What follows are extracts from Annex J: Exposures and Effects of the Chernobyl Accident of the UNSCEAR 2000 report to the General Assembly. The issue of the Chernobyl disaster is discussed here. For the full report, copies can be found at all good libraries.
From chapter V: Late Health Effects of the Chernobyl Accident, section A: Cancer, subsection 1: Thyroid cancer
(c) Summary
308. There can be no doubt about the relationship between the radioactive materials released from the Chernobyl accident and the unusually high number of thyroid cancers observed in the contaminated areas during the past 14 years. While several uncertainties must be taken into consideration, the main ones being the baselines rates used in the calculations, the influence of screening, and the short follow-up, the number of cases is still higher than anticipated based on previous data. This is probably partly a result of age at exposure, iodine deficiency, genetic predisposition, and uncertainty that surrounds the role of 131I compared with that of short-lived radioiodines. The exposure to short-lived radioiodines is entirely dependent on the distance from the release and mode of exposure, i.e. inhalation or ingestion. It was only in the Gomel region, the area closest to the Chernobyl reactor, that Astakhova et al. found a significantly increased risk of thyroid cancer. It has been suggested that the geographical distribution of thyroid cancer cases correlates better to the distribution of shorter-lived radioisotopes (e.g. 132I, 133I and 135I) than to that of 131I.
309. The identification of a genomic fingerprint that shows the interaction of the specific target cell with a defined carcinogen is a highly desirable tool in molecular epidemiology. However, a specific molecular lesion is almost always missing, probably because of the large number of factors acting on tumour induction and progression. Signaling via protein tyrosine kinases has been identified as one of the most important events in the cellular regulation, and rearrangements of the tyrosine kinase domain of the RET proto-oncogene have been found in thyroid cancers thought to be associated with ionizing radiation. However, the biological and clinical significance of RET activation remain controversial, and further studies of molecular biology of radiation-induced thyroid cancers are need before the carcinogenic pathway can be fully understood.
From subsection 2: Leukaemia
331. Summary. Although leukaemia has been found to be one of the early carcinogenic effects of ionizing radiation with a latency period of not more than 2-3 years, no increased risk of leukaemia related to ionizing radiation has been found among the recovery operation workers or in residents of contaminated areas. Numerous reports have compared incidence and mortality from the registers described in Chapter IV with national rates not taking the difference in reporting into consideration. A case-control study would dimish this bias, and a recent paper by Ivanov et al. failed to show an increased risk of leukaemia related to ionizing radiation in 48 cases of leukaemia in recovery operation workers identified through the Russian National Registry.
From subsection 3: Other solid tumours
342. Summary. The occurrence of solid tumours other than thyroid cancers in workers or in residents of contaminated areas habe not so far been observed. The weaknesses in the scientific studies, the uncertainties in the dose estimates, the latency period of around 10 years and the protracted nature of the exposures probably explain why no radiation-associated cancers have been noticed so far. Some increase in incidence of solid tumours might have been anticipated in the more highly exposed recovery operation workers.
From section B: Other Somatic Disorders, Subsection 1: Thyroid abnormalities
356. Summary. Other than the occurrence of thyroid nodules in workers and in children, which is unrelated to radiation exposure, there has been no evidence of thyroid abnormalities in affected populations following the Chernobyl accident. Even the large screening programme conducted by the Chernobyl Sasakawa Health and Medical Cooperation Project in 1991-1996, involving 160,000 children, less than 10 years of age at the time of the accident, there was no increased risk of hypothyroidism, hyperthyroidism or goiter that could be related to ionizing radiation. Neither was an increase in thyroid antibodies noticed, which is in contradiction with some other minor studies.
From subsection 3: Immunological effects
375. Summary. With the exception of the increase risk of thyroid cancer in those exposed at young ages, no somatic disorder or immunological defect could be assoicated with ionising radiation caused by the Chernobyl accident.
From section D: Pregnancy outcome
383. Summary. Several studies on adverse pregnancy outcomes related to the Chernobyl accident have been performed in the areas closest to the accident and in more distant regions. So far, no increase in birth defects, congenital malformations, stillbirths, or premature births could be linked to radiation exposures caused by the accident.
From section D: Psychological and other accident-related effects
394. Summary. The Chernobyl accident caused long-term changes in the lives of people living in the contaiminated area, since measures intended to limit radiation dose included resettlement, change in food suplies, and restrictions on the activities of individuals and families. These changes were accompanied by important economic, social, and political changes in the affected countries, brought about by the disintegration of the former Soviet Union. The anxiety and emotional stress among parents most likely influenced the children and unfavourable psychosocial factors probably explain the difference between the exposed and non-exposed groups.
From section E: Summary
395. A majority of the studies completed to date on the health effects of the Chernobyl accident are of the geographic correlation type that compare average population exposure with the average rate of health effects or cancer incidence in the time periods before and after the accident. As long as the individual dosimetry is not performed no reliable quantitative estimates can be made. The reconstruction of valid individual doses will have to be a key element in future research on health effects related to the Chernobyl accident.
396. The number of thyroid cancers in individuals exposed in childhood, particularly in the severely contaminated areas of the three affected countries, is considerably greater than expected based on previous knowledge. The high incidence and the short induction period have not been experienced in other exposed populations, and factors other than ionizing radiation are almost certainly influencing the risk. Some such factors include age at exposure, iodine intake and metabolic status, endemic goitre, screening, short-lived isotopes other than 131I, higher doses than estimated, and, possibly, genetic predisposition. Approximately 1,800 thyroid cancer cases have been reported in Belarus, the Russian Federation and Ukraine in children and adolescents for the period 1990-1990. Age seems to be an important modifier of risk. The influence of screening is difficult to estimate. Approximately 40%-70% of the cases were found through screening programmes, and it is unclear how many of these cancers would have otherwise gone undetected. Taking the advanced stage of the tumours at time of diagnosis into consideration, it is likely that most of the tumours would have been detected sooner or later.
397. The present results from several studies indicate the majority of the post-Chernobyl childhood thyroid carcinomas show the intrachromosomal rearrangements characterized as RET/PTC1 and 3. There are, however, several questions left unanswered, e.g. the influence of age at exposure and time since exposure on the rate of chromsome rearragenments.
398. The risk of leukaemia has been shown in epidemiological studies to be clearly increased by radiation exposure. However, no increased risk of leukaemia linked to ionizing radiation has so far been confirmed in children, in recovery operation workers, or in the general population of the former Soviet Union or other areas with measureable amounts of contamination from the Chernobyl accident.
399. Increases in a number of non-specific detrimental health effects other than cancer in recovery operation workers and in residents of contaminated areas have been reported. It is diffiult to interpret these findings without referring to a known baseline or background incidence. Because health data obtained from official statistical sources, such as mortality or cancer incidence statistics, are often passively recorded and are not always complete, it is not appropriate to compare them with data for the exposed populations, who undergo much more intensive and active health follow-up than the general population.
400. Some investigators have interpreted a temporary loss of ability to work among individuals living in contaminated areas as an increase in general morbidity. High levels of chronic disease of the digestive, neurological, skeletal, muscular and circulatory systems have been reported. However, most investigators relate these observations to changes in the age structure, the worsening quality of life, and post-accident countermeasures such as relocation.
401. Many papers have been published in the last decade on the immunological effects of exposure to radiation from the Chernobyl accident. Since it is unclear, however, if possible confounding factors have been taken into account, including, in particular, infections and diet, it is difficult to interpret these results.
From the Conclusions
402. The accident of 26 April 1986 at the Chernobyl nuclear power plant, located in Ukraine about 20km south of the border with Belarus, was the most serious ever to have occurred in the nuclear industry. It caused the deaths, within a few days or weeks, of 30 power plant employees and firemen (including 28 with acute radiation syndrome) and brought about the evacuation, in the 1986, of about 116,000 people from areas surrounding the reactor and the relocation, after 1986, of about 220,000 people from Belarus, the Russian Federation and Ukraine. Vast territories of those three countries (at that time republics of the Soviet Union) were contaminated, and trace deposition of released radionuclides was measurable in all countries of the northern hemisphere. In this Annex, the radiation exposures, of the population groups most closely involved in the accident have been reviewed in detail and the health consequences that are or could be associated with these radiation exposure have been considered.
403. The populations considered in this Annex are (a) the workers involved in the mitigation of the accident, either during the accident itself (emergency workers) or after the accident (recovery operation workers) and (b) members of the general public who either were evacuated to avert excessive radiation exposures or who still reside in contaminated area. The contaminated areas, which are defined in this Annex as being those where the average 137Cs ground deposition density exceed 37 kBq m-2 (1 Ci km-2), are found mainly in Belarus, in the Russian Federation and in Ukraine. A large number of radiation measurements (film badges, TLDs, whole-body counts, thyroid counts, etc.) were made to evaluate the exposures of the population groups that are considered.
404. The approximately 600 emergency workers who were on the site of the Chernobyl power plant during the night of the accident received the highest doses. The most important exposures were due to external irradiation (relatively uniform whole-body gamma irradiation and beta irradiation of extensive body surfaces), as the intake of radionuclides through inhalation was relatively small (except in two cases). Acute radiation sickness was confirmed in 134 of those emergency workers. Forty-one of these patients received whole-body doses from external irradiation of less than 2.1 Gy. Ninety-three patients received higher doses and had more severe acute radiation sickness: 50 persons with doses between 2.2 and 4.1 Gy, 22 between 4.2 and 6.4 Gy, and 21 between 6.5 and 16 Gy. The skin doses from beta exposures, evaluated for eight patients with acute radiation sickness, were in the range of 400-500 Gy.
405. About 600,000 persons (civilian and military) have received special certificates confirming their status as liquidators (recovery operation workers), according to laws promulgated in Belarus, the Russian Federation and Ukraine. Of those, about 240,000 were military servicemen. The principal tasks carried out by the recovery operation workers included decontamination of the reactor block, reactor site and roads, as well as construction of the sarcophagus and of a town for reactor personnel. These tasks were completed by 1990.
406. A registry of recovery operation workers was established in 1986. This registry includes estimates of effective doses from external irradiation, which was the predominant pathway of exposure for the recovery operation workers. The registry data show that the average recorded doses decreased from year to year, being about 170 mSv in 1986, 130mSv in 1987, 30mSv in 1988 and 15mSv in 1989. It is, however, difficult to assess the validity of the results that have been reported because (a) different dosimeters were used by different organizations without any intercalibration; (b) a large number of recorded doses were very close to the dose limit; and (c) there were a large number of rounded values such as 0.1, 0.2 or 0.5 Sv. Nevertheless, it seems reasonable to assume that the average effective dose from external gamma irradiation to recovery operation workers in the years 1986-1987 was about 100mSv.
407. Doses received by the general public came from the radionuclide releases from the damaged reactor, which led to ground contamination of large areas. The radionuclide releases occurred mainly over a 10-day period, with varying release rates. From the radiological point of view, the releases of 131I and 137Cs, estimated to have been 1,760 and 85 PBq, respectively, are the most important. Iodine-131 was the main contributor to the thyroid doses, received mainly via internal irradiation within a few weeks after the accident, while 137Cs was, and is, the main contributor to the doses to organs and tissues other than the thyroid, from either internal or external irradiation, which will continue to be received, at low dose rates, during several decades.
408. The three main contaminated areas, defined as those with 137Cs deposition density greater than 37 kBq m-2 (1 Ci km-2), are in Belarus, the Russian Federation and Ukraine; they have been designated the Central, Gomel-Mogilev-Bryansk and Kaluga-Tula-Orel areas. The Central area is within about 100km of the reactor, predominantly to the west and northwest. The Gomel-Mogilev-Bryansk contaminated area is centred 200 km north-northwest of the reactor at the boundary of the Gomel and Mogilev regions of Belarus and of the Bryansk region of the Russian Federation. The Kaluga-Tula-Orel area is in the Russian Federation, about 500km to the northeast of the reactor. All together, territories from the former Soviet Union with an area of about 150,000 km2 were contaminated with 137Cs deposition density greater than 37 kBq m-2. About five million people reside in those territories.
409. Within a few weeks after the accident, more than 100,000 persons were evacuated from the most contaminated aras of Ukraine and Belarus. The thyroid doses received by the evacuees varied according to their age, place of residence, dietary habits and date of evacuation. For example, for the residents of Pripyat, who were evacuated essentially within 48 hours after the accident, the population-weighted average thyroid dose is estimated to be 0.17 Gy and to range from 0.07 Gy for adults to 2 Gy for infants. For the entire population of evacuees, the population-weighted average thyroid dose is estimated to be 0.47 Gy. Doses to organs and tissues other than the thyroid were on average, much smaller.
410. Thyroid doses also have been estimated for the residents of the contaminated areas who were not evacuated. In each of the three republics, thyroid doses are estimated to have exceeded 1 Gy for the most exposed infants. For residents of a given locality, thyroid doses to adults were smaller than those to infants by a factor of about 10. The average thyroid dose was approximately 0.2 Gy; the variability of the thyroid dose was two orders of magnitude, both above and below the average.
411. Following the first few weeks after the accident, when 131I was the main contributor to the radiation exposures, doses were delivered at much lower dose rates by radionuclides with much longer half-lives. Since 1987, the doses received by the populations of the contaminated areas came essentially from external exposure from 134Cs and 137Cs deposited on the ground and internal exposure due to contamination of foodstuffs by 134Cs and 137Cs. Other, usually minor, contributions to the long-term radiation exposures include the consumption of foodstuffs contaminated with 90Sr and the inhalation of aerosols containing plutonium isotopes. Both external irradiation and internal irradiation due to 134Cs and 137Cs result in relatively uniform doses in all organs and tissues of the body. The average efffective doses from 134Cs and 137Cs that were received during the first 10 years after the accident by the residents of contaminated areas are estimated to be about 10 mSv.
412. The papers available for review by the Committee to date regarding the evaluation of health effects of the Chernobyl accident have in many instances suffered from methodological weaknesses that make them difficult to interpret. The weaknesses include inadequate diagnoses and classification of diseases, selection of inadequate control or reference groups (in particular, control groups with a different level of disease ascertainment than the exposed groups), inadequate estimation of radiation doses or lack of individual inadequate estimation of radiation doses or lack of individual data and failure to take screening and increased medical surveillance into consideration. The interpretation of the studies is complicated, and particular attention must be paid to the design and performance of epidemiological studies. These issues are discussed in more detail in Annex I, "Epidemiological evaluation of radiation-induced cancer".
413. Apart from the substantial increase in thyroid cancer after childhood exposure observed in Belarus, in the Russian Federation and in Ukraine, there is no evidence of a major public health impact related to ionizing radiation 14 years after the Chernobyl accident. No increases in overall cancer incidence or mortality that could be associated with radiation exposure have been observed. For some cancers no increase would have been anticipated as yet, given the latency period of around 10 years for solid tumours. The risk of leukaemia, one of the most sensitive indicators of radiation exposure, has not been found to be elevated even in the accident recovery operation workers or in children. There is no scientific proof of an increase in other non-malignant disorders related to ionizing radiation.
414. The large number of thyroid cancers in individuals exposed in childhood, particularly in the severely contaminated areas of the three affected countries, and the short induction period are considerably different from previous experience in other accidents or exposure situations. Other factors, e.g. iodine deficiency and screening, are almost certainly influencing the risk. Few studies have addressed these problems, but those that have still find a significant influence of radiation after taking confounding influences into consideration. The most recent findings indicate that the thyroid cancer risk for those older than 10 years at the time of the accident is leveling off, the risk seems to decrease since 1995 for those 5-9 years old at the time of the accident, while the increase continues for those younger than 5 years in 1986.
415. There is a tendency to attribute increases in cancer rates (other than thyroid) over time to the Chernobyl accident, but it should be noted that increases were also observed before the accident in the affected area. Moreover, a general increase in mortality has been reported in recent years in most areas of the former USSR, and this must also be taken into account in interpreting the results of the Chernobyl-related studies. Because of these and other uncertainties, there is a need for well designed, sound analytical studies, especially of recovery operation workers from Belarus, the Russian Federation, Ukraine and the Baltic countries, in which particular attention is given to individual dose reconstruction and the effect of screening and other possible confounding factors.
416. Increases of a number of non-specific detrimental health effects other than cancer in accident recovery workers have been reported, e.g. increase suicide rates and deaths due to violent causes. It is difficult to interpret these findings without reference to a known baseline or background incidence. The exposed populations undergo much more intensive and active health follow-up than the general population. As a result, using the general population as a comparison group, as has been done so far in most studies, is inadequate.
417. Adding iodine to the diet of populations living in iodine-deficient areas and screening the high-risk groups could limit the radiological consequences. Most data suggest that the youngest age group, i.e. those who were less than five years old at the time of the accident, continues to have an increase risk of developing thyroid cancer and should be closely monitored. In spite of the fact that many thyroid cancers in childhood are presented at a more advanced stage in terms of local aggressiveness and distant metastases than in adulthood, they have a good prognosis. Continued follow-up is necessary to allow planning of public health actions, to gain a better understanding of influencing factors, to predict the outcomes of any future accident, and to ensure adequate radiation protection measures.
418. Present knowledge of the late effects of protracted exposure to ionizing radiation is limited, since the dose-response assessments rely heavily on high-dose exposure studies and animal experiments. The Chernobyl accident could, however, shed light on the late effects of protracted exposure, but given the low doses received by the majority of exposed individuals, albeit with uncertainties in the the dose estimates, any increase in cancer incidence or mortality will most certainly be difficult to detect in epidemiological studies. The main goal is to differentiate the effects of the ionising radiation and effects that arise from many other causes in exposed populations.
419. Apart from the radiation-associated thyroid cancers among those exposed in childhood, the only group that received doses high enough to possibly incur statistically detectable increased risks is the recovery operation workers. Studies of these populations have the potential to contribute to the scientific knowledge of the late effects of ionizing radiation. Many of these individuals receive annual medical examinations, providing a sound basis for future studies of the cohort. It is, however, notable that no increased risk of leukaemia, an entity known to appear within 2-3 years after exposure, has been identified more than 10 years after the accident.
420. The future challenge is to provide reliable individual dose estimates for the subjects enrolled in epidemiological studies and to evaluate the effects of doses accumulated over protracted time (days to weeks for thyroid exposures of children, minutes to months for bone-marrow exposures of emergency and recovery operation workers, and months to years for whole-body exposures of those living in contaminated areas). In doing this, many difficulties must be taken into consideration, such as (a) the role played by different radionuclides, especially the short-lived radioidodines; (b) the accuracy of direct thyroid measurements; (c) the relationship between ground contamination and thyroid doses; and (d) the reliability o f the recorded or reconstructed doses for the emergency and recovery operation workers.
421. Finally, it should be emphasized that although those exposed as children and the emergency and recovery operation workers are at increased risk of radiation-induced effects, the vast majority of the population need not live in fear of serious health consequences from the Chernobyl accident. For the most part, they were exposed to radiation levels comparable to or a few times higher than the natural background levels, and future exposures are diminishing as the deposited radionuclides decay. Lives have been disrupted by the Chernobyl accident, but from the radiological point of view and based on the assessments of this Annex, generally positive prospects for the future health of most individuals should prevail.