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U. S. NUCLEAR REGULATORY COMMISSION |
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No. 96-100 FOR IMMEDIATE RELEASE
(Thursday, July 11, 1996)
NOTE TO EDITORS:
The Nuclear Regulatory Commission has received the attached report
from its Advisory Committee on Nuclear Waste. The report, in the form of
a letter, provides comments on the health effects of low levels of
ionizing radiation.
#
Attachment:
As stated
. July 10, 1996
The Honorable Shirley Ann Jackson
Chairman
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001
Dear Chairman Jackson:
SUBJECT: HEALTH EFFECTS OF LOW LEVELS OF IONIZING RADIATION
The health effects of ionizing radiation are central to many of
the regulations that are promulgated by the Nuclear Regulatory
Commission (NRC). The validity of the linear- no threshold (LNT)
dose-response relationships in the area of low doses and low dose
rates has been questioned. This letter supports the Commission's present
course of action of a review and analysis by the National Council on
Radiation Protection and Measurements (NCRP) leading to an evaluation of
this important issue.
Our discussion and recommendations concerning this subject derive
from the first meeting of the Joint Subcommittee of the Advisory
Committee on Reactor Safeguards (ACRS) and the Advisory Committee
on Nuclear Waste (ACNW) held on March 26, 1996. Presentations
were made by members of the NRC staff, including the visiting
medical fellow, and representatives from institutions and agencies such
as the NCRP, the Health Physics Society, and the Massachusetts Emergency
Management Agency. Written comments were also received from the public.
Most national and international scientific committees dealing
with the subject take the view that the safest approach to
regulation is one that relies on the LNT model of response to
doses of ionizing radiation. This model holds that the ill
health effects observed at high doses and high dose rates (mainly
among atomic bomb survivors) can be extrapolated linearly to low
doses and low dose rates, down to the smallest doses. The NRC
staff prepares regulations on the basis of this model. One of the basic
questions in this field is whether the LNT model
is valid at the low doses and rates normally encountered in many
of the regulatory domains. The increasing emphasis placed by the
Commission on risk-informed regulation makes it imperative that
the actual health risk of low levels of ionizing radiation be
assessed accurately.
The NRC is currently funding a contract with the NCRP to make a
critical evaluation of the LNT assumption. The ACNW has not
reached conclusions on the validity of the LNT model, and will
continue to study the matter. We see the NCRP study as an
opportunity to obtain an independent review of the data and their
quality.
The presence of unavoidable background radiation and the need for
very large samples have made it difficult in the past to obtain
definitive data on the validity of the LNT model. As with all
small-effects phenomena, the quality of the data and the statistical
interpretation of the results govern the ability of any study to
contribute to the testing of the model. However, investigators in the
field have recently been able to account for the effect of such
confounding factors such as variation in background radiation. Some
studies in the United States, as well as in China, Sweden, Poland, and
Canada, have arrived at conclusions that do not support the LNT model.
Other research concludes that it is likely that at least a threshold or
perhaps a corresponding zero equivalent point with beneficial risk decre-
ments (hormesis) exists at lower doses.
A notable example of the latter is a ten-year study by Johns
Hopkins University of U.S. nuclear shipyard workers which, we
were told, showed lower mortality, no increase in malignancies
among workers exposed to radiation when compared to those who
were not exposed, and no "healthy worker effect." This study may
be particularly significant since the investigators were looking for
evidence to support the LNT model. Another study, of Canadian women
patients in tuberculosis sanitariums who underwent repeated fluoroscopy
to monitor response to therapy, is used frequently to show the validity
of the LNT model, but examination of data at lower doses shows
significant beneficial effects. The 1994 report of the United Nations
Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)
contains an extensive appendix detailing cellular repair mechanisms
(called "adaptive response mechanisms") that could contribute to an
explanation of a threshold, or, if such cellular responses were
stimulated by low doses of radiation, to an explanation of beneficial
effects.
In contrast, some of the public comments received by the Subcommittee
suggested that the LNT model underestimates the harmful effects of low
doses of radiation. Also NCRP Report No. 121, "Principles and
Application of Collective Dose in Radiation
Protection," issued November 1995 and discussed with the Joint
Subcommittee at its March 26, 1996 meeting, finds that "from the
point of view of the scientific bases of collective doses for
radiation protection purposes, it is prudent to assume the effect
per unit dose in the low-dose region following single acute
exposures or low-dose fractions is a linear response."
In the face of conflicting views, the general belief of the
national and international committees dealing with the matter has
been that using the LNT model for regulatory purposes is a safe
and conservative approach and, if there is error, it is on the
side of enhanced protection. However, if there is a health
benefit at low doses, this logic is incorrect. Even if there is
no evident health benefit, there are significant societal costs
associated with this conservatism that could be avoided or
reduced if a threshold level could be established below which no
harm occurs. A basic principle of risk-informed regulation is to
prevent a situation in which scarce resources are misspent
to avoid negligible risks, while significant risks remain
unattended for want of resources to deal with them. Owing to the
potentially significant costs of the present conservatism, we
conclude that a reexamination of the regulatory model is
appropriate.
It is obvious that agreement on an appropriate dose-response
model is made more difficult by the differing voices on this
subject within the scientific community and those outside of this
community, including regulators, policy makers, and members of
the public. The first task required to reach such an agreement
is an impartial review of the data and their quality in the face
of the extensive application of the LNT model in regulations and
scientific opinion.
We recommend that the need for special attention be conveyed to
the NCRP regarding its study. Such attention should include:
(1) assurance that the study includes scientists other than those who are
"recognized experts" with a reputation built on the LNT model, (2) an
evaluation of the data by an entity with expertise in statistics or
information science, but no prior position on LNT - such as the National
Institute of Standards and Technology (NIST) as well as the NCRP study
committee, and (3) consideration of essentially all studies that could
relate to the LNT.
The Committee strongly believes in the NCRP goal of critically evaluating
data related to low dose health effects. We will follow the program
through interaction with NRC's Office of Research and will report to the
Commission on the study and its implications.
Sincerely,
/S/
Paul W. Pomeroy
Chairman, ACNW
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