May 2, 1997
EPA-SAB-EHC-97-004
Honorable Carol M. Browner
Administrator
U.S. Environmental Protection Agency
401 M Street, SW
Washington, DC 20460
Subject: Science Advisory Board's review of the revised Guidelines for
Neurotoxicity Risk Assessment
Dear Ms. Browner:
The proposed Neurotoxicity Guidelines for Risk Assessment were developed as
part of an interoffice guidelines development program under the auspices of the Risk
Assessment Forum within EPA's Office of Research and Development. The Draft
Guidelines were developed initially by an Agency work group composed of scientists
from throughout the Agency, and selected drafts were peer reviewed internally and by
experts from universities, environmental groups, industry and other government
agencies. The proposed Guidelines are based on recommendations derived from
these reviews and on those made at various scientific meetings and workshops on
neurotoxicology. The proposed Guidelines were published for public comment in the
October 4, 1995, issue of the Federal Register.
The Science Advisory Board's (SAB) Environmental Health Committee (EHC)
met on July 18, 1996 in Washington DC to review the proposed Guidelines.
In general, the Committee considers the revised Guidelines document to be
quite successful, and, all things considered, well suited to its intended task. It ad-
dresses a wide range of subjects of considerable complexity and (often) of consider-
able subtlety, and constitutes a clear step forward in the state of the art. Naturally, as
with any such ambitious undertaking, the Committee's review has identified areas
where improvements could be made. Specifics on such improvements, which generally
lie outside the area of the specific Charge for the review, and reflect the thoughts of
individual Committee Members rather than a Committee consensus, have been
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provided separately to Agency staff.
The various issues comprising the Charge are addressed in detail in the body of
the enclosed report, and are summarized (with the specific Charge question italicized)
below:
a) The combining of hazard identification and dose-response evaluation to
reflect more accurately the process used for noncancer health effects. It
was noted that this issue had been addressed in depth during the EHC
review of the reproductive toxicity guidelines and in following discussions
of the SAB Executive Committee. There was a clear consensus that EPA
should follow the SAB position articulated in the recent "Commentary on
Hazard identification" from the Executive Committee (EPA-SAB-EC-COM-
96-001, December 8, 1995) which called for keeping hazard identification
as an identifiable qualitative step in the risk assessment process. The
Committee recommended that revisions be made to decouple the qualita-
tive step of hazard identification from the more quantitatively rigorous
steps of exposure evaluation and dose response assessment. The
Committee encourages the Agency to adopt an approach similar to that
used in the proposed Cancer Risk Assessment Guidelines.
b) The issue of compensation and recovery of function in neurotoxicological
studies and how to account for compensation in neurotoxicology risk
assessment. The Committee believes that reversibility can not be ignored
in risk assessment, but that an assessor had to be very careful and
search out particularly good evidence if recovery or an apparent transient
effect is cited to support evidence for relatively benign effects. In general,
assessors should be advised to look carefully at instances where revers-
ibility is involved and be alert to the possibility of re-occurrence of effects.
The Committee recommended that EPA address specifically differences
related to type of damage, nature of the insulting agent, and age of the
organism, as well providing case examples relating to possible reversibil-
ity.
c) The use of blood and/or brain acetylcholinesterase activity as an indication
of neurotoxicity for risk assessment and Considering the available data
and the state of the science, does the SAB agree with the recommenda-
tion that inhibition of RBC and/or plasma cholinesterase can serve only as
a biomarker of exposure? The Committee addressed these two issues
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together because of their close relationship. The EHC concurred with the
findings of previous SAB reviews regarding the consideration of data on
the inhibition of RBC and/or plasma cholinesterase. In the absence of
clinical signs in humans or animals or the absence of morphological data
in animals, the quantitative nature of the inhibition of red blood cell (RBC)
and/or plasma cholinesterase is considered unreliable for assessing
significant biological adverse changes, but can be used as a biomarker of
exposure. The Committee also recommended that a noted decline in
brain ChE should be evaluated by risk assessors in terms of possible
effects that are biologically significant, and that the term "statistically
significant" needed to be better explicated - perhaps in terms of the
benchmark dose or by some measure which reflected information about
the distribution of the effect under study. The Committee also suggested
that further details concerning reversibility and possible tolerance effects
(which could enhance sensitivity to other agents) be provided.
d) Are there endpoints indicative of neurotoxicity that may not be covered by
these proposed Guidelines, e.g., endocrine disruption or neuroendocrine-
mediatedneurotoxicity? The Committee considered that the description
of endpoints indicative of neurotoxicity was extensive and it does not
suggest additional endpoints based on current scientific knowledge. The
Committee also recognizes that some chemical agents may cause both
neurotoxic and other effects such as endocrine disruption; however
endocrine disruption without evidence of a neurochemical or neuro-
physiological causation would not be a basis to label that chemical agent
as a neurotoxicant. This notwithstanding, the Guidelines should recog-
nize that a finding of neuroendocrine dysfunction might have to be ampli-
fied by neurobehavioral assessment techniques not discussed or de-
scribed in the current draft. Observation of a widening or narrowing of
male-female differences on certain neurotoxic endpoints could be a
reflection of interference with neuroendocrine function. Specific recom-
mendations should be included in the Guidelines for dealing with such
multi-system actions because risk assessors will typically not be sensitive
to their ramifications.
e) Are the descriptions of the endpoints used in human and animal
neurotoxicological assessments complete? In the broadest sense, the
Committee found the treatment of this issue by the Guidelines document
to be "..not complete, but complete enough for its purposes," meaning
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that, although the Committee has many refinements and improvements to
suggest, the Guidelines provided sufficient information on various end-
points to enable a risk assessor to do his/her job. More specifically, the
Committee found that coverage of human studies was quite good, but that
the treatment of animal studies (particularly with regard to warning cave-
ats) was, by comparison, weaker. It was also suggested that better
coverage be given to various possible tests (including references for
various test methodologies).
g) Treatment of the possibility of no threshold for some neurotoxic agents.
There was considerable debate/discussion as to what the concept of a
threshold, or of no threshold, actually meant in terms of a risk assess-
ment, and whether the concept itself was meaningful or useful in this
context. It was proposed that the term "non-linear (at low doses) dose-
response curve for most neurotoxicants" be substituted for the term
"threshold." The EPA was urged to harmonize its treatment of this (as
well as several other issues) with the presentation and positions taken in
the draft Carcinogen Risk Assessment Guidelines currently out for public
comment.
g) Adequacy of the treatment of susceptible populations and individuals by
the proposed Guidelines. The Committee noted that the aged were the
only specific sub-population noted, and that the Guidelines do not discuss
means of modifying NOAELS or dose-response curves in this context. It
was suggested that the Guidelines be modified to identify other suscepti-
ble groups (e.g., those with poor nutritional status) and provide sugges-
tions as to dealing with them.
h) The use of the Benchmark Dose in Neurotoxicity Risk Assessment. There
was considerable discussion within the Committee and between the
Committee, EPA staff, and members of the audience as to the nature of
the results yielded by the application of the benchmark dose methodology
and how it compared with the more common NOAEL measure. No
consensus on this aspect of the issue was reached. The Committee did
note that definition of the benchmark dose in the Guidelines was not clear
and should be improved; it further suggested that the basic position of the
Guidelines be retained, i.e., that the use of the benchmark dose should
be explored in specific situations, but that the NOAEL measure was the
default approach.
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Finally, during the SAB Executive Committee (EC) review of this report, several
Members raised one additional concern which we believe should be brought to EPA's
attention. The Guidelines note that there are a large number of indicators of neurotox-
icity and neurobehavioral effects. They recognize that the complexity of these multiple
indicators requires a great deal of scientific judgement, including expertise both within
and without the field of neurotoxicology. We recommend that the Guidelines describe
a mechanism for risk assessors and other users to obtain the resources for expert
advice.
We appreciate the opportunity to review this document, and look forward to your
response to the issues we have raised.
Dr. Genevieve Matanoski
Chair, Science Advisory Board
/signed/
Dr. Emil Pfitzer
Chair, Environmental Health Committee
ENCLOSURE
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United States Science Advisory Board EPA-SAB-EHC-97-004
Environmental Washington DC May 1997
Protection Agency
&EPA AN SAB REPORT: GUIDE-
LINES FOR NEUROTOXICITY
RISK ASSESSMENT
REVIEW OF THE OFFICE OF
RESEARCH AND DEVELOPMENT'S
GUIDELINES FOR NEUROTOXICITY
RISK ASSESSMENT BY THE ENVI-
RONMENTAL HEALTH COMMITTEE
-------
NOTICE
This report has been written as a part of the activities of the Science Advisory
Board, a public advisory group providing extramural scientific information and advice to
the Administrator and other officials of the Environmental Protection Agency. The
Board is structured to provide balanced, expert assessment of scientific matters
relating to problems facing the Agency. This report has not been reviewed for approval
by the Agency and, therefore, the contents of this report do not necessarily represent
the views and policies of the Environmental Protection Agency, nor of other agencies in
the Executive Branch of the Federal government, nor does mention of trade names or
commercial products constitute a recommendation for use.
-------
ABSTRACT
The proposed Neurotoxicity Guidelines for Risk Assessment were developed by
the Risk Assessment Forum within EPA's Office of Research and Development. The
Science Advisory Board's (SAB) Environmental Health Committee met on July 18, 1996
in Washington DC to review the proposed Guidelines.
In general, the Committee considers the revised Guidelines document to be
quite successful, and, all things considered, well suited to its intended task. On
specific issues, the Committee noted that:
a) EPA should keep hazard identification as an identifiable qualitative step
in the risk assessment process.
b) Reversibility can not be ignored in risk assessment, but that an assessor
had to be very careful and search out particularly good evidence if recov-
ery or an apparent transient effect is cited to support evidence for rela-
tively benign effects.
c) In the absence of clinical signs in humans or animals or the absence of
morphological data in animals, the quantitative nature of the inhibition of
red blood cell (RBC) and/or plasma cholinesterase is considered unreli-
able for assessing significant biological adverse changes, but can be
used as a biomarker of exposure.
d) The description of endpoints indicative of neurotoxicity was extensive and
it does not suggest additional endpoints based on current scientific
knowledge.
e) The Guidelines provided sufficient information on various endpoints to
enable a risk assessor to do his/her job.
g) The term "non-linear (at low doses) dose-response curve for most
neurotoxicants" be substituted for the term "threshold." The EPA was
urged to harmonize its treatment of this (as well as several other issues)
with the presentation and positions taken in the draft Carcinogen Risk
Assessment Guidelines currently out for public comment.
g) The Guidelines be modified to identify other susceptible groups (e.g.,
those with poor nutritional status) and provide suggestions as to dealing
with them.
h) The definition of the benchmark dose in the Guidelines was not clear and
-------
should be improved, and the basic position of the Guidelines be retained,
i.e., that the use of the benchmark dose should be explored in specific
situations, but that the NOAEL measure was the default approach.
KEYWORDS: neurotoxicology; neurobehavioral; risk assessment; benchmark dose;
threshold.
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U.S. ENVIRONMENTAL PROTECTION AGENCY
SCIENCE ADVISORY BOARD
ENVIRONMENTAL HEALTH COMMITTEE
MEETING
July 18, 1996
CHAIR
Dr. Emil Pfitzer, Research Institute for Fragrance Materials, Inc., Hackensack, NJ*
MEMBERS
Dr. Adolfo Correa, The Johns Hopkins University, Baltimore, MD
Dr. Michael A. Gallo, UMDNJ-Robert Wood Johnson Medical School, Piscataway, NJ
Dr. Ernest E. McConnell (SAP)**, Raleigh, NC
Dr. Mark J. Utell, University of Rochester Medical Center, Rochester, NY
Dr. Lauren Zeise, California Environmental Protection Agency, Berkeley, CA
CONSULTANTS
Dr. Steven C. Lewis, Exxon Biomedical Sciences, Inc., East Millston, NJ
Dr. Bernard Weiss, University of Rochester Medical Center, Rochester, NY
FEDERAL EXPERT
Dr. David Gaylor, National Center for Toxicological Research, Jefferson, AR
DESIGNATED FEDERAL OFFICIAL
Mr. Samuel Rondberg, Executive Secretary, Environmental Health Committee, Science
Advisory Board (1400), U.S. Environmental Protection Agency, Washington, D.C.
20460
STAFF SECRETARY
Ms. Mary L. Winston, U.S. Environmental Protection Agency, Science Advisory Board,
(1400), Washington, D.C. 20460
*Dr. Donald R. Mattison served as Chair of the public meeting, but resigned from the
Board prior to development of this report; Dr. Pfitzer suceeded him as Chair.
**Chair of the Scientific Advisory Panel
IV
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TABLE OF CONTENTS
1. EXECUTIVE SUMMARY 1
2. BACKGROUND AND CHARGE 4
2.1 Background 4
2.2 Charge 4
3. SPECIFIC ISSUES 8
3.1 Combining Hazard Identification and Dose-response Evaluation 8
3.2 Compensation and Recovery of Function in Neurotoxicological Studies . . 9
3.3 Blood And/or Brain Acetylcholinesterase Activity as an Indication of
Neurotoxicity for Risk Assessment 10
3.4 Other Endpoints Indicative of Neurotoxicity 12
3.5 Description of Endpoints for Neurotoxicological Assessments 13
3.6 The Possibility of No Threshold for Some Neurotoxic Agents 13
3.7 Treatment of Susceptible Populations and Individuals 14
3.8 The use of the Benchmark Dose 15
4. SUMMARY 17
REFERENCES R-1
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1. EXECUTIVE SUMMARY
In general, the Committee finds the revisions to the Guidelines document to be
quite successful, constituting a clear step forward in the state of the art. As with any
such ambitious undertaking, the Committee has identified some areas where
improvements could be made. The areas were, in general, outside the area of the
specific Charge for the review, and, in some instances, reflect the thoughts of individual
Committee Members rather than a Committee consensus. Specific technical comments
of this nature were provided to Agency staff.
The various items of the Charge are addressed in detail in the body of this report
(Section 3), and are summarized (with the specific question italicized)below:
a) The combining of hazard identification and dose-response evaluation to
reflect more accurately the process used for noncancer health effects.
There was a clear consensus that EPA should follow the SAB position
articulated in the recent "Commentary on Hazard identification" from the
Executive Committee (EPA-SAB-EC-COM-96-001, Decembers, 1995)
which called for keeping hazard identification as an identifiable qualitative
step in the risk assessment process. The Committee recommended that
the qualitative hazard identification be decoupled from the more
quantitatively rigorous steps of exposure evaluation and dose response
assessment, and that the Agency adopt an approach similar to that used
in the proposed Cancer Risk Assessment Guidelines.
b) The issue of compensation and recovery of function in neurotoxicological
studies and how to account for compensation in neurotoxicology risk
assessment.
The Committee believes that reversibility can not be ignored in risk assessment,
but that assessors should be advised to look carefully at instances where
reversibility is involved and be alert to the possibility of re-occurrence of effects.
EPA should address specifically differences related to type of damage, nature of
the insulting agent, and age of the organism, as provide case examples relating
to possible reversibility.
c) The use of blood and/or brain acetylcholinesterase activity as an indication
of neurotoxicity for risk assessment and Considering the available data
1
-------
and the state of the science, does the SAB agree with the
recommendation that inhibition of RBC and/or plasma cholinesterase can
serve only as a biomarker of exposure?
The Committee concurred with the findings of previous SAB reviews
regarding the consideration of data on the inhibition of RBC and/or
plasma cholinesterase. The quantitative nature of the inhibition of red
blood cell (RBC) and/or plasma cholinesterase is considered unreliable
for assessing significant biological adverse changes, but can be used as
a biomarker of exposure. A noted decline in brain ChE should be
evaluated by risk assessors in terms biologically significant effects. The
Committee also suggested that further details concerning reversibility and
possible tolerance effects (which could enhance sensitivity to other
agents) be provided.
d) Are there endpoints indicative of neurotoxicity that may not be covered by
these proposed Guidelines, e.g., endocrine disruption or neuroendocrine-
mediated neurotoxicity?
The Committee does not suggest additional endpoints based on current
scientific knowledge. The Committee also recognizes that some chemical
agents may cause both neurotoxic and other effects such as endocrine
disruption; however endocrine disruption without evidence of a
neurochemical or neurophysiological causation would not be a basis to
label that chemical agent as a neurotoxicant. A finding of neuroendocrine
dysfunction might have to be amplified by neurobehavioral assessment
techniques not discussed or described in the current draft. Specific
recommendations should be included in the Guidelines for dealing with
such multi-system actions because risk assessors will typically not be
sensitive to their ramifications.
e) Are the descriptions of the endpoints used in human and animal
neurotoxicological assessments complete?
Although the Committee has many refinements and improvements to
suggest, the Guidelines provided sufficient information on various
endpoints to enable a risk assessor to do his/her job. More specifically,
the Committee found that coverage of human studies was quite good, but
that the treatment of animal studies (particularly with regard to warning
-------
caveats) was. by comparison, weaker.
g) Treatment of the possibility of no threshold for some neurotoxic agents.
The EHC suggests that the term "non-linear (at low doses) dose-response
curve for most neurotoxicants" be substituted for the term "threshold."
Again, EPA is urged to harmonize its treatment of this (as well as several
other issues) with the presentation and positions taken in the draft
Carcinogen Risk Assessment Guidelines.
g) Adequacy of the treatment of susceptible populations and individuals by
the proposed Guidelines.
The Committee noted that the aged were the only specific sub-population
identified. The Guidelines should be modified to identify other susceptible
groups (e.g., those with poor nutritional status) and provide suggestions
as to dealing with them.
h) The use of the Benchmark Dose in Neurotoxicity Risk Assessment.
No consensus on the issue of comparing the results yielded by
applications of the benchmark dose methodology and the more common
NOAEL measure. The Committee noted that definition of the benchmark
dose in the Guidelines was not clear and should be improved and
suggested that the basic position of the Guidelines be retained, i.e., that
the use of the benchmark dose should be explored in specific situations,
but that the NOAEL (No-Observed-Adverse-Effect-Level) measure
remained the default approach.
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2. BACKGROUND AND CHARGE
2.1 Background
The proposed Guidelines were developed as part of an interoffice guidelines
development program under the auspices of the Risk Assessment Forum within EPA's
Office of Research and Development. Draft Guidelines were developed by an Agency
work group composed of scientists from throughout the Agency, and selected drafts
were peer reviewed internally and by experts from universities, environmental groups,
industry and other government agencies. A preliminary draft underwent peer review in
a workshop held on June 2-3, 1992, and has received internal review by the
Concordance and Oversight Subcommittees of the Risk Assessment Forum. These
proposed Guidelines also were reviewed on August 15, 1995, by the Committee on the
Environment and Natural Resources of the Office of Science and Technology Policy.
The proposed Guidelines are based on recommendations derived from these reviews
and on those made at various scientific meetings and workshops on neurotoxicology.
The proposed Guidelines were published for public comment in the October 4, 1995,
issue of the Federal Register.
After completion of the SAB review, the proposed Guidelines will be revised
according to the public comments and responses to the SAB. The revised Guidelines
will then undergo Agency review before being published in final form in the Federal
Register.
2.2 Charge
a) The combining of hazard identification and dose-response evaluation to reflect
more accurately the process used for noncancer health effects (Section I).
The draft Guidelines recognize that, in practice, hazard identification for
noncancer health effects is usually done in conjunction with an evaluation of dose-
response relationships in studies to identify hazard. This process of hazard
characterization provides an evaluation of a hazard within the context of the dose,
route, duration and timing of exposure. With one exception, public comment supported
the combining of hazard identification and dose-response evaluation.
In 1994, EPA requested SAB comment on this issue during the EHC's review of
the draft Guidelines for Reproductive Toxicity Risk Assessment (July 19, 1994 Meeting;
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EPA-SAB-EHC-95-014, and Decembers, 1995 Memorandum; EPA-SAB-EC-COM-96-
001). EPA's draft proposed Neurotoxicity Guidelines reflect the SAB's earlier opinions
by incorporating the EHC/SAB comments into these proposed Guidelines.
b) The issue of compensation and recovery of function in neurotoxicological
studies and how to account for compensation in neurotoxicology risk assessment
(Section II)
Once damaged, neurons, particularly in the central nervous system, have a
limited capacity for regeneration. Reversibility of effects resulting from cell death or
from destruction of cell processes may represent an activation of repair capacity,
decreasing future potential adaptability. The nervous system has a reserve capacity
that, once exceeded, results in permanent, irreversible loss of nervous system function
and structure. Reversible neurotoxic effects should be of concern to the risk assessor.
c-1) The use of blood and/or brain acetylcholinesterase activity as an indication
of neurotoxicity for risk assessment (Section I HA).
As written, the proposed Guidelines state that statistically significant decreases
in brain cholinesterase can be considered to be a biologically significant event.
However, the proposed Guidelines indicate that there is a lack of consensus as to
whether RBC and/or plasma cholinesterase activity represents biologically significant
events. Instead, inhibition of RBC and/or plasma cholinesterase can serve as a
biomarker of exposure.
Many comments were received from the public on this issue, including many that
supported the position that a 20% inhibition in brain acetylcholinesterase should be
considered as evidence of neurotoxicity and that inhibition of erythrocyte, whole-blood,
or plasma acetylcholinesterase could provide only indirect evidence of neurotoxicity.
Measures of blood acetylcholinesterase could be used to support clinical observations
of cholinesterase inhibition.
c-2) Considering the available data and the state of the science, does the SAB
agree with the above recommendations?
d) Endpoints indicative of neurotoxicity that may not be covered by these
proposed Guidelines, e.g., endocrine disruption or neuroendocrine-mediated
neurotoxicity
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The proposed Guidelines are inclusive of the major neurotoxicity endpoints of
concern and the Agency does not plan to develop additional topics. Few public
comments were received on this issue although several that were received supported
this position. No additional neurochemical, neurophysiological, or structural endpoints
were suggested. Comments indicated no need to consider endocrine disrupters
differently then other potential neurotoxic agents.
e) The completeness of the description of the endpoints used in human and
animal neurotoxicological assessments (Section III).
The proposed Guidelines are relatively complete in describing endpoints used in
human and animal neurotoxicological assessments. Some modification will be made to
the human studies section to include discussion on the evaluation of more subjective
effects, such as changes in mood, irritability, and well-being as well as additional
discussion on neurobehavioral and neurochemical measures.
f) The possibility of no threshold for some neurotoxic agents.
In general, a threshold is assumed for the dose-response curve for most
neurotoxicants. This is based on the known capacity of the nervous system to
compensate for or to repair a certain amount of damage at the cellular, tissue, or organ
level. In addition, because of the multiplicity of cells in the nervous system, multiple
insults at the molecular or cellular level may be required to produce an affect on the
whole organism.
Recent data suggest that there may be no threshold for some suspected
neurotoxic agents such as developmental exposure to lead. During the SAB's July
1994 review of the Guidelines for Reproductive Toxicity Risk Assessment, the EHC
recommended that use of the threshold assumption should occur only after an
evaluation of the likely biological mechanisms and available data provide evidence that
linear responses would not be expected. The Agency agrees that all available
mechanistic information should be reviewed carefully and analyzed to determine the
most suitable approach when information is available to make such a decision.
However, in the absence of data to make a judgment based on biological mechanisms,
the Agency will assume a threshold for neurotoxic effects as a default. This approach
is consistent with the Agency's long-standing practice used in other EPA guidelines for
health effects other than cancer.
g) Adequate treatment of susceptible populations and individuals by the
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proposed Guidelines (Section IV).
Several public comments indicated that additional categories of susceptible
populations, e.g., the elderly, should be addressed. EPA agrees, but considers parallel
questions such as modification of uncertainty factors for susceptible populations to be
outside the scope of these proposed Guidelines.
h) The use of the Benchmark Dose in Neurotoxicity Risk Assessment (Section
IVB).
There was consensus among public commentors in support of the Agency
exploring the use of other models for quantitative risk assessment. However, a majority
of respondents indicated that the Benchmark Dose (BMD) was not yet ready for actual
use in neurotoxicity risk assessment. Several responders provided a number of
qualifiers that should be included in the Guidelines' discussion on the BMD, including
the conservative nature at the lower end of the dose-response curve, the need for
computational formulas, guidance as to how the BMD would be used in risk
assessment, and the need for studies that compare the performance of the BMD
relative to the RfD approach.
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3. SPECIFIC ISSUES
3.1 Combining Hazard Identification and Dose-response Evaluation
The presence or absence of a dose response relationship is an important
consideration in the hazard identification and characterization step of a risk
assessment. Further, a discussion of the hazard in terms of dose, route, duration and
timing of exposure issues can be important feature of the hazard characterization in
some cases. However, this can be achieved without combining the hazard
identification and dose response steps, as has been done in the draft neurotoxicity
guidelines. Hazard characterization and dose response should remain distinct,
separate steps in risk assessment. The EPA, in its proposed cancer guidelines has
identified an approach which accomplishes this while providing for
considerations of dose response issues at the hazard
identification/characterization stage, and the EHC encourages the Agency to
adopt this approach in guidance documents for non-cancer endpoints as well.
The importance of maintaining hazard characterization and dose response
evaluation as separate steps in risk assessment has been previously discussed, in the
SAB Environmental Health Committee findings on the Guidelines for Reproductive Risk
Assessment SAB, 1995), and in the subsequent December 1995 letter by the SAB
chair, Dr. Matanoski to Administrator Browner, which further clarifies the SAB position
on this issue (SAB, 1996). The Agency released the draft neurotoxicity guidelines in
October 1995, before the letter from the SAB chair was sent to Administrator Browner.
In revising the draft neurotoxicity guidelines, the Agency should consider the
SAB position on this matter, as outlined in the above noted letter from the SAB chair:
"The National Academy of Sciences (MAS) risk assessment framework,
described in their 1983 report Decision Making in the Federal Government:
Managing the Process has proven a useful and durable tool for assessing risk.
It identifies easily understandable and recognizable steps for assessing risk and
using the results for decision-making. In 1994, the National Academy of
Sciences report Science and Judgment in Risk Assessment, recommended that
EPA and others should broaden the types of information considered in the
hazard identification phase of risk assessment. The SAB supports EPA's intent
to expand the hazard identification and evaluation phases to include additional
data. However we believe that hazard identification/characterization is too
8
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multidimensional to be merged into dose-response assessment in a fixed
manner, and the Committee recommend that the phases of the hazard
identification process remain clearly discernable.
The hazard identification should continue to be essentially qualitative in nature
on the following where data are available: overall consistency of data, nature of
effects observed, relevance of the effect(s) to human health, mechanisms of
action if known, pattern of dose response relationships in the studies reviewed,
and phamacokinetic data where appropriate (especially in terms of qualitative
differences in metabolic pathways between species). In contrast, the
dose-response analysis step evaluates in quantitative terms the relationship
between dose or exposure and severity or probability of effect in humans."
Finally we note in passing that it can be useful to characterize an agent as a
"neurotoxicant," and the Agency has characterized agents as such throughout the
current draft guidelines. Yet, the current Guidelines would appear to call for such
characterizations only with the indication of the conditions of exposure under which
such characterizations would apply.
3.2 Compensation and Recovery of Function in Neurotoxicological Studies
Dealing with reversible neurotoxic effects is a matter of considerable concern to
the risk assessor. In some organ systems, such as the liver, functional recovery and
tissue regeneration can follow even extensive injury. Tissue regeneration does not
occur in nerve cells, however. When functional recovery appears, as after a stroke, it is
due basically to the remaining neurons compensating for those lost to injury. This is an
extreme example of how apparent reversibility may prove deceptive. A more subtle
illustration may be found by examining the post-polio syndrome. Individuals who
suffered an episode of polio in their early years, then evidently recovered, experience a
reappearance of symptoms as they age. The most popular hypothesis to account for
this phenomenon posits compensatory mechanisms as the culprit. Remaining nerve
cells in the spinal cord are believed to develop additional connections to overcome the
loss of the failed cells. The added metabolic burden, combined with the natural loss of
nerve cells with aging, erodes the compensatory margin and the previously masked
deficit then reemerges.
Another setting in which apparent reversibility could prove deceptive is the
workplace. Recurrent episodes of clinically detectable neurotoxicity due to solvent
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exposure, although each is followed by evident recovery, may eventually lead to
enough cumulative impairment to be detectable. Perhaps such a history accounts for
some
proportion of the deficits observed in workers exposed to solvents during past eras in
which monitoring of exposure levels was absent, haphazard, or intermittent.
Laboratory data add another dimension-the aftermath of exposure during early
brain development. In rats, brain injury suffered via lesions or ionizing radiation early
in life may produce behavioral deficits visible late in life. During adulthood, the deficits
often become submerged. During senescence, they emerge again. Prenatal drug
treatment can produce similar fluctuations in function over the lifespan.
Stage of the lifespan, in fact, is another variable influencing reversibility. Older
animals recover from treatment with MPTP (methylphenyltetrahydraopyridine), AChE
antagonists, and other chemical agents less completely and rapidly than younger
animals. Apparent reversibility in young organisms cannot be extrapolated to older
organisms.
Given such data, the SAB supports the presumption that what appear to be
reversible neurotoxic effects, especially those arising from gestational or neonatal
exposure and observed before adulthood, should not be dismissed as of little practical
consequence. They may be indices of silent toxicity that emerge later in life or may
suggest more robust and enduring responses in aged individuals.
3.3 Blood And/or Brain Acetylcholinesterase Activity as an Indication of
Neurotoxicity for Risk Assessment
The proposed Guidelines state that statistically significant decreases in brain
cholinesterase can be considered to be a biologically significant event, and the Charge
for this review asked for the Committee's position on this issue. There is a historic lack
of consensus on this issue, however, as to whether RBC and/or plasma cholinesterase
activity represents biologically significant events or is primarily a biomarker of
exposure.
Many comments were received from the public on this issue, including many that
supported the position that a 20% inhibition in brain acetylcholinesterase should be
considered as evidence of neurotoxicity and that inhibition of erythrocyte, whole-blood,
or plasma acetylcholinesterase could provide only indirect evidence of neurotoxicity.
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Measures of blood acetylcholinesterase could be used to support clinical observations
of cholinesterase inhibition.
The subject of neurotoxic criteria for acetylcholinesterase (AChE) inhibitors was
addressed by the SAB in 1990 (SAB, 1990). This review addressed many of the issues
posed to the Committee by the Charge for the review of the current document. The
1990 review concluded that RBC and plasma cholinesterase levels serve as indices of
exposure rather than as direct measures of neurotoxicity or other biologically significant
effects. This conclusion contradicts the position that any "statistically significant" (a
term which is discussed below) reduction in these peripheral ChE levels is evidence of
toxicity. The Committee believes that the position taken by the SAB in 1990 is still
valid. The 1990 report also noted that reduced brain ChE is not necessarily inherently
adverse because of the large functional reserves of AChE in the brain, and
compensatory mechanisms such as up- and down-regulation of receptor populations.
Inhibitory muscarinic AChE receptors have been located presynaptically, for example,
and act as autoreceptors regulating acetylcholine release. Prolonged exposure to
cholinergic agonists or AChE inhibitors such as DFP reduces the density of these
receptors, even to the point of inducing behavioral deficits despite other indications of
tolerance. Defining neurotoxicity on the basis of a statistically significant change in
brain ChE activity, however, evades the question of biological significance. As is true
for peripheral ChE measures, the brain contains large reserves of AChE. A fall in brain
ChE, however, does merit a thorough appraisal of the criteria noted above so that a
more decisive relationship between the degree of ChE inhibition, its time course, and
its consequences can be established; it is not the same for all organophophates. Such
data will indicate the biological significance of a specified fall in ChE. It would help to
establish a margin of exposure for the selected agent.
The Guidelines, in discussing changes in ChE levels, use the term "statistically
significant." Although seemingly precise, this term is actually vague and undefined.
One way to overcome this problem would be to adopt a more tangible and quantitative
index, such as the Benchmark Dose and its variations, as discussed in the Guidelines.
Various methods are now available to derive such indexes. A promising approach for
AChE inhibitors is that outlined by Gaylor and Slikker (1990). It examines the
distribution of some measure in a control sample, then determines, in an exposed
sample, the proportion of responses beyond a specified measure (e.g., three standard
deviations of the control mean). It would lend itself to risk quantification of criteria
ranging, say, from 10% to 90% reductions in AChE.
Two additional facets of AChE neurotoxicity should also be discussed in the
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Guidelines because they may pertain to other classes of chemicals as well and to the
reversibility question (discussed in general terms above). The first is the development
of tolerance, possibly arising from down-regulation of receptors, which may mask
enhanced sensitivity to other agents or be followed by withdrawal compensation. Or, it
may reflect behavioral adaptation, a phenomenon seen with other kinds of agents such
as opiates. Again, this is a problem in experimental design and methodology that
cannot be dissociated from the discussion in the Guidelines. The second facet,
reversibility, is confounded with the enduring consequences of an acute exposure great
enough to have induced neurotoxic signs. Long after apparent recovery, neuro-
behavioral deficits may still be detectable with sensitive test methods. These lingering
effects have been observed in both human and animal studies.
In summary, the Committee does not accept the current position of the
Guidelines vis-a-vis the significance of ChE decreases. In the absence of clinical
signs in humans or animals or the absence of morphological data in animals, the
quantitative nature of the inhibition of red blood cell (RBC) and/or plasma
cholinesterase is considered unreliable for assessing significant biological
adverse changes, but can be used as a biomarker of exposure.
3.4 Other Endpoints Indicative of Neurotoxicity
The proposed Guidelines are inclusive of the major neurotoxicity endpoints of
concern and the Agency does not plan to develop additional topics. Few public
comments were received on this issue although several that were received supported
this position. No additional neurochemical, neurophysiological, or structural endpoints
were suggested. Comments indicated no need to consider endocrine disrupters
differently then other potential neurotoxic agents.
The Committee recognizes that the description of endpoints indicative of
neurotoxicity is extensive, and, in most circumstances, reflects the actions of
conventionally neurotoxic agents. The Committee also recognizes the mutual
dependence and influences among the nervous, immune, and endocrine systems that
have been revealed during the past two decades. In addition to such indirect effects,
many agents identified as neurotoxic also exert potent effects on immune and
endocrine function. Such broad systemic, interactive effects might not necessarily imply
that an agent provoking impaired endocrine function, for example, would need
concurrently to be assayed for neurotoxicity. That need would depend upon the nature
of the effect. The Guidelines should recognize, however, that a finding of
neuroendocrine dysfunction might have to be amplified by neurobehavioral
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assessment techniques not discussed or described in the current draft. Observation of
a widening or narrowing of male-female differences on certain neurotoxic endpoints
could be a reflection of interference with neuroendocrine function. Specific
recommendations should be included in the Guidelines for dealing with such
multi-system actions because risk assessors will typically not be sensitive to their
ramifications.
3.5 Description of Endpoints for Neurotoxicological Assessments
In general the descriptions of endpoints used in human and animal
neurotoxicological assessments are thorough, easy to read, adequately documented
and, in fact, appear to be exhaustive. There are a few areas for improvement,
however. The observations in the functional observational battery (FOB) could be
"sharpened;" FOBs should be conducted according to the need for inter-observer
reliability and thoroughly defined responses.
As a caution to the inexperienced reader on the use of single animal
observations of a behavioral parameter to reach a conclusion on the NOAEL, the
Guidelines state (correctly) that "It is reasonable to assume that a NOAEL or LOAEL
could be based on one or more of these endpoints." It would be useful to move this
sentence from its current location (the third column, middle of page 52044) to be closer
to the discussion of the relevance of statistically significant test results at the bottom of
the page.
Finally, adding caveats about the need to be sure the data are reliable,
particularly for case reports and neurologic examinations, to the discussion of tests
used in clinical studies would improve the document, and changing the word
"Summary" in the title of Table 5 to "Examples" would make it consistent with the other
tables.
3.6 The Possibility of No Threshold for Some Neurotoxic Agents
In general, a threshold is assumed for the dose-response curve for most
neurotoxicants. This is based on the known capacity of the nervous system to
compensate for or to repair a certain amount of damage at the cellular, tissue, or organ
level. In addition, because of the multiplicity of cells in the nervous system, multiple
insults at the molecular or cellular level may be required to produce an affect on the
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whole organism.
Recent data suggest that there may be no threshold for some suspected
neurotoxic agents such as developmental exposure to lead. During the SAB's July
1994 review of the Guidelines for Reproductive Toxicity Risk Assessment, the EHC
recommended that use of the threshold assumption should occur only after an
evaluation of the likely biological mechanisms and available data provide evidence that
linear responses would not be expected. The Agency agrees that all available
mechanistic information should be reviewed carefully and analyzed to determine the
most suitable approach when information is available to make such a decision.
However, in the absence of data to make a judgment based on biological mechanisms,
the Agency assumes a threshold for neurotoxic effects as a default. This approach is
consistent with the Agency's long-standing practice used in other EPA guidelines for
health effects other than cancer.
A strict threshold is not always clear in the human population because of the
wide variation in background levels of neurobehavioral function. Cumulative
neuro-damage may alter the response of some individuals within a special population.
Therefore, EPA risk assessors should have the option of establishing one level for the
general healthy population and a second level to protect a separate population of
compromised individuals. It must be remembered that the concept of "threshold" is
physiological and structural in the context of general neurological function, and the
wide range of "normal" has to be surpassed before damage is considered clinically
significant. This is not necessarily true in laboratory animals where function was
apparently normal in the face of data to the contrary at autopsy.
Although the Committee does not disagree with the guidelines assumption
of a threshold as a default for neurotoxic effects, it urges EPA to harmonize its
treatment of this issue with the 'weighing of evidence' positions taken in the draft
Carcinogen Risk Assessment Guidelines (which, at the time of the public meeting were
out for public comment).
3.7 Treatment of Susceptible Populations and Individuals
Several public comments indicated that additional categories of susceptible
populations, e.g., the elderly, should be addressed. EPA agreed, but considered
parallel questions such as modification of uncertainty factors for susceptible
populations to be outside the scope of the proposed Guidelines.
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There was, however, a consensus among the Committee members that the
elderly should be considered a susceptible population. The elderly might be at
increased risk of toxic effects for several age-related reasons, including:
a) loss of neurons with age which may result in a decline in the reserve
capacity for the development of compensatory neural pathways required
with various types of injury;
b) decrease in the ability to detoxify or excrete xenobiotics with age which
may result in increased body levels of the biologically active agent for the
same exposure; and
c) increase with age in the use of medications which may increase the
potential for synergistic interactions between toxic chemicals and
medications.
There may be other special populations that also deserve consideration as
potentially susceptible to the effects of neurotoxicants. These populations include:
a) individuals with other chronic and debilitating conditions (e.g., stroke,
cancer, renal failure);
b) certain groups of workers with potential exposure to other chemicals that
may be neurotoxic such as solvents or pesticides;
b) some ethnic subgroups because genetic polymorphism for susceptibility
to toxicity in humans may vary with ethnicity;
c) some disadvantaged communities because of their interactions with the
environment (e.g., may cluster around exposures to mixtures of other
chemicals), or diet (e.g., subclinical nutritional deficiencies or a diet that
may be rich in an item prone to certain contamination).
In the absence of data indicating whether or how the uncertainty factor
should be modified, the risk assessor should be advised to consider the extent to
which the exposed population includes susceptible subgroups and, if possible,
conduct risk assessment separately for such subgroups.
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3.8 The use of the Benchmark Dose
This issue was the subject of prolonged discussion at the Committee's public
meeting. There were differences within the EHC on many aspects of the use and
interpretation of the "Benchmark Dose" (BMD), and there was not a consensus that
the BMD was ready for immediate incorporation into adjustment-factor-based
safety assessment and could serve as a substitute or replacement for the more
familiar No-Observed-Adverse-Effect-Level (NOAEL). Research and development
on the BMD should be aggressively encouraged and actively supported by both the
public and private sectors. In the meantime, trials to gain experience with the BMD and
its use in safety assessment should also be encouraged. Attention should be directed
at developing a broad consensus among informed scientists on:
a) guidance on the characteristics and qualities of specific data sets that
make them appropriate (or inappropriate) for use in BMD estimation
b) a definition for the term, "observable range" and guidance on the limits for
that range
c) bases for selecting from among the various methods and models to
calculate the BMD
d) guidance on how to incorporate information on the dose-response slope
into safety assessment judgments
e) means to prevent inappropriate extrapolation of BMD statistics
f) mathematically robust and biologically meaningful means to deal with
continuous, as well as quantal, data
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4. SUMMARY
In general, the Committee finds the revised Guidelines document to be quite
successful, and, all things considered, well suited to its intended task. It addresses a
wide range of subjects of considerable complexity and (often) of considerable subtlety,
and constitutes a clear step forward in the state of the art. Naturally, as with any such
ambitious undertaking, the Committee's review has identified areas where
improvements could be made. Specifics on such improvements, which generally lie
outside the area of the specific Charge for the review, and reflect the thoughts of
individual Committee Members rather than a Committee consensus, have been
provided to Agency staff.
The various items of the Charge are addressed in detail in the body of this report
(Section 3), and are summarized (with the specific question italicized)below:
a) The combining of hazard identification and dose-response evaluation to
reflect more accurately the process used for noncancer health effects.
There was a clear consensus that EPA should follow the SAB position
articulated in the recent "Commentary on Hazard identification" from the
Executive Committee (EPA-SAB-EC-COM-96-001, Decembers, 1995)
which called for keeping hazard identification as an identifiable qualitative
step in the risk assessment process. The Committee recommended that
revisions be made to decouple the qualitative of hazard identification from
more quantitatively rigorous steps of exposure evaluation and dose
response assessment. The Committee encourages the Agency to adopt
an approach similar to that used in the proposed Cancer Risk
Assessment Guidelines.
b) The issue of compensation and recovery of function in neurotoxicological
studies and how to account for compensation in neurotoxicology risk
assessment. The Committee took the position that reversibility can not be
ignored in risk assessment, but that an assessor had to be very careful
and search out particularly good evidence if recovery or an apparent
transient effect is cited to support evidence for relatively benign effects.
In general, assessors should be advised to look carefully at instances
where reversibility is involved and be alert to the possibility of re-
occurrence of effects. The Committee recommended that EPA address
specifically differences related to type of damage, nature of the insulting
agent,
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and age of the organism, as well providing case examples relating to
possible reversibility.
c) The use of blood and/or brain acetylcholinesterase activity as an indication
of neurotoxicity for risk assessment and Considering the available data
and the state of the science, does the SAB agree with the
recommendation that inhibition of RBC and/or plasma cholinesterase can
serve only as a biomarker of exposure? The Committee addressed these
two issues together because of their close relationship. The EHC
concurred with the findings of previous SAB reviews regarding the
consideration of data on the inhibition of RBC and/or plasma
cholinesterase. In the absence of clinical signs in humans or animals or
the absence of morphological data in animals, the quantitative nature of
the inhibition of red blood cell (RBC) and/or plasma cholinesterase is
considered unreliable for assessing significant biological adverse
changes, but can be used as a biomarker of exposure. The Committee
also recommended that a noted decline in brain ChE should be evaluated
by risk assessors in terms of possible effects that are biologically
significant, and that the term "statistically significant" needed to be better
explicated - perhaps in terms of the benchmark dose or by some
measure which reflected information about the distribution of the effect
under study. The Committee also suggested that further details
concerning reversibility and possible tolerance effects (which could
enhance sensitivity to other agents) be provided.
d) Are there endpoints indicative of neurotoxicity that may not be covered by
these proposed Guidelines, e.g., endocrine disruption or neuroendocrine-
mediatedneurotoxicity? The Committee considered that the description
of endpoints indicative of neurotoxicity was extensive and it does not
suggest additional endpoints based on current scientific knowledge. The
Committee also recognizes that some chemical agents may cause both
neurotoxic and other effects such as endocrine disruption; however
endocrine disruption without evidence of a neurochemical or
neurophysiological causation would not be a basis to label that chemical
agent as a neurotoxicant. This notwithstanding, the Guidelines should
recognize that a finding of neuroendocrine dysfunction might have to be
amplified by neurobehavioral assessment techniques not discussed or
described in the current draft. Observation of a widening or narrowing of
male-female differences on certain neurotoxic endpoints could be a
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reflection of interference with neuroendocrine function. Specific
recommendations should
be included in the Guidelines for dealing with such multi-system actions
because risk assessors will typically not be sensitive to their ramifications.
e) Are the descriptions of the endpoints used in human and animal
neurotoxicological assessments complete? In the broadest sense, the
Committee found the treatment of this issue by the Guidelines document
to be "..not complete, but complete enough for its purposes," meaning
that, although the Committee has many refinements and improvements to
suggest, the Guidelines provided sufficient information on various
endpoints to enable a risk assessor to do his/her job. More specifically,
the Committee found that coverage of human studies was quite good, but
that the treatment of animal studies (particularly with regard to warning
caveats) was. by comparison, weaker. It was also suggested that better
coverage be given to various possible tests (including references for
various test methodologies); the Committee also agreed with the comment
by EPA staff that the Guidelines were not a "Testing Guideline," and that
details of specific tests were not called for. Re tests, the EHC also
suggested that Table V list the cited tests in alphabetical order to avoid
the implication of priority noted in the current arrangement. It was also
noted that addition treatment of maternal toxicity and possible effects on
offspring, particularly re interpreting dose-response curves, would be
useful.
g) Treatment of the possibility of no threshold for some neurotoxic agents.
There was considerable debate/discussion as to what the concept of a
threshold, or of no threshold, actually meant in terms of a risk
assessment, and whether the concept itself was meaningful or useful in
this context. It was proposed that the term "non-linear (at low doses)
dose-response curve for most neurotoxicants" be substituted for the term
"threshold." The EPA was urged to harmonize its treatment of this (as
well as several other issues) with the presentation and positions taken in
the draft Carcinogen Risk Assessment Guidelines currently out for public
comment.
g) Adequacy of the treatment of susceptible populations and individuals by
the proposed Guidelines. The Committee noted that the aged were the
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only specific sub-population noted, and that the Guidelines do not discuss
means of modifying NOAELS or dose-response curves in this context. It
was suggested that the Guidelines be modified to identify other
susceptible groups (e.g., those with poor nutritional status) and provide
suggestions as to dealing with them.
h) The use of the Benchmark Dose in Neurotoxicity Risk Assessment. There
was considerable discussion within the Committee and between the
Committee, EPA staff, and members of the audience as to the nature of
the results yielded by the application of the benchmark dose methodology
and how it compared with the more common NOAEL measure. No
consensus on this aspect of the issue was reached. The Committee did
note that definition of the benchmark dose in the Guidelines was not clear
and shout be improved; it further suggested that the basic position of the
Guidelines be retained, i.e., that the use of the benchmark dose should
be explored in specific situations, but that the NOAEL measure was the
default approach.
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REFERENCES
Crump, K. S. 1984. A new method for determining allowable daily intakes.
Fundamental & Applied Toxicology 4: 854-871.
Faustman, E.M., Allen, B.C., Kavlock, R. J., Kimmel, C.A. 1994. Dose-response
assessment for developmental toxicity: I. Characterization of data base and
determination of NOAELs. Fundamental & Applied Toxicology 23: 478-486
Gaylor, D.W., and Slikker, W. Jr. 1990. Risk assessment for neurotoxic effects.
Neurotoxicology 11:211-218.
National Research Council (NRC). 1994. Science and judgement in risk assessment,
Committee on Risk Assessment of Hazardous Air Pollutants, Board on
Environmental Studies and Toxicology, National Academy Press, Washington
DC.
National Research Council (NRC). 1983. Risk Assessment in the Federal
Government: Managing the Process. National Academy Press, Washington,
DC.
SAB. 1995. (EPA-SAB-EHC-95-014) Review of the Guidelines for Reproductive Risk
Assessment.
SAB. 1996. (EPA-SAB-EC-COM-96-001) Letter, Dr. G. Matanowski to Administrator
Browner, which further clarifies the Science Advisory Board position on the issue
of combining hazard identification and hazard characterization.
SAB. 1990. (EPA-SAB-EC-90-014). Joint Science Advisory Board/Scientific Advisory
Panel Review of Cholinesterase Inhibition and its Effects.
Searle, S. R., Casella, G. and McCulloch, C.E. 1992. Variance Components. New
York: John Wiley
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