Federal Register  /  Vol. 51.  No. 185 / Wednesday. September  24. 1988  / Notices


 Guidelines for the Heatth Risk
 Assessment of Chemical Mixtures

 AGENCY: U.S. Environmental Protection
 Agency (EPA).
 ACTION: I-'inal Guidelines for the Health
 Risk Assossment of Chemical Mixtures.

 SUMMARY: The U.S. Environmental
 Protection Agency is today Issuing five
 guidelines for assessing the health risks
 of environmental pollutants. These are:
 Guidelines for Carcinogen Risk
 Guidelines for Estimating Exposures
 Guidelines for Mutagenicity Risk
 Guidelines for the Health Assessment of
   Suspect Developmental Toxicants
 Guidelines for the Health Risk
   Assessment of Chemical Mixtures
   This notice contains the Guidelines
 for the Health Risk Assessment of
 Chemical Mixtures; the other guidelines
 appear elsewhei-e in  today's Federal
   The Guidelines for the Health Risk
 Assessment of Chemical Mixtures
 (hereafter "Guidelines") are intended to
 guide Agency analysis of information
 relating to health effects data on
 chemical mixtures in line with the
 policies and procedures established In
_illS-?.tatujes,adml.nisteredj)y th_e..EPA»	
 These Guidelines wera developed as
 part of an interoffice guidelines
 development program under the
 auspices of the Office of Health and
 Environmental Assessment (OHEA) in
 '.he  Agency's Office of Research and
 Development. They reflect Agency
.consideration of public and Science
 Advisory Board (SAB) comments on the
 Proposed Guidelines for the Health Risk
 Assessment of Chemical Mixtures
 published January 9,1985 (50 FR 1170).
   This publication completes the first
 round of risk assessment guidelines
 development. These  Guidelines will be
 revised, and new guidelines will be
 developed, as appropriate.
 EFFECTIVE DATE: The Guidelines will be
 effective September  24.1988.
 Dr.  Richard Hertzberg, Methods
 Evaluation and Development Staff,
 Environmental Criteria and Assessment
 Office, U.S. Environmental Protection
 Agency, 26 W. St. Clair Street,
 Cincinnati, OH 45208, 513-569-7582.
 the  National Academy of Sciences
 (NAS) published its book entitled Risk
 Assessment in the Federal Government:
                        Managing the Process. In that book, the
                        NAS recommended that Federal •
                        regulatory agencies establish "foference
                        guidelines" to ensure consistency and
                        technical quality in risk assessments
                        and to ensure that the risk assessment
                        process was maintained as a scientific
                        effort separate from risk management. A
                        task force within EPA accepted that
                        recommendation and requested titat
                        Agency scientists begin to develop such


                         The guidelines published today ara
                        products of a two-year Agencywida
                        effort, which has included many
                        scientists from the larger scientific
                        community. These guidelines s«t forth
                        principles and procedures to guide EPA
                        scientists in the conduct of Agency rink
                        assessments, and to inform Agency
                        decision makers and the public about
                        these procedures. In particular, the
                        guidelines emphasize that risk
                        assessments will be conducted on a
                        case-by-case basis, giving full
                        consideration to all relevant scientific
                        Information. This  case-by-case approach
                        means that Agency experts review the
                        scientific information on each agent and
                        use the most scientifically appropriate
                        interpretation to assess risk. The
                        guidelines also stress that this
                        information will be fully presented in
                        Agency risk assessment documents, and
                        that Agency scientists will identify the
                        strengths and weaknesses of each
                        assessment by describing' uncertainties,
                        assumptions, and limitations, ar well as
                        the scientific basis and rationale for
                        each assessment.
                         Finally, the guidelines are formulated
                        in part to bridge gaps in risk assessment
                        methodology and data. By identifying
                        these gaps and the importance of She
                        missing information to the risk
                        assessment process, EPA wishes to
                        encourage research and analysis that
                        will lead to new risk assessment
                        methods and data.
                        Guidelines for Health Risk Assessment
                        of Chemical Mixtures

                         Work on the Guidelines for the Health
                        Risk Assessment-of Chemical Mixtures
                        began in January  1984. Draft guidelines
                        were developed by Agency work group*
                        composed of expert scientists from
                        throughout the Agency. The drafts were
                        peer-reviewed by expert scientists in the
                        fields of toxicology, pharmacoldnetics.
                        and statistics from universities.
                        environmental groups, industry, labor,
                        and other governmental agencies. They
                        were then proposed for public comment
                        in the Federal Register (50 FR 1170}. On
                        November 9,1984. the Administrator
                        directed that Agency offices use the
 proposed guidelines in performing risk
 assessments until final guidelines
 become available.
  After the close of the public comment
 period. Agency staff prepared
 summaries of the comments, analyses of
 the major Issues presented by the
 commentors, and preliminary Agency
 responses to those comments. These
 analyses were presented to review
 panels of the SAB on March 4 and April
 22-23, 1985, and to the Executive
 Committee of the SAB on April 25-26.
 1965. The SAB meetings were  -   »
 announced in the Federal Register as
 follows: February 12. 1985 (50 FR 5811)
 and April 4. 1985 (50 FR 13420 and
  In a letter to the Administrator dated
 June 19, 1985, the Executive Committee
 generally concurred on all five of the
 guidelines, but recommended certain
 revisions, and requested that any
 revised guidelines be submitted to the
 appropriate SAB review panel chairman
 for review and concurrence on behalf of
 the Executive Committee. As described
 in the responses to comments (see Part
 B: Response to the Public and Science
 Advisory Board Comments), each
 guidelines document was revised, where
 appropriate, consistent with the SAB
 recommendations, and revised draft
 guidelines were submitted to the panel
 chairmen. Revised draft Guidelines for
 the Health Risk Assessment of chemical
 mixtures were concurred on in a letter
•letters are available at the Public
 Information Reference Unit. EPA
 Headquarters Library, a's" indicated
 elsewhere In this notice.
   Following this Preamble are two parts:
 Part A contains the Guidelines and Part
 B. the Response to the Public and
 Science Advisory Board Comments (a
 summary of the major public comments,
 SAB comments, and Agency responses
 to those comments).
   The SAB requested that the Agency
 develop a technical support document
 for these Guidelines. The SAB identified
 tha need for this type of document due
 to th« limited knowledge on interactions
 of chemicals in biological systems.
 Because of this, the SAB commented
 that progress in improving risk
 assessment will be particularly
 dependent upon progress in the science
 of interactions.
   Agency staff have  begun preliminary
 work on the technical support document
 and expect it to be completed by early
 1987. The Agency is continuing to study
 the risk assessment issues raised in the
 guidelines and will revise these
 Guidelines in line with new information
 as appropriate.


                Federal  Register / Vol.  51. No. 185 /  Wednesday, September 24. 1986 / Notices
  References, supporting documents,
and comments received on the proposed
guidelines, as well as copies of the final
guidelines, are available for inspection
and copying at the Public Information
Referenco Unit (202-382-5926). EPA
Headquarters Library, 401 M Street, SW.
Washington, DC. between the hours of
8:00 a.m.-and 4:30 p.m.
  I certify that these Guidelines are not
major rules as defined by Executive
Order 12:291, because they are
nonbinding policy statements and have
no direct effect on the regulated
community. Therefore  '.hey will have no
effect on costs or prices, and they will
have no other significant adverse effects
on the economy. These Guidelines were
reviewed by  the Office of Management
and Budget under Executive Order
  Dated: August 22, 190fl.
Le« M.Tl>omn«,
Administrator.           '
Part  A:  Guidelines  for  tho  Health  Risk
  Assessment of Chemical Mixtures
/. Introduction
II. Proposed Approach
  A. Dntn Available on the Mixture of Con-
  B. Data Available on Similar Mixtures
  C. Datu Available Only on Mixture Com-
   1. Systemic Toxicants
   2. Carcinogens
   3. Interactions
   4. Uncertainties
     a. Health Effects         	.
     b. Exposure Uncertainties
     c.  Uncertainties Regarding Composi-
        tion of the Mixture
///. Assumptions and Limitations
  A. Information on Interactions
  B. Additivity Models
IV. Mathematical Models and the Measure-
  ment of Joint Action
  A. Dose Addition
  C. Response Addition
  C. Interactions
V. Hvfpn'nces
Part B: Response to Public and Science Advi-
  sory Board Comments
   /. Introduction
   II. Recommended Procedures
  A.. Definitions
  B. Mixtures of  Carcinogens and  Systemic
///. Additivity Assumption
  A. Complex Mixtures
  B. Dose Additivity
  C. Interpretation of the Hazard Index
  D. Use of Interaction Data
IV. Uncertainties  and the Sufficiency of the
  Datu Vase
V Need for a Technical Support Document
 Part A: Guidelines for the Health Risk
 Assessment of Chemical Mixtures
 /. Introduction

   The primary purpose of this document
 is to generate a consistent Agency
 approach for evaluating data on the
 chronic and subchronic effects of
 chemical mixtures. It is a procedural
 guide that emphasizes broad underlying
 principles of the various  science
 disciplines (toxicology, pharmacology,
 statistics) necessary for assessing health
 risk from chemical mixture exposure.
 Approaches to be used with respect to
 the analysis and evaluation of the
 various data are also discussed.
   It is not the intent of these Guidelines
 to regulate any social or  economic
 aspects concerning risk of injury to
 human health or the environment
 .caused by exposure to a  chemical
 agent{s). AH such action  is addressed in
 specific statutes and federal legislation
 and is independent of these Guidelines.
   While some potential environmental
 hazards involve significant exposure to
 only a single compound,  most instances
 of environmental contamination involve
 concurrent or sequential exposures to a
 mixture of compounds that may induce
 similar or dissimilar effects over
 exposure periods ranging from short-
 term to lifetime. For the purposes of
 these Guidelines, mixtures will be
 defined as any combination of two or
 more chemical substances regardless of
 source or of spatial or temporal
 proximity. In some instances, the
-•mbctttres are-highry complex-correhrlmg -
 of scores of compounds that are
 generated simultaneously as by-
 products from a single source or process
 (e.g., coke oven emissions and diesel
 exhaust). In other cases, complex
 mixtures of related compounds are
 produced as commercial products (e.g.,
 PCBs, gasoline and pesticide
 formulations) and eventually released to
 the environment. Another class of
 mixtures consists of compounds, often
 unrelated chemically or commercially,
 which are placed in the samo area for
 disposal or storage, eventually come
 into contact with e^ch other, and are
 released as a  mixture to  the
 environment. The qualily snd quantity
 of pertinent information  evailable for
 risk assessment varies considerably for
 different mixtures. Occasionally, the
 chemical composition of a mixture is
 well characterized, levels of exposure to
 the population are known, and detailed
 toxicologic data on the mixture are
 available. Most frequently, not all
 components of the mixiure are known,
 exposure data are uncertain, and
 toxicologic data on the known
 components of the mixture are limited.
 Nonetheless, the Agency may be
 required to take action because of the
 number of individuals at potential risk
 or because of the known toxicologic
 effects of these compounds that  have
 been identified in the mixture.
   The prediction of how specific
 mixtures of toxicants will interact must
 be based on an understanding of the
 mechanisms of such interactions. Most
 reviews and texts that discuss toxicant
 interactions attempt to discuss the  '
 biological or chemical bases of the
 interactions (e.g., Kteaseen and Doull,
 1990; Levine. 1973: Goldstein et al.. 1974;
 NRC, 1980a; Veldslra. 1956; Withey,
 1981). Although different authors use
 somewhat different classification
 schemes when discussing the ways in
 which toxicants Interact. It generally Is
 recognized that toxicant interactions
 may occur during any of the toxicologic
 processes that take place with a single
 compound: absorption, distribution,
 metabolism, excretion, and activity at
 the receptor site(s). Compounds  may
 interact chemically, yielding a new toxic
 component or causing a change  in the
 biological availability of the existing
 component. They may also interact by
 causing different effects at different
 receptor sites.
   Because of the uncertainties inherent
 in predicting the magnitude and nature
 of toxicant interactions, the assessment
• of health risk from chemical mixtures —•
 must include a thorough discussion of
 all assumptions. No single approach Is
 recommended in these Guidelines.
 Instead, guidance is given for the use of
 several approaches depending on the
 nature and quality of the data.
 Additional mathematical details are
 presented in section IV.
   In addition to these Guidelines, a
 supplemental technical support
 document is being developer! which will
 contain a thorough review of all
 available Information on the toxicity of
 chemical mixtures  and a discussion of
 research needs.

 //. Proposed Approach
   No single approach can be
 recommended to risk assessments for
 multiple chemical exposures.
 Nonetheless, general guidelines  can be
 recommended depending on the type of
 mixture, the known toxic effects of its
 components, the availability of toxicity
 data on the mixture or similar mixtures,


Federal Register / Vol. 51, No'.' 185  /Wednesday, September 24, 1988 / Notices
 the known or anticipated interactions
 amoiig components of the mixture, and
 the quality of the exposure data. Given
 the complexity of this issue and the
 relative paucity of empirical data from
 which sound generalizations can.be
 constructed, emphasis must be placed
 on flexibility, judgment, and a clear
 articulation of the asrumptions and
 limitations in any risk assessment that is
 developed. The proposed approach Is
 summarized in Table 1 and Figure 1 and
 is detailed below. An alphanumeric
 scheme for ranking the quality of the
 data used in the risk assessment is given
 In Table 2.

 A. Data Available on the Mixture of
   For predicting the effects of  •
 subchronic or chronic exposure to
 mixtures, the preferred approach usually
 will be to use subchronic or chronic
 health effects d&ia oil the mixture of
 concern and adopt procedures similar to
 those used for single  compounds, either
 systemic toxicants or cavcinogens (see
 U.S. EPA, 1986a-c). The risk assessor
 must recognize, however, that dose-
 response models used for single
 compounds are often based on
 biological mechanisms of the toxicity of
 single compounds, and may not be as
 well justified when applied to the
 mixture as a whole. Such data are most
 likely to be available on highly complex
 mixtures, such as coke oven emissions
 or diesf:! exhaust, which are generated
 in large quantities and associated with
- orsttspected of causing adverse health •
 effects. Attention should also be given
 to the persistence of  the mixture in the
 environment as well  as to the variability
                          of the mixture composition over time or
                          from different sources of emissions. If
                          the components of the mixture are
                          known to partition Into different
                          environmental compartments or to
                          degrade or transform at different rates
                          in the environment, then those factors
                          must also be taken into account, or the
                          confidence in and applicability of the
                          risk assessment is diminished.
                          T»bl« 1.—Risk Assessment Approach for
                          Chemical Mixture*
                            1. As»e»« the quality of the data on
                          Interactions, health effects, and exposure (see
                          Table 2).       .
                            a. If adequate, proceed to Step 2.
                            b. If Inadequate, proceed to Step 14.
                            2. Health effects Information Is available
                          on the chemical mixture of concern.
                            a. If yes. proceed to Step 3.
                            b. If no, proceed to Step 4.
                          •  3. Conduct risk assessment on the mixture
                          of concern based on health effects data on
                          the mixture. Use the same procedures as
                          those for single compounds. Proceed to Step 7
                          (optional) and Step 12.
                            4. Health effects Information Is available
                          on a mixture that Is similar to the mixture of
                            a. If yes. proceed to Step 5.
                            b. If no, proceed to Step 7.
                            S. Assess the similarity of the mixture on
                          which health effects data are available to the
                          mixture of concern, with emphasis on any
                          differences In components or proportions of
                          components, as well as the effects that such
                          differences would have on biological activity.
                            a. If sufficiently similar, proceed to Step 6.
                            b. If hot sufficiently similar,.proceed to
                          Step 7.
                            6. Conduct risk assessment onjhe mixture
                          of concern based on health effects data on
                          the similar mixture. Use the same procedures
                          as those for single compounds. Proceed to
                          Step 7 (optional) and Step 12.
  7. Compile health effects and exposure
information on the components of the
  8. Derive appropriate Indices of acceptable
exposure and/or risk on the Individual
components In the mixture. Proceed to Step 9.
  9. Aosess data on Interactions of
components In the mlKtursr-
  a. V sufficient quantitative data are
available on the interactions of two or more
components In the mixture, proceed to Step
  b. If sufficient quantitative data are not
available, use whatever Information is
available to qualitatively Indicate the nature
of potential Interactions. Proceed to Step 11.  .
  10. Use an appropriate Interaction moilol to
combine risk assessments on compounds for
which data are adequate, find use an
addltlvlty assumption Tor the remaining
compounds. Proceed to Slop 11 (optional) and.
Step 12.
  11. Develop v risk assessment based on an
addltlvlty approach for all compounds In the
mixture. Proceed to Step 12. .
  12. Compare risk assessments conducted In
Steps 5. 8. and 9. Identify and justify the
preferred assessment, and quantify
uucertalnty, if possible. Proceed to Step 13.
  13. Develop an Integrated summary of the
qualitative and quantitative assessments
with special emphasis on uncertainties and
assumptions. Classify the overall quality of
the risk assessment, as Indicated in Table 2.
  14. No risk assessment can be conducted
because of Inadequate data on Interactions,
health effects, or exposure. Qualitatively
assess the nature of any potential hazard and
detail the types of additional data necessary
to support a risk assessment. Stop.
  Note.—Several decisions usad here,
especially those concerning adequacy of data
amd similarity batween'lwo mixtures, are not
precisely characterized and will require
considerable judgment. See text


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Federal Register / Vol. 51, No: 185 /  Wednesday,  September 24. 1886  / Notices
Table 2.—Classification Schema for the
Quality of the Risk Assessment of the

Information on Interactions
  I.'Assessment ts based on data on the
mixture of concern.
  II. Assessment Is based on data on a
sufficiently similar mixture.
  HI. Quantitative interactions of components
are well characterized.
  IV. The assumption of addltlvlty Is justified
based on the'nature of the health effect? end
on the number of component compounds.
  V. An assumption of addltlvlty cannot bo
justified, and no quantitative risk assessment
can be conducted.

Health Effects Information
  A. Full health effects data ere available
and relatively minor extrapolation Is
.  B. Full health effects data are available but
extensive extrapolation ts required for route
or duration of exposure or for species
differences. These extrapolations are
supported by pharmacoklnelic
considerations, empirical observations, or
other relevant Information.
  C. full health effects data are available.
but extensive extrapolation is required for
route or duration of exposure or for species
differences. These extrapolations are not
directly supported by the-Information
  D. Certain important health effects data are
lacking and extensive extrapolations are
required for route or duration of exposure or
for species differences.
  E. A lack of health effects information on
the mixture and Us components In the
mfxlure'precludes a quantitative risk

Exposure 'Information'
  1. Monitoring information either alone or in
combination witK modeling information is
sufficient to accurately characterize human
exposure to the mixture or Its components.
  2. Modeling information is sufficient to
reasonably chararisrize human exposure to
the mixUire or Its components.
  3. Exposure estimates for some components
are lacking, uncertain, or variable.
Information on health effects or
environmental chemistry ruggest  that this
limitation is not akely to substantially affect
the risk assessment.
  4. Not all components in the mixture have
been Identified or levels of exposure are
highly uncertain or variable. Information on
health effects or environmental chemistry is
not sufficient to assess the effect of this
limitation on the risk assessment.
   5. The available exposure information is
tastilficient for conducting a riak assessment.
  'See text ford'«cus»ic-.i>f ic-ffic'c'-l similarity.
 .••dequacy of data, an''. j>:>tKicMian for t^dilivity
  * See the Agcncj « .. •it?'y.:ni r f"f I .:'imating
 r^'tr.tires (U.S. E"\."  '>ij f'.»r on.rp -omplcte
 fi-,r;r.-'7'.ior on perio   .  ..-C'.r - asjensmnnU
 and . -limiting the quality   T. .o»ure dels.
                         B. Data Available on Similar Mixtures
                           If the risk assessment is based on
                         data from a single mixture that is known
                         to be generated with varying
                         compositions depending on time or
                         different emission sources, then the
                         confidence in the applicability of the
                         data to a risk assessment also is
                         diminished. This can be offset to some
                         degree'if data are available on several
                         mixtures of the same components that
                         have different component ratios, which
                         encompass the temporal'or spatial
                         differences in composition of the
                         mixture of concern. If such data are
                         available, an attempt should be made to
                         determine if significant and systematic
                         differences exist among the chemical
                         mixtures. If significant differences  are
                         noted,  ranges of risk can be estimated
                         based on the toxicologic data of the
                         various mixtures. If no significant
                         differences are noted, then a single risk
                         assessment may be adequate, although
                         the range of ratios of the components in
                         the mixtures to which the risk
                         assessment applies should also be given.
                           If no data are available on the
                         mixtures of concern, but health effects
                         data are available on a similar mixture
                         (i.e., a mixture having the same
                         components but in slightly different
                         ratios,  or having several common
                         components but lacking one or more
                         components, or having one or more
                         additional components), a decision must
                         be made whether the mixture on which
                         health  effects data are availablais.or is
                         not "sufficiently similar" to the mixture
                         of concern to permit a risk assessment.
                         The  determination of "sufficient
                         similarity" muut be made on a case-by-
                         case basis, considering not only the
                         uncertainties associated with using datn
                         on a dissimilar mixture  but also the
                         uncertainties of using other approaches
                         such as additivity. In determining
                         reasonable similarity, consideration
                         should be given to any information on
                         the components that differ or are
                         contained in markedly different
                         proportions between the mixture on
                         which health effects data are available
                         and the mixture of concern.  Particular
                         emphasis should be placed on any
                         toxicologic or pharmacokinetic data on
                         the components or the mixtures which
                         would be useful in assessing the
                         significance of any chemical difference
                         between the similar mixture and the
                         mixtures of concern.
                            Even if a risk assessment can be made
                         using data on the mixtures of concern or
                         a reasonably similar mixture, it may be
                         desirable to conduct a risk assessment
                         based on toxicity data on the
                         components in the mixture using the
                         procedure outlined in section H.B. In the
case of a mixture containing carcinogens
and toxicants, an approach based on the
mixture data alone may not be
sufficiently protective in all cases. For
example, this approach for a two-
component mixture of one carcinogen
and on* toxicant would use toxicity
data ou the mixture of the two
compounds. However, in a chronic study
of such a mixture, the presence of the
toxicant could mask the activity of !.'••"
carcinogen. That is to.say, at doses  of
the mixture sufficient to induce a
carcinogenic effect,' the toxicant could
Induce mortality so that at the maximum
tolerated dose of the mixture,  no
carcinogenic effect could be observed.
Since carclnogenlclty is considered by
the Agency to be a nonthreshold effect,
it may not be prudent to construe the
negative results of such a Moassay  as
indicating the absence of risk  at lower
doses. Consequently, the mixture
approach should be modified to allow
the risk assessor tc evaluate the
potential for masking, of c:ie effect  by
another, on a case-by-case basis.

C. Data Available Only on Mixture

  If data are not available on  an
identical or reasonably similar mixture,
the risk assessment may be based on
the toxic or carcinogenic properties of
the components in the mixture. When  •
little or no quantitative information is
available on the potential Interaction
(defined In the next section) are
recommended for systemic '.oxicantc.
Several studies have demonstrated that
dose additive models often predict
reasonably well the lox.ir.itie-: of
mixtures composed of a substantial
variety of both similar and dissimilar
compounds (Pozzani et al., 1959; Smyth
et al., 1969.1970; Murphy, 1980). The"
problem of multiple toxicant exposure
has.been addressed by the American
Conference of Governmental  Industrial
Hygienists (ACG1H, 1983). the
Occupational Safety and Health
Administration (OSHA, 1983), the World
Health Organization (WHO. 1981),  and
the National Research Council (NRC,
1980a,'b). Although the focus  and
purpose of each group was somewhat
different, all groups that recommended
an approach elected to adopt some type
of dose additive model. Nonetheless, as
discussed in section IV, dose  additive
models are not the most biologically
plausible approach if the compounds do
not  have the same mode of toxicologic
action. Consequently, depending on the
nature of the risk assessment and the
 available information on modes of
 action and patterns of joint action, the


                Federal Register / Vol.. 51. Np.  185 /Wednesday.'September'24. 1986 / Notices          34010
most reasonable additive model should
be used.
  1. Systemic Toxicants. For systemic
toxicants, the current risk assessment
methodology used by the Agency for
single compounds most often results in
the 'derivation of an exposure level
which is not anticipated to cause
significHnt adverse effects. Depending
on the route of exposure, media of
concern, and the legislative mandate
guiding the risk assessments, these
exposure levels may be expressed In a
variety of ways such as acceptable daily
intakes (ADIs) or reference doses
(UfDs). levels associated with various
margins of safety (MOS), or acceptable
concentrations in various media. For the
purpose of this discussion, the term
"acceptable level" (AL) will be used to
indicate any such crileri- or advisories
derived by the Agency. Levels of
exposure (E) will be estimates obtained
following  the mosi current Agsncy
Guidelines for Estimating Exposures
(U.S. EPA. 1930d). For such estimates,
the "hazard index"  (HI) of a mixture
based on the assumption of dose
addition may be defined ssr
                  r. . . +E./AL,  (11-1)
E, = exposure level to the ilh toxicant* and
ALi=maximum acceptable level for the iu
Since the assumption of dose addition is
most p'-.^perly applied to compounds
that induce the same effect by similar
modes of action, a separate hazard
index should be generated for each end. .
point (if concern. Dose addition for
dissimilar effects does not have strong
scientific support, and, if done,  should
be justified on a case-by-case basis  in
terms of biological plausibility.
   The assumption of dose addition in
most clearly justified when the
mechanisms of action of the compounds
under consideration are known to be the
same. Since the mechanisms of action
for most compounds are not well
understood, the justification of  the
assumption of dose addition will often
be limited  to similarities in
pharmacokinclic and toxicologic
characteristics. In any event, if  a hazard
index is generated, the quality of the
experimental evidence supporting the
assumption of dose addition must be
clearly articulated.
   The hazard index provides a  rough
measure of likely toxicity and requires
cautious interpretation. The hazard
index is only a numerical indication of
the nearness to acceptable limits of
exposure or UIH degree to which
acceptable exposure levels are
exceeded. As this Index approaches
unity, concern for the potential hazard
of the mixture increases. If the index
exceeds unity, the concern is the same
as if an individual chemicpl exposure
exceeded Its acceptable level by the
same proportion. The hazard index does
not define dose-response relationships,
and Its numerical value should not be
construed to be a direct estimate of risk.
Nonetheless,  If sufficient data are
available to derive Individual
acceptable levels for a spectrum of
effects (e.g., MFO induction, minimal
effects !n several organs, reproductive
effects, and behavioral effects), the
hazard index may suggest what types of
effects might  be expected from the
mixture exposure. If the components'
variabilities of the acceptable levels arc
known, or if the acceptable levels are
given as  ranges (e.g., associated with
different margins of safety), then the
hazard index should be presented with
corresponding estimates of variation or
  Most studies on systemic toxicity
report only descriptions of the effects in
each dose group. If dose-response
curves are estimated for systemic
toxicants, however, dose-additive or
response-additive assumptions can be
used, with preference given to the ir.v>st
biologically plausible assumption (see
section IV for the mathematical details).
  2. Carcinogens. For carcinogens,
whenever linearity of the individual
doserr_espon5ej;uryjEs.has bne,n assjurned
(usually  restricted to low doses), the
increase in risk P (also called excess or
incremental risk), caused by exposure d,
is related to carcinogenic potency B. as:
For multiple compounds, this equation
may be generalized to:
  * See the Agcucy'i guideline (U.S. EPA. 190rtd)
 for informBlion on how to estimate this value.
This equation assumes independence of
action by the several carcinogens and is
equivalp.nt to the aosumption of dose
addition as well as to response addition
with completely negative correlation of
tolerance, as long as P < 1 (see section
IV). Analogous to the  procedure used in
equation II-l for systemic toxicants, an
index for n carcinogens can be
developed by dividing exposure levels
(E) by doses (DR) associated with a set
level of risk:
H1 = E,/DR, + E,/DR,+.  .  .+E./DR.  (H-4)
Note that the less linear the dose-
response curve is, the less appropriate
equations II-3 and II-4 will be, perhaps
even at low doses. It should be
emphasized that because of the
uncertainties in estimating dose-
response relationships for single
compounds, and the additional
uncertainties in combining the
individual estimate to assess response
from exposure to mixtures, response
rates ant? hazard indices may hava merit
in comparing risks but should net be
regarded as measures of absolute risk.
  3. Interactions. None of Ihe above
equations Incorporates any form of
synerglstic or antagonistic interaction.
Some types of Information, however,
may be available that suggest that two
or more components in the mixture may
interact. Such information must be*
assessed in terms of both its relevance
to subchronlc or chronic hazard and Us
suitability for quantitatively altering tho
risk assessment.
  For example, if chronic or subchronic
toxicity or carcinogenlclty studies have
been conducted that permit a
quantitative estimation of Interaction for
two chemicals, then It may be desirable
to consider using equations detailed in
section IV, or modifications  of these
equations,.to treat ths two compounds
as a single toxicant with greater or
lesser potency than would be predicted
 from additivity. Other coraponentb i
 the  mixture, on which no such
 interaction data are available, coulrt
 then be separately treated in an additive
 manner. Ucfore such n procedure is
 adopted, howevsr, a discussion should
 be presented of the likelihood that other
 compounds in the mixture may interfere
 with the interaction of the two toxicants
.. on. which .quantitative .interaction dale
 are available. If the weight of evidence
 suggests that interference is likely, then
 a quantitative alteration of the risk
 assessment rt.ay not be justified. In such
 casec, the risk assessment may only
 indicate the likely nature of interaction^
 either synergistic or antagonistic, and
 not quantify their magnitudes.
   Other types ofinformati.on, such as
 •.hose relating 10 mechanisms of toxicant
 interaction, or quantitative estimates of
 interaction between two chemicals
 derived from acute studies,  are even less
 likely to be of use In the quantitative
 assessment of long-term health risks.
 Usually it will be appropriate only to
 discuss these types of Information,
 Indicate the relevance of the Information
 to subchronic or chronic exposure, and
 indicate, if possible, the nature of
 potential interactions, without
 attempting to quantify their magnitudes.
   When the interactions are expected to
 have a minor influence on the mixture's
 toxicity, the assessment should Indicate,
 when possible, the compounds most
 responsible for the predicted toxicity.
 This judgment should be based on
 predicted toxicity of each component.


   34020	     Federal  Register / Vol. 51. No; 185 / Wednesday.  September 24. 1988  /  Notices
   based on exposure and toxic or
   carcinogenic potential. This potential
   alone should not be used as an indicator
   of the chemicals posing the most hazard.
    4. Uncertainties. For each risk
   assessment the uncertainties should be
   clearly discussed and the overall quality
   of the risk assessment should be
   characterized. The scheme outlined in
   Table 2 should be used to express the
   degree of confidence in the quality of
   the data on interaction, health effects.
   and exposure.
    a. Henlth Effects—In some cases,
   when ht-alth effects data are incomplete,
   it may bo possible to argue by analogy
   or quantitative structure-activity
   relatiomihipa that the compounds on
   which no health effectii data are
   available are not likely to significantly
   affect the toxicity of trie mixture, if a
   risk assessment includes such an
   argument, the limitations of the
   approach must ha clearly articulated.
   Since a methodology has not been
   adopted for estimating an acceptable
   level (e.j(., ADI) or carcinogenic
   potential for single compounds based
   either on quantitative structure-activity
   relationships or on the results of short-
   term screening tests, such methods are
   not at present recommended as th« sole
  basis of a risk assessment on chemical
    b. Exposure Uncertainties—The
  general uncertainties in exposure
  assessment have been addressed in the
  Agency's Guidelines for Estimating
. J^o.sures.(U.S. EPA, 19S0d). The risk  .  -
  assessor should discuss these exposure
  uncertainties in terms of the strength of
  the evidence used to quantify the
  exposure. When appropriate, the
  assessor should also compare
  monitoring and modeling data and
  discuss any inconsistencies as a source
  of uncertainty. For mixtures, these
  uncertainties may be increased as the
  number of compounds of concern
    If levels of exposure to certain
  compounds known to be in the mixture
  are not available, but information on
  health effects and environmental
  persistence and transport suggest that
  Jhes^ compounds are not likely to be
  significant in affecting the toxicity of the
  mixture, then a risk assessment can be
  conducted based on the remaining
  compounds in the mixture, with
  appropriate caveats. If such an argument
  cannot be supported, no final risk
  assessment can be performed until
  adequate monitoring data are available.
  As an interim procedure,  a risk
  assessment may be conducted for those
  components in  the mixture for which
  adequate exposure and health effects
  data are  available. If the interim risk
 assessment does not suggest a hazard,
 there ia still concern about the risk from
 such a mixture because not all
 components in the mixture have been
   c. Uncertainties Regarding
 Composition of the Mixture—In perhaps
 a worst case scenario, information may
 be lacking not only on health effects and
 levels of exposure, but also on the
 identity of some components of the
 mixture. Analogous to the procedure
 described in the previous paragraph, an
 interim risk assessment can be
 conducted on those components of the
 mixture for which adequate health
 effects and exposure Information are
 available. If the risk is considered
 unacceptable, a conservative approach
 is to present the quantitativa estimates
 of risk, along with appropriate
 qualifications regarding the
 incompleteness of the data. If no hazard
 is Indicated by this partial assessment,
 the riok assessment should not be
 quantified until better health effects and
 monitoring data are available to
 adequately characterize the mixture
 exposure and potential hazards.

 ///. Assumptions and Limitations

 A. Information on Interactions

   Most of the data available on toxicant
 interactions are derived from acute
 toxicity studies using experimental
 animals in which mixtures of tv/o
 compounds were tested, often in onjy_a
 single combination. Major areas of
 uncertainty with the use of such data
 involve the appropriateness of
 interaction data from an acute toxicity
 study for quantitatively altering a risk
 assessment for subchronic or chronic
 exposure, the appropriateness of
 interaction data on two component
 mixtures for quantitatively altering  a
 risk assessment on a mixture of several
 compounds, and the accuracy of
 interaction data on experimental
 animals for quantitatively predicting
 interactions in humans.
  The use of interaction data from acute
 toxicity studies  to assess the potential
 interactions on chronic exposure is
 highly questionable unless the
 mechanism(s) of the interaction on acute
 exposure were known to apply  to low-
 dose chronic exposure. Most known
 biological mechanisms for toxicant
 interactions, however, involve some
 form of competition between the
 chemicals or phenomena involving
 saturation of a receptor site or metabolic
pathway. As the doses of the toxicants
are decreased, it is likely that these
mechanisms either no longer will exert a
significant effect or will be decreased to
 an extent that cannot be measured or
   The use of information from two^
 component mixtures to assess the
 interactions in a mixture containing
 mere than two compounds also is
 questionable from a mechanistic
 perspective. For example. If two
 compounds are known to interact, either
 cynergistically or antagonistically.
 because of the effects of one compound
 on the metabolism or excretion of the
 other, the addition of a third compound
 which either chemically alters or affects
 the absorption of one of the first two
 compounds could substantially alter the
 degree of the toxicologlc interaction.
 Usually, detailed studios quantifying
 toxicant interactions are not available
 on multicomponont mixtures, and the
 few studies that are available on such
 mixtures (e.g., Gullino et a!., 1958) do not
 provide sufficient information to assess
 the effects of interactive interference.'
   Concerns with the use of interaction
 data on experimental mammals to
 assess interactions in humans i.? based
 on the increasing appreciation for
 systematic differences among species in
 their response to individual chemicals. If
 systematic differences in toxic
 sensitivity to single chemicals exist
 among species, then it seems reasonable
 to suggest that the magnitude of toxicant
 interactions among species also may
 vary In a systematic manner.
 Consequently, even if excellent chronic
 data are.availabLe.jQnJthje-Diagnitude.of.,
 toxicant interactions in a species of
 experimental mammal, there is
 uncertainty that the magnitude of the
 interaction will be the same in humans.
 Again, data are not available to properly
 assess the significance of this
  Last, it should be emphasized that
 none of the models for toxicant
 interaction can predict the magnitude of
 toxicant interactions in the absence of
 extensive data. If sufficient data are
 available to estimate interaction
 coefficients as described in section  IV,
 then the magnitude of the toxicant
 interactions for .various proportions of
 the same components can be predicted.
The availability of an interaction ratio
 (observed response divided by predicted
response) is useful only in assessing the
 magnitude of the toxicant interaction for
 the specific proportions of the mixture
which was used to generate the
interaction ratio.
  The basic assumption in the
recommended approach is that ilsk
assessments on chemical mixtures are
best conducted using toxicologic data on
the mixture of concern or a reasonably
similar mixture. While such risk


                 Federal Register / Vol. 5X No. 185 / Wednesday. September 24.  1986 / Notices
  assessments do not formally consider
  to.xicologic interactions as part of a
  mathematical model,It is assumed that
  responses in experimental mammals or
  human populations noted after exposure
  to the chemical mixture can be used to
  conduct risk assessments on human
  populations. In bioassays of chemical
  mixtures using experimental mammals,
  the same limitations inherent in species-
  to-species extrapolation for single
  compounds  npply io mixtures. When
  using health effects data on chemical
  mixtures from studies on exposed
  human populations, the limitations of
  cpidemiologic studies in the risk
  assessment of single compounds also
  npply to mixtures. Additional limitations
  mny be involved when using health
  effects data  on chemical mixtures if the
  components  in (he mixture are not
  conslnnt or if the components partition
 in the environment.
 D. Addilivlly Models

   If sufficient data are not available on
 the effects of the chemical mixture of
 concern or a  reasonably similar mixture,
 the proposed approach is to assume
 additivily. Dose additivity is based on
 the assumption that  the components in
 the mixture have the same mode of
 action and elicit the  same effects. This
 assumption will not hold true in most
 cases, at least for mixtures of systemic
 toxicants. For systemic toxicants,
 however, most single compound risk
 assessments  will result in the derivation
 of acceptable levels, which, as currently
 defined, cannot be adapted to the
 different forms of response additivity as
 described in section  IV.**"-
   Additivity  models  can be modified to
 incorporate quantitative data on
 toxicant interactions from subchronic or
 chroni- studies using the models given
 in section IV  or modifications of those
 models. If this approach is taken,
 however, it will be under the assumption
 that other components in the mixture do
 not interfere with the measured
 interaction. In practice, such subchronic
 or chronic interactions data seldom will
 be available.  Consequently, most risk
 assessments (on mixtures) will be based
 on an assumption of additivity. as long
 as the components elicit similar effects.
  Dose-additive and  response-additive
 assumptions can lead to substantial
 errors in risk  estimates if synergistic or
 antagonistic interactions occur.
 Although dose additivity has been
shown to predict the  acute toxicities of
 many mixtures of similar and dissimilar
compounds (e.g., Pozzani et al., 1959;
Smyth et al.. 1909,1970; Murphy, 1980),
some marked  exceptions have been
noted. For example, Smyth et al. (1970)
tested the interaction of S3 pairs of
  industrial chemicals based on acute
  lethality in rats. For most pairs of
  compounds, the ratio of t!«j predicted
  LDM to observed LDM did not vary by
  more than a factor of 2. The greatest
  variation was seen with an equivolume
  mixture of morpholine and toluene, in
  which the observed LDM was about
  fives times less than the LDH predicted
  by dose addition. In a study by
  Hammond et al. (1979). the relative risk
  of lung cancer attributable to smoking
  was 11. while the relativb risk
  associated with asbestos exposure was
  5. The relative risk of lung cancer from
  both smoking and asbestos exposure
  was 53, indicating a substantial
  synergistic effect. Consequently, in some
  cases,  additivity assumptions may
  substantially underestimate risk. In
  other cases, risk may be overestimated.
  While  this is certainly an unsatisfactory
  situation, the available data on mixtures
  are insufficient for estimating the
 magnitude of these errors. Based on
 current information, additivity
 assumptions are expected to yield
 generally neutral risk estimates (i.e.,
 neither conservative nor lenient) and are
 plausible for component compounds that
 Induce similar types of effects at the
 same sites of action.

 IV. Mathematical Models and the
 Measurement of Joint Action

   The simplest mathematical models for
 Joint action assume no interaction in
 any mathematical sense. They describe
 either dose addition or response
 addition and are motivated by data on
 acute lethal-effects 'of mixtures of Two
 A. Dose Addition

   Dose addition assumes that the
 toxicants  in a mixture behave as If they
 were dilutions or concentrations of each
 other, thus the tr;u slopes of the dose-
 response curves for the individual
 compounds are identical, and the
 response elicited by  the mixture can be
 predicted  by summing the Individual
 doses after adjusting for differences in
 potency; this is defined as the ratio of
 equitoxic doses. Probit transformation
 typically makes this  ratio constant at all
 doses when parallel  straight lines are
 obtained. Although this assumption can
 be applied to any model (e.g..  the one-hit
 model in NRC. 1980b). it has been most
 often used in toxicology with the log-
 dose probit response model, which will
 be used to illustrate the assumption of
 close addition. Suppose the*, two
 toxicants show the following log-dose
probit response equations:
=0.3+3 log Zi
 where Yj is the probit response
 associated with a dose of Z, (1=1, 2).
 The potency, p, of toxicant #2 with
 respect to toxicant #1 is defined by the
 quantity Zi/Z, when Y,=Y, (that is
 what is meant by equitoxic doses). In
 this example, the potency, p. Is
 approximately 2. Dose addition assumes
 that the response. Y. to any mixture of
 these two toxicants can be predicted by;
       Y«=0.3+3log(Zi+pZt)     (IV-3)

 Thus, since p is defined as Z,/Z,,
 equation IV-3 essentially converts Z,
 into an equivalent dose of Z, by
 adjusting for the difference In potenay.
 A more generalized form of this
 equation for any nupber of toxicants in:
 Y-a, + blog(f,+ .
 a,— the y-intercept of the dose-response
     equation for toxicant #1
 b = the »lope of the dose-response linos for
     the toxicants
 f,»the proportion of the I" toxicant In the
 Pi—the potency of the I"1 toxicant with
     respect to toxicant #1 (I.e.. Z,/Z,). and
 Z=the sum of the Individual doses In the

 A more detailed discussion of the
 derivation of the equations for dose
 addition Is presented by Finney (1971).

 B. Response Addition

   The other form of additivity is
 referred to as response addition. As
 detailed by Bliss (1939), this  type of joint
 action assumes that the two toxicants
 act on different receptor systems and
 that the correlatibn'of individual
 tolerances may range from completely
 negative (r=—1) to completely positive
 (r= +1). Response addition assumes
 that the response to a given
 concentration of a mixture of toxicants
 is completely determined by the
 responses to the components and the
 pairwise correlation coefficient. Taking
 P as the proportion of organisms
 responding to a mixture of two toxicants
 which evoke Individual responses of P,
 and P,, then
 P=P, lfr=landP,>P,
 P=P, lfr=landP,

Federal Register / Vol. 61, No. 185  / Wednesday, September 24, 1S88 / Notices
  following modification of equation IV-4
  for dose addition:

  Y = a,+b log (f.+pf.+K [pf,f,]ci]+b log Z

  where a,. b. fu f,. p. and Z are defined as
  before, und K is the coefficient of
  interaction. A positive value of K
  indicates  synergism. a negative value
  indicates  antagonism, and a value of
  zero corresponds to dose addition as In
  equation IV-4. Like other proposed
  modifications of dose addition (Hewlett.
  1909), the equation assumes a consistent
  interaction throughout the entire range
  of proportions of individual components.
  To account for such asymmetric patterns
  of interaction as those observed by
  Alstott <;t  al. (1973). Durldn (1981)
  proposed  the following n.jdification to
  equation IV-6:
  Y-a.-i-b log (r,4pf«+K1f,(Pf,f,]»'+K,r,
             (pf.f^l + blogZ       (IV-10)

  In which Kf.pf.f,)" is divided into  two
  components, K,f,(pf1f,)0-«nnd KJ,(pf,f,)
  ". Since Kt and Ki need not have the
  same sign, apparent instances of
  antagonism at one receptor site and
  synergism at another receptor site can
  be estimated. When K, and K. are equal,
  equation 1V-10 reduces to Equation
    It should be noted that to obtain a
  reasonable number of degrees of
  freedom in the estimation of K in
  equation IV-9 or K, and K, in equation
  IV-10, trie  toxicity of several different
  combinations of the two components
 'must be assayed along with assays of
..ihe.toxiclty of the individual   	•
  components. Since this requires
  experiments with large numbers of
  animals, such analyses have been
  restricted for the most part to data from
  acute bioassays using insects (e.g..
  Finney, 1971) or aquatic organisms
  (Durkin, 1979). Also, because of the
  complexity of experimental design and
  the need for large numbers of animals,
  neither equation IV-6 nor equation IV-
  10 has been generalized or applied to
  mixtures of more than two toxicants.
  Modifications of response-additive
  models to include interactive terms have
  also been proposed, along with
  appropriate statistical tests for the
  assumption of additivity (Korn and Liu,
  1983:  Wahrendorf et al.. 1981).
   In the epidemlologic literature.
  measurements of the extent of toxicant
  interactions. S. can be expressed as the
 ratio of observed relative risk to relative
 risk predicted by some form of
 additivity assumption. Analogous to the
 ratio of interaction in classical
 toxiocology studies. S = 1 indicates no
 interaction. S>1  indicates synergism.
                         and S<1 indicates anagonism. Several
                         models for both additive and
                         multiplicative risks have been proposed
                         (e.g.. Hogan et al.. 1978; NRG, 1980b;
                         Walter, 197B). For instance, Rothman
                         (1978] has discussed the use of the
                         following measurement of toxicant
                         interaction based on the assumption of
                         risk additivity:
                                                )     (IV-ll)

                         where Rw is the relative risk from
                         compound #1 in the absence of
                         compound #2. R., is the relative risk
                         from compound #2 in the absence of
                         compound #1, and Ru is the relative risk
                         from exposure to both compounds. A
                         multiplicative risk model adapted from
                         Walter and Holford (1978, equation 4)
                         can be stated as:
                                8 - Ru/(R,.R,,)
                        As discussed by both Walter and
                        Holford (1973) and Rothman (1976). the
                        risk-additive model is generally applied
                        to agents causing diseases while the
                        multiplicative model is more appropriate
                        to agents that prevent disease. The
                        relative merits of these and other
                        indices have been the subject of
                        considerable discussion in the
                        epidemiologic literature (Hpgan et al..
                        1978: Kupper and Hogan. 1978; Rothman.
                        1978: Rothman et al., 1980; Walter and
                        Holford, 1978). There seems to be a
                        consensus that for public health
                        concerns regarding causative (toxic)
                        agents, the additive model is more
                          Both the additive and multiplicative
                       . models assume statistical independence
                        in that the risk associated with exposure
                        to both compounds in combination can
                        be predicted by the risks associated
                        with separate exposure to the individual
                        compounds. As illustrated by
                        Siemlatycki and Thomas (1981) for
                        multistage carcinogeneais, the better
                        fitting statistical model will depend not
                        only upon actual biological Interactions.
                        but also upon the stages of the disease
                        process which the compounds affect.
                        Consequently, there is no a priori basis
                        for selecting either type of model in a
                        risk assessment. As discussed by Stara
                        et al. (1983). the concepts of multistage
                        carcinogenesls and the effects of
                        promoters and cocarcinogens on risk are
                        extremely complex issues. Although risk
                        models for promoters have been
                        proposed (e.g., Burns et al., 1983), no
                        single approach can be recommended at
                        this time.
                        V. Reference!
                        ACGIH (American Conference of
                          Governmental Industrial Hygienists). 1983.
                          TLVs: threshold limit values for chemical
                          substances and physical agents in the work
    environment with intended changes for
    1983-1884. Cincinnati, OH. p. 68.
  Alstott RJ-, MJJ. Tarranf, and R.B. Forney.
    197$. The acute toxldties of 1-
    methybcanthine, ethanol. and 1-
    methylxanthine/ethanol combinations in
    the mouse. ToxicoL Appl. PharmacoL
  Blist, C.L 1039. The toxicity of poisons
    applied jointly. Ann. Appl. Biol. 28:585-615.
  Burnt, FM R. Albert, E. Altschuler, and E,
    Morris. 1983. Approach to risk assessment
    for jjenotoxic carcinogens based on data
    from the mouse skin initiation-promotion
    model. Environ. Health Perspect. 50:309-
  Durkin. P.R. 1979. Spent chlorlnatlon liquor
    and chlorophenollcs: a study In
    detoxicatlon and Joint action using
    Daphnta magnet. PhfD. Thesis. Syracuse.
    NY: State University of New York College
    of Environmental Science and Forestry, p
  Durkin. P.R. 1P81. An approach to the
    analysis of toxicant Interactions In the
    aquatic environment. Proceedings of the
    4th Annual Symposium on Aquatic
    Toxicology. American Society for Testing
  •  and Materials, p. 388-401.
  Finnoy. D.J. 1942. The analysis of toxicity
    tests on mixtures of poisons. Ann. Appl.
    Biol. 29:82-94.
  Finney. D.J. 1971. Problt analysis. 3rd ed.
    Cambridge, Great Britain: Cambridge
    University Press, 333 p.
  Goldstein. A.. L. Aronow. and S.M. Kalman.
    1974. Principles of drug action: the basis of
    pharmacology, 2nd ed. New York, NY: John
    Wiley and Sons. Inc.. 854 p.
  GuHtno. P., M. Winltz. S.M. Birnbaum. J.
    Cornfield, M.C. Otey. and J.P. Greenstein.
    1958. Studies on the metabolism of amlno
    acids and related compounds in vivo. I.
..  Toxicity of essential amlno-aelds.	
    individually and In  mixtures, and the
   protective effect of  L-arglnlne. Arch.
   Biochem. Blophys. 54:319-332.
  Hammond. E.C.. I.V. Sellkoff. and H.
   Seidman. 1979. Asbestos exposure.
   cigarette smoking and death rates. Ann. NY
   Acad. Scl. 330:473-490.
  Hewlett, P.S. 1969. Measurement of the
   potencies of drug mixtures. Biometrics
  Hogan, M.D., L. Kupper, B. Most, and J.
   Haseman. 1978. Alternative approaches to
   Rothman's approach for assessing
   synerglsm {or antagonism) In cohort
   studies. Am. J. Epldemlol. 108(l):60-67.
  Klaassen. C.D.. and J.  Doull. 1980. Evaluation
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   Doull. C.D.  Klaassen. and M.O. Amdur. eds.
   Toxicology: the basic science of poisons.
   New York. NY: Macmillan Publishing Co..
   Inc., p. 11-27.
 Korn, E.L., and P-Y. Liu. 1983. Interactive
   effects of mixtures of stimuli in life table   .
   analysis. Biometrika. 70:103-110.
 Kupper. L, and M.D. Hogan. 1978. Interaction
   In epidemiologlc studies. Am. J. Epldemiol.
 Levine, R.E. 1973. Pharmacology: drug actions
   and reactions. Boston. MA: Little. Brown
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 Murphy, S.D. 1980. Assessment of the-
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 NRC (National Research Council). 1980a.
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  Washington. DC: National Academy Press,
  p. 27-28.
 NRC (National Research Council). 1980b.
  Principles of lexicological interactions
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  exposures. Washington, DC: National
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 OSHA (Occupational Safety and Health
  Administration). 1983. General Industry
  Standards. Subpart 2, Toxic and Hazardous
  Substnnces. Code of Federal Regulations.
  40:1810.1000 (d)(2)(I). Chapter XVII—
  Occupational Safety and Health
  Administration, p. 667.
 Plackctt. R.L. and P.S. Hewlett. 1948.
  Statistical aspects of the independent joint
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 Pozznni. U.C.. C.S. Well, and C.P. Carpenter.
  195!). The toxicological basis of threshold
  valuns: 5. The experimental inhalation of
  vapor mixtures by rats, with  notes upon the
  relationship between single dose inhalation
  and single dose ora! data. Am. Ind. Hyg.
  Asaoc. J. 20:304-369.
 Rothman. K. 1976. The estimation of synergy
  or antagonism. Am. J. Epidemiol. 103(5):
 Rolhmiin, K. 1978. Estimation versus
  detection in the assessment of synergy.
  Am. f. Epidemiol. 108(1 )S-ll.
 Rothman. K., S. Greenland, and A. Walker.
  1980. Concepts of interaction. Am. J.
  Epidamiol. 112(4):467-470.
 Siemialycki. J.. and D.C. Thomas. 1981.
  Biological models and statistical
  interactions: An example from multistage
  carcinogenesis. Int. J. Epidemiol. 10(4):383-
Smyth. H.F.. C.S. Weil. J.S. West, and C.P.
  Carpenter. 1909. An exploration of joint
  toxic action: I. Twenty-seven industrial
  chemicals intubatod in rats in all possible
  pairs. Toxicol. Appl. Pharmacol. 14:340-347.
Smyth. H.F.. C.S. Weil. J.S. West and C.P.
  Curpenter. 1970. An exploration of joint
  toxic action. II. Equiloxic versus
  equivolume mixtures. Toxicol. Appl.
  Pharmncol. 17:498-503.
Slnra. J.F.. D. Mukerjee. R. McGaughy. P.
  Durkin, and M.L. Dourson. 1983. The
  current use of studies on promoters and
  cocarcinogens in quantitative risk
  asse.'isment. Environ. Health Perspecl.
U.S. EPA. 1986a. Guidelines for carcinogen
  risk assessment. Federal Register.
U.S. ETA. 1988b. Guidelines for mutagenicity
  risk assessment. Federal Register.
U.S. EPA. 1986c. Guidelines for the health
  assessment of developmental toxicants.
  Federal Register.
U.S. EI'A. 1980d. Guidelines for estimating
  exposures. Federal Register.
Veldstra, H. 1956. Synergism and potentiation
  with special reference to the combination
  of sli-uctural analogues. Pharmacol. Rev.
 Wahrendorf, J., R. Zentgrof. and C.C. Brown.
   1981. Optimal designs for the analyils of
   interactive effects of two carcinogens or
   other toxicants. Biometrics. 37:45-R4.
 Waltar, S.D. 1976. The estimation end
   interpretation of attributable risk in health
   research. Biometrics. 32:829-849.
 Walter, S.D., and T.R. Holford. 1978.
   Additive, multiplicative, and other models
   for disease risks. Am. J. EpldemloL 108:341-
 Withey. J.R. 1981. Toxicodynamlcs and
   biotransformation. In: International
   Workshop on the Assessment of
   Multlch'emical Contamination. Milan, Italy.
   (Draft copy courtesy of JJi. Withey)
 WHO (World Health Organization). 1981.
   Health  effects of combined exposures In
   the work environment. WHO Tech. Rayort
   Series No. 602.

 Part B. Response to Public and Science
 Advisory Board Comments

 /.  Introduction

   This section summarizes  some of the
 major issues raised In public comments
 on the Proposed Guidelines for the
 Health Risk Assessment of Chemical
 Mixtures published on January 9.1985
 (50 FR 1170). Comments were lecelved
 from 14 individuals or organizations. An
 issue paper reflecting public and
 external review comments was
 presented to the Chemical Mixtures
 Guidelines Panel of the Science
 Advisory Board (SAB) on March 4,1985.
 At its April 22-23,1985, meeting, the
 SAB Panel provided the Agency with
 additional suggestions and
 recommendations concerning the
 Guidelines." ThisVection also
 summarizes the issues raised by the
   The SAB and public commentors
 expressed diverse opinions  and
 addressed issues from a variety of
 perspectives. In response to comments,
 the Agency has modified or clarified
 many sections of the Guidelines, and is
 planning to develop a technical support
 document in line with the SAB
 recommendations. The discussion that
 follows highlights significant Issues
 raised in the comments, and the
 Agency's response to them.  Also, many
 minor recommendations, which do not
 warrant discussion here, were adopted
 by the Agency.

//. Recommended Procedures
A. Definitions

   Several comments were received.
 concerning the lack of definitions for
certain key items and the general
understandability of certain sections.
Definitions have been rewritten for
several terms and the text has been
significantly rewritten to clarify the
Agency's intent and meaning.
   Several commenlors noted the lack of
 a precise definition of "mixture," even
 though several classes of mixtures are
 discussed. .In the field of chemistry, the
 term "mixture" is usually differentiated
 from true solutions, with the former
 defined as nonhomogencous
 multicomponent systems. For these
 Guidelines, the term "mixture" is
 defined as ". . . any combination of two
 or more chemicals regardless of spatial
 or temporal homogeneity of source"
 (section 1). These Guidelines are
 intended to cover risk assessments for
 any situation where the population Is
 exposed or potentially exposed to two
 or more compound's of concern.
 Consequently, the introduction has been
 revised to clarify the intended breudth
 of application.
   Several commentors expressed
 concern that "sufficient similarity" was
 difficult to define and  that the
 Guidelines should give more details
 concerning similar mixtures. The
 Agency agrees end is planning research
 projects to improve on the definition.
 Characteristics such as composition and
 toxic end-effects are certainly
 important, but the best indicators of
 similarity in terms of risk assessment
 have yet to be determined. The
 discussion in the Guidelines emphasizes
 case-by-case judgment until the
 necessary research can be performed.
 The Agency considered but rejected
 adding an example, because it is not
 likely that any single example would be
 adequate to illustrate the variety in the
 data and types of judgments that will be
 required in applying this concept.
 Inclusion of examples  is being
 considered for the technical support

 B. Mixtures of Carcinogens and
 Systemic Toxicants

  The  applicability of trie-preferred
 approach  for a mixture of carcinogens
 and systemic (noncarclnogenic)
 toxicants was a concern of several
 public  commentors as well as the SAB.
The Agency realizes that the preferred
 approach of uslr^ test  data on the
 mixture Itself may not  be sufficiently
 protective in all cases. For example,
 take a  simple two-component mixture of
one cardnogtm and one toxicant.' The
preferred approach would lead to using
 toxicity data on the mixture of the two
compounds. However,  It Is possible to
set the proportions of each component
so that in a chronic bioassay of such a
mixture, the presence of the toxicant
could mask the activity of the
carcinogen. That is to say, at doses of
the mixture sufficient for the carcinogen
to induce tumors in the small
                                         BEST  COPY AVAILABLE


Federal  Register. / Vol. 51...No...185, / Wednesday. September 24. 1986 / Notices
 experimental group,:the toxicant could
 Induce mortality. At a lower dose In the
 same study, no adverse effects would be
 observed, Including no carcinogenic
"effects. The data would, then suggest use
 of a threshold approach. Since
 carclnogenicity Is considered by the
 Agency to.be a honthresriold effect. It
 may not be prudent to construe the
 negative results of such a bibassay as
 Indicating the absence of risk at lower
 doses. Consequently, the Agency has
 revised the discussion of the preferred
 approach to allow the risk assessor to
 .evaluate the potential for masking of
 • carclno'flenlcity or other effects on a
 case-by-case basis.
   Another difficulty  occurs with such a
 mixture when the risk assessment needs
 to be based on data  for the mixture
 components. Carcinogens and systemic
  toxicants are evaluated by the Agency
 •using different approaches and generally
  arc described by different types of data:
  respbnne rales'for carcinogens vs. effect
;,. descriptions for toxicants. The Agency
  recognizes this difficulty and
  recommends research to develop a new
• assessment model for combining these
  "dissimilar data sets  Into one risk
  estimate. One suggestion In the interim
 ' is to present'separate risk estimates for
  the'.dis similar end points. Including
 • carcinogenic, teratogenic, routagenic,
  and systemic toxicant components.
   ///. Additivity Assumption
     Numerous comments were received
  'concerning the assumption of additivity.
     a. the applicability of additivity to
   "complex" mixtures;
     b. the use of dose  additivity for
   compounds that induce different effects;
     c. the intepretation of the Hazard
   Index: and
     d. the use of interaction data.
   Parts of the discussion in the proposed
   guidelines concerning the use of
   addilivity assumptions were vague and
   have been revised in the final
   Guidelines to clarify the Agency's Intent
   and position.
   A. Complex Mixtures
      The issue of the applicability of an
   assumption of additivity to complex
   mixtures containing tens or hundreds of
   components was raised In several of the
    public comments. The Agency and its
    reviewers agree that as the number of
   .compounds in the mixture increases, an
    assumption of additivity will become
    less  reliable in estimating risk-This is
    based on the  fact that each component
    estimate of risk or an acceptable level is
    associated with some error and
    uncertainty. With current knowledge,
    the uncertainty will increase as the
                        number of components Increases. In any
                        event, little experimental data fire
                        available to determine the general
                        change in the error as the mixture
                        contains more components. The Agency
                        has decided that a limit to the number of
                        components should not be set in these
                        Guidelines. However, the Guidelines do
                        explicitly state that as the number of
                        compounds in the mixture increases, the
                        uncertainty associated with the risk
                        assessment is also likely to Increase.

                        B. Dose Additivity
                           Commentor* were concerned about
                        what appeared to be a recommendation
                        of the use of dose additivity for
                         compounds that induct) different effects.
                        The discussion following the dose
                         additivity equation was clarified to
                         indicate that the act of combining all
                         compounds, even if they induce
                         dissimilar effects, la a screening
                         procedure and not the preferred
                         procedure in developing a hazard Index.
                         The  Guidelines were further clarified to
                         state that dose (or response) additivity
                         is theoretically sound, and therefore
                         best applied for assessing mixtures of
                         similar acting components that do not
                         C. Interpretation of the Hazard Index
                            Several comments addressed the
                         potential for misinterpretation of the
                         hazard Index, and some questioned its
                         validity, suggesting that it mixes science
                         and value judgments by using
                         "acceptable" levels in the calculation.
                         The Agency agrees with  the possible
                          confusion regarding its use and has
                          revised the Guidelines for clarification.
                          The hazard index is an easily derived
                          restatement of dose addilivity, and Is,
                          therefore, most accurate when used with
                          mixture components that have similar
                          toxic  action. When used with
                          components of unknown or dissimilar
                          action, the hazard index is less accurate
                          and should be interpreted only as a
                          rough Indication of concern. As with
                           dose  addition, the uncertainty
                           associated with the hazard Index
                           Increases as the number of components
                           Increases, so that It Is less appropriate
                           for evaluating the toxlclty of complex
                           D. Use of Interaction Data
                             A few commentors suggested that any
                           Interaction data should be used to
                           quantitatively alter the risk assessment.
                           The Agency disagrees. The. current
                           information on interactions is meager,
                           with only a few studies comparing
                           response to the mixture with that
                           predicted by studies on components.
                           Additional uncertainties include
                           exposure variations due to changes in
composition, mixture dose, and species
differences in the extent of the
interaction. The Agency is constructing
an Interaction :data base in an attempt to
answer some of these issues. Other
comments concerned the use of different
types of interaction data. The Guidelines
restrict the use of Interaction data to
that obtained from whole animal  .
bloassays of a duration appropriate to
the risk assessment. Since such data are
frequently lacking, at least for chronic or
subchronic effects, the issue is whether
to allow for ths use of other Information
such as acute date. In vitro data, or
 structure-activity relationships to  *
 quantitatively alter the risk assessment,
 perhaps by use of a safety factor. The
 Agency believes that sufficient scientific
 support does not exist for the use of
 such data in any but a qualitative
 discussion of possible synorgistic or
 antagonistic effects.
 IV. Uncertainties and the Sufficiency of
 the Data Base
   In the last two paragraphs of section II
 of the .Guidelines, situations are
 discussed In which the risk assessor is
 presented with Incomplete toxiclty.
• monitoring, or exposure data. The SAB.
 as well as several public commentors,
 recommended that the "risk
 management" tone of this section be
 modified and that the option of the risk
 assessor to decline to conduct a risk
  assessment be made more explicit.
    This is a difficult issue that must
  consider not only the quality of the
  available data for risk assessment, but
  also the needs of the Agency in risk
  management. Given the types of poor
  data often available, the risk assessor
  may indicate that the risk assessment is
  based on limited information and thus
  contains no quantification of risk.
   Nonetheless, in any risk assessment,
   substantial uncertainties exist. It is the
   obligation of the risk assessor to provide
   an assessment, but also to ensure that
   all the assumptions and uncertainties
   are articulated clearly and quantified
   whenever possible.
     The SAB.artlculated several o'her
   recommendations related to
   uncertainties, all of which have been
   followed in the revision of the.
   Guidelines. One recommendation was
   that the summary procedure table also
   be presented as a  flow chart so that all
   options are clearly displayed. The SAB
   further recommended the development
   of a system to express the level of
   confidence in the various steps of the
   risk assessment.
     The Agency has revised the summary
   table to present four major options: risk
   assessment using data on the mixture


                Federal Register / V^l. 51.  No. 185  /  Wednesday, September 24. 1980 / Notices
itself, data on a similar mixture, data on
the mixture's components, or declining
to quantify the risk when the data are
inadequate. A flow chart of this table
has also been added to more clearly
depict the various options and to suggest
the combining of the several options to
indicate the variability and uncertainties
in the risk assessment.
  To determine the adequacy of the
data, the SAD also recommended the
development  of a system to express the
level of confidence associated with
various steps In the risk assessment
process. The Agency has developed a
rating scheme to describe data quality in
three areas: interaction, health effects,
and exposure. This classification
provides a range of five levels of data
quality for each of the three areas.
Choosing the  last level in any area
results in declining to perform a
quantitative risk assessment due to
inadequate data. These last levels are
described as follows:
  An assumption of additivity cannot be
    justified, and no quantitative risk
    assessment can be conducted.
Health effects:
  A lack of health effects information on
    the mixture and its components
    precludes a quantitative risk
  The available exposure information is
    insufficient for conducting a risk
  Several commentors. Including the
 SAB,  emphasized the importance of not
 losing these classifications and
 uncertainties farther along in the risk
 management process. The discussion of
 uncertainties has been expanded in the
 final Guidelines and Includes the
 recommendation that a discussion of
 uncertainties  and assumptions be
 included at every step of the regulatory
 process that uses risk assessment.
  Another SAB comment was that the
 Guidelines should Include additional
 procedures for mixtures with more than
one end point or effect.  The Agency
agrees that these are concerns and
revised the Guidelines to emphasize
these  as additional uncertainties worthy
of further research.

 V. Need fora Technical Support
  The third major SAB comment
concerned the necessity for a separate
technical support document for these
Guidelines. The SAB pointed out that
the scientific and technical background
from which these Guidelines must draw
their validity Is so broad and varied that
it cannot reasonably be synthesized
within the framework of a brief set of
guidelines. The Agency is developing a
technical support document that will
summarize the available information on
health effects from chemical mixtures,
and on interaction mechanisms, as well
as Identify and develop mathematical
models and statistical techniques to
support these Guidelines. This document
will also identify critical gaps and
research needs.
  Several  comments addressed the need
for examples on the use of the
Guidelines. The Agency has decided to
include examples in the technical
support document.
  Another Issuo raised by the SAB
concerned the identification of research
needs. Because little emphasis has been
placed on  the toxicology of mixtures
until recently, the Information on
mixtures Is limited. The SAB pointed out
that Identifying research needs Is critical
to the risk assessment process, and the
EPA should ensure that these needs are
considered in the research planning
process. The Agency will Include a
section in the technical support
document that Identifies research needs
regarding both methodology and data.
[FR Doc. 86-19003 Filed 9-23-88; 8:45 am]