UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D-C, 20460
EPA-SAB-RAC-LTR-92-003
January 9, 1992
OFFICE OP
TT , , ,17.,,. rr n -11 THE ADMINISTRATOR
Honorable William K. Reilly
Administrator
U.S. Environmental Protection Agency
401 M Street, SW
Washington, D.C. 20460
Re; Revised Radon Eisk Estimates and Associated Uncertainties
Dear Mr. Reilly:
With this letter we are responding to the request of the Office of Radiation
Programs (ORP) dated April 15, 1991 for review of two documents supporting the
ORP reassessment of radon associated risks to the general population. This
review is based on the documents entitled "EPA's National Residential Radon
Survey Preliminary Results" and "Proposed Revisions in EPA Estimates of Radon-
Risks and Associated Uncertainties." These documents were provided to the
Radiation Advisory Committee in April, and the Committee was briefed on them
by ORP staff at the committee meeting in Montgomery, Alabama, on May 20,
1991.
These documents represent an important step forward by the Agency in
assessing the health risks of exposure to radon and its decay products, in review-
ing and utilizing the recent scientific results and deliberations on issues affecting
the dose and risk, and in attempting to quantify the attendant uncertainties. The
National Residential Radon Survey is a success story, since it represents the first
significant nationwide survey of an indoor air pollutant with a firm statistical
basis for its design and implementation. The original design of this survey was
brought to the Board for review in October 1987 [SAB-RAC-88-002], The Agency
adopted many of the suggestions made by the Board, and has provided periodic
status reports on its progress. The initial analyses have been completed. It
appears the survey will provide a wealth of important data on annual average
radon concentrations and on housing stock characteristics.
In its letter of transmittal, ORP requested that the Board address seven
issues. These questions and the Board responses are detailed below. Overall, the
Board has found that the methods and analyses used by the Agency for the
assessment of radon risk are generally appropriate. The nationwide average of
radon concentrations in U.S. homes, based on the National Residential Radon
Survey, represents the best available data. The Agency's proposed adjustment of
the exposure/dose relationship between miners and the general population,
obtained from the recent EPA-sponsored National Research Council report,
PriiUtd on Riti'.
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"Comparative Dosimetry of Radon in Miners and Homes," (National Academy
Press, Washington, DC, 1991) is appropriate as is the proposed value for the
residential radon- equilibrium fraction. The average home occupancy factor is -
reasonable and the Agency's method for deriving an estimate of uncertainty
associated with the number of annual radon-related lung cancers is deemed
acceptable,
The Board has some reservations and comments regarding the dose-response
model used for the estimation of overall risk. Notwithstanding the SAB's earlier
recommendation [SAB-RAC-88-042] that an average of the relative risk models
presented in the National Research Council's 1990 report, "Health Effects of
Radon and Other Internally Deposited Alpha-Emitters-BEIR IV," and in the
International Commission on Radiological Protection report ICRP-50 be used for
making this estimate, the sole use of the BEIE IV model is now recommended
with no adjustment for a potential increased risk for individuals exposed as
children and young adults, OEP staff concurs with this recommendation. The
effect of implementing this recommendation would be to lower the central estimate
of the overall risk. The possible variation of risk with age it exposure could be
incorporated in the uncertainty analysis.
The Agency's estimate of overall U.S. risk of lung cancer from radon
exposure is calculated for the total population consisting of both smokers and non-
smokers. Since smokers are significantly more sensitive to the effects of radon
exposure than are non-smokers and former smokers, the Board recommends that
the Agency review periodically the national lung cancer estimates in view of
changes in smoking habits as well as changes in the underlying lung cancer
incidence rate. Detailed discussion of the various issues is provided below.
a. Is the methodology which will be used to derive the nationwide
average of radon concentrations in U.S. homes appropriate?
Yes. The Board had previously reviewed and provided input to the design
for the National Eesidential Radon Survey, Based on the preliminary results, the
Survey appears to have met its original objectives. The initial survey result of use
to the task of estimating the nationwide risks associated with radon exposure is
the average radon concentration in U.S. homes* Since data were collected from
multiple locations in each house, four methods of describing the average radon
concentration for a particular house were presented. The Board feels that the
method selected by QRP, that of averaging overall frequently occupied levels, is a
reasonable approach.
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b. Given the finding to date of higher risk to atomic bomb survivors ex-
posed as children, is it prudent for EPA to continue to follow the
previous SAB advice to average the risk estimates from BEIR IV and
ICRP 50 to reflect a potential higher risk to children?
No, The earlier Science Advisory Board advice [SAB-RAC-88-042] should be
revised at this time. The Radiation Advisory Committee (RAC) unanimously
recommends that the Agency now use only the BEIR IV Model for its risk
assessment for residential exposure to radon. This recommendation differs from
the Committee's previous advice to average the estimates based on the BEIR IV
and the ICRP 50 models. This current recommendation is based on the following
rationale:
1, The earlier recommendation that projections of the BEIR IV
model be averaged with those of ICRP 50 was based, in part, on the assumption
by the International Commission on Radiological Protection (ICRP) committee that
radon exposures of children and young adults below 20 years of age may produce
a greater risk for cancer of the lung than do exposures of adults. This assumption
was based largely on earlier (1950-1978) Life Span Study data on lung cancer from
whole-body exposure to low-LET radiation from the atomic bomb explosions in
Hiroshima and Nagasaki as reported by Kato and Schull (1982). However, the
subsequently published (1990) National Research Council report, "Health Effects of
Exposure to Low Levels of Ionizing Radiatioji-BEIR V," using Life Span Study
data through 1985 concluded that there was no dependence on age at exposure for
lung cancer,
A report on lung cancer in Chinese tin miners exposed to radon gas
(published in Cancer Research, January 1, 1990, by Jay H. Lubia, You-lin Qiao, et.
al.) also failed to show a dependence on age at exposure for individuals less than
13 years of age. The recent (1991) National Research Council analysis "Compara-
tive Dosimetry of Radon in Mines and Homes" states that "although the K-factors
[the ratio of the exposure-to-dose relationship in homes to that of mines] for
children and infants were somewhat greater than those for adults, none of the
values was above unity," The K-factor values of 0,7 for adults and 0.8 for children
do not support the greater risk to children assumed in the ICRP 50 model.
2, The BEIR IV model is based on four reasonably well-documented
cohorts of underground miners and represents the consensus of a National
Academy of Sciences/National Research Council Committee of well-established and
qualified scientists.
3. The BEIR IV model incorporates a decrease in lung cancer risk
with time after exposure, a phenomenon that was also accounted for in the 1984
NCRP model and which has been further demonstrated in epidemiological studies
published subsequent to the BEIR IV report. The ICRP 50 model does not
incorporate this decrease in risk with time after exposure,
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c. Is the EPA proposed adjustment of the exposure-dose relationship
between miners and persons In the general population, based on the
1991 NAS report, appropriate?
Yes, the adjustment of the risk coefficient using a K-factor of 0,7 to account
for the estimated lower bronchial dose per unit exposure in homes compared with
that in mines is appropriate. The Board notes that recent data on nasal deposi-
tion of ultra-fine aerosols suggest that the S-factor may decrease to about 0.6.
d. Is the proposed EPA value for the residential radon equilibrium ratio
reasonable?
The Agency's evaluation and selection from the available data set of the
value of 0.5 is reasonable. The uncertainty range of 0.35-0.55 is appropriate.
This value of the equilibrium factor may turn out to be lower as further data are
collected from U.S. homes.
e. Is the proposed occupancy factor reasonable in view of available data?
Yes, the Agency has selected a value of 75%, with an associated uncertainty
range of 65 to 80%,
f. Is BPA*s method for deriving an estimate of uncertainty associated
with the number of annual radon-related lung cancer deaths appropri-
ate?
Yes, although the Board does recommend that, because smoking has a
significant effect on risk, the Agency attempt to incorporate smoking into the
uncertainty analysis, The uncertainty analysis conducted by the Agency begins
with a central estimate of lung cancer deaths based on the BEIR IV risk model
and the results of the residential radon survey. Following an approach utilized by
The National Institutes of Health for quantifiable uncertainties, the uncertainty
ranges were distributed log-normally without correlations and the log-transformed
variances were then added. The quantified uncertainties included the variability
in risk coefficients derived from different cohorts, uncertainty about the decrease
in relative risk following cessation of exposure, and uncertainty about the relation-
ship between exposure and risk in homes versus that in underground mines. The
quantified uncertainties about exposure included the statistical reliability of the
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survey, uncertainty about the equilibrium factor, and uncertainty about the percent
of time people spend in their residences. Variables not treated quantitatively
include estimates of exposure in mines, the effect of exposure rate on risk and the
effects of smoking status and gender. The knowledge base about these uncertain-
ties is not well developed,
g. Is EPA's methodology for deriving the central estimate and range of
radon-related lung cancer deaths in the U.S. population appropriate?
As has been discussed in addressing the earlier questions, the Board has
found the methods and data to be appropriate, with the exception of the dose-risk
methodology. The Board now recommends the sole use of the BEIR IV model for
this purpose. This model has already served as the basis for the uncertainty
analysis presented by ORP. Although alternative methods or assumptions could
lead to different central estimates, in general these differences are not large
compared with the range of the uncertainty. The present analysis, with the use of
the single BEIE IV model, will yield a solid, well-documented and defensible
central estimate of the radon-related lung cancer deaths in the U.S. and the
probable range of uncertainties. The possible variation of risk with age may also
be addressed in the uncertainty analysis.
General Recommendations:
OEP has identified and utilized most of the recent data or assessments on
the factors influencing the risk assessment, including the K-factor and the equilib-
rium factor. As pointed out briefly in the discussion of these individual issues,
both are topics of (or their values are influenced by) current research.
The Agency's estimate of overall U.S. risk of lung cancer from radon
exposure is calculated for the total population consisting of both smokers and non-
smokers. It is well-established that smokers are significantly more sensitive to the
effects of radon exposure than are non-smokers and former smokers. As noted in
the ORP discussion of uncertainties, since the dose-risk estimate is based on a
relative risk model, changes in the underlying cancer incidence rate, including the
results of changes in smoking habits, will cause changes in the number of lung
cancer deaths attributable to radon. The Board recommends that the Agency
review these cancer risk estimates and the underlying data periodically to update
the estimates. The Board believes the current methodology and the reliance on
BEIR IV are sufficiently robust, so that these reassessments would primarily need
to focus on updating the various factors.
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We appreciate the opportunity to review this issue and are ready to provide
review comments on any significant revisions to the subject documents. We look
forward to your response on the major points we have raised.
Sincerely,
Raymond C,
Science Advisory Board
Oddvar F, Nyga^^f Chairman
Radiation Advisory Committee
Enclosure: FY91 Roster of the Radiation Advisory Committee
cc; Assistant Adminiitrators
Director, Office of Radiation Programs
Director, Office of Drinking Water
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U.S. ENVIRONMENTAL PROTECTION AGENCY
SCIENCE ADVISORY BOARD
RADIATION ADVISORY COMMITTEE
ROSTER
CHAIRMAM
Dr. Oddvar F, Nygaard
Division of Biochemical Oncology
Department of Radiology
University Hospitals of Cleveland
2058 Abington Road
Cleveland, Ohio 44106
MEMBERS
Dr. Kelly H. Clifton
Department of Human oncology and Radiology
University of Wisconsin Clinical Cancer center
K4/33Q, Clinical Science Center
600 Highland Avenue
Madison, Wisconsin 53792
Dr. James E. Martin
Assistant Professor of Radiological Health
University of Michigan
School of Public Health
Ann Arbor, Michigan 48109
Dr. Genevieve M. Matanoski
Professor of Epidemiology
The Johns Hopkins University
School of Hygiene and public Health
Department of Epidemiology
624 North Broadway, Room 230
Baltimore, Maryland 21205
Dr. H. Robert Meyer
C.N.S.I.
750 East ParK Drive
Suite 200
Harrisburg, Pennsylvania 17111
Dr. Richard G. Sextro
Building Ventilation and
Indoor Air Quality Program
Lawrence Berkeley Laboratory
Building 90, Room 3058
Berkeley, California 94720
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Mr. Paul G. Voilleque
MJP Risk Assessment, Inc.
Historic Federal Building
591 Park Avenue
Idaho Falls, Idaho 83405-0430
DESIGNATED FEDERAL .OFFICIAL
Mrs. Kathleen W. Conway
Science Advisory Board
U.S, Environmental Protection Agency
401 M Street, S.W., A-101P
Washington, D.C. 20460
STAFF SECRETARY
Mrs, Dorothy M. Clark
Secretary, Science Advisory Board
U.S. Environmental Protection Agency
401 M Street, s.w,, A-101F
Washington, D.C. 20460
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