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Chernobyl According To UNSCEAR

Zbigniew Jaworowski
Central Laboratory for Radiological Protection, Warsaw
jaworo@clor.waw.pl



For the past twelve years the health effects of the Chernobyl accident have been studied by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). The results are presented in a 115-page annex to the Committee's 1,220-page magnum opus: the report to the General Assembly "Sources and Effects of Ionizing Radiation", published in September 2000 (1). The two important points that the report makes are, first, comparison of the radiation doses that the average inhabitant of the Earth receives from all types of natural and man-made sources (figure 1); and second, an estimate of the health effects caused by the Chernobyl accident, probably the largest possible catastrophe that can occur at a nuclear power station. This offers the reader a way to compare the man-made radiation hazards, such as Chernobyl or nuclear weapons tests, with the ubiquitous natural radiation. The effects of the Chernobyl were also discussed at the session of UNSCEAR held in April 2001.

According to UNSCEAR 2000, the global-average individual whole-body radiation dose from natural sources is 2.4 mSv per year, with a typical range between 1 and 10 mSv per year. However, in many regions of the world the natural annual doses are much higher, reaching for example, about 35 mSv in India, 80 mSv in France, 150 mSv in the city of Ramsar (Iran) or more than 700 mSv at the Brazilian beaches. No adverse health effects related to radiation, such as cancers or hereditary diseases, were observed among humans and other organisms exposed chronically to such high natural doses (2, 3, 4, 5, 6, 7, 8, 9, 10). Also no adverse hereditary effects of exposure to short-term high-dose irradiation from atomic explosions in Hiroshima and Nagasaki were detected in the children of survivors (11).

An acute radiation syndrome developed in 134 employees of the Chernobyl nuclear power plant and firemen, who received short-term whole-body radiation doses ranging from 800 to 16, 000 mGy. Among these persons 28 died within the first four months of the accident due to radiation sickness, and two more persons died as a result of thermal and mechanical injuries. In the period 1986 - 1995 the average whole-body radiation doses from Chernobyl fallout received by about 190,000 persons living in the most contaminated areas (where the deposition concentration of cesium-137 was more than 555 kBq-m-2), were 5.2 mSv per year in Belarus, 4.0 mSv per year in the Russian Federation, and 9.2 mSv per year in Ukraine. The average doses for the persons living in the less contaminated areas (37 to 555 kBq m-2), were 0.77 mSv per year in Russia, 0.88 mSv per year in Belarus, and 1.1 mSv per year in Ukraine. About 5,160,000 persons are living in these areas, and 336,000 were evacuated. The whole-body doses received by them fit the lower part of the typical range of natural radiation doses. Therefore it seems proper to expect that Chernobyl irradiation of the general population of these three countries will not cause any epidemics of radiation-induced diseases.

Between 1986 and 1989 about 381,000 workers from Belarus, Russia and Ukraine were involved in recovery operations. The average whole-body radiation dose received by these workers was about 100 mSv. Among them about 18,500 workers were irradiated with dose range of 200 - 500 mSv, and 713 emergency workers and accident witnesses received doses higher than 500 mSv. No increase of leukemia and solid cancers, other than thyroid ones, was found among them. According to UNSCEAR 2000, the increased medical surveillance and active follow-up of these workers most likely influenced the thyroid results, particularly when observed numbers were contrasted to national background rates in a not-so-surveyed population.

UNSCEAR (1) stated that 14 years after the Chernobyl accident, apart from thyroid cancers in children, there is no evidence of a major health impact induced by radiation in the general population of contaminated areas. "No increases in overall cancer incidence or mortality have been observed that could be attributed to ionizing radiation. The risk of leukemia (which is the first cancer to appear after radiation exposure owing to its short latency time of 2 - 10 years) does not appear to be elevated ... Neither is there any proof of other non-malignant disorders that are related to ionizing radiation. However, there were widespread psychological reactions to the accident, which were due to fear of the radiation, not to the actual radiation doses". These reactions led to psychosomatic disorders and epidemic symptoms such as headaches, depression, sleep disturbances, inability to concentrate and emotional imbalance, not unlike similar social reactions following major non-nuclear catastrophes, such as floods, earthquakes or landslides.

New epidemiological data analyzed during the UNSCEAR session in April 2001 indicate that all cancer incidence rates among the exposed Belarusian population (recovery operation workers, evacuees from 20-km zone and persons resettled) are lower than that in the general population (standardized incidence ratio, SMR, in 1999 ranging from 0.299 to 0.941). Similar results were found in the Russian residents of contaminated areas and evacuees from the 30-km zone (SMR in 1994-1997 ranging from 0.65 to 0.83).

The number of thyroid cancers (about 1800) 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 are unusual. Other factors may be influencing the risk (1). The increase in the number of thyroid cancers in children and some adults is very likely due to a screening effect. The normal incidence of "occult" thyroid cancers that, while not causing any visible clinical disturbance, are histologically malignant and aggressive, is very high in most countries. They are usually discovered in the course of post mortem pathological examination or an imaging study. The autopsy prevalence of occult thyroid cancer in various countries ranges from 4.5% to 36% (12, 13). These numbers should be compared with the highest incidence of thyroid cancer recorded in the highly contaminated areas of three countries. The highest incidence of 0.027% appeared in the Bryansk region of Russia, where the average thyroid dose in children was 37 mGy. The highest incidence in Belarus of 0.018% was found in Gomel region (thyroid dose 177 mGy), and in Ukraine of 0.005% in Kiev region (thyroid dose in eight districts plus Pripyat 37 mGy) of 0.005% (1). The incidence does not seem directly proportional to thyroid dose.

An indication that the increased incidence rate of thyroid cancers in the three affected countries is an effect of screening comes from new epidemiological data presented at the 2001 session of UNSCEAR. A similar increase of risk as for thyroid cancers was found for lymphatic leukemia, a disease known as not being caused by ionizing radiation. Also among the recovery operation workers, subject to intensive screening, exposed between August and December 1986, i.e. after more than ten half-lives of iodine-131, when its level in air, water and food was already near zero, the risk of thyroid cancer incidence increased similarly as in children in the highly contaminated areas of Belarus, Ukraine and Russia.

In its 2001 report (14), the Committee discussed the increased frequency of Down's syndrome observed in West Berlin in January 1987, and of neural tube defects in small series in Turkey. These observations were not confirmed in larger and more representative series in Europe. The induction of minisatellite mutations in human germ cells was observed in the progeny of those who were resident in heavily contaminated areas in Belarus, but several confounding factors in these studies (e.g., heavy metal poisoning and improper control groups) question the true significance of this finding. Since these are not protein-coding genes, their relevance for risk estimation cannot be readily discerned. The search for genetic effects associated with Chernobyl exposures provide no unambiguous evidence for an increase of Down's syndrome, congenital anomalies, miscarriages and perinatal mortality.

The Committee stated that no increase in genetic diseases was demonstrated that could be an effect of radiation exposure resulting from the Chernobyl accident (14).

According to UNSCEAR, the people in the contaminated regions need not live in fear of serious health consequences from the Chernobyl fallout. They were exposed to radiation levels comparable to natural background, and future exposures are diminishing as the deposited radionuclides decay. From the radiological point of view and based on UNSCEAR's assessments, generally positive prospects for their future health should prevail.




REFERENCES:

1 UNSCEAR, "Sources and Effects of Ionizing Radiation. United Nations Scientific Committee on the Effects of Atomic Radiation UNSCEAR 2000 Report to the General Assembly, with Scientific Annexes" (United Nations, 2000).

2 M. Sohrabi, Recent radiological studies of high level natural radiation areas of Ramsar, J. U. A. M. Sohrabi, and S.A. Durrani, Ed., High Levels of Natural Radiation, Ramsar, Iran (IAEA, 1990).

3 L. Wei, et al., Epidemiological investigation in high background radiation areas of Yangjiang, China, M. Sohrabi, J. U. Ahmed, S. A. Durrani, Eds., High Level of Natural Radiation, Ramsar, Iran (International Atomic Energy Agency, 1990).

4 M. Jamali, Z. Sahabi, The effects of high natural radioactivity on the bone marrow activities of Rattus - rattus in Ramsar: Chromosomal Study, N. Sohrabi, J. U. Ahmed, S. A. Durrani, Eds., High Levels of Natural Radiation, Ramsar, Iran (International Atomic Energy Agency, 1990)

5 T. A. Fazeli, et al., Cytogenetic studies of inhabitants of high level natural radiation area of Ramsar, Iran, M. Sohrabi, J. U. Ahmed, S. A. Durrani, Eds., High Levels of Natural Radiation, Ramsar, Iran (International Atomic Energy Agency, 1990).

6. K. M. K. Nair, et al., Radiation Research 152, S145-S148 (1999).

7. M. V. Thampi, et al., Cytogenetic and epidemiological studies on newborns in the high level natural radiation areas of Kerala, Radiobiology 2000 - International Conference on Radiobiology, Trivandrum, India (2000).

8. G. Jaikrishan, et al., Radiation Research 152, S149-S153 (1999).

9. P. C. Kesavan, Indian research on high levels of natural radiation: pertinent observations for further studies, L. Wei, T. Sugahara, Z. Tao, Eds., High levels of Natural Radiation 1996. Radiation Dose and Health Effects, Beijing, China (Elsevier, Amsterdam, 1996).

10. V. D. Cheriyan, et al., Radiation Research 152, S154-S158 (1999).

11. U. 2001, "Hereditary Effects of Radiation. Scientific annex of UNSCEAR 2001 report to the General Assembly" A/AC.82/R.613 (United Nations Scientific Committee on the Effects of Atomic Radiation, 2001).

12. M. Moosa, E. L. Mazzaferri, The Cancer Journal 10 (1997).

13. G. H. Tan, H. Gharib, Annals of internal Medicine 126, 226-231 (1997).

14 UNSCEAR, "Hereditary Effects of Radiation. Scientific annex of UNSCEAR 2001 Report to the General Assembly" (United Nations Scientific Committee on the Effects of Atomic Radiation, 2001)



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