| 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.
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
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.
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, 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. 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
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.
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 internal sources
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). A figure 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. The fact
that both external and internal doses were roughly proportional to the
deposition density is reflected in similarities between the two other figures.
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
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. A figure 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. There are 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; 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
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 an other figure 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
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 the detonation.
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, and epidemiology.
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
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.
Estimating Your Thyroid Cancer Risk
A Web-based calculator developed by the National
Cancer Institute is available to anyone wishing to estimate individual
thyroid cancer risks associated with exposure to I-131 radiation in fallout
from the Nevada Test Site, for persons who lived in the U.S. during the
1950s. The calculator can be accessed through the Internet at its stand-alone
Web page (http://ntsi131.nci.nih.gov/) or through the main NCI Web site
(http://www.cancer.gov/i131), which provides more general information about
the NTS, I-131 and radioactive fallout. Information required for the calculation
includes gender, age at exposure, places of residence during the years
1951–71, and sources and approximate amounts of milk consumed.