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
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
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)
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.
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