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1
INTRODUCTION
A major function of the National Toxicology Program (NTP) is the
selection and testing of chemicals for toxicity. NTP considers chemicals
for testing on a continuing basis, as candidates are nominated by the
federal agencies served by the programs, by state and local governments,
and by academic, industrial, and labor groups. Such candidates are of
possible interest to NTP because of their potential for human exposure
and possible public-health impact.
This portion of the study, the determination of toxicity-testing
needs, was undertaken because NTP recognizes that the number of
substances, both natural and man-made, in the human environment is large
and is increasing with no clear indication of the nature and amount of
toxicity information that might be needed on these substances to
ascertain potential for adverse effects on human health. It is useful
for NTP to know as precisely as practicable the toxicity-testing needs
for substances to which humans are potentially exposed. Consequently,
NTP asked the National Research Council to address these matters.
A major objective of the study was to estimate the amount and type of
toxicity testing needed in the defined select universe of substances. To
accomplish this objective, three operating goals had to be met by the
Committee on Statistical Sampling Methods (commonly called the Committee
on Sampling Strategies) and the Committee on Characterization of Status
of Toxicity Data Elements for a Select Universe of Compounds (commonly
called the Committee on Toxicity Data Elements):
· To estimate the proportion of compounds in the select universe on
which toxicity data exist.
· To estimate the proportion of compounds in the select universe on
which toxicity testing has been adequate or that NTP should consider for
additional toxicity testing.
· To determine the nature of any additional toxicity testing that
may be required for particular classes of compounds.
The size of the select universe precluded retrieving and evaluating
existing toxicity data on all its constitutents to determine the extent
to which additional data are needed. To approach an understanding of the
status of toxicity information on the select universe, a scheme using
carefully conceived sampling techniques, as described in Chapter 2, was
developed. A probability sample was extracted from the select universe
and later analyzed to learn the extent and quality of toxicity data on
substances in the sample. The primary objective of creating the sample
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was to permit the characterization of the status of toxicity information
on chemicals in the sample, the characterization of the quantitative
distribution of toxicity data in the sample, and the estimation of the
proportions of substances in the select universe on which there are
various degrees of toxicity data. This knowledge was then used to
estimate the types and amounts of toxicity testing that might be
required. The Committee on Sampling Strategies, composed primarily of
experts in statistics, was responsible for evaluating sampling methods,
for selecting the most appropriate sampling approach to be used in this
study, for generating a sample, for assembling the data, and for
assisting in the interpretation of the results of sample evaluation.
To estimate the percentage of substances that might need further
testing, each substance in the final sample was the subject of an
exhaustive search for toxicity information, as described in Chapter 3 and
in Appendix M. m e Committee on Toxicity Data Elements, composed mainly
of experts in the toxicologic sciences, was responsible for establishing
criteria by which the toxicity information on chemicals in the sample was
to be characterized, for applying the criteria to the sample, and for
interpreting the results in relation to the select universe.
By blending their expertise, the two committees jointly estimated the
toxicity-testing needs of the select universe. The description of
toxicity-testing needs includes an estimate of the proportion of the
select universe with a degree of information defined later in this report
as minimal for the evaluation of possible hazard to human health.
m e methods for documentation, protocol evaluation, and testing-need
determination used by the committees are presented in this report. These
methods were structured and detailed in such a fashion that new
information and different judgments can be entered into the process, so
that interpretations can be revised.
Part 1 of this report, which contains the approaches and procedures
used to ascertain the toxicity-testing needs of a select universe of
chemical substances, begins with a detailed description of the contents
of the select universe as a sampling frame, from which a sample and
subsample were drawn. The sampling process is then presented, with
particular attention to the choice of sampling method and the sample
size. This is followed by an accounting of procedures for data
identification, acquisition, and organization, including a description of
the operating policies established by the two committees. The
organizational framework for the recording and evaluation of data is
documented with a consideration of the committees' adopted principles for
the evaluative process. The specific standards and process of data
evaluation used by the committees are then described. Finally, after a
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presentation of the statistical techniques used for data interpretation
and analysis, the committees explain in detail the results of their
findings, with estimates of the quality and quantity of toxicity testing
available for the entire select universe and of unmet needs.
The report of the Committee on Toxicity Data Elements and the
Committee on Sampling Strategies does not include an evaluation of the
toxicity itself of the substances examined by the two committees, nor of
the predictive power (false-positive and false-negative rates) of tests,
the nature of toxicity end points (e.g., the proportion of substances in
the select universe that are carcinogenic), or the nature of exposure end
points (e.g., the proportion of substances that have high exposure
potential). To the extent that it includes an examination of individual
substances, it does so only for their usefulness as representatives of a
larger population, namely the select universe.
The decisions, criteria, and findings of this study were based solely
on scientific principles. The study did not include the use of statutory
processes of the various federal agencies with regulatory interest in the
substances contained in the select universe. The agencies may have
regulatory constraints that could alter their use of the information
contained in this report; such use would not necessarily be based solely
on scientific evaluation.
In the detailed explanations of each phase, the committees present
their procedures and assumptions with cautionary statements where
appropriate, alerting readers to the possibility of misinterpretation.
Because the process used by the committees was complex and integrative,
readers are advised to note the details and cautionary statements with
equal care to safeguard their understanding of the committees' findings
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Representative terms from entire chapter:
toxicity data
