or methodology has developed acute values or 

benchmarks for all of the potential compounds of concern." The same can also be said of chronic toxicity 

data, so there is no point in making this statement. The acute inhalation exposure criteria, Acute 

Exposure Guideline Levels (AEGLs) and Emergency Response Planning Guides (ERPGs), are only 

available for a limited number of compounds. Their derivation is also highly controversial in the 

scientific community, and many of the published values are the subject of ongoing litigation with 

USEPA. Therefore, due to the conservative nature of acute inhalation exposure criteria, the acute hazard 

quotient may significantly overestimate hazards associated with acute inhalation exposure to stack or 

fugitive emissions. 

With regard to the recommended use of AEGL, ERPG, ATEL, Temporary Emergency Exposure Limit 

(TEEL), and SCAPA toxicity-based approach values (in order of preference) as acute toxicity 

benchmarks for evaluating acute hazards from emissions from hazardous waste incinerators, it was 

suggested that AEGL, ERPG, TEEL, and SCAPA toxicity-based approach values are inappropriate. 

Noting that they were developed for use in emergency response situations involving accidental chemical 

releases, the assumptions on which these benchmarks are based (i.e., that an individual will be exposed 

for only a one-hour period followed by a recovery period of two to four weeks and that there are no 

background exposures) are inappropriate. ATEL criteria (also called Acute Reference Exposure Levels) 

were recommended as the most appropriate for evaluating acute hazards from emissions from hazardous 

waste incinerators which can occur for a longer duration than one hour and more frequently than every 

two weeks. 

It was noted that criteria cited in the draft HHRAP are out of date, and suggested that since all sources of 

the proposed Acute Inhalation Exposure Criteria (AIEC) continually develop and revise acute exposure 

criteria, the HHRAP should explicitly state that risk assessors should obtain the most current values. 

It was also suggested that evaluating a smaller number of constituents in more depth would be better than 

including the large number of constituents for which there is limited toxicity data. 
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EPA Response: 

The state of development of acute toxicity benchmarks may not be known as well to most of the risk 

assessor users as it is to the person providing the comment and therefore, the sentence remains in the 

HHRAP. 

The issue of the most appropriate criteria was addressed during the external peer review, and we’ve 

revised the HHRAP, taking into account the most recent science in the area of acute risk assessment. As 

with many parameters and equations provided in the HHRAP, the scientific basis and reference are 

provided. A facility is then able to evaluate whether to propose and justify to the permitting authority an 

alternative they consider more appropriate than that recommended in the HHRAP. 

Regarding the suggestion to limit the assessment to a smaller number of constituents, given that the fate, 

transport, and toxicity information is provided in the HHRAP, we believe it is more efficient to run the 

selected COPCs through the risk assessment process, rather than go through the arduous process of 

justifying elimination of COPCs from the assessment. 

Issue: Acute Risk Assessment - Calculating Acute and Long-term Fugitive Emissions 

The suggestion was made that acute and long-term fugitive emission rates for components handling 

dedicated, invariable waste streams be calculated and speciated identically. Components handling 

multiple or variable waste streams require different calculations for acute and long-term speciated fugitive 

emission rates. A long-term fugitive emission rate should be speciated according to the volume-weighted 

average annual composition of waste passing through a fugitive component. An acute fugitive emission 

rate should be speciated according to the most "conservative" (with regard to toxicity values and 

volatility) waste stream handled by the fugitive component. 

EPA Response: 

Calculating acute emission rates for fugitive emissions is beyond the scope of the HHRAP. 

Issue: Acute Risk Assessment - Health Effects from Short-term Exposures (See RC Section 3.4.1) 

It was suggested that the term "acute effects” is ambiguous. It could mean either short-term effects from 

short-term exposures, or the effects that result from acute exposures since it is possible for short-term 

exposures to induce long-term and/or irreversible effects in addition to immediate, reversible effects. It 

was recommended that a more robust explanation of the range of potential effects of concern due to brief 

exposure be included in the HHRAP. 

EPA Response: 

This is beyond the scope of the HHRAP.

Issue: Acute Risk Assessment - Particulate Matter Inhalation 

Is particulate matter (e.g., PM10 or PM2.5) to be assessed with respect to one-hour inhalation exposures? 

If so, EPA should be aware that none of the sources of AIEC include particulate matter, nor does 

Occupational Safety and Health Administration (OSHA), National Institute of Occupational Safety and 

Health (NIOSH), or (ACGIH) have a relevant short-term criterion. 
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EPA Response: 

Acute exposure to PM10 and PM2.5 in not addressed in the HHRAP. However, speciated particulate 

matter is evaluated with respect to one-hour exposures. The HHRAP has been revised taking into 

account the most recent science in the area of acute risk assessment. As with many parameters and 

equations provided in the HHRAP, the scientific basis and reference are provided. A facility is then able 

to evaluate whether to propose and justify to the permitting authority an alternative they consider more 

appropriate than that recommended in the HHRAP. 

 Issue: Acute Risk Assessment - Cumulative Risk Calculation 

Additional information was requested regarding the calculation of an acute hazard index, noting that the 

draft HHRAP indicates that a COPC’s acute hazard quotient should be compared to a target level of 1. 

Does this mean that for the acute analysis, the COPCs should be evaluated separately and not summed to 

obtain an acute hazard index, keeping in mind that if the acute exposure results in injury to a particular 

organ, then evaluating the COPCs separately may underestimate risk from acute exposure? 

EPA Response: 

This is not part of the HHRAP, but rather part of the U.S. EPA Region 6 Risk Management Addendum 

(July 1998; http://www.epa.gov/earth1r6/6pd/rcra_c/protocol/r6add.pdf). 

 Issue: Acute Risk Assessment - Site Workers

Clarification was requested regarding whether non-facility workers who work on or adjacent to a site 

where a combustion facility is located or will be located should be evaluated in the acute assessment. 

EPA Response

We’ve further clarified our recommendations regarding when and when not to assess workers in a risk 

assessment. In addition, please remember that the HHRAP is guidance and contains no requirements. As 

with many parameters and equations provided in the HHRAP, the scientific basis and reference are 

provided. A facility is then able to evaluate whether to propose and justify to the permitting authority an 

alternative they consider more appropriate than that recommended in the HHRAP. 

2.4.4 Exposure Scenario Identification – Selecting Exposure Scenario Locations

Issues pertaining to selecting exposure scenario locations include: 

• Use of a Cartesian grid to model receptor exposure; 

• Defining the radius of evaluation; 

• Calculation of single point exposures; and 

• Cumulative risks from multiple stacks. 

Issue: Selecting Exposure Scenario Locations – Receptor Grid 

Issues regarding receptor grids include: 

• The computer requirements to meet the needs of the model based on a Cartesian grid of 100 meter 

spacing from the stack to 3 km from the stack and of 500 meter spacing from 3 km to 10 km from 
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the stack are prohibitive and excessive, requiring too much computer time to evaluate. The receptor 

grid specified in previous guidance, which resulted in more than 2000 nodes seemed more than 

adequate. 

• It was suggested that there may be more than one way to design a receptor grid, depending on the 

location of sources and receptors, and a statement should be added to the HHRAP to allow for 

alternative receptor grid node arrays if approved by appropriate permitting agencies. 

• Clarification was requested regarding how receptor locations are to be defined. It is assumed that 

the selection of receptor locations is unchanged from earlier versions of guidance in that, based on 

land use evaluations, actual or reasonable future receptors are located and then maximum model 

deposition rates or air concentrations within that area are identified. However, this is not clearly 

stated in the draft HHRAP. 

EPA Response: 

We strongly disagree that computer run time is a factor. Since the time of this comment, the computer 

clock speed has greatly improved. Also, runs can be broken up and run on multiple computers at once. 

Currently, using actual terrain data, runs are being completed in less than 8 hours. We agree with the 

recommendation that alternate receptor grid node arrays may be appropriate. Selection of receptor 

locations is a site-specific decision. Default selection guidance of receptor locations is presented in 

Section 4.3 of the HHRAP. 

Issue: Selecting Exposure Scenario Locations - Defining Radius of Evaluation 

It was suggested that by recommending in the draft HHRAP that the exposure setting be defined by a 50-

km radius, many water bodies and properties that would not be impacted by emissions from a facility 

could be included. A radius of 3 to 10 km represents an adequate area in which to evaluate any emissions 

from a combustor facility. Evaluating a 50-km radius would significantly increase the level of effort 

without any effect on the risk assessment outcome. 

Further clarification was requested regarding facilities on large controlled sites, such as a DOE site that 

may be as large as 800 km2

, about whether it would be appropriate to further extend the limit of the 

ISCST3 modeled receptor grid node array to reflect the fact that areas that could be assumed to be 

residential may be more distant. 

EPA Response: 

The draft HHRAP states that efforts should be focused within a 3-km radius of the facility. However, 

when drinking water sources or the closest farm or water body fall outside of this radius, it may be 

essential to evaluate these potential exposure pathways and/or receptors. In response to the request that 

a larger radius be used for a controlled site, please note that the recommendation in the guidance is for a 

minimum of 10-km. Also recommended in the guidance, depending on site-specific conditions, the 

potential future land use of a currently controlled site may affect numerous choices in the risk assessment, 

including the size of the ISCST3 receptor grid. 

Issue: Selecting Exposure Scenario Locations - Single Point Exposures 

It was suggested that air quality impacts are too focused on maximum single-point exposures, and should 

instead rely on a distribution. 
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EPA Response: 

EPA Guidance for probabilistic risk assessments, “Guiding Principles for Monte Carlo Analysis” (U.S. 

EPA, 1997d, EPA/630/R-97/001), recommends a deterministic evaluation be performed at the screening 

level, which is consistent with the HHRAP methodology. Also, concentration difference at grid nodes 

spaces 500 meters apart have been shown to have little variation. Distributions can be used. However, 

the EPA guidance states that they should be used along side deterministic evaluations. Practical 

implementation of the HHRAP has shown that distributions are of more use in evaluating uncertainty of a 

few key parameters in a sensitivity analysis after completing the deterministic analysis. 

Issue: Selecting Exposure Scenario Locations - Multiple Stacks 

The validity of considering cumulative emissions from multiple stacks was questioned. It was suggested 

that to assume risks are cumulative requires that the receptor be the same for all stacks. Because this is 

not the case, each stack should be assessed separately, not cumulatively. 

EPA Response: 

If a facility has multiple stacks, the HHRAP recommends performing air dispersion modeling for each 

stack separately. This modeling generates air concentrations and deposition rates at various points 

(potential receptor locations) within the study area for each stack (or source) separately. When choosing 

exposure scenario locations, the HHRAP recommends identifying the receptor locations with the highest 

air concentrations and deposition rates for each source separately, as well as for all sources combined. 

We agree that the receptor location of maximum exposure for one stack is not necessarily the receptor 

location of maximum exposure for another stack, even if both stacks are at the same facility. That is one 

reason it is valuable to assess each source separately. As discussed further in the HHRAP, this may lead 

to assessing risk at multiple locations. 

However, any one location is impacted to a greater or lesser extent by all sources. It is therefore 

appropriate to also assess locations by summing the air concentration and deposition rate contributions 

of all sources to each location. Because concentration and deposition are defined with multiple 

parameters (e.g., wet vapor deposition, dry vapor deposition), each with its own point of highest impact, 

this may lead to assessing multiple receptor locations. Please see the HHRAP for further discussion. 

2.4.5 Exposure Scenario Identification - Special Subpopulations

Issues regarding characterization of risks to sensitive subpopulations include: 

• Assessing risks to sensitive subpopulations; 

• Describing the subsistence farmer; 

• Ingestion of breast milk; and 

• Identification of sensitive subpopulations. 
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Issue: Special Subpopulations - Assessing Risks to Sensitive Subpopulations 

The requirement that "the risk assessment may need to directly address special subpopulations in 

impacted areas because of characteristics of the exposure setting or to address specific community 

concerns." was considered vague and unnecessary, because the assumptions specified in the draft HHRAP 

have been developed to protect human health, including special subpopulations. 

EPA Response: 

The HHRAP is guidance and contains no requirements, although the evaluation of sensitive individuals 

may be warranted, depending on site-specific conditions. We made the recommendation because we 

consider it wise to at least consider the issue of special subpopulations. 

Issue: Special Subpopulations - Subsistence Farmer Description 

It was recommended that a clear description of the subsistence farmer be provided because this class, or 

their children, will probably drive the assessment for many compounds. Such factors as the likelihood of 

the farm resident consuming products from a combination of species would be influenced by choice. A 

hypothetical construct for the subsistence farmer might be worthwhile to reduce misunderstanding and to 

provide a degree of uniformity among assessments. 

EPA Response: 

Review of this issue during the external peer review concluded that the daily consumption amounts 

associated with the farmer scenario, which help to define the scenario, are more comparable to 

reasonable versus subsistence amounts. The HHRAP has been revised to remove references to 

”subsistence” farming. We’ve also further clarified the definitions of all recommended exposure 

scenarios, including the farmer and farmer child. 

Issue: Special Subpopulations - Ingestion of Breast Milk 

It was recommended that a discussion about the relevant aspects concerning the ingestion of breast milk 

exposure pathway be included in the HHRAP as a separate section in the chapters on exposure scenarios, 

media concentrations, and quantifying exposure. Although it is stated in draft HHRAP Chapter 4 that the 

ingestion of breast milk exposure pathway is evaluated separately in Chapter 2 (Facility Characterization), 

infant exposure to PCDDs and PCDFs via the ingestion of their mother's breast milk is only briefly 

discussed in Chapter 2 and Appendix C. In any event, a full evaluation of breast milk exposures would be 

out of place in Chapter 2. 

EPA Response: 

In addition to discussions in Chapter 2 and Appendix C, the nursing infant exposure pathway is discussed 

in Chapter 4 under the adult exposure scenarios.

Issue: Special Subpopulations - Identification 

In the draft HHRAP on page 4-10, top paragraph from previous page, it is specifically indicated that 

“...special subpopulations in such areas should be identified.” Guidance was requested regarding how 

these subpopulations are to be identified and who is responsible for this. It was also suggested that 

consideration and discussions be included in the HHRAP for subpopulations, such as: 
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• Those whose activities may result in the assessment of additional exposure pathways or adjustment 

of consumption rates to reflect behavioral patterns, such a Native Americans; 

• Those outside the assessment area, such as downstream, that may be using water contaminated in 

the assessment area (draft HHRAP Section 4.1.2 Water Bodies and Their Associated Watersheds, 

page 4-6, line 7 and page 4-7, line 9); 

• Children with asthma; and 

• Those populations significantly exposed to similar types of chemicals from other sources for which 

the risk might be additive. 

EPA Response: 

The HHRAP suggests identifying special subpopulations in the assessment area, based on the location of 

schools, hospitals, nursing homes, day care centers, parks, community activity centers, etc. If available 

information indicates there are children exhibiting pica behavior (defined for risk assessment purposes as 

“an abnormally high soil ingestion rate”) in the assessment area, these children could represent a 

special subpopulation. Fetuses, infants and children, and the elderly are examples of human life stages 

(i.e., subpopulations) that might be more sensitive to COPC exposure. You might consider some Native 

American groups to be a special subpopulation because their focus on fish as a food source increases 

their exposure. Our recommendations in the HHRAP, such as the protective nature of the recommended 

exposure scenarios (see Section 4.2) and the use of RfDs developed to account for toxicity to sensitive 

receptors, also protect the health of special subpopulations. However, we cannot guarantee that all 

subpopulations are covered. You may also need to specifically address special populations that are 

located in impacted areas because of unique characteristics of the exposure setting or to address 

particular community concerns. For example, a day care center or hospital may be located in an area 

that is directly impacted by the facility stack emissions. Receptors at these locations may be especially 

sensitive to the adverse effects and/or the exposure setting may be particularly conducive to exposure. 

2.4.6 Exposure Scenario Identification – Land Use

Issue: Land Use - Identifying Future Land Use 

Although the draft HHRAP requires that the risk assessment be performed for all six required receptors 

and perhaps additional receptors at both current and future exposure scenario locations, it was noted that 

the draft HHRAP does not provide explicit guidance regarding future land use designations. For current 

land use, local data sources can be consulted to determine areas that are residential or agricultural and 

water bodies that are frequented by fishers. However, for future land use, considerable professional 

judgment is required. For example, should future use based solely on zoning be addressed or should only 

the future use that is planned, funded, etc., be included? The extent to which future scenarios should be 

evaluated should also be clarified. 

EPA Response: 

The HHRAP is guidance and contains no requirements. Determining future land use is a site-specific 

decision. We provide potential ways of gathering information to address this issue in Section 4.1.1 of the 

HHRAP. 
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2.4.7 Exposure Scenario Identification – Clarification / Editorial

Issue: Clarification - Child Scenario 

Clarification was requested with regard to the terms adult fisher child and adult resident child scenarios. 

The terms should be corrected to be "child of the adult fisher" or “child of the adult resident.” It was the 

reviewers’ understanding that the child scenario in the draft HHRAP is not the adult fisher/resident as a 

child (as indicated by the text where the child is described as the "adult resident child scenario"). If so, 

then it would only be appropriate to add the cancer risk of the child to that of the adult to obtain a 

cumulative lifetime cancer risk. Rather, the child scenario is evaluated separately, as indicated earlier in 

the text, to address increasing concerns regarding infant and child exposures. It was recommended that 

this issue should be clarified by indicating that the child is the child of the adult fisher/resident and not the 

adult fisher/resident child scenario. 

EPA Response: 

Terms utilized in the HHRAP have been changed to farmer child, fisher child, and resident child. We 

have further clarified their definitions as the child member of a farmer, fisher, or resident family. 

Issue: Clarification - Particulates

There are inconsistencies in the draft HHRAP with regard to particulates and acute inhalation risks. For 

instance, Section 7.5 refers to "vapor phase and particle phase COPCs," whereas section 4.2.7 refers to 

"vapors and particles" and Table 4-1 refers to "vapors and particulates." If EPA simply means the total 

vapor and particle-bound concentrations of specific chemicals, then the wording should be clarified. 

EPA Response: 

We’ve made the terms consistent in the HHRAP sections noted. 

Issue: Clarification - Arbitrary Safety Factors

A reference in the draft HHRAP to the use of “arbitrary safety factors” is unclear and requires 

clarification. 

EPA Response: 

Reference to the use of arbitrary safety factors has been removed from the HHRAP. 

Issue: Editorial 

• Draft HHRAP Table 4-1 Recommended Exposure Scenarios for Evaluation in a Human Health 

Risk Assessment: By including the "c" footnote, this table indicates that the ingestion of breast milk 

pathway should be included for all of the child scenarios. This appears to be a typographical error, 

as a child would not be able to breast-feed an infant. 

• Draft HHRAP Section 4.2 Recommended Exposure Scenarios: The word "conducive" used in the 

fourth sentence of the second paragraph should be replaced. 

EPA Response: 

Comments are noted. We have reviewed editorial errors and made corrections to the HHRAP, where 

applicable. 
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2.5 ESTIMATING MEDIA CONCENTRATIONS [draft HHRAP Section 5] 

To complete the exposure assessments described in Section 4 of the draft HHRAP, concentrations of 

COPCs in relevant media must be determined. These media include air, soil, produce, beef and dairy 

products, pork, chicken and eggs, and drinking water and fish. In Chapter 5 of the draft HHRAP, EPA 

provides guidance on equations for estimating COPC concentrations in these media, as well as on 

recommended parameter values for these equations. 

An extensive range of comments was received regarding the equations and parameters. These comments 

were categorized into: 

• General Modeling Issues; 

• Recommended Input Values for Equation Parameters; 

• Equations for Estimating COPC Levels in Various Media; 

• Conservation of Mass; 

• Screening of COPC; and 

• Clarification / Editorial. 

2.5.1 General Modeling Issues

General modeling issues included: 

• Model Validation; 

• Use of Site-Specific Models; 

• Climate Issues; 

• Approaches for Cancer versus Non-cancer Effects; 

• Modeling of Water Bodies; 

• Incorporation of Metabolism into Models; and 

• Development of New Models. 

Issue: General - Model Validation 

Air: It was noted that EPA builds upon the air dispersion and deposition modeling (draft HHRAP 

Chapter 3) to estimate COPC concentrations in various environmental media that people might contact. 

By necessity, EPA relies on a series of sequential (sometimes linked) models, which are a combination of 

theoretical and empirical algorithms. These models remain largely unvalidated. In reviewing similar 

models for the Hazardous Waste Identification Rule, EPA's Science Advisory Board (SAB) recognized 

the crucial need for model validation. Commenters recommended that EPA act on the SAB's 

recommendation and develop an internal, multidisciplinary effort to validate multi-pathway transport 

models where possible, and then apply the models consistently across the various regulatory programs. 

Water: It was noted that the water body modeling should be validated because the models are highly 

theoretical. The assumed equilibrium in the distribution of COPCs between the water column and 

sediment layer has not been subject to verification, but is critical to the accuracy of model predictions. 

While the processes are modeled as steady-state, they are actually time-dependent, varying by season and 
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individual storm event. The consequences of implicitly averaging this time-dependence are not known. 

It is recommended that EPA examine all aspects of the water body modeling against empirical data, 

where possible. 

Beef and Dairy: It was stated that the models used to estimate COPC levels in beef and dairy products are 

based on flawed assumptions. 

 

EPA Response: 

Air: Comment noted. 

Water: Comment noted. Water body modeling has been reviewed by EPA’s Office of Research and 

Development, which has commented that the modeling as given is appropriate. As with many parameters 

and equations provided in the HHRAP, the scientific basis and reference are provided. A facility is then 

able to evaluate whether to propose and justify to the permitting authority an alternative they consider 

more appropriate than that recommended in the HHRAP. 

Beef and Dairy: All equations have been reviewed and updated as appropriate. As with many parameters 

and equations provided in the HHRAP, the scientific basis and reference are provided. A facility is then 

able to evaluate whether to propose and justify to the permitting authority an alternative they consider 

more appropriate than that recommended in the HHRAP. 

 

Issue: General - Use of Site Specific Models 

It was recommended that to assess infiltration rates, the HHRAP should allow for site-specific models to 

determine infiltration. Such models have been used for this purpose in other regulatory programs. In 

addition, resuspension of particulates and revolatilization of volatile and semi-volatile COPCs are not 

adequately addressed and need to be reassessed. 

EPA Response: 

We agree. As with many parameters and equations provided in the HHRAP, the scientific basis and 

reference are provided. A facility is then able to evaluate whether to propose and justify to the permitting 

authority an alternative they consider more appropriate than that recommended in the HHRAP. We 

agree that resuspension of particulates and revolatilization of volatile and semi-volatile chemicals could 

be important on a case-by-case basis, and have given the HHRAP the flexibility to include addressing 

them in a risk assessment on a site-specific basis. 

Issue: General - Climate Issues 

It was noted that EPA has not addressed climatic differences that impact many of the model parameters. 

While EPA has provided some equations in which site-specific values can be inserted, these are relatively 

few and the impact of such substitution is unclear. It is recommended that EPA conduct sensitivity 

analyses on these equations and simplify, where possible. 

In addition, much of the exposure via the water-related pathways appears to be dependent on a large 

number of parameters that do not distinguish between warm (e.g., Texas) and cold (e.g., northern 

Minnesota) climates. In considering deposition to surface water as well as issues such as dilution, EPA 
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should examine whether it has accounted for the different climatic conditions in the country that could 

lead to seasonal variations that could affect many parameters, including direct deposition. 

It was suggested that in draft HHRAP Section 5.7.1.2, page 5-66, use of an annual average water body 

temperature of 250

 C is unduly high. The annual average water temperature in most parts of the country 

is more likely to be 100 C to 150 C. 

Vapor phase diffusion (dry deposition) should be affected by winter conditions. Temperature has a direct 

effect on this parameter, but the assumption is made that all surface water bodies have a temperature of 

25° C (or approximately 78° F).

EPA Response: 

Site-specific data are not only allowed, but also encouraged, throughout the HHRAP. If the air and 

terrestrial fate and transport models in HHRAP are applied properly, they should account for differences 

among regions with varying climates. Concentrations and exposure in the HHRAP are based on longterm equilibrium assumptions. Variations in results based on seasonal effects are accounted for with 

site-specific inputs to the air and terrestrial fate and transport models. Equations have been simplified 

where possible. However, default values provided for many parameters could be replaced by site-specific 

information, if available. Sensitivity analyses are encouraged on a site-specific basis. As with many 

parameters and equations provided in the HHRAP, the scientific basis and reference are provided. A 

facility is then able to evaluate whether to propose and justify to the permitting authority an alternative 

they consider more appropriate than that recommended in the HHRAP. EPA also notes that deposition in 

various weather conditions is accounted for in the air model. 

The water body temperature used to calculate vapor phase COPC diffusion (dry deposition) load to a 

water body can be site-specifically added in a calculation. A default value of 25 C was recommended to 

be consistent with previous EPA guidance. 

Dry deposition of vapor was re-evaluated during the external peer review, and alternative default 

deposition velocities have been provided. 

Issue: General - Approaches for Cancer versus Non-cancer Effects 

The draft HHRAP states that separate soil concentrations are required for carcinogenic chemicals and 

non-carcinogenic chemicals, because averaging chemical dose over the lifetime assesses carcinogenic 

risks and non-carcinogenic effects are assessed by the use of a Reference Dose. The last year of the 

combustor's operation must be used for the non-carcinogenic soil concentration, because this yields the 

highest chemical concentrations. This requirement will result in two entirely separate risk assessments 

with double calculations for all exposure pathways that have soil concentration as an input, including soil 

ingestion, produce ingestion, beef ingestion, milk ingestion, pork ingestion, poultry ingestion, egg 

ingestion, and fish ingestion. 

In addition, in the section “Calculating Cumulative Soil Concentration,” EPA states, "non-carcinogenic 

COPCs are based on a reference dose rather than a lifetime exposure." Chronic reference doses are 

specifically derived to be protective of long-term exposures (7 years to lifetime for chronic). It is 
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recommended that the sentence be corrected to indicate that the hazard quotient associated with noncarcinogenic effects is based on threshold and not as currently stated. 

EPA Response: 

A detailed evaluation of the soil concentration equations has been conducted by EPA’s Office of Research 

and Development and is available upon request, but is beyond the scope of the HHRAP. Also, as with 

many parameters and equations provided in the HHRAP, the scientific basis and reference are provided. 

A facility is then able to evaluate whether to propose and justify to the permitting authority an alternative 

they consider more appropriate than that recommended in the HHRAP. For the sentence regarding 

hazard quotients, we agree and have corrected the sentence.

Issue: General - Modeling of Water Bodies 

For the modeling of water bodies, commenters noted that partitioning of organics should use the total 

organic carbon concentration in the water column, rather than just the suspended fraction, because 

organics will sorb onto both soluble and particulate carbon in the water column and be washed out of the 

system. 

EPA Response: 

Comment noted. Water body modeling has been reviewed by EPA’s Office of Research and Development, 

which has commented that the modeling is appropriate as given. As with many parameters and equations 

provided in the HHRAP, the scientific basis and reference are provided. A facility is then able to evaluate 

whether to propose and justify to the permitting authority an alternative they consider more appropriate 

than that recommended in the HHRAP. 

Issue: General - Incorporation of Metabolism into Models 

It was stated that relevant information may be overlooked in favor of the application of uniform 

procedures among chemicals (e.g., lack of consideration of metabolism of COPCs, such as PAHs). It was 

recommended that empirical checking of model estimates be applied to as many COPCs as possible, 

especially those predicted to be risk drivers, and that metabolism factors should be established for COPCs 

based on available pharmacokinetic data. 

Phthalate metabolism: Whether the metabolism factor (MF) for bis(2-ethylhexyl)phthalate should be 

applied to other phthalates, such as di-n-octyl phthalate, was questioned. 

EPA Response: 

EPA’s Office of Solid Waste has endeavored to empirically check the model, including PAH and 

phthalate metabolism. Some changes have been made to the HHRAP in response to these efforts. The 

HHRAP, however, is only guidance and is meant to help provide a documented consistent approach to 

risk assessment. As with many parameters and equations provided in the HHRAP, the scientific basis and 

reference are provided. A facility is then able to evaluate whether to propose and justify to the permitting 

authority an alternative they consider more appropriate than that recommended in the HHRAP. 

 

No MF for di-n-octyl phthalate is given in the HHRAP. 
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Issue: General - Development of New Models 

EPA was urged to consider focusing its resources on coordinated development of the TRIM Multipathway Model and to coordinate and build on the existing expertise and evolving approaches to multipathway modeling. 

EPA Response: 

Comment noted. 

Issue: General - Animal Feed 

It was recommended that the terms used for animal feeds be defined consistent with those normally used 

in agriculture. Such factors as which feeds are used would be influenced by choice. The use of the 

terms “forage” and “silage” in the draft HHRAP do not appear to be consistent with the definitions used 

in the animal production industry and in academic animal science. “Grain” is satisfactory, but the more 

usual term is “concentrate.” Swine are not fed silage and this plant type should be removed from Section 

5.5.1.2. Realistically, ground grains with the appropriate protein, mineral, and vitamin supplements are 

the only feeds used for swine and poultry. As noted for concentrates in cattle, it is likely that these feeds 

would be obtained from a source off-site for most small operators. 

EPA Response: 

The terms “forage,” “silage,” and “grain” were adopted to be consistent with past usage throughout 

EPA guidance documents. The feeding of silage to hogs is documented in the literature and referenced in 

the HHRAP. 

Issue: General - Cattle Exposure Pathways 

It was recommended that the HHRAP should indicate whether inhalation and water consumption are 

considered significant sources of exposure of contaminants to cattle (draft HHRAP page 5-18). If not, 

then the basis of this should be stated. If so, then a cross-reference should be given to the specific section 

where this is discussed. 

EPA Response: 

This issue was put before the external peer review panel, which concluded these exposure pathways for 

cattle were not significant. The reasoning presented by the panel was that: 1) for water, COPC’s, which 

would tend to bioaccumulate would not be in the water column, but rather be in the sediments, and 2) for 

air, direct concentrations inhaled and not bioaccumulated in the food chain would be insignificant. 

Issue: General - Produce Exposure Pathway 

It was recommended that providing a reference indicating the basis for root uptake of COPCs to be the 

primary mechanism through which aboveground-protected produce becomes contaminated, as indicated 

in Section 5.3.1 Aboveground Produce Concentration Due to Direct Deposition (Pd), page 5-25. 

EPA Response: 

We’ve modified the HHRAP to reference the source for this assumption (i.e., “Methodology for Assessing 

Health Risks Associated with Multiple Pathways of Exposure to Combustor Emissions” (U.S. EPA, 

1998d). 
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Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 80 

Issue: General - Soil Loss Constant 

It was noted that in the statement in draft HHRAP Section 5.2.2 Calculating the COPC Soil Loss Constant 

(ks), page 5-9, lines 1-10, “...a first-order loss constant may be adequate to describe the loss of COPC 

from soil...” it is not clear what “may be” means in terms of the error to the overall process. It should be 

indicated here how this assumption affects error on this parameter, and whether there could or could not 

be significant differences if kinetics other than first-order were considered. 

EPA Response: 

What is indicated in the referenced section is that a first order kinetic loss may be adequate to describe 

the loss of COPC from soil at the low concentrations generally associated with hazardous waste 

combustion. Zero order or second order kinetics may be more appropriate on a site-specific basis. 

References for this assumption are provided in the HHRAP. 

2.5.2 Recommended Input Values for Equation Parameters

Commenters provided input on many of the parameters for the equations given in Chapter 5 of the draft 

HHRAP. Some commenters recommended specific values for parameters, while others recommended 

that EPA make more use of site-specific information rather than focusing on default values. The 

comments on site-specificity are discussed first. Next, parameter-specific comments are presented. 

Issue: Input Values - Feed Intake 

It was noted that the discrepancy in feed intake values used for various animal species has arisen because 

values that were reported in individual research studies were selected for application on a more general 

basis. Feed intake is a complex function that involves animal size, growth rate, productivity (milk or 

eggs), energy, density of the feed, and ambient temperature (National Research Council, 1987). It might 

be useful to establish “standard animals” for each species. This would ensure uniformity in the factors 

used for converting data in the literature to the form used in the assessment. The National Research 

Council publication provides a comprehensive review the feed intake literature for all species of interest, 

and it should be considered the most authoritative document on this subject. 

EPA Response: 

Comment noted. 

Issue: Input Values - Soil Ingestion

It was stated that the soil ingestion values presented in the draft HHRAP should be considered maximum 

values. In practice the values may be considerably less with changes in the assumptions concerning sitespecific conditions and the normal range of animal management practices. 

EPA Response: 

The soil ingestion values for cattle presented in the HHRAP are average values, not maximum values. 

Documentation of how they were derived is presented in the HHRAP such that site-specific conditions 

can be compared to the default assumptions presented. 
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Multimedia Planning and Permitting Division Office of Solid Waste 

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Issue: Input Values - Runoff Loading 

It was noted that in draft HHRAP Section 5.7.1.3 Runoff Load from Impervious Surfaces, page 5-67, 

runoff loading from impervious surfaces (LRI) is assumed to occur with perfect efficiency. While this 

may be appropriate for a surface adjacent to surface water bodies, runoff from more distance surfaces are 

likely to be considerably less efficient. An adjustment for large watersheds, similar to that applied to 

sediment transport, should be considered. 

A correction should be applied to the impervious watershed area to account for such issues as gutters, 

which generally disperse water. 

EPA response: 

The total watershed area that contributes water to a water body can be very extensive relative to the area 

impacted from facility emissions. When evaluating an area, the HHRAP recommends that the risk 

assessor consider evaluating an effective watershed rather than the entire watershed area. The area 

extent of a watershed is generally defined by topographic highs that result in down slope drainage into 

the water body. Many times this is within a few hundred feet of the water body due to the presence of 

dikes or roads. The effective watershed is generally not expansive, and therefore, impervious runoff is 

very efficient. Gutters from roofs can as easily drain to roadways as they do to yards. The estimation of 

the amount of a watershed that is impervious is site-specific, as recommended in the HHRAP.

Issue: Input Values - Specific Parameters 

Issues were raised regarding the following parameters: 

• Untilled mixing; 

• Contact fractions for beef and milk; 

• Fraction of wet deposition that adheres to plant; 

• Water body loadings due to direct deposition; 

• Plant Surface Loss Coefficient (kp); 

• Correction factors for calculating plant uptake to account for the ratio of surface area of skin to 

whole plant; 

• Percentage of plant material consumed by beef; 

• Correction factors for hay, grain, and corn; 

• Default sediment foc values; and 

• Use of VGag and VGrootveg as empirical correction factors for organic uptake. 

EPA response: 

As with many parameters and equations provided in the HHRAP, the scientific basis and reference for 

our recommendations are provided. A facility is then able to evaluate whether to propose and justify to 

the permitting authority an alternative more appropriate than that recommended in the HHRAP. 

Issue: Input Values - Produce Interception Fraction 

EPA’s calculated value for the default interception fraction of 0.39 for the edible portion of produce in the 

draft HHRAP does not appear to be correct. It is recommended that EPA review its calculations. 
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U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

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Algorithms to calculate plant uptake due to particulate deposition include a term for the interception 

fraction of the edible portion of above ground vegetation, Rp. Input values presented in the guidance 

reflect estimations based on an algorithm for pasture grass. No data were presented for above ground 

crops nor were any data presented to indicate whether the value for pasture grass is appropriate for other 

species. 

EPA response: 

Calculations have been verified to be correct. As noted in the HHRAP, one of the primary uncertainties 

associated with this variable is whether the algorithm, based on studies of pasture grass, accurately 

represents aboveground produce. Baes et al. (1984) noted that their approach to developing classspecific Rp values is “at best ad hoc,” but stated that this approach was justified, because the 

consequences of using Rp estimates that are independent of productivity are “serious.”

Issue: Input Values - Bioavailability Factors 

It was recommended that risk assessors be allowed to use appropriately documented bioavailability 

factors. It is further recommended that EPA modify the equation labeling to identify that trial burn 

emission rate Q is unadjusted so that it may not be directly incorporated into the media concentration 

equations.

A default soil bioavailability factor (Bs) of 1 should not be recommended for all COPCs, given that data 

on relative bioavailability are readily available in the open literature for a number of chemicals. In 

addition, the definition of Bs appears to be incorrect. 

The draft HHRAP assumption that the bioavailability of all chemicals is 100% must be changed to 

prevent overestimates of exposure, particularly for nonvolatile compounds.

EPA response: 

We agree with the first comment. 

Soil ingestion is unlikely to be a determining factor driving a risk at a level of concern. Soil 

bioavailability factors for soil ingestion are not well presented in the literature. However, if 

documentation can be provided, facilities can provide data to support use of a chemical specific 

bioavailability factor. The definition for Bs has been changed. 

The assumption of 100% bioavailability is a conservative default assumption. As with many parameters 

and equations provided in the HHRAP, the scientific basis and reference are provided. A facility has the 

option to propose and justify to the permitting authority an alternative they consider more appropriate 

than that recommended in the HHRAP. 

Issue: Input Values - Dioxin Congener Photodegradation 

The HHRAP should consider photodegradation of dioxin congeners on plant surfaces.
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Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 83 

EPA response: 

We do not believe that the amount of dioxins adhered to plant surfaces significantly affects risk estimates, 

but as with other parameters, site-specific values may be applicable.

Issue: Input Values - Soil Loss 

The draft HHRAP concludes that the soil loss due to volatilization constant should be set equal to zero 

until more applicable models are identified and validated. EPA should further evaluate the source of the 

equations originally proposed for calculating the volatilization loss to determine their applicability or 

identify other suitable equations in the literature. In addition, the terms for runoff and leaching are 

inadequate to address total soil loss. There are situations where EPA’s default value of zero for the 

erosion loss rate constant kse is inappropriate. 

EPA response: 

The ksv term has been revised from supporting a default of zero to providing a chemical-specific equation 

to calculate the ksv. The erosion loss rate constant kse is set equal to zero as a default, citing essentially 

equivalent erosion rates onto and off of impacted receptor sites. 

Issue: Input Value – Degradation Rate Constants 

For several COPCs, ksg values (degradation rate constants) are set at zero because of "lack of data.” If 

there are insufficient data in the single reference cited by EPA, EPA should (1) identify data gaps, (2) 

solicit data from interested parties, (3) use degradation data from structurally similar compounds, or (4) 

estimate needed rate constants using available methodology. 

EPA response: 

When a ksg value is not provided, it is assumed to be zero. If a chemical is found to be a risk driver that 

has a ksg equal to zero, a literature search may be warranted. To date, EPA is unaware of any instance 

where this is the case. Using degradation data from structurally similar compounds might be approved 

on a site-specific basis, at the discretion of the permitting authority.

Issue: Input Value - Fish Pathway 

EPA has made a clear demarcation between water column concentrations and benthic sediment 

concentrations. A similar distinction should be made between the type of fish consumed (i.e., a bottom 

feeding fish species is more likely to be affected by sediment content than a non-bottom feeder). 

EPA response: 

Choice of the trophic level of fish eaten is a site-specific determination. The HHRAP provides a 

screening default approach to help focus where site-specific information may be most effective. Bottom 

feeding fish are not necessarily more tied to sediment concentrations than pelagic fish. Fish are tied to 

exposure mostly through their food source. Concentrations of chemicals in the food source will depend 

on which food chain a chemical will travel and the extent of accumulation.

Issue: Input Value - Fish Lipid 

The draft HHRAP requires that a value of 7% for fish lipid be used, which is unreasonably high for warm 

water locations and should not be required input. 
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Multimedia Planning and Permitting Division Office of Solid Waste 

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EPA response:

We disagree. The HHRAP recommends a default value of 7%, but it does not require it. 

Issue: Input Value – Total Suspended Solids (TSS) 

EPA recommends that water body-specific measured total suspended solids (TSS) values be used. No 

recommended default value is provided and, for the majority of water bodies, this value will not be 

readily available. 

EPA response: 

A method to calculate TSS for lakes and ponds is provided in the HHRAP. We disagree that measured 

concentrations are not available for streams and rivers. This is standard data collected by the States and 

USGS.

 2.5.3 Equations for Estimating COPC Levels 

Several comments were received with regard to the equations for estimating media concentrations of 

COPCs given in Chapter 5. Commenters expressed a broad range of concerns, ranging from the scientific 

defensibility of the algorithms presented in Chapter 5, to more specific comments on algorithms related to 

fate and transport. 

Issue: Estimating COPC Levels - Flawed Equations (See RC Sections 3.5.3, 3.4.3 and 3.5.5) 

Commenters recommended that the pork, chicken, and fish algorithms undergo substantial revision, as 

equations for several of the media are flawed. Specifically, it was noted that the soil concentration 

algorithms are not scientifically defensible, and the pork and chicken algorithms are based on multiple, 

flawed assumptions. The approach in the draft HHRAP for estimating COPC concentrations in fish is 

inconsistent with other guidance and scientific literature on chemical accumulation in fish. In addition, it 

was stated that there is no accounting for chemical or biological degradation in the various equations in 

this section. 

EPA Response: 

We disagree with the view that the soil concentration equations are flawed. A detailed evaluation of the 

soil concentration equations has been conducted by EPA’s Office of Research and Development and is 

available upon request, but is beyond the scope of the HHRAP. For the other media mentioned, all 

equations have been reviewed and updated as appropriate. EPA states that degradation is accounted for 

in the soil equations. The assumption of ksv equal to zero has been replaced by an equation which 

calculates ksv. The erosion loss rate constant, kse is set equal to zero as a default citing essentially 

equivalent erosion rates onto, and off of, impacted receptor sites. As with many parameters and 

equations provided in the HHRAP, the scientific basis and reference are provided. A facility is then able 

to evaluate whether to propose and justify to the permitting authority an alternative they consider more 

appropriate than that recommended in the HHRAP. 

Issue: Estimating COPC Levels - Considering COPC Concentration in Water 

It was recommended that consideration be given to normalizing the concentration of TSS by its organic 
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Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 85 

carbon content as well as the dissolved organic carbon content. Both of these parameters have been 

found to be important in predicting the bioaccumulation of hydrophobic organic COPCs, and are 

considered in EPA evaluations of that phenomenon. 

EPA Response: 

The HHRAP recommends site-specific determination of TSS, as this parameter has been found to vary by 

more than two orders of magnitude across water bodies. Site-specific values should account for organic 

carbon content, as well as dissolved organic carbon content. 

Issue: Estimating COPC Levels - Overall Total Water Body Dissipation Rate Constant 

It was suggested that in draft HHRAP Section 5.7.4.2, page 5-75, the overall water body dissipation rate 

(kwt) could include losses due to biotic and abiotic degradation. While it may be reasonable to neglect 

degradation in the water column due to the short residence times implied by the calculations, it is less 

appropriate in aquatic sediments where residence times can be significant and losses due to degradation 

have been documented.

EPA Response: 

Biotic and abiotic degradation in sediments are chemical specific, and highly variable based on sitespecific parameters. It is also unclear whether breakdown products of COPC’s are less toxic than the 

parent compounds. As with all equations and recommended values in the HHRAP, site-specific values 

can be documented and utilized in the risk assessment. 

Issue: Estimating COPC Levels - HHRAP Equations May Result in Increased Risk 

 (See RC Section 3.5.3) 

The following equations included in the draft HHRAP may result in higher concentrations in the relevant 

media and overall increase in risk: 

• A new equation for produce concentration due to root uptake for below ground produce is 

contained in the draft HHRAP which will result in higher concentrations in produce, and therefore, 

in higher risk values. 

• Equations for estimating the COPC concentrations in pork, eggs, and poultry are included. While 

the inclusion of these pathways will increase the final risk, they are not expected to drive the risk 

because their concentrations and associated risks should be less than those resulting from the 

ingestion of the beef and milk pathways.

• Equations for forage, silage, and grain consumption by beef and dairy cattle are included in the 

draft HHRAP. Previous guidance included only consumption of forage. This will result in higher 

concentrations in beef and dairy cattle, and therefore higher risk values.

EPA Response: 

Comments noted. 

Issue: Estimating COPC Levels - Biotransfer Equations (See RC Section 3.5.3) 

Several commenters were concerned that the guidance on biotransfer is flawed. Specifically, it was noted 

that: 
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U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 86 

• EPA has recently questioned the use of the Travis and Arms algorithm for certain highly lipophilic 

compounds. In a recent memorandum, EPA noted that the Travis and Arms (1988) algorithms 

yield increasing biotransfer values with increasing logKow while opposite trends have been reported 

for dioxin congeners with logKow values ranging from 6.5 to 8.0. As a result, EPA recommended 

against using the Travis and Arms algorithms for these highly lipophilic agents. There may be 

several COPCs included in the draft HHRAP with logKow values exceeding 6.5. The use of the 

Travis and Arms algorithms for these chemicals is highly questionable. 

• The correlation algorithms developed by Travis and Arms to calculate biotransfer factors for beef 

and milk involved only 36 chemicals for beef and 28 for milk. The scientific validity of extending 

correlation data from limited data to the large number of potential COPCs is questionable. 

Furthermore, the coefficient of linear regression for the Travis and Arms algorithms was only 0.81 

for beef and 0.74 for milk, suggesting considerable variability. Finally, the default cattle 

consumption rate should not be used in conjunction with the Travis and Arms biotransfer rate. 

• Algorithms to estimate constituent concentrations in beef and milk include the quantity of plant 

material consumed by plant type. EPA recommends use of 1 1.8 kg DW/d for beef cattle and 20.0 

kg DW/d for dairy cattle. These quantities include forage, silage and grain. These algorithms also 

include factors for feed-to-beef biotransfer (Ba beef) and feed-to-milk biotransfer (Ba milk). These 

biotransfer factors were calculated from the correlation algorithms presented in Travis and Arms 

(1988). Travis and Arms developed their correlation algorithms from available bioconcentration 

studies in beef and dairy cattle. To develop their biotransfer factors, Travis and Arms multiplied 

the constituent concentration in feed (mg/kg) by a feed consumption rate (kg/d) to arrive at daily 

intake (mg/d). As such, biotransfer factors are defined as follows: Biotransfer Factor = 

concentration in beef tissue (mg/kg) / daily constituent intake (mg/d). However, it is important to 

note that feed consumption rates were NOT measured in the studies used by Travis and Arms. As 

such, these authors used assumed feed consumption rates of 8.0 kg DW/d for beef cattle and 16 kg 

DW/d for dairy cattle. Based on these assumed ingestion rates, Travis and Arms developed the 

correlation equation used in the guidance relating Ba-beef and Ba-milk to logKow. It is clearly 

inappropriate to use a different set of consumption rates in the guidance than those used for the 

underlying correlation algorithm developed by Travis and Arms. EPA should either reduce feed 

consumption rates in the guidance to be consistent with those used by Travis and Arms or 

recalculate the Travis and Arms (1988) correlation algorithm using the consumption rates presented 

in the draft HHRAP. 

• A correction factor is needed in the plant uptake algorithm for particulate deposition. Plant uptake 

algorithms for vapors include an air-to-plant biotransfer factor, B. These were calculated using an 

algorithm derived from studies in azalea plants with a very limited number of chemicals. Only 14 

organic compounds were examined in these studies. Thus, the air-to-plant transfer algorithm used 

in the method reflects a correlation equation based on a limited number of chemicals. It is 

scientifically and mathematically invalid to use a correlation algorithm derived from such a limited 

number of chemicals to calculate transfer factors for the approximately 200 chemicals which fall 

into very diverse chemical classes covered by the HHRAP. 

• The use of the soil-to-aboveground plant transfer approach developed by Travis and Arms (1988) is 

technically insupportable for many of the COPCs recommended for evaluation. 
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• The combination of the equations for calculating COPC concentrations in beef, pork, chicken and 

eggs and the method of estimating the biotransfer factors results in substantial underestimation of 

dioxin concentrations in these tissues. This may also be true for other COPCs. 

• To develop biotransfer factors for pork and chicken, biotransfer factors for beef were adjusted by 

the ratio of pork or chicken fat content to the fat content of beef. EPA provides no data to justify 

use of such adjustments. With the known anatomical differences and the expected absorption 

differences among species, EPA should review the proposed methodology and provide justification 

for use of ruminant biotransfer factors for any non-ruminant species. 

EPA Response: 

We disagree that the method for estimating biotransfer factors for pork and chicken are flawed. The 

basis for these estimates is consistent with lipid partitioning theory. The HHRAP has been revised to 

include newly developed biotransfer equations for beef and milk. Caps have also been implemented for 

other biotransfer equations at the high and low ends of the Kow range used to develop the biotransfer 

equations. EPA continues to work on refining and updating biotransfer equations to reduce error and 

minimize violation of the conservation of mass. As with many parameters and equations provided in the 

HHRAP, the scientific basis and reference are provided. A facility is then able to evaluate whether to 

propose and justify to the permitting authority an alternative they consider more appropriate than that 

recommended in the HHRAP. 

Issue: Estimating COPC Levels - Description of the Evolution of the Equations 

Commenters observed that draft HHRAP Chapter 5 includes the reasoning behind the selection of the 

equations used to estimate the concentration in media due to the deposition of stack emissions. However, 

it was recommended that soil modeling equations be presented in a more informative manner. As 

currently written, the presentation of the various equations to estimate COPC concentrations in soil is 

confusing, especially since only a few of the equations (5-1 C, 5-1 D, and 5-1 E) are ultimately 

recommended for use. The commenters recommended that if the earlier equations (5-1, 5-1A, and 5-1 B) 

are presented for historical perspective, some description of the evolution of the equations should be 

given. In addition, the deposition term in equations 5-1 C, 5-1 D, and 5-1 E includes both wet and dry 

deposition of both particles and vapors; none of the earlier equations contain all four terms. 

EPA Response: 

The purpose of presenting multiple soil concentration equations is to differentiate the different equations 

presented in previously released guidance documents. A detailed evolution of the equations is beyond the 

scope of the HHRAP. However, one has been prepared by EPA’s Office of Research and Development 

and is available upon request. 

Issue: Estimating COPC Levels - Equations for Plants 

Several issues were raised regarding the algorithms for estimating COPC levels in plants. These issues 

include: 

• The exposure pathways include contamination of plants from volatilization from the soil. 

However, subsequent discussion on the mechanisms of uptake in aboveground plants does not 

include this pathway. It is recommended that this discrepancy be clarified. 
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• The overall intake from depositions and vapor transfer to plants does not consider the duration of 

depositions (years of facility operation) as presented in the equations. For facilities with a very 

short lifespan (e.g., 3 years), this will greatly overestimate cancer and non-cancer risks. The 

equations model deposition and vapor transfer to plants as if they continue over the entire exposure 

duration. It is recommended that the HHRAP include the duration of operations to more accurately 

depict actual exposures. This may be done by averaging the plant concentrations similar to that of 

the soil concentrations in Section 5.3 or by calculating two separate intakes in Section 6.1 for the 

periods during operations and after operations. 

• Only one equation is provided for modeling uptake into belowground produce (draft HHRAP 

Equation 5-20B). However, this equation should only be applied when modeling uptake for 

organic compounds. It is recommended that language be added which indicates that Equation 5-

20A should be used for evaluating uptake of metals into belowground produce. 

• It is unclear whether the plant uptake algorithm for particulate deposition describes particulate 

deposition onto plants and subsequent consumption of particulate material along with plant material 

or whether there is an implicit assumption that particulate constituents are absorbed through leaf or 

vegetable/fruit skin. If direct consumption of particulate material is assumed, there appears to be an 

implicit assumption that fruits and vegetables are not washed, peeled, or cooked prior to 

consumption. While this assumption may be appropriate for animal feeds, it is clearly 

inappropriate for fruits/vegetables consumed by humans. However, if absorption is assumed, EPA 

has provided no data to address the bioavailability of particulate material for absorption, nor has 

EPA included a correction factor (as was included in air-to-plant and root uptake algorithms) to 

address dilution of COPCs throughout the plant. 

• The use of the root uptake correlation equations developed by Briggs et al. (1982) is technically 

insupportable for many of the compounds recommended for evaluation 

EPA Response: 

We agree that uptake in above-ground plants from volatilization from soil requires clarification and have 

revised the Chapter accordingly. We have also added language indicating that Equation 5-20A should be 

used for evaluating uptake of metals into below-ground produce. For duration of deposition and 

regarding root uptake correlations, as with many other parameters and equations provided in the 

HHRAP, the scientific basis and reference are provided. A facility is then able to evaluate whether to 

propose and justify to the permitting authority an alternative they consider more appropriate than that 

recommended in the HHRAP. We note that direct consumption of plants is assumed and that 

consumption rates of food are adjusted for preparation including preparation and cooking losses. 

Issue: Estimating COPC Levels - Dry Vapor Phase Diffusion to Soil and Water (See RC Section 3.3.5) 

The equation (draft HHRAP equation 5-lA) for calculating soil concentrations contains a dry vapor phase 

diffusion load to soil (Ldif) that is no longer accounted for in the recommended equations for calculating 

soil concentration (Equations 5-1C through 5-lE). Although the use of the Ldif term contributes 

negligibly to the soil concentration and can be reasonably excluded from the soil concentration 

calculation, this should be more explicitly stated in the text. 
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Commenters also noted that dry deposition of vapors to the water body is modeled incorrectly. The 

equation to estimate dry deposition of vapors (Ldif) to the water body considers a one-way transfer of the 

COPC from the air into the water body. This is an incorrect method for estimating mass transfer. 

Instead, mass transfer from the air into the water body should be modeled as the product of a mass 

transfer coefficient and the difference between two concentrations - that in the air and the effective 

concentration in the water, which serves as a resistance to mass transfer across the air-water interface. 

Given the EPA model construction, loadings into the water body from soil erosion and runoff could be 

large enough to serve as a source of emissions to the air from the water body. By not allowing for the 

outward flux of vapors from the water to the air, EPA's model in some cases could incorrectly predict the 

trapping of a volatile chemical within the surface water body. 

EPA Response: 

The dry deposition of vapor to soil was addressed during the external peer review of the HHRAP. 

Changes have been made according to recommendations of the external peer review panel. Regarding 

dry deposition of vapors to water, the scientific basis and reference are provided. A facility is then able 

to evaluate whether to propose and justify to the permitting authority an alternative they consider more 

appropriate than that recommended in the HHRAP. We developed the HHRAP with the objective of 

acquiring information necessary to aid a permit writer in constructing a permit for hazardous waste 

burning facilities that allows the setting of emission or waste feed limits that are protective of human 

health and the environment. If volatile chemicals were trapped in the water body, a conservative 

estimation of drinking water and fish would result. If this is a problem, a more detailed site-specific fate 

and transport analysis could be conducted. However, we are unaware of any actual case where this 

situation has occurred. 

2.5.4 Conservation of Mass

The following issues related to conservation of mass were raised: 

• General Issues; 

• COPC Degradation in Soil; 

• Soil Erosion; 

• COPCs in Produce; 

• COPCs in Chickens and Eggs; and 

• Combining Steady State and Non-Steady State Models. 

Issue: Conservation of Mass - General 

The methods in the draft HHRAP do not account for conservation of mass. Commenters recommended 

that mass balance be incorporated into the exposure assessment. 

EPA Response: 

The question of mass balance was addressed during the external peer review. Conservation of mass 

checks have been put throughout the HHRAP. Dry deposition of vapor has been revised based on 

comments received regarding the conservation of mass. We have refined biotransfer equations to reduce 
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error and minimize violation of the conservation of mass. 

Issue: Conservation of Mass - COPC Degradation in Soil (See RC Sections 3.5.4 and 3.5.5) 

Commenters pointed out internal inconsistencies with the soil and plant equations in the draft HHRAP, 

resulting in violations of conservation of mass. Specifically, there is no provision for COPC degradation 

in soil, despite noting COPCs uptake by plants, consumption by animals, and runoff and erosion. 

Commenters were concerned that this may result in “double counting” these COPCs. Commenters 

recommended that EPA carefully examine all of the equations to ensure that the principles of 

conservation of mass are obeyed and that the terms and equations are used in a consistent manner 

throughout the HHRAP. 

EPA Response: 

We disagree that the HHRAP does not allow for both biotic and abiotic degradation in soil. The HHRAP 

provides chemical-specific values to help the user estimate loss due to degradation. 

Issue: Conservation of Mass - Soil Erosion (See RC Sections 3.5.4 and 3.5.5) 

Commenters recommended that erosion and volatilization pathways be included in the soil loss equation. 

The draft HHRAP does not include loss due to erosion and volatilization. While eliminating erosion from 

the soil loss equation, the draft HHRAP includes it as an input to water bodies, violating the conservation 

of mass. The soil volatilization factor, ksv, has been temporarily set to 0 for all chemicals, resulting in the 

unlikely situation in which VOCs end up driving the risk through root uptake into plants. 

Soil modeling assumptions should be consistent with other aspects of environmental fate and transport 

modeling. Recommendations for two of the soil loss coefficients are inconsistent with other parts of the 

draft HHRAP, and could, under certain conditions, result in substantial overestimates of COPCs. The 

value of 0 for kse (draft HHRAP page 5-12) is recommended because soil can erode both onto and away 

from a soil parcel (erosion from a soil parcel can be replaced by soil erosion onto it from an adjacent 

parcel). However, by not accounting for erosion of soil before it enters the water body (i.e., by assuming 

kse = 0), EPA is double-counting a portion of the COPC load. EPA is effectively assuming that COPCs 

build up in soil without any loss due to erosion, then erode to the water body, but then effectively remain 

at a constant concentration in soil because the losses to the water body have not been considered. 

The EPA recommendation that kse be set equal to zero results in higher soil, water, and fish 

concentrations. 

The draft HHRAP states that Kse should be set equal to zero in soil COPC concentration calculations for 

the fish pathway. However, a convincing argument for inclusion or exclusion is not included in the 

guidance document and needs justification.

EPA Response: 

The ksv assumption of zero has been replaced by an equation to calculate ksv. The external peer 

reviewers concluded that setting erosion equal to zero would not affect the conservation of mass at the 

tops of hills. 

 
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With regard to the second and third comments, this issue was addressed during the external peer review. 

The peer reviewers disagreed with the commenters. Soil erosion is not set equal to zero for the loading to 

the water body. It is set equal to zero for determining soil concentrations. The peer reviewers disagreed 

with the conclusion that this results in higher soil concentrations, with the possible exception of at the top 

of a hill. 

We agree that a convincing argument for setting Kse equal to 0 is warranted. Further justification has 

been added. 

Issue: Conservation of Mass - COPCs in Produce (See RC Section 3.5.4) 

EPA's model for deposition of COPCs to vegetation fails to account for removal of COPCs from the 

vegetation (for example, by rain), thus violating conservation of mass. In addition, the scientific 

foundation for the model parameters is limited. Further, the draft HHRAP relies on partitioning equations 

and coefficients to estimate concentrations in the different media, with assumptions of steady-state 

conditions. The use of equilibrium equations and coefficients can lead to mass balance problems. For 

example, for some of the PAHs, the air-to-plant transfer coefficients are so large that they can result in 

more material in the plants than is emitted from the facility. The commenters are concerned that the 

magnitude of error introduced by these assumptions might be quite large for vegetation with high surface 

area and that such error could be magnified in subsequent models that consider consumption of plants by 

animals. The commenters recommend that both equations used to estimate media concentrations and the 

equations used to calculate the coefficients should be re-evaluated. 

EPA Response: 

This issue was addressed during the external peer review. Conservation of mass can be violated as 

COPC’s are transferred into food items, but it is within the error bars of biotransfer parameters. To 

specifically address this issue, EPA commissioned a study to revise and update the biotransfer factors for 

beef and milk. In addition, biotransfer factors for plants are capped at the transfer factor associated with 

the highest and lowest log Kow values in the Travis and Arms studies. EPA expects these updates to 

dramatically reduce the error bars at the extremities of the Log Kow/biotransfer factor relationship. As 

with many parameters and equations provided in the HHRAP, the scientific basis and reference are 

provided. A facility is then able to evaluate whether to propose and justify to the permitting authority an 

alternative they consider more appropriate than that recommended in the HHRAP. 

Issue: Conservation of Mass - COPCs in Chickens and Eggs (See RC Section 3.5.3) 

Certain COPCs (e.g., PAHs) may undergo metabolism, for example by chickens (as well as other 

animals). EPA should reconcile its model predictions with empirical knowledge whenever possible. The 

commenters recommend that EPA's models be evaluated against pharmacokinetic data and other 

information that might lead to departures from the assumed behavior of Kow-based models of 

bioaccumulation that consider solely the lipophilic nature of organic chemicals. 

EPA Response: 

A mass balance check performed on the chicken and egg pathway showed that predicted COPC 

concentrations are within the error of the recommended biotransfer factors. We will continue our effort 

to update and revise the biotransfer equations. As with many parameters and equations provided in the 
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HHRAP, the scientific basis and reference are provided. A facility is then able to evaluate whether to 

propose and justify to the permitting authority an alternative they consider more appropriate than that 

recommended in the HHRAP. Although it is agreed that PAHs are metabolized to some extent, we are 

also concerned that the toxicity of the metabolites is not being directly assessed, and therefore, it is 

reasonable to maintain the current biotransfer factors until more scientific research can support adoption 

of a metabolism factor. 

Issue: Conservation of Mass - Combining Steady State and Non-Steady State Models 

 (See RC Section 3.5.4) 

It was recommended that the multi-pathway analysis be revised because of the combination of steady 

state and non-steady state modeling that can result in violations of the conservation of mass. 

EPA Response: 

We have made an extensive effort to provide the most accurate yet implementable guidance for 

conducting risk assessments. There is a mixing of steady state with non-steady state assumptions within 

the guidance. However, every effort has been made to bound estimates and preserve conservation of 

mass. The question of mass balance was addressed during the external peer review, and conservation of 

mass checks have been included throughout the HHRAP. 

2.5.5 Screening of VOCs

The issue raised under the screening of COPCs relates to the inclusion of volatile organic compounds 

(VOCs) in indirect exposure pathways. The draft HHRAP recommends that VOCs should be modeled 

through both the direct and indirect exposure pathways in the combustion risk assessments. While VOCs 

are clearly COPCs for the direct inhalation pathway, the evaluation of VOCs through the indirect foodchain pathways is not technically defensible. According to EPA Region 6, "One argument for excluding 

these COPCs (VOCs) from evaluation is that there is no empirical evidence that VOC emissions pose a 

hazard via indirect pathways. U.S. EPA OSW agrees that it is not aware of such evidence; however, 

OSW is similarly unaware of a lack of evidence to the contrary.” 

Further, it was noted that carrying VOCs, such as benzene and toluene, through multi-step food chain 

models is erroneous if Kow-based correlations are used to predict biotransfer factors. Similar to polycyclic 

aromatic hydrocarbons, many VOCs are metabolized, and not simply transferred to and from the fatty 

tissues of animals. Thus, even if certain VOCs were ingested by a pig, those VOCs would not necessarily 

result in human exposure via consumption of pork. Finally, many of the Kow-based correlations are 

derived from data on lipophilic compounds, which generally have high values of Kow. Chemicals with 

low Kow values, such as benzene require such correlations to be extrapolated outside of the range of Kow

for which they are applicable. Commenters recommended VOCs not be evaluated via the indirect 

exposure pathways, and should only be included as COPCs for the direct inhalation pathway: 

EPA Response: 

Given that the fate, transport, and toxicity information is provided in the guidance, we believe it is more 

efficient to run the selected COPCs through the risk assessment process, rather than go through the 
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arduous process of justifying elimination of COPCs from portions of the assessment. In addition, if 

chemicals do not transport up various exposure routes, the fate and transport properties of these 

chemicals will cause them to result in insignificant risk. 

2.5.6 Estimating Media Concentrations - Clarification / Editorial

Issue: Clarification - Figure on Concentrations of COPCs in Pork 

Draft HHRAP Figure 5-5 implies that the concentration of COPC in the grain and silage ingested by 

swine comes from root uptake only. The text, however, suggests that the silage is also affected by direct 

deposition and vapor transfer of COPCs. It was recommended that EPA consider revising the figure to 

more clearly indicate that silage concentrations are affected by root uptake, direct deposition, and vapor 

transfer, and that grain concentrations are affected by root uptake only. 

EPA Response: 

Figure 5-5 has been updated to reflect the concerns in the comment. 

Issue: Clarification - Summary Table for the Model Results 

It was suggested that in draft HHRAP Chapter 5, it would be useful to have a summary (perhaps in 

tabular form) at the end indicating whether data are available which indicate whether the predicted values 

using the models and assumptions presented in the chapter have been shown to agree or disagree with 

actual field measurements. Presenting these data, even though it is likely to be very limited, would 

present some confidence in the process. 

EPA Response: 

Comment noted. Although we agree that this would be a valuable effort, it is beyond the scope of the 

HHRAP. 

Issue: Clarification - Consistent Computation of Deposition Velocity 

The present approach for computing deposition velocities is too subjective to be used effectively. The 

draft HHRAP should be revised to reference an established method that will promote consistency. 

EPA response: 

Default dry deposition of vapor velocities has been changed and documentation provided. 

Issue: Clarification - Particulate Inhalation 

With regard to Section 5.1, the draft HHRAP is unclear about whether it suggests that all particles are 

inhaled, irrespective of size. If so, then why? We know that the smaller particles are more effectively 

inhaled than the larger. 

EPA Response: 

All particles are inhaled irrespective of size. Size does however affect transport of a particle in the 

atmosphere, and it is unlikely that a large fraction of what is being inhaled with respect to a combustion 
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unit is in the large particle size range. Also note that not all toxics that are inhaled have a primary effect 

on the lungs. For example, nasal tumors are a common health endpoint for several inhaled toxics. 

Issue: Clarification - Unit Soil Loss (Xe) 

The draft HHRAP indicates that parameter values used in calculating Xe should be re-evaluated if the 

calculated benthic burial rate constant (kb) value exceeds 1.0. However, no recommendations are 

provided to detail how to do the re-evaluation. 

EPA response: 

Based on experience, the value calculated for Xe is the most likely reason for determining a large and 

potentially unrealistic benthic burial rate. Information about determining site-specific values and 

variables for calculating Xe are in the references cited in Section 5.7.2. We’ve clarified this in Section 

5.7.4.7. 

Issue: Clarifications - Appropriate Annual Soil Concentration 

The use of the potentially highest annual soil concentration over the operating lifetime of the unit for 

evaluation of non-carcinogenic effects does not seem appropriate. The highest annual level over the 

exposure duration of each adult receptor is recommended as an alternative. 

EPA response: 

We agree. We’ve changed the HHRAP to recommend the highest annual level over the exposure duration 

of each adult receptor. 

Issue: Editorial 

Editorial issues include: 

• The draft HHRAP states that an above ground correction factor (VGag) of 0.01 should be used. 

However, the text of Section 5.3.2 (page 5-33) needs to be changed to reflect this assumption, as it 

currently states that 0.001 is being used for lipophilic organics.

• Draft HHRAP Section 5.7.4.1, 2nd & 3rd paragraphs following Equation 5-36C parameters, 

requires clarification to explain exactly when and how the parameter values used in calculating Xe

should be re-evaluated, as well as when Equation 5-37 should be used to determine Obs and when 

the recommended value should be used.

• Draft HHRAP page 5-75: EPA states at the top that the bed sediment porosity is based on a bed 

sediment concentration of 1 kg/L. In the subsequent paragraph, EPA states that "... 0.1 kg/L is a 

reasonable concentration for most applications of the bed sediment." Please clarify.

• A metabolism factor is included in the estimation of the chemical concentration in beef, milk, and 

pork to account for loss of the chemical from fat and muscle. EPA recommends a value of 0.01 for 

bis(2-ethylhexyl)phthalate and 1.0 for all other COPCs.

• Better descriptors are recommended for unitized yearly average wet deposition (Dydw) and 

(Dyww) in draft HHRAP Equation 5-1B.

• Draft HHRAP Section 5.3.1.2, page 5-29, line 2: a constant is missing.
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• Draft HHRAP Section 5.2.3, Equation 5-11, Calculating the Deposition Term (Ds) Comment: The 

term Cyv is reported in the wrong units. Recommendation: Cyv should be in units of ug-s/g-m3.for 

proper unit cancellation. These are also the units generated by the dispersion modeling.

• Draft HHRAP Section 5.3.1.2 Calculation of COPC Concentrations in Produce 2: the units of the 

parameter kp (yrs-1) should be indicated in the text and in the gray box on page 5-29.

• Draft HHRAP Section 5.7 Calculation of COPC Concentrations in Drinking Water and Fish: A 

typographical error exists in the description of the parameter Dytwp on page 5-65. The parameter 

should refer to total particle deposition rather than total vapor deposition.

EPA response: 

Comments have been noted, and editorial errors and text revisions have been corrected, where 

applicable. The 0.1 kg/L has been changed to 1 kg/L in the text. 

2.6 Quantifying Exposure [draft HHRAP Section 6]

The calculation of COPC-specific exposure rates for each exposure pathway evaluated includes the 

following factors: 

• The estimated COPC media concentrations calculated in Chapter 5; 

• Consumption rate; 

• Receptor body weight; and 

• The frequency and duration of exposure. 

This calculation is repeated for each COPC and for each exposure pathway included in an exposure 

scenario. 

Comments regarding quantifying exposure were categorized into: 

• General Comments; 

• Exposure Duration; 

• Exposure Pathways (Inhalation, Dermal, Ingestion, and Water); and 

• Clarification / Editorial. 

2.6.1 Quantifying Exposure - General

General issues pertaining to quantifying exposure include: 

• Corrections and recommendations; and 

• Assumptions that may overestimate risk. 

Issue: General - Corrections and Recommendations 

General comments related to corrections include: 

• The statement that ‘a receptor is exposed via the indirect exposure pathways for as long as the 

receptor remains in the assessment area.’ is not entirely correct (draft HHRAP Chapter 6, page 6-
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13). In the equations provided, the exposures resulting from indirect exposures would be identical 

for facilities that operate 1 year or 40 years. Indirect exposures may or may not continue after the 

facility ceases operation. 

• Draft HHRAP Chapter 6 should include a summary of specific recommendations and processes to 

follow for quantifying exposure. Much of this may consider site-specific factors and the type of 

metric to be used for carcinogenic versus non-carcinogenic endpoints. Nevertheless, the specific 

guidelines should be made. 

• EPA should coordinate and build on the existing expertise and evolving approaches to multipathway modeling. 

EPA Response: 

The statement on page 6-13 has been changed from “is exposed” to “may be exposed.” We disagree 

with the characterization that exposure would be identical regardless of facility emission duration. The 

period before emission, duration of emissions, and duration of exposure are all time-related components 

of the recommended equations for estimating soil concentrations. These time-dependent soil 

concentrations then contribute to other indirect exposure media concentrations. In any individual year 

during facility operation, COPC losses from the soil do not necessarily equal depositions, and 

concentrations tend to build with time. When emissions (and hence depositions) cease, soil 

concentrations and the indirect exposure media concentrations dependent on them tend to decrease. 

Recommended exposure rates and frequencies are presented and referenced in Chapter 6. Specific 

guidelines on how to generate site-specific exposure factors are beyond the scope of the HHRAP.

We note the comment regarding coordinating and building on existing expertise and evolving 

approaches. 

2.6.2 Quantifying Exposure - Exposure Duration

Issues pertaining to quantifying exposure duration include: 

• Combined exposure from childhood through adulthood; and 

• Use of highest annual soil concentration. 

Issue: Exposure Duration - Combined Exposure from Childhood through Adulthood 

The draft HHRAP discusses the scenario in which a child is exposed to facility emissions and then grows 

to adulthood with additional exposure. However, a recommended approach for addressing this issue is 

not provided. 

EPA Response: 

Exposure duration recommendations are presented. They do not include a mixed scenario of 6 years 

child and 24 years adult. This could be a scenario evaluated on a site-specific basis. Averaging-time 

recommendations are presented in HHRAP Section 6.5, with further clarification provided in Appendix 

C. 
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Issue: Exposure Duration - Use of Highest Annual Soil Concentration 

The use of the potentially highest annual soil concentration over the operating lifetime of the unit for 

evaluation of non-carcinogenic effects was questioned. Given permit renewal requirements, EPA will 

have several opportunities to review incinerator operation and subsequent potential impacts prior to the 

end of the incinerator’s service. It was noted that the application of a maximum annual soil concentration 

deviates from the intended non-cancer hazard analysis when facilities with short life spans are evaluated. 

As an alternative, the highest annual level over the exposure duration of each adult receptor was 

recommended. 

EPA Response: 

As with many parameters and equations provided in the HHRAP, the scientific basis and reference are 

provided. A facility is then able to evaluate whether to propose and justify to the permitting authority an 

alternative they consider more appropriate than that recommended in the HHRAP. We agree with the 

recommendation of the highest annual level over the exposure duration of each receptor as an 

alternative, and we’ve changed page 5-4 of the HHRAP accordingly. 

2.6.3 Quantifying Exposure – Exposure Pathways

Comments were received regarding issues related to the following exposure pathways: 

• Inhalation; 

• Dermal; 

• Ingestion (pica behavior); and 

• Water 

Issue: Exposure Pathways – Inhalation 

Issues related to quantifying inhalation exposure include:

• It is incorrect to assume that 100% of the inhaled particles are retained and taken up by the body. 

• Draft HHRAP Section 6.2.1, the 4th paragraph Air Exposure Pathways indicates that direct 

inhalation exposures are preferentially characterized by using inhalation unit risk factors (URF) and 

RfCs. This section should explain whether inhalation CSFs and inhalation RfDs are to be 

converted to URFs and RfCs, respectively, if these toxicity data are the only values available. 

• Any site-specific conditions or setting characteristics that may require consideration of the soil 

inhalation pathway resulting from dust resuspension should be indicated in draft HHRAP Section 

6.2.3.3, page 6-9. 

EPA Response: 

As with many parameters and equations provided in the HHRAP, the scientific basis and reference are 

provided. A facility is then able to evaluate whether to propose and justify to the permitting authority an 

alternative they consider more appropriate than that recommended in the HHRAP. With regard to how 

direct inhalation exposures are characterized, this information is provided in Section 7.3 and chemicalspecific examples are provided throughout Appendix A. Although we do briefly discuss inhalation of 
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resuspended dust in Section 6.2.3.3, a full discussion of an exposure pathway we do not normally 

recommend is beyond the scope of the HHRAP.

Issue: Exposure Pathway - Dermal 

Issues pertaining to quantifying dermal exposure include: 

• The draft HHRAP statement "available data indicate that the contribution of dermal exposure to 

soils to overall risk is typically small,” should be revised to state that “available data indicate that 

the contribution to overall risk from dermal exposure to soils impacted from hazardous waste 

combustion facilities is typically small relative to contributions resulting from exposures via the 

food chain.” 

• The validity of using the highest annual average soil concentration to calculate dermal exposure 

was questioned, as this contradicts several other discussions concerning the non-carcinogenic 

hazard analysis. A revision of the methodology was recommended to be consistent with the 

exposure period to be evaluated (7 years to 70 years) and average daily exposure levels over the 

specific exposure duration derived for use with the RfDs/RfCs. 

EPA Response: 

With regard to the statement “available data indicate...” we have changed the HHRAP as suggested. As 

with many parameters and equations provided in the HHRAP, the scientific basis and reference are 

provided. A facility is then able to evaluate whether to propose and justify to the permitting authority an 

alternative they consider more appropriate than that recommended in the HHRAP. 

Issue: Exposure Pathway - Ingestion 

It was recommended that draft HHRAP Section 6.2.3.1 Soil Ingestion include a statement indicating that 

pica behavior must be considered as part of the risk assessment and that the lack of this consideration be 

supported by documentation, rather than consideration only on a case-by-case and site-specific basis. 

Based on what is presented in the draft HHRAP, it seems unreasonable to rule out pica behavior, 

especially since it is considered “a normal part” of a child’s development and children may be one of the 

most susceptible groups. Without such a statement, this potential source of exposure may be largely 

ignored. 

EPA Response: 

We disagree. For risk assessment purposes, pica is typically defined as “an abnormally high soil 

ingestion rate” and is believed to be uncommon in the general population. If available information 

indicates that there are children exhibiting pica behavior in the assessment area, and you determine that 

these children represent a special subpopulation potentially receiving significant exposure, it may be 

prudent to include these children in the risk assessment. We recommend you make this evaluation on a 

case-by-case basis, based on site-specific exposure setting characterization.

Issue: Exposure Pathway - Water 

Issues pertaining to quantifying the water exposure pathway include: 

• It is not clear how contaminant concentrations in cistern water are to be estimated, nor does it seem 

possible to make accurate estimates. It was noted that the area of the cistern would not be likely to 
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collect a significant amount of contaminants. This pathway could result in overly conservative risk 

assessments. In the event that it is necessary to maintain consideration of exposure to drinking 

water via cisterns, clarification is needed on whether this pathway should only be considered in 

cases where a cistern was actually observed during a site visit or when use of a cistern has been 

specifically reported. 

• Additional guidance was requested about how water bodies should be included in the evaluation of 

the water exposure pathway. This is particularly important when faced with a location which is 

surrounded by numerous surface water bodies large and small. For example, the following 

conditions should be considered: 

- The predominant wind direction, if applicable. 

- Flow rates, in that flowing water bodies (with large flow rates) tend to have lower estimated 

COPC concentrations because of a greater rate of volumetric replacement. This is not to say 

that rivers should be excluded from evaluation. 

• Priority should be given to those water bodies for which relevant risk pathways are complete (e.g., 

trophic level 4 fish occur and are likely to be sought by anglers) while the average rate of COPC 

deposition to watershed is high relative to the likely dilution available. This can present a challenge 

as these criteria tend to be mutually exclusive. An extensive fishery may depend on a large 

watershed while the highest COPC concentrations are likely to occur in small watersheds located 

close to the source. 

• The discussion in draft HHRAP Section 6.2.4 should also be placed in Section 4.1.2 since it will 

assist the risk assessor in selecting the appropriate water bodies for analysis. 

• Different RfDs can be applied for the evaluation of PCBs and it was noted that the draft HHRAP 

does not address which RfD to use when evaluating ingestion of water-soluble (lower chlorinated) 

congeners. The recommendation is for the use of the RfD for Aroclor 1016 of 7x10-5 milligram per 

kilogram-day to evaluate the non-carcinogenic PCBs in the water ingestion pathway, given that 

only the lower chlorinated PCB congeners are soluble in water. This approach would be consistent 

with recommendations concerning the evaluation of carcinogenic PCB congeners in the water 

ingestion pathway. (See RC Section 3.4.5) 

• Further clarification is needed on the use of safety factors for the water ingestion pathway, given 

that only the lower chlorinated PCB congeners are soluble in water. The more highly chlorinated 

PCBs will adhere to soil and sediments and the safety factor of 2 should be applied to evaluate risks 

through the food chain, sediment, or soil exposures unless at least 99.5 percent of the mass of the 

released PCB mixture has fewer than four chlorine atoms per molecule. 

EPA Response: 

We disagree with the comment regarding cistern water. Cistern water may be a viable exposure pathway 

in some site-specific situations. How or to what extent a cistern is considered is up to the permitting 

authority. The HHRAP provides guidance as to what should be considered. The quantitative evaluation 

of water concentrations in cisterns has been clarified. 

Selection of the water bodies to be evaluated is a site-specific decision. The HHRAP provides several 

potential variables to consider when selecting water bodies in the assessment. 
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Chapters 3 through 6 should be reviewed when selecting water bodies for the assessment. Air modeling 

(Chapter 3) and environmental fate and transport (Chapter 5) are just as important when selecting water 

bodies to evaluate as are exposure scenario selection (Chapter 4) and exposure pathway evaluation 

(Chapter 6). 

The issue of evaluating coplanar PCB congeners was reviewed during the external peer review. The peer 

review panel agreed that evaluating coplanar PCB congeners using dioxin toxicity equivalency factors 

(TEFs) is appropriate and reasonable. The peer review panel also agreed that using the health 

benchmarks for Aroclor 1254 and Aroclor 1016, depending on the congener composition of actual 

emissions, is reasonable. We recommend using the World Health Organization (WHO) TEFs for dioxin 

and furan congeners. 

As with many parameters and equations provided in the HHRAP, the scientific basis and reference are 

provided. With regard to safety factors for the water ingestion pathway, a facility may evaluate whether 

to propose and justify to the permitting authority an alternative they consider more appropriate than that 

recommended in the HHRAP. 

2.6.4 Quantifying Exposure - Clarification / Editorial 

Issue: Clarification - Consumers of Contaminated Food

In draft HHRAP Section 6.2.2.3 Percentage of Contaminated Food, page 6-6, it is assumed that no other 

receptors in addition to the subsistence farmer and subsistence farmer child consume the contaminated 

animal tissues. Is there some basis for this assumption? If so, it should be referenced. 

EPA Response: 

Section 6.2.2.3 of the draft HHRAP referenced earlier guidance in recommending particular exposure 

pathways for each exposure scenario. We’ve modified Section 6.2.2.3 to deal exclusively with the title 

subject: the percentage of a particular food type to assume is contaminated. The issue of which exposure 

pathways (i.e., food types) to assign to which exposure scenarios is now dealt with exclusively in HHRAP 

Chapter 4, Section 4.2 (Recommended Exposure Scenarios). 

Issue: Clarification - Dose Metric

More discussion was recommended in draft HHRAP Section 6.1 GENERIC EXPOSURE RATE 

EQUATION, pages 6-1 to 6-2 about the proper dose metric to be used for quantifying exposure, and the 

HHRAP should consider characteristics of the particular chemical, as well as the endpoint(s) of concern. 

Dose to the receptor can be quantified in a variety of metrics, such as daily intake, total body burden, or 

body burden averaged over a given period of time. The metric used should clearly match the particular 

toxic endpoint. Developmental effects may use a metric which describes exposure at a particular time of 

development. On the other hand, responses such as cancer may be best described either by an exposure 

over time or a particular body burden. The appropriate metric used may also depend on the half-life of 

the particular chemical. 
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EPA Response: 

The discussion in Section 6.1 of the HHRAP is meant only as a generic intake rate overview specific only 

to what is generally utilized in risk assessment practice. Variations of this equation as they pertain to 

receptor and chemical specific exposures are presented in detail through Appendix C. 

2.7 RISK AND HAZARD CHARACTERIZATION [draft HHRAP Section 7] 

Risk characterization is the probability that a receptor will develop cancer, based on exposure to 

chemicals over time. Risk calculations are based on cancer slope factors that are estimates of lifetime 

probabilities of an individual developing cancer as a result of exposure to particular levels of a potential 

carcinogen. The risk characterization is then compared to an acceptable target risk level, usually in the 

range of 1 x 10-4 to 1x 10-6. 

Hazard characterization is a measure, calculated as a ratio, of a receptor’s potential exposure relative to a 

standard exposure level, typically the reference dose (RfD) or reference concentration (RfC). Often (but 

not always), a hazard ratio above 1 is generally considered unacceptable. However there are additional 

analyses that can be performed, such as subdividing hazards by target organs. 

For multiple chemical exposures, risks and hazards are generally added together. There is flexibility 

inherent in the risk and hazard characterization process, so that site-specific information can be included. 

Comments regarding risk and hazard characterization were categorized into: 

• General Issues; 

• Risk and Hazard Toxicity Factor Determination; 

• Route-to-Route Extrapolation of Toxicity Benchmarks; 

• Inhalation Toxicity Benchmarks; 

• Non-cancer Effects Estimation; and 

• Clarification / Editorial. 

2.7.1 Risk and Hazard Characterization - General

General comment issues include: 

• Additive effects of contaminants of potential concern (COPCs); 

• Impacts of background and other man-made sources; 

• Use of average daily contaminants of potential concern intake (ADI) via inhalation in calculating 

cancer risk and Hazard Quotient (HQ); 

• Characterization of receptors; 

• Incremental lifetime cancer risks; 

• Consideration of radionuclides; 

• IEUBK model application for lead risks; 

• Probabilistic risk assessment; 
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• Application of site-specific data in risk and hazard characterization; 

• Separate carcinogenic and non-carcinogenic soil calculations; 

• Integrated risk assessment process; 

• Contaminants derived from raw materials; and 

• Evaluation of non-cancer dioxin hazards. 

Issue: General - Additive Effects of COPCs 

It was suggested that the statement in draft HHRAP section 7.3, page 7-6, “This summation methodology 

assumes that the health effects of the various COPCs to which a receptor is exposed are additive” is a 

very conservative assumption, especially for different endpoints of concern. Although the possibility 

exists, there is only very limited evidence for any of the health effects of these chemicals to be additive, 

and in fact, it is probably unlikely for many health effects. 

EPA Response: 

The HHRAP recommends a conservative assumption of additivity as a first cut in the risk assessment, but 

also allows for grouping by target organ.

Issue: General - Background and Other Man-made Sources 

It was suggested that background exposure levels and exposure from other man-made sources are issues 

not adequately addressed in the draft HHRAP (Section 7.3, page 7-6, lines 4-5). The draft HHRAP 

suggests that consideration of other sources should be largely left up to the discretion of the permitting 

authority. It was recommended that a requirement be made to at least consider other sources of exposure. 

This would necessitate a statement in the HHRAP that the hazard quotient (HQ) has or has not been 

adjusted based on the determined, or at least estimated, absence or presence of other significant sources of 

exposure. The importance of other sources should also be considered for cancer risk, but may be 

especially important for effects where thresholds are involved, especially if the exposure is near such a 

threshold. 

EPA Response: 

We consider the issue of whether/how to include background to be a risk management decision, and 

therefore beyond the scope of the HHRAP. An example of how background and cumulative risk are being 

addressed is presented in the Region 6 Risk Management Addendum (U.S. EPA Region 6, 1998, EPA-R6-

98-002). 

Issue: General - Use of ADI in Calculating Cancer Risk and Hazard Quotient 

 [draft HHRAP Appendix C] 

Comments suggested that total air concentrations of COPCs, along with unit risk factors (URFs), are 

more appropriate than using the average daily intake (ADI) in the calculation of cancer risks and hazard 

quotients. 

EPA Response: 

Using the ADI allows for altering the exposure duration, that is built into the URF. As with many 

parameters and equations provided in the HHRAP, the scientific basis and reference are provided. A 
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facility is then able to evaluate whether to propose and justify to the permitting authority a site-specific 

option, or other alternative to that recommended in the HHRAP. 

Issue: General - Characterization of Receptors 

A concern was expressed that calculations based on hypothetical receptors are overly conservative, or that 

assumptions in the risk assessment do not reflect the distribution of receptors relative to the location of 

the stacks within the facility. Based on the draft HHRAP text which states that risks and hazards from all 

combustion units be summed across all receptor exposure pathway combinations for each receptor, one 

commenter questioned the reasonableness of summing risks and hazards from multiple stacks and using 

the assumption of collocation of all receptors, which is unlikely to be supported by site-specific data. 

EPA Response: 

The draft HHRAP recommends basing the evaluation of receptors on current and reasonable potential 

land use (See draft HHRAP Section 4). Furthermore, receptors are located where they live or could 

reasonably be expected to live in the future. Exposure is through appropriate direct and indirect 

pathways for that receptor. It does not and should not be interpreted that all receptors live at one 

location. This may be done for a conservative screening, but is far from the procedures recommended in 

the draft HHRAP. 

Issue: General - Incremental Lifetime Cancer Risk 

The text should make clear that the assessment of cancer risks is for incremental risks over a lifetime. 

EPA Response: 

We agree, and have made the recommended changes. 

Issue: General - Consideration of Radionuclides 

Additional equations relevant to calculating risks due to radionuclides, as found in NESHAPS, should be 

added to the HHRAP. 

EPA Response: 

The HHRAP is consistent with the guidance used in the Superfund program rather than NESHAPS. The 

HHRAP only gives a brief overview of radionuclides. A separate guidance would need to be developed to 

conduct a risk assessment of a hazardous waste combustion facility emitting these constituents. 

Issue: General - IEUBK Model Application for Lead Risks 

Several concerns about application of the IEUBK model to address lead impacts from incinerator 

emissions included: 

• The current version of the IEUBK model is inadequate for screening purposes. It was suggested 

that the 1994 USEPA guidance value of 400 mg/kg be applied first as a screening value, and the 

IEUBK model be applied when levels exceed 400 mg/kg. 

• Lead should be evaluated by media-specific health-based criteria, such as USEPA Region 6’s 

values for soil (100 mg/kg) and air (0.2 pg/m3

). 
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• Site-specific levels of lead estimated in the air, drinking water, and foodstuffs should be included 

when applying the IEUBK model. 

EPA Response: 

Detailed recommendations on what assumptions to apply to the IEUBK model are site-specific. A 

screening method for addressing this is presented in the risk management addendum developed by EPA 

Region 6. Development of guidance on how to implement the IEUBK model with regard to changes in 

exposure parameters and model default assumptions is beyond the scope of the HHRAP. 

Issue: General - Probabilistic Risk Assessment 

It was suggested that the use of probabilistic risk assessment (Monte Carlo Analysis) be more strongly 

supported as an option for risk analysis. 

EPA Response: 

Consistent with other U.S. EPA guidance, the HHRAP allows for probabilistic risk assessment, but 

recommends that a deterministic assessment be done as well. Experience in conducting risk assessments 

for combustion facilities has shown that probabilistic assessments are most valuable in the sensitivity 

analysis of sensitive parameters for compounds and exposure pathways driving the risk. 

Issue: General - Application of Site-specific Data to Risk and Hazard Characterization 

Concern was expressed that the flexibility inherent in the recommended risk and hazard characterization 

will cloud the certainty of the risk estimate. 

It was recommended that site-specific conditions that would indicate calculation of population risks 

should be specified in the last sentence of the 1st full paragraph on page 7-2 of the draft HHRAP. 

EPA Response: 

As with many parameters and procedures provided in the HHRAP, the scientific basis and reference are 

provided. A facility is then able to evaluate whether to propose and justify to the permitting authority a 

site-specific option, or other alternative to that recommended in the HHRAP. 

Developing methodologies to capture the site-specific conditions that may validate the appropriateness of 

calculating population risks is beyond the scope of the HHRAP. 

Issue: General - Separate Carcinogenic and Non-carcinogenic Soil Calculations 

Requiring separate soil concentrations for carcinogenic chemicals and non-carcinogenic chemicals 

requires separate calculations for all exposure pathways that have soil concentration as an input. 

EPA Response: 

A detailed evaluation of the soil concentration equations has been conducted by EPA’s Office of Research 

and Development and is available upon request, but is beyond the scope of what is presented in the 

HHRAP. Also, as with many parameters and procedures provided in the HHRAP, the scientific basis 

and reference are provided. A facility is then able to evaluate whether to propose and justify to the 

permitting authority a site-specific option, or other alternative to that recommended in the HHRAP. 
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Issue: General - Integrated Risk Assessment Process 

Consideration of the reasonableness of the risk assessment should be integral throughout the risk 

assessment process, and not just a final consideration. 

EPA Response: 

We agree. This is why it was so important for us to clearly document the parameters and procedures set 

forth in the HHRAP. A facility is then able to evaluate the basis for the parameter, offer an alternate 

parameter, if applicable, and be able to evaluate the uncertainty of the parameter used in the risk 

assessment. 

Issue: General - Contaminants Derived from Raw Materials 

The guidance document requires that the evaluation consider emissions from raw materials and cements, 

in addition to the hazardous material products. The commenter assumed this is to understand potential 

emissions from the manufacturing process to better assess the incremental risk from hazardous waste 

combustion. 

 

EPA Response: 

This assumption is correct. 

Issue: General - Evaluation of Non-cancer Dioxin Hazards 

[draft HHRAP Section 2.3] (See RC Section 3. 4.4) 

It was suggested that EPA exclude from the HHRAP the non-cancer evaluation of dioxin, in particular 

because the draft Dioxin Reassessment has never been finalized. 

EPA Response: 

The only non-cancer assessment recommended in the HHRAP is a margin of exposure comparison of 

dioxin in breast milk and background levels noted in the U.S. Authors of the Dioxin Reassessment have 

reviewed the HHRAP. 

2.7.2 Risk and Hazard Toxicity Factor Determination

Risk and hazard toxicity factor determination issues include: 

• Selection of COPCs; 

• Realistic and current toxicity values; 

• Basis of toxicity values; 

• Absorption assumptions; 

• Calculation of toxicity factors; 

• Separate versus cumulative target organ toxicity; 

• Evaluation of products of incomplete combustion (PICs); and 

• Toxicity equivalent factor (TEF) values for coplanar PCBs, PCDD, and PCDF. 
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Issue: Toxicity Factor Determination - Selection of COPCs 

It was suggested that the criteria air pollutants (sulfur dioxide, nitrogen dioxide, ozone and carbon 

monoxide) should not be included in a quantitative risk assessment because they lack toxicity factors. No 

toxicity values are listed in draft HHRAP Appendix A, IRIS, or HEAST. 

EPA Response: 

Availability of chemical-specific toxicity information is a criterion in our recommended 5-step process for 

choosing which COPCs to assess in a risk assessment (see HHRAP Chapter 2 Facility Characterization). 

We recommend performing a quantitative risk assessment on any chemical that satisfies all criteria in our 

5-step process. As with many parameters and equations in the HHRAP, we’ve provided the scientific 

basis and reference behind our process. A facility is then able to evaluate whether to propose and justify 

to the permitting authority a site-specific option, or other alternative to that recommended in the HHRAP. 

Issue: Toxicity Factor Determination - Realistic and Current Values 

Comments regarding the selection of toxicity factors for use in assessing risks and hazards included: 

• The suggestion that available toxicity values may not match real-world exposures. In some 

instances, toxicity values cannot be applied to the hazardous waste incineration model because the 

likely exposure scenarios do not match.

• A noted discrepancy in the draft HHRAP regarding the order of preference for obtaining health 

benchmarks. A different order is specified in Volume I, Sections 7.2 and 7.3 as compared to 

Appendix 3. Specifically, Agency for Toxic Substances Disease Registry (ATSDR) is not 

mentioned in Appendix 3. Additionally, the most recent edition of HEAST is not referenced. 

• Toxicity values are subject to change over time with the addition of new data, and updates are 

needed. Tables containing out-of-date toxicity values should be removed. 

EPA Response: 

We have revised the hierarchy of recommended sources of toxicity information, and have updated the 

references. The hierarchy of recommended sources of toxicity information is presented in Appendix A. 

The HHRAP has been revised taking into account the most recent science. 

We recommend using the best available data throughout the risk assessment process. Any part of the 

guidance can and will go out of date, however, seldom do toxicity factors change by significant amounts 

across the board. Therefore, the numbers provided serve as a useful sounding board with which to 

conduct an initial analysis. As with many parameters and procedures provided in the HHRAP, the 

scientific basis and reference are provided. A facility is then able to evaluate whether to propose and 

justify to the permitting authority a site-specific option, or other alternative to that recommended in the 

HHRAP. 

Issue: Toxicity Factor Determination - Basis of Toxicity Values 

Comments regarding the basis for the derivation of toxicity values include: 

• The tables do not indicate whether the reference doses are based on chronic or subchronic data, and 

it was recommended that these references be included. 
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• Cancer toxicity data for COPCs not classifiable as carcinogens have been labeled as “ND”, which is 

misleading. 

• More detailed information should be provided on how to determine chemical-specific parameters 

for COPCs not included in draft HHRAP Appendix A3. 

EPA Response: 

The reference we have provided for each value can be used to investigate the basis of the toxicity 

information. As with many parameters and procedures provided in the HHRAP, the scientific basis and 

reference are provided. A facility is then able to evaluate whether to propose and justify to the permitting 

authority a site-specific option, or other alternative to that recommended in the HHRAP. 

 

The methods we used to obtain chemical-specific parameters, including the scientific bases, references, 

and equation, are provided in the draft HHRAP. Similar methods and references may be followed to 

obtain information on more chemicals. The chemicals presented in the draft HHRAP are those typically 

measured by the analytical methodologies carried out in a trial or risk burn. 

Issue: Toxicity Factor Determination - Absorption Assumptions 

It was noted that in the draft HHRAP, 100% absorption for route-to-route extrapolations is generally 

assumed. Therefore, the basis for requiring chronic RfDs or RfCs be applied to subchronic exposures was 

questioned, and clarification was requested about the justification for modifying the absorption factor. 

 

EPA Response: 

Assuming 100% absorption is a screening assumption to be used when chemical-specific data are not 

available. Also note, that the HHRAP is guidance and does not require chronic RfD’s or RfC’s be 

applied to sub-chronic exposures. As with many parameters and procedures provided in the HHRAP, the 

scientific basis and reference are provided. A facility is then able to evaluate whether to propose and 

justify to the permitting authority a site-specific option, or other alternative to that recommended in the 

HHRAP.

Issue: Toxicity Factor Determination - Calculation of Toxicity Factors 

A question was raised about the statement that risk assessors should develop acute inhalation exposure 

criteria (AIECs) for compounds not addressed by available criteria. 

EPA Response: 

The HHRAP does not state that AIECs should be calculated but only states that they can be calculated, 

and it provides a reference with the method to do so.

Issue: Toxicity Factor Determination - Separate versus Cumulative Target Organ Toxicity 

The question was raised about when it is appropriate to evaluate target organ toxicity for individual 

COPCs versus cumulative COPCs. The evaluation of COPCs separately could result in underestimating 

risk. 
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EPA Response: 

This is not part of the HHRAP, but rather part of the risk management addendum developed by EPA 

Region 6. 

Issue: Toxicity Factor Determination - Evaluation of PICs 

The basis of categorizing chemicals with regard to low, medium, or high toxicity was questioned. It was 

suggested that such classification might be used to eliminate PICs. 

EPA Response: 

Whether or not to evaluate PIC’s is a site-specific decision. General metrics, which can be used in such 

an evaluation, are presented in Section A1.7. As with many parameters and procedures provided in the 

HHRAP, the scientific basis and reference are provided. A facility is then able to evaluate whether to 

propose and justify to the permitting authority a site-specific option, or other alternative to that 

recommended in the HHRAP. 

Issue: Toxicity Factor Determination - TEF Values for Coplanar PCBs, PCDD and PCDF 

[draft HHRAP Section 2.3] (See RC Section 3.4.6) 

The toxicity equivalency factors (TEFs) listed for coplanar PCBs, PCDD, and PCDF do not reflect the 

most up-to-date revisions made by the World Health Organization (WHO) (draft HHRAP Chapter 2, 

pages 2-42 and 2-49). These TEFs are based on the most recent science and were made available at 

Dioxin '97, 17th International Symposium on Chlorinated Dioxins and Related Compounds, Indianapolis, 

Indiana, USA, August 25-29, 1997. 

It was suggested that it is not appropriate to evaluate potential non-carcinogenic effects of PCBs by using 

the oral reference dose (RfD) for Aroclor 1254 or Aroclor 1016 (draft HHRAP Chapter 2, page 2-51). 

EPA Response: 

We have edited the HHRAP to reflect the most up-to-date TEFs. As with many parameters and 

procedures provided in the HHRAP, the scientific basis and reference are provided. A facility is then 

able to evaluate whether to propose and justify to the permitting authority a site-specific option, or other 

alternative to that recommended in the HHRAP. 

2.7.3 Route-to-route Extrapolation of Toxicity Benchmarks 

 [draft HHRAP Sections 7.2 and 7.3, Appendix A.3] (See RC Section 3.4.2)

Route-to-route extrapolation issues included: 

• Caution should be exercised when making route-to route extrapolations; 

• The need for additional information on how to conduct a qualitative route-to-route extrapolation; 

• The suggestion that route-to-route extrapolation should not be conducted because of the inherent 

uncertainty; and 

• Clarification about the application of route-to-route extrapolation to carcinogens. 
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Issue: Route-to-Route Extrapolation - Cautions for Using Route-to-Route Extrapolations 

Commenters suggested caution should be exercised in the calculation of CSFs from URFs or of URFs 

from CSFs assuming route-to-route extrapolation because in many cases, COPCs can elicit route-of-entry 

effects or undergo first-pass metabolism in the respiratory system. Similarly, caution should be exercised 

in the calculation of RfDs from RfCs or of RfCs from RfDs 

It was recommended that data from the National Center for Environmental Assessment (NCEA) research 

effort be incorporated into the HHRAP to more accurately define where route-to-route extrapolations 

would be appropriate. 

EPA Response: 

We agree that caution should be exercised in conducting route-to-route extrapolations. The NCEA 

authors of the “Methods for Derivation of Inhalation Reference Concentrations and Application of 

Inhalation Dosimetry” (1994a, EPA/600/8-90/066F) were consulted on the methodology presented in the 

HHRAP and agreed with it as a screening methodology. This issue was reviewed during the external 

peer review, and the peer reviewers also agreed with it as a screening methodology. 

Issue: Route-to-Route Extrapolation - Additional Information 

It was suggested that additional information should be provided regarding how a qualitative assessment of 

route-to-route extrapolation should be performed. 

EPA Response:

We’ve added to this section a reference to the EPA document, “Methods for Derivation of Inhalation 

Reference Concentrations and Application of Inhalation Dosimetry” (1994a, EPA/600/8-90/066F), to 

provide insight into what should be considered in evaluating route-to-route extrapolations. 

Issue: Route-to-Route Extrapolation - Inherent Uncertainty 

It was suggested that it is inappropriate to use oral toxicity values to evaluate inhalation risk or hazard 

because these calculations are not scientifically valid or they contradict other EPA guidance (draft 

HHRAP Chapter 7, page 7-4). If route-to-route extrapolations are to be performed, information should be 

given to scientifically defend the method used for each individual chemical. 

EPA Response: 

The HHRAP recommends that caution be exercised in calculating RfDs from RfCs or RfCs from RfDs 

assuming route-to-route extrapolation, since in many cases, COPCs can elicit route-of-entry effects or 

undergo first-pass metabolism in the respiratory system (U.S. EPA 1994a, EPA/600/8-90/066F). 

However, the HHRAP recommends evaluating as many COPCs as possible in the risk assessment. 

Hence, there is a need to conduct route-to-route extrapolations.

The authors of the Methods for Derivation of Inhalation Reference Concentrations and Application of 

Inhalation Dosimetry were consulted on the methodology presented in the HHRAP and agreed with it as 

a screening methodology. This issue was reviewed during the external peer review, and the peer 

reviewers also agreed with it as a screening methodology. 
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Issue: Route-to-Route Extrapolation - Application to Carcinogens 

It was noted that the draft HHRAP contains route-to-route extrapolation to assess non-carcinogenic 

effects from inhalation in the absence of RfCs. However, it seems that the same is not done for 

carcinogens as there is no discussion in draft HHRAP Section 7.2. It was recommended that EPA 

examine why route-to-route extrapolation for carcinogenic COPCs is not contained in Section 7.2. 

EPA Response: 

Section 7.2 of the peer review draft of the HHRAP presents one example of route-to-route extrapolation. 

We discuss all methods of route-to-route extrapolation used in the HHRAP in Appendix A Section A3.6.3. 

2.7.4 Inhalation Toxicity Benchmarks

Issues that were raised regarding inhalation toxicity benchmarks included: 

• Inhalation RfD versus Inhalation RfC; and 

• Calculation of RfC and URFs from RfDs and CSFs 

Note that several comments about inhalation toxicity benchmarks are included in this Response to 

Comments document, Section 2.7.3 Route-to-Route Extrapolations. 

Issue: Inhalation Toxicity Benchmarks - Inhalation RfD versus Inhalation RfC 

Clarification is needed on the difference between the inhalation RfD and the inhalation RfC, and how the 

inhalation RfD is to be used in risk assessment.

EPA Response: 

The inhalation RfD is used in the risk assessment so that site-specific exposure durations can be assessed. 

RfC's are not sensitive to exposure duration. The inhalation RfD differs from the oral RfD in the toxicity 

information used to derive it. Our recommendation has been revised to utilize the RfC rather than the 

inhalation RfD. We provide an equation to calculate the inhalation RfC from the RfD in HHRAP 

Appendix A, Section A3.6.3. 

Issue: Inhalation Toxicity Benchmarks - Calculation of RfC and URFs from RfDs and CSFs 

Several questions were raised about the methodology for calculating RfCs from RfDs, mainly the 

methodology to convert CSFs and RfDs to URFs and RfCs, as well as what procedure should be used if 

CSFs and RfDs are not available. Clarification is needed regarding whether the CSF is to be calculated 

from the URF only when the URF is available and if not, whether the URF should be calculated from the 

oral CSF. 

 

EPA Response: 

The HHRAP includes information to calculate URFs and RfCs in Section 7.3, and chemical-specific 

examples are provided in Appendix A. The procedure for handling the calculations in the absence of 

CSFs and RfDs is explained in A3.6.3 under number 6 b.
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2.7.5 Non-cancer Effects Estimation

Issues regarding estimation of non-cancer effects included: 

• Selection and inclusion of COPCs and the identification of target organs; 

• Calculation of hazard quotients (HQs); and 

• Consideration of long-term/lifetime exposures in the calculation of HQs. 

Issue: Non-cancer Effects Estimation - Selection of COPCs and the Identification of Target Organs 

EPA was asked to provide the basis and the method used to identify target organs for specific chemicals 

included in draft HHRAP Table A-2. How COPCs and target organs might be excluded from Table A-2 

should also be explained. 

EPA Response: 

We present Table A-2 as a starting point for the refined assessment of non-carcinogens. As stated in 

Table A-2, the list of chemicals is by no means exhaustive. It is correct that more than one target organ is 

often listed. This seems reasonable in that there are different routes of potential exposure. 

We’ve provided a general description of how critical effects are used in selecting RfD and RfC values 

following the target organ table in Appendix A-2. 

Issue: Non-cancer Effects Estimation - Calculation of Hazard Quotients (HQs) 

It was suggested that there may be contradictions in the recommendations for calculating the Hazard 

Index. The text should be reworded so that average exposure duration is calculated. 

EPA Response: 

We’ve reviewed the subject text and found no contradictions. As with many parameters and procedures 

provided in the HHRAP, the scientific basis and reference are provided. A facility is then able to evaluate 

whether to propose and justify to the permitting authority an alternative they consider more appropriate 

than that recommended in the HHRAP.

Issue: Non-cancer Effects Estimation - Consideration of Long-term/Lifetime Exposures 

The statement in the draft HHRAP suggesting that hazard assessments for non-carcinogenic COPCs are 

not based on lifetime exposures is incorrect, as chronic reference doses are derived to be protective of 

long-term (7 years to lifetime) exposures. 

EPA Response: 

We agree, and have corrected the sentence. 

2.7.6 Risk and Hazard Characterization - Clarification / Editorial

Issue: Editorial 

In draft HHRAP Chapter 7 Introduction, 2nd paragraph, 2nd sentence: Change “and” to “an”. In the 3rd
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sentence, add “during his lifetime” after “developing cancer.” 

EPA Response: 

Comments have been noted, and editorial errors and text revisions have been corrected, where 

applicable. 

2.8 UNCERTAINTY INTERPRETATION FOR HUMAN HEALTH RISK ASSESSMENT 

[draft HHRAP Section 8] 

Uncertainty may be introduced into a health risk assessment at all of the stages described in the draft 

HHRAP. It may be classified into four types: variable uncertainty, model uncertainty, decision-rule 

uncertainty, and variability. Some sources of uncertainty may be quantifiable. Others may not. In both 

cases, it is important for risk assessments of facilities that burn hazardous wastes to fully explain the areas 

of uncertainty in the risk assessments and to identify the key assumptions used in conducting the 

assessments. 

Comments received regarding uncertainty in risk assessments included: 

• General Issues; 

• Qualitative Uncertainty; 

• Quantitative Uncertainty; and 

• Clarification / Editorial. 

2.8.1 Uncertainty Interpretation - General Uncertainty Comments

General issues included: 

• Completeness of the Guidance; 

• Uncertainty’s Effects on Estimation of Exposure; and 

• Uncertainty and Risk Management. 

Issue: General - Completeness of the Guidance 

It was suggested that the discussion in draft HHRAP Chapter 8 about the need for uncertainty analyses is 

neither sufficient in detail nor emphasis to convey that this is an integral and necessary element of the risk 

assessment process. It was recommended that EPA incorporate both sensitivity studies and uncertainty 

analyses into its risk assessment protocol. Uncertainty of a given variable, parameter, or process should 

be assessed relative to its impact on the overall uncertainty in the associated risk that results from all 

sources, and it should be further assessed relative to the uncertainty in each of the other components of 

the risk assessment process. This means that uncertainty of a variable needs to be considered in the 

context of the sensitivity of the estimated risk to that variable and its uncertainty.
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EPA Response: 

Descriptions of both qualitative and quantitative methods for conducting uncertainty analyses are 

presented in the HHRAP. Sensitivity studies are one tool a risk assessor can use to quantitatively bound 

the uncertainty of an input parameter. The HHRAP includes references to sensitivity studies, about which 

EPA is aware, within the technical sections to which they pertain (e.g., ISCST3 air model input sensitivity 

analysis in Chapter 3). Within the HHRAP, we’ve identified uncertainties and limitations within the 

discussion of specific technical issues (e.g., TOE, estimates of emission rates, COPC selection process, 

quantifying non-detects) as they are presented in their respective sections. We present the limitations 

associated with parameter values and inputs to equations in Appendices A, and B and C, respectively. 

Issue: General - Uncertainty’s Effects on Estimation of Exposure 

It was stated that uncertainty about indirect exposure pathways would result in overestimating exposure. 

Specific examples were requested about when risk estimates would be considered overly conservative.

EPA Response: 

The HHRAP provides a formal discussion of uncertainty in Chapter 8 and descriptions of all of the 

parameters used to calculate risk and estimate exposure in Appendices A, B and C. We acknowledge that 

uncertainty may result in overestimation or underestimation of risk. 

Issue: General - Uncertainty and Risk Management 

It was noted that uncertainty in a risk assessment might exist because of the lack or quality of information 

about pathways and the toxicity of chemical substances. Concern was expressed that, although the draft 

HHRAP provides qualitative and quantitative guidance to assess uncertainty and variability in the risk 

assessment process, it does not detail how the uncertainty in these parameters and in general may affect 

permitting decisions and the risk management process. 

EPA Response:

How uncertainty in a risk assessment is interpreted is a risk management decision, beyond the scope of 

the HHRAP. Section 8.5 provides a description of the materials we recommend be summarized and 

presented to the permit writer for introduction into the decision making process.

Issue: General - Uncertainty and Risk Management 

It was suggested that risk assessments should only be performed for facilities and units that are part of the 

permitting process. 

EPA Response: 

This is a risk management decision, and beyond the scope of the HHRAP. 

2.8.2 Uncertainty Interpretation - Qualitative Uncertainty [draft HHRAP Section 8.3]

Issue: Qualitative Uncertainty - TICs 

Clarification was requested about how information from tentatively identified compounds (TIC) could be 

used in uncertainty analysis. 
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EPA Response: 

The state-of-the-science for TIC’s has been presented in the HHRAP. Currently, TIC analysis in risk 

assessment is highly uncertain. 

2.8.3 Uncertainty Interpretation - Quantitative Uncertainty [draft HHRAP Section 8.4]

Issue: Quantitative Uncertainty - Quantitative Uncertainty Analysis 

It was stated that quantitative uncertainty analysis might require more work than is reasonable for 

assessing risk. The extensive level of quantitative assessment described in the draft HHRAP should not 

be required to account for uncertainty. 

EPA Response: 

The HHRAP presents only guidance. The level of detail necessary for a risk assessment is site-specific 

and should be negotiated with the permitting authority. (Examples of risk assessment reports accepted by 

permitting authorities are available on the EPA Region 6 web site.) 

Issue: Quantitative Uncertainty - Probabilistic versus Deterministic Analyses 

It was stated that the HHRAP should allow for the use of distributional analysis and probabilistic methods 

instead of relying on deterministic analysis. The use of maximum single point exposures in air quality 

impacts was specifically noted as an area where distributional analysis would be more effective. It was 

suggested that risks and hazards should be presented as distributions rather than point estimates, and EPA 

should conduct a general sensitivity and uncertainty analysis before finalizing the draft HHRAP. 

EPA Response: 

As guidance, the HHRAP is not in the position of “allowing” or “disallowing” anything. The HHRAP is 

flexible enough to include probabilistic risk assessment, but recommends that a deterministic analysis be 

performed as well. Experience in conducting risk assessments for combustion facilities has shown that 

probabilistic assessments are most valuable in the sensitivity analysis of the most sensitive parameters, 

such as compounds and exposure pathways driving risk. Deterministic evaluations are effective in 

helping to identify these sensitive parameters. 

Issue: Quantitative Uncertainty - Adequacy of the Guidance 

It was recommended that the HHRAP include more information on how to conduct a quantitative 

uncertainty analysis, unless the reader is referred to another source (e.g., EPA's “Draft Risk Assessment 

Guidance for Superfund Volume 3 - Part A: Process for Conducting Probabilistic Risk Assessment” 

(1999c)). It was noted that the reader is not given any guidance in the draft HHRAP for deciding when a 

detailed quantitative treatment of uncertainty is required. While there is enough guidance in Chapter 8 

about the qualitative description of uncertainties, there is no discussion about how this type of 

information should be incorporated into the risk-based decision making process. 

EPA Response: 

We’ve modified the HHRAP to refer the reader to EPA's “Draft Risk Assessment Guidance for Superfund 

Volume 3 - Part A: Process for Conducting Probabilistic Risk Assessment” (December 1999c). Whether 
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a detailed quantitative treatment of uncertainty is required is a decision of the permitting authority. 

Interpretation of how qualitative uncertainty analyses are incorporated into the risk-based decision 

making process is also the responsibility of the permitting authority. 

2.8.4 Uncertainty Interpretation – Clarification / Editorial

Issue: Clarification - “Typical” Accuracies 

Draft HHRAP Section 8.5: It was stated that the sentence on “typical” accuracies of dispersion model 

estimates is arbitrary without further clarification of the conditions under which such accuracies are 

achieved. Many situations achieve much poorer accuracies. 

EPA Response: 

A reference is provided for use of the word typically as follows: “When applied properly, air dispersion 

models are typically accurate to ± 10 to 40 percent and can be used to yield a “best estimate” of air 

concentrations (Title 51 CFR Appendix W).” 

Issue: Clarification - Relative Quantification

Draft HHRAP Section 8.5, page 8-7, lines 5-6 from bottom: It was recommended that some relative 

quantification of what is meant by high, medium, and low effect be given. For instance, will a high effect 

give an uncertainty of 2 orders of magnitude versus 5-fold? 

EPA Response: 

A relative quantification of what is meant by high, medium, and low is beyond the scope of the HHRAP. 

The relative quantification of the magnitude of an effect is parameter-specific and while a variation of 

one order of magnitude for one input parameter may be considered low, for another input parameter one 

order of magnitude may be quite high. 

Issue: Editorial

Draft HHRAP Section 8.5, 2nd paragraph: TOE (total organic emissions) is not included in the acronym 

list 

EPA Response: 

We’ve added TOE to the acronym list. 

2.9 COMPLETION OF RISK ASSESSMENT AND FOLLOW-ON ACTIVITIES 

[draft HHRAP Section 9] 

Issue: Follow-on Activities - Activities Following the Risk Assessment 

It was suggested that the statements in Section 9.2, page 9-2, last paragraph leave too much to the 

discretion of the facility and/or permitting authority. This section should specify a minimum amount of 

time between reviews and the types of changes in process and procedure that would require a review of 

the facility.
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EPA Response: 

The recommendations made by the commenter are the purview of the permitting authority. 

Issue: Follow-on Activities - Conclusions 

It was suggested that it might be useful to include in the Conclusions the major sources of uncertainty and 

an estimate of how many fold the error might be.

EPA Response: 

The HHRAP recommends that the conclusions interpret the results of the risk and hazard 

characterization in light of the uncertainty analysis. 

2.10 CHEMICAL SPECIFIC DATA (draft HHRAP Appendix A) 

Draft HHRAP Appendix A details a range of chemical specific data in four areas: 

• Chemicals for consideration as compounds of potential concern (COPC); 

• Target organs and critical effects for compounds with reference doses; 

• Compound specific parameter values; and 

• Acute benchmarks. 

A significant number of comments were received regarding the chemical specific data. These comments 

included: 

• Accuracy and Completeness of Data; 

• Accuracy of Equations; 

• Inconsistencies; 

• COPCs and PICs; 

• Missing Material; and 

• Specific Issues. 

Issue: Chemical Specific Data - Accuracy of the Data in Appendix A 

It was generally noted that draft HHRAP Appendix A had numerous errors. Commenters felt that some 

of the values in Appendix A conflicted with the chemical specific data found in other EPA documents and 

guidance, and that the guidance lacked internal consistency. Commenters recommended that EPA 

undertake a review of the data to ensure consistency and accuracy and that EPA establish one set of 

physical, chemical, and toxicological parameters that has been both internally and externally peer 

reviewed. 

EPA Response: 

Appendix A parameters have been reviewed in their entirety and have been corrected or updated where 

appropriate. 
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Issue: Chemical Specific Data - Errors and Incomplete Information 

Commenters noted errors and incomplete information in chemical-specific parameters in the following 

areas: 

• Acute inhalation exposure criteria are cited incorrectly or are out of date. 

• Chemical-specific inputs for Aroclor 1016 and 1254 are incorrect. 

• Chemical-specific inputs for Benzo(a)Pyrene are incorrect. 

• Chemical-specific inputs for 2,3,7,8-Tetrachlorodibenzo(p)dioxin are incorrect. 

• The values for the fraction of dioxin congener assumed to be in vapor state are incorrect for 2,3,7,8-

TCDD and OCDD. 

• The biota sediment accumulation factor (BSAF) values on draft HHRAP page A-3-38 all have the 

same name. 

• The dioxin toxicity equivalency factors (TEFs) are not based on the most recent World Health 

Organization recommended TEFs. 

• The TEFs for polychlorinated biphenyls are not based on the most recent World Health 

Organization recommended TEFs. 

• The tables in draft HHRAP Appendix A do not indicate whether the listed reference doses are 

chronic or sub-chronic. 

• It is misleading for COPCs not classifiable as carcinogens to be labeled “ND,” implying that cancer 

slope factors are not available for them. 

• The last paragraph of draft HHRAP Section 5.3.2 should state that VGag is 0.01 for lipophilic 

COPCs rather than the 0.001 shown. The correct value of 0.01 is stated in the second to the last 

paragraph in Section 5.3.2.1. 

• The vapor pressure (Vp) shown for 2,3,7,8-TCDD in draft HHRAP Table A-3-176 does not 

correspond with that recommended by EPA in the referenced document (cited as U.S. EPA, 1994a 

in the draft HHRAP). The referenced document shows that EPA recommends a Vp of 7.4 x 10-10

mm Hg at 250

C (9.7 x 10-13 atm). These values are presented on page 2-13 (Volume II) and page 7-

66 (Volume 3) in USEPA (1994a). 

 

EPA Response: 

Appendix A parameters have been reviewed in their entirety and have been corrected or updated where 

appropriate. TEFs have been updated with the latest information from the World Health Organization. 

With regard to the lack of subchronic and chronic detail for the reference doses, the reference for each 

RfD value is provided and can be used to investigate the basis of the toxicity information. 

With regard to COPCs not classifiable as carcinogens, as with many parameters and equations provided 

in the HHRAP, the scientific basis and references are provided. A facility is then able to evaluate 

whether to propose and justify to the permitting authority an alternative they consider more appropriate 

than that recommended in the HHRAP. 

 

With regard to the VGag and Vp values, editorial errors have been reviewed and corrected where 

applicable. 
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Issue: Chemical Specific Data - Accuracy of Equations

Commenters noted apparent inaccuracies in draft HHRAP Appendix A equations: 

• Appendix A-1, Section A3.6.3 Explanation of Calculated Toxicity Benchmark Values Comment: 

There is a typographical error in the equation for calculating the inhalation URF (A3.6.3 5). If it is 

the inhalation URF that is being calculated, then this same number cannot be part of the equation. 

• Equation 5-1C should reduce to a form similar to equation 5-lE when T1 is zero. This is not the 

case, and hence equation 5-1C should be checked for computational and dimensional correctness. 

• The formula for the fraction of a COPC in the vapor phase (Fv) in draft HHRAP Table B-1-1 (page 

9 of 11) has an error. The correct formula is presented in equation A-3-10 in Appendix A-3. 

EPA Response: 

Comments noted. Editorial errors have been reviewed and corrected, where applicable. 

Issue: Chemical Specific Data - Inconsistencies 

Commenters noted the following areas where draft HHRAP Appendix A was different from other 

sections of the draft HHRAP: 

• Commenters found an inconsistency between draft HHRAP Figure 2.3 and the information 

presented in Appendix A for considering COPCs and PICs. 

• The order of preference for obtaining health benchmarks is different in Appendix A than that 

described in the body of the guidance. 

• Appendix A fails to reference the 1997 version of HEAST as being preferred to the 1995 version. 

• Appendix A references different material from the body of the text with regard to soil concentration 

factors for aboveground produce. The reference does not appear in the document. 

• Draft HHRAP Section 2.3, page 2-38, line 5 references Table A-1.9-5, but this table is missing 

from Appendix A. 

EPA Response: 

Appendix A parameters have been reviewed in their entirety and have been corrected or updated where 

appropriate. Figure 2.3 was correct, and the Appendix was modified to reflect that fact. The order of 

preference for obtaining health benchmark information has been updated so that it is consistent. The 

1997 version of HEAST is noted as being preferred over the 1995 version. The references have been 

clarified. Reference to Table A-1.9-5 has been removed from the text in Chapter 2.

Issue: Chemical Specific Data - COPCs and PICs 

Commenters noted the following problems with the information provided in draft HHRAP Appendix A 

regarding COPCs and PICs: 

• The information provided in Table A-1 is not useful for identifying COPCs from hazardous waste 

emissions. The information provided in Table A1:9-1, Table A1:9-2A, and Table A1:9-2B is 

confusing.

• The guidance does not specify how to determine chemical-specific parameters for COPCs not 

included in Appendix A.
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EPA Response: 

Tables A-1.9-1 through A-1.9-5, and all references to them, have been removed from the HHRAP 

Methods including the scientific basis and reference for obtaining chemical-specific parameters and 

equations are provided in HHRAP. Similar methods and references may be followed to obtain 

information on more chemicals. The chemicals selected for presentation in the HHRAP are those 

typically measured by the analytical methods carried out in a trial or risk burn. 

Issue: Chemical Specific Data - COPCs and PICs 

Commenters stated that compounds with low toxicity values are listed as those that were deleted from the 

list of PICs. Clarification was requested about how to determine whether a compound has a low, 

medium, or high toxicity. 

EPA Response: 

Evaluation of PICs is a site-specific determination. General metrics that can be used in such an 

evaluation are presented in Section A1.7. As with many parameters and equations provided in the 

HHRAP, the scientific basis and references are provided. A facility is then able to evaluate whether to 

propose and justify to the permitting authority an alternative they consider more appropriate than that 

recommended in the HHRAP. 

 

Issue: Chemical Specific Data - Material Missing from a Previous Draft 

It was noted that a table in the Appendix of a previous draft of the HHRAP document had been removed. 

This table contained an extensive list of chemicals for consideration as COPCs. 

EPA Response: 

Comments received on the original draft requested that the table be removed so that it could not be 

misinterpreted in the selection of COPCs. We agreed with the potential for confusion, and removed table. 

Issue: Chemical Specific Data - PAH 

Commenters stated that, to avoid confusion, chemical-specific inputs for benzo(a)pyrene (BaP) only 

should be provided in Appendix A, and the data for the other six carcinogenic PAHs should be removed. 

EPA Response: 

We disagree. Fate and transport characteristics of different PAH congeners result in different 

distribution characteristics in the environment. It is important to model each congener separately before 

normalizing them to BaP equivalents. 

Issue: Chemical Specific Data - Applicability of Partition Coefficients for Soil Water 

It was stated that the draft HHRAP default sediment foc values might be too high for many water bodies 

in Texas. 
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EPA Response: 

As with many parameters and equations provided in the HHRAP, the scientific basis and reference are 

provided. A facility is then able to evaluate whether to propose and justify to the permitting authority an 

alternative they consider more appropriate than that recommended in the HHRAP. 

Issue: Chemical Specific Data - The Use of Route-to-Route Extrapolation Values 

 (See RC Section 3.4.2) 

EPA was requested to consider specifying exactly what should be included in the qualitative analysis that 

discusses route-to-route extrapolation of toxicity benchmarks. 

Commenters requested that route-to-route extrapolation values not be included in the chemical specific 

input tables as their inclusion gives them almost equal significance to verifiable RfCs. Alternatively, if 

route-to-route extrapolation values are included in the chemical specific inputs, the fact that the values are 

uncertain and potentially unverifiable should be emphasized in each of corresponding input tables. 

EPA Response: 

A reference to the EPA document, "Methods for Derivation of Inhalation Reference Concentrations and 

Application of Inhalation Dosimetry" (1994a, EPA/600/8-90/066F) has been added to provide insight as 

to what one should consider in the evaluation of route-to-route extrapolations. 

The HHRAP clearly states that rout-to-route extrapolations should be used with caution. Authors of the 

"Methods for Derivation of Inhalation Reference Concentrations and Application of Inhalation 

Dosimetry" were consulted on the methodology presented in the HHRAP and agreed with it as a 

screening methodology. This issue was reviewed during the external peer review, and the peer reviewers 

also agreed with it as a screening methodology. 

Issue: Chemical Specific Data - Modifying IRIS Risk Values 

Appendix 1A of the updated Exposure Factors Handbook (EPA/600/P-95/002Fa, August 1997) discusses 

procedures for ensuring that assumptions about population parameters in the dose-response analysis are 

consistent with the population parameters used in the exposure analysis. It may be necessary to adjust the 

IRIS values for certain contaminants. 

EPA Response: 

Comment noted. As with all parameters and methodologies presented in the HHRAP, documentation and 

references are provided such that one can make a determination of when new science or site-specific 

parameters are more appropriate than the recommended default. 

Issue: Chemical Specific Data - Explanation of Calculated Toxicity Benchmark Values 

Draft HHRAP Table A-3: For “Health Benchmarks,” the type of toxic effect on which the benchmark 

values are based should be briefly indicated for each compound. Where values are missing, it is assumed 

that values for these health benchmarks are not available. If so, it would be useful to clearly indicate this 

in a key at the beginning of the table. It is noted that for the halo-substituted dibenzodioxin and 

dibenzofurans, no health benchmarks are listed. These values should be calculated on the basis of TEFs 

in comparison to the 2,3,7,8-tetrachlorodibenzo-p-dioxin (as has been performed for the PAHs). 
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EPA Response: 

Each table in Appendix A is footnoted to indicate whether a value is not applicable or whether there is no 

data available. For dioxin and furan congeners, TEF values are provided in Appendix A in order to 

calculate toxicity using the TCDD health benchmarks. 

Issue: Chemical Specific Data - Compound Specific Parameter Values 

HHRAP Page A-3-33, section A3.5.4.2: The exact difference between BAF as defined here and BCF as 

defined in A3.5.4.1 is not clear. This should be explained in simple terminology. 

EPA Response: 

The difference noted in the HHRAP between a BCF and a BAF is that a BCF accounts for chemical 

transfer from the water through the gills, whereas a BAF accounts for chemical transfer from water and 

sediment through the gills as well as chemical transfer through food consumption. BCF’s and BAF’s will 

generally be similar for chemicals that do not bioacumulate in the food chain as well as and at low 

torphic levels of the food chain. 

Issue: Chemical Specific Data - Compound Specific Parameter Values 

Draft HHRAP Page A-3-2, last sentence: Was any consideration given to variability? Depending on the 

variability and goodness of the data, using the geometric mean often may not be the best approach. 

EPA Response: 

Compound-specific parameters were selected following the hierarchy found in the Superfund Chemical 

Data Matrix (SCDM). They do consider variability and where the geometric mean was not appropriate, 

it was not utilized. 

2.11 MEDIA CONCENTRATION EQUATIONS (draft HHRAP Appendix B) 

Issue: Media Concentration Equations - USLE Cover Management Factor (C) Value 

Draft HHRAP page B-256: The USLE cover management factor (C) can be a very important aspect of the 

estimation of erosion loading to a water body (Renard et al., 1997). Application of a C value of 0.1 for an 

entire watershed is much more appropriate than higher values especially as most row crops are estimated 

to justify a C of 0.1. Higher values are likely to be overly conservative. 

EPA Response: 

The HHRAP recommends that the universal soil loss equation (USLE) for cover management be 

determined and documented on a site-specific basis.

Issue: Media Concentration Equations - COPC in Air and Water 

Draft HHRAP B-251: The draft HHRAP separately considers diffusional loading to the water body and 

volatilization losses from the water body. Consideration should be given to collapsing these two 

approaches to estimate their net effect given COPC concentrations in air and water. 
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EPA Response: 

Comment noted. The recommendation was passed on to the EPA’s Office of Research and Development. 

Issue: Media Concentration Equations - Sediment Enrichment Ratio (ER) 

Draft HHRAP page B-211: The sediment enrichment ratio (ER) is intended to account for the more ready 

erosion of light, relatively high organic soils. While this phenomenon has been observed in agriculture, it 

has not been demonstrated at the low concentrations and diverse watersheds likely to be of interest to the 

HHRAP. More support should be provided for an estimate of ER of 3 for organic compounds. This 

might include review of the agricultural literature or a comparison of COPC concentrations on in-situ 

soils versus those being eroded. 

EPA Response: 

Comment noted. The recommendation was passed on to the EPA’s Office of Research and Development. 

2.12 RISK CHARACTERIZATION EQUATIONS [draft HHRAP Appendix C]

Issue: Risk Characterization Equations - Inhalation Cancer Risk Calculation 

Draft HHRAP Table C-2-1 Inhalation Cancer Risk for Individual Chemicals: The draft HHRAP states 

that Cancer Risk(i) = ADI CSF(i). It was recommended that the total COPC air concentration (Ca) be 

directly used with unit risk factors (URFs), where available, rather than calculating an internal dose from 

inhalation of air concentrations of these COPCs (“Methods for Derivation of Inhalation Reference 

Concentrations and Application of Inhalation Dosimetry” (1994a, EPA/600/8-90/066F)). 

EPA Response: 

The HHRAP has been revised such that the total COPC concentration in air is directly used with unit risk 

factor. As with many parameters and equations provided in the HHRAP, the scientific basis and 

reference are provided. A facility is then able to evaluate whether to propose and justify to the permitting 

authority an alternative they consider more appropriate than that recommended in the HHRAP. 

 

Issue: Risk Characterization Equations - Inhalation Hazard Quotient 

Table C-2-2 Inhalation Hazard Quotient for COPCS: Noncarcinogens Issue. The HHRAP states that 

HQ(i) = ADI(i) / RfD(i). It was recommended that the total COPC air concentration (Ca) be directly used 

with reference concentrations (RfCs), where available, rather than calculating an internal dose from 

inhalation of air concentrations of these COPCs (“Methods for Derivation of Inhalation Reference 

Concentrations and Application of Inhalation Dosimetry” (1994a, EPA/600/8-90/066F)). 

EPA Response: 

The HHRAP has been revised such that the total COPC concentration in air is directly used with 

reference concentrations. As with many parameters and equations provided in the HHRAP, the scientific 

basis and reference are provided. A facility is then able to evaluate whether to propose and justify to the 

permitting authority an alternative they consider more appropriate than that recommended in the 

HHRAP. 
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3.0 PEER REVIEW COMMENTS ON MAJOR ISSUES RELATED TO 

THE DRAFT HHRAP

This section contains summaries of the external peer reviewers’ responses to 30 major issues provided to 

them by EPA. These major issues were based on public comments on the draft HHRAP for which EPA 

wanted outside expert input and opinion to fully address the public comments. The 30 issues are 

organized to generally map to topics discussed in specific chapters in the draft HHRAP, primarily 

Chapters 2 through 6 and Appendices A, B, and C. Of the 30 major issues, the peer reviewers had written 

comments for 13 issues that demonstrated consensus among the peer review panel members. These 

written comments are summarized in Section 3 as “Summary of Peer Review Written Comments.” The 

remaining 17 major issues were addressed in the peer review workshop, held in May 2000. For each of 

these 17 issues, a summary of the general consensus reached by the peer panel during their workshop 

discussion is provided. The workshop consensus opinions are presented in Section 3 as “Workshop 

Synopses.” 

3.1 GENERAL ISSUES 

The external peer reviewers were given six general issues to address about the format and content of the 

draft HHRAP. The peer reviewers responded to these issues in writing. The general issues were not 

discussed during the peer review workshop. The following sections summarize the written responses 

received from the external peer reviewers regarding the general issues. The issues include: 

1. Organization and Documentation; 

2. Alignment and Purpose of Methods; 

3. Scientific Soundness; 

4. Credibility of Results; 

5. Major Data Gaps and Limitation; and 

6. Long-Term Research. 

3.1.1 General Issue 1: Organization and Documentation

Comment on the organization of the section you reviewed. Is the presentation of information clear and 

concise considering the technical complexity of the subject and intended audience? 

 

Summary of Peer Review Written Comments: 

The unanimous opinion of the external peer review panel was that the draft HHRAP procedures are 

generally well written and completely described. Considering the technical complexity of the material, 

the draft HHRAP is presented in a clear and relatively concise manner, and the organization follows a 

logical format. The level and presentation of the material are appropriate for an intended audience of risk 

practitioners. It should also be a useful resource for permit writers, risk managers, and community 

relations’ personnel, to varying degrees. 
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EPA Response: 

Comment noted. 

3.1.2 General Issue 2: Alignment of Purpose and Methods

Does the purpose of the HHRAP as stated in the Introduction (Draft HHRAP Chapter 1) accurately reflect 

the presented methodologies and scope? 

Summary of Peer Review Written Comments: 

The external peer review panel cited the stated purposes in the HHRAP to be: 

• Provide an explanation of how risk assessments should be performed at hazardous waste 

combustion facilities and to provide a comprehensive source of data needed to complete the 

assessment; 

• Provide risk assessors with a tool for completing quality, consistent, and defensible risk 

assessments in a short amount of time; 

• Provide the tools needed to clearly communicate the procedures, results, and limitations of the risk 

assessment process; and 

• Present a user-friendly set of procedures for performing risk assessments. 

 

With regard to these stated purposes, the peer reviewers offered the following comments. 

While the HHRAP provides an understanding of the risk assessment process and is a general guidance, 

reading the HHRAP alone does not equip one to do a risk assessment. The HHRAP does pull together 

existing methodologies and points to where all the details can be found if one wishes or needs to do a risk 

assessment. However, there are many cases, such as at complex sites and facilities, where site-specific 

conditions will warrant additional analyses, for which little or no guidance is provided in the document, 

and expert judgment on the part of the risk assessor will be extremely important. 

EPA Response: 

Although any process which requires integration of several technical disciplines (i.e., combustion 

engineering, air modeling, terrestrial fate and transport modeling, exposure assessment, and toxicology) 

to complete can be viewed as cumbersome and unreasonable, regional and state personnel have been 

implementing the guidance since the draft was released and have not found the guidance, with the 

exceptions brought out in the external peer review, particularly burdensome to use. Specifics of 

exceptions are addressed in the responses to technical comments made by the peer reviewers in Sections 

2 and 3 of this Response to Comments document. 

3.1.3 General Issue 3: Scientific Soundness 

Evaluate critically the scientific aspects (within your expertise) of the document, including 

methodologies, exposure factors and scenarios, parameters and defaults, and risk characterization. 
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Interpretations of scientific conclusions should be based on sound biological principles and accurate and 

legally defensible scientific support. 

Summary of Peer Review Written Comments: 

In general, the external peer reviewers agreed that overall, the draft HHRAP represents a comprehensive 

presentation of methodologies and variables reflecting over a decade of interagency research dedicated to 

the field of risk assessment of combustor emissions. EPA and EPA Region 6 should be commended for 

developing an outstanding risk assessment guidance document that is very detailed and comprehensive. 

The breadth and depth of information presented in this guidance document is unprecedented as a standalone EPA risk assessment guidance document. With some key exceptions noted by the peer reviewers, 

generally dealt with during the external peer review workshop, the methods and assumptions represent a 

reasonable position based on scientific consensus and available data. Scientific aspects of the exposurerelated sections appear to conform to accepted EPA approaches for conducting risk assessments, much of 

which has also been reviewed and supported by the Science Advisory Board. 

EPA Response: 

Comment noted. 

3.1.4 General Issue 4: Credibility of Results 

Give your opinion whether the risk assessment supports not only a credible interpretation of what is 

known, but also a credible interpretation of the hazard and risk that is predicted. 

Summary of Peer Review Written Comments: 

The credibility of the interpretations will depend on how the results of the risk assessment are applied. 

The conservatism of the assumptions and the uncertainties and gaps in the underlying data will generally 

overstate the risk. Thus, it is important that the results and uncertainties are properly described and 

characterized. The results of the assessments can only be used as general guides for decision making and 

should not be viewed as definitive results used as the sole basis for making decisions. 

EPA Response: 

Comment noted. 

3.1.5 General Issue 5: Major Data Gaps and Limitations 

As with any risk assessment, there are always additional data and method development efforts that could 

be undertaken to reduce the level of uncertainty. However, are there any major data or methodological 

gaps within this guidance specific to the sections reviewed that would preclude using the HHRAP for 

regulatory decision-making? If so, how should they be addressed? 
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Summary of Peer Review Written Comments: 

In response to this question, the external peer reviewers stated that the seriousness of these methodology 

and data gaps is a function of the degree of uncertainty that can be tolerated in the final assessment. If the 

risk assessment is only used as one factor of many in making a decision, the data gaps become less 

serious. 

 

EPA Response: 

Comment noted. 

3.1.6 General Issue 6: Long-Term Research 

What long-term research would you recommend that could significantly improve risk assessments of this 

type in the future? 

Summary of Peer Review Written Comments: 

1) It is important to develop methods to integrate cumulative risk estimates from ambient air and other 

pathways, to provide a basis for comparing the contributions from various sources and for 

contrasting the risk levels among different locations with different types of sources. The next 

important step would be to develop approaches for conducting integrated assessments that consider 

different types of possible effects; not only human and ecological health risks, but also social, 

cultural, and economic impacts. 

2) Methods for assessing childhood risks need to more directly account for children’s unique 

susceptibilities and sensitivities to specific contaminants and chemical mixtures. In particular, there 

is a need to improve knowledge regarding the relative importance of exterior dust and soil as lead 

exposure sources for children in various residential environments. 

3) Further evaluate the idea of simply reducing the acceptable risk range and hazard benchmark level 

as a means to realistically account for background contributions from exposures to a wide range of 

contaminants. 

4) There is a need for chemical mixture-related research, especially with respect to predicting the 

health effects of the unidentified organic compounds in the total organic emissions fraction. The 

most relevant research would be toxicity studies of the whole mixture of emitted chemicals from 

representative facilities. 

5) Because of the uncertainty associated with including unspeciated total organic emission data when 

estimating stack emission rates, research is also needed to evaluate this component of the facility 

characterization. For example, what is the composition of the gravimetric fraction typically, and 

how appropriate is the approach for attributing a risk to this unknown portion of emissions? 
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6) Because of the increasing involvement of stakeholders in the risk assessment and management 

process, improved approaches are needed for managing environmental risk data and 

communicating risk-related information. A variety of tools should be utilized and further 

developed (e.g., World-Wide Web, geographic information systems) to not only disseminate risk 

results, but also facilitate an exchange of information with various stakeholder groups. 

7) There is a need to collect environmental monitoring data for model input parameters that drive the 

exposure assessment to validate the modeling approach. In particular, fate and transport data are 

needed to track long-term exposures and health effects from combustion emissions. Large-scale 

epidemiological studies should be carried out to verify that the health effects results predicted by 

these risk assessments are valid. 

8) To address the substantial uncertainties inherent in mercury risk assessments, EPA's Mercury 

Research Strategy (U.S. EPA, 1999d, NCEA-I-0710) identified a prioritized list of research needs. 

This report describes a research program that provides information, methods, models, and data for 

addressing key scientific questions, which are clearly needed to reduce the uncertainties in the 

HHRAP approach to estimating mercury exposures and risks. 

9) Assess how well the route-to-route extrapolations of slope factors and reference doses actually 

work and if there are additional parameters that could improve this. 

10) The litmus test for any risk assessment is to compare the predicted risks with the actual incidence of 

health effects. For example, when applying the conventional EPA risk assessment methodology 

(similar to the HHRAP) to estimate risks for exposure to naturally occurring levels of arsenic in 

U.S. soils using the 95UCL of the mean background concentration, it is estimated there would be 

slightly more than 1 million cases of arsenic-induced tumors (lung cancer, hyperplastic keratosis, 

etc.) every year. Since we obviously do not observe 1 million cases of arsenic-induced cancer from 

soil in the United States each year, it is apparent the risk assessment model is incorrect. Likewise, a 

long-term study needs to be initiated at hazardous waste combustion facilities to confirm that the 

HHRAP model is correctly predicting risk estimates. It would be difficult to conduct 

epidemiological studies for cancer, but biomarker studies could be used to at least determine 

whether exposures are actually occurring. It will also be necessary to confirm that the modeled 

concentrations actually exist in all the environmental media for all the intake sources in the risk 

assessment. A sensitivity analysis needs to be conducted to confirm that the indirect pathways are 

truly significant and warrant inclusion in the risk assessment. The sensitivity analysis should not be 

mathematical, but should be based on actual biological measurements from foodstuffs (e.g., eggs, 

chickens, meat, milk, etc.) 

 

11) Priorities for long-term research should be based on the toxicity and volume of compounds released 

from an incinerator. Also, as a long-term goal, there should be continuing reviews of the literature 

so that new information on persistence, metabolism, and transport can be incorporated in a timely 

manner. Efforts should also be made to draw inferences concerning metabolism and 

bioaccumulation in farm animals from the results of research with laboratory animals. 
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12) There has been an effort in recent years to evaluate polychlorinated biphenyls (PCBs) as individual 

compounds rather than as Aroclor mixtures, especially for those congeners with dioxin-like 

activity. Serious consideration should be given to providing individual PCB congener data in 

Appendix A for those PCBs of greatest toxicological concern. 

13) Risk assessments of this type would benefit greatly from the research being performed on fate and 

exposure of pesticides. In particular, the agricultural literature contains many efforts at quantifying 

mass balance of pesticides in various media and in adjacent locations following controlled 

applications. While significant portions of the existing methodology (e.g., the application of the 

Universal Soil Loss Equation) derive from the agricultural disciplines, much work has been done 

since the development of these simple algorithms. A major uncertainty and likely bias toward high 

exposures in the existing methods is the efficiency of constituent runoff (contributed by the 

parameterization of the sediment delivery ratio, the sediment enrichment ratio, and the extremely 

high efficiency of runoff from impervious surfaces). This uncertainty could very likely be reduced 

by a careful review of the current literature on pesticide runoff from croplands. Behavior of 

pesticides in adjacent, uncultivated areas subject to over-spray should also be a productive topic for 

further review. 

14) The risk assessment methodology should integrate the recent findings and models of mercury fate 

and transport. Of particular relevance are the ongoing research efforts in south Florida and by the 

Electric Power Research Institute, both of which are examining the importance of atmospheric 

loadings, watershed processes, aquatic geochemistry, and the structure of the food chain in the 

accumulation of mercury in fish tissue. The efficiency of mercury runoff from the watershed to the 

water body should be evaluated in the context of these studies. These issues are the most likely 

ones to contribute to uncertainty in assessing risks due to mercury emissions. 

EPA Response: 

Comments noted. EPA will forward these recommendations to its Office of Research and Development.

3.2 COMBUSTION ENGINEERING [draft HHRAP Chapter 2] 

Issues regarding combustion engineering that were addressed by the external peer review panel include: 

• Process Upsets; 

• Selecting COPCs; 

• Unspeciated Total Organic Emissions (TOE); 

• 95th Percentile Emission Rates; and 

• Quantifying Non-detect Compounds. 
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3.2.1 Combustion Engineering: Process Upsets 

 

Comments were received regarding guidance presented for considering process upsets in estimating 

emission rates (draft HHRAP Section 2.2.5). Is additional detail and clarification of guidance specific to 

this issue required? 

Summary of Peer Review Written Comments: 

The default CARB upset factors are excessively conservative, and worse than "worst case maximum 

potential risk." Existing studies indicate a very large discrepancy between the CARB default values and 

actual field results regarding frequency and duration of upsets. No hazardous waste combustion facility 

would be allowed to operate in upset conditions for 20% (organics) and 5% (metals) of the time. Also, 

hazardous waste combustion is prohibited under RCRA during startup and shutdown periods by the 

automatic waste feed cut off (AWFCO) system. Available technical information indicates that upset 

emissions are not close to 10-times normal emissions. Upsets occur fairly frequently during trial 

burn/risk burn (TB/RB) sampling conditions with minor apparent impact on measured concentrations. 

However, many smaller, simpler facilities have the disadvantage of not having upset duration data or 

emissions estimates during upset conditions. Thus, per guidance, these less threatening facilities would 

be required to use the excessively conservative CARB default values. This approach would be very 

difficult to defend in a court. 

EPA Response: 

The HHRAP is guidance and will continue to contain no requirements. Historic information has been 

provided and used routinely to adjust the default-upset factors. However, the HHRAP recommends 

generating a site-specific upset factor, where possible. As with many parameters and equations provided 

in the HHRAP, the scientific basis and reference are provided. A facility is then able to evaluate whether 

to propose and justify to the permitting authority an alternative they consider more appropriate than that 

recommended in the HHRAP. 

3.2.2 Combustion Engineering: Selecting COPCs 

Comments were received regarding guidance presented for selection of compounds of potential concern 

(COPCs) (Section 2.3). Review and comment on the combustion engineering aspects of the COPC 

selection process presented in the guidance. These aspects would include guidance provided regarding 

inclusion of (1) compounds initially present in the hazardous waste feed stream and not completely 

destroyed in the combustion process, (2) compounds that are formed during the combustion process, and 

(3) compounds in the waste feed stream that should be evaluated as potential products of incomplete 

combustion (PICs) precursors. 

Summary of Peer Review Written Comments: A. Methods of Selecting COPCs 

Response to this question considers two methods of selecting COPCs: (1) selection of COPCs using 

analytical results from test burn/risk burn (TB/RB) on existing facilities, and (2) selection of COPCs for a 

pre-TB/RB emissions inventory for a new or currently non-operating facility. 
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(Method 1) Selection of COPCs from Analytical Results from TB/RB on Existing Facilities: This 

process/procedure is well defined in Section 2.3 of the guidance document. Overall, this is a very 

reasonable approach. The primary suggestion was to add a discriminator phase to the second step (i.e., 

Review the waste stream analysis and add as a COPC any compound present in the waste even if that 

compound is not detected in the stack sample analytical results), with a provision for excluding 

compounds that may be present in the waste in low concentrations. The relative thermal refractivity of 

the COPC of interest should also be considered, e.g., Class 1 compounds from the University of Dayton 

System might use 4-9s DRE. Less refractive compounds, such as nitrobenzene, might use 5-9s DRE. 

(Method 2) Selection of COPCs for New or Currently Non-Operating Facilities: In the current draft 

guidance, essentially nothing is said concerning how the pre-RB/TB emission inventory should be 

prepared. Estimating the stack concentration of organic constituents fed with the waste is relatively 

straight-forward using a DRE-type calculation (i.e., Mass Feed Rate of Constituent i x 0.0001 = Mass 

Emission Rate of constituent i in the stack). However, identifying and estimating COPCs which could be 

emitted as PICs is much less clear. 

The guidance should provide a short procedure that lays out an approach that EPA would accept, without 

requiring, for example, any nitrogen-containing compound from Table A-1 in the draft HHRAP to be 

included as a COPC if organic nitrogen is present in the waste. While these types of PICs could be 

present conceptually, some means of selecting more likely candidates is needed, especially in view of 

EPA's use of high RDL values to estimate low concentration COPCs. 

EPA Response: 

With regard to Method 1, the proposed approach may be acceptable on a site-specific basis. However, 

we don’t recommend this approach in the guidance because we believe it’s preferable to carry a 

compound through the risk assessment and allow factors, such as quantity and fate and transport 

characteristics, to demonstrate quantitatively if the risk falls below levels of concern. Additionally, 

determining the deminimus concentration of a compound as it applies to potential risk would depend on 

the toxicity characteristics of the individual compound, as well as site-specific exposure characteristics. 

With regard to Method 2, the permitting process for new hazardous waste combustion facilities includes 

submitting information of sufficient detail for the regulatory authority to evaluate compliance with 

existing regulations, guidance, and standards of protectiveness. Stack (or other source) locations and 

dimensions, design flow and emission rate estimates, waste feed characteristics, surrounding building 

dimension data, facility plot plans, and terrain data are frequently reviewed and used in a pre-operation 

risk assessment. This will assist in decision-making and designing permit requirements. 

We generally recommend reviewing design, emission rates, waste feed characteristics, and other design 

data, along with supplementary documentation, to make sure they’re representative, accurate, and 

comprehensive. Good engineering practice dictates a check of, and comparison with, data from similar 

existing units. Stack test reports for facilities of similar technology, design, operation, capacity, auxiliary 

fuels, waste feed types, and APCSs can be useful to estimate COPC emission rates for new facilities that 

haven’t been constructed. In addition to design data, particle size distribution data from a similar type 

unit that’s operational may be useful. Estimated emission rates used to complete pretrial burn risk 
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assessments are frequently compared to the measured emission rates from actual trial burns completed 

after the new facility receives a permit and is constructed. 

If surrogate data from similar facilities aren’t available, some state environmental agencies enforce 

emission rate limits based on state laws. Since these limits can’t be exceeded, you might use them to 

develop emission rate estimates for the risk assessment. A trial or risk burn could then demonstrate that 

facility emissions are less than those considered in the permit and risk assessment. 

Summary of Peer Review Written Comments: B. Selecting Specific COPCs

The following comments pertain to the guidance provided on the seven categories of potential COPCs: 

PCDD/PCDF and PAHs: The peer reviewer agreed that dioxin/furan sampling and analysis are important 

and should be included in the first TB/RB/RA for every HWC facility. This comment is based on the 

assumption that EPA will not insist on detection limits for non-detects that would suggest significant risk 

even from non-detects. Test every facility at least once. Retest for dioxin/furan at appropriate repermitting, etc., stages, but only when the first test reveals a credible risk. One reservation to this pertains 

to COPCs which have driven risk assessments only because they were included at very high RDL 

concentrations. This comment also applies to the other constituent categories such as PAHs, PCBs, etc. 

EPA Response: PCDD/PCDF and PAHs 

With regard to PCDD/PCDF, guidance for quantifying dioxin and furan non-detects is consistent with 

that of other similar compounds. Using RDL’s is based on the data quality objective of setting an 

achievable permit limit.

With regard to PAH’s, PCB’s, etc., the high risk values at detection limits were the result of conservative 

biotransfer factors and not the detection limits. With the adoption of the new biotransfer equations in the 

HHRAP (see discussion below), we don’t expect that risks above regulatory levels of concern will result 

from evaluating compounds at their reliable detection limits. 

PCBs: With few exceptions, the peer reviewers agreed with guidance concerning stack testing for PCB. 

However, the last sentence on Page 2-47 does not appear to agree with the previous discussion, i.e., 

consider requiring stack testing for PCBs even if the conditions favorable for emitting PCBs do not exist 

at a given site. Also, "highly chlorinated waste stream" needs some further definition, i.e., >60% Cl. 

Further, the presence or possible presence of PCBs in the waste should also have a threshold level. The 

presence or possible presence of PCBs at 100 ppm may not warrant stack testing for PCBs for risk 

assessment purposes when DRE reductions are considered. It should also be noted that the references to 

PCB stack gas emissions being equal to or greater than those for dioxins and furans is based solely on 

mass and not on the toxicity of the PCBs. Further, a review of selected referenced documents indicates 

that some of the data may be in error. If PCB testing is conducted, it should examine the coplanar PCBs, 

which are reported to exhibit dioxin-like toxicity. Review of the limited available literature examining 

the 2,3,7,8-TCDD TEQ contribution due to PCBs suggests that PCBs are insignificant PICs of non-PCB 

waste streams. 
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EPA Response: PCBs

With regard to PCBs, comment noted. We’ve continued reviewing the issue of PCB emissions from 

combustors, and have revised the HHRAP to include (1) a summary of a study indicating de novo PCB 

formation based upon Stieglitz et al., 1989, and (2) a summary of a study indicating PCB formation in 

combustion units to be similar to dioxin and furan formation (Lemieux et al., 1999). 

Nitroaromatics, HCB and PCP: The procedure for including these compounds appears to be reasonable, 

especially if a waste concentration threshold is provided. For instance, 10 ppm nitrobenzene in the waste 

would be very unlikely to result in increased risk, especially after considering that these compounds must 

survive a >99.99% DRE combustion process. 

Phthalate: The peer reviewer disagreed that the absence of these compounds in the stack "should always 

be verified by stack testing." Some knowledgeable practitioners in the combustion engineering field 

believe that detected phthalates in a stack sample are usually the result of laboratory, sampling, or sample 

transport extraction from plastics used or from cross-contamination from the background. These possible 

false positives can be risk drivers in the risk assessment process. The same concept of a threshold 

concentration in the waste should also apply here. Even if a facility burns plastics, phthalates are 

generally used in low concentrations and the phthalates present must survive a >>99.99% DRE process 

(for phthalates, the ‘>>' symbol is added due to their combustible nature) to reach the stack. 

VOCs: This section is weak overall, but may be the best possible at this time. The substantial database 

available to EPA should be mined to provide a more focused view of VOCs as a risk driver in risk 

assessments. Results of current risk assessments have generally shown that VOCs are not risk drivers. 

EPA Response: Nitroaromatics, HCB and PCP; Phthalate; and VOCs

With regard to Nitroaromatics, HCB and PCP, and VOCs, comments noted. With regard to Phthalates, 

the high-risk values at detection limits were the result of conservative biotransfer factors. With the 

adoption of the new biotransfer equations in the HHRAP (see discussion below), we do not expect that 

risk above regulatory levels of concern will result from evaluating compounds at low levels. 

3.2.3 Combustion Engineering: Unspeciated Total Organic Emissions (TOE)

Comments have been received by the EPA regarding the guidance presented for inclusion of the 

“unknown” or unspeciated Total Organic Emission (TOE) data when estimating stack emission rates 

(Section 2.2.1.3). Considering the technical complexity of the TOE issue, is guidance on quantifying 

unspeciated TOE data for use in the risk assessment adequate and presented clearly? Provide comments 

as to whether the guidance on assigning toxicity values to the “unknown” or TOE portion of the 

emissions is adequate and scientifically sound. 

 Peer Review Workshop Synopsis

The external peer reviewer panel reached a consensus. The peer reviewers believed that EPA should 

evaluate the TOE issue qualitatively in the Uncertainty Chapter of the draft HHRAP and that it would still 
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be conservative. In addition, development of new analytical methods is needed to get better numbers for 

doing a risk assessment. 

The peer reviewers recommended that an approach be taken to better characterize the unknown mixtures. 

They also recommended that guidance should focus on looking more closely at the unspeciated fraction, 

rather than saying that the risks here are unacceptable. 

EPA Response: 

The HHRAP states that organic compounds that cannot be identified by laboratory analysis cannot be 

defensibly treated as COPC’s in the risk calculations. However, these compounds might still contribute 

significantly to the overall risk, and so it is reasonable to consider them qualitatively in the risk 

assessment. The HHRAP recommends using the TOE factor in the uncertainty section of the risk 

assessment report to evaluate the risks from the unknown fraction of organics. Both regulated industry 

and U.S. EPA have expressed some concern that the gravimetric fraction may over-report the organic 

fraction. It has been suggested that the gravimetric fraction may consist of organic and/or inorganic 

mass not directly attributable to organic incinerator emissions. The U.S. EPA Office of Research and 

Development (ORD) National Risk Management Research Laboratory (NRMRL) recently conducted a 

series of experiments to investigate this issue. The results indicate that it is indeed possible for inorganic 

mass to become soluble and ultimately be retained in the TOE train methylene chloride extract. More 

importantly, the ORD/NRMRL research identified and demonstrated techniques for successfully 

mitigating this problem. Characterization of unknown mixtures is an ongoing area of research at EPA 

and as new methodologies are developed it seems appropriate to incorporate them into the risk 

assessment. Although the HHRAP states that the permitting authority might not request analysis of the 

organic compounds with all possible analytical methods, it also says one may wish to consider the effects 

that gaps in compound-specific identification may have on the computation of the TOE factor. We agree 

that the better the upfront characterization of emissions is, the better the risk characterization will be and 

the less of an impact the TOE analysis will have.

3.2.4 Combustion Engineering: 95th Percentile Emission Rate

Comments have been received by the EPA concerning the definition of the 95thpercentile emission rate in 

the risk assessment (Section 2.2). Provide comments as to whether the guidance on quantifying emission 

rates of compounds for use in the risk assessment is adequate and scientifically sound. Should the 

guidance specify use of the 95th percentile or the 95th upper confidence limit (UCL) of the mean? 

 Peer Review Workshop Synopsis

The external peer review panel appeared to be in agreement about the use of the mean or max instead of 

the 95th UCL, but did not reach consensus about using the 95th percentile for quantifying compound of

emission rates. This was primarily due to the limited number of proposed data sets found in the guidance 

document. 
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EPA Response 

We’ve altered the HHRAP to recommend that, for the chronic portion of the risk assessment; you have the 

option of using either the long-term average or maximum emission rates. The emission rates are to be 

adjusted for process upsets and represent long-term averages. Long-term average emission rates are 

typically based on tests of the combustor burning representative, yet worst-case or challenging feeds, at 

operating conditions that are representative of normal operating conditions over a long-term period. 

This is an update to Section 8.1.2 of the Risk Burn Guidance for Hazardous Waste Combustion Facilities. 

For the acute portion of the risk assessment, we generally recommend using the maximum emission rates, 

adjusted for process upsets. 

3.2.5 Combustion Engineering: Quantifying Non-detect Compounds

Comments were received regarding guidance presented for quantifying non-detect compounds when 

estimating stack emissions rates (Section 2.4). Is the guidance on quantifying non-detect compounds for 

use in risk assessment adequate and scientifically sound? Provide comments on the recommendations of 

detection limit related issues: (1) can instrument detection limit (IDL) be substituted for the method 

detection limit (MDL) in determining the Reliable Detection Level (RDL) for metals, (2) can sample 

condensates be combined to lower detection limits without affecting the quality assurance and control of 

the data generated from analysis, and (3) how should J-flagged or qualified data be used if it’s below the 

calculated RDL? 

 Peer Review Workshop Synopsis

The external peer review panel recommended using one-half of the MDL for organic non-detect 

compounds. The panel did not support the use of the RDLs and SQLs in the risk assessment. They were 

willing to support the use of RDLs if the compound was a “risk driver” in the risk assessment. For metals 

analysis, they felt that the IDL was appropriate in determining the RDLs for metals. 

The peer reviewers agreed that the use of non-detects should be discussed additionally in the Uncertainty 

chapter of the HHRAP. If a non-detect indicates a risk, it should be discussed in the uncertainty section 

of the risk assessment report. If the use of an RDL shows a risk, the risk assessor should rerun the risk 

assessment with the MDL or one-half of the MDL and explain this in the uncertainty section of the report. 

It was also recommended that the risk assessment report should not discuss the risk associated with nondetect compounds until the uncertainty section. 

A recommendation was made that EPA should inform the public more about the use of RDLs. It was 

believed that the public does not understand the concept. Additional communication regarding RDLs 

with a larger group of the regulated community could make a tremendous difference in the understanding 

of and resistance to RDLs, rather than thinking EPA is wrong in their use. 

 

Parts #2 and #3 of this major issue question were not discussed by the external peer reviewers during the 

workshop. 
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EPA Response: 

The HHRAP will continue to recommend using RDLs in the risk assessment based on the data quality 

objectives of the assessment. Although we agree that using one-half the MDL for organic non-detected 

compounds would result in a more accurate calculation of risk, one purpose of the risk assessment is to 

determine an acceptable permit limit. Since facilities would not be able to meet permit limits set at onehalf the MDL, we recommend evaluating the RDL in the risk assessment. 

We’ve modified the HHRAP to recommend using the Instrument Detection Limits (IDLs,) rather than 

Reliable Detection Level (RDL) for metals (see Section 2.4.2, Use in the Risk Assessment of Data 

Reported As Non-Detect). We’ve added this recommendation because the peer review draft HHRAP did 

not specifically address quantification of non-detects for metals. 

3.3 AIR DISPERSION AND DEPOSITION MODELING (draft HHRAP Chapter 3) 

Issues regarding air dispersion and deposition modeling that were addressed by the external peer review 

panel include: 

• Modeling Vapor Phase, Particle Phase, and Particle-Bound Phase Emissions; 

• Estimating Fugitive Emissions and Acute Exposure; 

• Particle and Vapor Phase Emission Partitioning; 

• Air Modeling Input Default Values; 

• Dry Vapor Phase Deposition Modeling; and 

• Particle Size Distribution. 

3.3.1 Air Dispersion and Deposition Modeling: Modeling Vapor Phase, Particle Phase, and 

Particle-Bound Phase Emissions 

Comments were received on guidance presented for conducting separate modeling runs for evaluating 

vapor phase, particle phase (mass weighting), and particle-bound phase (surface area weighting) 

emissions (Sections 3.4 and 3.9.3). Review and comment on guidance presented for modeling vapor, 

particle, and particle-bound phases. Does conducting separate modeling runs for each emissions phase 

noted above provide better resolution of air parameter inputs for use in the risk assessment? Also, review 

and comment on the scientific validity regarding use of the fraction in vapor (Fv) for partitioning 

emissions between particle and vapor phases. 

Summary of Peer Review Written Comments: 

The issue is not whether separate model runs are required. Rather, the issue would appear to be whether 

it is important to distinguish among the atmospheric concentration of each of the three phases. In terms 

of indirect exposure through deposition and uptake, as well as direct exposure through inhalation, it is 

important to distinguish between the vapor and particle phases for COPCs. With reference to direct 

exposure, it is important to apportion airborne particle concentrations according to particle size and 

number. There is an in-house version of ISC called HWIR that provides outputs of airborne 
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concentrations and surface deposition by particle size, which can be used with some difficulty to 

apportion airborne concentrations and surface depositions according to phase, particle size, and source, 

although this requires each phase and particle size(s) to be treated as a separate source group in the ISC 

model. OAQPS has indicated a willingness to consider making more widely available the HWIR version 

of the code. 

EPA Response: 

Comment noted. The HHRAP has been updated to include the latest version of ISCST3. 

3.3.2 Air Dispersion and Deposition Modeling: Air Dispersion Modeling Charge: Estimating 

Fugitive Emissions and Acute Exposure 

 

Comments were received regarding guidance and examples provided for air dispersion modeling and 

estimating of fugitive emissions (Sections 2.2.6 and 3.10) and acute exposures. Review and comment on 

guidance presented for air modeling of fugitive emissions and emissions used to evaluate acute exposure. 

Will conducting air dispersion modeling of fugitive emissions add unwarranted complexity to the risk 

assessment due to the setup and air model runs required considering the usefulness of information 

obtained in estimating potential risks? For facilities permitted under RCRA Subpart AA, BB, and CC 

that have already met requirements for monitoring and evaluating fugitive emissions, is it reasonable to 

assume that volatile emissions from these sources would be released in quantities that would result in an 

off-site impact? 

Summary of Peer Review Written Comments: 

The draft HHRAP does not focus on truly acute events, except to incorporate their emissions into "longterm average emission rates adjusted for upsets ... or reasonable maximum emission rates measured 

during trial burn conditions." The ISCST3 dispersion model has the ability to deal with certain types of 

events that yield acute exposure, particularly those where the emissions can be appropriately 

characterized by hourly averages. However, for true acute risk assessment, a non-steady state dispersion 

model (e.g., ALPUFF) may provide a more representative estimate of concentration and deposition given 

this model's ability to treat time-varying emissions and time-and space-varying meteorological fields. 

The peer reviewer did not view assessing risks from fugitive emissions as necessarily complex or 

unwarranted. Given the nature of the risk assessment process and the goal of providing the public with 

objective information, this analysis is reasonable and appropriate. The need to model fugitive emissions 

can best be addressed through consideration of the sensitivity of the risk to these emissions and their 

associated uncertainties. 

 

EPA Response: 

We recommend using the latest version of the ISCST3 model in most situations to conduct air dispersion 

and deposition modeling for conducting a risk assessment (see introduction of Chapter 3, Air Dispersion 

and Deposition Modeling). Review of this issue by the External Peer Review Panel concluded that the 

latest version of ISCST3 is the appropriate air model to support conducting a risk assessment. We agree 

that the ISCST3 is only useful to deal with certain types of acute exposure events appropriately 
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characterized by hourly averages. Additional modeling of acute emissions may be warranted in sitespecific situations. 

3.3.3 Air Dispersion and Deposition Modeling: Particle and Vapor Phase Emission Partitioning 

 

Comment on the scientific validity regarding use of the fraction in vapor (Fv) for partitioning emissions 

between particle and vapor phases. 

Summary of Peer Review Written Comments: 

If the fraction in vapor, Fv, can be reasonably well described or defined, then it is appropriate to use this 

parameter to partition emissions between particle and vapor phases. The challenge is to define a 

representative fraction through measurements in the hot plume environment where this fraction is likely 

to undergo significant change. 

EPA Response: 

Comment noted. We agree that the Fv would be difficult to determine in a hot plume. However, the 

HHRAP assumes the plumes will equilibrate to ambient conditions and portions of semi-volatiles will 

adhere to the surface area of particles and the remaining portions will remain in the vapor state. Fv 

values in the HHRAP are based on a compounds fraction of vapor at ambient conditions (25 degrees C). 

3.3.4 Air Dispersion and Deposition Modeling: Air Modeling Input Default Values 

Comment on default values provided in the guidance air modeling inputs. 

Summary of Peer Review Written Comments: 

It was assumed the question pertains to the appropriateness of the default values contained in Tables 3-1 

through 3-4. 

Table 3-1 provides generalized particle-size distribution data. The default size distribution intended for 

use at facilities with electrostatic precipitators or bag houses appears appropriate in a generic sense for the 

finer sizes. However, the larger (15 micron) particles may be too heavily represented since they are 

typically effectively captured by particle control systems. EPA was urged to provide clear guidance 

regarding where and when either the default size distribution or a source-specific distribution is 

allowable. 

 

Table 3-2 provides representative values of the albedo for different surface types during each of the four 

seasons. As default values, they are appropriate. 

Table 3-3 provides representative seasonal values of the daytime Bowen ratio. This is a difficult 

parameter to estimate without site-specific precipitation and wind data. Using site-specific data wherever 

possible was recommended. Note also that Bowen ratio is incorrectly defined on page 3-35. It is the ratio 

of the sensible heat flux to the evaporative or latent heat flux at the ground surface. 
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Table 3-4 estimates default values of the anthropogenic heat flux for different types of urban areas. These 

are appropriate for use as default values. There are more recent summary papers that might be referenced 

here as well, e.g., the work of Klysik et al. (1999). 

EPA Response: 

We’ve modified the title of Table 3-1 and the HHRAP text to clarify the role of Table 3-1 as hypothetical 

data, and to recommend that the data in Table 3-1 not be used as a default particle size distribution when 

modeling site-specific emissions. We’ve also corrected the definition of Bowen ratio as recommended. 

3.3.5 Air Dispersion and Deposition Modeling: Dry Vapor Phase Deposition Modeling

Comments have been received regarding recommendations on (dry) vapor phase deposition modeling 

(draft HHRAP Sections 3.1.1 and 3.5.1.7; and 5.7.1.2). 

• Is the guidance provided for estimating (dry) vapor phase deposition technically valid as applied? 

• Review and comment on the scientific validity regarding the assumption that wet deposition and 

precipitation rates are linear. 

• Are the equations to estimate dry deposition of vapors (Ldif) to water bodies technically valid or do 

they incorrectly consider only one-way transfer of pollutants from the air to the water body? 

Peer Review Workshop Synopsis 

1) The peer reviewers agreed the draft HHRAP guidance is not technically valid because it’s out of 

date. Specifically, at the time the Peer Review Draft HHRAP was written, there were unresolved 

issues among experts about modeling dry deposition. Therefore, U.S. EPA decided to use a 

calculated or literature value estimate of deposition velocity for vapor transport to the ground. One 

issue with this approach is that the balance of mass is not maintained because the deposited mass is 

not taken into account in the air concentration modeling. 

2) The peer reviewers recommended using the more updated 1999 ISCST3 model. The dry vapor 

deposition velocity can be inputted directly into this version of ISCST3, rather than incorporating it 

into the Appendix B estimating media concentration equations (see Section 3.1.1, History of 

HHRAP Air Dispersion Models). The peer reviewers concluded that including the dry vapor 

deposition velocity directly into the ISCST3 modeling provides a better representation of 

conservation of mass because the revised model accounts for the mass depletion due to deposition. 

The peer reviewers commented that the value of 3-cm/sec-deposition velocity used in the draft 

HHRAP to represent every compound is unrealistically conservative. Based on their experience 

and review of the literature, measured values are generally much lower than 3 cm/sec. 

3) In theory, wet deposition and precipitation rates are believed to not be linearly proportional. 

However, the peer reviewers concluded that in practice, it is appropriate for risk assessment 

applications to use empirical relationships that relate wet deposition to precipitation rate in a linear 

fashion. The peer reviewers concluded that it is necessary to default to this practical assumption 
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because the hydrometeor and in-cloud scavenging process parameters are very difficult to measure 

and model. 

4) The peer reviewers believe the 2-way transfer of pollutants between air and water bodies should be 

considered. However, the peer reviewers believe this is not a first-order problem, considering the 

other uncertainties in the whole risk assessment process. 

EPA Response: 

1) We’ve modified the HHRAP to recommend inputting the dry vapor deposition velocity directly 

into the latest version of the ISCST3 model, rather than incorporating it into the Appendix B 

estimating media concentration equations (see Section 3.1.1, History of HHRAP Air Dispersion 

Models). This change also requires that the product term of the dry deposition velocity and 

unitized yearly average air concentration from vapor phase (Vdv * Cyv) be replaced with the 

ISCST3 air parameter value for dry vapor deposition. 

It is important to note that although we recommend inputting the dry vapor deposition velocity 

directly into the ISCST3 modeling, the user still has the option to execute the dry gas deposition 

algorithms within ISCST3 to calculate a deposition velocity. However, the user is cautioned to 

read Section 3.1.2 (Preprocessing Programs) and Section 3.4.6 (Solar Radiation), which note 

additional data needs, and potential limitations. Executing the dry gas deposition algorithms 

within ISCST3 to calculate a deposition velocity may also require the user to conduct compoundspecific air modeling runs and deviate from the unit emission rate approach as outlined in the 

HHRAP. 

The HHRAP provides alternative default dry deposition of vapor velocities. We’ve revised both 

the ksv and dry deposition of vapor velocity sections of the HHRAP. As advances are made in air 

modeling, one should always take into account the most recent state-of-the-science. The modeling 

reported in the Mercury Study Report to Congress (U.S. EPA, 1997b) used a value of 0.3 cm/s for 

nighttime dry deposition of divalent mercury, but used daytime values ranging from 0.20 to 4.83 

cm/s depending upon atmospheric stability and land-use category. The daytime values were 

based on data developed from nitric acid data, not from measurements of divalent mercury. U.S. 

EPA (1997b) used an average ISC model-calculated dry deposition velocity of 2.9 cm/s for 

divalent mercury vapor and 0.06 cm/s for elemental mercury. Higher values were expected for 

chemicals with greater reactivity than acetic acid or formic acid, but no measured values were 

identified for any organic compounds higher than 1.1 cm/s. As a result, we recommend the 

default of 2.9 for divalent mercury. We recommend a dry vapor deposition velocity of 0.5 

centimeter per second (cm/s) for organic contaminants, chlorine, and HCl. The recommended 

dry vapor deposition velocity value of 0.5 cm/s for organic contaminants is consistent with the 

range specified for pesticides (0.01 - 1.1 cm/s) and dioxins and furans (0.27 - 0.78 cm/s). 

2) We acknowledge the practicality of assuming linearity between wet deposition and precipitation 

rates while recognizing that in theory, they are not linearly proportional. 
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3) Assuming that volatile chemicals are trapped in the waterbody encourages conservative 

estimates of drinking water and fish concentrations. If this tended to result in risk/hazard levels 

above the benchmark, a more detailed site-specific fate and transport analysis could be 

conducted. However, we’re unaware of any case where this has actually occurred. As with many 

parameters and equations provided in the HHRAP, the scientific basis and reference is provided. 

A facility is then able to evaluate whether 2-way transfer of pollutants between air and water 

bodies is sufficiently important to risk assessment outcomes to recommend and justify to the 

permitting authority a site-specific method, or other alternative to that recommended in the 

protocol. 

3.3.6 Air Dispersion and Deposition Modeling: Particle Size Distributions

Comments have been received by EPA regarding guidance on determination of stack-specific particle size 

distributions recommended for use in the air dispersion modeling (Section 3.4). Provide comments as to 

whether inclusion of stack-specific particle size distributions are warranted, or could general or default 

distributions be applied without inducing additional uncertainty in the risk assessment?

 Peer Review Workshop Synopsis

The external peer review panel recommended that the HHRAP use four or five sets of data as a 

categorical set of default choices, which would be better than using the single default distribution. Data 

profiles should be obtained for the different categories among waste incinerators, boilers, liquid or solid 

waste fuel incinerators, etc. If these data profiles are not available, particulate size distributions (PSDs) 

need to be divided into different categories for evaluation. However, stack specific analysis may still be 

required for some facilities.

EPA Response: 

We included the generalized particle size distribution in Table 3-1 in the HHRAP to document example 

calculations. We’ve modified the title of Table 3-1 and the HHRAP text to clarify the role of Table 3-1 as 

hypothetical data, and to recommend that the data in Table 3-1 not be used as a default particle size 

distribution when modeling site-specific emissions. 

3.4 EXPOSURE ASSESSMENT [draft HHRAP Chapter 4 & 6, Appendix C] 

Issues regarding exposure assessment that were addressed by the external peer review panel include: 

• Acute Inhalation Exposure Criteria (AIEC) Values and Evaluating Acute Toxicity; 

• Route-to-Route Extrapolation; 

• Atmospheric Degradation of Contaminants; 

• Exposure Scenario Locations; 

• Non-Carcinogenic Risk of Dioxins Benchmark; 

• Using TEFs for Coplaner PCBs; 

• Dioxins in Breast Milk; and 
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• Inhalation Rate Value. 

3.4.1 Exposure Assessment: Acute Inhalation Exposure Criteria (AIEC) Values and Evaluating 

Acute Toxicity 

 

Comments were received regarding guidance presented for evaluation of acute toxicity and the Acute 

Inhalation Exposure Criteria (AIEC) values presented in the draft HHRAP (Appendix A-4). Review and 

comment on issues pertaining to evaluating acute toxicity and recommended AIEC values, specifically (1) 

should AIEC values presented in Appendix A-4 be removed to prevent use of potentially out-of-date 

values, (2) should AIEC values be updated to include values provided in California Office of 

Environmental Health Hazard Assessments (OEHHA) 1998 revised draft, (3) should a more complete 

description of the effects of concern when evaluating acute risks be provided, and (4) are OEHHA's 

Reference Exposure Levels (REL) comparable to other AIECs. Comment on the hierarchy of AIEC 

values recommended in the guidance, including the use of Acute Exposure Guideline Levels (AEGL) at 

the top of the hierarchy. 1

Summary of Peer Review Written Comments: 

The purpose of the acute exposure scenario is the important factor in determining which values are the 

most appropriate, and it was recommended that the draft HHRAP sections dealing with the acute 

exposure scenario be revised to clearly specify the purpose of the scenario. If the goal of the acute 

exposure scenario is to estimate the risks from upset conditions, it was recommended that the draft 

HHRAP be revised after reconsidering how to best accomplish that goal. It would be useful if EPA 

created and maintained a web page with links to web pages with the current values, and referred to that 

web page in the HHRAP. Links to support documents would also be good. 

 

1 The AEGLs characterized as "short-term threshold or ceiling exposure values intended for the protection of the general public, including 

susceptible or sensitive individuals, but not hypersusceptible or hypersensitive individuals." The AEGL-1 value is "the airborne concentration 

(expressed as parts per millions (ppm) or milligrams/cubic meter of a substance at or above which it is predicted that the general population, 

including ``susceptible'' but excluding ``hypersusceptible'' individuals, could experience notable discomfort. Airborne concentrations below 

AEGL-1 represent exposure levels that could produce mild odor, taste, or other sensory irritations." AEGL values are given for specific durations 

of exposure (0.5, 1, 4 and 8 hr). The HHRAP assumes a 1-hr exposure duration, so the 1-hr values would be appropriate. 

The Relative Exposure Levels (REL) values were established by California Office of Environmental Health Hazard Assessments 1998 draft. 

RELs are defined as "The concentration level at or below which no adverse health effects are anticipated for a specified exposure duration is 

termed the reference exposure level (REL). RELs are based on the most sensitive, relevant, adverse health effect reported in the medical and 

toxicological literature. RELs are designed to protect the most sensitive individuals in the population by the inclusion of margins of safety. Since 

margins of safety are incorporated to address data gaps and uncertainties, exceeding the REL does not automatically indicate an adverse health 

impact." RELs are based on 1 hr exposure. 

Emergency Response Planning Guides (ERPGs) have been developed by the American Industrial Hygiene Association. Unlike the RELs and 

AEGLs, the ERPGs do not include safety factors and they are designed to predict response (and therefore plan appropriate emergency responses), 

not protect health. ERPG-1 is defined as "the maximum concentration in air below which it is believed nearly all individuals could be exposed 

for up to one hour without experiencing other than mild transient adverse health effects or perceiving a clearly defined objectionable odor." 

ERPGs are based on a 1 hr exposure. Temporary Emergency Exposure Limit (TEEL) values are estimated from existing data, such as toxicity 

and lethality data and occupational exposure limit values and are meant to serve as substitutes for ERPGs until ERPGs are developed. TEELs are 

developed by the Department of Energy. 
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The order in which the different guidelines should be consulted is a difficult question, particularly without 

knowing the objective of the acute risk scenario. Assuming the acute exposure scenario is to consider the 

immediate effects of the emissions that occur on a routine basis, the REL values seem most suited to the 

purpose and would take precedence over ERPG-1 values. The ERPGs are geared towards accidental 

releases, not releases that occur from predictable operations, which for a hazardous waste incinerator are 

upset conditions such as start-up, shut-down, and other upsets as described in the draft HHRAP. In 

contrast, the REL values consider the effects to the population from non-emergency routine releases. 

To address the question of comparability of the RELs to other AIECs, the values for a variety of 

compounds from the various guidelines were compared. The characterization of the AEGL-1 and REL 

values for some compounds suggest that they would result in similar values. Other comparisons show 

that with the exceptions of vinyl chloride and acrylic acid, the REL values are more conservative than are 

the ERPG-1/TEEL-1 values, and often considerably so. 

The California OEHHA comments that the REL values are designed for preparation of risk assessments 

for non-emergency routine releases and that the AEGL values are for emergency planning purposes. The 

approach taken in determining the REL focuses on the most sensitive effect. Both use similar techniques 

to adjust for time of exposure. The ERPG-1 value allows an individual to experience transient, mild 

adverse effects. Thus, it is not intended to be as protective as the REL. The TEEL-1 values are 

extrapolated from other toxicity values and incorporate simplifying assumptions. Nonetheless, excluding 

an outlier in each sub-group (vinyl chloride in the TEEL-1 group and chloroform in the ERPG-1 group), 

TEEL-1 and ERPG-1 each predicted the REL similarly (the ratio of REL to TEEL-1 or ERPG-1 was 15% 

for TEEL-1 vs. 11% for ERPG-1; 12% for the two combined). 

The acute toxicity risk assessment is done using a hazard quotient. For that, effects are summed for a 

particular response. To do that, the risk assessor must know to what response each guidance value 

applies. This is clearly specified for the AEGL-1 and the REL values, but not for the TEEL-1 values. To 

be able to use the TEEL-1 values, the effects of concern would need to be provided. 

EPA Response: 

EPA has created a database to maintain current values of toxicity and fate and transport parameters. 

After reviewing the existing hierarchical approaches, we now recommend the following approach. 

Because of the daily operations of most combustion units and the potential for upset conditions to 

sometimes occur during operations, we consider acute values that address intermittent exposures more 

appropriate and more protective than values that are based on the assumption that acute exposures will 

be one-time only. We recommend using values from all of the sources that are based on one-hour 

exposures: 

1. Cal/EPA Acute RELs - the concentration in air at or below which no adverse health effects are 

anticipated in the general population, including sensitive individuals, for a specified exposure period 

(On-Line Address – http://www.oehha.ca.gov/air/pdf/acuterel.pdf) 

2. Acute inhalation exposure guidelines (AEGL-1) - “the airborne concentration of a substance above 

which it is predicted that the general population, including susceptible individuals, could experience 
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notable discomfort, irritation, or certain asymptomatic nonsensory effects. However, the effects are not 

disabling and are transient and reversible upon cessation of exposure.” 

(On-Line Address – http://www.epa.gov/oppt/aegl/chemlist.htm) 

3. Level 1 emergency planning guidelines (ERPG-1) - “the maximum concentration in air below which 

it is believed nearly all individuals could be exposed for up to one hour without experiencing other than 

mild transient adverse health effects or perceiving a clearly defined objectionable odor.” 

(On-Line Address – http://www.bnl.gov/scapa/scapawl.htm) 

4. Temporary emergency exposure limits (TEEL-1) - “the maximum concentration in air below which it 

is believed nearly all individuals could be exposed without experiencing other than mild transient adverse 

health effects or perceiving a clearly defined odor.” 

(On-Line Address – http://tis-hq.eh.doe.gov/web/Chem_Safety/teel.html) 

5. AEGL-2 values - “the airborne concentration of a substance above which it is predicted that the 

general population, including susceptible individuals, could experience irreversible or other serious, 

long-lasting adverse health effects or an impaired ability to escape.” AEGL-2 values are to be used only 

if lower ERPG-1 or TEEL-1 values are not available. (On-Line Address – 

http://www.epa.gov/oppt/aegl/chemlist.htm) 

The hierarchy is presented in order of preference, from 1 (most preferred) to 5 (least preferred). The 

HHRAP now recommends using the Acute Reference Exposure Levels (Acute RELs, developed by 

Cal/EPA) as the first choice for acute inhalation values. If no acute REL value is available for a given 

COPC, you can work down the list in order. If no AEGL-1 value is available, but an AEGL-2 value is 

available, select the AEGL-2 only if it is a more protective value (lower in concentration) than an ERPG1, or a TEEL-1 value if either of these values is available. If no acute values are available for a COPC, 

an acute value can be developed following the toxicity-based approach used by SCAPA (Tier 5). 

3.4.2 Exposure Assessment: Route-to-Route Extrapolations

Comments have been received regarding assumptions governing determination of route-to-route 

extrapolations of toxicity benchmarks presented in the draft HHRAP (Appendix A-3). Provide comments 

as to whether route-to-route extrapolation is appropriate and conservative in determining benchmark 

values for use in an initial screen of potential toxicity of compounds for which peer reviewed toxicity 

benchmarks are not available. 

 Peer Review Workshop Synopsis

The external peer review panel encouraged EPA to employ route-to-route extrapolations for toxicity 

during the first run of the risk assessment. If a chemical develops a significant risk during the first run, 

the risk assessor should review the defensibility of the extrapolation for the “risk drivers.” 

Peer reviewers noted that route-to-route extrapolations did not consistently underestimate or overestimate 

toxicity. They suggested that it may be necessary during the risk assessment process to examine the 

extrapolations to see if they indicate that something in the analysis should have been done differently. 
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The peer review panel also recommended that TELs could be used as additional guidance for chemicals 

for which there is not a reference dose or reference concentration. TELs could help the risk assessor in 

selecting the appropriate values and risk drivers. 

EPA Response: 

Comment noted. EPA will continue to use route-to-route extrapolations as a screening methodology. 

3.4.3 Exposure Assessment: Atmospheric Degradation of Contaminants

Comments were received regarding the guidance on assessing contaminant losses due to atmospheric 

degradation. Should atmospheric degradation be incorporated, and if so, which documented 

methodologies and associated parameter values should be included in the guidance for implementation? 

Would changes to the guidance for air dispersion modeling be required? 

 Peer Review Workshop Synopsis

The peer review panel agreed that degradation does occur in the atmosphere and that, in theory, it should 

be in the HHRAP. However, there are no models currently available to calculate atmospheric degradation 

sufficiently. 

EPA Response: 

Comment noted. 

3.4.4 Exposure Assessment: Exposure Scenario Locations

Comments were received on guidance provided for selection of exposure scenario locations (Section 

4.3.). Provide your comments to this subject. 

 Peer Review Workshop Synopsis

The external peer review panel agreed that the term subsistence needs to be better defined in the HHRAP. 

They agreed that the HHRAP needs to have flexibility built into the subsistence exposure scenario, and 

the default values for the subsistence exposure scenario need to be reevaluated. 

EPA Response: 

We’ve deleted the descriptive "subsistence" from the recommended exposure scenarios (see Section 4.2, 

Recommended Exposure Scenarios, and Appendix B). Review of this issue during the external peer review 

concluded that the subsistence terminology isn’t consistent with the actual mass per day amounts of 

ingested food items evaluated in the recommended exposure pathways. 
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3.4.5 Exposure Assessment: Non-Carcinogenic Risk of Dioxins Benchmark

Comments were received concerning the recommendation to evaluate non-carcinogenic risk of dioxins by 

comparing exposures to national average background exposure levels, using 1 pg/kg/day for adults 

(Section 2.3.1.2). Is the recommended benchmark appropriate for evaluating non-carcinogenic risk of 

dioxins for adults? 

 Peer Review Workshop Synopsis

The external peer review panel basically agreed that it is reasonable to use 1 pg/kg/day, with the caveat 

that there may actually be health effects at this level. 

EPA Response: 

Comment Noted 

3.4.6 Exposure Assessment: Using TEFs for Coplaner PCBs

Comments were received regarding the recommendation to evaluate coplaner polychlorinated bi-phenyls 

(PCBs) congeners in the risk assessment using dioxin toxicity equivalency factors (TEFs) (Section 2.3.3). 

Is the guidance for evaluating coplaner PCB congeners in the risk assessment using TEFs scientifically 

valid? Also comment on the technical validity of the recommendation to use toxicity benchmarks for 

Arochlor 1254 and Arochlor 1016 for evaluating other PCB congeners. 

 Peer Review Workshop Synopsis

The external peer review panel agreed that evaluating coplanar PCB congeners in the risk assessment 

using dioxin TEFs is appropriate and reasonable. They also agreed that using Aroclor 1254 and Aroclor 

1016 is a reasonable estimation, depending on the congener composition of actual emissions. 

EPA Response: 

We’ve modified the HHRAP to recommend using the World Health Organization (WHO) toxicity 

equivalency factors (TEF) for dioxin and furan congeners. TEF values for the 17 congeners are listed in 

the following table.
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Dioxin Congener TEF 

(unitless)

Furan Congener TEF 

(unitless)

2,3,7,8-Tetrachlorodibenzo(p)dioxin 1 2,3,7,8-Tetrachlorodibenzofuran 0.1

1,2,3,7,8-Pentachlorodibenzo(p)dioxin 1 1,2,3,7,8-Pentachlorodibenzofuran 0.05

1,2,3,4,7,8-

Hexachlorodibenzo(p)dioxin

0.1 2,3,4,7,8-Pentachlorodibenzofuran 0.5

1,2,3,6,7,8-

Hexachlorodibenzo(p)dioxin

0.1 1,2,3,4,7,8-

Hexachlorodibenzofuran

0.1

1,2,3,7,8,9-

Hexachlorodibenzo(p)dioxin

0.1 1,2,3,6,7,8-

Hexachlorodibenzofuran

0.1

1,2,3,4,6,7,8-

Heptachlorodibenzo(p)dioxin

0.01 1,2,3,7,8,9-Hexchlorodibenzofuran 0.1

1,2,3,4,6,7,8,9-

Octachlorodibenzo(p)dioxin

0.0001 2,3,4,6,7,8-

Hexachlorodibenzofuran

0.1

 1,2,3,4,6,7,8-

Heptachlorodibenzofuran

0.01

 1,2,3,4,7,8,9-

Heptachlorodibenzofuran

0.01

 1,2,3,4,6,7,8,9-

Octachlorodibenzofuran

0.0001

Source: WHO. 1997. Meeting on the Derivation of Toxic Equivalency Factors (TEFs) for PCBs, PCDDs, 

PCDFs, and Other Dioxin-like Compounds for Humans and Wildlife. Institute of Environmental 

Medicine, Karolinska Institute. Stockholm, Sweden. June 15-18. Draft Report, July 30, 1997. 

Van den Berg et al. 1998. Toxic Equivalency Factors (TEFs) for PCBs, PCDDs, PCDFs for Humans and 

Wildlife. Environmental Health Perspectives, Volume 106, Number 12:775-792.

3.4.7 Exposure Assessment: Dioxins in Breast Milk

Comment on the guidance for addressing the non-cancer effects from dioxins and furans (Section 2.3.1.2). 

Is the default value for breast milk fat intake for the breast milk pathway appropriately set, or are there 

new data available to suggest that different value may be more appropriate? Is there enough scientific 

evidence to show that depletions in the concentration of dioxin in breast milk over the first year of nursing 

should be accounted for in this pathway? 

 Peer Review Workshop Synopsis

The external peer review panel stated that there may be enough literature to account for the depletion of 

30-50 percent of dioxin over the first year, and they felt that EPA should look at the relevant studies in 

relation to this topic. It was not clear to the panel that the cited research was referenced in the draft 

HHRAP. 
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EPA Response: 

EPA’s review of the data did not indicate there was enough data in the literature to derive a factor to 

account for dioxin concentration depletion in breast milk, nor was any provided by the peer review panel. 

3.4.8 Exposure Assessment: Inhalation Rate Value

The EPA would like to solicit comments from the panel on their attempts to incorporate the most current 

risk assessment guidance. For instance, an inhalation rate of 15 cubic meters per day (“Exposure Factor 

Handbook” (U.S. EPA 1997c)) has been proposed, as opposed to the 20 cubic meters per day used by the 

Superfund Program. Does the panel have any recommendations about the necessity for this document in 

applicability of the values? 

 Peer Review Workshop Synopsis

The external peer review panel did not reach a consensus on the use of a particular inhalation rate, but 

they did agree that 20 cubic meters of air was generally too high to be representative of a daily average 

adult inhalation rate. Some of the peer reviewers recommended using RFCs rather than converting air 

concentrations into a dose of milligrams per kilogram per day. Peer reviewers also generally agreed that 

using inhalation rates should be guided by the type of risk (chronic and carcinogenic elements) and the 

most appropriate inhalation rate for long-term exposure. 

EPA Response: 

We’ve modified the HHRAP to recommend using RfC’s rather than converting air concentrations into a 

dose of milligrams per kilogram day. 

3.5 ENVIRONMENTAL FATE AND TRANSPORT 

[draft HHRAP Chapter 5, Appendices A&B] 

Issues regarding exposure assessment that were addressed by the external peer review panel include: 

• Water Ingestion by Cows; 

• Speciating and Modeling Mercury; 

• Biotransfer (Ba) Values; 

• Organic Compound Conservation of Mass; 

• Losses to Soil due to Erosion; and 

• Acute Benchmarks Based on Mild Adverse Health Effects. 

3.5.1 Environmental Fate and Transport: Water Ingestion by Cows Pathway 

Comments were received regarding not including the water ingestion by cows as a potential COPC uptake 

mechanism (Sections 4.2 and 5.4). Should ingestion of contaminated water by cows be included in the 
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calculation of exposure concentrations in beef and milk? If ingestion of contaminated water by cows is 

included as a pathway, will adjustments to the recommended Babeef and Bamilk values be required? 

Summary of Peer Review Written Comments: 

Assuming the same source of water, direct exposures of human receptors to contaminants in drinking 

water is likely to be a more significant pathway than secondary exposures from meat and milk. Because 

the draft HHRAP focuses on the most significant exposure pathways, it is probably not necessary to 

account for the ingestion of contaminated water by cows unless pasture grasses are also irrigated with 

contaminated groundwater from shallow wells or surface water. Exposure pathways involving 

groundwater are not generally evaluated for combustion units, unless site-specific conditions dictate 

otherwise (e.g., infiltration of COPCs into very shallow aquifers). The water component of oral ingestion 

is not an important consideration for hydrophobic compounds like PCDD/Fs and PCBs because the low 

solubility of these compounds limits the potential intake. Compounds with lower log Kow values might be 

present in surface waters consumed by animals, but it is likely that these compounds would not be 

accumulated because of rapid rates of metabolism and/or urinary excretion. 

EPA Response: 

EPA agrees with the external peer review panel and won’t recommend including ingestion of 

contaminated water by cows in the calculation of exposure concentrations in beef and milk. 

3.5.2 Environmental Fate and Transport: Speciating and Modeling Mercury

Comments were received regarding guidance presented for speciating and modeling of mercury in the risk 

assessment (Section 2.3.8.3, Appendix B, and Appendix C). Provide comments on the technical validity 

of key elements of mercury modeling. 

Peer Review Workshop Synopsis

According to the external peer review panel, the total mercury loading of a combustion facility in the 

watershed is overestimated using the processes and equations outlined in the draft HHRAP. One reason 

for this overestimation is the way Equation B-4-27 in the draft HHRAP guidance is written, where the 

concentration of methyl mercury in fish is being determined by the dissolved water concentration of both 

methyl mercury and inorganic mercury times the bioaccumulation factor (BAF) for methyl mercury. A 

second reason is that the use of the Universal Soil Loss Equation (USLE) model overestimates the soil 

erosion factor, thereby overestimating the runoff of methyl mercury into the surface water. In addition, it 

appears that the derivation and application of the draft HHRAP equations do not account for the 

methylmercury contributions from wetlands. 

The external peer reviewers also concluded that the draft HHRAP modeling methods appear to violate 

mass balance requirements. In some applications of the draft HHRAP guidance, more mercury has been 

calculated in soil and going into a water body than was originally present in the system, giving the 

appearance that the facility being modeled is creating mercury. 
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The peer reviewers recommended that the HHRAP guidance should focus on modeling of mercury more 

than the speciation of mercury, emphasizing improvement in deposition and runoff/erosion parts of the 

modeling effort. The USLE erosion model was specifically mentioned again as needing adjustment. 

A recommendation was also made to not assume that every fisher and recreational fisher is eating Tier 4 

fish in every water body. 

EPA Response: 

We’ve modified the HHRAP to recommend estimating the dissolved water concentration for divalent 

mercury Cdw (Hg2+) and methyl mercury Cdw (MHg) (see Table B-4-24) by applying the default mercury 

speciations (15 percent methyl mercury and 85 percent divalent mercury) directly to the dissolved water 

concentration calculated for divalent mercury versus speciating each water body loading, as currently 

provided in the draft HHRAP and August 2, 1999 errata. This results in a Cdw (Mercury Total) being 

calculated using the fate and transport parameters specified for mercuric chloride (see Appendix A-3 

COPC tables for mercuric chloride). Then, as indicated below, Cdw (Mercury Total) should be 

apportioned based on an 85 percent Hg2+ and 15 percent MHg speciation split in the water body 

(consistent with Chapter 2 of the draft HHRAP). 

Cdw Hg2+ = Cdw (Mercury Total) * 0.85 

Cdw MHg = Cdw (Mercury Total) * 0.15 

We now recommend estimating the concentration of mercury species in fish (Cfish) using species-specific 

bioaccumulation factors and dissolved water concentrations. This requires revising the equation in Table 

B-4-27 (provided specifically to calculate Cfish (MHg)) as follows: 

Cfish Hg2+ = Cdw Hg

2+ * BAF fish (Hg 2+

)

Cfish MHg = Cdw MHg * BAF fish (MHg) 

3.5.3 Environmental Fate and Transport: Biotransfer (Ba) Values

Comments were received regarding the recommended determination and application of biotransfer (Ba) 

values, including (Chapter 5, Appendix A-3, and Appendix B). Considering the scientific literature 

currently available, provide comments on the technical validity of guidance presented for determination 

and application of Ba values, Baegg values for PAHs, Babeef, and Bamilk. For biotransfer values for 

lipophilic compounds, such as PAH’s and dioxins, are the equations presented in Travis and Arms (1988) 

and Baes et al. (1984) for the estimation of Ba values appropriate as applied in the guidance? 

Considering how the applicable bioconcentration values reported in Stephens et al. (1995) were 

determined, should the total feed consumption rate of 0.2 mg/kg be multiplied by the fraction of feed 

found in the soil, 0.1 before calculating the Bachicken and Baeggs for dioxins? 
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 Peer Review Workshop Synopsis

Panel discussions on this charge focused on a number of different issues. 

2) The external peer review panel agreed that the Ba values should refer to total dietary intake. The 

value for the concentration in the total diet already includes soil. They also agreed that water should 

be considered for compounds when appropriate. For lipophilic compounds, however, water 

solubility is so low that it essentially supplies no input to the equation. 

3) As to using the high dose verses the low dose in the Stephens’ equation, the external peer review 

panel concluded that the high dose should be used. In the case of the high-dose value, the equation 

would still be essentially linear, unless very high doses are obtained that have toxicological effects 

on the animal. If the low dose is used, computational problems can arise for many of the 

congeners. Using the low dose may also result in relatively large analytical errors in the data sets. 

4) With regard to soil consumption values for free-range chickens, the external peer review panel 

concluded that studies conducted on wild turkeys indicate a default value of between 9 and 10 

percent of dietary intake. Since wild turkeys and free-range chickens are closely related, the 

guidance default value of ten percent for free-range chickens is a good assumption and should be 

used as an upper bound. However, if the guidance is going to be consistent in relation to all 

species, the bioconcentration factor (BCF) value should be based on total dry matter intake. 

5) With regard to those compounds with log Kow values greater than 6, the external peer review panel 

agreed that bioaccumulation decreases. They noted research on dioxins and PCBs which 

demonstrate that, as chlorination in the compound increases, the dietary uptake of the compounds in 

the body decreases. Thus, the panel concluded that bioconcentration decreases as the degree of 

chlorination increases. 

In addition, the external peer review panel noted that the log Kow values do not indicate whether a 

compound is being metabolized in the body, which appears to be a weakness in the Travis 

equations. Compounds that are metabolized do not bioaccumulate. Therefore, it was 

recommended that the risk assessor look at any compounds that are indicating a high risk and 

determine if the risk assessment should be adjusted to account for those that are likely to be 

metabolized. 

As to the log Kow values themselves, the external peer review panel agreed that there is no guidance 

on how to select them. Therefore, it is hard to put any correlation together for the log Kow values if 

the numbers are not coming from the same laboratory using the same method for the derivation of 

the data. In saying that, the peer review panel conceded that there was no one best method to obtain 

log Kow values. 

6) The external peer review panel concluded that there were three major problems with using the 

information derived from bioaccumulation research of substances in beef tissue (Babeef). First, in 

that research, the majority of the animals were diary cows, which are lactating. Lactation would 

change the nature of the bioaccumulation process and would not represent tissue concentrations in 
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non-lactating animals. Second, the duration of the research was not long enough to have reached a 

steady state on tissue concentrations. Third, the research did not take in to account that animals 

typically are growing. The research ignored the fact that growing animals consume more feed as 

they grow. Instead, researchers assumed a constant milligram per day intake. 

7) With regard to the values for pork, the external peer review panel did not find specific problems, 

but they noted that there is still the same growing animal problem as encountered with beef. They 

recommended that, because there are not many good data on persistent compounds in pork, if a 

compound found in pork tissue is driving the risk assessment, the risk assessor should carefully 

examine the factors most affecting the risk calculation. 

8) With regard to the Ba values for PAH’s and phthalates, the external peer review panel concluded 

that there was enough literature data to indicate that these compounds would never show up in beef, 

milk, or eggs. PAHs and phthalates are readily metabolized in the body. If they do show up in that 

tissue, it would probably be the result of post-contamination. 

EPA Response: 

We’ve modified the HHRAP to recommend reducing the biotransfer factor values for chicken (Bachicken) 

and poultry eggs (Baeggs) by one order of magnitude for all dioxin and furan congeners reported in the 

Appendix A-2 COPC tables (see Section A.2.5.3, Biotransfer Factors for Bachicken and Baegg). 

Based on the significant comments received during the peer review concerning biotransfer factors, we 

commissioned a study to validate the Travis and Arms beef and milk biotransfer factors. Comments noted 

by the peer reviewers were taken into account when designing the study. This study did validate the 

Travis and Arms equations for a limited range of Kow values, but indicated a large variability associated 

within the data set. The study also generated non-linear biotransfer equations to replace the Travis and 

Arms equations and dramatically reduce variability in the data set. We’ve updated the HHRAP with the 

new biotransfer equations for beef and milk. In addition, all biotransfer equations (including plant 

biotransfer equations) have been capped at the highest and lowest Kow value used in generating the 

equations. We believe this is a conservative cap that lessens the possibility of violating conservation of 

mass. 

3.5.4 Environmental Fate and Transport: Organic Compound Conservation of Mass

Comments were received stating a violation in conservation of mass exists based on guidance presented 

for calculating exposure concentrations of some organic compounds via indirect pathways, based on not 

accounting for removal of the contaminant fraction assumed to deposit on vegetation (Sections 5.2 and 

5.3, Appendix A-3, and Appendix B). Does application of the recommended Ba values violate 

conservation of the mass of contaminants emitted to mass concentrated in plant or tissues? 

 Peer Review Workshop Synopsis

The external peer review panel agreed that a violation in the conservation of mass has occurred based on 

the equations for calculating exposure concentrations. The potential solution for dioxin and other organic 

vapors would be to transform the air modeling equations so that you subtract out the mass in the air as the 
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contaminants accumulate elsewhere. The panel recommended that EPA develop a check to ensure mass 

balance. The panel also recommended that EPA expand the incorporation of the approach that is being 

used for surface water. Using that approach everywhere, EPA would have a limit to the rate of transfer 

from the air and to the soil, plants, and water, and a boundary limiting the movement of material from one 

pathway to another.

EPA Response: 

As noted more than a decade ago in EPA guidance for calculating health risks associated with indirect 

exposure to emissions, fate and transport and food chain algorithms that don’t rigorously maintain mass 

balance checks can result in mass balance violations for certain compounds in certain exposure 

situations. EPA guidance has generally implied that modelers conduct mass balance checks to ensure 

that reasonable predictions are being made. However, we recognize the need for more internal mass 

balance checks within the biotransfer algorithms. 

To address the above issue, we’ve revised sensitive and affected parameters of the fate and transport 

models we recommend in the HHRAP. For example, uptake factors for most organic compounds are 

calculated using regression equations derived from empirical data sets that correlate to compound 

specific octanol/water partitioning coefficient (Kow) values. Sometime those data sets were rather limited, 

and compounds with Kow values well outside the range of the data set are the most common examples of 

potential mass balance violations. To limit this problem, uptake factor values for compounds with Kow

values outside the range of empirical data used to derive the correlation equations have been bound more 

in line with the upper or lower values of the empirical data set. 

Additionally, the HHRAP provides literature-documented ranges of site-specific input parameters and, 

where appropriate, bounding values to help the risk assessor perform mass balance checks. For 

example, the range of USLE rainfall (or erosivity) factors documented in the literature is between 50 and 

300 yr-1. Values outside of this range would need to be evaluated closely to prevent potential mass 

balance violations in the calculation of unit soil loss. An example of a bounding parameter would be the 

default value provided in the HHRAP for the suspended solids deposition rate utilized to prevent 

erroneous errors from implying that streams will fill in from sedimentation to and unrealistic level. 

3.5.5 Environmental Fate and Transport: Losses to Soil Due to Erosion

Comments were received regarding the guidance on accounting for losses to soil due to erosion (Sections 

5.2.2 and Appendix B). Are the presented recommendations regarding losses to soil due to soil erosion 

technically valid for evaluation of all terrestrial exposure scenarios?

 Peer Review Workshop Synopsis

The external peer review panel suggested a reasonable approach to deal with soil erosion is to assume that 

erosion to an area is equal to that from an area and thereby set erosion equal to zero. However, the 

HHRAP should also provide an equation for calculating loss due to erosion for site-specific situations in 

which this assumption is not the case. 
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EPA Response: 

We’ll continue to recommend that the loss from soil due to erosion be set equal to zero. This is based on 

the assumption that soil in a specified area will lose as much soil contaminant (erosion out of the area) as 

it will gain in soil contaminant from erosion into the area of concern. If this assumption is not met, the 

HHRAP provides an equation that can be used with the Universal Soil Loss Equation to estimate loss of 

contaminant due to erosion. We also recommend using the Universal Soil Loss Equation (USLE) to 

calculate the amount of contaminant entering a water body via erosion. The USLE is calculated on a 

site-specific basis. 

3.5.6 Environmental Fate and Transport: Acute Benchmarks Based on Mild Adverse Health 

Effects

U.S. EPA solicited comments from the peer review panel on attempts to set acute benchmarks based on 

mild adverse health effects, such as irritation. 

Peer Review Workshop Synopsis

The Panel recommended that EPA contact the States about their frequencies of upset events, and evaluate 

whether they occurred frequently enough to warrant use of an acute benchmark based on mild adverse 

health effects. 

It was suggested that EPA could improve the assessment of the likelihood of mild acute health effects by 

using two approaches: (1) evaluating upset conditions combined with the worst meteorological conditions 

and (2) evaluating upset emissions over typical atmospheric conditions. 

Another suggestion was that EPA could use the 95th percentile value, and that would be a blend between 

the worst case and things that occur more frequently. It would still give the HHRAP a conservative value. 

This method was further substantiated by the fact that several States have taken the percentile approach to 

determine acute benchmarks. 

The point was made that mild health effects, even if not debilitating to individuals, may result in hundreds 

of visits to local hospital emergency rooms, and be something that should be considered seriously when 

setting acute benchmarks. 

EPA Response: 

After evaluating the comments from the external peer review panel, we decided to stay with the approach 

of recommending acute benchmarks based on mild health effects. We realize that better modeling and 

exposure assessment methodologies may be appropriate in some cases, but the HHRAP methodology is 

presented only as a broad screen of acute effects. The methodology in the HHRAP was selected based on 

ease of implementation and screening capabilities. The air dispersion modeling conducted to evaluate 

chronic health effects generates maximum one-hour air concentrations used in the acute assessment. We 

believe that using the maximum one-hour air concentration and the mild health effects acute benchmark 

will provide a reasonable screening method without being too burdensome on facilities. 
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4.0 REFERENCES

Baes, C.F., R.D. Sharp, A.L. Sjoreen, and R.W. Shor. 1984. ”Review and Analysis of Parameters and 

Assessing Transport of Environmentally Released Radionuclides Through Agriculture.” U.S. Department 

of Energy Oak Ridge National Laboratory. Oak Ridge , TN. 

Briggs, G.G., R.H. Bromilow, A.A. Evans, and M. Williams. 1982. “Relationships between Lipophilicity 

and Root Uptake and Distribution of Non-ionized Chemicals in Barley Shoots Following Uptake by the 

Roots.” Pesticide Science. 14:492-500. 

California Air Resources Board. 1990. Health Risk Assessment Guidelines for Nonhazardous Waste 

Incinerators. Sacramento, CA: Department of Health Services, Stationary Source Division and Air Toxics 

Unit. 

California Environmental Protection Agency (Cal/EPA). 1998. Evaluation of Acute Toxicity and AIEC, 

Revised Draft. Office of Environmental Health Hazard Assessments (OEHHA). 

Dulin, D., H. Drossman, and T. Mill. 1986. “Products and Quantum Yields for Photolysis of 

Chloroaromatics in Water.” Environmental Science and Technology. 20:72-77. 

Kaleri, C.J. 2000. “Implementation Issues: Mercury Transport & Fate, Combustion Risk Assessments in 

Region 6.” Proceeding of the 12th International Conference on Incineration and Thermal Treatment 

Technologies. May 14-18. 

Klysik, K. and K. Fortuniak. 1999. “Temporal and Spatial Characteristics of the Urban Heat Island of 

Lodz, Poland.” Atmospheric Environment. 33(24-25):3885-3895. 

Lemieux, P.M., C.W. Lee, J.D. Kilgore, and J.V. Ryan. 1999. “Emissions of Polychlorinated Biphenyls 

as Products of Incomplete Combustion from Incinerators.” Presented at the 1999 International 

Conference on Incineration and Thermal Treatment Technologies. Orlando, Florida. May. 

National Academy of Sciences (NAS). 1983. Risk Assessment in the Federal Government: Managing the 

Process. National Research Council, Committee on the Institutional Means for Assessment of Risks to 

Public Health, Washington, D.C. 

National Research Council. 1987. Predicting Feed Intake of Food-Producing Animals. Subcommittee on 

Feed Intake, Committee on Animal Nutrition, Board on Agriculture. National Academy Press, 

Washington, D.C. 

 

Paine, R.J., R.F. Lee, R. Brode, R.B. Wilson, A.J. Cimorelli, S.G. Perry, J.C. Weil, A. Venkatram, and 

W.D. Peters. 1998. Model Evaluation Results for AERMOD. U.S. EPA. December 17. 
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Presidential/Congressional Commission on Risk Assessment and Risk Management. 1997a. “Framework 

for Environmental Health Risk Management and Risk Assessment.” Risk World. January/February 

Presidential/Congressional Commission on Risk Assessment and Risk Management. 1997b. “Risk 

Management in Regulatory Decision-Making.” Risk World. March/April. 

Renard, K.G., G.R. Foster, G.A. Weesies, D.K. McCool, and D.C. Yoder. 1997. “Predicting Soil Erosion 

by Water: A Guide to Conservation Planning with the Revised Universal Soil Loss Equation (RUSLE).” 

U.S. Department of Agriculture Agricultural Handbook. No. 703. U.S. Government Printing Office, 

Washington, D.C. 

Secrest, J.A. 1997. Model Parameter Sensitivity Analysis. The Air Group, U.S. EPA Region 6 Center for 

Combustion Science and Engineering. Prepared for Lynette Collins, Project Manager, Tetra Tech EM, 

Inc., Dallas, TX. May 23. 

Stephens, R.D., M. Petreas, and G.H. Hayward. 1995. "Biotransfer and Bioaccumulation of Dioxins and 

Furans from Soil: Chickens as a Model for Foraging Animals." The Science of the Total Environment. 

175: 253-273. July 20. 

Stieglitz, L., G. Zwick, J. Beck, H. Bautz, and W. Roth. 1989. “Carbonaceous Particles in Fly Ash-A 

Source for the De-Novo-Synthesis of Organochlorocompounds.” Chemosphere. 19: 283-290. 

Travis, C.C. and A.D. Arms. 1988. “Bioconcentration of Organics in Beef, Milk, and Vegetation.” 

Environmental Science and Technology. 22:271-274. 

U.S. EPA. 1994a. Methods for Derivation of Inhalation Reference Concentrations and Application of 

Inhalation Dosimetry. EPA/600/8-90/066F. 

U.S. EPA. 1995a. Compilation of Air Pollutant Emission Factors: Volume I, Stationary Point and Area 

Sources. Research Triangle Park, North Carolina. Fifth Edition. AP-42. January. 

U.S. EPA. 1996 Guideline on Air Quality Models. Federal Register 61:41838. August. 

U.S. EPA. 1997a. Risk Assessment for the Waste Technologies Industries (WTI) Hazardous Waste 

Incineration Facility (East Liverpool, Ohio). EPA-905-R97-002. May. 

U.S. EPA. 1997b. Mercury Study Report to Congress, Volumes I through VIII. Office of Air Quality 

Planning and Standards and Office of Research and Development. EPA/452/R-97-001. December. 

U.S. EPA. 1997c. Exposure Factors Handbook. Office of Research and Development. EPA/600/P95/002Fc. August. 

U.S. EPA. 1997d. “Guiding Principles for Monte Carlo Analysis.” U.S. EPA Risk Assessment Forum, 

Washington, D.C. EPA/630/R-97/001. 
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U.S. EPA. 1998a. Human Health Risk Assessment Protocol for Hazardous Waste Combustion Facilities, 

Peer Review Draft. EPA530-D-98-001A. July. 

U.S. EPA, 1998b. Science Policy Council Handbook for Peer Review. Washington, DC. 

U.S. EPA. 1998c. Guidance on Collection of Emissions Data to Support Site-Specific Risk Assessments at 

Hazardous Waste Combustion Facilities, Internal Review Draft. Prepared by EPA Region 4 and the 

Office of Solid Waste. 

U.S. EPA. 1998d. Methodology for Assessing Health Risks Associated with Multiple Pathways of 

Exposure to Combustor Emissions. EPA/600/R-98/137. December. (Update to: U.S. EPA’ s Methodology 

for Assessing Health Risks Associated with Indirect Exposure to Combustor Emissions. EPA/600/6-

90/003) 

U.S. EPA. 1998e. Guidance on Collection of Emissions Data to Support Site-Specific Risk Assessments at 

Hazardous Waste Combustion Facilities. EPA 530D-98-002. August. 

U.S. EPA. 1999a. “40CFR Part 60, et al. NESHAPS: Final Standards for Hazardous Air Pollutants for 

Hazardous Waste Combustors; Final Rule.” Federal Register, September 30, 1999. p. 52828 

U.S. EPA, 1999b, Cumulative Exposure Project. http://www.epa.gov/ttnatw01/cep/fsheet.html 

U.S. EPA. 1999c. Draft Risk Assessment Guidance for Superfund (RAGS) Volume III – Part A: Process 

for Conducting Probabilistic Risk Assessment. Risk Assessment Superfund, U.S. EPA. 

U.S. EPA. 1999d. Mercury Research Strategy, Workshop Review Draft. NCEA-I-0710. November. 

U.S. EPA. 2000 Exposure and Human Health Reassessment of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin 

(TCDD) and Related Compounds, Draft Final. EPA/600/P-00/001B. September. 

U.S. EPA. 2001a. NATA – Evaluating the National-Scale Air Toxics Assessment for 1996 – An SAB 

Advisory. An advisory by the U.S. EPA Science Advisory Board (SAB). EPA-SAB-EC-ADV-02-001. 

December. 

U.S. EPA. 2001b. Risk Burn Guidance for Hazardous Waste Combustion Facilities. EPA530-R-01-001. 

July. 

U.S. EPA Region 6. 1998. Risk Management Addendum. EPA-R6-98-002. July. 

Van den Berg et al. 1998. “Toxicity Equivalency Factors (TEFs) for PCBs, PCDDs, PCDFs for Humans 

and Wildlife.” Environmental Health Perspectives. 106(12):775-792. 
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World Health Organization. 1997. Meeting the Derivation of Toxic Equivalency Factors (TEFs) for 

PCBs, PCDDs, PCDFs, and Other Dioxin-Like Compounds for Humans and Wildlife. Institiute of 

Environmental Medicine, Karolinska Institute. Stockholm, Sweden. June 15-18. Draft Report. July 30. 
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Appendix A

Public and Peer Reviewers of the 

Human Health Risk Assessment Protocol for Hazardous Waste Combustion Facilities (HHRAP) 

Peer Review Draft, July 1998

A1. Internal U.S. EPA and State Reviewers 

The Peer Review Draft HHRAP (U.S. EPA, 1998a) was developed by the U.S. Environmental Protection 

Agency (EPA) Region 6 and the Office of Solid Waste. Jeff Yurk from EPA Region 6 was the primary 

author/editor of this document with Tetra Tech EM, Inc, providing the bulk of the background research 

under Contract No. 68-W-99-018. Karen Pollard and Timothy Taylor of the Office of Solid Waste 

provided authorship, editorial input, and overall coordination with experts in other EPA Regions, 

Headquarters, and the Office of Research and Development. The following U.S. EPA and state agency 

personnel provided comments on earlier draft versions of the HHRAP. 

U.S. EPA REVIEWERS

1. Office of Solid Waste 

3⁄4 Economics, Methods, and Risk Analysis Division 

David Cozzie 

Virginia Colten-Bradley 

Becky Daiss 

Stephen Kroner 

David Layland 

Alec McBride 

3⁄4 Permits and State Programs Division 

Rosemary Workman 

Val De LaFuente 

3⁄4 Municipal and Industrial Solid Waste Division 

Bill Schoenborn 

3⁄4 Hazardous Waste Minimization and Management Division 

Fred Chanania 

2. Office of Solid Waste and Emergency Response 

3⁄4 Office of the Assistant Administrator 

Dorothy Canter 
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3. Office of Research & Development 

3⁄4 National Center for Environmental Assessment/Cincinnati 

Eletha Brady-Roberts 

Randy Bruins 

David Reisman 

Glenn Rice 

Sue Schock 

Jeff Swartout 

3⁄4 National Center for Environmental Assessment/DC 

David Cleaverly 

Jim Cogliano 

Matthew Lorber 

Anne Sergeant 

3⁄4 National Center for Environmental Assessment/RTP 

Judy Strickland 

3⁄4 National Exposure Research Laboratory 

Robert Ambrose 

Larry Johnson 

Donna Schwede 

3⁄4 National Risk Management Research Laboratory 

Paul Lemieux 

Jeffrey Ryan 

3⁄4 National Health and Environmental Effects Research Laboratory/RTP 

John Nichols 

4. Office of Air Quality Planning and Standards 

3⁄4 Air Quality Monitoring Group 

Joe Touma 

5. Office of Pollution, Prevention and Toxics 

3⁄4 Risk Assessment Division 

Vince Nabholtz 

6. Office of General Council 

3⁄4 Solid Waste and Emergency Response Law Office 

Karen Kraus 
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7. EPA Region 1 

3⁄4 Office of Ecosystem Protection Division 

Jui-Yu Hsieh 

8. EPA Region 2 

3⁄4 Division of Environmental Planning and Protection 

John Brogard 

9. EPA Region 3 

3⁄4 Waste and Chemicals Management Division 

Gary Gross 

10. EPA Region 4 

3⁄4 Waste Management Division 

Beth Antley 

Rick Gillam 

11. EPA Region 5 

3⁄4 Waste, Pesticide and Toxic Division 

Gary Victorine 

Mario Mangino 

12. EPA Region 6 

3⁄4 Multimedia Planning and Permitting Division 

Cynthia Kaleri 

David Weeks 

3⁄4 Superfund Group 

Ghassan Khoury 

Jon Rauscher 

Susan Roddy 

13. EPA Region 7 

3⁄4 Air, RCRA, and Toxics Division 

John Smith 

14. EPA Region 8 

3⁄4 Hazardous Waste Program 

Carl Daly 

Tala Henry 

15. EPA Region 9 

3⁄4 Waste Management Division 

Stacy Braye 
Response to Public Comments on the Human Health Risk Assessment Protocol June 2005 ________________________________________________________________________________________________________ 

 

________________________________________________________________________________________________________

U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 161 

16. EPA Region 10 

3⁄4 Office of Environmental Assessment 

Marcia Bailey 

Roseanne Lorenzana 

3⁄4 Office of Waste and Chemicals 

 Catherine Massimino 

STATE REVIEWERS

1. Texas Natural Resource Conservation Commission 

3⁄4 Toxicology and Risk Assessment Section 

 Larry Champagne 

 Lucy Frasier 

 Roberta Grant 

 Laurie Haws 

 Loren Lund 

 Torin McCoy 

 Robert Opiela 

 Mark Rudolph 

 Dom Ruggeri 

 Andrew Tachovsky 

2. Arkansas Department of Pollution Control and Ecology 

Phillip Murphy 

Tammi Hynum 

3. Colorado Department of Health 

3⁄4 Hazardous Materials and Waste Management Division 

Joe Schieffelin 

R. David Waltz 

4. Utah Department of Environmental Quality 

Christopher Bittner 

5. Alabama Department of Environmental Management 

3⁄4 Air Division 

John Rogers 

Nathan Hartman 

3⁄4 Division of Epidemiology 

 Brian Hughes 
Response to Public Comments on the Human Health Risk Assessment Protocol June 2005 ________________________________________________________________________________________________________ 

 

________________________________________________________________________________________________________

U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 162 

A2. Public Commenters

Commenter Affiliation

Robin Anderson Superfund 

Dr. Howard Vyvyan Fetal & infant Toxico-Pathology 

University of Liverpool 

Liverpool L69 7ZA, 

United Kingdom 

Louisiana Chemical Association Louisiana Chemical Association (LCA) BIF workgroup 

One American Place, Suite 2040 

Baton Rouge, LA 70825 

(225) 344-2609 

Roger Rudolf 

Drew Lyle 

Laura Peters 

Department of the Army 

Center for Health Promotion and Preventive Medicine 

5158 Blackhawk Road 

Aberdeen Proving Ground, Md 21010-5422 

Thomas Traceski Department of Energy 

Washington, DC 20585 

Dr. Craig Stevenson Environmental Chemistry 

Institute of Environmental and Scientific Research 

17 Kelly Street 

Mt. Eden Auckland, 

New Zealand 

Official Public Comments on the 

Risk Burn guidance 

Various organizations 

Richard Richter ExponentFailure Analysis 

3187 Red Hill Ave, 

 Suite 100 

Costa Mesa, CA 92626 

Michelle Lusk Cement Kiln Recycling Coalition 

1225 Eye street, N.W, 

Suite 300 

Washington, DC 20005 

(202) 789-1945 
Response to Public Comments on the Human Health Risk Assessment Protocol June 2005 ________________________________________________________________________________________________________ 

 

________________________________________________________________________________________________________

U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 163 

Commenter Affiliation

Tim Bingman 

Nancy G. Doerrer 

American Industrial Health Council 

Suite 780 

2001 Pennsylvania Ave. 

Washington, DC 20004-1850 

(202) 833-2131 

Melvin Keener Coalition for Responsible Waste Incineration 

1133 Connecticut Ave. 

Suite 1023 

Washington, DC 20036 

(202) 775-9869 

C. T. “Kip” Howlett, Jr. 

Ann M. Mason 

Chlorine Chemical Council (CCC) 

1300 Wilson Boulevard 

Arlington, VA 22209 

(703) 741-5000 

Thomas Nilan Chemical Manufacturers Association 

1300 Wilson Blvd. 

Arlington, VA 22209 

(703) 741-5000 

Joe Lysonski Anderson-Mulholland & Associates, Inc. 

110 Corporate Park Drive 

White plains, NY 10604 

George Bollweg Region 5 

Waste Management Branch 

Rex Jameson 

Chuck Kellett 

Holnam, Inc 

6211 north Ann Arbor 

Dundee, MI 48131 

 

Safety Kleen Systems 

1 Brinkman Way 

Elgrin, Ill 60123-7857 

Environmental Technology 

Council 

Environmental Technology Council 

734 15th Street, NW 

Washington, DC 20005 

(202) 783-0870 
Response to Public Comments on the Human Health Risk Assessment Protocol June 2005 ________________________________________________________________________________________________________ 

 

________________________________________________________________________________________________________

U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 164 

Commenter Affiliation

Lisa Bailey, PhD Menzie-Cura & Associates 

Environmental Consultants 

1 Couthouse lane, Suite 2 

Chelmsford, MA 01824 

Steven G. Zemba, PhD Cambridge Environmental Inc. 

53 Charles Street 

Cambridge, MA 02141 

(617) 225-0810 

Roger Rudolph US Army Center for Health Promotion and Preventative 

Medicine (USACHIPPM) 

5158 Blakhawk Road 

Aberdeen Proving Ground, Md. 21010-5422 

Roy Wood Kodak Park Environ. Services 

Health Safety, and Environment 

Eastman Kodak Company 

Rochester, New York 14652-6263 

State of Texas Texas Natural Resource Conservation Commission 

Toxicology & Risk Ass. Section 

Office of Chief Engineer 

Modeling Section 

Remediation Division 

New source Review Division 

Office of Air Quality 

Resource & Environmental Assessment Section 

Office of Resource Management 
Response to Public Comments on the Human Health Risk Assessment Protocol June 2005 ________________________________________________________________________________________________________

________________________________________________________________________________________________________

U.S. EPA Region 6 U.S. EPA 

Multimedia Planning and Permitting Division Office of Solid Waste 

Center for Combustion Science and Engineering 165 

A3. External Peer Reviewers 

On May 25 and 26, 2000, an external peer review workshop was held. The peer review and workshop 

was organized, convened and conducted by Tech Law, an EPA contractor under the EPA contract 

number: 68-W-99-017. Panel members and reviewers at this workshop include the following. 

EXTERNAL PEER REVIEWERS

1. Combustion Engineering 

Dr. William Schofield (Focus Environmental) 

2. Atmospheric Modeling 

Dr. Walter Dabberdt (National Center of Atmospheric Research) 

3. Human Health Toxicology 

Dr. Thomas A. Gasiewicz (University of Rochester) 

Dr. Mary Davis (West Virginia University) 

4. Human Health Exposure 

Dr. James P. Butler (Argonne National Laboratory/University of Chicago) 

Dr. Richard L. DeGrandchamp (University of Colorado) 

Mr. Steven Washburn (Environ Corporation) 

5. Chemical Fate and Transport 

 Dr. George F. Fries (U.S. Department of Agriculture (retired)) 

Dr. Douglas Smith1

 (ENSR International) 

The following team supplied comments to panel member Dr. Douglas Smith 

Dr. Ishrat Chaudhuri, DABT (ENSR International) 

Dr. Don Fontaine (Dow Chemical) 

Dr. Lucy Fraiser (MFG, Inc.) 

Dr. Andrew Friedmann (ENSR International) 

Mr. Marcus Garcia (ENSR International) 

Dr. Mark Gerath (ENSR International) 

Dr. Ken Heim (ENSR International) 

Dr. Michael Mills (ENSR International) 

Ms. Betsy Ruffle (ENSR International)