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AN SAB REPORT: REVIEW
OF HEALTH RISKS FROM
LOW-LEVEL ENVIRONMENTAL
EXPOSURES TO RADIO-
NUCLIDES (FGR-13 REPORT)
REVIEW OF THE OFFICE OF RADIATION
AND INDOOR AIR'S FEDERAL
GUIDANCE REPORT 13 - PART 1,
INTERIM VERSION (FGR-13)
U.S. Environmental Protection Agency
Region 5, Library (PL-12J)
77 West Jackson Boulevard, 12th Floor
Chicago, ft 60694-3590
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, UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
'\ WASHINGTON, D.C- 20460
«*o^
December 23, 1998
OFFICE OF TWE AOMWISnUTOR
EPA-SAB-RAC-99-009 SCIENCE ADVISORY 3c*Ro
Honorable Carol M. Browner
Administrator
U.S. Environmental Protection Agency
401 M. Street, SW
Washington, DC 20460
Re: An SAB Report: Review of the Health Risks from Low-Level Exposure to
Radionuclides, Federal Guidance Report No. 13 - Part 1, Interim Version CFGR
13 - Part 1)
Dear Ms. Browner:
The accompanying report was developed by the FOR 13 Review Subcommittee of the
Radiation Advisory Committee (RAC) of the Science Advisory Board (SAB) in response to a
request from the Office of Radiation and Indoor Air (ORIA) to review the technical aspects of
the interim version of Federal Guidance Report 13 - Part 1, dated January, 1998. The
Subcommittee met on May 6-7, 1998, and held two subsequent conference calls, to review the
interim document with respect to the following Charge:
a) Is the methodology employed for calculating health risks from radionuclide
intakes and external exposures acceptable?
b) In light of scientific information, have the major uncertainties been identified
and put into proper perspective?
c) Is the proposed method for extending the list of radionuclides to include all
those tabulated in Federal Guidance Reports 11 and 12 reasonable?
FGR 13 - Part 1 provides tabulations of unit risk estimates, or 'risk coefficients", for
cancer morbidity and mortality attributable to exposure to any of approximately. 100
radionuclides via various environmental media. Radiation doses are calculated as an
intermediate step in estimating risks, but are not tabulated in the report. The risk coefficients
apply to populations that approximate the age, gender, and mortality experience characterized
by the current U.S. population. The report is intended by the Agency to promote consistency
in assessments of the risks to health from radiation and to help ensure that radiological risk
assessments are based on sound scientific information.
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In our enclosed report, we address these issues and offer a few suggestions in addition
to our response to the Charge. The report focuses on specific technical issues that the
Subcommittee believes should be addressed prior to publication of the final version of FGR 13
- Pan 1 and makes suggestions for future consideration by the Agency. The following
findings and recommendations are viewed as particularly important with respect to each
Charge element:
a) Methodology
1) The methodology employed for calculating health risks from
radionuclide intakes and external exposures was found to be acceptable
in the conrxt of: 1) current Agency science policy choices, such as the
applicability of a linear, no-threshold dose-response model for cancer
risks at low doses of radiation; and 2) its intended use to support broad
federal radiation protection programs such as environmental impact
statements or generic rulemaking. However, the Subcommittee is
concerned about reliance on unpublished reports for some of the detailed
techniques employed, absence of absorbed dose rate information, and
other points discussed below.
2) The lack of published reports for some of the techniques employed is a
weakness of this work, for both the internal and -external exposure
calculations. The Subcommittee recommends that evidence of
. verification and quality assurance procedures be included in an Appendix
to the final version of FGR 13 - Part 1.
3) The Subcommittee is concerned about the absence of radiation dose
information in FGR 13 - Part I. This makes it difficult for users to
update the risk coefficients as new information on risk or dosimetry
becomes available, or to employ risk models other than the linear-no
threshold model used by the Agency. The Subcommittee recommends
that dose information be published in electronic form.
4) Some potentially valuable information has not been addressed in FGR
13. For example, a published dosimetry model for the esophagus and
trachea is not used for the internal dose calculations or discussed in the
document. Risks calculated in FGR 13 - Part 1 are based on mortality
(death) data, but information on cancer incidence (morbidity) is
generally more accurate.
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b) Uncertainty
1) Although the document does well in identifying many of the major
uncertainties and describing them qualitatively, it does not provide the
reader with a good sense of the overall uncertainty entailed when a
specific risk coefficient is used to predict the risks of low-level exposure
to radiation. The Subcommittee recommends at a minimum that: 1) the
individual radionuclides of highest concern for environmental exposures
be placed into categories associated with low, moderate, high, or very
high uncertainty; 2) identification of these uncertainty categories be
contained within the tables of risk estimates: 3) general guidance about
the limits of. application be provided for those radionuclides that cannot
be placed into an uncertainty category, and; 4) the final version of FGR
13 - Part 1 include an appendix that describes the steps that may be taken
to improve uncertainty estimates for the risk coefficients.
2) For future revisions of the Agency's reports, such as those in the FGR
13 series, the Subcommittee recommends that quantification of
uncertainty be considered at the outset of the project.
3) Although the lack of scientific consensus on the estimation of lifetime
cancer risks from low-level radiation is acknowledged in FGR 13 - Part
1, the Subcommittee recommends adding a concise, balanced discussion
that lays out the arguments for and against the possibility of a threshold
or other non-linear behavior at low dose and low dose rate.
c) Expansion
1) The Subcommittee believes that the Agency's proposed method for
extending the list of radionuclides to include those tabulated in FGRs 11
and 12 is reasonable, but recommends that risk coefficients also be
presented for the inhalation of radon and its short-lived decay products.
The Subcommittee would prefer that these risk coefficients be included
in the final version of Part 1 if feasible.
The Subcommittee also identified and addressed several issues not raised in the Charge.
These issues (and where appropriate) recommendations are listed below:
a) The Subcommittee observes that use of the FGR 13 - Part 1 risk coefficients
could contribute to debate or ambiguity as to whether a regulated site meets
regulatory limits, especially in cases where a regulation is stated in terms of
dose limits but the supporting technical documentation might use risk
calculations.
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b) The Subcommittee recommends that risk coefficients for external exposure,
applicable specifically to the Uranium Mill Tailings Radiation Control Act
(UMTRCA) standard, be calculated and tabulated in the final version of FGR
13 - Part 1 or in a separate document.
c) The Subcommittee recommends that the title of the document be changed to
"Estimated Health Risks from Low-Level Environmental Exposure to
Radionuclides, Federal Guidance Report 13 - Pan 1: Cancer" (emphasis
added). This change conveys the important points that the tabulated risk values
are estimates and that cancer is the only health effect treated in the document.
We commend the Agency on its leadc :;hip and efforts in using up-to-date scientific
methods and data to develop the health risk estimates in FGR 13 - Part 1. We believe that our
recommendations, except those noted in the report for future consideration, can be
incorporated prior to publication of the,final version and will strengthen the report and its
credibility. We strongly support the Agency's stated intent to publish supporting information
in an electronic form to accompany release of the final version of FGR 13 - Part I, and we
recommend that it include the data, models, and dose information used in formulating the risk
coefficients.
The RAC and its Federal Guidance Report Review Subcommittee (FGRRS) appreciate
the opportunity to provide this report to you and we hope that it will be helpful. We look
forward to your response to this report in general and to the specific comments and
recommendations in this letter in particular.
Sincerely,
. Joan M. Daisey, Q&r Dr. Stephen L. Brown, Chair
Science Advisory Board Radiation Advisory Committee
Science Advisory Board
.
Dr. Thomas F. Gesell, Chair
Federal Guidance Report Review Subcommittee
Radiation Advisory Committee
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NOTICE
This report has been written as part of the activities of the Science Advisory Board
(SAB), a public advisory group providing extramural scientific information and advice to the
Administrator and other officials of the Environmental Protection Agency (EPA). The Board
is structured to provide balanced, expert assessment of scientific matters related to problems
facing the Agency. This report has not been reviewed for approval by the Agency and, hence,
the contents of this report do not necessarily represent the views and policies of the EPA nor
of other agencies in the Executive Branch of the Federal government. In addition, the mention
of trade names or commercial products does not constitute a recommendation for use.
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ABSTRACT
On May 6-7, 1998, the Federal Guidance Report Review Subcommittee
(PGRRS) reviewed technical aspects of the draft document, "Health Risks from Low-Level
Environmental Exposure to Radionuclides," Federal Guidance Report 13 - Part 1 - Interim
Version (FGR 13 - Part 1). This document provides tabulations of unit risk coefficients for
cancer morbidity and mortality attributable to exposure to approximately 100 radionuclides
through various environmental media, in a population approximated by the age, gender, and
mortality experienced in the United States.
The Subcommittee found the report to be well organized and well written and to have
used up-to-date scientific methods and data to determine the health risk estimates. Although
most of the important limitations of the risk estimates are noted in FGR 13 - Part 1, they are
not sufficiently prominent in the current draft, given the potential for misuse or
misinterpretation of the estimates. In particular, the magnitudes of the uncertainties in the
computed numbers are difficult to ascertain. Other concerns included partial reliance on
unpublished methodologies, lack of dose information, insufficient discussion of alternatives to
the linear, no-threshold risk model, and several other technical issues. The Subcommittee
found that the Agency's plan to calculate risk coefficients for an extended list of radionuclides
was appropriate, except that radon and its decay products should also be included. The
Subcommittee strongly supports the Agency's stated intent to publish supporting information in
electronic form to accompany release of the final version of FGR 13 - Part 1, and recommends
'that it include the data, models, and dose values used in formulating the risk coefficients.
KEYWORDS: radionuclides; cancer; risk assessment
u
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U.S. ENVIRONMENTAL PROTECTION AGENCY
SCIENCE ADVISORY BOARD
FEDERAL GUIDANCE REPORT REVIEW SUBCOMMITTEE
of the RADIATION ADVISORY COMMITTEE
ROSTER
Chair
Dr. Thomas F. Gesell, Idaho State University, Pocatello, CD
Members
Dr. Stephen L. Brown, Risks of Radiation &. Chemical Compounds, Oakland, CA
Dr. Raymond A. Guijmette, Lovelace Respiratory Research Institute, Albuquerque, NM
Dr. F. Owen Hoffman, SENES Oak Ridge, Inc., Oak Ridge, TN
Dr. Janet Johnson, Shepherd Miller, Inc., Ft. Collins. CO
Dr. Ellen Mangione, Colorado Department of Public Health and Environment, Denver, CO.
Dr. Paul J. Merges, New York State Department of Environmental Conservation, Albany, NY
Dr. John W. Poston, Sr., Texas A&M University, College Station, TX
Dr. Genevieve S. Roessier, Radiation Consultant, Elysian, MN
Dr. James E. Watson, Jr., University of North Carolina at Chapel Hill, Chapel Hill, NC
Dr. Arthur Upton, Robert Wood Johnson Medical School, Piscataway, NJ
Science Advisory Bn^rd Staff
Dr. K. Jack Kooyoomjiah, Designated Federal Officer, Science Advisory Board (1400), U.S.
EPA, 401 M Street, S.W., Washington, DC 20460
Mr. Samuel Rondberg, Designated Federal Officer, Science Advisory Board (1400), U.S.
EPA, 401 M Street, S.W., Washington, DC 204601
Ms. Diana L. Porun, Management Assistant, Science Advisory Board (1400), U.S. EPA, 401
M Street, S.W., Washington, DC 20460
Provided editorial support fot this report, but did not participate in the review.
Ill
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TABLE OF CONTENTS
L EXECUTIVE SUMMARY 1
2. INTRODUCTION 7
2.1 Background 7
2.2 Charge '. . 8
3. DETAILED FINDINGS 9
3.1 Assessing Methodology for Calculating Health Risks - Charge Element (a) .... 9
3.1.1 Overall Findings 9
3.1.2 Use of Unpublished Methods 9
3.1.3 Check of Numerical Results , 10
3.1.4 Absence of Dose Information 10
3.1.5 Information Not Addressed in FGR 13 - Part 1 11
3.1.6 Use of Mortality Data vs Morbidity 12
3.1.7 Absorption Types for Inhaled Radionuclides 13
3.1.8 Assumption of Full-time, Unshielded Exposure to Radionuclides
in Soil ,. . . . 13
3.1.9 Soil Ingestion ' 13
3.1.10 Appropriateness of Diet Averaging 14
3.2 Assessing the Treatment of Uncertainty - Charge (b) 14
3.2.1 Overall Findings 14
3.2.6 Comprehensive Uncertainty Calculation for Classes of Biokinetic .
Models 18
3.3 Assessing the Method for Extending List of Radionuclides - Charge (c) . . 19
3.3.1 Overall Findings . . 19
3.4 Additional Topics Identified by the Subcommittee 19
3.4.1 Recommended Change in Report Title 20
3.4.2 Contribution of FGR 13 - Part 1 to Regulatory Debate and Ambiguity 20
3.4.3 Domain of Applicability 20
3.4.4 Clarification of the Risk Tables 21
3.4.5 Tabulation of Predominant Cancer Types 21
3.4.6 Adequacy of Soil Geometries for Use with the Mill Tailings Standards 22
3.4.7 Relanonship-TBetween Exposure and Dose 22
3.5 General Conclusions 22
APPENDDC A - DETAILED TECHNICAL COMMENTS
APPENDIX B - ACRONYMS
REFERENCES ......'
iv
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1. EXECUTIVE SUMMARY
As pan of the EPA Science Advisory Board's (SAB) Radiation Advisory Committee
(RAC), the FOR 13 Review Subcommittee (the Subcommittee) reviewed technical aspects of
"Health Risks from Low-Level Environmental Exposure to Radionuclides," Federal Guidance
Report 13 - Part 1 - Interim Version (FGR 13 - Part I). This first report in the series provides
tabulations of risk estimates, or "risk coefficients," for cancer morbidity and mortality
attributable to exposure to approximately 100 radionuclides through various environmental
media. Radiation doses are calculated as an intermediate step in estimating risks, but are not
tabulated in the report. The risk coefficients apply to populations that approximate the age,
gender, and mortality experience characterized by the 1989-91 U.S. decennial life tables.
FGR 13 - Part 1 is intended by the Agency to promote consistency in assessments of the risks
to health from radiation and to help ensure that radiological risk assessments are based on
sound scientific information.
The Subcommittee was charged by the Agency's Office of Radiation and Indoor Air .
(ORIA) to focus its review on the following questions:
a) Is the methodology employed for calculating health risks from radionuclide
intakes and external exposures acceptable?
b) In light of scientific information, have the major uncertainties been identified
and put into proper perspective?
c) Is the proposed method for extending the list of radionuclides to include all
those tabulated in Federal Guidance Reports 11 and 12 reasonable?
The Subcommittee's report addresses these questions and offers some suggestions
beyond the charge. FGR 13, Part 1 is a useful addition to the complement of Federal
Guidance reports relevant to the evaluation of the risks of radiation and makes important
improvements in the calculation of radiation risks. The report is well organized and well
written, although difficult to comprehend for all but highly technical audiences. The careful
reader will note that most of the important caveats to the use of the document's results are
included in the text, but not as prominently as they probably should be. In particular, the
magnitudes of the uncertainties in the computed numbers are difficult to ascertain.
This review focuses on specific technical issues that the Subcommittee believes should
be addressed prior to publication of the final version of FGR 13 - Part 1 and makes
suggestions for future consideration by the Agency. These major findings and
recommendations are summarized below (with references to the corresponding sections of the
report appearing in parentheses).
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Charge element a) addressed the methodology employed for calculating health risks
from radionuclide intakes and external exposures. The proposed approach was found to be
acceptable in the context of a) current Agency science policy choices, such as the applicability
of a linear, no-threshold dose-response model for cancer risks at low doses of radiation, and b)
its intended use to support broad Federal radiation protection programs such as environmental
impact statements or generic rulemaking. However, as discussed below, the Subcommittee is
concerned about unpublished reports being cited for some of the detailed techniques employed,
absence of dose information, and the incomplete treatment of uncertainty. (Section 3.1.1)
The lack of published reports for some of the techniques employed is a weakness of
FGR 13 - Part 1, for both the internal and external exposure calculations. In particular,
because dose calculation (DCAL) software is such an integral part of FGR 13 - Part 1, the
Subcommittee recommends that the DCAL software and a users manual be published in
electronic form prior to, or concurrently with, the final version of FGR 13 - Part 1. (Section
3.1.2)
The Subcommittee considered whether or not it would be possible to check the
numerical results of the risk coefficients in some way. It concluded chat a check of the risk
estimates, or merely a verification of the coding of the model equations, would be a
formidable task outside the scope and level of resources available to the Subcommittee. The
Subcommittee recommends that evidence of verification and quality assurance procedures be
included in an appendix to the final version of FGR 13 - Part 1, or be made available in a
separate document. (Section 3.1.3)
The Subcommittee is concerned about the absence of radiation dose information in
FGR 13 - Pan 1. While the direct tabulation of risk estimates may provide a useful shortcut
for the preparation of rulemaking and environmental documents, it may make the report
difficult to follow by those who think of radiation safety in terms of dose and will make it
difficult for users to update the risk coefficients as new information on risk or dosimetry
becomes available. This approach also makes it difficult for users to employ risk models other
than the linear-no threshold model used by the Agency. The Subcommittee recommends that
dose information and corresponding uncertainties be published in electronic form.
Alternatively the authors could identify another source of this information, such as the
anticipated publication of dose information in electronic form by the International Commission
for Radiation Protection (ICRP). (Section 3.1.4)
FGR 13 - Part 1 does not use or address some potentially valuable information not
already incorporated into the ICRP models, such as an internal dosimetry model suggested for
the esophagus and trachea. Also, recent calculations of the absorbed energy in the
gastrointestinal tract could replace the very conservative assumptions used in ICRP Publication
30 and FGR 13 - Part 1. (Section 3.1.5)
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Risks calculated in FOR 13 - Pan I are based on mortality data. However, information
on cancer incidence (morbidity) is generally much more accurate than is information on cancer
mortality derived from death certificates. Although the Subcommittee understands that a
change to morbidity-based estimation is a major undertaking with potentially significant policy
implications, and therefore may be impossible in the short run, we recommend it be given
serious consideration for future updates of FGR 13 - Part 1 and other radiation risk activities in
the Agency. (Section 3.1.6)
In FGR 13 - Part 1, risk coefficients are calculated only for the default absorption types
given by ICRP and for the absorption types adjacent to the default types, resulting in an
incomplete list of risk coefficients. The Subcommittee recommends that: a) risk coefficients,
at a minimum, be calculated for each absorption type considered by ICRP for all
radionuclides, b) the default absorption types recommended by ICRP be specifically identified
in the tables of FGR 13 - Part I, and c) preference be given to using material-specific
knowledge in lieu of default values when appropriate and practicable. (Section 3.1.7)
In the risk calculation for direct radiation exposure from contaminated ground and for
immersion in a radioactive gas or vapor, no shielding from structures is assumed. Because
most people spend the majority of their time indoors, the calculated risks in FGR 13 - Pan 1
may overestimate the real risk. The document should provide some general guidance to the
user as to a reasonable range of risk reduction factors and point the user to methods and
references for estimating an appropriate value within that range. (Section 3.1.8)
Soil ingestion has not been considered as an exposure pathway in FGR 13 - Part I.
The Subcommittee recommends that FGR 13 - Part 1 be expanded at some time in the future
to include soil intake. (Section 3.1.9)
The risk coefficients for radionuclides in food are expressed as lifetime probabilities of
cancer morbidity or mortality from ingestion of 1 Bq of the radionuclide. Except for separate
tables for radioiodines in milk, the radionuclides are assumed to be distributed in foods such
that radionuclide activity intake per unit caloric intake would remain constant with age and
gender. In real exposure scenarios, this assumption might not be correct. Although the
Subcommittee is not suggesting a suite of risk factors for different food types, it recommends
that the Agency include differential food scenarios in the list of risk analyses for which the
FGR 13 - Part I tables are not appropriate. (Section 3.1.10)
The second Charge element (b) addressed the treatment of uncertainty. The major
uncertainties have not entirely been identified and put into proper perspective. Although the
document does well in identifying many of the major uncertainties and describing them
qualitatively, it does not provide the reader with a good sense of the overall uncertainty
entailed when a specific risk coefficient is used to predict the risks of low-level exposure to
radiation. The Subcommittee recommends that the Agency attempt to convey better the
overall impact of the multiple sources of uncertainty on the final risk numbers. In particular,
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it should more prominently note the debate about the applicability of the linear no-threshold
model at very low doses, even though the Agency's own evaluations may discount the
likelihood of alternative models. It should also be made clearer how the reliability of the
calculations varies from such highly studied radionuclides as a9Pu or I31I to those for which
most of the parameters are based on analogies with surrogates. (Section 3.2.1)
Quantitative estimates of uncertainty are clearly unnecessary for nominal values
intended only for an initial screening calculation. However, the values in FGR 13 - Part 1 are
not intended for screening; rather, they are intended to provide a realistic best estimate of the
average risk per unit exposure. Given the broad application expected for these risk estimates,
the Subcommittee recommends at a minimum that: a) the individual radionuclides of highest
concern for environmental exposures be placed into categories associated with low, moderate,
high, or very high uncertainty; b) these categories be described quantitatively (e.g., a factor of
less than three, three to 10, 10 to 100, and more than 100, respectively; c) these uncertainty
categories be listed in the tables of risk estimates; d) general guidance about the limits of
application be provided for those radionuclides for which information is insufficient to permit.
placement into a given uncertainty category, and e) the final version of FGR 13 - Pan i
include an appendix that describes the steps that may be taken to improve uncertainty estimates
for the risk coefficients to reflect future advances in the state of knowledge of internal and
external dosimetry and of the response to low doses of ionizing radiation. (Section 3.2.2)
The present state of the art in exposure, dose and risk assessment employs the use of
probabilistic methods to propagate the uncertainty of all major model components through to a
final estimate of dose and risk. In the case of FGR 13 - Part I, the Subcommittee recognizes
that such calculations, due to inherent complexity of the Agency's methodology, could be a
major undertaking requiring extensive new resources. For future improvements of the
Agency's reports, such as revisions of FGR 13, the Subcommittee recommends that
quantification of uncertainty be considered at the outset of the project. (Section 3.2.3)
Although the lack of scientific consensus on the estimation of lifetime cancer risks from
low-level radiation is acknowledged in FGR 13 - Part 1, the Subcommittee recommends
adding a concise, balanced discussion that lays out the arguments for and against the possibility
of a threshold or other non-linear behavior at low dose and low dose rates. (Section 4.4)
Model uncertainties, i.e., uncertainties in the structure of a biofanetic, dosimetric or
dose-response model, are often more difficult to assess than are parameter uncertainties.
Significant uncertainty can be attributed to the models themselves, apart from uncertainties in
parameter values, and model uncertainty should be reflected in the text of FGR. 13 - Part 1.
(Section 4.5)
Although making a detailed probabilistic uncertainty evaluation for every element
would be a formidable task, the Subcommittee recommends that a comprehensive uncertainty
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calculation be performed for one example radionuclide for each class of biokineac model. It
might be of use also to show a calculation for the inhalation of specific radon decay products
and some examples for ill-studied radionuclides to illustrate the range of uncertainties that are
present in the calculations. (Section 4.6)
The third and last major element of the Charge (c) dealt with extending the list of
radionuclides to include those incorporated in previous versions of FGR 13 - Part 1. The
radionuclides addressed in the interim report were primarily those included in ICRP
Publications 56, 66, 67, 69, and 71, which contained the models used for calculating dose
rates to organs and tissues. The Subcommittee believes that the Agency's proposed method for
extending the list of radionuclides to include those tabulated in FGRs 11 and 12 is reasonable,
but recommends that risk coefficients also be presented for the inhalation of radon and its
short-lived decay products, the largest source of collective radiation exposure (and
presumably, radiation risk) to the U.S. population as a whole. If it is not feasible to include
these risk coefficients in the final version of FGR 13 - Part 1, their omission should be
explained and the risk coefficients included in a future revision of FGR 13 - Part I. (Section
3.3)
During the course of the public meeting, the Subcommittee identified several additional
areas where improvements or clarifications could be made to the EPA draft report, and has
provided recommendations to that effect. First, the Subcommittee recommends that the title of
the document be changed from "Health Risks from Low-Level Environmental Exposure to
Radionuclides, Federal Guidance Report 13 - Part r.to "Estimated Health Risks from Low-
Level Environmental Exposure to Radionuclides, Federal Guidance Report 13 - Part 1:
Cancer" (emphasis added). This change conveys two important points: a) The tabulated risk
values are estimates and b) cancer is the only health effect treated in the document. (Section
3.4.1)
The Subcommittee also noted that use of the FGR 13 - Part 1 risk coefficients could
contribute to debate or ambiguity as to whether a regulated site meets regulatory limits,
especially in cases where a regulation is stated in terms of annual (or per incident) dose limits
but the supporting technical documentation, such as an environmental impact statement, might
use risk calculations. The Subcommittee observes that some confusion may result in any
attempt to apply both the FGR 11 and 12 methodologies and the FGR 13 - Part 1 meth'odology
on the same regulatory issue. This complication could arise even if the Agency's regulatory
action itself used only one method, because an intervener from the regulated community or the
public interest community could use the other method to contest the regulation. (Section
3.4.2)
The Subcommittee is concerned with the potential .for misapplication of the FGR 13 -
Part 1 document. As noted in the document, its approach to risk analysis is not intended to be
applied to the analysis of risks to individuals associated with either past or future exposures.
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The Subcommittee recommends that the preface of FOR 13 - Part I include a more thorough
discussion of the potential for misapplication and the limitations of use for FOR 13 - Part 1
risk estimates. (Section 6.3)
The Subcommittee believes that tables in any document should be understandable
without reference to the text. For instance, the need for separately considering ingrowth of
chain members in an environmental medium before they enter the body (e.g., by ingestion,or
inhalation) and the absence of uncertainty bounds in the tables are critically important to
explain to users. (Section 3.4.4)
It is not clear to the Subcommittee that the listing of a single predominant cancer type
in the risk tables is either useful or correct. Alternatives to the current presentation might be:
a) eliminating the columns completely or b) selecting '"or example, the three tissues or
organs considered to be at greatest risk, but without g: -g a quantitative estimate of the risk
fraction associated with those tissues and organs. (Sec-. :n 3.4.5)
It is stated in FGR 13 - Part 1 that "Because nsk coefficients for external exposure to
soil contaminated to 15 cm would differ only slightly from those for concentrations to an
infinite depth, it would not be useful to provide tabulations of nsk coefficients for both
situations." The Uranium Mill Tailings Radiation Control Act (UMTRCA) specifies a
standard for contaminated soil of 5 pCi g'1 in the top 15 cm and 15 pCt g"1 below 15 cm. The
Subcommittee recommends that risk coefficients for external exposure, applicable specifically
to the UMTRCA standard, be calculated and tabulated in the final version of FGR 13 - Part 1
or in a separate document. (Section 3.4.6)
In conclusion, the Subcommittee commends the Agency on its leadership and efforts in
using up-to-date scientific methods and data to make the health risk estimates in FGR 13 - Part
1. We believe that our recommendations, except those noted for future consideration, can be
incorporated prior to publication of the final version and will strengthen the report and its
credibility. We strongly support the Agency's stated intent to publish supporting information
in electronic form to accompany release of the final version of FGR 13 - Part 1, and we
recommend that it include the data, models, and dose information used in formulating the risk
coefficients.
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2. INTRODUCTION
2.1 Background
Since the mid-1980s the United States Environmental Protection Agency has issued a
series of Federal guidance documents for the purpose of providing technical information to
assist Federal agencies in their implementation of radiation protection programs (EPA 1984;
1988; 1993; 1998). The newest in the series, collectively to be called Federal Guidance
Report'13 (FGR 13), is intended by the Agency to promote consistency in assessments of the
risks to health from radiation and to help ensure that radiological risk assessments are based on
sound scientific information. The document reviewed in this Science Advisory Board (SAB)
report is the interim version of Part 1 of FGR 13 (FGR 13 - Part 1), which is limited to cancer
as a health outcome (EPA, 1998). The FGR 13 documents are to use state-of-the-Ln methods
and models for estimating the risks to health from internal or external exposure to specific
radionuclides, taking into account the age- and gender-specific aspects of radiation risk. FGR
13,- Pan I provides tabulations of risk estimates, or "risk coefficients," for cancer morbidity
and mortality attributable to exposure to each of approximately 100 radionuclides via various
environmental media. These risk coefficients apply to populations that approximate the age,
gender, and mortality experience characterized by the 1989-91 U.S. decennial life tables.
The tabulations in the final version of Part 1 are expected to extend the methodology of
the interim version to the other radionuclides included in FGRs 11 and 12. Subsequent parts
of FGR 13 may extend the exposure pathways and health endpoints addressed. As necessary,
these publications are to be reissued to update the information provided. The Agency chose to
issue Part 1 of FGR 13 as an interim report in order to provide governmental agencies and
other interested parties an opportunity to become familiar with it and its supporting
methodology and to provide comments for the Agency's consideration before publishing the
final version.
The Agency intends that the risk estimates tabulated in FGR 13 - Part 1 be used
mainly for prospective assessments of estimated cancer risks from long-term exposure to
radionuclides in environmental media, such as preparation of environmental impact statements
and development of assessments in support of generic rulemaking for control of radiation
exposure. Although it is recognized that these risk coefficients are likely also to be used in
retrospective analyses of radiation exposures of populations, the Agency emphasized that such
analyses should be limited to estimation of total or average risks in large populations. The
tabulations are not intended for application to specific individuals or to age or gender
subgroups, for example, children, and should not be used for that purpose. Also, these risk
coefficients should not be applied to accident cases involving high doses and dose rates, either
in prospective or retrospective analyses. Finally, some risk assessment procedures are
established as a matter of policy, and additional steps may be needed before using these risk
coefficients. For example, the Agency recommends that radiation risk assessments for sites on
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the National Priorities List under the Comprehensive Environmental Response, Compensation,
and Liability Act (CERCLA) be performed using the Health Effects Assessment Summary
Tables.
In FOR 13 - Part 1, estimated risks are tabulated directly against the radionuclide
quantities without providing radiation dose information to the user. The linear, no-threshold
dose-response hypothesis and the individual organ and tissue risk coefficients are integrated
into the tabulated results. This approach represents a significant departure from the two
previous Federal Guidance Reports (EPA 1988; 1993) in which dose quantities were tabulated
as a function of radionuclide intake or soil concentration, leaving it to die user to apply the
risk coefficients. For users who desire a simple way to estimate risks to populations directly
from radionuclide concentrations, the approach used in FGR 13 - Part 1 represents a useful
step forward. For experts in the Meld, however, the lack of radiation dose information and
other details in the interim version of FGR 13 - Part 1 is of concern. However, the Agency
has stated its intention to make available detailed supporting information in electronic form,
coincident with the release of the final version of FGR 13 - Part 1.
The RAC was briefed on the subject: on March 3, 1998, and subsequently formed the
FGR 13 Review Subcommittee (the Subcommittee). The Subcommittee held a public meeting
in Washington, DC on May 6 and 7, 1998, at which time it was briefed by, and had technical
discussions with, the Office of Radiation and Indoor Air (ORIA) staff. In addition, a public
teleconference was held on June 2, 1998 to discuss an internal working draft.
2.2 Charge
The SAB was asked by ORIA to focus on the following questions:
a) Is the methodology employed for calculating health risks from radionuclide
intakes and external exposures acceptable?
b) In light of scientific information, have the major uncertainties been identified
and put into proper perspective?
c) Is the proposed method for extending the list of radionuclides to include all
those tabulated in Federal Guidance Reports 11 and 12 reasonable?
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3. DETAILED FINDINGS
3.1 Assessing Methodology for Calculating Health Risks - Charge Element (a)
The first element of the charge asked the Subcommittee to assess the methodology
employed for calculating health risks from radionuclide intakes and external exposures.
3.1.1 Overall Findings
The methodology is acceptable in the context of a) current Agency science policy
choices, such as the applicability of a linear, no-threshold dose-response model for cancer risks
at low doses of radiation and b) its intended use to support broad Federal radiation protection
programs such as environmental impact statements or generic rulemaking. The preface of the
draft report points out mat the tabulations are not intended for application to specific
individuals or age or gender subgroups, especially for retrospective analyses. The
Subcommittee also cautions that they may not be appropriate for site- or situation-specific
analyses for which the assumptions of the document may not be valid.
The internal dose assessment and biokinetic models used in the calculations are from
the most recent publications (numbers 56, 67, 68, 69, and 71) of the ICRP (1989; 1993a;
1994b; 1995a; 1995b). In addition, the most recent model of the respiratory system (ICRP
Publication 66, 1994a) is used, where appropriate. In some cases, where data are lacking and
or incomplete, the parameters in the several parts of ICRP Publication 30 (1979; 1980; 1981;
1988) are used in the models. The models, many of the important parameter values, and the
default situations are summarized in tables presented in Chapters 4 and 5 of FOR 13 - Part 1.
These calculations appear to have been made using standard techniques and represent "good
science." External exposure calculations also appear to have been made using standard
techniques, many of which have been used in other calculations for the Agency. However, as
discussed below, unpublished reports were cited for some of the detailed techniques employed.
Some specific and more detailed comments on the methodology are given in the following
sections.
3.1.2 Use of Unpublished Methods
The lack of published reports for some of the techniques employed is a weakness of this
work, for both the internal and external exposure calculations. This weakness did not hamper
our evaluation because a Subcommittee member had been supplied with draft copies of these
reports by the authors. Nevertheless, this weakness should be addressed before publication of
the final version of FOR 13 - Part 1.
In particular all computations of dose and risk were performed using the dose
calculation (DCAL) software developed by staff at Oak Ridge National Laboratory (ORNL),
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which is to be published' as an ORNUTM report. DCAL is a comprehensive computer
program of biokinetic-dose-risk models and computational systems used for radiation
dosimetry and risk analyses. While DCAL has been extensively tested, and was used to
support five published ICRP reports, it still needs to be published in order to become openly
available and widely accepted by the scientific community. Because DCAL is such an integral
part of FOR 13 - Part I, the Subcommittee recommends that the DCAL software and a users
manual be published in electronic form prior to, or concurrently with, the final version of
FGR-13, Part 1.
3.1.3 Check of Numerical Results
The Subcommittee considered whether or nc" it would be possible to check the
numerical results of the risk coefficients in some w. Checking the risk estimates in their
entirety would require observing cancer rates in an exposed cohort, which is clearly
impossible. Given far more time and resources than is available to the Subcommittee, it may
be possible to check discrete components of the risk estimates, such as the time-dependent
organ burden of a specific radionuclide as a result of a given intake, or the specific health
outcomes resulting from a given dose to a specific tissue. Another approach would be an
exercise through which several qualified assessment teams independently predict the lifetime
risk per Bq of intake along with estimates of uncertainty, and their results and rationales
compared with each other. This type of exercise has been performed internationally for
environmental transfer models (BIOMOVS, 1993; BIOMOVS II, 1996; IAEA, 1995; IAEA,
1996; Thiessen et al., 1997) but such an inter-assessment comparison by independent experts
has not been undertaken for the disciplines of internal dosimetry and risk. Other possibilities
would be to assign one or two members of the Subcommittee the task of asking questions and
working with the group at ORNL to more completely understand what was done, or to have
another group run the code for a few specific test cases.
The Subcommittee concluded that a check of the risk estimates, or merely a verification
of the coding of the model equations, would be a formidable task outside the scope and level
of resources available to the Subcommittee. As an alternative, the Subcommittee recommends
that evidence of verification and quality assurance procedures be included in an appendix to the
final version of FGR 13 - Part 1, or be made available in a separate document.
^
3.1.4 Absence of Dose Information
r-
t
Although absorbed dose rates were calculated by the authors of FGR 13 - Part 1 as an
intermediate step in the estimation of risk, dose information is not provided in the document.
The Subcommittee understands that the committed effective dose approach was not used in
preparation of FGR 13 - Part 1, but is concerned that the lack of dose information may make -
the report difficult to follow by those who think of radiation safety in terms of dose and will
make it difficult for users to update the risk coefficients as new information on risk or
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dosimetry becomes available. The lack of dose information will also make it difficult to
employ risk models other than the linear-no threshold model used by the Agency.
The Subcommittee recommends that the authors publish comprehensive absorbed dose
rate or other appropriate dose information in electronic form concurrently with the release of
the final version of FGR 13 - Part 1. Alternatively the authors could identify another source
of this information, such as the anticipated publication of dose information in electronic form
by the ICRP. The Subcommittee also recommends that the dose information be accompanied
with estimates of uncertainty. This would allow the reader to evaluate the importance of
uncertainty in the dosimetry model as compared to the uncertainty in the dose response. The
Subcommittee further recommends that the final version of FGR 13 - Part 1 include a
discussion that compares and contrasts the methodologies of FGR Reports 11 and 12 (F-PA,
1988; 1993) with the methodology of FGR 13 - Part I.
3.1.5 Information Not Addressed in FGR 13 - Part 1
There are instances in which potentially valuable information, not already incorporated
into the ICRP models, is not addressed or used in FGR 13 - Pan 1. For example, a dosimetry
model has been proposed for the esophagus and trachea for many years (Lewis and Ellis,
1979). In Chapter 5 of FGR 13 - Part I, the authors state that no model of the esophagus has
been incorporated in the mathematical phantom for internal dose calculations, whereas in
Chapter 6, the discussion indicates that the esophagus has been incorporated into the
mathematical model for external dose calculations (but with no literature citation). One would
expect there to be some commonality between the mathematical phantom used for internal dose
assessment and the phantom used in the external dose assessment portion of the work. The
Subcommittee recommends that the final version of FGR 13 - Part I include a discussion of
why an esophagus model was not considered appropriate to include in the model for internal
dose rate calculations. The Subcommittee further recommends that a literature citation or a
short description be given of the model of the esophagus used in the mathematical phantom for
external exposure.
Recent calculations of the absorbed energy for monoenergetic electrons in the
gastrointestinal tract could provide an improvement over the very conservative assumptions
used in ICRP Publication 30 and FGR 13 - Part 1 (see Poston et al. 1996a, 19965). These
two papers have provided a better understanding of the energy dependence of the absorbed
fractions in the wall of the gastrointestinal tract. For example, these data show that at 5 MeV,
the absorbed fraction of energy (AF) for electrons in the gastrointestinal tract wall reaches
0.15. For lower energies, the absorbed fraction can be significantly less. For example, at 1
MeV, the AF is about 0.1 and decreases monotonically to a value of about 2 x 10"5 at 0.01
MeV. These data must be contrasted with the ICRP approach used in the calculations for FGR
13 - Part 1. It is clear that, for many beta-emitting radionuclides, the ICRP approach
significantly over-estimates the absorbed energy in the wall of the tract and, therefore, over-
estimates dose (and the risk). Here, the uncertainty in dose and risk is not merely a factor of
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two or three, but could be orders of magnitude. The Subcomnvttee recommends that the
approach described in Poston et al. (1996a; 1996b) be considered for the final version of FOR
13-Parti.
3.1.6 Use of Mortality Data vs Morbidity Data
Mortality and morbidity risk coefficients are both presented in FOR 13 * Part 1. The
mortality risk coefficients were determined based on data from the Japanese atomic bomb
survivors and other study groups. However, the morbidity risk coefficients were calculated by
taking each site-specific mortality risk and dividing it by its respective lethality fraction, that
is, the fraction of radiogenic cancers at that site which are fatal. A better approach would be
to develop morbidity risk coefficients directly from populations such as the Japanese bomb
survivors.
Information on cancer incidence (morbidity) has long been recognized to be generally
much more accurate than is information on cancer mortality derived from death certificates. •
Typically, incident cancers are historically confirmed, with the information gathered by
individuals trained in the collection of such data from a wide variety of sources. They are
usually collected in a timely manner following diagnosis and are unaffected by variations in
survival fraction geographically and over time.
Cancer mortality, data, gathered from death certificates for most cancers, are often
collected at a time remote from diagnosis, making epidemiologic studies of exposures
problematic. Cause of death is often recorded by those to whom secondary cause of death
information is not available, and it is generally not confirmed histologically or otherwise.
Their accuracy varies substantially with cancer site. All of these problems are expected to
hold in some measure for the principal studies of radiation-induced cancers.
The availability of cancer incidence data from studies of the atomic bomb survivors and
other irradiated populations provides an opportunity to base cancer risk estimates (either for
morbidity or mortality) on these more reliable statistics. Such an endeavor would also have
the advantage of making the radiation cancer risk estimation process more in harmony with
that used for chemicals. Although the Subcommittee understands that a change to
morbidity-based estimation is a major undertaking with potentially significant policy
implications, and therefore may be impossible in the short run, we recommend it be given
serious consideration for future updates of FOR 13 - Part 1 and other radiation risk activities in
the Agency.
Additional, more detailed discussions of the advantages of developing risk coefficients
from morbidity data rather than mortality data are given in the SAB/RAC report on
uncertainties in radiogenic cancer risk (SAB, 1998).
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3.1.7 Absorption Types for Inhaled Radionuclides
As indicated in FGR 13 - Part 1, there are significant uncertainties in estimating
absorption types for radionuclides in forms likely to be encountered in the environment. In
FGR 13 - Part 1, risk coefficients are calculated only for the default absorption types given by ........
ICRP and for the absorption types adjacent to the defaults in the ICRP publication. This
approach results in an incomplete list of risk coefficients. An alternative approach, which
would provide the user of FOR 13 - Part I the most flexibility, would be at a minimum to
calculate the risks for all of the absorption types considered by ICRP for each of the listed
radionuclides, and to indicate the ICRP default type. In this way, the user would be made
aware of the recommended ICRP default values, which would most likely be used in the
absence of material-specific data. With more information, however, the user could also use
one of the alternative risk coefficients.
Specifically the Subcommittee recommends that: a) risk coefficients be calculated for
each absorption type considered by ICRP, for all radionuclides, b) the default absorption
types recommended by ICRP be specifically identified in the tables of FGR 13 - Part 1, and c)
preference be given to using material-specific knowledge in lieu of default values when
appropriate and practicable. These additions should make the report easier to use.
3.1.8 Assumption of Full-time, Unshielded Exposure to Radionuclides in Soil
In the risk calculation for direct radiation exposure from contaminated ground and for
immersion in a radioactive gas or vapor, no shielding from structures is assumed. However,
most people spend the majority of their time indoors. Therefore, the calculated risks in FGR
13 - Part 1 may overestimate the real risk by factors ranging from less than two to over ten.
The reduction in dose due to the inherent shielding characteristics of structures depends not
only on the type, thickness and configuration of the building materials but also on the type and
energy of the radiation. The external exposure calculations are made assuming that the
exposed individual is outdoors for the entire period. This assumption could result in additional
bias in the range of a factor of two to three. The Subcommittee realizes that it would be
infeasible to incorporate structural shielding factors into the risk tables. However, the
Subcommittee recommends that the document include some general guidance to the user as to
a reasonable range of risk reduction factors for shielding and point the user to methods and
references for estimating an appropriate value.
3.1.9 Soil Ingestion
Soil ingestion has not been considered as an exposure pathway in FGR 13 - Part 1. For
most radionuclides and soils, the bioavailability from soils would be likely to be lower than for
foods. Applying the ingestion risk factors for food to soil intake is likely to overestimate the
dose. However, the average soil intake rate for children may be higher than for adults. The
relative ratios of food intake by children to adult intake may be skewed if soil intake is
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included. That is, the fraction of the lifetime intake attributable to early childhood may be
underestimated when soil is a significant route of exposure. The Subcommittee recommends
that FGR 13 - Part 1 be expanded at some time in the future to include soil intake.
3.1.10 Appropriateness of Diet Averaging
The risk coefficients for radionuclides in food are expressed as lifetime probabilities of
cancer morbidity or mortality from ingestion of 1 Bq of the radionuclide, presumed to be
spread evenly over a lifetime or, if acute, spread evenly across a stationary population. The
risk coefficients take into account the variations in caloric intake per unit body weight as a
function of age and gender, so that the contribution of a constant concentration of the
radionuclide in food can have differing impacts on risk at different ages. Except for separate
tables for radioiodines- in .milk, the radionuclides are assumed in FGR 13 - Part 1 to be
distributed in foods such that radionuclide activity intake per unit caloric intake remains
constant with age and gender.
In real exposure scenarios, this assumption of constant intake factors is incorrect, as the
Agency has recognized by computing separate risk factors for milk, a food well known to be
consumed differentially by children and adults. In response to Subcommittee questions, the
Agency further stated orally that no other food group stood out as having a significant
variability not predicted by caloric intake. The Subcommittee did not determine what level of
food disaggregation was used, but pointed out that differences could well be significant for
some types of food, such as fish and apple juice. During the discussion, it became clear that
the types of scenarios envisioned in FGR 13 - Part 1 for application of the risk factors were
not ones that would concentrate radionuclides in only a few narrow food groups. Although the
Subcommittee is not suggesting a suite of risk factors for different food types, it recommends
that the Agency include differential food scenarios in the list of risk analyses for which the
FGR 13 - Part 1 tables are not appropriate. To the extent that all sources of exposure have not
been identified, this approach may either overestimate, or underestimate, the actual exposure.
1 i
3.2 Assessing the Treatment of Uncertainty'Charge (b)
The second element of the charge asked the Subcommittee to determine if, in light of
scientific information, the major uncertainties have been identified and put into proper
perspective
3.2.1 Overall Findings
The major uncertainties have not entirely been identified and put into proper
perspective. Although the document does well in identifying many of the major uncertainties
and describing them qualitatively, it does not provide the reader with a good sense of the
overall uncertainty entailed when a specific risk coefficient is used to predict the risks of
low-level exposure to radiation. It is possible for a user to conclude equally that the estimates
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are quite reliable or almost totally uncertain. Without a better understanding of the plausible
limits of uncertainty, any use of the estimates for regulatory purposes could lead to
inappropriate decisions and lack of public trust. Disclosure of quantitative uncertainty is
important whenever an estimate of health risk is intended to be realistic rather than a
conservative screening value.
Although the Subcommittee is aware of the scientific and computational difficulties of
undertaking a detailed quantitative uncertainty analysis that would examine all the potentially
important sources of uncertainty, it recommends that the Agency attempt to convey better the
overall impact of the multiple sources, of uncertainty on the final risk numbers, as it is
currently attempting to do for the uncertainties in the risk model (SAB, 1998, report in
preparation). In particular, it should more prominently note the debate about the applicability
of the linear no-threshold model at very low doses, even though the Agency's own evaluations
may discount the likelihood of alternative dose-response models. It should also be made
clearer how the reliability of the calculations varies from such highly studied radionuclides as
^Pu or 131I to those for which most of the parameters are based on analogies with surrogates.
Detailed guidance for uncertainty analysis has been published in NCRP (1996). This
guidance acknowledges that the quantification of the state of knowledge for exposure and risk
assessment models is inherently dependent on subjective judgment. The authors of FGR 13 -
Pan 1 have extensive experience in the development and application of dosimetric and risk
models for specific radionuclides and should be able to quantify the state of knowledge in a
defensible manner, using subjective probability distributions. Alternatively, expert elicitation
can be used to derive these distributions as recommended in the SAB/RAG review (SAB,
1998) of the December 1997 Draft Addendum on the uncertainty analysis in estimating
radiogenic cancer risks (EPA, 1997), It is emphasized that the uncertainty should be
characterized and quantified specifically for the application of the methodology in FGR 13 -
Part 1 to the general U.S. population.
When providing estimates of uncertainty, the Agency should also provide guidance and
incentives for the user to incorporate updates of these estimates, given improvements in the
state of knowledge. Presumably, based upon its statement in the preface of the interim version
of FGR 13 - Part 1, the Agency will permit updating of the risk estimates and statements of
uncertainty in specific regulatory cases.
Specific and more detailed comments on aspects of uncertainty are given below.
r-
Specification of quantitative uncertainty estimates is increasingly seen as very important
for decision making. Quantitative estimates of uncertainty are clearly unnecessary for nominal
values intended only for an initial screening calculation and when it is already known that the
screening values are not likely to lead to a substantial underestimate of the true but unknown
risk (see NCRP, 1996).
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However, the values in FOR 13 - Pan 1 are not intended to be used for screening
calculations; rather, they are intended to provide a realistic best estimate of the average risk
per unit exposure. The Subcommittee recommends that, at a minimum, the individual
radionuclides of highest concern for environmental releases and exposures be categorized into
those associated with low, moderate, high, or very high uncertainty. The Agency should
define the numerical bounds of low, moderate, high, or very high uncertainty, but the
following guidance is offered: Low uncertainty might be assigned to any coefficient likely to
be in a range less than a factor of three either side of a best estimate; moderate uncertainty
would be greater than a factor of three, but less than a factor of ten; high uncertainty would be
greater than a factor of ten, but less than a factor of 100, and very high uncertainty would
exceed a factor of 100. The Subcommittee recognizes that in some cases, the uncertainty may
not be symmetrical about the best estimate. In this case, two estimates of uncertainty could be
provided (e.g., the estimate of calculated risk is unlikely to underestimate the true risk by
more than a factor of 3, but under some circumstances may overestimate the true risk by a
factor of 10 to 100).
The Subcommittee also recommends that identification of categories of uncertainty for
radionuclides likely to be of highest concern for environmental releases and exposures be
included in the tables of risk coefficients, rather than in separate tables. The Subcommittee
further recommends that general guidance be given about the limits of application for those
radionuclides for which information is currently insufficient to support their placement into a
specific category of uncertainty and that the final version of FOR 13 - .Part 1 include an
appendix that indicates steps that may be taken to improve estimates of uncertainty to account
for future advances in the state of knowledge for internal and external dosimetry and the dose-
response for exposure to low levels of ionizing radiation.
The Subcommittee recognizes that the present state-of-the-art in exposure, dose and
risk assessment employs the use of probabilistic methods to propagate the uncertainty of all
major model components through to a final estimate of dose and risk. The result is usually a
subjective probability distribution from which a 90% or 95% subjective confidence interval
(sometimes referred to as a credibility interval) is obtained. This information is sometimes
accompanied by a table or pie chart that identifies the most important variables that dominate
the overall estimate of uncertainty in the risk per unit intake or per unit concentration in the
environment.
In the case of FOR 13 - Part 1, the initial methodology was developed without
considering the need to propagate uncertainty using probabilistic methods. In the current form
of the risk models used in FOR 13 - Part I, the Subcommittee recognizes that such uncertainty
calculations, due to inherent complexity of the Agency's methodology, could be a major
undertaking requiring extensive new resources. However, if the uncertainty propagation is
limited to those model components already known to dominate the overall result, then the
process of uncertainty analysis can be made more efficient.
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For future improvements of the Agency's reports, such as revisions of FGR 13, the
Subcommittee recommends that quantification of uncertainty be considered at the outset of the
project and that the investigators be provided with sufficient funding and resources to •
characterize the state of knowledge for uncertain model components in a defensible manner. ;
The computer algorithms should be designed at the beginning of the project to facilitate the ' \
propagation of uncertainty through to the final result. " '" \
The lack of scientific consensus on the estimation of lifetime cancer risks from 3
low-level radiation is acknowledged in FGR 13, Pan 1. Some scientists believe in an effective i
threshold for health effects at sufficiently low doses and low dose rates, citing numerous I
studies that have not demonstrated a risk at levels within the range of natural background ]
(BEIR, 1990; Modan, 1991). Others acknowledge the possibility of a non-linear relationship ;
not adequately treated with the dose and dose-rate effectiveness factor (DDREF) approach.
Concluding a discussion on uncertainty of risk at low dose, the BEIR V report (BEIR, 1990)
states "Moreover, epidemiologic data cannot rigorously exclude the existence of a threshold in
the millisievert dose range. Thus the possibility that there may be no risks from exposures
comparable to external natural background cannot be ruled out. At such low doses and dose
rates, it must be acknowledged that the lower limit of the range of uncertainty in the risk
estimates extends to zero." The Subcommittee recommends that a concise, balanced discussion
that lays out the arguments for and against the possibility of a threshold or other non-linear
behavior at low dose and dose rates be included in the final version of FGR 13, Part i. This
issue has been addressed in detail by the Agency in its unpublished December 1997 Draft
Addendum on the uncertainty analysis in estimating radiogenic cancer risks (EPA, 1997) and
these arguments could be summarized in FGR 13 - Part I.
The position that the Agency has taken with respect to the risk of low-LET radiation at
low doses and low dose rates is somewhat comparable to the position taken by the National
Council on Radiation Protection and Measurement's Report Number 126 (NCRP, 1997) in
which the DDREF for whole body radiation spans an interval from one to five with values less
than one and greater than five being given negligible weight. The Agency states that a more
careful consideration of the uncertainty in die DDREF may be needed for cases' where the dose
is heavily concentrated in a few specific target tissues. The DDREF approach is defended in
FGR 13 - Part 1 (p. Ill) by appealing to either the dual-action theory or the saturable repair
theory. The Subcommittee recommends that the authors acknowledge in FGR 13 - Part 1 the
possibility of alternative models in which the reduction factor for low doses and low dose rates
might not be the same, for example, the genomic instability model proposed by Scott (1997) or
models that recognize a continuous dose-dose rate-response surface.
All models by definition are a simplification of reality and hence some uncertainty is
unavoidable. Model uncertainties, i.e., uncertainties in the structure of a biokinetic,
dosimetric or dose-response model, are often more difficult to assess than are parameter
uncertainties. The current efforts by the Human Alimentary Tract Model Task Group of ICRP
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Committee 2 to revise the gastrointestinal tract model point out the recognized need to update
the Eve model (Eve, 1966; Dolphin and Eve, 1966).
The previous respiratory tract model (TOLD 1966) was not adopted by the ICRP until
1979, 13 years after its publication (ICRP, 1979), and was not codified in the United States
(10CFR20) until 1991, 25 years after its publication. By that time, deficiencies in the TOLD
(1966) model were well recognized. These deficiencies, plus the large amount of new
research results, were the impetus for the development of the newer ICRP Publication 66
modeh(ICRP, 1994a).
Uncertainties in systemic biokinetic models probably vary among elements, depending
on our understanding of the metabolism of the individual elements and on the amount of data
available to construct and parameterize them. Model uncertainties also depend on the specific
models that are to be used. For example, none of the current generation of biokinetic models
are physiologically based toxicokinetic models, in which the biokinetics are described based on
models constructed with true physiological variables and values.
The important point is that there is significant uncertainty that can be attributed to
models themselves, apart from uncertainties in parameter values. Continuing scientific review
will elucidate strengths and weaknesses of the models, which will need to be taken into
account as they are identified. The Subcommittee recommends that model uncertainty be
discussed in the text of FOR 13 - Part 1, most logically in the uncertainty section.
3.2.6 Comprehensive Uncertainty Calculation for Classes of Biokinetic Models
There is no question that making a detailed probabilistic uncertainty evaluation for
every element would be a formidable task. However, the Subcommittee recommends that a
comprehensive uncertainty calculation be performed for one example radionuclide for each
class of biokinetic model. Possibilities include well-studied radionuclides such as 131I, "Sr,
239Pu, 137Cs, 3H, and perhaps °*U and 22ARa. It might be of use also to show a calculation for
the inhalation of specific radon decay products. To the extent feasible, the Agency should also
consider some examples for less well-studied radionuclides to illustrate the range of
uncertainties that are present in the calculations. This effort is necessary for the user to
appreciate the relative magnitude of the uncertainties associated with these calculations.
Also, the Subcommittee recommends that uncertainty calculations using the ICRP
generic model for "calcium-like" elements (ICRP, 1993a) be presented. This model is used
extensively in FOR 13 - Part 1, and an uncertainty evaluation would be extremely informative.
This task would not be easy but is feasible.
Usually, a parameter value is chosen from a range of possible values found in the open
literature. It would be helpful to present examples showing the range of values and indicating
the specific value chosen for each parameter. In this regard, the Agency might benefit from
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the model of uncertainty analysis used in the KRC (1998) uncertainty analysis of internal
dosimetry.
3.3 Assessing the Method for Extending List of Radionuciides - Charge (c)
The third element of the charge asked for the Subcommittee's assessment of the
proposed method for extending the list of radionuclides to include all those tabulated in Federal
Guidance Reports 11 and 12 (EPA, 1988; 1993).
3.3:1 Overall Findings
FGR 13 - Part 1 is not very explicit about how the extension to other radionuclides
would be accomplished, but the Subcommittee did receive a briefing on the Agency's plans for
the extension at the public meeting. Although the proposed method for extending the lists of
radionuclides to those in FGRs 11 and 12 is reasonable, the Subcommittee recommends that
risk coefficients also be presented for the inhalation of radon and its short-lived decay
products. These radionuclides, when inhaled, are the largest source of collective radiation
exposure (and presumably, radiation risk) to the U.S. population as a whole. In determining
the radon risk coefficients, consideration should be given to the risk estimates found in the
BEIR VI report (BEIR 1998) and in the ICRP Publication 65 on protection against radon
(ICRP 1993b). The Subcommittee recommends that these risk coefficients be included in the
final version of Part 1, if feasible.
The interim version of the report does not include a tabulation of risk coefficients for
all of the radionuclides covered in FGRs 11 and 12. The radionuclides addressed in the
interim report are primarily those included in ICRP Publications 56, 66, 67, 69, and 71 (ICRP
1989; I994a; 1993a; I993a; 1995b), which contained the models used for calculating dose
rates to organs and tissues.
The Agency proposes to calculate risk coefficients for intakes of the remaining
radionuclides in FGR 11 and for external exposures to those remaining in FGR 12. The
models in ICRP Publication 72 (ICRP 1996) wffl be feed to calculate dose rates for intakes of
the remaining radionuclides in FGR II. These dose rates will be used to calculate risk
coefficients in the same manner as for the radionuclides in the interim report. The
methodology for calculating the external exposure risk coefficients will also be the same as
used in the interim report.
t •
3.4 Additional Topics Identified by the Subcommittee
The following items are outside the charge to the SAB but were identified as important
by the Subcommittee.
19
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3.4.1 Recommended Change in Report Title
The Subcommittee recommends that the title of the document be changed from "Health
Risks from Low-Level Environmental Exposure to Radionuclides, Federal Guidance Report 13
- Part I" to 'Estimated Health Risks from Low-Level Environmental Exposure to
Radionuclides, Federal Guidance Report 13 - Part 1: Cancer" (emphasis added). This change
conveys two important points: 1) The tabulated risk values are estimates and 2) cancer is the
only health effect treated in the document. Note that the "Blue Book" (EPA 1994), from
which-the health risks used in FGR 13 - Part 1 were derived, is titled "Revised EPA
Methodology for Estimating Radiogenic Cancer Risks." (emphasis added)
3.4.2 Contribution of FGR 13 • Part 1 to Regulatory Debate and Ambiguity
Public comments on the document expressed concern that use of the FGR 13 - Part 1
risk coefficients could contribute to debate or ambiguity as to whether a regulated site meets
regulatory limits, especially in cases where a regulation is stated in terms of annual (or per
incident) dose limits but the supporting technical documentation (such as an environmental
impact statement) might be required to use risk calculations. The Agency responded orally
that it had clearly stated that dose-based regulations would still use FGRs 11 and 12 for
implementing the dose calculations and that use of the FGR 13 - Part 1 risk coefficients was
not mandated for any purpose. The Subcommittee observes that some confusion may result in
any attempt to apply both the FGR 11 and 12 methodologies and the FGR 13 - Part 1
methodology on the same regulatory issue, because the projection of the dose calculations to
risk using standard dose-to-risk conversion factors can result in large differences .from the FGR
13 - Part 1 method, as acknowledged by Agency staff. This complication could arise even if
the regulatory agency action itself used only one method, because an intervener from the
regulated community or the public interest community could use the other method to contest
the regulation. An example is Superfund risk assessments in which dose-based limits are being
used as "Applicable or Relevant and Appropriate Requirements" (ARARs) but risks are also
calculated. Although the Subcommittee makes no recommendation regarding the propriety of
having potentially conflicting guidance in place, it does suggest that the caveats in FGR 13 -
Part 1 be strengthened to minimize any such potential for conflicts to occur.
3.4.3 Domain of Applicability
The Subcommittee is concerned with the potential fbr misapplication of FGR 13 - Part
1. As noted in the document, its approach to risk analyses is not intended to be applied to the
analysis of risks to individuals associated with either past or future exposures. The
Subcommittee recommends that the preface of FGR 13 - Part 1 include a more thorough
discussion of the potential for misapplication and the limitations of use for FGR 13 - Part 1
risk estimates.
20
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The Preface of FOR 13 - Part 1 (page iv, second full paragraph, beginning with: "The
risk estimates—" through to the bottom of the page) contains an important statement
describing the intended use and limitations to the applicability of FOR 13 - Part 1
methodology. Since users may fail to read the preface, the Subcommittee recommends that
this important paragraph be repeated in the main body of the text at least once.
3.4.4 Clarification of the Risk Tables
The Subcommittee believes that tables in any document should be understandable
without reference to the text. In FGR 13 • Part 1, however, the explanatory material required
may be more than can reasonably be placed in the titles and footnotes. As an alternative, each
of the risk coefficient'tables could have a note, perhaps in parentheses under the title, that
states "Refer to the explanation of entries in the text preceding this table for important
information necessary for interpreting the table entries."
Two explanations were identified by the Subcommittee as critically important for users
of the tables:
f
a) Ingrowth of chain members in the environmental medium is not incorporated
into the risk estimates; and
b) Because scientists disagree regarding the reliability of estimates of lifetime
cancer risk from low-level exposure to radiation, and because the error in such
estimates may vary substantially from one radionuclide to another and from one
exposure scenario to another, no attempt is made in the tables to characterize the
overall uncertainty associated with any given risk coefficient. (Obviously this
last statement would be modified if the authors of FOR 13 - Pan 1 are able to
implement some of the suggestions made in section 3.2 regarding uncertainty.)
The Subcommittee recommends that these two statements be linked clearly with the
tables of risk coefficients in the final version of FGR IS - Part I.
3.4.5 Tabulation of Predominant Cancer Types
It is not clear to the Subcommittee that the listing of a single predominant cancer type
in the risk tables is either useful or correct. The scientific basis for making such predictions is
lacking in cases where the tissues at risk are limited, the type of radiation and its dose patterns
(spatial and temporal) are different from those used to develop the risk coefficients (which are
based predominantly on the atomic bomb survivor data), and radionuclide-specific data are
sparse or absent. Alternatives to the current presentation might be: a) eliminating the columns
completely, and b) selecting, for example, the three tissues or organs considered to be at
greatest risk, but without giving a quantitative estimate of the risk fraction associated with
those tissues and organs. This approach at least would dampen the criticism that the
21
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predominant cancer type is over-specified. The Subcommittee's recommendation is to
eliminate the predominant cancer columns because of insufficient knowledge to specify the
tumor locations reliably.
3.4.6 Adequacy of Soil Geometries for Use with the Mill Tailings Standards
It is stated in FOR 13 - Part 1 that "Because risk coefficients for external exposure to
soil contaminated to IS cm would differ only slightly from those for concentrations to an
infinite depth, it would not be useful to provide tabulations of risk coefficients for both
situations" (p. 56, last sentence). The Uranium Mill Tailings Radiation Control Act
(UMTRCA) specifies a standard for contaminated soil of 5 pCi g'1 in Ac top 15 cm and 15 pCi
g'1 below 15 cm. Because of the wide application of this standard, even beyond its original
intent, it would be useful to provide tabulations of risk coefficients for dose rates from soil that
is uncontaminated for the first 15 cm, but contaminated to an infinite depth below 15 cm.
These risk coefficients could then be added to those for soils contaminated to an infinite depth
to readily obtain overall risk coefficients for sites where the UMTRCA standards apply. This.
approach would need to be developed only for radionuclides and their decay products to which
the UMTRCA standard is applied. The Subcommittee recommends that risk coefficients for
external exposure, applicable specifically to the UMTRCA standard, be calculated and
tabulated in the final version of FOR 13 - Part 1 or in a separate document.
3.4.7 Relationship Between Exposure and Dose
In example 4 of Appendix E in FGR 13 - Part 1, a risk is derived from a measured
exposure rate (in /zR h"1) using a factor of 1 rem per roentgen. Use of this factor results in an
overestimate of the risk, since some self-shielding of critical organs is provided by the body
itself. For example, if the sum of the dose conversion factors for. an infinitely thick layer of
soil for the BIU decay series found in FGR 12 (EPA 1993) is compared to the exposure rate
from an infinite soil source with Z3VU in equilibrium with its decay products found in Huffext
and Miller (1995), a conversion factor from pR h*1 to prem h*1 of approximately 0.7 is
inferred. Some realistic guidance for converting measured exposure to risk rates should be
provided in FGR 13 - Part 1 if such examples are to be included.
3.5 General Conclusions
The interim version of FGR 13 - Part 1 is a useful addition to the complement of
Federal Guidance reports relevant to the evaluation of the risks of radiation and makes
important improvements in the calculation of radiation risks for appropriate regulatory
decisions. The report is well organized and well written, although difficult to comprehend for
all but r ighly technical audiences. The inclusion of sample calculations in the appendices
should be useful for those attempting to understand the details of the extensive calculations,
especially if they need to modify the computed values, to fit a situation for which the standard
calculations are not suited. The careful reader will also note that most of the important caveats
22
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to the use of the document's results are included somewhere in the text. However, the caveats
are not as prominent as they probably should be, given the potential for misuse or
misinterpretation of the document's risk estimates. In particular, the magnitudes of the
uncertainties in the computed numbers are difficult to ascertain.
The Subcommittee commends the Agency on its leadership and efforts in using up-to-
date scientific methods and data to develop the health risk estimates in FGR 13 - Part 1. We
believe that our recommendations, except those noted for future consideration, can be
incorporated prior to publication of the final version and will strengthen the report and its
credibility. We strongly support the Agency's slated intent to publish supporting information
in electronic form to accompany release of the final version of FGR 13 - Part 1, and we
recommend that it include the data, models, and dose values used in formulating the risk
coefficients.
23
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APPENDIX A - DETAILED TECHNICAL COMMENTS
The Subcommittee offers the following specific technical comments:
a) Suitability of Assumed Particle Diameter P. 53: An activity median
aerodynamic diameter (AMAD) of 1 Mm is assumed for the calculations.
Presumably, situations will arise in which the AMAD will be different. The
Subcommittee recommends that comment on this issue be made in the document
and that a procedure for adjusting the coefficients (or perhaps the air
concentration) appropriately be included. Another possibility is to include this
information in the electronic files to be issued by the Agency.
b) Gastrointestinal Absorption Factors: P. 62*63: The gastrointestinal (GI)
absorption factors for ingestion are usually equal to that for inhalation of
absorption type F compounds. Presumably, elements are sometimes chemically
bound in a relatively insoluble form, even in food or water. The document
should explain how the ingestion absorption factors were selected and, if
appropriate, suggest when and how an adjustment should be made if conditions
indicate a lower absorption factor.
c) Compound Speciation: P. 71: Surrogate information for the biokinetic models
is described as being based on a "chemical analogue of the element in humans."
Compound speciation may be important for an element. FOR 13 - Part 1
should state to what extent the surrogate data are based on an appropriate
analogue compound (as opposed to element).
d) Effective Alpha Particle RBE: The document states that an "ef: xtive" alpha
particle RBE of 1 is used for leukemia (P. 98). It is not clear wnether another
such adjustment has been made for the effect of radium on bone, where the
unadjusted ICRP methods do not predict radium risks correctly. More broadly,
the difficulties with the whole RBE structure are not sufficiently discussed in
this section.
e) Selection of Units of Risk: A statement about the selection of the unit in which
risk is expressed should be given. The Subcommittee offers the following for
consideration by the authors: 'Risk coefficients are presented in units of Bq"1
because the Bq is the fundamental SI unit of activity."
A-1
-------
APPENDIX B - ACRONYMS
AF
AMAD
ARARS
BEIR
BIOMOVS
Bq
CERCLA
Ci
cm
Cs-137
DC AL
DDREF
EPA
FOR
GI
g
h
HRTM
I3'l
IAEA
ICRP
LET
on .
MeV
NCRP
NRC
NUREG
ORNL
OR1A
p
22*Ra
R
RAC
RBE
rem
Absorbed Erection of Energy
Activity Median Aerodynamic Diameter
Applicable or Relevant and Appropriate Requirements
Biological Effects of Ionizing Radiation
Biosphenc Model Validation Study
Comprehensive Environmental Response Compensation and Liability Act
Curie
Centimeters
Cesium-137
Dose Calculation (Software)
Dose and Dose-Rate Effectiveness_£actor
U.S. Environmental Erotection Agency (U.S. EPA)
Eederal Guidance Report (Includes FGR 11, 12, 13)
gastrointestinal
Hour
Human Respiratory Iract Model
Iodine -131 '.
International Atomic Energy Agency
International Commission on Radiological Protection
Linear Energy Transfer
Meter
Million electron Yolts
Rational Council on Radiation Erotection and Measurements
U.S. Nuclear Regulatory Commission (U.S. NRQ
Huclear Regulatory (U.S. NRC Documents)
CWc Ridge Nationaliaboratory
Office of Radiation and Indoor Air (U.S. EPA)
Pjco (one trillionth, e.g., 1CT12 Ci is a picocurie )
PJutonium-239
Radium-226
Roentgen
Radiation Advisory Committee (RAC/U.S. EPA/SAB/RAQ
Relative Biological Effectiveness
the unit of the quantity dose equivalent
B-l
-------
SAB Science Advisory fioard (U.S. EPAySAB)
SI Systeme International d' Unites (international system of units)
^Sr Sttontium-90
a*U Uranium
UMT Uranium Mill lailings
UMTRCA Uranium Mill lailings Radiation Control Act
TJ c United States
Micro (A millionth of given unit) [e.g., ^m - Micrometer (A millionth of am);
Mrem - Microrem (A millionth of a rem); ^ - Micro Roentgen (A millionth of
aR)]
>«
B-2
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