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OCR for page 1
Executive Summary
INTRODUCTION
Naturally occurring radionuclides are found throughout the earths
crust, and they form part of the natural background of radiation to which all
humans are exposed. Many human activities such as mining and milling of
ores, extraction of petroleum products, use of groundwater for domestic
purposes, and living in house& alter the natural background of radiation either
by moving naturally occurring radionuclides from inaccessible locations to
locations where humans are present or by concentrating the radionuclides in the
exposure environment. Such alterations of the natural environment can increase,
sometimes substantially, radiation exposures of the public.
Exposures of the public to naturally occurring radioactive materials
(NORM) that result from human activities that alter the natural environment can
be subjected to regulatory control, at least to some degree. The regulation of
public exposures to such technologically enhanced naturally occurring
radioactive materials (TENORM) by the US Environmental Protection Agency
(EPA) and other regulatory and advisory organizations is the subject of this
study by the National Research Council's Committee on the Evaluation of EPA
Guidelines for Exposures to Naturally Occulting Radioactive Materials.
The committee has used the term technologically enhanced naturally
occurring radioactive materials to refer to He materials of concern to this study
and has defined this term as follows:
Technologically enhanced naturally occurring radioactive
materials are any naturally occurring radioactive materials not
subject to regulation under the Atomic Energy Act whose
radionuclide concentrations or potential for human exposure have
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2
GUIDELINES FOR EXPOSURE TO TENORM
been increased above levels encountered in the natural state by
human activities.
The exclusion of NORM subject to Atomic Energy Act jurisdiction
from the definition of TENORM means that this study is not concerned with
evaluating guidelines developed by EPA or other federal agencies that apply to
NORM associated with the production and use of nuclear fuels, including
uranium and thorium mill tailings, naturally occurring radionuclides released to
the environment during operations of nuclear fuel-cycle facilities, or natural
uranium or thorium in the form of source material. The most important
radionuclides in TENORM as defined in this study include the long-lived,
naturally occurring isotopes of radium, thorium, and uranium and their
radiologically important decay products (such as radon), as well as potassium-
40.
PURPOSE AND SCOPE OF STUDY
This study resulted from considerations by EPA and other
organizations of guidelines for controlling exposures of the public to TENORM.
Specifically, there has been a concern that EPA and other organizations have
arrived at different numerical values for guidelines for essentially the same
exposure situations but that the reasons for the differences, especially the extent
to which they were based on scientific and technical considerations, were not
apparent.
In light of that concern, the National Research Council committee was
asked to address the following questions:
· Whether the differences in the guidelines for TENORM developed by
EPA and other organizations are based upon scientific and technical
information, or on policy decisions related to risk management.
· If the guidelines developed by EPA and other organizations differ in
their scientific and technical bases, what the relative merits of the
different scientific and technical assumptions are.
. Whether there is relevant and appropriate scientific information that
has not been used in the development of contemporary risk analysis for
NORM.
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EXECUTIVE SUMMARY
3
In view of the concern about guidelines for TENORM and their
scientific and technical bases, the committee has considered approaches to risk
assessment for NORM only in regard to their use in developing and
implementing guidelines for controlling radiation exposures of the public. The
committee has not addressed other issues of risk assessment for NORM that
may arise in attempting to estimate real risks posed by actual exposures of
individuals or populations.
The guidelines evaluated in this study include those for indoor radon
and those for any other TENORM. Those two types of NORM generally have
been regulated separately. After a thorough review of the subject, we have
concluded that the differences between regulatory agencies' and other
organizations' guidances for the control of TENORM have little basis in science
but reflect differences in risk-management approaches and organizational
. .
mlsslons.
This study is not concerned with evaluations of nonscientific issues of
importance to the development of guidelines for NORM, such as costs and
policy judgments in risk management. However, in evaluating whether
differences in guidelines for NORM developed by EPA and other organizations
are based on scientific and technical information, the committee found it
necessary to identify important policy judgments in risk management that have
influenced development of the guidelines, even though the merit of any such
judgments is not considered.
The EPA's current guidelines for indoor radon, however, are easily
identified. In the case of TENORM other than indoor radon, it was not a simple
matter for the committee to define what the current EPA guidelines are. Some
existing guidelines clearly are outdated and do not represent EPA's current
views on suitable approaches to regulating TENORM; proposals for revising
some of them have been published, but the proposals have not been issued in
final form, so there is uncertainty about what the new guidelines might be. Some
guidelines are in the form of legally enforceable regulations, but EPA's
preferred approach to regulating TENORM in some cases is indicated only by
policy statements, and there are no published guidelines for some important
exposure situations. Thus, judgment was required by the committee in selecting
EPA guidelines for TENORM other than indoor radon to be emphasized in the
comparisons with similar guidelines developed by other organizations.
In general, the committee has emphasized the most recent statements
by EPA concerning guidelines for TENORM other than indoor radon, regardless
of their form or status. However, the committee also has attempted to discuss all
EPA guidelines, in whatever form, in an effort to provide a reasonably complete
picture of EPA's current policies.
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4
GUIDELINES FOR EXPOSURE TO TENORM
RESPONSES TO COMMITTEE CHARGE
The following sections summarize the committee's responses to the
three parts of its charge.
Technical Basis for Differences in Guidelines for TENORM
The committee has reviewed existing or proposed guidelines for
TENORM developed by EPA and similar guidelines developed by other
regulatory or advisory organizations in the United States and elsewhere. The
other organizations whose guidelines have been considered are the National
Council on Radiation Protection and Measurements (NCRP), the International
Commission on Radiological Protection (ICRP), other federal agencies, state
agencies and organizations, regulatory authorities in other nations, and other
national and international advisory organizations. In keeping with its charge, the
committee paid particular attention to the bases of the various guidelines
developed by the different organizations.
There clearly are differences in the numerical values of the most recent
guidelines for TENORM developed by EPA and some of the guidelines for
similar exposure situations developed by other organizations. Differences are
found in the guidelines for indoor radon and for TENORM other than indoor
radon. Furthermore, where there are differences, EPA guidelines tend to be
more restrictive, that is to correspond to lower levels of exposure and therefore
presumably lower risks to the public. Whether the differences between the EPA
guidelines and those developed by other organizations are significant is entirely
a matter of judgment.
On the basis of its review, the committee finds that the differences
between EPA guidelines for TENORM and similar guidelines developed by
other organizations are not based on scientific and technical information. This
conclusion is based primarily on the following considerations:
.
All organizations that have developed guidelines for indoor
radon have assumed approximately the same risk associated with
exposure to radon and its short-lived decay products based on
epidemiologic data obtained from studies of underground miners and
extrapolation of these data to exposures to radon in indoor residences.
· All organizations that have developed guidelines for TENORM
other than indoor radon have assumed approximately the same risk
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EXECUTIVE SUMMARY
associated with uniform irradiation of the whole body based on
epidemiologic data obtained primarily from studies of the Japanese
atomic-bomb survivors and extrapolation of these data to the low doses
of concern in environmental exposures.
.
All organizations that have developed guidelines for TENORM
have assumed a linear, no-threshold dose-response relationship at low
levels of exposure.
5
Thus the committee finds that differences in the guidelines for
TENORM developed by EPA and other organizations are based essentially on
differences in policy judgments for risk management. That is not to say that
EPA and other organizations have used the same methods and assumptions in
estimating risks posed by radiation exposure. For indoor radon, different
organizations have assumed somewhat different lifetime risks of lung cancer
associated with exposure to short-lived radon decay products in air based, for
example, on differences in the assumed risk-projection models, and the risk
estimates have changed over time. EPA also has given greater attention than
other organizations to the dependence of lung-cancer risk on an individual's
smoking history.
Similarly, for radionuclides other than radon, EPA has used methods
and assumptions that differ from those normally used by other organizations in
estimating risks associated with chronic lifetime exposure. As a result, EPA's
current estimate of the risk posed by external exposure is slightly higher than the
risk estimate currently used by most other organizations, but EPA's risk
estimates for internal exposure to the important long-lived alpha-emitting
radionuclides in TENORM are, in some cases, substantially lower than risk
estimates obtained with the methods and assumptions of other organizations. An
example is the risk from ingestion or inhalation of thorium.
However, the differences between EPA guidelines for TENORM and
the guidelines developed by other organizations are not a reflection of
differences in the methods and assumptions for risk assessments for radon and
other naturally occurring radionuclides. That is, EPA's current approach to risk
assessment, as it differs from the approaches normally used by other
organizations, was not an important factor in developing the numerical values of
its current guidelines for TENORM.
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6
Relative Merits
Assumptions
GUIDELINES FOR EXPOSURE TO TENORM
of Different Scientific and Technical
Given that the differences between the guidelines for TENORM
developed by EPA and other organizations are not, in the committee's opinion,
based on scientific information, the second question in the charge to the
committee is moot. However, the committee has considered the differences
between EPA's current methods and assumptions for risk assessment and the
approaches normally used by other organizations, even though these differences
have not had a substantial influence on the development of guidelines for
TENORM. The committee's views on the approaches to risk assessment are
summarized later.
Development of Contemporary Risk Analysis for NORM
The third part of the charge to the committee was to consider whether
there is relevant and appropriate scientific information that has not been used in
the development of contemporary risk analysis for NORM, especially risk
analysis for purposes of developing and implementing guidelines for radiation
exposure. A particular concern expressed to the committee is that some of the
important radionuclides are parents of long decay chains involving a complex
mixture of radioisotopes of different chemical elements and that exposures to
such mixtures might necessitate novel approaches to methods of risk estimation.
The committee is not aware of any evidence that the properties of
NORM differ from the properties of any other radionuclides in ways that would
necessitate the development of different approaches to risk assessment. In
regard to radiological properties, if one accepts the view currently held by all
regulatory and advisory organizations involved in radiation protection that
estimates of absorbed dose in tissue are the fundamental physical quantities that
determine radiation risks for any exposure situation, there is no plausible
rationale for any differences in risks due to ionizing radiation arising from
naturally occurring and any other radionuclides, because absorbed dose in tissue
depends only on the radiation type and its energy, not on the source of the
radiation.
The decay chains of some naturally occurring radionuclides are
considerably more complex than the decay chains of other radionuclides with
regard to the number of decay products and chemical elements involved.
However, contemporary methods of risk assessment that estimate doses and
risks related to ingestion or inhalation of radionuclides by assuming that decay
OCR for page 7
EXECUTIVE SUMMARY
7
products produced in the body are redistributed and retained in the body
according to the metabolic behavior characteristic of particular chemical
elements take the added complexity into account by using the same methods
that are applied to other radionuclides with many fewer decay products.
Thus, in general, there should be no difference between NORM and
other radioactive materials with regard to suitable approaches to estimating
doses and risks related to external or internal exposure. However, because
naturally occurring radionuclides are ubiquitous in the exposure environment,
there might be more opportunity than there is with many human-made
radionuclides to use observational data on natural levels in different
environmental comparknents (such as soil, water, air, plants, and animals) and
the fluxes between compartments to calibrate exposure-pa~way models for
TENORM. On the other hand, the ability to use such natural analog data for
exposure pathway analysis must be tempered by the recognition that the
physical and chemical forms of TENORM could be substantially different from
those for the same elements in the natural environment. In that case,
observations on He behavior of radionuclides in natural systems might not be
relevant to the exposure situation of concern.
OTHER CONCLUSIONS AND RECOMMENDATIONS
During this study, the committee considered other issues related to the
development of guidelines for TENORM. The committee's views on some of
these issues are summarized below.
Policy Judgments for Risk Management
The committee has concluded that the differences between EPA
guidelines for TENORM and similar guidelines developed by other
organizations are based essentially on differences in policy judgments for risk
management, rather than differences in scientific and technical information. An
evaluation of the relative merit of the differences in policy judgments for risk
management was not part of this study, but the committee needed to identify
these judgments in reaching the conclusion that the differences in the guidelines
do not reflect differences in scientific and technical information.
From its considerations of the various guidelines for TENORM
developed by EPA and other organizations, the committee believes that the
differences often are based, at least in part, on two factors that are strictly
matters of policy:
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8
GUIDELINES FOR EXPOSURE TO TENORM
· Differences in judgments about acceptable risks related to
exposure to TENORM.
· Differences in judgments about levels of TENORM in the
environment, doses, or risks that are reasonably achievable.
Judgments about what is reasonably achievable in controlling exposures to
TENORM take into account such factors as the costs of reducing exposures in
relation to the benefits in reduced health risks to the public and other societal
concerns. A particularly important consideration in regulating TENORM is the
pre-existing levels of naturally occurring radionuclides in the environment and
associated human exposures.
An additional factor that has been important in developing guidelines
for TENORM is a judgment about the extent to which existing guidelines for
particular exposure situations can be transferred to other situations. For
example, some organizations have developed guidelines for TENORM in soil
based on the concentration limits in current EPA guidelines for cleanup of soil
contaminated with radium at uranium mill tailings sites developed under the
Atomic Energy Act. TransferabilitY of standards developed for a specific class
of TENORM waste is limited bY the extent that the physical and chemical
properties of the TENORM in issues as well as projected exposure pathways are
substantially similar to those considered for uranium mill tailings.
Other policy judgments for risk management also have been important
causes of the differences between EPA guidelines and guidelines developed by
other organizations. Some guidelines are concerned primarily with reducing
risks to individuals who receive the highest exposures, and others with reducing
risks to whole populations. Some guidelines include exposures to natural
background, and others do not. Finally, in accordance with legislative mandates,
some EPA guidelines apply only to a particular environmental medium (such as
air) or a particular exposure pathway (such as drinking water), whereas
guidelines developed by most other organizations, especially those expressed in
terms of dose or risk, apply to all environmental media and all exposure
pathways combined.
The importance of differences in policy judgments for risk
management in determining differences in guidelines for TENORM is
illustrated by the following three examples.
First, the EPA guideline for mitigation of radon in homes, 150 Bq/m3
(4 pCi/L), is lower than mitigation levels recommended by NCRP and ICRP.
The differences result from differences in the primary focus of the guidelines.
NCRP and ICRP were concerned primarily with mitigation of risks to
individuals who receive the highest exposures, and their recommended action
OCR for page 9
EXECUTIVE SUMMARY
9
levels were based primarily on judgments about the maximum tolerable risk,
with some consideration of the feasibility of achieving concentrations below the
action levels. EPA also was concerned with limiting individual risks but
emphasized reducing exposures in the greatest number of homes possible, on
the basis of judgments about levels that were reasonably achievable in most
homes.
Second, EPA has issued proposed guidance on radiation protection
of the public that includes a limit on annual dose equivalent of 1 mSv from all
controlled sources combined, including TENORM, naturally occurring
radionuclides from the nuclear fuel cycle, and human-made radionuclides but
excluding radon. In contrast, NCRP has recommended an annual dose
equivalent of 5 mSv as a remedial-action level for all natural sources, including
natural background and TENORM but excluding radon. EPA's annual dose limit
of 1 mSv for all controlled sources combined normally should be considerably
more restrictive than NCRP's recommended remedial-action level of 5 mSv for
all natural sources.
The difference between the two guidelines is due entirely to
differences in policy judgments for risk management because EPA and NCRP
assumed the same risk per unit dose. The most important difference is in the
judgments about the maximum tolerable risk posed by exposure to TENORM.
EPA regards TENORM other than indoor radon as a type of controlled source
that should be regulated in the same manner as human-made sources, whereas
NCRP essentially regards sources of TENORM, even if they are controllable, as
a form of natural background that should be controlled differently from human-
made sources. The other important difference in policy judgments is that EPA's
dose limit excludes the dose from undisturbed natural background, whereas
NCRP's remedial-action level includes the dose from background.
The third example concerns guidelines for cleanup of radioactively
contaminated sites. The Nuclear Regulatory Commission has issued regulations
that specify that sites are acceptable for unrestricted use if the annual dose
equivalent from all exposure pathways, including the use of groundwater as a
source of drinking water, does not exceed 0.25 mSv. EPA has objected to the
Nuclear Regulatory Commission standards on two grounds. First, EPA believes
that the annual dose from all exposure pathways should be limited to 0.15 mSv
to achieve an acceptable level of risk. Second, in addition to the dose constraint
for all exposure pathways, concentrations of radionuclides in groundwater
should be limited in accordance with current standards for public drinking-water
supplies unless compliance with drinking-water standards is not feasible.
The difference of opinion between EPA and the Nuclear Regulatory
Commission about the adequacy of the Nuclear Regulatory Commission's
cleanup standards for contaminated sites is strictly a matter of differences in
policy judgments for risk management. Those judgments include the
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10
GUIDELINES FOR EXPOSURE TO TENORM
determination of a limit on acceptable risk and therefore dose for this exposure
situation and the determination of whether separate requirements are needed for
protection of groundwater resources.
Consistency of Radiation Guidelines
Many diverse guidelines have been developed by EPA and other
organizations for TENORM and human-made radionuclides. The fundamental
purpose of the guidelines is to limit risks to exposed individuals and
populations. However, when the various guidelines are compared, the levels of
acceptable risk corresponding to the numerical criteria in the guidelines appear
to be inconsistent. The committee has considered the issue of consistency of
radiation guidelines with regard to risk and offers the following observations.
First, although the desire for consistency of guidelines with regard to
levels of acceptable risk is understandable, the committee has identified several
important reasons why such a consistency should not be expected:
· Differences in statutory and judicial mandates for guidelines,
especially the fundamental difference between a regulatory limit, as
embodied in some guidelines, and a regulatory goal that can be relaxed
on the basis of other considerations as embodied in other guidelines.
· Differences in the primary bases of guidelines, especially judgments
about acceptable risk versus judgments about risks that are reasonably
achievable.
· Differences in the applicability of guidelines, especially guidelines
that apply to all sources of exposure combined versus guidelines that
apply only to specific sources or practices, or to particular
environmental media and comparisons of guidelines that apply to quite
different sources or practices.
· Differences in the population groups of primary concern, especially
individuals who receive the highest exposures versus whole
populations.
· Differences in the considerations of natural background.
The committee believes that it is important to understand those
factors when comparing different guidelines.
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EXECUTIVE SUMMARY
11
Second, the numerical criteria in the guidelines, whether they are in
the form of limits or goals, do not appear to be the most important factor in
determining acceptable risks to individuals or populations. Rather, without
regard for the substantial differences in risks corresponding to the various
guidelines and without regard for the different factors that result in these
differences, as described above, the principle that exposures of individuals and
populations should be maintained as low as reasonably achievable (ALARA),
economic and social factors being taken into account, appears to be the most
important factor in determining risks actually experienced for any exposure
situation that is subject to regulatory control. Therefore, to the extent that the
ALARA objective is applied consistently to all exposure situations, all
guidelines would be consistent with regard to the risks actually achieved, even
though the risks that are ALA MA can depend significantly on the particular
exposure situation.
Importance of Natural Background for Guidelines for
TENORM
Regulation of TENORM is a unique problem among all radioactive
materials in that the radionuclides of concern occur naturally in all
environmental media. Therefore, guidelines for TENORM must correspond to
levels of naturally occurring radionuclides in the environment at which it is
practical to distinguish the radionuclides resulting from human activities from
those in the undisturbed natural background. Furthermore, determinations of
practical levels for identifying and controlling TENORM must take into account
the variability of natural levels in different environmental media, as well as the
average values.
Importance of Knowledge of Sources of TENORM
Sources of TENORM other than indoor radon result from a wide
variety of human activities, and the physical and chemical properties of the
radioactive materials that result from those activities vary widely. Furthermore,
some sources are discrete and thus localized, whereas others are diffuse and
very large in volume. Especially when guidelines for TENORM might be
expressed in terms of concentrations in environmental media (such as soil),
rather than dose or risk, development of the guidelines should take into account
the properties of the various sources of concern. It would be inappropriate to
apply a guideline developed for a particular exposure situation to other
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12
GUIDELINrES FOR EXPOSURE TO TENORM
situations when there are important differences in the properties of the
radioactive materials.
In addition, exposure-pathway analysis for TENORM generally should
take into account the leachability, sorption, and biologic availability of the
particular physical and chemical forms of the materials. In some cases, the
characteristics of TENORM differ substantially from the characteristics of
naturally occurring materials in their undisturbed state.
Differences in Approaches to Radiation Risk Assessment
As part of this study, the committee considered the approaches
currently used by EPA in estimating cancer risks posed by radiation exposure,
as documented in Federal Guidance Report No. 13 (Part 1, interim version), in
comparison with the approaches used by most other organizations whose
guidelines were evaluated. Most other organizations estimate cancer risks on the
basis of methods developed by ICRP either the current methods represented in
ICRP Publications 60 and 72 and supporting documents or, in the case of other
federal agencies and state organizations, the methods represented in ICRP
Publications 26 and 30, which have been superseded by Publications 60 and 72.
The issue of differences in approaches to risk assessment is of concern
only for radionuclides other than radon because the risks posed by exposure to
radon can be estimated, with some uncertainty, from epidemiologic data without
the need to estimate the dose to radiosensitive tissues per unit exposure and the
risk per unit dose for different types of radiation. In addition, for external
exposure, the differences between risks estimated by EPA and the risks
estimated by other organizations and based on ICRP methods generally are
insignificant.
For internal exposure, EPA's approach to risk estimation differs from
approaches based on current or outdated ICRP methods in three important
respects.
· First, EPA's risk models take into account the age dependence of
the absorbed dose rate in body tissues and the age dependence of the
radiogenic risks, whereas risks calculated with ICRP methods are based
on calculated committed effective doses or effective dose equivalents,
which are not intended to provide accurate representations of cancer
risks for individual organs and tissues of the body. The difference
between the two approaches is particularly important for long-lived
radionuclides with long retention times in the body.
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EXECUTIVE SUMMARY
Second, EPA's assumptions about mortality for all cancers and for
other competing causes of death, which are used in estimating
radiogenic risks, are based on data on the U.S. population, which differ
from the data used by ICRP, and there are other differences in the risk
estimates for particular cancers.
· Third, the other federal agencies and state governments that
normally estimate risks based on the outdated ICRP methods do not
take into account age-specific dosimetric and biokinetic models and
current models for the redistribution and retention of radioactive decay
products in the body.
13
The differences between EPA's risk estimates for internal exposure and
estimates obtained with ICRP methods are particularly important for the long-
lived, alpha-emitting radionuclides found in TENORM (such as thorium). If
EPA's risk estimates are compared with estimates based on the outdated ICRP
methods normally used by other federal agencies and state governments, the
differences are well in excess of a factor of 10 in some cases. In comparison
with current ICRP methods based on age-specific committed effective doses, the
differences can be as large as a factor of 5. EPA's risk estimates are lower in all
cases.
The EPA uses its methodologically more rigorous approaches to risk
assessment only in assessing risks for purposes of reaching decisions on rule-
making, including decisions on the feasibility of establishing guidelines and the
effects of alternative guidelines. However, when guidelines are expressed in
terms of dose, as is often the case, EPA uses standard ICRP calculations of
committed effective dose equivalents for adults, based on the methods and data
in ICED Publications 26 and 30, for purposes of demonstrating compliance with
the guidelines to maintain a stable and uniform framework for the regulated
community.
The committee generally supports the current EPA approaches to
estimating risks posed by radiation exposure. They should be appropriate for the
current US population, and the methods for estimating risks posed by internal
exposure are methodologically more rigorous than those used by other
organizations on the basis of current or outdated ICRP methods incorporating
committed doses.
EPA's current methods of risk assessment as they differ from the
methods used by most other organizations-especially for long-lived, alpha-
emitting radionuclides have not had a direct influence on the development of
guidelines for TENORM expressed in terms of dose. That is mainly because
guidelines expressed in terms of dose apply to both external and internal
exposure, EPA's risk estimate for external exposure is nearly the same as
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14
GUIDELINES FOR EXPOSURE TO TENORM
ICRP's, and external exposure often is important in scenarios for TENORM.
Furthermore, policy judgments for risk management and a desire for consistency
in regulation have been more important for the development of guidelines for
TENORM than any differences in estimated risks based on different methods.
However, if EPA chose to develop guidelines for TENORM in the form of
concentrations in environmental media, its current methods for risk assessment
could be used to derive the guidelines from an assumed limit on acceptable dose
or risk. Such an approach could be suitable for TENORM, because only a few
radionuclides are of concern. However, the analysis of exposure pathways and
dose would need to account for the various physical and chemical forms of
radionuclides that may be encountered in the environment.
Use of Linear' No-threshold Dose-Response Hypothesis
At the present time, there is considerable debate over the validity of the
linear, no-threshold dose-response hypothesis for low levels of exposure. It
remains as an assumption used in developing all radiation guidelines, including
those for TENORM, in spite of the current debate over the validity of this
hypothesis, including the possibilities that there is an effective threshold for
radiation risks and that there are beneficial effects at low doses.
The committee does not have any new insights into the validity of the
linear, no-threshold dose-response hypothesis. However, the committee
understands that it is used because it represents a prudent approach to health
protection of the public in the absence of definitive information on radiation
risks at the dose levels of concern for routine exposures. The committee also
notes that the central issue here is the risks due to incremental increases in dose
above background, not the risks due to the incremental increases themselves.
Directions for Further Research on TENORM
The committee has noted a number of subjects on which additional
scientific information would be beneficial in developing guidelines for
TENORS.
Although models for exposure and dose assessment generally are well
developed, the models for TENORM, especially the models for exposure
assessment, possibly could be improved through validation of parameters. Much
information on exposure-pathway models for naturally occurring radionuclides
was obtained in studies on uranium mill tailings, but this information may be
inappropriate for other exposure situations involving substantially different
physical and chemical forms of radionuclides. Differences in chemical and
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EXECUTIVE SUMMARY
15
physical forms of TENORM also could affect the estimates of dose from
ingestion and inhalation of radionuclides.
There is need for improved methods for locating and measuring
discrete and diffuse TENORM in the environment, especially if guidelines for
TENORM correspond to levels in the environment that are only marginally
above the levels of natural background.
Finally, given the importance of the linear, no-threshold dose-response
hypothesis, an understanding of radiation carcinogenesis and the validity of the
hypothesis remains an important scientific need for radiation protection,
specifically for estimating the probabilities of adverse human health effects at
the levels of natural background.
Representative terms from entire chapter:
epa guidelines
