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.
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.
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.
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.
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.
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 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.
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.
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).
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.
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.
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.