Fallout from Nuclear Weapons Tests and Cancer Risks
Exposures 50 years ago still have health implications today that will continue into the future
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.