April 26, 2006, was the 20th anniversary of the Chernobyl accident, the second major single exposure to radiation of a substantial population. The accident produced a significant international response whose effectiveness is the subject of debate. It is relevant to the current view of the consequences of Chernobyl to reflect on the understanding in 1965 of the health consequences of the first major event, radiation from the atomic bombs in Hiroshima and Nagasaki, Japan, in 1945. The only significant consequences observed in survivors 20 years after the atomic bombs were increases in leukemia and thyroid cancer, and the general view of the future was reassuring. In 1974, a significant increase in solid cancers was detected, and nearly 50 years after the event, an unexpected increase was found in non-cancer diseases. Today, leukemia and thyroid cancer form only a small fraction of the accepted total radiation-related health detriment.
In 1990, four years after the Chernobyl accident, an increase in thyroid cancer was found in children exposed to fallout from the accident. Two years later, the first reports in the Western literature of an increase in childhood thyroid cancer (CTC) in Belarus were published. In 2000, about 2,000 cases of thyroid cancer had been reported in those exposed as children in the former Soviet Socialist Union, and in 2005, the number was estimated at 4,000; the latest estimate for the year 2056 ranges from 3,400 to 72,000. The effects are not limited by national borders; Poland has recorded cases in spite of a rapid precautionary distribution of stable iodine. The causative agent,131I, was detected in many European countries with as yet unknown effects. Interestingly, a significant increase in leukemia has not been reliably reported in the three most affected countries.
This dramatic contrast between the two incidents is in part due to the different types of radiation exposure, but both show that the effects of massive exposures to radiation are immensely complex. In comparing the health effects after Chernobyl with those after the atomic bombs, it must be remembered that apart from workers in or close to the power plant, the Chernobyl accident involved mainly exposure to radioactive isotopes, and the atomic bombs primarily involved direct exposure to gamma rays and neutrons. Because of the prominence given to thyroid carcinoma after Chernobyl, less attention has been given to whole-body exposure from the ingestion and inhalation of all isotopes, together with the shine from the radioactive cloud and deposited radioactivity. Consideration of the health effects of Chernobyl must take into account both tissue-specific doses due to isotope concentration and whole-body doses.
The most prominent tissue-specific dose is that to the thyroid, largely from 131I, with a smaller contribution from short-lived isotopes of iodine. For many in the 30-km zone (135,000), there were relatively high absorbed doses to other organs as well as the thyroid until evacuation, and for those living in the contaminated areas around the 30-km zone (5 million), relatively high dose rate exposure (days to weeks) was followed by prolonged (years) exposure to a low dose rate. This exposure was a complex mixture of external radiation and internal emitters. For others living farther from the accident, in Western Europe, for example, their average exposure was equivalent to an additional ≤ 50% of average annual natural background level of radiation. About 600,000 liquidators assisted with the cleanup. Those working at the site shortly after the accident (200,000) received substantial doses. For all of these groups, estimates of numbers of fatal cancers can be derived from the collective doses. However, such estimates depend on the assumed risk coefficient, but of the order of 60,000 such fatalities in total can be estimated, based on the collective dose estimated by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), less than half of which would derive from the declared contaminated areas. A more recent estimate of the numbers of fatal cancers based on a collective dose of less than half the UNSCEAR estimate gives a central value of 16,000 (95% confidence interval, 7,000–38,000).
Firmly established health consequences
By far, the most prominent health consequence of the accident is the increase in thyroid cancer among those exposed as children. The medical authorities in Belarus and Ukraine were aware in 1990 that the incidence of the rare (typically about 1/106children/year) CTC was increasing, particularly in children living close to the reactor. Initially, various non-Chernobyl-related causes were suggested for the increase in thyroid carcinoma. In terms of radiation dose, the most likely culprit was 131I, a copious product of nuclear fission with an 8.1-day half-life. 131I is rapidly taken up by the thyroid but was widely regarded as carrying little risk of thyroid neoplasia. Swedish epidemiologic studies of the widespread use of 131I in diagnosis and treatment of thyroid disease found no significant risk of thyroid cancer. Other isotopes of iodine and tellurium-132 were also released in very large amounts, but because of their much shorter half-lives, their most significant contribution to the thyroid dose occurred only in those living near the reactor.
The first reports of the increase in Belarus were received with skepticism by the scientific community, but the risks were shown to be real. Analysis of thyroid carcinogenesis after X-ray exposure also showed clearly that the younger the subject at exposure, the higher the risk. The almost complete lack of children in the Swedish studies thus accounted for the apparent lack of a carcinogenic risk from 131I. It has since become increasingly clear that 131I is as carcinogenic in children as X rays. The child’s thyroid is one of the most sensitive human tissues to cancer induction by radiation. Because iodine is a volatile element, its release from fractured fuel rods is inevitable.
Much has been made of the fact that differentiated thyroid cancer is an eminently curable disease. Only a very small number of deaths from Chernobyl-related thyroid carcinoma have occurred so far. However, the preferred treatment regime, total thyroidectomy followed by 131I treatment to destroy metastases, is not always fully effective. Death from papillary carcinoma of the thyroid is rare, usually of the order of 5–10%. Because of the slow growth of the tumor, it is premature to assume that the even lower death rate for current Chernobyl-related cases will be maintained, particularly for cases yet to occur. An older age at onset can be associated with a less favorable prognosis. Currently, those exposed as small children are now adolescents or young adults but continue to carry an increased risk of developing thyroid carcinoma. The incidence of thyroid cancer in those who were adults at the time of exposure is reported to have increased in the many exposed populations, although the relationship to radiation is not clear. Screening has become more sophisticated, and increased ascertainment may be a major factor. The concentration of effort on the major increase in those exposed as children has meant that the possible much smaller risk to adults has not been adequately investigated.
Acute radiation sickness
A small group of liquidators and plant workers received very high whole-body doses. Among these, about 150 individuals were treated for acute radiation sickness; 28 of these died within a relatively short time. Approximately 20 more have since died from probable radiation-related diseases.
Psychological effects are of considerable importance. They arise from an understandable fear of exposure to an unknown amount of an intangible but potentially dangerous agent, fear for exposed children, mistrust of reassurances from the authorities, and for hundreds of thousands of people, the consequences of forced evacuation from home and land. For some, the stress from these experiences has precipitated psychological illness; for others, an increased consumption of alcohol and cigarettes; and for still others, dietary changes to avoid perceived contamination. Some deaths from suicide, cirrhosis, or lung cancer could be regarded as indirect consequences of the accident and the subsequent measures taken. Whatever the view the nuclear industry may have about the irrationality of these consequences, they are real and have an important impact on public health, and so deserve greater attention.
Another consequence, not as firmly established as thyroid cancer, is mini-satellite instability (MSI) in children born to exposed fathers after Chernobyl. MSI is not a classical genetic effect, and its implications for health are far from clear. A similar effect has been seen in the children and grandchildren of men exposed to weapons testing in Semipalatinsk and a parallel phenomenon, tandem repeat instability, occurs in laboratory mice. MSI has not been observed in the survivors of the atomic bombings in studies of Chernobyl cleanup workers or in radiotherapy patients. MSI is considerably more frequent in relation to radiation dose than classical genetic effects and apparently does not become diluted in subsequent generations. Although its clinical significance is uncertain it is of some concern, certainly more than the Chernobyl Forum gave it credit for.
These issues are particularly relevant in view of developments in radiobiological research over the past 15 years. The apparently simple relationship between radiation dose and its effects are being reappraised. In the early 1990s, two previously unacknowledged effects of radiation were reported, genomic instability and the bystander effect. These effects are not accommodated by the current theoretical framework. Also in 1986, the risk per unit dose accrued from Chernobyl would have been assumed to be half that estimated from the atomic bombs in Japan. A recent detailed analysis of the Japanese experience suggested that the risk for those exposed to the lower doses could even be supralinear. Furthermore, the accuracy of the standard models for inferring doses from internal exposure have been questioned by the U.K. Committee Examining Radiation Risk of Internal Emitters. There is, therefore, considerable uncertainty in translating collective dose to health detriment and fatalities.
There have been many claims of an increased incidence of congenital anomalies in children born shortly after the accident. Some cases reported in the press show abnormalities similar to those following the use of thalidomide in pregnancy, and thalidomide was apparently available in the Soviet Union. It is not possible to separate Chernobyl-related abnormalities from those due to other causes or from the effects of increased ascertainment. Although a slight increase in minor conditions has been observed, there does not appear to have been a major increase in serious conditions such as limb deformities.
Intensive efforts have been made to detect an increase in leukemia, which is strongly associated with radiation. No statistically significant increases of those forms associated with radiation have been reported, but increases in chronic lymphatic leukemia, a no-radiation-related disease of older age, may testify to increased case ascertainment. However, the level of increase expected, given the received doses, anticipated risk factor, and the rarity of the condition, would only be detected by large analytical—as opposed to ecological—epidemiology studies.
In the future
Experience from Japan shows that many effects of whole-body radiation exposure may not be apparent for decades. While the short initial latent period associated with the thyroid carcinoma after Chernobyl, together with the very large amounts of radioactive isotopes of iodine released, have led to a huge effort to reconstruct thyroid doses, much less attention has been paid to whole-body doses. Measurements of the initial exposure phase for those in the 30-km zone, while confused, point to absorbed doses to the whole body of many individuals that were > 1 Gray (Gy), with average doses to some 25,000 Belarusian evacuees of a substantial fraction of a Gray. Doses received by infants evacuated from the 30-km zone are estimated to be in the range of 0.03–2 sieverts (Sv), well within the range that led to a significant rise in cancer incidence after the atomic bombs. As well as the thyroid, other organs show some concentration of iodine. One particularly important tissue is breast epithelium, which can concentrate iodine and receive radiation from isotopes in the lung or thyroid. Some particular groups at exposure may show an excess incidence of breast cancer now or in the future. A significant rise in incidence of a range of malignancies in the population exposed to high levels of fallout, particularly those exposed as children, is clearly possible. All too often the phrase “no increase has been observed” conceals the lack of an adequate study.
The full complexity of the exposure regime has not been adequately explored, and the estimation of whole-body and many tissue-specific doses is imprecise or unknown. The radiation dose received from the atomic bombs was still being revised 50 years after the event. Taking into account the results of new research and the CERRIE report, it is very difficult to derive with any confidence the likely levels of health detriment from the estimated dose levels. It is also too soon to make an accurate assessment of longer-term effects from those already observed.
In the light of this level of uncertainty, the case is compelling for international research surveillance of the millions of people exposed to fallout from Chernobyl and selective follow-up of those exposed to high levels similar to that following the atomic bombings in Japan.
This article is derived largely from Keith Baverstock and Dillwyn Williams, The Chernobyl Accident 20 Years On: An Assessment of the Health Consequences and the International Response, Environmental Health Perspectives, 114(9):1312:1317 (September 2006) doi:10.1289/ehp.9113. Topic editors and authors for the Encyclopedia of Earth have edited its content or added new information. That article is used here under the terms of its Creative Commons Attribution License. The use of information from the original article should not be construed as support for or endorsement by its author for any new information added by EoE personnel, or for any editing of the original content.
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