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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

How People Are Exposed to Fallout

Figure 3. Wind shear...Click to Enlarge Image

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.

Figure 4. One major means...Click to Enlarge Image

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.

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