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OCR for page 75
4
Role of Exposure and Dose or Risk Assessments in
Developing Radiation Standards
This chapter discusses the use of exposure and dose or risk assessments
in providing a technical basis for standards for radionuclides in the environment.
It does not discuss in detail the current approaches to exposure and dose or risk
assessments of radionuclides, such as their use in demonstrating compliance
with standards.
The standards for radionuclides in the environment discussed in this
report are directed only at protection of humans. According to available data
and modeling, radiation standards that would provide acceptable protection of
humans would ensure that other species are not put at unreasonable risk.
although individual members of a species occasionally might be harmed (IDEA
1992; ICRP 1991~. Risks to biota other than humans and the issue of whether
separate standards are needed for their protection are not considered in this
study.
The chapter begins with a general discussion of the risk-assessment
process for carcinogens and the application of the process to radionuclides. That
is followed by a discussion of the calculational elements of dose or risk
assessments for radionuclides in the environment and the use of such
assessments in developing standards. The chapter ends with the committee's
views on suitable approaches to exposure and dose or risk assessments for
purposes of developing standards for radionuclides in the environment.
RISK ASSESSMENT OF CARCINOGENS
As described, for example, by the National Research Council (1983)
and the Office of Science and Technology Policy (OSTP 1985) the process of
75
OCR for page 76
76
ROLE OF ASSESSMENTS IN DEVELOPING STANDARDS
risk assessment of carcinogens, including radionuclides, generally consists of
the following four components:
· Hazard identification.
· Estimation of dose-response relationship.
.
Exposure assessment.
· Risk characterization.
Brief descriptions of these components and the usual approaches to
addressing them for radionuclides are presented in the following sections.
Hazard Identification
Hazard identification for carcinogens entails a qualitative evaluation of
the data bearing on an agent's ability to produce carcinogenic effects and the
relevance of this information for humans. This component of the risk-
assessment process is particularly important for chemical carcinogens because
data on possible carcinogenic effects on humans often are lacking. However,
hazard identification is not an important concern for radionuclides, including
naturally occulting radionuclides found in TENORM, because widespread
epidemiologic data have shown conclusively that ionizing radiation can cause
cancer in humans (for example, National Research Council 1990; 1988~.
Noncarcinogenic health effects also are potentially important for some
radionuclides that can be found in TENORM. Particularly for uranium,
chemical toxicity in the kidney after exposures above some threshold is clearly
demonstrated by a large amount of animal and human data (for example,
Leggett 1989~. For uranium, an important issue is whether current radiation
standards for the public would prevent chemical toxicity in the kidney.
Resolution of that issue depends on a number of factors including: the assumed
threshold for chemical toxicity in the kidney; selection of an appropriate safety
factor below the assumed threshold for protection of public health, including
protection of unusually sensitive populations (such as infants and children); and
the relationship between kidney burden and radiation dose from uranium. On
the basis of consideration of those factors and current data and models, Kocher
(1989) concluded that chemical toxicity generally should be considered in
developing health-protection standards for the public with respect to ingestion
and inhalation of soluble or insoluble uranium. For example, the Environmental
Protection Agency has based a proposed standard for uranium in drinking water
on prevention of chemical toxicity in the kidney, as well as limitation of
radiation dose (see chapter 7~. However, the chemical toxicity of natural
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GUIDELINES FOR EXPOSURE TO TENORM
77
uranium appears to be potentially important only if the limit on radiation dose
from exposure to uranium is greater than about 0.25 mSv (25 mrem) per year
and if the dose from uranium results primarily from ingestion and inhalation
rather than external exposure (Kocher 1989~.
The chemical toxicity of naturally occurring radionuclides found in
TENORM and its effect on setting radiation standards is not considered further
in this report.
Estimation of Dose-Response Relationship
Estimation of the dose-response relationship for carcinogens is the
process of estimating the magnitude, or an upper bound on the magnitude, of the
carcinogenic effect of any given dose. For radiation exposure, best estimates of
the carcinogenic effect of a given dose normally are emphasized.
Estimation of the carcinogenic response to exposure to particular
radionuclides is greatly facilitated by the generally held view (which is based on
observation and modeling) that the absorbed dose in tissue is the fundamental
physical quantity that determines the response to any exposure. The absorbed
energy in tissue depends only on the radiation type and its energy (NCRP
1993a; ICRP 1991), not on the source of the radiation. Therefore, in contrast
with the current situation for chemical carcinogens, animal or human studies to
estimate the dose-response relationship for exposure to every radionuclide of
potential concern are not required. Rather, the dose-response relationships
developed for different types of radiation (such as photons, electrons, and alpha
particles) can be applied to exposures to any radionuclide once the energies and
intensities of all the kinds of radiation emitted in the decay of the radionuclide
are known.
Current estimates of the dose-response relationships for radiation
exposure obtained, for example, by the National Research Council's Committee
on the Biological Effects of Ionizing Radiations (National Research Council
1990; 1988) are based on studies of human populations that received radiation
doses considerably above environmental levels. The most important study
groups include the Japanese atomic-bomb survivors, who received high doses
from photon exposures of all organs and tissues of the body, and various groups
of miners, who received high doses to the lung from exposure to alpha particles
emitted by the short-lived decay products of radon.
Applying the dose-response relationships for radiation exposure
estimated by such groups as the BEIR committee to the considerably lower
doses of concern in controlling routine exposures of the public requires
assumptions about extrapolation to doses and dose rates beyond the range of
direct observation (ICRP 1991~. For purposes of risk management, the
carcinogenic response to environmental radiation generally is assumed to be a
OCR for page 78
78
ROLE OF ASSESSMENTS IN DEVELOPING STANDARDS
linear function of dose without a threshold. Although the assumption has often
been challenged, the consensus among regulatory authorities and expert
organizations is that there is insufficient evidence to support a departure from
the linear, no-threshold dose-response relationship for regulatory purposes. This
issue has not been investigated in the present study of standards for TENORM.
Exposure Assessment
Exposure assessment is the process of identifying a group or groups of
individuals who might be at risk and describing for the~on the basis of
assumptions, observations, or modeling- the various routes (pathways) of
exposure and their magnitude, frequency, and duration. Exposure assessment is
essentially the same for radionuclides and hazardous chemicals except that
external (direct) exposure to penetrating radiation (such as photons) is an
important pathway for radionuclides but not for hazardous chemicals.
The process of exposure assessment for radionuclides in the
environment is considered in more detail later in this chapter.
Risk Characterization
Risk characterization is the process of combining the information
obtained from hazard identification, estimation of the dose-response
relationship, and exposure assessment to describe the carcinogenic risk
associated with expected or assumed human exposures to the agent of interest.
Risk characterization for radiation exposure is relatively
straightforward and is almost always expressed in terms of the numerical
probability of cancer induction in an individual or the number of cancers in a
population over a defined time period. Some approaches to risk characterization
for radiation exposure focus on fatal cancer as the end point of concern, whereas
others focus on cancer incidence; the difference between the two is not
important for most organs and tissues at risk (ICRP 1991~. Other issues in risk
characterization include the risks that might be experienced by particularly
susceptible populations and alternative approaches to expressing risk. Those
issues have not been considered in this study.
ELEMENTS OF RADIATION RISK ASSESSMENT
This section discusses the calculational elements that normally make up
an assessment (estimation) of risk posed by exposure to radionuclides in the
environment. It does not apply to risk assessments for exposure to radon and its
short-lived decay products in air. As noted in chapter 1, risk assessments of
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GUIDELINES FOR EXPOSURE TO TENORM
79
radon usually are based directly on epidemiologic data without the need to
estimate dose from a given exposure and risk per unit dose.
There is some overlap between this discussion and the general
discussion of risk assessment for carcinogens above. However, we are
discussing radiation risk assessment separately mainly because the approaches
to estimating radiation risks were developed long before the general risk-
assessment process for carcinogens was formally laid out. Radiation risk
assessment usually includes estimates of dose from a given exposure, which are
generally not part of risk assessments of chemical carcinogens.
An exposure and dose or risk assessment of radionuclides in the
environment addresses the relationship between the concentrations of
radionuclides in various media (air, water, soil, or other materials) at particular
locations and the resulting radiation doses or risks to exposed individuals or
populations. Knowledge of this relationship can be used either to derive
estimates of dose or risk corresponding to given concentrations of radionuclides
in the environment or to derive estimates of concentrations of radionuclides in
the environment corresponding to a given dose or risk.
As shown in figure 4.1, an assessment of risk corresponding to given
concentrations of radionuclides in the environment, or vice versa, consists of
three elements:
An assessment of internal and external exposures, which provides
estimates of intakes of radionuclides by ingestion and inhalation, and
estimates of exposures to external radiation per unit concentration of
particular radionuclides in environmental media.
· An estimate of radiation dose, given the estimates of radionuclide
intakes and external exposures.
.
An estimate of risk, given the estimate of dose from internal and
external exposures.
As noted previously, the measure of risk posed by radiation exposure
normally is assumed to be cancer mortality, although cancer incidence
(morbidity) is calculated in some cases. In many environmental-radiation
assessments, the desired end point for the calculations is dose, rather than cancer
risk, especially when an assessment is used to develop a radiation standard
expressed in terms of dose or to demonstrate compliance with a dose standard.
When dose is the desired end point, only the first two elements listed above are
included in the assessment. Furthermore, as noted previously, dose often is
calculated as an intermediate step even when risk is the desired end point for the
assessment.
OCR for page 80
80
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OCR for page 81
GUIDELINES FOR EXPOSURE TO TENORM
81
The approach to estimating dose and risk for given internal and
external exposures (the second and third elements of a risk assessment listed
above) often is straightforward. Estimates of committed doses from internal
exposure per unit activity intakes of radionuclides by ingestion and inhalation
can be obtained, for example, from federal guidance (Eckerman and others
1988~. Similarly, for radionuclides dispersed in air, water, or soil, estimates of
external dose rates per unit activity concentration are given in federal guidance
(Eckerman and Ryman 1993~. Estimates of dose from internal and external
exposure then can be converted to estimates of risk by using the nominal fatal-
cancer risk per unit dose discussed in chapter 7. These simplified approaches to
estimating dose and risk generally are adequate for purposes of controlling
radiation exposures (that is, in developing standards and in demonstrating
compliance with standards), but they might not be adequate for estimating doses
and risks for real exposure situations (see chapter 11~.
The approach to estimating internal and external exposures per unit
concentration of radionuclides in various environmental media (the first element
of a risk assessment) generally is more complicated than the approach to dose
and risk estimation described above, especially for internal exposure. Estimates
of ingestion intakes of radionuclides can require estimates of transfers of
radionuclides through various terrestrial or aquatic food-chain pathways to
humans, including transfers into food crops, meat, milk, and fish; and estimates
of inhalation intakes can require estimates of transfers of radionuclides from
various environmental media, such as soil, into the air. Exposure assessment
also requires estimates of so-called usage factors, including intakes of
contaminated water, air, soil, foodstuffs, or other materials by humans or
livestock and residence times and shielding factors for external exposure.
The general approach to exposure assessment for radionuclides in the
environment is depicted in figure 4.2. The source compartments can include
water, air, soil, or other materials (such as contaminated buildings or
equipment), and the exposure compartments can include the source
compartments plus various foodstuffs or other materials into which radioactivity
is transferred. The particular exposure pathways that should be considered in
any assessment depend on such factors as Me characteristics of the site, the
source compartments of concern, the physical and chemical forms of
radionuclides, and the assumptions about living habits of exposed individuals or
populations. Examples of important exposure pathways for different source
compartments are as follows.
For contaminated soil, important exposure pathways generally include:
· Consumption of vegetables, fruits, and grains grown in the
contaminated soil.
OCR for page 82
82
ROLE OF ASSESSMENTS IN DEVELOPING STANDARDS
CONCENTRATIONS |
_ IN SOURCE
COMPARTMENTS
r
TRANSPORT AND
INTERACTION
PROCE SSES
CONCENTRATIONS
EXTERNAL ~IN EXPOSURE
EXPOSURE COMPARTMENTS
1 1
l DOSE TO l I
HUMANS |
_ INTERNAL
EXPOSURE
Figure 4.2. Relationships between concentrations of radionuclides in source
compartment and resulting doses to humans from internal and external exposure.
OCR for page 83
GUIDELINES FOR EXPOSURE TO TENORM
· Consumption of milk and meat obtained from livestock that
eat pasture grass grown in the contaminated soil.
.
Direct consumption of the contaminated soil.
· Inhalation of radionuclides from contaminated surface soil
suspended in the air.
.
External exposure to the contaminated soil.
83
For contaminated groundwater, additional important pathways of
internal exposure generally include:
.
Direct ingestion of water from the contaminated source.
· Use of the contaminated source as a water supply for agricultural
purposes, such as watering of livestock; irrigation of vegetables, fruits,
and grains, and irrigation of pasture grass consumed by livestock.
For contaminated surface water, additional important exposure
pathways generally include:
.
The ingestion pathways listed above for contaminated
groundwater.
.
Consumption of aquatic foodstuffs (such as fish) obtained from the
contaminated source.
.
External exposure to the contaminated source during such
activities as swimming, boating, and residence along contaminated
shorelines.
For contaminated air, important exposure pathways generally include:
· Inhalation and external exposure to airborne radionuclides in the
source compartment.
· Exposure to radionuclides deposited on vegetation and the ground
surface, including external exposure and ingestion of radionuclides
incorporated into foodstuffs (vegetables, fruits, grains, meat, and milk).
.
air.
Inhalation of deposited radionuclides that are resuspended in the
Finally, if the source compartment consists of contaminated structures
or equipment, credible scenarios for ingestion, inhalation, and external exposure
OCR for page 84
84
ROLE OF ASSESSMENTS IN DEVELOPING STANDA=S
would need to be developed and evaluated on the basis of the characteristics of
the particular exposure situation of concern.
Most models for terrestrial and aquatic food-chain pathways assume
that the concentration of a radionuclide in the exposure comparDnent of interest
(such as milk) is a constant multiple of the concentration in the source
compartment (such as soil). Thus, food-chain pathway models generally use
radionuclide-specif~c transfer factors (also called concentration factors or
concentration ratios in some cases). For pathways involving exposure to
contaminated soil, the long-term retention of radionuclides in surface soil,
which depends on the solid-solution distribution coefficient (Kd) for the
radionuclides, also is important. The models and parameter values for the
various internal and external exposure pathways of concern in exposure
assessments for radionuclides in the environment are discussed in a number of
references (for example, IAEA 1996b, 1994; NCRP 1984d; Till and Meyer
1983~.
Additional issues are particularly important with respect to TENORM.
Soluble radionuclides are more available for biologic uptake than those sorbed
on soils or sediments, so the partitioning of radionuclides among these phases-
between groundwater and aquifer materials; between river water and suspended
or bottom sediments-is important in pathway modeling. A laboratory-derived
distribution coefficient, K<3, is typically used, where
K = concentration of radionucTide sorbed on sediment
concentration of radionucTide remaining in solution
.
For a given element, the coefficient can be expected to vary with the chemical
speciation of the element, the solute chemistry of the water, the mineralogy and
surface area of the solids, redox conditions, and pH. For example, the presence
of competing ions in solution can decrease the sorption of a radionuclide. That
effect was seen in the sorption of 226Ra from a sodium chloride oil-production
brine by soils and marsh sediments where the percentage of radium sorbed
increased with brine dilution (Landa and Reid 1983~. In groundwater, uranium
partitioning to the solid phase can be expected to increase with more-reducing
conditions along a Towpath. Laboratory-derived K<3 values should attempt to
simulate field conditions at the TENORM site, and generic, literature-derived Kin
values should be used with caution.
Much of the nutrients locked up in unweathered rock fragments might
not be available for plant uptake; the chemical forms of radionuclides in
TENORM might greatly influence their environmental mobilities and biologic
availabilities. Total radionuclide concentrations in TENORM might not be
OCR for page 85
GUIDELINES FOR EXPOSURE TO TENORM
85
reflective of the biologically-labile pool, and some subsegment of the total could
be of greater value in assessing biologic uptake and resulting dose. For example,
in assessing the hazard due to inhalation of radon decay products, radon that
does not escape the radium-bearing mineral matrix is not a concern. The radon
emanation coefficient is the fraction of the radon formed in a radium bearing
solid that escapes to the atmosphere. It can vary widely in natural and industrial
materials; for example, although 226Ra coprecipitated with barium sulfate
generally emanates less than 1% of the 222Rn generated, that coprecipitated with
or sorbed on iron hydrous oxide can emanate nearly 100% (Hahn 1936~. For
TENORM with high emanation coefficients, radon release and radon decay-
product inhalation might be the pathway of concern. For TENORM with low
emanation coefficients, whole-body gamma exposure might yield the limiting
dose.
For the inhalation or ingestion pathway, the solubility of the TENORM
particles in bodily fluids (such as lung serum for inhaled particles, or stomach
acid) and in the soil and aquatic environments where TENORM resides can
exert a major influence on the direct and indirect uptake of radionuclides by
humans. In viva solubility can be affected by several factors such as particle
size, position of nuclides in less-accessible particle interiors versus those on
particle surfaces (for example, heat-volatilized 2~0Pb/2'0Po condensed onto
cooling particle surfaces in flue dust of thermal-process phosphate plants and in
coal-combustion fly ash), and kinetics. In soil and aquatic environments,
radionuclide solubility can vary with pH, presence of complexing or
precipitating ions in contact solutions, and redox conditions. Plant uptake of
radionuclides differs between different members of a decay series and between
species for a given nuclide; the biologic uptake of radium by terrestrial plants
has been reviewed by Simon and Ibrahim (1990~. If biologic-uptake data are not
available, leaching tests with dilute acids to assess readily dissolvable fractions,
or salt solutions to assess ion-exchangeable fractions can be used to provide an
index of biologic availability.
USE OF DOSE OR RISK ASSESSMENT IN DEVELOPING RADIATION
STANDARDS
In principle, radiation standards expressed in terms of dose or risk can
be developed without the need to consider exposure-pathway and dose or risk
assessments for the exposure situations of concern. If a standard is expressed
directly in terms of risk, all that is required in developing the standard is a
judgment about a limit on acceptable risk. Similarly, if a standard is based only
on a judgment about acceptable risk but is expressed in terms of dose as a
OCR for page 86
86
ROLE OF ASSESSMENTS IN DEVELOPING STANDA=S
surrogate for risk, the only additional requirement is an assumption about the
risk per unit dose. For purposes of developing standards expressed in terms of
dose, the risk per unit dose normally is assumed to be independent of the
particular radionuclides and exposure pathways of concern.
In practice, however, some type of exposure-pathway and dose or risk
assessment normally is used in developing radiation standards, even when the
standards are expressed only in terms of dose or risk. For example, such
assessments are required by the National Environmental Policy Act whenever
imposition of a standard would have substantial economic or environmental
impacts. More generally, dose or risk assessments are important in developing
radiation standards because risks posed by environmental exposures are not
directly observable and dose often cannot be measured, especially the dose from
internal exposure. Dose or risk assessments are important in developing
radiation standards in the following ways.
First, if a standard is expressed in terms of dose or risk and is based on
a judgment about acceptable risk, as described above and discussed further in
chapter 5, a dose or risk assessment for the exposure situations of concern can
be used to demonstrate whether compliance with the standard is reasonably
achievable. Such demonstrations help in gaining acceptance of the standard by
the public and affected parties.
Second, if a standard is expressed in terms of dose or risk but is based
on a judgment about the achievability of doses or risks for the exposure
situations of concern, as described above and discussed further in chapter 5, a
dose or risk assessment clearly is required to support the conclusion that the
standard is reasonably achievable. In making such a judgment, some type of
cost-benefit analysis often is performed in which the costs of achieving various
doses or risks are compared with the corresponding benefits in health risks
averted in exposed populations (for example, Wolbarst and others 1996; ICRP
1 989b).
Third, radiation standards might be expressed in terms of directly
measurable quantities, such as concentrations of radionuclides in environmental
media or external exposure rates, rather than in terms of dose or risk which
cannot be measured. Standards expressed in terms of measurable quantities
often are developed to facilitate demonstrations of compliance, especially in the
case of standards for naturally occurring radionuclides (see chapter 7~. If such a
standard is based on an assumed limit on dose or risk, exposure and dose or risk
assessments clearly are required in deriving the limits on measurable quantities.
However, an exposure and dose or risk assessment is not necessarily
required even in developing environmental standards expressed in terms of
directly measurable quantities. Consider, for example, current standards for
alpha-emitting radionuclides in drinking water, radioactivity in liquid discharges
from particular mines or mills, control and cleanup of residual radioactive
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GUIDELINES FOR EXPOSURE TO TENORM
87
materials at uranium and thorium mill-tailings sites, and indoor radon, all of
which are concerned with exposures to naturally occurring radionuclides (see
chapter 7~. In each standard, the quantitative criteria are expressed in terms of
measurable quantities, and the criteria were developed on the basis of
considerations of levels of radioactivity that are reasonably achievable for the
exposure situations of concern, given existing background levels and the ability
of current technologies to reduce them. However, the cost-benefit analysis used
in developing the criteria in each case was performed with respect to levels of
radioactivity, rather than with respect to the corresponding doses or risks to
individuals or populations. Thus, the values of measurable quantities that were
judged reasonably achievable were derived essentially without concern for the
resulting doses or risks. Those examples illustrate that an exposure-pathway and
dose or risk assessment is required in developing standards expressed in terms
of measurable quantities only when the standards are based on a judgment about
acceptable dose or risk.
Additional discussions on the various ways that standards for
TENORM might be expressed and the implications of the different forms of
standards for exposure-pathway and dose or risk assessment are presented in
chapter 1 1.
SUITABLE APPROACHES TO RISK ASSESSMENT IN DEVELOPING
STANDARDS
As indicated earlier, it is not the purpose of this chapter to discuss in
detail the kinds of exposure and dose or risk assessments that might be used in
developing a technical basis for radiation standards, especially standards for
TENORM. However, the committee offers the following general observations
on suitable approaches to risk assessment in developing radiation standards.
First, in developing radiation standards, it is appropriate to use stylized
methods of exposure and dose or risk assessment for assumed reference
conditions, provided that the assumed conditions are reasonably representative
of the exposure situations of concern and that the regulations permit the use of
alternative and more realistic approaches for specific exposure situations,
especially in demonstrating compliance with the standards. The use of stylized
methods of exposure, dose, and risk assessment in developing environmental
standards for radionuclides is consistent with the approach used in developing
secondary limits (limits on intakes of radionuclides or concentrations of
radionuclides in air) for control of radiation exposures in the workplace (for
example, Eckerman and others 1988~.
Second, exposure and dose or risk assessments used in developing
standards should be reasonably realistic, particularly the assumptions about
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88
ROLE OF ASSESSMENTS IN DEVELOPING STANDARDS
pathways for internal and external exposure, including transfers of radionuclides
among various environmental compartments, transfers through terrestrial and
aquatic food chains, and the various usage factors for internal and external
exposure pathways. That is, the assumptions should not be intended to greatly
overestimate or underestimate actual outcomes for the exposure situations of
concern. The need for reasonably realistic assessments is particularly important
if standards are based on cost-benef~t analyses with respect to dose or risk. A
dose or risk assessment that is unreasonably conservative or nonconservative
could lead to standards that either are not reasonably achievable or are not
adequately protective of public health.
Representative terms from entire chapter:
radiation standards
