Fallout from Nuclear Weapons Tests and Cancer Risks
Exposures 50 years ago still have health implications today that will continue into the future
Prior to 1950, only limited consideration was given to the health
impacts of worldwide dispersion of radioactivity from nuclear
testing. But in the following decade, humanity began to
significantly change the global radiation environment by testing
nuclear weapons in the atmosphere. By the early 1960s, there was no
place on Earth where the signature of atmospheric nuclear testing
could not be found in soil, water and even polar ice.
Cancer investigators who specialize in radiation effects have, over
the intervening decades, looked for another signature of nuclear
testing—an increase in cancer rates. And although it is
difficult to detect such a signal amid the large number of cancers
arising from "natural" or "unknown" causes, we
and others have found both direct and indirect evidence that
radioactive debris dispersed in the atmosphere from testing has
adversely affected public health. Frequently, however, there is
misunderstanding about the type and magnitude of those effects. Thus
today, with heightened fears about the possibilities of nuclear
terrorism, it is worthwhile to review what we know about exposure to
fallout and its associated cancer risks.
The first test explosion of a nuclear weapon, Trinity, was on a
steel tower in south-central New Mexico on July 16, 1945. Following
that test, nuclear bombs were dropped on Hiroshima and Nagasaki,
Japan, in August of 1945. In 1949, the Soviet Union conducted its
first test at a site near Semipalatinsk, Kazakhstan. The U.S., the
Soviet Union and the United Kingdom continued testing nuclear
weapons in the atmosphere until 1963, when a limited test ban treaty
was signed. France and China, countries that were not signatories to
the 1963 treaty, undertook atmospheric testing from 1960 through
1974 and 1964 through 1980, respectively. Altogether, 504 devices
were exploded at 13 primary testing sites, yielding the equivalent
explosive power of 440 megatons of TNT (see Figure 2).
The earliest concern about health effects from exposure to fallout
focused on possible genetic alterations among offspring of the
exposed. However, heritable effects of radiation exposure have not
been observed from decades of follow-up studies of populations
exposed either to medical x rays or to the direct gamma radiation
received by survivors of the Hiroshima and Nagasaki bombs. Rather,
such studies have demonstrated radiation-related risks of leukemia
and thyroid cancer within a decade after exposure, followed by
increased risks of other solid tumors in later years. Studies of
populations exposed to radioactive fallout also point to increased
cancer risk as the primary late health effect of exposure. As
studies of biological samples (including bone, thyroid glands and
other tissues) have been undertaken, it has become increasingly
clear that specific radionuclides in fallout are implicated in
fallout-related cancers and other late effects.
Nuclear Explosions: The Basics
Nuclear explosions involve the sudden conversion of a small portion
of atomic nuclear mass into an enormous amount of energy by the
processes of nuclear fission or fusion. Fission releases energy by
splitting uranium or plutonium atoms, each fission creating on
average two radioactive elements (products), one relatively light
and the other relatively heavy. Fusion, triggered by a fission
explosion that forces tritium or deuterium atoms to combine into
larger atoms, produces more powerful explosive yields than fission.
Both processes create three types of radioactive debris: fission
products, activation products (elements that become radioactive by
absorbing an additional neutron) and leftover fissionable material
used in bomb construction that does not fission during the explosion.
A nuclear explosion creates a large fireball within which everything
is vaporized. The fireball rises rapidly, incorporating soil or
water, then expands as it cools and loses buoyancy. The radioactive
debris and soil that are initially swept upwards by the explosion
are then dispersed in the directions of the prevailing winds.
Fallout consists of microscopic particles that are deposited on the ground.
How People Are Exposed to Fallout
The radioactive cloud usually takes the form of a mushroom, that
familiar icon of the nuclear age. As the cloud reaches its
stabilization height, it moves downwind, and dispersion causes
vertical and lateral cloud movement. Because wind speeds and
directions vary with altitude (Figure 3), radioactive
materials spread over large areas. Large particles settle locally,
whereas small particles and gases may travel around the world.
Rainfall can cause localized concentrations far from the test site.
On the other hand, large atmospheric explosions injected radioactive
material into the stratosphere, 10 kilometers or more above the
ground, where it could remain for years and subsequently be
deposited fairly homogeneously ("global" fallout). Nuclear
tests usually took place at remote locations at least 100 kilometers
from human populations. In terms of distance from the detonation
site, "local fallout" is within 50 to 500 kilometers from
ground zero, "regional fallout" 500-3,000 kilometers and
global fallout more than 3,000 kilometers. Because the fallout cloud
disperses with time and distance from the explosion, and
radioactivity decays over time, the highest radiation exposures are
generally in areas of local fallout.
Following the deposition of fallout on the ground, local human
populations are exposed to external and internal irradiation.
External irradiation exposure is mainly from penetrating gamma rays
emitted by particles on the ground. Shielding by buildings reduces
exposure, and thus doses to people are influenced by how much time
one spends outdoors.
Internal irradiation exposures can arise from inhaling fallout and
absorbing it through intact or injured skin, but the main exposure
route is from consumption of contaminated food. Vegetation can be
contaminated when fallout is directly deposited on external surfaces
of plants and when it is absorbed through the roots of plants. Also,
people can be exposed when they eat meat and milk from animals
grazing on contaminated vegetation. In the Marshall Islands,
foodstuffs were also contaminated by fallout directly deposited on
food and cooking utensils.
The activity of fallout deposited on the ground or other surfaces is
measured in becquerels (Bq), defined as the number of radioactive
disintegrations per second. The activity of each radionuclide per
square meter of ground is important for calculating both external
and internal doses. Following a nuclear explosion, the activity of
short-lived radionuclides is much greater than that of long-lived
radionuclides. However, the short-lived radionuclides decay
substantially during the time it takes the fallout cloud to reach
distant locations, where the long-lived radionuclides are more important.
Iodine-131, which for metabolic reasons concentrates in the thyroid
gland, has a half-life (the time to decay by half) of about eight
days. This is long enough for considerable amounts to be deposited
onto pasture and to be transferred to people in dairy foods
(Figure 4). In general, only those children in the U.S.
with lactose intolerance or allergies to milk products consumed no
milk products, particularly in the 1950s and 1960s when there were
fewer choices of prepared foods. Radioiodine ingested or inhaled by
breast-feeding mothers can also be transferred to nursing infants
via the mother's breast milk.
The two nuclear weapons dropped on Hiroshima and Nagasaki were
detonated at relatively high altitudes above the ground and produced
minimal fallout. Most of the injuries to the populations within 5
kilometers of the explosions were from heat and shock waves; direct
radiation was a major factor only within 3 kilometers. Most of what
we know about late health effects of radiation in general, including
increased cancer risk, is derived from continuing observations of
survivors exposed within 3 kilometers.
Understanding Radiation Dose
Radiation absorbed dose is the energy per unit mass imparted to a
medium (such as tissue). Almost all radionuclides in fallout emit
beta (electron) and gamma (photon) radiation. A cascade of events
follows once tissue is exposed to radiation: The initial radiation
scatters, and atoms in the body are ionized by removal of weakly
bound electrons. Radiation can damage DNA by direct interaction or
by creating highly reactive chemical species that interact with DNA.
The basic unit of the system used internationally to characterize
radiation dose is the gray (Gy), defined as the absorption of 1
joule of energy per kilogram of tissue. (The international system of
units is gradually supplanting the previous system based on dose
units of rad, but conversion is easy: 1 Gy = 100 rad.) For
perspective, it is helpful to remember that the external dose
received from natural sources of radiation—from primordial
radionuclides in the earth's crust and from cosmic
radiation—is of the order of 1 milligray (mGy, one-thousandth
of a gray) per year; the dose from a whole-body computer-assisted
tomographic (CT) examination is about 15-20 mGy, and that due to
cosmic rays received during a transatlantic flight is about 0.02 mGy.
Examples of Fallout Exposures
Doses from fallout received in the 1950s and 1960s have been
estimated in recent years using mathematical exposure assessment
models and historical fallout deposition data. There have been only
a few studies involving detailed estimation of the doses received by
local populations; the exceptions include some towns and cities in
Nevada and adjacent states, a few villages near the Soviet
Semipalatinsk Test Site (STS), and some atolls in the Marshall Islands.
Marshall Islands. One of the 65 tests conducted in the
Marshall Islands, the explosion of a U.S. thermonuclear device
code-named BRAVO (March 1, 1954), was responsible for
most—although not all—of the radiation exposure of local
populations from all of the tests. The fallout-related doses
received as a result of that one test at Bikini Atoll are the
highest in the history of worldwide nuclear testing.
Wind shear (changes in direction and speed with altitude) and an
unexpectedly high yield resulted in heavy fallout over populated
atolls to the east of Bikini rather than over open seas to the north
and west. About 31/2 hours after the detonation, the radioactive
cloud began to deposit particulate, ash-like material on 18 Rongelap
residents who were fishing and gathering copra on Ailinginae Atoll
about 135 kilometers east of the detonation site, followed 2 hours
later by deposition on Rongelap Island 65 kilometers farther to the
east, affecting 64 residents. The fallout arrived 21/2 hours later
at Rongerik Atoll another 40 kilometers to the east, exposing 28
American weathermen; about 22 hours after detonation, it reached the
167 residents of Utrik Atoll.
Doses received by the Rongelap group were assessed by ground and
aerial exposure rate measurements and radioactivity analysis of a
community-pooled urine sample. The doses received before evacuation
were essentially due to external irradiation from short-lived
radionuclides and internal irradiation from ingestion of short-lived
radioiodines deposited on foodstuffs and cooking utensils. Thyroid
doses, in particular, were very high: At Rongelap they were
estimated to be several tens of Gy for an adult and more than 100 Gy
for a one-year old. Estimated thyroid doses at Ailinginae were about
half those at Rongelap, and doses at Utrik were about 15 percent of
those at Rongelap. The external whole-body doses estimated were
about 2 Gy at Rongelap, 1.4 Gy at Ailinginae, 2.9 Gy at Rongerik and
0.2 Gy at Utrik. Much lower exposures have been estimated for most
of the other Marshall Islands atolls.
Twenty-three Japanese fishermen on the fishing vessel Lucky
Dragon were also exposed to heavy fallout. Their doses from
external irradiation were estimated to range from 1.7 to 6 Gy. Those
doses were received during the 14 days it took to return to harbor;
about half were received during the first day after the onset of fallout.
Semipalatinsk, Kazakhstan. The Semipalatinsk Test Site, in
northeastern Kazakhstan near the geographical center of the Eurasian
continent, was the Soviet equivalent of the U.S. Nevada Test Site;
88 atmospheric tests and 30 surface tests were conducted there from
1949 through 1962. The main contributions to local and regional
environmental radioactive contamination are attributed to particular
atmospheric nuclear tests conducted in 1949, 1951 and 1953.
Doses from local fallout originating at the STS depended on the
location of villages relative to the path of the fallout cloud, the
weather conditions at the time of the tests, the lifestyles of
residents, which differed by ethnicity (Kazakh or European), and
whether they were evacuated before the fallout arrived at the
village. Some unique circumstances included strong winds that
resulted in short fallout transit times and little radioactive decay
before deposition for at least one test. Also, the residents of the
area were heavily dependent on meat and milk from grazing animals,
including cattle, horses, goats, sheep and camels.
Dose-assessment models predict a decreasing gradient in the ratio of
external radiation doses to internal doses from inhalation and
ingestion with increasing time from detonation to fallout arrival.
The relatively large particles that tend to fall out first are not
efficiently transferred to the human body. At more distant locations
in the region of local fallout, internal dose is relatively more
important because smaller particles that predominate there are
biologically more available. For example, in rural villages along
the trajectory of the first test (August 1949) at the Semipalatinsk
Test Site, average estimated radiation dose from fallout to the
thyroid glands of juvenile residents decreased with increasing
distance from the detonation, but the proportion of that total due
to internal radiation sources increased with distance. At 110
kilometers from the detonation site, the average dose was 2.2 Gy, of
which 73 percent was from internal sources, whereas at 230
kilometers, 86 percent of the average dose of 0.35 Gy was from
Nevada Test Site (NTS). The NTS was used for surface and
above-ground nuclear testing from early 1951 through mid-1962.
Eighty-six tests were conducted at or above ground level, and 14
other tests that were underground involved significant releases of
radioactive material into the atmosphere.
In 1979 the U.S. Department of Energy described a methodology for
estimating radiation doses to populations downwind of the NTS. Doses
from internal irradiation within this local fallout area were
ascribed mainly to inhalation of radionuclides in the air and to
ingestion of foodstuffs contaminated with radioactive materials.
Doses from internal irradiation were, for most organs and tissues,
substantially smaller than those from external irradiation, with the
notable exception of the thyroid, for which estimated internal doses
were substantially higher. Estimated thyroid doses were ascribed
mainly to consumption of foodstuffs contaminated with iodine-131
(I-131) and, to a lesser extent, iodine-133 (I-133), and to
inhalation of air contaminated with both I-131 and I-133. Thyroid
doses varied according to local dairy practices and the extent to
which milk was imported from less contaminated areas.
Bone-marrow doses less than 50 mGy were estimated for communities in
a local fallout area within 300 kilometers of the NTS, where
ground-monitoring data were available, and an order of magnitude
less for other communities in Arizona, New Mexico, Nevada, Utah and
portions of adjoining states.
Investigators at the University of Utah estimated radiation doses to
the bone marrow for 6,507 leukemia cases and matched controls who
were residents of Utah. Average doses were about 0.003 Gy with a
maximum of about 0.03 Gy. Subsequently, thyroid doses were estimated
to members of a cohort exposed as school children in southwestern
Utah and who are part of a long-term epidemiology study. The mean
thyroid dose was estimated to be 0.12 Gy, with a maximum of 1.4 Gy.
Among children who did not drink milk, the mean thyroid dose was on
the order of 0.01 Gy.
In response to Public Law 97-414 (enacted in 1993), the U.S.
National Cancer Institute (NCI) estimated the absorbed dose to the
thyroid from I-131 in NTS fallout for representative individuals in
every county of the contiguous United States. Calculations
emphasized the pasture-cow-milk-man food chain, but also included
inhalation of fallout and ingestion of other foods. Deposition of
I-131 across the United States was reconstructed for every
significant event at the NTS using historical measurements of
fallout from a nationwide network of monitoring stations operational
between 1951 and 1958. Thyroid doses were estimated as a function of
age at exposure, region of the country and dietary habits. For
example, for a female born in St. George, Utah, in 1951 and residing
there until 1971, the thyroid doses are estimated to have been about
0.3 Gy if she had consumed commercial cow's milk, 2 Gy if she had
consumed goat's milk, and 0.04 Gy if she had not consumed milk. For
a female born in Los Angeles, California, at the same time, the
corresponding values would have been 0.003, 0.01, and 0.0004 Gy. (A
link to these data is available in the bibliography.)
Following the publication of the NCI findings in 1997, the U.S.
Congress requested that the Department of Health and Human Services
extend the study to other radionuclides in fallout and to consider
tests outside the U.S. that could have resulted in substantial
radiation exposures to the American people. Examples of results
extracted from the report (a link is available in the bibliography)
are shown in Figures 7 through 9 and 11. Figure 7 shows the pattern
of deposition of cesium-137 (Cs-137), a radionuclide traditionally
used for reference, resulting from all NTS tests in the entire
United States. Fallout decreased with distance from the NTS along
the prevailing wind direction, which was from west to east. Very
little fallout was observed along the Pacific coast, which was
usually upwind from the NTS. Estimated bone-marrow and thyroid doses
are illustrated in Figure 8. The fact that both external and
internal doses were roughly proportional to the deposition density
is reflected in similarities between the two figures. Estimates of
average thyroid and of bone-marrow doses for the entire U.S.
population are presented in Figure 11; the thyroid doses from I-131
are much higher than the internal doses from any other radionuclide
and also much higher than the doses from external exposure.
Global fallout within the U.S.Global fallout originated
from weapons that derived much of their yield from fusion reactions.
These tests were conducted by the Soviet Union at northern latitudes
and by the U.S. in the mid-Pacific. For global fallout, the mix of
radionuclides that might contribute to exposure differs from that of
NTS fallout, largely because radioactive debris injected into the
stratosphere takes one or more years to deposit, during which time
the shorter-lived radionuclides largely disappear through
radioactive decay. Of greater concern are two longer-lived
radionuclides, strontium-90 and cesium-137, which have 30-year
half-lives and did not decay appreciably before final deposition.
Examples of the doses received from global fallout are shown
in Figures 9 and 11. Figure 9 shows the pattern of deposition of
Cs-137 from global fallout, as well as the total dose to red bone
marrow, which is roughly proportional to the deposition. A
comparison of Figures 9 and 7 shows very different patterns of
Cs-137 in global fallout (related to rainfall patterns) and NTS
fallout, which depended mainly on the trajectories of the air masses
originating from the NTS. Estimates of average thyroid and
bone-marrow doses for the entire U.S. population from global fallout
are presented in Figure 11; the thyroid dose from I-131 is higher
than the internal doses from any other radionuclide, but it is no
greater than the doses from external irradiation.
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.
Fallout and Radiological Terrorism
Concern about the possible use of radioactive materials by
terrorists has been heightened following the attacks on the World
Trade Center and the Pentagon on September 11, 2001, and other acts
elsewhere in the world. Conventional attacks, including use of a
dirty bomb—that is, a conventional explosive
coupled with radioactive material—seem more likely (because
they are easier to carry out) than a fission event, but it is still
useful to ask ourselves "What lessons from our research on
fallout are applicable to events of radiological terrorism?"
The potential for health damage downwind of a terrorist event
involving any degree of fission will be dominated by exposure to
early highly radioactive fallout.
Accurately projecting fallout patterns requires knowledge of the
location and altitude at which the device is exploded, and the local
meteorology—particularly a three-dimensional characterization
of the wind field in the vicinity of the explosion. Logistics would
likely lead a terrorist organization to explode a small-scale,
fission-type nuclear device at ground level. According to the
National Council on Radiation Protection and Measurements, an
explosive yield of only 0.01 kiloton would cause more physical
damage than the explosion that destroyed the Oklahoma City Federal
Building in 1995. Persons within 250 meters of a 0.01-kiloton
nuclear detonation would receive whole-body doses of 4 Gy from the
initial radiation, resulting in the mortality of almost half of
those exposed. The same dose would be received within one hour from
exposure to fallout by those who remained within 1.3 kilometers of
Acute life-threatening effects would dominate treatment efforts
within the initial weeks of a terrorist event. Later, increase
levels of chronic disease, including cancer, would be expected to
contribute to radiation-related mortality and morbidity among
survivors, including those with lesser exposures. Among all persons
in the U.S. and most other developed countries, cancer causes about
1 in 4 deaths. The total additional cancer risk from exposure to
radioactive fallout is relatively small, although follow-up of the
Japanese atomic bomb survivors has shown that elevated cancer risks
continue throughout the remainder of life.
Fallout—What We've Learned
Over the more than five decades since radioactive fallout was first
recognized as a potential public-health risk, it has stimulated
interdisciplinary research in areas of science as diverse as nuclear
and radiation physics, chemistry, statistics, ecology, meteorology,
genetics, cell biology, physiology, exposure and risk assessment,
Individual radionuclides in fallout were recognized early on as
opportune tracers by which the kinetic behavior of elements could be
studied, both among components of ecosystems and in their transport
to people. The phenomenon of fallout, while contributing only
modestly to our overall understanding of radiation risks, has taught
us much about pathways of exposure and about cancer risks to the
public in settings outside the medical and occupational arenas. And
in particular, fallout studies helped increase our understanding of
health risks from specific radionuclides, for example, I-131. This
has made possible the development of the National Cancer Institute's
thyroid dose and risk calculator (see "Estimating Your
Thyroid Cancer Risk," below).
In the U.S., it took a number of years for the differences in dose
and cancer risk from regional and global fallout to be understood.
We have learned that the internal doses from global fallout were
considerably smaller for the thyroid, but greater for the red bone
marrow, than those from Nevada fallout, whereas the doses from
external irradiation were similar for Nevada and for global fallout.
We estimate that in the U.S. the primary cancer risks from past
exposure to radioactive fallout are thyroid cancer and leukemia,
whereas in a very few cases—for example, the Marshall
Islands—large internal doses as a result of ingestion of
radionuclides have led to significant risks of cancers in the
stomach and colon. Our research has quantified the likely number of
cancer cases to be expected in the U.S. from Nevada exposures and
has contributed to the assessment of risk at other worldwide locations.
Nuclear testing in the atmosphere began 60 years ago. It ended in
1980, in part because of public concerns about involuntary exposure
to fallout. By that time, increased cancer risk had been established
as the principal late health effect of radiation exposure, based
primarily on studies of populations exposed to medical x rays, to
radium and radon decay products from the manufacture of luminescent
(radium) watch dials and in uranium mining, and to direct radiation
from the atomic bombings of Hiroshima and Nagasaki. Since then,
organ-specific dose-response relationships for radiation-related
risks of malignant and more recently benign disease (for example,
cardiovascular disease and benign neoplasms of various organs) have
been increasingly well quantified with further follow up of these
and other populations, and it is increasingly clear that
radiation-related risk may persist throughout life. Fallout studies
have substantially clarified the consequences of exposure to
specific organs from internal contamination with radioactive
materials—for example, I-131 in the thyroid gland—and
there is every reason to believe that, on a dose-specific basis,
increased risks from fallout should be similar to those from other
radiation sources. Our improved understanding of individual
radionuclides, radiation dose and related health risk is due in part
to decades of study of fallout from nuclear testing; that same
understanding today makes us better prepared to respond to
nuclear terrorism, accidents or other events that could disperse
radioactive materials in the atmosphere.
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