EPA/100/R11/0001

Final 

Recommended Use of Body Weight3/4

as the Default Method in Derivation of the

Oral Reference Dose 

Office of the Science Advisor

Risk Assessment Forum

U.S. Environmental Protection Agency

Washington, DC 20460
DISCLAIMER

This document has been reviewed in accordance with U.S. Environmental Protection 

Agency policy. Mention of trade names or commercial products does not constitute endorsement 

or recommendation for use.
Table of Contents

LIST OF TABLES ........................................................................................................... V

LIST OF FIGURES ......................................................................................................... V

PREFACE...................................................................................................................... VII

EXECUTIVE SUMMARY ............................................................................................IX

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

2 BACKGROUND ......................................................................................................... 2

3 TOXICOLOGICALLY EQUIVALENT DOSES IN THE RFC AND RFD......... 5

3.1 Estimating toxicologically equivalent Doses Extrapolation and Dosimetry in 

the RfC......... ................................................................................................... 5

3.2 Extrapolation in the RfD Derivation Process...................................................... 7

4 BW3/4 SCALING FOR DERIVING TOXICOLOGICALLY EQUIVALENT 

DOSES IN THE RFD ................................................................................................. 8

4.1 Scaling in Cross-Species Extrapolation.............................................................. 8

4.2 Interspecies BW3/4 Scaling and Life Processes................................................... 8

4.3 Interspecies BW3/4 Scaling And Toxicity Processes........................................... 9

5 CONSIDERATIONS ON USING BW3/4 SCALING AS A DEFAULT FOR 

ESTIMATING TOXICOLOGICALLY EQUIVALENT DOSES....................... 11

5.1 Metabolism and Clearance................................................................................ 11

5.2 Measure Of Delivered Dose: Choice Of The Appropriate Dose Metric ......... 11

5.3 Early Lifestages ................................................................................................ 12

5.4 Toxicokinetics and Toxicodynamics in Toxicological Equivalence ................ 14

5.5 Acute Scenarios ................................................................................................ 15

5.6 Portal-of-entry Issues for Oral Exposure .......................................................... 16

5.7 Summary of Limitations in BW3/4 Scaling ....................................................... 16

6 CONCLUSIONS ....................................................................................................... 18

6.1 Default Procedure in the Hierarchy of Approaches.......................................... 18

6.2 Optimal and Intermediate Approaches ............................................................. 20

6.3 The HED And Interspecies Uncertainty Factor, UFA....................................... 20
6.4 Summary........................................................................................................... 22

REFERENCES................................................................................................................ 23

APPENDIX A. BW3/4 SCALING VERSUS BW1/1 – A COMPARISON................... 28

APPENDIX B. THE INTERSPECIES BW3/4 ORAL DOSIMETRIC 

ADJUSTMENT FACTOR (DAF) – BWA

1/4 / BWH1/4

............................................ 30

APPENDIX C. THE ORAL DAF AND PORTAL-OF-ENTRY 

CONSIDERATIONS ................................................................................................ 32

APPENDIX D. HIERARCHY OF APPROACHES FOR INHALATION 

DOSIMETRY AND INTERSPECIES EXTRAPOLATION ............................... 35

GLOSSARY..................................................................................................................... 36
v

LIST OF TABLES

Table 4-1 Cross-species body weight scaling for various metabolic and physiological 

functions

Table 6-1 Hierarchical framework of approaches for interspecies extrapolation

Table A-1 Comparison of BW1/1 and BW3/4 in estimating oral exposure in humans from a 10 

mg/kg exposure to rats, mice, and a dog. 

Table B-1 The animal:human BW1/4 ratio as the Dosimetric Adjustment Factor (DAF) used 

in deriving an HED from an oral animal exposure

LIST OF FIGURES

Figure 3-1 Schematic of procedures in the RfC (a.) and previous RfD (b.) processes

Figure 6-1 Procedures for (a.) the current RfD and (b.) recommended RfD processes
vi

LIST OF ABBREVIATIONS AND ACRONYMS

AUC area under the curve

BMD benchmark dose 

BW body weight

Cmax maximum tissue concentration

DAF dosimetric adjustment factor

HEC human equivalent concentration

HED human equivalent dose

HEE human equivalent exposure

LOAEL lowest-observed-adverse-effect-level

MOA mode of action

NOAEL no-observed-adverse-effect -level

NRC National Research Council

PBTK physiologically-based toxicokinetic

POD point of departure

RDDR regional deposited dose ratio

RfC reference concentration

RfD reference dose

RGDR regional gas dose ratio

TD toxicodynamic

TK toxicokinetic

UFA uncertainty factor (animal to human extrapolation)

USEPA U.S. Environmental Protection Agency
vii

PREFACE

This document is intended to be used in combination with other risk assessment tools, 

guidance, and guidelines. U.S. Environmental Protection Agency (EPA or the Agency) risk 

assessments may be conducted differently than envisioned in this document for many reasons 

including, for example, new information, new scientific understanding, or different science 

policy judgment. The practice of assessing interspecies differences from exposure to toxicants 

continues to develop, and specific components of this guidance may become outdated or may 

otherwise require modification in individual settings. It is EPA’s intent to use, to the extent 

practicable and consistent with Agency statutes and regulations, the best available science in its 

risk assessments and regulatory actions, and this guidance is not intended to provide any 

substantive or procedural obstacle in achieving that goal. Therefore, this guidance has no binding 

effect on EPA or on any regulated entity. Where EPA does use this guidance in developing 

exposure and risk assessments, it will be because EPA has decided, in the context of that 

assessment, that the approaches from this guidance are suitable and appropriate. This judgment 

will be tested through peer review, and the risk assessment will be modified to use different 

approaches, if appropriate.

This guidance does not establish any substantive ―rules‖ under the Administrative 

Procedure Act or any other law and has no binding effect on EPA or any regulated entity, but 

instead represents a non-binding statement of policy.
viii

Risk Assessment Forum Technical Panel

Authors:

Gary L. Foureman, Office of Research and Development (Retired)

Elaina M. Kenyon, Office of Research and Development

Risk Assessment Forum Technical Panel:

Vicki L. Dellarco, Office of Pesticide Programs, Co-Chair

Deirdre L. Murphy, Office of Air and Radiation, Co-Chair 

Brenda P. Foos, Office of Children’s Health Protection

Gary L. Foureman, Office of Research and Development (Retired)

Elaina M. Kenyon, Office of Research and Development

Gregory G. Miller, Office of Policy, Economics, and Innovation

Contributors:

Hugh A. Barton, Office of Research and Development

Annie M. Jarabek, Office of Research and Development

Risk Assessment Forum Staff:

Michael W. Broder

Kathryn Gallagher

Resha M. Putzrath

William P. Wood (retired)

Elizabeth Lee Hofmann

Gary Bangs (retired)
ix

EXECUTIVE SUMMARY

This document is a product of efforts by the Agency to develop and promote biologically 

based and harmonized approaches for all toxicological endpoints used in human health risk 

assessments. EPA risk assessors routinely use body weight (BW) as the basis for scaling toxicity 

data from animal models for human health risk assessments. However, the Agency has used 

BW1/1 for non-cancer endpoints and, at various times BW2/3 or BW3/4 for cancer endpoints to 

normalize dose across species. This document promotes the use of BW3/4 as a default method to 

convert data between species for both categories of endpoints. A hierarchy of methods for 

interspecies scaling is presented along with the rationale for selection of the scaling factor and

guidance on how to conduct the conversion.

EPA endorses a hierarchy of approaches to derive human equivalent oral exposures from 

data from laboratory animal species, with the preferred approach being physiologically-based

toxicokinetic modeling. Other approaches may include using chemical-specific information, 

without a complete physiologically-based toxicokinetic model. In lieu of data to support either 

of these types of approaches, body weight scaling to the 3⁄4 power (i.e., BW3/4) is endorsed as a 

general default procedure to extrapolate toxicologically equivalent doses of orally administered 

agents from all laboratory animals to humans for the purposes of deriving an oral Reference 

Dose (RfD). Use of BW3/4scaling in combination with a reduced default interspecies 

uncertainty factor, UFA , is recommended as the Agency default approach to replace the previous 

default approach for this purpose which involved BW1/1 scaling with a full uncertainty factor

(i.e., a UFA value of 10). Use of BW3/4 in derivation of RfD values is consistent with its current 

Agency use in derivation of oral cancer slope factors. Thus, this default scaling procedure is a 

point of harmonization between the two main Agency oral dose-response procedures. 

While recognizing that, as supported by the available information for a given substance, 

priority be given to more data-reliant approaches to interspecies extrapolation, the scope of this 

document is limited to discussion of the generic default procedure that is viewed as an informed,

species-specific, dosimetric adjustment factor (DAF) addressing predominately toxicokinetic and 

some toxicodynamic aspects of the interspecies uncertainty factor, UFA. The DAF is multiplied 

by the animal exposure (in mg/kg-day) to achieve the human equivalent exposure (in mg/kgday). A detailed derivation of the DAF described in this document results in the following: 

DAF = (BWa / BWh)

1/4

, 

where the subscripts ―a‖ indicates animal, ―h‖ indicates human, and the 1⁄4 exponent results from 

the application of BW3/4 scaling to exposure in units of mg/kg-day (rather than mg/day) such that
x

BW3/4 / BW1/1 = BW-1/4 

.

This procedure results in derivation of a human equivalent exposure, specifically a 

human equivalent dose (HED), that is an estimate of the animal exposure of interest translated to 

a biologically-motivated common scale for use in derivation of the RfD in a manner parallel to 

the human equivalent concentration (HEC) in derivation of a Reference Concentration (RfC). A 

reduced interspecies UFA (with a default value of 3) is then applied to the HED as part of the 

RfD calculation. The quantitative significance of this procedure with regard to the magnitude of

a RfD will depend on the body weight of the species (as well as the value assigned to the UFA)

and may be more or less than the current procedure of dividing by the default composite UFA of 

10. The Appendix A (Table A1) contains example calculations of numeric impact on RfDs for 

different body weight of the animals.

BW3/4 scaling for derivation of the HED is recommended as the default approach for 

RfDs for remote, as well as portal-of-entry effects. It is noted that this scaling is not inclusive of 

lethal or frank effects for which maximum concentration (Cmax) may be the most appropriate 

dose metric and that such effects are not among those effects recommended for use in deriving 

RfDs (USEPA, 2002). This default approach generally applies to different durations of 

exposure. The basis for the overarching assumption is outlined in Sections 3 and 4. The 

limitations inherent in the application of this approach are related to acute durations of exposure, 

early life stages, and clearly frank effects such as lethality. The limitations are detailed in 

Section 5.

The reader is encouraged to read the document carefully in order to fully understand how 

to apply the procedure appropriately. Additionally, although non-oral RfDs can be estimated 

(e.g., a dermal RfD), this document focuses only on oral RfDs and for this document the 

acronym will refer only to RfDs for oral exposure. The overarching assumption in this default 

approach is that measurable characteristics of anatomy and physiology scale as a function of 

BW3/4.

It is recognized that this procedure, as all default procedures, may not always predict oral 

exposures associated with precise toxicologically-equivalent doses for specific chemicals. It 

should be emphasized that other biological information not discussed in this document may

inform interspecies adjustments. As a general default procedure, however, it may be anticipated 

to provide a reasonable description of average behavior of many chemicals much of the time. As 

with the HEC, appropriate chemical-specific data and information would supersede or modify 

this default procedure for the HED, with the optimal approach being use of a physiologicallybased toxicokinetic (PBTK) or other biologically-based model. Thus the recommended 

hierarchy of approaches is as follows:
xi

• Preferred approach: PBTK modeling

• Intermediate approaches: use of chemical-specific data 

• Default approach: use of BW3/4 scaling and reduction of uncertainty factor 

default value from 10 to 3
1

1 INTRODUCTION

In using animal data in carcinogen risk assessment, the U.S. Environmental Protection 

Agency (EPA or the Agency) has been scaling oral exposures from animals to humans on the 

basis of equivalence of mg/kg

3/4

-d (milligrams of the agent normalized by the 3/4 power of body 

weight per day) since the early 1990s (USEPA, 1992a). As part of more recent Agency efforts to 

harmonize human health risk assessment for cancer with that for other toxicological endpoints, 

this document explores the use of allometric scaling using the 3/4 power of body weight (BW3/4) 

as the default approach for oral exposures associated with noncancer endpoints. Allometric 

scaling is scaling of physiological rates or quantities to relative growth and size (mass or 

volume) of one animal species relative to another animal species. The specific purpose of this 

document and the associated technical report (Rhomberg and Lewandowski, 2004, 2006) is to 

assess such a broadened application of allometric scaling using the BW3/4 as the Agency default 

approach in light of current scientific knowledge. Aspects of such application across 

toxicological endpoints, such as different exposure conditions, toxicokinetics, and different lifestages, have also been considered to the extent allowed by the current information.

This document is a product of efforts by the Agency to develop and promote biologicallybased and harmonized approaches for all toxicological endpoints used in human health risk 

assessment. Specifically, this document describes approaches, inclusive of the use of BW3/4

scaling, to estimate oral exposures in terms of human equivalents for all toxicological endpoints 

for the purpose of deriving an oral Reference Dose (RfD)

1

. In doing so, this work is intended to 

follow and be concordant with the mode of action (MOA) as the guiding paradigm for 

toxicological evaluations, both cancer and noncancer (USEPA, 1994; 2002; 2005). That the dose 

at the target site, the internal dose, ―is the ultimate determinant of risk‖ (NRC, 1994), is a 

fundamental generalization of this paradigm. Use of BW3/4 combined with consideration of a 

reduced interspecies uncertainty factor is recommended here as the default approach in deriving 

RfDs from animal data in replacement of the previous Agency default approach which involved 

BW1/1 scaling with a full uncertainty factor. The procedure would by default (in lieu of other 

information) apply to any and all laboratory species, although in practice, the rat is a

predominant species, with mice, rabbits, and dogs also represented.

This document is intended to be consulted for future EPA risk assessments. The decision 

to apply this approach retroactively is left to the discretion of the Agency’s programs.

 

1

Although non-oral RfDs can be estimated (e.g., a dermal RfD), this document focuses only on oral RfDs

and, for this document, the acronym will refer only to RfDs for oral exposure. 
2

2 BACKGROUND

Human risk assessments are often based on toxicity data from laboratory animal species,

thereby necessitating several extrapolations for estimating the exposure conditions for which a 

similar toxicity is projected. A critical step is relating the exposure-dose-response relationships 

for laboratory animals to those pertaining to humans, that is, the need to adjust the exposure used 

in an animal study to a human equivalent exposure (HEE). The most scientifically sound 

approach by which this may be accomplished is through the use of chemical- and speciesspecific toxicokinetic and toxicodynamic information to estimate the internal dose at the target 

tissue(s). Note that this approach is based on and concordant with the mode-of-action paradigm 

mentioned above. In most cases, however, there are insufficient toxicokinetic and/or 

toxicodynamic data available to compare internal dose between different species. In these cases, 

science-based intermediate and default approaches are needed to derive the estimate of human 

equivalent dose or concentration. 

The methods used by the Agency to extrapolate from animal experimental exposures or 

doses to human equivalent estimates have evolved since the assessments of the early 1980s. 

There have been differences in that evolution for oral versus inhalation exposures, as well as for 

cancer versus non-cancer assessment approaches. The Agency’s cancer risk guidelines have and 

continue to endorse the application of scaling procedures, either on body surface area using 

(BW2/3) or on body weight (BW), for purposes of default interspecies extrapolation in calculation 

of an HEE. This document recognizes and uses the term HEE in referring collectively to human 

exposures

2

via any route. When exposures are via the oral route, the more specific term Human 

Equivalent Dose (HED) may be applied; when via the inhalation route, the term Human 

Equivalent Concentration (HEC) may be applied.

In EPA cancer assessments for the oral route prior to 1992, extrapolation from laboratory 

animal exposures to a human cancer risk estimate were typically adjusted based on surface area 

scaling (derived using BW2/3 as in Anderson et al., 1983; USEPA, 1980; 1986; 1992a). In cancer 

assessments for the inhalation route prior to development of the Agency’s inhalation dosimetry 

methodology (USEPA, 1989; 1994), interspecies extrapolation from the animal exposure 

concentration differed depending on judgment regarding the chemical’s absorption following 

 

2 The term ―exposure‖, as used here, is roughly analogous here to the terms ―administered‖ or ―potential 

dose‖ as used in the Guidelines for Exposure Assessment (USEPA, 1992b).
3

inhalation. For chemicals judged to be completely absorbed upon inhalation, surface area 

scaling (BW2/3) was employed in the extrapolation, e.g., dichloromethane (USEPA, 1985).3

The interspecies extrapolation practice for cancer assessments was recommended in 

19924 when the EPA, in collaboration with the Food and Drug Administration and the Consumer 

Product Safety Commission, proposed the use of a cross-species scaling factor for administered 

dose based on equivalence of body weight to the 3⁄4 power per day (BW3/4/day) (USEPA, 1992a). 

While acknowledging that empirical data on comparative carcinogen potency was compatible 

with scaling either by body weight or surface area (BW2/3/day), the document proposed use of 

BW3/4 because of its underlying basis in established allometric interspecies variation in anatomy 

and physiology, much of which is explored and documented further in this policy paper and 

accompanying appendices. Similarly, the Agency’s 2005 cancer risk guidelines (USEPA, 2005) 

endorse the application of BW3/4 for purposes of interspecies extrapolation when chemicalspecific data are absent.

The EPA (1992a) action provided a source of standardization for predictive cancer risk 

assessment via the oral route. However, the procedures employed for derivation of the RfD, the 

Agency’s traditional dose-response method for noncancer effects from a toxic agent via the oral 

route were, and remained, quantitatively different. In the derivation of an RfD, the dose 

administered orally is expressed in mg/kg-day and is considered directly proportional across 

species on a body weight basis, (i.e., BW1/1 versus BW3/4). An uncertainty factor, UFA, is 

subsequently applied to the laboratory animal exposure for consideration for interspecies 

extrapolation. 

Since 1989, extrapolation from laboratory animal inhalation exposure concentrations to 

HECs has been performed as per Interim Methods for Development of Inhalation Reference 

Doses (USEPA, 1989) and Methods for Derivation of Inhalation Concentrations and Application 

of Inhalation Dosimetry (USEPA, 1994). The methods described in these documents give

preference to the use of toxicokinetic modeling for extrapolation, but also recognize the use of 

intermediate and default options for exposure concentration adjustment approaches, based on 

consideration of physicochemical characteristics of a given agent as key determinants of 

interaction with the respiratory tract and ultimate disposition of the agent in the body. In these 

procedures, which may be considered as inhalation dosimetry, particles and gases are treated 

separately, and the type of toxicity observed, respiratory tract (i.e., portal of entry) or toxicity 

remote to the portal-of-entry tissues, determines the adjustment procedure applied. Since its 

 

3

Subsequent toxicokinetic analysis with dichloromethane (USEPA, 1987; 1989) informed the Methods for 

Derivation of Inhalation Concentrations and Application of Inhalation Dosimetry (USEPA, 1994) development, 

leading to different default adjustments for remote acting gases.

4 The Agency has since reaffirmed the method in carcinogen risk assessment (see USEPA, 2005).
4

inception, derivation of the HEC for this extrapolation has been utilized in cancer and noncancer 

inhalation assessment procedures. With regard to the latter, the Reference Concentration, RfC 

(initially termed an ―inhalation Reference Dose‖ or RFDi), originated with the publication of 

these documents (USEPA, 1989; 1994). Concordant with the RfD process, these methodologies 

include recognition of an uncertainty factor for uncertainties associated with interspecies 

extrapolation, UFA, with the default value of this UF automatically reduced (i.e., by half, 

logarithmically) in recognition of the dosimetric adjustment employed to estimate the HEC.
5

3 TOXICOLOGICALLY EQUIVALENT DOSES IN THE RFC AND RFD

3.1 ESTIMATING TOXICOLOGICALLY EQUIVALENT DOSES 

EXTRAPOLATION AND DOSIMETRY IN THE RFC

As previously discussed, dose-response assessment for human health, by the Agency as 

well as the risk assessment community, often uses toxicological effect information from 

laboratory animals, requiring extrapolation to humans. The goal of one of the extrapolation 

procedures is to determine toxicologically equivalent doses for animals and humans, ideally by 

―matching‖ with respect to the internal dose, or in other words, determining the externally 

applied exposure for humans that would result in the same internal dose. In derivation of the 

RfC,5this extrapolation is accomplished through application of a suite of procedures that range 

from application of a sophisticated physiologically-based toxicokinetic (PBTK) model of sitespecific dosimetry to default procedures. 

These default procedures are described fully in the Agency’s 1994 document and are 

represented here schematically in Figure 3-1a

6

. Dosimetric adjustment factors (DAFs) are used 

to extrapolate laboratory animal exposure concentrations to human equivalent exposure 

concentrations. The DAFs differ with chemical categories and include the Regional Gas Dose 

Ratio (RGDR) for gases and the Regional Deposited Dose Ratio (RDDR) for particles. When 

the default approaches are employed, the interspecies uncertainty factor is reduced. While the 

remaining uncertainty may include elements of toxicokinetics as well as toxicodynamics, the 

dosimetric adjustment, for simplicity is generally described as addressing cross-species 

differences in chemical disposition or toxicokinetics. A more in-depth discussion of the aspects 

of inter- and intra-species extrapolation considered to be accounted for with the default 

dosimetric adjustments is presented elsewhere (USEPA, 1994; Jarabek, 1995a; 1995b; 

Bogdanffy and Jarabek, 1995).

Figure 3-1a shows the experimental animal exposure of an agent, in ppm or mg/m3, 

extrapolated to an HEC, via application of DAFs. In this case, the animal exposure extrapolated 

is a point of departure (e.g., NOAEL, LOAEL, or BMDL) derived from analysis of the findings 

from an animal study. DAFs are based on the determinants of disposition considered most 

 

5 The same suite of procedures is applied in determining the HEC when deriving the inhalation cancer risk 

estimate (USEPA, 2005).

6

It is noted that this description of the default approach is, for the purposes here, a simplification of the 

recommended process that is based on the selection of the measure of dose which best expresses the internal dose at 

the target. Consideration of duration adjustment, e.g., from discontinuous to continuous exposures, is not described 

here (See USEPA, 1994, for the full recommended methodology). 
6

Figure 3-1. Schematic of procedures in the RfC (a.) and previous RfD (b.) processes. An

HEC is derived in the case of the RfC, but an HED has not been derived for the RfD. UFA

denotes the animal-to-human uncertainty factor, with UF representing other factors applied for 

various extrapolations (as per USEPA, 1994). In the RfC pathway, TK denotes toxicokinetic and 

TD toxicodynamic components of the UFA.

influential to differences between animals and humans. For example, with inhaled agents that 

affect the upper airways, DAFs are constructed from the surface areas of various regions of the 

airways and the minute inhalation volume of the species involved. For inhaled agents that 

involve transport by blood to affect systemic tissues, DAFs are constructed from partition 

coefficients of the agent (e.g., blood:gas and blood:tissue). The DAFs listed above are actually 

ratios constructed of animal and human values for these default determinants of disposition. As 

application of this ratio is projected to result in the human exposure that would bring about the 

same internal dose as the laboratory animal exposure to which it is applied, the DAF may also be 

considered as a factor used to ―normalize‖ an animal external exposure to the corresponding 

human external exposure under the guiding paradigm that a common internal dose is the ultimate 

determinant of risk (see Appendix D).

In the RfC process, the default application of DAFs is considered to produce HECs 

associated with toxicokinetically equivalent doses to the human tissue. Application of the 

default DAFs is not generally described as accounting for the toxicodynamic portion of response,

as is indicated in Figure 3-1a, although as noted above, that is a simplification. Other chemicalspecific information may inform consideration of toxicodynamic differences. As a 

simplification, Figure 3-1a shows the elimination of the toxicokinetic (TK) component from the 

UFA in consideration of the use of an HEC and implies the residual is within the toxicodynamic 

(TD) component, although depending on the chemical assessment, this may or may not be the 

case. 

UF mg/kg-day - animal A

UF A mg/m HEC (TK x TD) 3

- animal

RGDR RDDR

X DAF RfC

RfD

a.)

b.)

UF

UF

UF mg/kg-day - animal A

UF A mg/m HEC (TK x TD) 3

- animal

RGDR RDDR

X DAF RfC

RfD

a.)

b.)

UF

UF
7

3.2 EXTRAPOLATION IN THE RFD DERIVATION PROCESS

Currently, no document or dosimetry procedures comparable to the the RfC methodology 

exist for the RfD. As with the RfC, derivation of this reference value is frequently reliant on 

experimental animal data. In general, the UFA had been applied along with other UFs to the 

animal experimental dose to give the RfD value. Figure 3-1b reflects this simplified process 

showing application of the UFA with no ―HED‖ formally calculated. Appendix A illustrates the 

numerical consequences of this practice (e.g., use of BW1/1 versus BW3/4). 
8

4 BW3/4 SCALING FOR DERIVING TOXICOLOGICALLY 

EQUIVALENT DOSES IN THE RFD 

The following subsections discuss various considerations in interspecies extrapolation, 

with focus on the use of BW3/4 scaling (long used for Agency cancer risk assessment) of oral 

exposures as a default approach in extrapolation for the derivation of RfDs to replace the use of 

BW1/1, which has been the Agency default for that purpose.

4.1 SCALING IN CROSS-SPECIES EXTRAPOLATION

Use of a fractional power of body weight, most often BW3/4, as a means to derive 

toxicologically-equivalent doses across species is an accepted risk assessment practice (e.g., 

USEPA, 1992a; 2005). The basis for this acceptance is along several lines. Literature exists on 

general allometric relationships between BW3/4 and physiological and biochemical processes, 

mostly related to kinetics (Kleiber, 1932; 1961). Empirical information on the kinetics and 

toxicology of pharmacologic agents has been examined in relation to BW (e.g., Dedrick et al., 

1970; Dedrick, 1973). Much of the information related to these arguments are described and 

explained in the report by Rhomberg and Lewandowski (2004, 2006) and in EPA’s Federal 

Register notice (USEPA, 1992a). Some of the more compelling information on BW scaling in 

relation to basic life processes and to the effects and kinetics of pharmaceuticals and toxic agents 

is reviewed below.

4.2 INTERSPECIES BW3/4 SCALING AND LIFE PROCESSES

Kleiber’s (1947) synthesis of data on energy utilization in mammals resulted in his 

observation that the allometric relationship of BW3/4 is an accurate reflection of ―metabolic body 

size‖ in mammals. Subsequent to his work, a large number of characteristics and functions of 

mammalian biological systems were examined for their relationship with BW. Table 4-1 shows 

some examples. Volumes and capacities tend to retain their proportionality across species, i.e., 

BW1/1. A number of physiological processes, in addition to those listed here, are proportional to 

BW3/4 (see, for example, West et al., 1997). 

From these relationships, it can be deduced that, in mice and humans, weight increases in 

direct proportion with characteristics such as blood volumes and organ weights. Other processes 

(e.g., those involving flow and energy production, and food and water consumption) increase in 

absolute values but in proportion only to the three-quarters power of the body weight. It may 

also be seen that in processes involving rates and time, a decrease in the absolute value may 

actually occur. Thus, although the body mass and absolute heart mass are both about 2300-fold 

greater in humans than in mice (scaling to BW1/1), cardiac output in humans is only about 
9

Table 4-1. Cross-species body weight scaling for various metabolic and physiological 

functions.

Function Units Species Scaling

Energy utilization kcal/day BW 3⁄4

Glomerular filtration L/min BW 3⁄4

Glucose turnover mg/min BW 3⁄4

Heart rate min-1 BW-1/4

Respiratory rate min-1 BW-0.26

Blood volume L BW1/1

Vital capacity mL BW1/1

Food consumption g/day BW 3⁄4

Water consumption L/day BW 3⁄4

300-fold greater than in mice (scaling to BW3/4), whereas the heart rate in humans is about 7-fold 

less than in mice (scaling to BW-1/4). The latter relationship, where certain processes between 

species are related one to another in an inverse manner (i.e., are decreased rather than increased),

follows from BW3⁄4allometry when processes varying by a three-quarters power are normalized 

against an aspect that varies directly, i.e, BW1/1, such that

BW3/4 / BW1/1 = BW-1/4 

This inverse relationship between the absolute rates of processes (e.g., glomerular filtration, 

minute ventilatory volume, and cardiac output) and BW is supported by the concept of 

physiological time across species (e.g., USEPA, 1992a). Thus, processes that are related by 

BW1/4, such as physiological time, are actually corollaries of BW3/4 scaling that have been: (1)

predicted mathematically and (2) substantiated by empirical observations. This concordance 

between hypothesis and observations imparts considerable credence to the overall relational 

theory of BW3/4 scaling.

4.3 INTERSPECIES BW3/4 SCALING AND TOXICITY PROCESSES 

The BW3/4 allometric scaling relationship of Kleiber (1932; 1961) was derived from 

general kinetic processes of living systems. The relevancy of these general processes to the more 

refined relationship of kinetics of pharmaceuticals and other xenobiotics, such as toxicants, has 

also been examined. In general, the information available in this area supports the three-
10

quarters power relationship (including the quarter power relationship) for toxicologic and 

toxicokinetic behavior of a number of compounds over a reasonable number and range of 

species. 

Travis and White (1988) undertook an analysis of the maximum tolerated dose from the 

dose-response relationship of 27 direct-acting agents (i.e., where the agent causing toxicity is the 

administered or parent chemical) administered orally (once per day, 5 days a week) in mice, rats, 

dogs, monkeys, hamsters, and humans. They then used regression techniques to determine the 

optimal power of body weight to achieve the best fitting relationship. They reported BW0.73

(95% confidence bounds BW0.69-0.77) as the geometric mean of the cross-species predictions. 

This study is a reanalysis of data sets from two other studies (Freireich et al., 1966; Schein et al., 

1970) and provides support to the relationship of BW3/4 in making interspecies extrapolations for 

direct-acting toxicologic agents. The analysis also rejected the relationships of BW1/1 (the 

exponent associated with direct proportionality) and BW2/3 (the exponent associated with body 

surface area scaling), although a subsequent report offered an analysis suggesting that BW2/3 still 

may fall within the confidence bounds (Watanabe et al., 1992). 

The analyses by Boxenbaum (1982) and Dedrick (1973) of elimination rate constants for 

8 drugs in 4 different species found this parameter to be proportional to BW0.22, which is 

reasonably close to the expected value for BW1/4 scaling related to time processes of BW3/4.

Kirman et al. (2003) employed PBTK models as tools to assess the performance of 

allometric scaling. These authors used PBTK models for 12 volatile and lipophilic compounds 

to estimate the kinetic disposition of these various agents in yielding a measure of internal dose 

(area under the curve or AUC) in mouse, rats, and humans. Model estimates were obtained 

under conditions of continuous and gavage dosing over a range of experimental exposures 

considered low (0.0001-1 mg/kg-day) and high (10-10,000 mg/kg-day). The estimates from the 

models were then compared to predictions calculated from allometric scaling of the administered 

dose based solely on BW3/4. The results indicated that BW3/4 generally performed better at 

relatively low administered doses (where metabolism is not saturated and clearance rates were 

pseudo-first order) than at high administered doses. The results also indicated that the scaling 

factors were applicable in oral administration not only to parent chemical but also for the 

formation of stable metabolites and amount metabolized by first-order pathways (see Metabolism 

and Clearance below). 
11

5 CONSIDERATIONS ON USING BW3/4 SCALING AS A DEFAULT FOR 

ESTIMATING TOXICOLOGICALLY EQUIVALENT DOSES 

This section discusses a variety of aspects pertinent to using BW3/4 as a default for 

estimating toxicologically equivalent doses for the purposes of deriving an RfD. This discussion 

is intended to be informative to this application (e.g., with regard to assumptions or limitations).

5.1 METABOLISM AND CLEARANCE

Observations of a fractional power relationship between body weights with regard to 

processes across species has been hypothesized to be related fundamentally to differences in 

exchange surfaces and distribution networks, which constrain internal concentrations and flux 

associated with general metabolic processes of endogenous substances and reactants for all living 

systems (West et al., 1997; Enquist et al.,1998).

The applicability of this general body weight scaling relationship to more specific 

metabolic processes, such as xenobiotic metabolizing systems involved in the clearance or 

activation of exogenous substances (where disproportional relationships among species often 

exist) would depend on the similarity in the kinetic behavior of the exogenous substance to 

endogenous reactants. The degree of similarity would reflect whether the parent or a stable or 

reactive metabolite is the relevant dose to the target tissue (O’Flaherty, 1989; USEPA, 1992a; 

Beck and Clewell, 2001) and on the specific kinetics of the clearance process, as to whether it is 

a first-order or capacity-limited process (O’Flaherty, 1989). Both of these factors prominently 

influence the dose to the target tissue over time, i.e., the AUC (see below).

Accordingly, BW3⁄4 scaling would apply most appropriately to those exogenous 

substances for which the unmetabolized parent or a stable metabolite is the relevant toxic species 

and clearance is according to first-order processes (USEPA, 1992a). Conversely, the 

applicability of BW3/4 scaling is less well supported when toxicity is a consequence of exposure 

to a very reactive parent compound or metabolite that is not removed from the site of formation

by biological processes (e.g., subsequent metabolism) but chemically reacts with cellular 

constituents (Travis, 1990; Beck and Clewell, 2001). 

5.2 MEASURE OF DELIVERED DOSE: CHOICE OF THE APPROPRIATE 

DOSE METRIC

As discussed above, the species BW scaling interrelationships among volumes (BW1/1), 

physiological processes (BW3/4), and rates (BW-1/4) have been shown to result in a normalization 

of dose across species (USEPA, 1992a) with respect to time. This normalization, based on the

concept of physiological time implicit in BW-1/4 scaling, is in terms of exposure to a 
12

concentration over some duration of time (e.g., AUC that is adjusted for species). Thus, the 

measure of dose that is being scaled for kinetic equivalency between species in this process is the 

AUC (O’Flaherty, 1989).

It is recognized that toxicants may not exert their effects through a single mode or 

mechanism of action and that, in some cases, measures of delivered dose other than AUC may be 

more appropriate. However, for agents causing cumulative irreversible damage, a common 

definition of toxicity, an expression of integrated exposure such as AUC may be considered 

appropriate (recognizing the caution raised in the previous section regarding toxicity at the portal 

of entry). 

5.3 EARLY LIFESTAGES

Investigations with BW scaling have dealt almost exclusively with interspecies scaling 

across adult organisms, consistent with much of the experimental toxicity information that is 

generated through exposure of adult organisms. The default application of BW3/4 scaling prior to 

this document has been for cancer assessment based upon adult animals (USEPA, 2005). This 

document extends that application to derivation of the RfD by scaling the administered exposure 

for the mature laboratory animal to that for the adult human. As the toxicant typically is 

administered to an adult animal, this practice would generally be employed even when the target 

is a fetus or developing pup, such as in developmental or multigenerational reproductive studies.

Exposure and internal dosimetry of pregnant, nursing, and growing animals may vary 

compared to adult animals, so use of the administered dose for toxicity studies involving these 

periods is associated with relatively greater uncertainty, absent lifestage-specific information. In 

some situations, the Agency may have data on effects resulting from exposure of young animals 

and be interested in derivation of an exposure value particular to infants or children as described 

in A Framework for Assessing Health Risk of Environmental Exposures to Children (USEPA, 

2006). In those instances, extrapolation from the young animal to a young human exposure may 

be desirable (Barton, 2005). When doing such an extrapolation, however, key developmental 

processes need to be matched in a species-dependent manner, because the temporal pattern of 

development (of physiological systems, organs, etc.) differs across species (Finlay and 

Darlington, 1995; Clancy et al., 2001; USEPA, 2002; 2006). Because of these differences in 

temporal patterns of development, such matching is usually done on an endpoint-specific basis

for both toxicokinetics and toxicodynamics, which may also differ (USEPA, 2002; 2006). The 

Agency’s Review of the Reference Dose and Reference Concentration Processes (USEPA, 2002) 

also discusses such considerations, with particular focus on the Agency’s toxicity testing 

protocols. Further analysis of this important aspect of interspecies extrapolation of early 
13

lifestages is not presented here, as it is anticipated to generally depend upon analyses with 

lifestage-specific information rather than application of default approaches.

Given that the default application of BW3/4 scaling would be to scale the administered 

exposure for the mature laboratory animal to that for the adult human, potential limitations were 

considered for this default application in deriving RfDs, which, by definition, apply to the human 

population inclusive of children. The following discussion focuses primarily on recent analyses 

of pharmacokinetic differences across ages of humans. An attempt to systematically evaluate 

quantitative scaling differences in toxicodynamic processes across lifestages (Hattis, 2004) 

observed that data for such an analysis are limited. Analyses of pharmacokinetic data on 45 

therapeutic drugs for different age groups of children indicate that a number of toxicokinetic 

parameters, including activity of various xenobiotic metabolizing enzymes, generally reflect 

adult values by 6 months of age, with the largest differences from the adult values occurring in 

the first 2 months (Ginsberg et al., 2002; 2004). Using largely the same database, Hattis et al.

(2004) analyzed clearance rates on a BW1/1 basis for a number of drugs representing a wideranging spectrum of clearance mechanisms in adults, children, and infants. These data showed 

that, from the age range of 2 months to 12 years, clearance rates were higher in children than in 

adults, whereas values for very young infants (1-8 weeks, full-term neonates, and premature 

neonates) were deficient relative to adults, ratios for premature infants being about 1/2 those for 

adults (Hattis et al., 2004; 2003a). Analysis of these clearance rate data on a BW3/4 basis

resulted in ratios that approximated 1 (children equal to adults) down to 6 months of age (Hattis 

et al., 2004). This analysis indicates that the occurrences of higher clearance rates in children 

(down to about 6 months) relative to adults are consistent with the use of BW3/4 allometry across 

ages, while not being consistent with such allometry for young infants. 

PBTK models are a method to integrate changes in physiological and biochemical factors 

(including development of metabolizing enzymes) to assess their impact on dosimetry by making 

predictions across ages and genders (Kedderis, 1997; Rodriguez et al., 2007). Clewell et al. 

(2004) used a PBTK lifestage model that integrated various age- and gender- specific differences

from birth to 75 years old and reported results by age categories including one for ―birth to 6 

months‖. The authors predicted measures of internal dose (e.g., parent, circulating metabolite, or 

reactive metabolite) for 6 different chemicals, and reported that values for each were within a 

factor of 2 across the age groups evaluated, with the larger transient variations predicted 

particularly during the neonatal period. Their results indicated that the most important agerelated PK factor appeared to be the potential for decreased clearance of a toxic chemical in the 

perinatal period, although this same factor could also result in reduced production of a reactive 

metabolite. 
14

Application of BW3/4 scaling to derive a human equivalent dose from another species by 

scaling to the body weights of children rather than adults would yield a higher equivalent dose. 

Scaling to children’s body weights might not be appropriate for RfD or short-term guidance 

value intended to apply to a population that includes young infants and children due to the 

comparatively slower clearance during this period and the limited toxicokinetic data available to 

assess the appropriateness of body weight scaling in early life. Similar analyses on potential 

sensitivity of toxicodynamic processes during developmental periods are not currently available.7 

5.4 TOXICOKINETICS AND TOXICODYNAMICS IN TOXICOLOGICAL 

EQUIVALENCE

Species differences in dose-response functions may be elicited both: (1) as a 

consequence of distribution of agent affecting the target-tissue dose between species and (2)

from intrinsic differences in the tissue response between species. Achieving ―toxicological 

equivalence‖ across species requires that aspects of both toxicokinetics and toxicodynamics be 

considered. Therefore using BW3/4 to achieve ―toxicological equivalence‖ for interspecies 

differences implies that scaling is inclusive of both aspects.

From the preceding discussion on allometry, it is apparent that many physiological 

processes relating to kinetics conform to a BW3/4 relationship. This is not to say, however, that 

BW3/4 scaling encompasses all kinetic processes related to toxicity. Neither does this statement 

intend to indicate that BW3/4 scaling does not address any dynamic aspects of toxicity. It has 

been established that there are processes considered to be toxicodynamic in nature, e.g., cellular 

repair and regeneration, signaling cascades, and proliferative responses that also scale as a 

fractional power of BW (see Rhomberg and Lewandowski, 2004, 2006; USEPA, 1992a, for 

additional examples). 

It is necessary to acknowledge the overlap in kinetic and dynamic factors addressed by 

BW3/4 scaling. That BW3/4 scaling applies only to metabolic types of kinetic processes between 

species is a misconception. Many potential modulating factors that may be considered as 

dynamic, such as the intrinsic sensitivity of the target site, may be highly species dependent. The 

 

7 While the focus of this document is on interspecies extrapolation (not intraspecies extrapolation), it is 

noted that inclusion in the RfD derivation of the UF for intraspecies variation in susceptibility due to the possibility 

that a given data base may not be representative of the exposure-response relationship for some susceptible 

subgroups within the overall population further addresses this toxicodynamics data gap (USEPA, 2002). As with 

the UFA, this UF is also considered to comprise both toxicokinetic and toxicodynamic components (Renwick and 

Lazurus, 1998). Additionally, the RfD methodology includes a database UF intended to account for the potential for 

deriving an inadequate RfD as a result of an incomplete characterization of the chemical's toxicity, including 

toxicity to early life developmental processes, and any associated toxicodynamic changes (Dourson et al., 1992; 

USEPA, 2002).
15

concept to be conveyed is that cross-species BW3/4 scaling for toxicologically equivalent doses 

predominately addresses factors involved in estimating toxicokinetics, as well as some 

toxicodynamic factors. This concept is critical to how the interspecies uncertainty factor (UFA) 

is applied in derivation of the RfD (see below). 

5.5 ACUTE SCENARIOS

The focus of this default procedure is for the oral RfD and cancer assessments, both of 

which are concerned with lifetime repeated exposure scenarios at the time of writing this 

document. Application of this procedure to scenarios of shorter durations, such as acute

exposures, is considered in this section (Hattis, 2003b).

As discussed above, BW3/4 scaling is understood to address aspects of both TK and TD, 

the latter being inclusive of many repair types of processes. BW3/4 scaling would not generally 

be relevant in the case of a single exposure eliciting sudden and severe toxicity resulting from 

immediate and intolerable damage to some critical biological pathway, and where repair 

processes (i.e., TD) would be overwhelmed. This case is in contrast to chronic exposures, where 

the organism or tissue has time available for repair processes to be elicited and functional, and 

where the level of damage is not severe (e.g., those routinely considered for an RfD).

Rhomberg and Wolff (1998) examined patterns in the correspondence of LD50 values 

from a single oral administration across several species. They found that direct scaling by body 

weight (i.e., BW1/1), rather than scaling to BW3/4, best fit the data. One limitation of this analysis 

is that most of the data were obtained in species of similar size (i.e., mouse, rat, guinea pig, and 

hamster) where the correlation was strongest. Rhomberg and Caprario (1999) extended the 

findings of Rhomberg and Wolff (1998) by examining information available on larger species 

via collating data from administration routes other than oral (i.e., intravenous, intraperitoneal,

and intramuscular). LD50 values of over 3,000 agents were evaluated in pair-wise species 

comparisons across eight species (mouse, rat, hamster, guinea pig, rabbit, cat, dog, and monkey). 

Results were similar to the oral LD50 analysis, indicating that for lethal acute exposures, scaling 

to BW1/1 provided a good extrapolation factor across species. 

Thus, due to these toxicodynamic considerations, BW3/4 would most likely not be 

applicable to scenarios under conditions of an acute exposure focused on the occurrence of 

immediate and severe or lethal effects. Application of BW3/4 is considered a reasonable 

approach, however, for acute exposures in which the operative physiological processes are 

comparable to those for the chronic scenario.
16

5.6 PORTAL-OF-ENTRY ISSUES FOR ORAL EXPOSURE

The utility and limitations of allometric scaling (BW3/4) for oral route portal-of-entry 

effects has not been systematically evaluated. Portal-of-entry effects are caused by direct action 

of a chemical or its metabolites on tissues in the respiratory or gastrointestinal tract (or the skin, 

but that is not the focus here). Considerable conceptual similarity exists between inhalation and 

oral portal-of-entry exposure scenarios. In both cases, exposures would occur due to agents 

entrained in the incoming media (e.g., inspired air or ingested materials) to the surface 

respiratory or gastrointestinal epithelial tissues. This scenario differs fundamentally from 

delivery of a chemical from circulating blood to organs throughout the body and uptake into 

target tissue. The relevant dose metric for systemic effects is generally the mass per tissue (i.e., 

mg/kg), whereas with portal-of-entry effects the most relevant dose metric would be based on

mass of agent per surface area (i.e., mg/cm2).

For respiratory portal-of-entry effects considered in the RfC methodology (USEPA, 

1994), species-specific surface areas for regions within the respiratory tract are used in 

calculating a DAF that is specific to that region. An approach for an interspecies DAF for oral 

portal-of-entry effects parallel to the inhalation portal-of-entry DAF process may be appropriate

(see Appendix C). Such an approach would utilize species-specific surface areas for the affected 

area of the gastrointestinal tract of the laboratory animal and human and take into account other 

unique physiological differences. For example, rodents have a physiologically distinct 

forestomach region which humans lack, as well as a different average gastrointestinal pH. 

Implementing such an RfC -like approach for the gastrointestinal tract, however, entails further 

development.

A BW3/4 relationship exists among species in studies where dose is administered in food, 

because interspecies food consumption follows a BW3/4 relationship (see Table 4-1). 

Consequently, exposure among species to an agent, both overall and to the gastrointestinal tract, 

present at a constant concentration in food would also follow a BW3/4 relationship. Therefore, it 

is reasonable to apply the BW3/4 approach for gastrointestinally related, portal-of-entry effects.

5.7 SUMMARY OF LIMITATIONS IN BW3/4 SCALING

From the preceding analysis, the following is summarized concerning the underlying 

limitations in the application of BW3/4 scaling.

• Use of BW3/4 scaling is most appropriate for toxicity where the measure of dose 

associated with the toxicity is the AUC for the parent chemical or stable active metabolite 

following oral exposure. Conversely, the applicability of BW3/4 scaling is less well 

supported when toxicity is a consequence of exposure to a very reactive chemical that is 
17

not removed from the site of formation by biological processes (e.g., metabolism), but by 

chemical reaction with cellular constituents.

• Differing allometric patterns among various sized individuals of the same species 

(Rhomberg and Lewandowski, 2004, 2006) may pose an uncertainty to intraspecies 

scaling, while differences across species in patterns of development (Finlay and 

Darlington, 1995; Renwick and Lazarus, 1998; Clancy et al., 2001) can complicate 

interspecies extrapolation from immature animals to humans. With regard to variation in 

toxicokinetic proceses, recent analyses suggest that a BW3/4 relationship is descriptive of 

some TK differences observed with pharmaceuticals among ages including early 

lifestages, down to about 6 months (Ginsberg et al., 2002; 2004; Hattis et al., 2004). 

Scaling to children’s body weights might not be appropriate for an acute RfD or shortterm guidance value intended to apply specifically to a population that includes young 

infants and children due to the comparatively slower clearance during this period and the 

limited toxicokinetic data available to assess the appropriateness of body weight scaling 

in early life. 

• Cross-species BW3/4 scaling for toxicologically equivalent doses predominately addresses 

factors involved in estimating toxicokinetics, as well as some toxicodynamic factors. 

• Use of BW3/4 scaling for orally administered acute exposures where effects such as 

lethality are manifest may be less accurate than for other exposures (Hattis et al., 2003b). 

It is considered more appropriate for non-lethal acute effects in which the functional 

status of physiological processes is comparable to the chronic scenario. 

• For oral portal-of-entry effects, development of a DAF involving aspects relating dose to 

a surface area at or within the portal may be appropriate. Additionally, the BW3/4

relationship exists for interspecies food consumption and diet-associated dosing. 
18

6 CONCLUSIONS

The recommended default procedure (in the absence of chemical-specific data or in lieu 

of information indicating an alternate approach) for extrapolating from laboratory animal oral 

exposure estimates to human equivalent estimates is body weight scaling to the 3/4 power 

(BW3/4). The overarching assumption in this default approach is that measurable characteristics 

of anatomy and physiology scale as a function of BW 3⁄4. The basis for the overarching 

assumption is outlined in Sections 3 and 4. The limitations inherent in the application of this 

approach are related to acute durations of exposure, early life stages, and clearly frank effects 

such as lethality. 

The limitations of this approach are described in detail in Chapter 5 and summarized in 

section 5.7. The arithmetic involved in the application of this procedure to a given exposure 

scenario in representative animal species using mature body weights is demonstrated in 

Appendix B. Implementation of this scientifically-based, default dosimetric adjustment, which is 

already employed in the derivation of oral cancer slope factors, provides a parallel to the default 

dosimetric approach employed in derivation of the HEC for inhalation exposures.

Pending, or in lieu of, the development of specific information to employ an interspecies 

dosimetric adjustment based on dose to the specific site of toxicity within the gastrointestinal 

tract, the BW3/4-based DAF is recommended as the default to derive a HED involving oral 

portal-of-entry toxicity (e.g., see Appendix C). This science-based policy decision provides 

consistency with methods used for scaling oral exposures for cancer assessment (USEPA, 2005).

6.1 DEFAULT PROCEDURE IN THE HIERARCHY OF APPROACHES

This default approach (as in the case of the HEC default approach) represents the base 

tier in the hierarchy of approaches (Table 6.1) to be considered in extrapolating from laboratory 

animal to human equivalent oral exposure scenarios for the purposes of developing a doseresponse assessment pertinent to human risk assessment. A customization of the hierarchy 

presented in Methods for Derivation of Inhalation Reference Concentrations and Application of 

Inhalation Dosimetry (USEPA, 1994; see also Appendix D), for the purposes of the oral route 

extrapolation, is shown in Table 6-1. 
19

Table 6-1. Hierarchical framework of approaches for interspecies extrapolation.

Optimal

If available, employ PBTK (or PBTK-TD) or other biologically based modeling.

Intermediate

Assess available information, considering what is known about species differences, and 

the toxicokinetic and toxicodynamics of the chemical. Use this information to derive an 

appropriate cross-species adjustment (e.g., a data-supported scaling function or a different UF or 

combination of the two). 1 Basic issues in this consideration include 

1. indications that BW3/4 scaling or an alternate approach would be preferred for 

interspecies extrapolation; and 

2. the best quantitative judgment of the residual uncertainty in animal-to-human 

extrapolation that remains after BW scaling. 

Examples of intermediate approaches include the use of chemical specific adjustment factors, as 

described in IPCS (2005), as well as the existing IRIS assessment for boron (USEPA, 2004).

Default

In lieu of useful information about the chemical being considered (see intermediate 

approach), the default is employed. 

1 Evaluate information available for laboratory animals compared to human with respect to: 

 whether the active toxicant is the parent or a metabolite,

 appropriate dose metric (e.g., Cmax, AUC, TACC [time-above-critical concentration], agerelated concentration x time interval),

 critical TD event(s), and

 critical effect, including consideration of portal-of-entry issues.
20

6.2 OPTIMAL AND INTERMEDIATE APPROACHES

An overriding aspect of the hierarchy in Table 6-1 is the incorporation of mechanistic 

data as feasible, ensuring that the methodology employed for a particular chemical assessment is 

commensurate with the available data. This is particularly important for extrapolations from 

developing animals to young humans due to the differences in developmental patterns across 

species. Whichever approach is employed, the individual chemical assessment is to include text 

clearly describing the consideration of all of the available and relevant information in the 

selection of approaches. When the default is employed, it is to be explicitly recognized that the 

default has its basis in our knowledge of other chemicals.

6.3 THE HED AND INTERSPECIES UNCERTAINTY FACTOR, UFA

With the implementation of this approach, the RfD, like the RfC, will be derived from an 

HEE for the critical effect(s) by consistent application of UFs. The UFs are applied to account 

for various recognized uncertainties in the extrapolations from the experimental data conditions 

to an estimate appropriate to the assumed human scenario (USEPA, 2002; USEPA, 1994). For 

the extrapolation from experimental animals, the RfC methodology (USEPA, 1994) currently 

recommends that the default value for the animal to human uncertainty factor be 3 when the 

default dosimetric adjustments are employed. Further, the RfC methodology recommends that 

the use of more rigorous dosimetric adjustments may allow additional modification of the UF. 

Although the RfC methodology generally describes the default dosimetric adjustment as 

accounting for variability in disposition or toxicokinetics, it also states that the processes 

pertinent to the uncertainty factor include both toxicokinetics and toxicodynamics, indicating that 

the toxicokinetics versus toxicodynamics assignment reflects these considerations. Additional 

discussion on this point appears in Jarabek (1995a) and Bogdanffy and Jarabek (1995).

In considering the recommendation regarding the default value for UFA when the default 

BW3/4 scaling approach is used (i.e., in lieu of information indicative of an alternate approach), 

various options were considered, ranging from no change in the default of 10 to 1. The 

conclusion to reduce the UF from its current default value of 10 is a science policy decision 

based on the qualitative recognition that current scientific knowledge indicates that BW3/4

scaling generally addresses the potential for species differences in both kinetic and dynamic 

processes, which the UFA had been intended to address. Additionally, it is recognized that in the 

situation in which defaults are invoked, uncertainties remain. Thus, when BW3/4 scaling is 

applied the recommendation is that the default value for the interspecies UFA be set at 3.

In the recommended process, an HED would be derived using BW3/4 as a DAF or kinetic 

equivalence factor that is to be applied in the derivation of an HED. As illustrated in Figure 6-1, 

UFA denotes the interspecies or animal-to-human uncertainty factor and is divided into 2 separate 
21

components without designation of either TK or TD, but equal to one half, logarithmically, of the 

101value typically assigned to this UF. With the application of the default HED approach, one 

component is dropped. In the absence of additional data informing consideration of interspecies 

differences, a residual default interspecies uncertainty factor of 3 remains.

This recommended reduction of the default value from 10 to 3 is generally consistent 

with the approach for this uncertainty factor in the RfC methodology (as described in the 

preceding paragraph). As discussed in the previous sections, the scaling of chronic oral exposure 

via BW3/4 addresses notable aspects of predominantly toxicokinetic and some toxicodynamic 

processes, yet leaving some residual uncertainty, which may flow from either area. As per the 

RfC methodology, it is recommended that, with the use of more rigorous dosimetric adjustments 

in deriving the HED (including the use of BW3/4 scaling rather than the historical default), there 

is a modification of the default value for the UFA. Implementation of the recommended default 

approach explicitly endorses a case-by-case evaluation with consideration of all available data in 

determining the modification of the uncertainty factor (with the default of 3 applied unless there 

are data indicative of an inappropriate scaling via BW3/4). Similar to its application for the RfC, 

processes pertinent to the consideration of this UF are recognized to include both toxicokinetics 

and toxicodynamics, including relevance of the laboratory animal model and species sensitivity. 

Thus, while different concepts are emphasized in the default oral versus inhalation dosimetric 

adjustment approaches, there are some similarities in the residual uncertainty and consequently 

in the recommended default value for the UFA.

Figure 6-1. Procedures for (a.) the current RfD and (b.) recommended RfD processes. In

the application of the recommended default approach, one component of UFA is dropped. In the 

absence of additional data informing consideration of interspecies differences, a residual default 

interspecies uncertainty factor of 3 remains.
22

6.4 SUMMARY

Instituting procedures by which the target tissue or some form of internal dose is 

considered, in this case indirectly through calculation of an HED, is conceptually compatible 

with the overarching paradigm that regards mode of action as central to understanding toxicity.

The default procedure described here involves first calculating an oral dosimetric 

adjustment factor or DAF that is then used to derive an HED that is subsequently used in 

derivation of the RfD. This procedure, of translating the animal exposure of interest to human 

equivalence using a biologically motivated approach, is parallel to derivation of the HEC in 

derivation of an RfC. 

Adopting BW3/4 scaling as a means to make dosimetric adjustments in calculating an 

HED, brings about harmonization of RfD procedures with that of the RfC methods (USEPA 

1989; 1994) wherein the HEC is derived. 

As with the RfC methodology, a hierarchy of models and procedures for interspecies 

extrapolation is established. BW3/4 is acknowledged as the default approach for the oral route 

with more sophisticated models being considered, as being more data-informed approaches by 

which the internal dose may be estimated.

Pending, or in lieu of, the development of specific information to employ an interspecies 

dosimetric adjustment based on dose to the specific site of toxicity within the gastrointestinal 

tract, the BW3/4-based DAF is recommended as the default to derive an HED involving oral 

portal-of-entry, as well as systemic, toxicity. 

With the calculation of an HED using the DAF approach, a default value of 3 is 

recommended for the UFA in the absence of additional, relevant information. 

The quantitative significance of this procedure for translating an animal exposure into 

human equivalence, in terms of the magnitude of an RfD, will depend on the body weight of the 

species (as well as the value assigned to the UFA) and may be more or less than the current 

procedure of dividing by the default composite UFA of 10. In the case of mice and small rats, 

and with use of the default UFA, the resulting RfD under the new approach will be lower than an 

RfD from the same data using the previous approach of a default UFA of 10, whereas the 

opposite is the case for dogs (See Table A1).

Using BW3/4 procedures in deriving an RfD harmonizes with its existing use as the 

default procedure in extrapolating from laboratory animal administered oral doses in oral 

quantitative cancer assessment as per the Cancer Guidelines (USEPA, 2005). Adoption of this 

procedure thus provides harmonization between the two main Agency dose-response assessment 

methodologies. 
23

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27

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28

APPENDIX A. BW3/4 SCALING VERSUS BW1/1 – A COMPARISON

Prior to this document, the use of an interspecies uncertainty factor (UFA) applied directly 

to an animal experimental dose reported in mg/kg-day was the same as scaling BW1/1 and 

factoring in uncertainty. When proceeding from a small to a larger animal, this procedure is not 

―conservative‖ in that it actually produces higher oral dose rates in comparison to BW3/4-scaling 

procedures, as shown in Table A-1 comparing BW1/1 and BW3/4 scaling from animals to humans. 

This analysis also demonstrates the nonproportionality of the BW3/4 scaling processes: The 

smaller the animal being scaled from, the smaller the scaled human dose, about 4-fold different 

from BW1/1, when scaling from rats, and about 7-fold different from BW1/1, when scaling from 

mice. 

A common point of confusion in understanding and performing BW scaling is the 

expression of the experimental dose that will be scaled. In Table A-1 (below) showing examples 

of BW3/4 scaling, note that what is being scaled is the absolute intake or exposure, in mg, not, for 

example, mg/kg. Therefore, it is the absolute exposure of 0.25 mg to the mouse that is scaled to 

the human by the BW3/4 animal-to-human scaling factor of 385 to arrive at the absolute (and 

scaled) exposure of 96.25 mg for human. The mg/kg value is then derived by applying the 

human weight to this exposure, 96.25 mg/ 70 kg (human body weight), to arrive at the 1.4 mg/kg 

scaled human intake. Scaling of mg/kg (which is actually a rate process) rather than absolute 

mg, may be undertaken directly but with transformations of the BW 3⁄4relationship that are used 

to scale rate processes, e.g., BW-1/4. This procedure is addressed in Appendix B in derivation of 

the DAF. The reports of Clewell et al. (2002) and O’Flaherty (1989) both contain clear 

examples and helpful specifics in performing BW scaling. 
29

Table A-1. Comparison of BW1/1 and BW3/4 in estimating oral exposure in humans from a 

10 mg/kg exposure to rats, mice, and a dog. 

Scaling

Absolute Animal 

Intake or 

Administered Dose

Species BW(h)/BW(a) BW Scaling 

Factor

BW Scaled Human Intake 

or Oral Dose (mg/kg)

BW3/4 0.25 mg/ 0.025 kg mouse 70/ 0.025 = 2800 28003/4 = 385 (385 x 0.25 mg =96 mg) 

96 mg /70 kg = 1.4 mg/kg

BW3/4 2.5 mg / 0.25 kg rat 70/ 0.25= 280 2803/4 = 68 (68 x 2.5 mg = 170 mg) 

170 mg/70 kg = 2.4 mg/kg

BW3/4 120 mg / 12 kg dog 70 / 12 = 5.8 5.83/4 = 3.7 (3.7 x 120 mg = 144 mg) 

444 mg/70 kg = 6.4 mg/kg

BW1/1 0.25 mg/ 0.025 kg mouse 70/ 0.025= 2800 28001/1 = 2800 (2800 x 0.25 mg = 700 mg) 

700 mg /70 kg = 10 mg/kg

BW1/1 2.5 mg / 0.25 kg rat 70/ 0.25 = 280 2801/1 = 280 (280 x 2.5 mg = 700 mg) 

700 mg /70 kg = 10 mg/kg

BW1/1 120 mg / 12 kg dog 70 / 12 = 5.8 5.81/1 = 5.8 (5.8 x 120 mg = 700 mg) 

700 mg /70 kg = 10 mg/kg

none 700 mg / 70 kg human - -

(700 x 1 mg = 700 mg) 

700 mg /70 kg = 10 mg/kg
30

APPENDIX B. THE INTERSPECIES BW3/4 ORAL DOSIMETRIC 

ADJUSTMENT FACTOR (DAF) – BWA

1/4 / BWH1/4

The application of a DAF in determining an HEE, or in the specific case of an oral 

exposure, an HED, would typically be applied to the laboratory animal dose (in mg/kg) as:

Laboratory animal exposure (mg/kg) x DAF = HED (mg/kg)

where,

DAF = (BWa

1/4 / BWh1/4)

This equation demonstrates the mechanics of application of the DAF to attain an HEE or 

HED. The procedure would apply to any and all laboratory species, although in practice, rat and 

mouse are the predominant species, with dogs also at times represented. An analysis of DAFs, 

such as those derived and then applied to derive an HEC (see USEPA, 1989; 1994, 2002),

reveals that they are composed of determinants that are intended to inform about the dose to the 

target tissue (the more accurate determinant of risk). A DAF is applied to adjust to the equivalent 

dose for animals and humans that is present at the target tissue (i.e., the internal dose). This 

analysis also reveals that the DAF is a ratio constructed of human and animal parameters that are 

predictive of processes that lead to the internal dose. Application of this ratio estimates the 

human exposure projected to elicit the same internal dose as the laboratory animal exposure. For 

adjustments based on BW3/4 scaling, the DAF would be a factor that would:

- reflect the scaling of the absolute exposure amount of the animal to the 

corresponding absolute exposure amount of a human; 

- reflect this human exposure on a mg/kg basis; and 

- be constructed as a ratio of animal and human determinants of internal dose. 

Table B-1 lists examples of scaling from various laboratory animals to humans based on 

a 10 mg/kg exposure in various species (see also Table A-1). The column showing the ratio of 

these scaled human-to-animal exposures reflects the scaling and normalization of this value to a 

mg/kg basis, corresponding to the requisites described above for a DAF. The column in Table 

B-1 labeled ― BWh

-1/4 / BWa-1/4 ‖ lists the body weights to the negative one quarter power of the 

laboratory animals and a 70-kg human. The human to animal ratio of these scaled weights can 

be seen to correspond to the ―Scaled Human to Animal Ratio‖.

Exposure expressed as a rate, such as mg/kg, rather than as an absolute amount is an 

example of this relationship. Table B-1 demonstrates when this exposure (actually an exposurerate or dose-rate) relationship is expressed as a ratio of humans to laboratory animals(s). There 

is predictably parallel equivalence between this ratio and the BW -1/4 scaling of human to animal 
31

ratio. This may be transformed into the corresponding positive (+) scaling relationship through 

inversion of the terms such that (BWh / BWa)

-1/4 is equivalent to (BWa / BWh)1/4

. 

Table B-1. The animal:human BW1/4 ratio as the Dosimetric Adjustment Factor (DAF) 

used in deriving an HED from an oral animal exposure. 

Species

BW3⁄4 Scaled Human 

Equivalent Exposure

(for a 70 kg human)

Scaled 

Human to Animal 

Dose Ratio

BWa

1/4/BWh1/4

= DAF

BW3/4 Scaling 

of a 

10 mg/kg-day 

Exposure

in a ...

Mouse

(0.025 kg) 1.4 mg/kg-day* 1.4 / 10 = 0.14 0.40 / 2.89 = 0.14**

Rat

(0.25 kg) 2.4 mg/kg-day 2.4 / 10 = 0.24 0.71 / 2.89 = 0.24

Dog

(12 kg) 6.3 mg/kg-day 6.3 / 10 = 0.63 1.86 / 2.89 = 0.63

* mice/human = 0.025 kg 1/4 / 70 kg 1/4 x 10 mg/kg-day

** The complete arithmetic calculation is

mice/human = 0.025 kg 1/4 / 70 kg 1/4 = 0.398/2.89 = 0.137, which is rounded to 0.14. 

Also (equivalently), human / mice = 70 kg -1/4 / 0.025 kg -1/4 = 0.345 / 2.51 = 0.137, also rounded to 0.14.

As pointed out in Section 4 of this document, rate-related processes scale across species 

in a manner related to both the direct (BW1/1) and BW3/4 aspects such that: 

BW3/4 / BW1/1 = BW-1/4

As BWh

-1/4 / BWa-1/4 can be readily calculated and applied to any combination of body weights, it 

is designated as the BW3/4-based DAF, such that:

 DAF = (BWh

-1/4 / BWa-1/4)

and, the equivalent,

DAF = (BWa

1/4 / BWh1/4).

In summary then, the laboratory animal exposure (mg/kg) x DAF = HED (mg/kg).
32

APPENDIX C. THE ORAL DAF AND PORTAL-OF-ENTRY 

CONSIDERATIONS

“Systemic” versus “portal-of-entry” considerations in toxicity from oral administration

Table 4-1 shows that interspecies allometric scaling via BW3/4 is applicable over a wide 

range of functions and responses including kinetics and systemic toxicity (e.g., body weight 

loss). This has generally been demonstrated with information available from oral administration 

of pharmaceuticals.

Allometric scaling has not been extensively evaluated with toxicities such as those that 

occur at portal-of-entry tissues. ―Portal of entry‖ is a descriptor used for those effects caused by 

direct action of an agent on barrier tissues at or proximal to the point of agent entry, i.e., the 

respiratory tract for an inhaled agent and the gastrointestinal tract for an ingested agent. Agents 

causing portal-of-entry effects are often highly water soluble and/or highly reactive, such that 

concentrations achieved in these barrier tissues may be much higher than in blood. 

Portal-of-entry considerations – empirical relationships in feeding studies

A BW3/4 relationship exists among species in studies where dose is administered in food,

because interspecies food consumption follows a BW3/4 relationship (see Table 4-1). 

Consequently, exposure among species to an agent, both overall and to the gastrointestinal tract, 

present at a constant concentration in food would also follow a BW3/4 relationship. As with 

systemic effects, this empirical relationship implies that portal-of-entry effects could occur in 

increasingly larger species at decreasingly lower dose-rates in mg/kg-day. By extension, it is 

reasonable to apply the BW3/4 approach for gastrointestinally related, portal-of-entry effects.

Portal-of-entry considerations in the RfC Methods – theoretical application to oral exposures

In the case of the respiratory tract, RfC Methods (USEPA, 2002) provides a hierarchy for 

performing respiratory tract (portal-of-entry) dosimetry and derivation of an HEC with the 

default process proscribing use of species-specific physiological and anatomical measures such 

as ventilation volumes and rates and surface areas of various regions within the respiratory tract. 

The default HEC derivation is based on specifically where effects occur within these portal 

tissues. The paradigm is that dose to the target tissue will be related to risk of toxicity. Further, 

the Agency’s RfC methodology specifies different dosimetric approaches for agents manifesting 

portal-of-entry versus systemic effects, giving considerable discussion as to why different 

approaches are advised. Use of these procedures derives a DAF would be used in calculating an 

HEE or HEC. 
33

Considerable physical and anatomic parallels exist between inhalation and oral portal-ofentry tissues. Both the respiratory tract and the gastrointestinal tract have a central lumen for the 

passage of the agent, a metabolically active epithelial cell layer covered by saliva/mucous that 

lines the lumen, and submucosal tissues containing blood vessels and other elements. The 

transport processes determining movement of agent from the lumen, such as convection, 

diffusion, and metabolic clearance, as well as those determining movement at the lumen-tissue 

interface, are also commonalities. Conceptual similarity exists between inhalation and oral 

portal-of-entry exposure scenarios. In both cases, exposures would occur due to agents entrained 

in the incoming media (e.g., inspired air or ingested materials) to the surface of epithelial tissues, 

either respiratory or gastrointestinal. This scenario differs fundamentally from systemic 

exposure scenarios, where delivery of an agent is from circulating blood to organs throughout the 

body and uptake into target tissue. In systemic toxicity the most appropriate measure of dose 

generally would be the mass of agent per tissue over the relevant period of time, e.g., body 

weight/day or mg/kg. Based on the portal-of-entry scenario, however, the most appropriate dose 

metric would likely be mass of agent per surface area, e.g., mg/cm2. 

A parallel approach to the RfC derivation process could thus be considered appropriate 

for an interspecies DAF for oral portal of entry. The basic relationship of measures specified in 

the default inhalation scenario is ventilatory rate divided by surface area of the respiratory tract. 

Using the ventilatory volume as a surrogate for mass of agent inhaled, this relationship would 

result in units of ―mass per surface area‖, as discussed above. The parallel oral construction to 

the inhalation default construct of ventilatory volume (VE ) divided by respiratory tract surface 

area (SART) could be ingestion rate (Q) divided by the surface area of a specified region of the 

gastrointestinal tract (SAGI), such that:

VE / SART || Q / SAGI

In this case, the ingestion rate is the surrogate for mass of agent ingested. This relationship would 

similarly result in ―mass per surface area‖.

Implementation of such an approach requires interpretive analysis of existing 

information, or possibly generation of other specific information. These analyses and data could, 

for example, address such concerns as interspecies anatomical differences (such as the lack of a 

human anatomical parallel to the rodent forestomach), surface areas of the gastrointestinal tract 

in laboratory animals and humans, rates and scenarios of ingestion, or diffusion rates. 

Integration of these data into appropriate models (for example, those estimating clearance or 

fractional penetration [as per Aharonson et al., 1974, or Hanna et al., 2001] or a valid –
34

physiologically-based toxicokinetic [PBTK] model parameterized for both animal and humans)

would also facilitate implementation.

In the case of the gastrointestinal tract, however, such specific considerations have yet to 

be developed for oral portal-of-entry effects. Nevertheless, because of the conceptual similarities 

in exposure between these portal-of-entry tissues, it is possible that some approaches could be 

applied for ingestion of toxicants.

Recommendations regarding DAF and oral “portal-of-entry” effects

For oral portal-of-entry effects occurring in laboratory animal studies in which the agent 

is administered via food, direct application of the BW3/4 approach is recommended. This 

approach is generally consistent with existing Agency procedures for cancer assessment in 

calculation of oral slope factors, since they do not use portal-of-entry tumors for estimating 

systemic carcinogenic potency.

There exist conceptual similarities between the inhalation and oral portals of entry that 

could theoretically support development of a more sophisticated and refined oral portal-of-entry 

DAF. However, in the absence of an adequately developed theory and the further definition of 

information to develop and characterize such factor, this pragmatic and reasonable approach of 

applying the BW 3⁄4is recommended. 
35

APPENDIX D. HIERARCHY OF APPROACHES FOR INHALATION 

DOSIMETRY AND INTERSPECIES EXTRAPOLATION

(Adapted from Table 3-6 of USEPA, 1994)

________________________________________________________________________

Optimal Approach 

- Is based on sufficient data to support a model structure that will describe all 

significant mechanistic determinants of chemical disposition, toxicant-target 

interaction, and tissue response.

- Uses chemical-specific and species-specific parameters.

- Describes the dose metric at level of detail commensurate to response data.

Immediate Approaches

Default Approach

- Is based on general (non-chemical specific) understanding of mechanistic 

determinants of chemical disposition, toxicant-target interaction, and tissue response.

- May use categorical or default values for chemical- and species-specific parameters.

- Describes the dose metric at a generic level commensurate to response data.
36

GLOSSARY8

Allometry

The study of the usual variation in measurable characteristics of anatomy and physiology 

as a function of overall body size.

Allometric scaling

Scaling of physiological rates or quantities to relative growth and size (mass or volume) 

of one animal species relative to another animal species. The relationship is generally written as 

A = a(B)k, where A is the physiological process, B is a measure of the size of the organism (e.g., 

body weight) and a and k are constants (e.g. a equals 1 and k equals the exponent 0.75 as used in 

this document)9.

Benchmark Dose (BMD) or Concentration (BMC)

A dose or concentration that produces a predetermined change in response rate of an 

adverse effect (called the benchmark response or BMR) compared to background. 

BMDL or BMCL:

A statistical lower confidence limit on the dose or concentration at the BMD or BMC, 

respectively. 

Dosimetric adjustment factor (DAF)

Generally, a multiplicative factor used to adjust observed experimental data to human 

equivalent concentration or dose for an assumed ambient scenario, as used in this document 

refers specifically to a default adjustment factor for conversion of an animal oral dose to a human 

oral dose.

Human Equivalent Concentration (HEC) or Dose (HED)

The human concentration (for inhalation exposure) or dose (for other routes of exposure) 

of an agent that is believed to induce the same magnitude of toxic effect as the experimental 

animal species concentration or dose. This adjustment may incorporate toxicokinetic information 

 

8 Unless otherwise specified, all definitions are taken directly from official Agency guidance documents 

and may be referenced at http://www.epa.gov/iris/ or EPA (2005).

9

See Dedrick (1973) and U.S. EPA (1992a).
37

on the particular agent, if available, or use a default procedure, such as assuming that daily oral 

doses experienced for a lifetime are proportional to body weight raised to the 0.75 power. 

Mode of Action (MOA)

A less-detailed description of the mechanism of action in which some but not all of the

sequence of biological events leading to a toxic effect is known

Physiologically-based toxicokinetic model

A model that estimates the dose to a target tissue or organ by taking into account the rate 

of absorption into the body, distribution among target organs and tissues, metabolism, and 

excretion. 

Point of departure

The dose-response point that marks the beginning of a low-dose extrapolation. This point 

can be the lower bound on dose for an estimated incidence or a change in response level from a 

dose-response model (BMD), or a NOAEL or LOAEL for an observed incidence, or change in 

level of response.

Portal of entry 

The tissue or organ of first contact between the biological system and the toxicant, 

including. oral tissues, stomach, nasal or pulmonary tissues, skin.

Reference Concentration (RfC)

An estimate (with uncertainty spanning perhaps an order of magnitude) of a continuous 

inhalation exposure to the human population (including sensitive subgroups) that is likely to be 

without an appreciable risk of deleterious noncancer health effects during a lifetime. The 

inhalation reference concentration is for continuous inhalation exposures and is appropriately 

expressed in units of mg/m3.

Reference dose (RfD)

An estimate (with uncertainty spanning perhaps an order of magnitude) of a daily oral 

exposure for a chronic duration (up to a lifetime) to the human population (including sensitive 

subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime. 

It can be derived from a NOAEL, LOAEL, or benchmark dose, with uncertainty factors 

generally applied to reflect limitations of the data used, generally used in EPA's noncancer health 

assessments.
38

Regional deposited dose ratio (RDDR)

The ratio of the deposited dose in a respiratory tract region (r) for the laboratory animal 

species of interest (RDDA) to that of humans (RDDH). This ratio is used to adjust the observed 

particulate exposure effect level for interspecies dosimetric differences.

Regional gas dose ratio (RGDR)

The ratio of the deposited gas dose in a respiratory tract region (r) for the laboratory 

animal species of interest to that of humans. This ratio is used to adjust the observed gas 

exposure level for interspecies dosimetric differences.

Toxicodynamic

The determination and quantification of the sequence of events at the cellular and 

molecular levels leading to a toxic response to an environmental agent (sometimes referred to as 

pharmacodynamics).

Toxicokinetic

The determination and quantification of the time course of absorption, distribution, 

biotransformation, and excretion of chemicals (sometimes referred to as pharmacokinetics).

Uncertainty

Uncertainty occurs because of a lack of knowledge. It is not the same as variability. For 

example, a risk assessor may be very certain that different people drink different amounts of 

water but may be uncertain about how much variability there is in water intakes within the 

population. Uncertainty can often be reduced by collecting more and better data, whereas 

variability is an inherent property of the population being evaluated. Variability can be better 

characterized with more data but it cannot be reduced or eliminated. Efforts to clearly distinguish 

between variability and uncertainty are important for both risk assessment and risk 

characterization.

Uncertainty Factor (UF)

One of several, generally 3- to 10-fold factors, used in operationally deriving the

inhalation reference concentration (RfC) or oral reference dose (RfD) from experimental data. 

UFs are intended to account for (1) the variation in sensitivity among the members of the human 

population (UFH), (2) the uncertainty in extrapolating laboratory animal data to humans (UFA), 

(3) the uncertainty in extrapolating from data obtained in a study that is of less-than-lifetime 
39

exposure, (4) the uncertainty in using LOAEL data rather than NOAEL data, and (5) the inability 

of any single study to adequately address all possible adverse outcomes in humans. The RfC 

methods use 3 for the UF for interspecies extrapolation due to the incorporation of default 

dosimetric adjustments.