FEATURE ARTICLE
Fallout from Nuclear Weapons Tests and Cancer Risks
Exposures 50 years ago still have health implications today that will continue into the future
Steven Simon, André Bouville, Charles Land
Fallout and Cancer Risk
Increased cancer risk is the main long-term hazard associated with
exposure to ionizing radiation. The relationship between radiation
exposure and subsequent cancer risk is perhaps the best understood,
and certainly the most highly quantified, dose-response relationship
for any common environmental human carcinogen. Our understanding is
based on studies of populations exposed to radiation from medical,
occupational and environmental sources (including the atomic
bombings of Hiroshima and Nagasaki, Japan), and from experimental
studies involving irradiation of animals and cells. Numerous
comprehensive reports from expert committees summarize information
on radiation-related cancer risk using statistical models that
express risk as a mathematical function of radiation dose, sex,
exposure age, age at observation and other factors. Using such
models, lifetime radiation-related risk can be calculated by summing
estimated age-specific risks over the remaining lifetime following
exposure, adjusted for the statistical likelihood of dying from some
unrelated cause before any radiation-related cancer is diagnosed.
Relatively little of the information on radiation-related risk comes
from studies of populations exposed mostly or only to radioactive
fallout, because useful dose-response data are difficult to obtain.
However, the type of radiation received from external sources in
fallout is similar to medical x rays or to gamma rays received
directly by the Hiroshima and Nagasaki A-bomb survivors, allowing
information from individuals so exposed to be used to estimate
fallout-related risks from external radiation sources. Estimates of
radiation-related lifetime cancer risk per unit dose from external
radiation sources to the organs and tissues of interest are shown in
Figure 10 for leukemia, thyroid cancer and all cancers combined.
Estimated risks, in percent, are given separately by sex, as
functions of age at exposure.
Thyroid cancer is a rare disease overall—with U.S. lifetime
rates estimated to be 0.97 percent in females and 0.36 percent in
males—and it is extremely rare at ages younger than 25.
Furthermore, the malignancy is usually indolent, may go long
unobserved in the absence of special screening efforts and has a
fatality rate of less than 10 percent. These factors make it
difficult to study fallout-related thyroid cancer risk in all but
the most heavily exposed populations. Thyroid cancer risks from
external radiation are related to gender and to age at exposure,
with by far the highest risks occurring among women exposed as young children.
The applicability of risk estimates based on studies of external
radiation exposure to a population exposed mainly to internal
sources, and to I-131 in particular, has been debated for many
years. This uncertainty relates to the uneven distribution of I-131
radiation dose within the thyroid gland and its protraction over
time. Until recently, the scientific consensus had been that I-131
is probably somewhat less effective than external radiation as a
cause of thyroid cancer. However, observations of thyroid cancer
risk among children exposed to fallout from the Chornobyl reactor
accident in 1986 have led to a reassessment. An Institute of
Medicine report concluded that the Chornobyl observations support
the conclusion that I-131 has an equal effect, or at least
two-thirds the effect of internal radiation. More recent data on
thyroid cancer risk among persons in Belarus and Russia exposed as
young children to Chornobyl fallout offer further support of this inference.
In 1997, NCI conducted a detailed evaluation of dose to the thyroid
glands of U.S. residents from I-131 in fallout from tests in Nevada.
In a related activity, we evaluated the risks of thyroid cancer from
that exposure and estimated that about 49,000 fallout-related cases
might occur in the United States, almost all of them among persons
who were under age 20 at some time during the period 1951-57, with
95-percent uncertainty limits of 11,300 and 212,000. The estimated
risk may be compared with some 400,000 lifetime thyroid cancers
expected in the same population in the absence of any fallout
exposure. Accounting for thyroid exposure from global fallout, which
was distributed fairly uniformly over the entire United States,
might increase the estimated excess by 10 percent, from 49,000 to
54,000. Fallout-related risks for thyroid cancer are likely to
exceed those for any other cancer simply because those risks are
predominantly ascribable to the thyroid dose from internal
radiation, which is unmatched in other organs.
External gamma radiation from fallout, unlike beta radiation from
I-131, is penetrating and can be expected to affect all organs.
Leukemia, which is believed to originate in the bone marrow, is
generally considered a "sentinel" radiation effect because
some types tend to appear relatively soon after exposure, especially
in children, and to be noticed because of high rates relative to the
unexposed. Lifetime rates in the general population, however, are
comparable to those for thyroid cancer (on the order of one
percent), whereas those for all cancers are about 46 percent in
males and 38 percent in females.
A total of about 1,800 deaths from radiation-related leukemia might
eventually occur in the United States because of external (1,100
deaths) and internal (650 deaths) radiation from NTS and global
fallout. For perspective, this might be compared to about 1.5
million leukemia deaths expected eventually among the 1952
population of the United States. About 22,000 radiation-related
cancers, half of them fatal, might eventually result from external
exposure from NTS and global fallout, compared to the current
lifetime cancer rate of 42 percent (corresponding to about 60
million of the 1952 population).
The risk estimates in Figure 10 do not apply to the extremely
high-dose fallout exposures experienced by 82 residents of the
Marshall Islands exposed to BRAVO fallout on Rongelap and Ailinginae
in 1954, because the total dose to the thyroid gland (88 Gy on
average) far exceeded those in any of the studies on which the
estimates are based. Other islands in the archipelago, with about
14,000 residents in 1954, had average estimated doses of 0.03 Gy to
bone marrow and 0.68 Gy to the thyroid gland. Altogether, excess
lifetime cancers are estimated to be three leukemias (compared to
122 expected in the absence of exposure, an excess of 2.5 percent),
219 thyroid cancers (compared to 126 expected in the absence of
exposure, an excess of 174 percent) and 162 other cancers (compared
to 5,400 expected, an excess of 3 percent).
It is important to note that, even though the fallout exposures
discussed here occurred roughly 50 to 60 years ago, only about half
of the predicted total numbers of cancers have been expressed so
far. The same can be said of the survivors of the atomic bombings of
Hiroshima and Nagasaki. Most of the people under study who were
exposed to fallout or direct radiation—for example, A-bomb
survivors—at very young ages during the 1940s, 1950s and 1960s
are still alive, and the cumulative experience obtained from all
studies of radiation-exposed populations is that radiation-related
cancers can be expected to occur at any time over the entire
lifetime following exposure.
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