National Center for Environmental Assessment
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`Office of Research and Development
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`Table of Contents
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`EFH
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`Chapter 1 Introduction
`
`
`H
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`Chapter 2 Variability and
`Uncertainty
`
`Chapter 3 Drinking Water
`Intake
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`Chapter 4 Soil Ingestion
`and Pica
`
`Chapter 5 Inhalation
`Route
`
`Chapter 6 Dermal Route
`
`Chapter 7 Body Weight
`Studies
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`I
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`Chapter 8 Lifetime
`
`Chapter 9 Intake of Fruits
`and Vegetables
`
`Chapter 10 Intake of Fish
`and Shellfish
`
`Chapter 11 Intake of
`Meat and Dairy Products
`
`Chapter 12 Intake of
`Grain Products
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`Chapter 14 Breast Milk
`Intake
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`Chapter 16 Consumer
`Products
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`Glossary
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`
`
`I
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`E
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`Chapter 13 Intake Rates
`for Various Home
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`Produced Food Items
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`Chapter 15 Activity
`Factors
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`Chapter 17 Residential
`Building Characteristics
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`About the Handbook
`
`The National Center for Environmental Assessment has prepared this handbook to address
`factors commonly used in exposure assessments. This handbook was first published in 1989
`in response to requests from many EPA Program and Regional offices for additional guidance
`on how to select values for exposure assessments.
`
`This document provides a summary of the available data on consumption of drinking water;
`consumption of fruits, vegetables, beef, dairy products, and fish; soil ingestion; inhalation rates;
`skin surface area; soil adherence; lifetime; activity patterns; body weight; consumer product use;
`and the reference residence.
`
`The handbook is equipped with a number of tools meant to help the user navigate through the
`Exposure Factors Handbook. The following is a description of these tools.
`
`Some of the links that appear throughout the document will transport the user to another
`portion of the handbook. An indication that the user has encountered a hypertext link is that the
`hand in the Adobe Acrobat Reader will change to a hand with a pointing finger or an arrow.
`
`Arrow buttons at the top of the screen are part of the Adobe Acrobat Reader program and will
`allow the user to move through files which have been opened. These arrows include:
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`QEEEEE
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`This button will move the user to the first page of a file.
`
`This button will move the user to the previous page.
`
`This button will move the user to the next page.
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`This button will move the user to the last page of a file.
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`This button will move the user to the last view displayed on the computer monitor.
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`This button will magnify the view on the screen. Push the button, move the mouse to
`the portion of the screen the user wants magnified, and click the left mouse button.
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`The user will need to use the last view button (the double arrow pointing to the left above) to
`maneuver from the tables to the text of the Exposure Factors Handbook. A more convenient
`way of maneuvering between the tables and text is being explored.
`
`At the left of each page in the Exposure Factors Handbook, the user will find a Bookmarks Panel
`containing bookmarks to jump to any other chapter, table, appendix, or figure in the handbook.
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`PREFACE
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`The National Center for Environmental Assessment (NCEA) of EPA’s Office of
`
`Research and Development (ORD) has prepared this handbook to address factors
`
`commonly used in exposure assessments. This handbook was first published in 1989 in
`
`response to requests from many EPA Program and Regional offices for additional
`
`guidance on how to select values for exposure factors.
`
`Several events sparked the efforts to revise the Exposure Factors Handbook. First,
`
`since its publication in 1989, new data have become available. Second, the Risk
`
`Assessment Council issued a memorandum titled, "Guidance on Risk Characterization for
`
`Risk Managers and Risk Assessors," dated February 26, 1992, which emphasized the use
`
`of multiple descriptors of risk (i.e., measures of central tendency such as average or mean,
`
`or
`
`high end), and characterization of individual
`
`risk, population risk,
`
`important
`
`subpopulations. A new document was issued titled "Guidance for Risk Characterization,"
`
`dated February 1995. This document is an update of the guidance issued with the 1992
`
`policy. Third, EPA published the revised Guidelines for Exposure Assessment in 1992.
`
`As part of the efforts to revise the handbook, the EPA Risk Assessment Forum
`
`sponsored a two-day peer involvement workshop which was conducted during the summer
`
`of 1993. The workshop was attended by 57 scientists from academia, consulting firms,
`
`private industry, the States, and other Federal agencies. The purpose of the workshop
`
`was to identify new data sources, to discuss adequacy of the data and the feasibility of
`
`developing statistical distributions and to establish priorities.
`
`As a result of the peer involvement workshop, three new chapters were added to
`
`the handbook. These chapters are: Consumer Product Use, Residential Building
`
`Characteristics, and Intake of Grains. This document also provides a summary of the
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`available data on consumption of drinking water; consumption of fruits, vegetables, beef,
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`dairy products, grain products, and fish; breast milk intake; soil ingestion; inhalation rates;
`
`skin surface area; soil adherence; lifetime; activity patterns; and body weight.
`
`A new draft handbook that incorporated comments from the 1993 workshop was
`
`published for peer review in June 1995. A peer review workshop was held in July 1995
`
`to discuss comments on the draft handbook. A new draft of the handbook that addressed
`
`comments from the 1995 peer review workshop was submitted to the Science Advisory
`
`Board (SAB) for review in August 1996. An SAB workshop meeting was held December
`
`19-20, 1996, to discuss the comments of the SAB reviewers. Comments from the SAB
`
`review have been incorporated into the current handbook.
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`FOREWORD
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`The National Center for Environmental Assessment (NCEA) of EPA's Office of
`
`Research and Development (ORD) has five main functions:
`
`(1) providing risk assessment
`
`research, methods, and guidelines; (2) performing health and ecological assessments;
`
`(3) developing, maintaining, and transferring risk assessment information and training;
`
`(4) helping ORD set research priorities; and (5) developing and maintaining resource
`
`support systems for NCEA. The activities under each of these functions are supported by
`
`and respond to the needs of the various program offices.
`
`In relation to the first function,
`
`NCEA sponsors projects aimed at developing or refining techniques used in exposure
`
`assessments.
`
`This handbook was first published in 1989 to provide statistical data on the various
`
`factors used in assessing exposure. This revised version of the handbook provides the
`
`up-to-date data on these exposure factors. The recommended values are based solely
`
`on our interpretations of the available data.
`
`In many situations different values may be
`
`appropriate to use in consideration of policy, precedent or other factors.
`
`Michael A. Callahan
`
`Director
`
`National Center for Environmental Assessment
`
`Washington Office
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`AUTHORS, CONTRIBUTORS, AND REVIEWERS
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`The National Center for Environmental Assessment (NCEA), Office of Research and
`
`Development was responsible for the preparation of this handbook. The original document
`
`was prepared by Versar Inc. under EPA Contract No. 68-02-4254, Work Assignment No.
`
`189. John Schaum, of NCEA-Washington Office, served as the EPA Work Assignment
`
`Manager, providing overall direction and coordination of the production effort as well as
`
`technical assistance and guidance. Revisions, updates, and additional preparation were
`
`provided by Versar Inc. under Contract Numbers 68-DO-0101, 68-D3-0013, and
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`68-D5-0051. Russell Kinerson and Greg Kew have served as EPA Work Assignment
`
`Managers during previous efforts of the update process. Jackie Moya served as Work
`
`Assignment Manager for the current updated version, providing overall direction, technical
`
`assistance, and serving as contributing author.
`
`AUTHORS
`
`DESKTOP PUBLISHING
`
`GRAPHICS
`
`Patricia Wood
`Linda Phillips
`Aderonke Adenuga
`Mike Koontz
`
`Harry Rector
`Charles Wilkes
`
`Maggie Wilson
`
`Susan Perry
`
`WORD PROCESSING
`
`Kathy Bowles
`Jennifer Baker
`
`CD-ROM PRODUCTION
`
`Valerie Schwartz
`
`Charles Peck
`
`Exposure Assessment Division
`Versar Inc.
`
`Springfield, VA
`
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`CONTRIBUTORS AND REVIEWERS
`
`The following EPA individuals have reviewed and/or have been contributing
`authors of this document.
`
`Michael Dellarco
`
`Robert McGaughy
`Amy Mills
`Jacqueline Moya
`Susan Perlin
`
`Paul Pinsky
`John Schaum
`
`Paul White
`
`Amina Wilkins
`
`Chieh Wu
`
`The following individuals were Science Advisory Board Reviewers:
`
`Members
`
`Dr. Joan Daisey
`Lawrence Berkley Laboratory
`Berkley, California
`
`Dr. Paul Bailey
`Mobil Business Resources
`
`Corporation
`Paulsboro, New Jersey
`
`Dr. Robert Hazen
`
`State of New Jersey Department of
`Environmental Protection and
`
`Energy
`Trenton, New Jersey
`
`Dr. Timothy Larson
`Department of Civil Engineering
`University of Washington
`Seattle, Washington
`
`Dr. Kai-Shen Liu
`
`California Department of Health
`Services
`
`Berkeley, California
`
`Dr. Paul Lioy
`Environmental Occupational Health
`Sciences Institute
`
`Piscataway, New Jersey
`
`Dr. Maria Morandi
`
`University of Texas School of Public
`Health
`
`Houston, Texas
`
`Dr. Jonathan M. Samet
`
`The Johns Hopkins University
`Baltimore, Maryland
`
`Mr. Ron White
`
`American Lung Association
`Washington, D.C.
`
`Dr. Lauren Zeise
`
`California Environmental Protection
`
`Agency
`Berkeley, California
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`Federal Experts
`
`Dr. Richard Ellis
`
`U.S. Department of Agriculture
`Washington, D.C.
`
`Ms. Alanna J. Moshfegh
`U.S. Department of Agriculture
`Washington, D.C.
`
`An earlier draft of this document was peer reviewed by a panel of experts at a peer-review
`workshop held in 1995. Members of the Peer Review Panel were as follows:
`
`Edward Avol
`
`Department of Preventive Medicine
`School of Medicine
`
`Patricia Guenther
`
`Beltsville Human Nutrition
`
`Research Center
`
`University of Southern California
`
`U.S. Department of Agriculture
`
`James Axley
`School of Architecture
`
`Yale University
`
`David Burmaster
`
`Alceon Corporation
`
`Steven Colome
`
`Integrated Environmental Services
`
`Michael DiNovi
`
`Chemistry Review Branch
`U.S. Food & Drug Administration
`
`Dennis Druck
`
`Environmental Scientist
`
`Center of Health Promotion &
`
`Preventive Medicine
`
`U.S. Army
`
`J. Mark Fly
`Department of Forestry, Wildlife, &
`Fisheries
`
`University of Tennessee
`
`Larry Gephart
`Exxon Biomedical Sciences, Inc.
`
`P.J. (Bert) Hakkinen
`Paper Product Development &
`Paper
`Technology Divisions
`The Proctor & Gamble Company
`
`Mary Hama
`Beltsville Human Nutrition
`
`Research Center
`
`U.S. Department of Agriculture
`
`Dennis Jones
`
`Agency for Toxic Substances &
`Disease Registry
`
`John Kissel
`
`Department of Environmental
`Health
`
`School of Public Health &
`
`Community Medicine
`
`Neil Klepeis
`Information Systems & Services,
`Inc.
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`Andrew Persily
`National Institute of Standards &
`
`Technologies
`
`Barbara Petersen
`
`Technical Assessment Systems,
`Inc.
`
`Thomas Phillips
`Research Division
`
`California Air Resources Board
`
`Paul Price
`
`ChemRisk
`
`John Risher
`
`Division of Toxicology
`The Agency for Toxic Substances &
`Disease Registry
`
`John Robinson
`
`University of Maryland
`
`Peter Robinson
`
`The Proctor & Gamble Company
`
`P. Barry Ryan
`Department of Environmental &
`Occupational Health
`Rollins School of Public Health
`
`Emory University
`
`Val Schaeffer
`
`U.S. Consumer Product Safety
`Commission
`
`Brad Shurdut
`
`DowElanco
`
`John Talbott
`
`U.S. Department of Energy
`
`Frances Vecchio
`
`Beltsville Human Nutrition
`
`Research Center
`
`U.S. Department of Agriculture
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`The following individuals within EPA have reviewed an earlier draft of this document
`and provided valuable comments:
`
`OFFICE
`
`REVIEWERS/CONTRIBUTORS
`
`Office of Research and Development Maurice Berry
`Jerry Blancato
`Elizabeth Bryan
`Curtis Dary
`Stan Durkee
`
`Manuel Gomez
`
`Wayne Marchant
`Sue Perlin
`
`James Quanckenboss
`
`Glen Rice
`
`Lance Wallace
`
`Office of Emergency and Remedial
`Response
`
`Jim Konz
`
`Office of Pollution, Pesticides and
`Toxic Substances
`
`Pat Kennedy
`Cathy Fehrenbacker
`
`Office of Water
`
`Office of Air Quality Planning and
`Standards
`
`EPA Regions
`
`Denis Borum
`
`Helen Jacobs
`
`Warren Peters
`
`Steve Ehlers - Reg. VI
`Maria Martinez - Reg. VI
`Mike Morton - Reg. VI
`Jeffrey Yurk - Reg. VI
`Youngmoo Kim - Reg. VI
`
`the National Exposure Research Laboratory (NERL) of the Office of
`In addition,
`Research and Development of EPA made an important contribution to this handbook by
`conducting additional analyses of the National Human Activity Pattern Survey (NHAPS)
`data. EPA input to the NHAPS data analysis came from Karen A. Hammerstrom and
`Jacqueline Moya from NCEA-Washington Office; William C. Nelson from NERL-RTP, and
`Stephen C. Hern, Joseph V. Behar (retired), and William H. Englemann from NERL-Las
`Vegas.
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`The EPA Office of Water made an important contribution by conducting an analysis of
`the USDA Continuing Survey of Food Intakes by Individual (CSFII) data. They provided
`fish intake rates for the general population. The analysis was conducted under the
`direction of Helen Jacobs from the Office of Water.
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`APPENDIX 1A
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`RISK CALCULATIONS USING EXPOSURE FACTORS HANDBOOK DATA
`
`AND DOSE-RESPONSE INFORMATION FROM THE
`
`INTEGRATED RISK INFORMATION SYSTEM (IRIS)
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`RISK CALCULATIONS USING EXPOSURE FACTORS HANDBOOK
`
`DATA AND DOSE-RESPONSE INFORMATION FROM IRIS
`
`APPENDIX 1A
`
`1.
`
`INTRODUCTION
`
`When calculating risk estimates for a specific population, whether the entire national
`population or some sub-population, the exposure information (either from this handbook
`or from other data) must be combined with dose-response information. The latter typically
`comes from the IRIS data base, which summarizes toxicity data for each agent separately.
`Care must be taken that the assumptions about population parameters in the dose-
`response analysis are consistent with the population parameters used in the exposure
`analysis. This Appendix discusses procedures for insuring this consistency.
`
`In the IRIS derivation of threshold based dose-response relationships (U.S. EPA,
`1996), such as the RfD and the RfCs based on adverse systemic effects, there has
`generally been no explicit use of human exposure factors.
`In these cases the numerical
`value of the RfD and RfC comes directly from animal dosing experiments (and occasionally
`from human studies) and from the application of uncertainty factors to reflect issues such
`as the duration of the experiment, the fact that animals are being used to represent
`humans and the quality of the study. However in developing cancer dose-response (D-R)
`assessments, a standard exposure scenario is assumed in calculating the slope factor
`(i.e., human cancer risk per unit dose) on the basis of either animal bioassay data or
`human data. This standard scenario has traditionally been assumed to be typical of the
`U.S. population: 1) body weight = 70 kg; 2) air intake rate = 20 m3/day; 3) drinking water
`intake = 2 liters/day; 4) lifetime = 70 years.
`In RfC derivations for cases involving an
`adverse effect on the respiratory tract, the air intake rate of 20 m3/day is assumed. The
`use of these specific values has depended on whether the slope factor was derived from
`animal or human epidemiologic data:
`
`- Animal Data: For dose-resopnse (D-R) studies based on animal data, scale
`animal doses to human equivalent doses using a human body weight assumption
`of 70 kg. No explicit lifetime adjustment is necessary because the assumption is
`made that events occurring in the lifetime animal bioassay will occur with equal
`probability in a human lifetime, whatever that might happen to be.
`
`- Human Data - In the analysis of human studies (either occupational or general
`population), the Agency has usually made no explicit assumption of body weight
`or human lifetime. For both of these parameters there is an implicit assumption
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`that the population usually of interest has the same descriptive parameters as the
`population analyzed by the Agency.
`In the rare situation where this assumption
`is known to be wrong, the Agency has made appropriate corrections so that the
`dose-response parameters represent the national average population.
`
`When the population of interest is different than the national average (standard)
`population, the dose-response parameter needs to be adjusted.
`In addition, when the
`population of interest is different than the population from which the exposure factors in
`this handbook were derived, the exposure factor needs to be adjusted. Two generic
`examples of situations where these adjustments are needed are as follows:
`
`A) Detailed study of recent data, such as are presented in this handbook, show that
`EPA’s standard assumptions (i.e., 70 kg body weight, 20 m3/day air inhaled, and 2 L/day
`water intake) are inaccurate for the national population and may be inappropriate for sub-
`populations under consideration. The handbook addresses most of these situations by
`providing gender- and age-specific values and by normalizing the intake values to body
`weight when the data are available, but it may not have covered all possible situations.
`An example of a sub-population with a different mean body weight would be females, with
`an average body weight of 60 kg or children with a body weight dependent on age.
`Another example of a non-standard sub-population would be a sedentary hospital
`population with lower than 20 m3/day air intake rates.
`
`B) The population variability of these parameters is of interest and it is desired to
`estimate percentile limits of the population variation. Although the detailed methods for
`estimating percentile limits of exposure and risk in a population are beyond the scope of
`this document, one would treat the body weight and the intake rates discussed in Sections
`2 to 4 of this appendix as distributions, rather than constants.
`
`2.
`
`CORRECTIONS FOR DOSE-RESPONSE PARAMETERS
`
`The correction factors for the dose-response values tabulated in the IRIS data base
`for carcinogens are summarized in Table 1A-1. Use of these correction parameters is
`necessary to avoid introducing errors into the risk analysis. The second column of Table
`1A-1 shows the dependencies that have been assumed in the typical situation where the
`human dose-response factors have been derived from the administered dose in animal
`studies. This table is applicable in most cases that will be encountered, but it is not
`applicable when: a) the effective dose has been derived with a pharmacokinetic model and
`b) the dose-response data has been derived from human data.
`In the former case, the
`subpopulation parameters need to be incorporated into the model.
`In the latter case, the
`correction factor for the dose-response parameter must be evaluated on a case-by case
`basis by examining the specific data and assumptions in the derivation of the parameter.
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`As one example of the use of Table 1A-1, the recommended value for the average
`consumption of tapwater for adults in the U. S. population derived in this document
`(Chapter 3), is 1.4 liters per day. The drinking water unit risk for dichlorvos, as given in
`the IRIS information data base is 8.3 x 10'‘‘ per pg/I, and was calculated from the slope
`factor assuming the standard intake,
`IWS, of 2 liters per day. For the United States
`population drinking 1.4 liters of tap water per day the corrected drinking water unit risk
`should be 8.3 x 10"‘ x (1 .4/2) = 5.8 x 10"‘ per ug/I. The risk to the average individual is
`then estimated by multiplying this by the average concentration in units of ug/I.
`
`Another example is when the risk for women drinking water contaminated with
`dichlorvos is to be estimated.
`If the women have an average body weight of 60 kg, the
`correction factor for the drinking water unit risk is (disregarding the correction discussed
`in the above paragraph), from Table 1A-1, is (70/60)2’3 = 1.11. Here the ratio of 70 to 60
`is raised to the power of 2/3. The corrected water unit risk for dichlorvos is 8.3 x 10'6 x
`1.11 = 9.2 x 10'5 per ug/I. As before, the risk to the average individual is estimated by
`multiplying this by the water concentration.
`
`When human data are used to derive the risk measure, there is a large variation in
`the different data sets encountered in IRIS, so no generalizations can be made about
`global corrections. However, the typical default exposure values used for the air intake
`of an air pollutant over an occupational lifetime are: air intake is 10 m3/day for an 8-hour
`shift, 240 days per year with 40 years on the job.
`If there is continuous exposure to an
`ambient air pollutant, the lifetime dose is usually calculated assuming a 70-year lifetime.
`
`3.
`
`CORRECTIONS FOR INTAKE DATA
`
`When the body weight, WP, of the population of interest differs from the body weight,
`WE, of the population from which the exposure values in this handbook were derived, the
`following model furnishes a reasonable basis for estimating the intake of food and air (and
`probably water also) in the population of interest. Such a model is needed in the absence
`of data on the dependency of intake on body size. This occurs for inhalation data, where
`the intake data are not normalized to body weight, whereas the model is not needed for
`food and tap water intakes if they are given in units of intake per kg body weight.
`
`The model is based on the dependency of metabolic oxygen consumption on body
`size. Oxygen consumption is directly related to food (calorie) consumption and air intake
`and indirectly to water intake. For mammals of a wide range of species sizes (Prosser and
`Brown, 1961), and also for individuals of various sizes within a species, the oxygen
`consumption and calorie (food) intake varies as the body weight raised to a power between
`0.65 and 0.75. A value of 0.667 = 2/3 has been used in EPA as the default value for
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`adjusting cross-species intakes, and the same factor has been used for intra-species
`intake adjustments.
`
`[NOTE: Following discussions by an interagency task force (Federal Register, 1992),
`the agreement was that a more accurate and defensible default value would be to choose
`the power to 3/4 rather than 2/3. A recent article (West et al., 1997) has provided a
`theoretical basis for the 3/4 power scaling. This will be the standard value to be used in
`future assessments, and all equations in this Appendix will be modified in future risk
`assessments. However, because risk assessors now use the current IRIS information,
`this discussion is presented with the previous default assumption of 2/3].
`
`With this model, the relation between the daily air intake in the population of interest,
`IAP = (m3/day)P, and the intake in the population described in this handbook, IAE = (m3/day)E
`is:
`
`IAP = If x (wP/wE)2’3.
`
`4.
`
`CALCULATION OF RISKS FOR AIR CONTAMINANTS
`
`The risk is calculated by multiplying the IRIS air unit risk, corrected as described in
`Table 1A-1, by the air concentration. But since the correction factor involves the intake
`in the population of interest (IAP), that quantity must be included in the equation, as follows:
`
`(Risk)P= (air unit risk)P x (air concentration)
`= (air unit risk)‘°’ x (IAP/20) x (70/WP)” x (air concentration)
`= (air unit risk)‘°’ x [( IAE x (WP/WE)”/20)] x (70/WP)” x (air concentration)
`= (air unit risk)‘°’ x (IAE/20) x (70/WE)” x (air concentration)
`
`In this equation the air unit risk from the IRIS data base (air unit risk)S, the air intake
`data in the handbook for the populations where it is available (IAE) and the body weight of
`that population ONE) are included along with the standard IRIS values of the air intake (20
`m3/day) and body weight (70 kg).
`
`For food ingestion and tap water intake, if body weight-normalized intake values from
`this handbook are used, the intake data do not have to be corrected as in Section 3 above.
`In these cases, corrections to the dose-response parameters in Table 1A-1 are sufficient.
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`5.
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`REFERENCES
`
`Federal Register. (1992) Cross-species scaling factor for carcinogen risk assessments
`based on equivalence of (mg/kg-day)3’4. Draft report. Federal Register, 57(109):
`24152-24173, June 5, 1992.
`Prosser, C.L.; Brown, F.A.
`(1961) Comparative Animal physiology, 2nd edition. WB
`Saunders Co. p. 161.
`Integrated Risk Information System
`U.S. EPA.
`(1996) Background Documentation.
`(IRIS). Online. National Center for Environmental Assessment, Cincinnati, Ohio.
`Background Documentation available from: Risk Information Hotline, National Center
`for Environmental Assessment, U.S. EPA, 26 W. Martin Luther King Dr. Cincinnati,
`OH 45268. (513) 569-7254
`West, G.B.; Brown, J.H.; Enquist, B.J. (1997) A general model of the origin of allometric
`scaling laws in biology. Science 276:122-126.
`
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`2.
`
`VARIABILITY AND UNCERTAINTY
`
`The chapters that follow will discuss exposure factors and algorithms for estimating
`exposure. Exposure factor values can be used to obtain a range of exposure estimates
`such as average, high-end and bounding estimates.
`It is instructive here to return to the
`general equation for potential Average Daily Dose (ADDp,,,) that was introduced in the
`opening chapter of this handbook:
`
`_ Contaminant Concentration x Intake Rate x Exposure Duration
`ADDp0l ‘ (Eqn- 2-1)
`
`With the exception of the contaminant concentration, all parameters in the above
`equation are considered exposure factors and, thus, are treated in fair detail in other
`chapters of this handbook. Each of the exposure factors involves humans, either in terms
`of their characteristics (e.g., body weight) or behaviors (e.g., amount of time spent in a
`specific location, which affects exposure duration). While the topics of variability and
`uncertainty apply equally to contaminant concentrations and the rest of the exposure
`factors in equation 2-1, the focus of this chapter is on variability and uncertainty as they
`relate to exposure factors. Consequently, examples provided in this chapter relate
`primarily to exposure factors, although contaminant concentrations may be used when they
`better illustrate the point under discussion.
`
`This chapter also is intended to acquaint the exposure assessor with some of the
`fundamental concepts and precepts related to variability and uncertainty, together with
`methods and considerations for evaluating and presenting the uncertainty associated with
`exposure estimates. Subsequent sections in this chapter are devoted to the following
`topics:
`
`- Distinction between variability and
`uncertainty;
`- Types of variability;
`- Methods of confronting variability;
`- Types of uncertainty and reducing uncertainty;
`- Analysis of variability and uncertainty; and
`- Presenting results of variability/uncertainty analysis.
`
`Fairly extensive treatises on the topic of uncertainty have been provided, for example,
`by Morgan and Henrion (1990), the National Research Council (NRC, 1994) and, to a
`lesser extent, the U.S. EPA (1992; 1995). The topic commonly has been treated as it
`relates to the overall process of conducting risk assessments; because exposure
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`assessment is a component of risk-assessment process, the general concepts apply
`equally to the exposure-assessment component.
`
`2.1. VARIABILITY VERSUS UNCERTAINTY
`
`While some authors have treated variability as a specific type or component of
`uncertainty, the U.S. EPA (1995) has advised the risk assessor (and, by analogy, the
`exposure assessor) to distinguish between variability and uncertainty. Uncertainty
`represents a lack of knowledge about factors affecting exposure or risk, whereas variability
`arises from true heterogeneity across people, places or time.
`In other words, uncertainty
`can lead to inaccurate or biased estimates, whereas variability can affect the precision of
`the estimates and the degree to which they can be generalized. Most of the data
`presented in this handbook concerns variability.
`
`Variability and uncertainty can complement or confound one another. An instructive
`analogy has been drawn by the National Research Council (NRC, 1994: Chapter 10),
`based on the objective of estimating the distance between the earth and the moon. Prior
`to fairly recent technology developments, it was difficult to make accurate measurements
`of this distance, resulting in measurement uncertainty. Because the moon's orbit is
`elliptical, the distance is a variable quantity.
`If only a few measurements were to be taken
`without knowledge of the elliptical pattern, then either of the following incorrect conclusions
`might be reached:
`
`- That the measurements were faulty, thereby ascribing to uncertainty what was
`actually caused by variability; or
`- That the moon's orbit was random, thereby not allowing uncertainty to shed light
`on seemingly unexplainable differences that are in fact variable and predictable.
`
`A more fundamental error in the above situation would be to incorrectly estimate the
`true distance, by assuming that a few observations were sufficient. This latter pitfall --
`treating a highly variable quantity as if it were invariant or only uncertain -- is probably the
`most relevant to the exposure or risk assessor.
`
`Now consider a situation that relates to exposure, such as estimating the average
`daily dose by one exposure route -- ingestion of contaminated drinking water. Suppose
`that it is possible to measure an individual's daily water consumption (and concentration
`of the contaminant) exactly, thereby eliminating uncertainty in the measured daily dose.
`The daily dose still has an inherent day-to-day variability, however, due to changes in the
`individual's daily water intake or the contaminant concentration in water.
`
`It is impractical to measure the individual's dose every day. For this reason, the
`exposure assessor may estimate the average daily dose (ADD) based on a finite number
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`of measurements, in an attempt to "average out" the day-to-day variability. The individual
`has a true (but unknown) ADD, which has now been estimated based on a sample of
`measurements. Because the individual's true average is unknown, it is uncertain how
`close the estimate is to the true value. Thus, the variability across daily doses has been
`translated into uncertainty in the ADD. Although the individual's true ADD has no
`variability, the estimate of the ADD has some uncertainty.
`
`The above discussion pertains to the ADD for one person. Now consider a
`distribution of ADDs across individuals in a defined population (e.g., the general U.S.
`population).
`In this case, variability refers to the range and distribution of ADDs across
`individuals in the population. By comparison, uncertainty refers to the exposure asse