United States

Environmental Protection

Agency

Prevention, Pesticides

and Toxic Substances

(7101)

EPA 712–C–95–244

August 1995

Health Effects Test

Guidelines

OPPTS 870.7485

Metabolism and

Pharmacokinetics

‘‘Public Draft’’
i

INTRODUCTION

This guideline is one of a series of test guidelines that have been

developed by the Office of Prevention, Pesticides and Toxic Substances,

United States Environmental Protection Agency for use in the testing of

pesticides and toxic substances, and the development of test data that must

be submitted to the Agency for review under Federal regulations.

The Office of Prevention, Pesticides and Toxic Substances (OPPTS)

has developed this guideline through a process of harmonization that

blended the testing guidance and requirements that existed in the Office

of Pollution Prevention and Toxics (OPPT) and appeared in Title 40,

Chapter I, Subchapter R of the Code of Federal Regulations (CFR), the

Office of Pesticide Programs (OPP) which appeared in publications of the

National Technical Information Service (NTIS) and the guidelines published by the Organization for Economic Cooperation and Development

(OECD).

The purpose of harmonizing these guidelines into a single set of

OPPTS guidelines is to minimize variations among the testing procedures

that must be performed to meet the data requirements of the U. S. Environmental Protection Agency under the Toxic Substances Control Act (15

U.S.C. 2601) and the Federal Insecticide, Fungicide and Rodenticide Act

(7 U.S.C. 136, et seq.).

Public Draft Access Information: This draft guideline is part of a

series of related harmonized guidelines that need to be considered as a

unit. For copies: These guidelines are available electronically from the

EPA Public Access Gopher (gopher.epa.gov) under the heading ‘‘Environmental Test Methods and Guidelines’’ or in paper by contacting the

OPP Public Docket at (703) 305–5805 or by e-mail:

guidelines@epamail.epa.gov.

To Submit Comments: Interested persons are invited to submit comments. By mail: Public Docket and Freedom of Information Section, Office

of Pesticide Programs, Field Operations Division (7506C), Environmental

Protection Agency, 401 M St. SW., Washington, DC 20460. In person:

bring to: Rm. 1132, Crystal Mall #2, 1921 Jefferson Davis Highway, Arlington, VA. Comments may also be submitted electronically by sending

electronic mail (e-mail) to: guidelines@epamail.epa.gov.

Final Guideline Release: This document is available from the U.S.

Government Printing Office, Washington, DC 20402 on The Federal Bulletin Board. By modem dial 202–512–1387, telnet: federal.bbs.gpo.gov

3001, or call 202–512–1530 for disks or paper copies. This guideline is

available in ASCII and PDF (portable document format).
1

OPPTS 870.7485 Metabolism and pharmacokinetics

(a) Scope—(1) Applicability. This guideline is intended to meet testing requirements of both the Federal Insecticide, Fungicide, and

Rodenticide Act (FIFRA) (7 U.S.C. 136, et seq.) and the Toxic Substances

Control Act (TSCA) (15 U.S.C. 2601).

(2) Background. The source material used in developing this harmonized OPPTS test guideline are 40 CFR 798.7485, OPP guideline

85–1, and OECD Guideline 417.

(b) Purpose. (1) Testing of the disposition of a test substance is designed to obtain adequate information on its absorption, distribution,

biotransformation, and excretion and to aid in understanding the mechanism of toxicity. Basic pharmacokinetic parameters determined from these

studies will also provide information on the potential for accumulation of

the test substance in tissues and/or organs and the potential for induction

of biotransformation as a result of exposure to the test substance. These

data can be used to assess the adequacy and relevance of the extrapolation

of animal toxicity data (particularly chronic toxicity and/or carcinogenicity

data) to human risk assessment.

(2) Metabolism data can also be used to assist in determining whether

animal toxicity studies have adequately addressed any toxicity concerns

arising from exposure to plant metabolites, and in the setting of tolerances,

if any, for those metabolites in raw agricultural commodities.

(c) Definitions. The following definitions apply to this guideline:

Metabolism (biotransformation) is the sum of the processes by which

a foreign chemical is subjected to chemical change by living organisms.

LOEL is the lowest observable effect level.

NOEL is the no observable effect level.

Pharmacokinetics is the quantitation and determination of the time

course and dose dependency of the absorption, distribution,

biotransformation, and excretion of chemicals.

(d) Good laboratory practice standards. The pharmacokinetics and

metabolism tests outlined in this guideline are to conform to the laboratory

practices stipulated in 40 CFR parts 160 and 792.

(e) Test Procedures. Test procedures presented below utilize a tier

system to minimize the use of resources and to allow flexibility in the

conduct of metabolism studies. The proposed tier system will consist of

a basic data set (Tier 1) and additional studies (Tier 2) which may be

requested based upon the existing toxicology data base and/or the results

of Tier 1 testing which are found to impact upon the risk assessment process. For Tier 1 testing, the oral route will typically be required; however,
2

if the use pattern results in other types of exposure, other routes (dermal

and/or inhalation) may be required for initial testing of the disposition of

a chemical substance. Complete descriptions of the test procedures for

these other routes of exposure can be found under paragraph (i) of this

guideline. Except in unusual circumstances, the tiered approach to metabolism testing should apply to all listed routes of exposure.

(1) Pilot studies. The use of pilot studies is recommended and encouraged for the selection of experimental conditions for the

pharmacokinetics and metabolism studies (mass balance, analytical procedures, dose-finding, excretion of CO2, etc.).

(2) Animal selection—(i) Species. The rat is normally used for testing because it has been used extensively for metabolic and toxicological

studies. The use of other or additional species may be required if critical

toxicology studies demonstrate evidence of significant toxicity in these

species.

(ii) Strain. Adult animals of the strain chosen are to be used for the

determination of adverse health effects associated with the test substance.

(3) Material to be tested—(i) Test substance. (A) A test substance

radiolabeled using 14C should be used for all material balance and

metabolite identification aspects of the study. Other radioactive isotopes

such as 35S, 36Cl, and 113Sn or stable isotopes such as 15N and 18O may

be used, particularly if the element is responsible for or is a part of the

toxic portion of the compound. If it can be demonstrated that the material

balance and metabolite identification requirements can be met using

unlabeled test substance, radiolabeled compound need not be used. If possible, the radiolabel should be located in a core portion of the molecule

which is metabolically stable (it is not exchangeable, is not removed metabolically as CO2, and does not become part of the one-carbon pool of

the organism). Labeling of multiple sites of the molecule may be necessary

to follow the metabolic fate of the compound.

(B) The label should follow the test compound and/or its major

metabolites until excreted. The radiopurity of the radioactive test substance

is to be the highest attainable for a particular test substance (ideally it

should be greater than 95 percent) and reasonable effort should be made

to identify impurities present at or above 2 percent. The purity, along with

the identity of major impurities which have been identified, is to be reported. For other segments of the study, nonradioactive test substance may

be used if it can be demonstrated that the analytical specificity and sensitivity of the method used with nonradioactive test substance is equal to

or greater than that which could be obtained with the radiolabeled test

substance. The radioactive and nonradioactive test substances are to be

analyzed using an appropriate method to establish purity and identity. Additional guidance will be provided in chemical specific test rules to assist
3

in the definition and specifications of test substances composed of mixtures

and methods for determination of purity.

(ii) Administration of test substance. Test substance should be dissolved homogeneously in a known vehicle which does not significantly

alter (positively or negatively) the absorption of test material. The customary method of administration will be by oral gavage; however, administration by gelatin capsule or as a dietary mixture may be advantageous

in specific situations. Verification of the actual dose administered to each

animal should be provided.

(4) Tier testing. (i) The multiplicity of metabolic parameters that impact the outcome of toxicological evaluations preclude the use of a universal study design for routine toxicological evaluation of a test substance.

The usefulness of a particular study design depends upon the biological

activity of a compound and circumstances of exposure. For these reasons,

a tiered system is proposed for evaluation of the metabolism/kinetic properties of a test substance.

(ii) The first tier data set is a definitive study by the appropriate route

of exposure conducted in male rats to determine the routes and rate of

excretion and to identify excreted metabolites. First tier data will also provide basic information for additional testing (Tier 2) if such testing is considered necessary. In the majority of cases, Tier 1 data are expected to

satisfy regulatory requirements for biotransformation and pharmacokinetic

data on test chemicals.

(iii) Second tier testing describes a variety of metabolism/kinetic experiments which address specific questions based on the existing toxicology data base and/or those results of Tier 1 testing impacting significantly on the risk assessment process. For conduct of these studies, individualized protocols may be necessary. These can be developed as a cooperative effort between Agency and industry scientists.

(f) Tier 1 data requirements (minimum data set). At this initial

level of testing, biotransformation and pharmacokinetic data from a single

low dose group will be required. This study will determine the rate and

routes of excretion and the type of metabolites generated.

(1) Number and sex of animals. A minimum of four male young

adult animals will be required for Tier 1 testing. The use of both sexes

may be required in cases where there is evidence to support significant

sex-related differences in toxicity.

(2) Dose selection. (i) A single low dose is required for each route

of exposure. The low dose should be nontoxic, but high enough to allow

for metabolite identification in excreta. If no other toxicity data are available for selection of the low dose, a dose identified as a fraction of the

LD50 (as determined from acute toxicity studies) may be used. The mag-
4

nitude of the low dose used and any other dose levels used in Tier 1

studies should be justified in the final report.

(ii) For test substances of low toxicity a maximum dose of

1,000 mg/kg should be used; chemical-specific considerations may necessitate a higher maximum dose and will be addressed in specific test rules.

(3) Measurements—(i) Excretion. (A) Data obtained under this

paragraph (percent recovery of administered dose from urine, feces, and

expired air) will be used to determine the rate and extent of excretion

of test chemical, to assist in establishing mass balance, and will be used

in conjunction with pharmacokinetic parameters to determine the extent

of absorption. The quantities of radioactivity eliminated in the urine, feces,

and expired air are to be determined separately at appropriate time intervals.

(B) If a pilot study has shown that no significant amount of radioactivity is excreted in expired air, expired air need not be collected in

the definitive study.

(C) Each animal is to be placed in a separate metabolic unit for collection of excreta (urine, feces and expired air). At the end of each collection period, the metabolic units are to be rinsed with appropriate solvent

to ensure maximum recovery of radiolabel. Excreta collection is to be terminated at 7 days, or after at least 90 percent of the administered dose

has been recovered, whichever occurs first. The total quantities of radioactivity in urine are to be determined at 6, 12, and 24 h on day 1 of

collection, and daily thereafter until study termination, unless pilot studies

suggest alternate or additional time points for collection. The total quantities of radioactivity in feces should be determined on a daily basis beginning at 24 h post-dose, and daily thereafter until study termination. The

collection of CO2 and other volatile materials may be discontinued when

less than 1 percent of the administered dose is found in the exhaled air

during a 24-h collection period.

(ii) Tissue distribution. At the termination of the Tier 1 study, the

following tissues should be collected and stored frozen: Liver, fat, kidney,

spleen, whole blood, and residual carcass. If it is determined that a significant amount of the administered dose is unaccounted for in the excreta,

data on the percent of the dose in these tissues as well as residual carcass

will be requested. Additional tissues are to be included if there is evidence

of target organ toxicity from subchronic or chronic toxicity studies. For

other routes of exposure, specific tissues may also be required, such as

lungs in inhalation studies and skin in dermal studies. Certain techniques

currently at various stages of development, e.g. quantitative whole-body

autoradiography, may prove useful in determining if a test substance concentrates in certain organs or in determining a specific pattern of distribution within a given tissue. The use of such techniques is encouraged, but
5

not required, and may be employed to limit the number of tissues collected

to those shown to contain a measurable amount of radioactivity.

(iii) Metabolism. Excreta are to be collected for identification and

quantitation of unchanged test substance and metabolites as described

under paragraph (f)(3)(i) of this guideline. Pooling of excreta to facilitate

metabolite identification within a given dose group is acceptable. Profiling

of metabolites from each time period is recommended. However, if lack

of sample and/or radioactivity precludes this, pooling of urine as well as

pooling of feces across several time points is acceptable. Appropriate qualitative and quantitative methods are to be used to assay urine, feces, and

expired air from treated animals. Reasonable efforts should be made to

identify all metabolites and to provide a metabolic scheme for the test

chemical. Compounds which have been characterized in excreta as comprising 5 percent or greater of the administered dose should be identified.

If identification at this level is not possible, a justification/explanation

should be provided in the final report. Identification of metabolites representing less than 5 percent of the administered dose might be requested

if such data are needed for risk assessment of the test chemical. Structural

confirmation should be provided whenever possible. Validation of the

methods used in metabolite identification should be included.

(g) Tier 2 data requirements. Studies at the Tier 2 level are designed

to answer questions about the disposition of test chemicals based on the

existing toxicology data base and/or results of Tier 1 testing which may

have a significant impact on the risk assessment for the test chemical.

Such studies may address questions regarding absorption, persistence, or

distribution of the test chemical, or a definitive alteration in the metabolic

profile occurring with dose which may be of toxicological concern. Flexibility will be allowed in the design of specific experiments as warranted

by technological advances in this field.

(1) Absorption. (i) In cases where the extent of absorption has not

been established from Tier 1 studies, or where greater than 20 percent

of the administered dose is present in feces and has been identified as

parent chemical, a study to determine the extent of absorption will be required. This can be accomplished either through intravenous administration

of test material and measurement of radioactivity in excreta or after oral

administration of test material and measurement of radioactivity in bile.

(ii) For the intravenous study, a single low dose of test chemical using

an appropriate vehicle will be administered in a suitable volume (e.g.

1 mL/kg) at a suitable site to at least three male rats (both sexes might

be used if warranted). The disposition of the test chemical should be monitored for oral dosing as outlined in paragraph (f)(3)(i) of this guideline.

Metabolite identification will not be required for this study.
6

(iii) If a biliary excretion study is chosen the oral route of administration will be requested. In this study, the bile ducts of at least three rats

(or of both sexes, if warranted) should be appropriately cannulated and

a single low dose of the test chemical should be administered to these

rats. Following administration of the test chemical, excretion of radioactivity in bile should be monitored as long as necessary to determine if a

significant percentage of the administered dose is excreted via this route.

(2) Tissue distribution time course. (i) The correlation of levels of

metabolites or parent chemical in target tissues with toxicity is often useful

in determining the mechanisms of toxicity, especially when evidence of

target organ toxicity or extended half-life is apparent. For this reason, a

time course of tissue distribution in selected tissues will be requested to

aid in the determination of possible mechanisms of toxicity. The selection

of tissues for this type of study will be based upon available evidence

of target organ toxicity and/or carcinogenicity, and the number of time

points required will be based upon pharmacokinetic information obtained

from Tier 1 data. Flexibility will be allowed in the selection of time points

to be used in the study.

(ii) For this type of study, three rats per time point will be administered an appropriate oral dose of test chemical, and the time course of

distribution monitored in selected tissues. Only one sex may be required,

unless target organ toxicity is observed in sex-specific organs. Assessment

of tissue distribution will be made using appropriate techniques for assessment of total amount distributed to tissue and for assessment of metabolite

distribution.

(3) Plasma kinetics. The purpose of this experiment is to obtain estimates of basic pharmacokinetic parameters (half-life, volume of distribution, absorption rate constant) for the test substance. In addition, kinetic

data can also be used to substantiate questions about bioavailability and

whether clearance is saturated in a dose-dependent fashion. For this experiment a minimum of three rats is required. At least two doses will be required, usually the NOEL and LOEL from the critical toxicology study.

Following administration of test substance, samples should be obtained

from each animal at 30 min and again at 2, 4, 8, and 24 h using appropriate

sampling methodology. Different time points may be utilized if found

more suitable for obtaining pharmacokinetic estimates. Total radioactivity

present (or total amount of chemical, for nonradioactive materials) should

be analyzed in whole blood and plasma using appropriate methods, and

the blood/plasma ratio should be calculated.

(4) Induction. (i) Studies addressing the possible induction of metabolizing enzymes will be requested if:
7

(A) Available evidence shows a relationship between metabolism of

the test chemical and toxicity (in the form of target organ toxicity and/

or carcinogenicity).

(B) There is a nonlinear relationship between dose and metabolism.

(C) The results of tissue distribution studies show identification of

a potentially toxic metabolite.

(D) Induction can plausibly be invoked as a factor in such effects.

(ii) Several in vivo and in vitro methods are available for assessment

of enzyme induction, and the experiments which best address the issue

at hand can be determined between Agency and industry scientists. If induction is demonstrated, the relationship of this phenomenon to toxicity

observed from subchronic and/or chronic toxicity studies will need to be

addressed (mechanism of toxicity will need to be established).

(iii) If toxicologically significant alterations in the metabolic profile

of the test chemical are observed through either in vitro or in vivo experiments, characterization of the enzymes involved (for example, Phase I enzymes such as isozymes of the Cytochrome P450–dependent monooxygenase system, Phase II enzymes such as isozymes of sulfotransferase

or uridine diphosphate glucuronosyl transferase, or any other relevant enzymes) will be requested. This information will help establish the relevance of the involved enzymes to human risk, as it is known that certain

isozymes are present in animal species which are not present in humans,

and vice versa.

(5) Physiologically-based modeling. Traditional methods of modeling have been used to determine kinetic parameters associated with drug

and xenobiotic disposition, but have assumed a purely mathematical construct of mammalian organisms in their operation. On the other hand, more

recent models which take into account the physiological processes of the

animal have been used with success in defining biological determinants

of chemical disposition as well as the relationship between tissue dose

and tissue response. These so-called physiologically-based models, also

allow for cross-species extrapolation which is often necessary in the riskassessment process. The use of physiologically-based modeling as an experimental tool for addressing specific issues related to biotransformation

and pharmacokinetics of a test substance is encouraged. Information as

derived from physiologically-based modeling experiments may aid in the

comparison of biotransformation and pharmacokinetics of a test substance

between animal species and humans, and in the assessment of risk under

specific exposure conditions.

(h) Reporting of study results. In addition to the reporting requirements specified in 40 CFR part 792, subpart J, the completed study (Tier

1 or Tier 2) should be presented in the following format:
8

(1) Title/Cover Page. Title page and additional requirements (requirements for data submission, good laboratory practice, statements of data

confidentiality claims and quality assurance) if relevant to the study report,

should precede the content of the study formatted below. These requirements are to be found in 40 CFR parts 158 and 160 or parts 790, 792,

and 799.

(2) Table of Contents. A concise listing is to precede the body of

the report, containing all essential elements of the study and the page and

table number where the element is located in the final report of the study.

Essential elements of the Table of Contents should include a summary,

an introduction, the materials and methods section, results, discussion/ conclusions, references, tables, figures, appendices, and key subsections as

deemed appropriate. The Table of Contents should include the page number of each of these elements.

(3) Body of the report. The body of the report must include information required under 40 CFR 158.85(a) or this guideline, organized into

sections and paragraphs as follows:

(i) Summary. This section of the study report is to contain a summary

and analysis of the test results and a statement of the conclusions drawn

from the analysis. This section should highlight the nature and magnitude

of metabolites, tissue residue, rate of clearance, bioaccumulation potential,

sex differences, etc. The summary should be presented in sufficient detail

to permit independent evaluation of the findings.

(ii) Introduction. This section of the report should include the objectives of the study, guideline references, regulatory history, if any, and a

rationale.

(iii) Materials and methods. This section of the report is to include

detailed descriptions of all elements including:

(A) Test substance. (1) This section should include identification of

the test substance—chemical name, molecular structure, qualitative and

quantitative determination of its chemical composition, and type and quantities of any impurities whenever possible.

(2) This section should also include information on physical properties including physical state, color, gross solubility and/or partition coefficient, and stability.

(3) The type or description of any vehicle, diluents, suspending

agents, and emulsifiers or other materials used in administering the test

substance should be stated.

(4) If the test substance is radiolabeled, information on the following

should be included: The type of radionuclide, position of label, specific

activity, and radiopurity.
9

(B) Test animals. This section should include information on the test

animals, including: Species, strain, age at study initiation, sex, body

weight, health status, and animal husbandry.

(C) Methods. This subsection should include details of the study design and methodology used. It should include a description of:

(1) How the dosing solution was prepared and the type of solvent

used.

(2) Number of treatment groups and number of animals per group.

(3) Dosage levels and volume.

(4) Route of administration.

(5) Frequency of dosing.

(6) Fasting period (if used).

(7) Total radioactivity per animal.

(8) Animal handling.

(9) Sample collection.

(10) Sample handling.

(11) Analytical methods used for separation.

(12) Quantitation and identification of metabolites.

(13) Other experimental measurements and procedures employed (including validation of test methods for metabolite analysis).

(D) Statistical analysis. If statistical analysis is used to analyze the

study findings, sufficient information on the method of analysis and the

computer program employed should be included so that an independent

reviewer/statistician can reevaluate and reconstruct the analysis. Presentation of models should include a full description of the model to allow

independent reconstruction and validation of the model.

(iv) Results. All data should be summarized and tabulated with appropriate statistical evaluation and placed in the text of this section. Radioactivity counting data should be summarized and presented as appropriate

for the study, typically as disintegrations per minute and microgram or

milligram equivalents, although other units may be used. Graphic illustrations of the findings, reproduction of representative chromatographic and

spectrometric data, and proposed metabolic pathways and molecular structure of metabolites should be included in this section. In addition the following information is to be included in this section if applicable:
10

(A) Justification for modification of exposure conditions, if applicable.

(B) Justification for selection of dose levels for pharmacokinetic and

metabolism studies.

(C) Description of pilot studies used in the experimental design of

the pharmacokinetic and metabolism studies, if applicable.

(D) Quantity and percent recovery of radioactivity in urine, feces,

and expired air, as appropriate. For dermal studies, include recovery data

for treated skin, skin washes, and residual radioactivity in the covering

apparatus and metabolic unit as well as results of the dermal washing

study.

(E) Tissue distribution reported as percent of administered dose and

microgram equivalents per gram of tissue.

(F) Material balance developed from each study involving the assay

of body tissues and excreta.

(G) Plasma levels and pharmacokinetic parameters after administration by the relevant routes of exposure.

(H) Rate and extent of absorption of the test substance after administration by the relevant routes of exposure.

(I) Quantities of the test substance and metabolites (reported as percent of the administered dose) collected in excreta.

(J) Individual animal data.

(v) Discussion and conclusions. (A) In this section the authors should:

(1) Provide a plausible explanation of the metabolic pathway for the

test chemical.

(2) Emphasize species and sex differences whenever possible.

(3) Discuss the nature and magnitude of metabolites, rates of clearance, bioaccumulation potential, and level of tissue residues as appropriate.

(B) The authors should be able to derive a concise conclusion that

can be supported by the findings of the study.

(vi) Optional sections. The authors may include additional sections

such as appendices, bibliography, tables, etc.

(i) Alternate routes of exposure for Tier 1 testing—(1) Dermal—

(i) Dermal treatment. One or more dose levels of the test substance are

to be used in the dermal portion of the study. The low dose level should

be selected in accordance with paragraph (f)(2) of this guideline. The der-
11

mal doses are to be dissolved, if necessary, in a suitable vehicle and applied in volumes adequate to deliver the doses. Shortly before testing, fur

is to be clipped from the dorsal area of the trunk of the test animals.

Shaving may be employed, but it should be carried out approximately 24

h before the test. When clipping or shaving the fur, care should be taken

to avoid abrading the skin, which could alter its permeability. Approximately 10 percent of the body surface should be cleared for application

of the test substance—a minimum of 6 cm2 is to be used. With highly

toxic substances, the surface area covered may be less than approximately

10 percent, but as much of the area as possible is to be covered with

a thin and uniform film. The same nominal treatment surface area is to

be used for all dermal test groups. The dosed areas are to be protected

with a suitable covering which is secured in place. The animals are to

be housed separately.

(ii) Dermal washing study. (A) A washing experiment is to be conducted to assess the removal of the applied low dose of the test substance

by washing the treated skin area with a mild soap and water. A single

dose is to be applied to two animals in accordance with paragraph (f)(2)

of this guideline. After application (2 to 5 min) the treated areas of the

animals are to be washed with a mild soap and water. The amounts of

test substance recovered in the washes are to be determined to assess the

effectiveness of removal by washing.

(B) Unless precluded by corrosiveness, the test substance is to be

applied and kept on the skin for a minimum of 6 h. At the time of removal

of the covering, the treated area is to be washed following the procedure

as outlined in the dermal washing study. Both the covering and the washes

are to be analyzed for residual test substance. At the termination of the

studies, each animal is to be sacrificed and the treated skin removed. An

appropriate section of treated skin is to be analyzed to determine residual

radioactivity.

(2) Inhalation. One or more concentrations of test substance are to

be used in this portion of the study. The low concentration should be selected in accordance with paragraph (f)(2) of this guideline. Inhalation

treatments are to be conducted using a ‘‘nose-cone’’ or ‘‘head-only’’ apparatus to prevent absorption by alternate routes of exposure. If other inhalation exposure conditions are proposed for use in a chemical-specific test

rule, justification for the modification must be documented. A single exposure over a defined period is to be used for each group—a typical exposure

is 4–6 h.