The long-term irradiation of a large number of population at low doses is to be considered the principal peculiarity of the Chernobyl accident. The medical activities are complicated significantly by the absence of verifiable individual dosimetric information, natural or forced migration of the population, insufficient development of radiation epidemiology, complicated social-economic situation in the country, and other factors which are inevitable at large-scaled catastrophes. Besides, many fundamental questions related to biological effects of action of low doses of ionizing radiation are still being studied.

According to the official estimations, the total release of radioactivity constituted 50MCi (without noble gases) - near 4% of the total in the reactor. This is 1,000 times as high as it was at the accident at Three-Mile-Island APS (USA) in 1979.

* Release on the 1st day of the accident. Activity is decay-corrected to 06.05.1986.

** Total release until 06.05.1986.

At present, the National Radiation-Epidemiological Register is acting in Russian Federation. The Russian Ministry on Emergencies is the general customer of the Register. The head organization is Medical Radiological Scientific Center of RAMS (Russian Academy of Medical Sciences). It fulfils collection of primary medical and dosimetric data through 24 regional centres [7].

Three principal data bases consist of the National Radiation-Epidemiological Register:

• Registration List of those who underwent irradiation. It is created according to special dosimetric categories;
• Chernobyl Register. In 1992, it was officially named Russian State Medical-Dosimetric Register (RSMDR);
• Register of inter-institutional expert councils.

In the present paper we will dwell on the radiation-epidemiological analysis of data of the Chernobyl Register of Russia.

During all years of functioning, the data bank of RSMDR was filled constantly with medical-dosimetric information. By 1 of December 1994, it comprehended 370,120 persons from all the Russian Federation. All persons registered in RSMDR are divided into 5 groups of primary account (GPA):

• 1 GPA - liquidators - 159,027 (43.0%);
• 2 GPA - evacuees - 8,091 (2.2%);
• 3 GPA - those who live or have lived on controlled territories - 185,912 (50.4%);
• 4 GPA - children born by liquidators of 1986-1987 - 16,226 (4.4%);
• 5 GPA - migrants from the zones of alienation (after 1986), resettlement and with right to resettlement - 864 (0.2%) [8].

Special attention is paid to observation of health state of the liquidators which have obtained the highest radiation loading as a result of accident. RSMDR contains data of external irradiation of 125,771 liquidators on the basis of certificates given in the zone of works on liquidation of the accident consequences. Regretfully, it is impossible now to establish types of dose values- absorbed or exposure one - put down in RSMDR. The maximum difference between exposure and absorbed doses for whole-body external irradiation can reach 30%, i.e. absorbed dose can constitute 0.7 exposure dose (this estimation was made on the results of measurements by the Institute of Biophysics of Minzdravmedprom of Russia in the 30-km zone of ChAPS). Considering that the error of each individual value of dose can be much higher than 30%, the mentioned possible difference due to dose types is not taken into consideration in further analysis. The unit of absorbed dose (cGy) is used in the analysis.

In order to understand clearly the character of dosimetric information put down in RSMDR, it is useful to describe briefly when and how the dosimetric surveillance of radiation loading was organized in the zones of liquidation of the accident consequences. After the ChAPS accident, the USSR Governmental Commission charged on 28 of May 1986 with the individual dosimetric surveillance (IDS) three institutions: USSR Ministry of Defence (MD), Ministry of Middle Machine-Industry and Ministry of Energy. The dosimetric surveillance was also carried out independently by USSR Ministry of Interior, USSR KGB (until September 1987), Academy of Sciences of Ukrainian SSR.

The staff of more than 600 organizations of 49 ministries and institutions of USSR was under IDS since 1986. The biggest share of them fell on units of USSR Ministry of Energy.

Before the ChAPS accident, condenser dosimeter of KID type and individual film dosimeter (IFD) were used for controlling irradiation of ChAPS staff. These dosimeters were created at the Institute of Biophysics of USSR Ministry of Health. The measured dose range of them was from 50 mR to 2 R. 400 persons were working in ChAPS at night shift on 26 April of 1986. They were equipped only with IFD [9]. Thereby, at the moment of the accident, ChAPS personnel was not equipped with necessary accident means of IDS as well as the firemen which came to put out the fire on the 4th reactor. Meanwhile, exposure dose rate (EDR) on the ChAPS roof and in premises reached after the explosion several hundreds of R/h.

By the level of reliability, the dosimetric data for liquidators can be divided into three principal groups in dependence of the used methods for dosage assessment:

• Exposure or absorbed dose obtained by using individual dosimeter: maximum error is near 50% (supposing its correct use);
• Group dose attributed to groups of people working within the zone based on individual dosimeter of one of them: maximum uncertainty by the group can reach 300%;
• Itinerary dose evaluated based on the average exposure dose rate in the zone of works and the period of stay there of group of persons: maximum uncertainty by the group can reach 500%.

As a result of the explosion of the 4th reactor of the Chernobyl APS, the territory of Russia was intensively polluted with radioactive substances released into the atmosphere. In the composition of mixture of radionuclides precipitated over the Russian territory, isotopes of iodine, cesium, strontium and transuranium elements are the most radiologically significant. A spot of high radioactive pollution formed in 6 western districts of the Bryansk region. The territory with density of soil pollution for ¹³⁷Cs over 15 Ci/km² stands out there. There are areas with radioactive pollution density up to 5-15 Ci/km² in the Bryansk, Tula, Kaluga and Orel regions. Besides, separate areas with pollution density from 1 to 5 Ci/km² for ¹³⁷Cs were found in the Belgorod, Voronezh, Kursk, Leningrad, Lipetsk, Penza, Ryazan, Smolensk, Tambov, Ulianovsk regions and the Republic of Mordovia. The ⁹⁰Sr content in soil of polluted areas of Russia is 10-100 times as low as that of ¹³⁷Cs. By the data of State Committee of Russian Federation on Hydrometeorology for 1991, the total area of radioactive pollution (more than 1 Ci/km² for ¹³⁷Cs) constituted in Russia 55,000 km² being bigger than that in Ukraine or Belarus.

From the moment of radioactive pollution of area, the population is subjected to external irradiation with gamma- and beta-radiation of mixture of radionuclides - products of nuclear fission and activation. The initial dose rate in the air has decreased tens of times by July 1986 and is determined thereafter mainly by gamma-radiation of ¹³⁷Cs and ¹³⁴Cs. ¹³¹I was the leading factor of internal irradiation of people in May 1986. It entered the organism with food, principally with local milk and vegetables. Radioactive iodine gathered in human thyroid and irradiated it selectively.

By the estimations, the collective dose of external and internal whole-body irradiation from the moment of the accident for the average duration of life can be equal to 5,000 man-Sv (for the districts of the Kaluga region where radioactive precipitation took place). This value constitutes near 10% of expected collective dose for all Russian territories polluted by the accident.

Individual examinations with TL-dosimeters were held also on the territory of western districts of the Bryansk region with pollution density from 0.6 to 4.0 MBq/m². 8,000 individual measurements were held in 1986-1993 in 44 settlements. This number of habitants constituted 90% of all population in the strict control zone.

The highest value of average irradiation dose for adult population was registered in village Vyshkov of the Zlynka district (236 mGy). This settlement is situated close to the zone of increased pollution (1480 kBq/m²) and is of interest in terms of dosimetric studies. The averaged dose for children in given settlement is substantially less (103 mGy). Considerable differences between irradiation doses of adults and children are observed also in other small settlements where the activity of population is connected mainly with agriculture, they do not differ practically among children in different settlements..

For Klintsy town the values of averaged dose for adults are considerably less than those for small settlements (123 mGy) and differ few from the values for children (105 mGy). Incidentally, low level of pollution with radionuclides (near 100 kBq/m² for ¹³⁷Cs [14]) is registered in Klintsy town as a whole, and the calculated averaged irradiation doses by the settlement are much less than measured ones.

RSMDR contains reports on 78 territorial regions of Russia: republics, lands, regions, Moscow, St. Petersburg, and compiled reports (annual) for all Russian Federation and for 5 regions (Bryansk, Smolensk, Tula, Kaluga and Orel) entering the zone of radioactive pollution of Russia - so-called “Chernobyl zone”.

It is stated - on the basis of data of these reports - that the mortality of participants of the ChAPS accident consequences liquidation from all causes (total mortality or simple mortality) increased from 4.6 to 6.9 per 1,000 persons in 1990-1992 period, i.e. the mortality index increased by 50%. This index increases by years for one regions and decreases for other regions, but does not on the average exceed the control level for Russia [13]. For example, until 1991, the death rate among liquidators in the Ryazan region was not higher than regional indices of death rate for the same age. In 1992-1993, the death rate among liquidators was higher than the regional indices 1.3-1.5 times and constituted 12.4 and 15.3 per 1,000 persons per year [14].

According to the results of study of 814 deaths in 1989-1995 of liquidators, the share of oncological diseases (12%) among the causes of death is not higher than the mean indices for Russian population. Cardiovascular pathology occupies the first place among somatic diseases (35%). The most frequent causes of death (over 50%) are accidents among which suicides, alcoholic intoxications and road accidents prevail. By the data of study, 34% of liquidators were at the moment of death in state of middle or strong stage of alcoholic intoxication. Among those who committed suicide, alcohol was found in blood only in 28% of cases, another 72% did it consciously. These data indicate the necessity of deep examination of liquidators in the field of psychoneurological sphere [15].

The forecast and interpretation of data on morbidity and disability indices of liquidators are much more complicated. Comparison of morbidity indices by the principal classes of diseases among liquidators and Russian population as a whole is adduced in Table 5. This table shows that the indices of morbidity among liquidators in a number of cases are many times as high as those among the population of Russia. Undoubtedly, the level, completeness and quality of medical examination of liquidators differ significantly from all-Russia practice. The most modern methods of diagnostics are applied in examining liquidators, and mostly skilled and competent specialists are engaged. By the data of MRSC RAMS, the revealing rate of primarily registered diseases by means of specialists of this institution is several times as high as that by the means of local medical personnel. In such situation it is quite difficult to pick up the adequate control group for comparing.

* - The standardized index on age distribution of liquidators for 1993 is adduced for malignant neoplasms.

It is known that factors of social and psychological character connected with the Chernobyl accident have great importance in the formation of pathological states and morbidity among liquidators. All this combined with radiation influence can be determined as “Chernobyl syndrome”. Attempts to analyse the weight of radiation factor from this very complicated syndromes are of great importance. Therefore, the authors [16] evaluated the indices of morbidity and disability by dosage groups - 0-5 cGy, 5-20 cGy and more than 20 cGy - grounding on dosimetric data of liquidators included in RSMDR. As internal control group the contingents of liquidators irradiated within 0-5 cGy interval were taken therewith.

As it is seen from Table 6, morbidity indices by a number of classes of diseases in groups of 5-20 cGy and more than 20 cGy turned out statistically higher than those in group of 0-5 cGy. It was established therewith that the group of more than 20 cGy consisted by 99.1% of liquidators of 1986-1987.

* - indices differ significantly (p < 0.001) from corresponding indices in group of 0-5 cGy;

** - indices differ significantly (p < 0.01) from corresponding indices in group of 5-20 cGy.

The liquidators of 1986-1987 constitute 91.2% of dosage group of 5-20 cGy and the smaller half of group of 0-5 cGy (48.9%). Therefore, 2 factors were studied within the framework of standard multifactor analysis: dosage (with 3 grades: 0-5 cGy, 5-20 cGy and more than 20 cGy) and date of entering the radiation zone (with 3 grades: liquidators of 1986, of 1987 and of 1988-1990). By the analysis of indices of rate of 3 classes of diseases (endocrine system, blood circulation organs and mental disorders), it was established that the factor of date of entering the radiation zone (1986, 1987, 1988-1990) is undoubtedly the determining one as compared with dosage factor from the point of view of its influence on morbidity. It means that first of all the health state of liquidators of 1986 and 1987 is of special worry.

The indices of disability of liquidators in dependence of obtained external irradiation doses are adduced in Table 7. The disability indices in second and third dose groups are significantly higher than the corresponding coefficients in first group (0-5 cGy). The fact is also worth attention that the index of disability of liquidators on the whole is 2.8-3.2 times as high as the index of all Russia.

Against the background of general aggravation of health state, decrease of birth-rate and duration of life of Russians in the last 10 years, Chernobyl brought a strong additional factor of immediate and mediate effect of ionizing radiation on present and following generations.

The direct result of radiation influence on human organism - beside of acute and sub-acute radiation sickness - is the injury of thyroid tissue by iodine radionuclides and, as a result, impetuous growth of thyroid cancer cases among children and adolescents of both sexes and women of genital age.

In the process of screening of children and adolescents from south-western districts of the Bryansk region in 1991-1995, the primary clinico-laboratoy information on 25,000 persons was obtained including the examination of pediatrist, endocrinologist, ultrasonic investigation of thyroid, analysis of TTH, FT4 content in serum as well as of antibodies to thyroglobulin and microsomal fraction, and the data on ¹³⁷Cs content in organism.

It was stated that thyroid pathology in given groups of persons occupies the leading place in general morbidity structure. Out of all thyroid pathology, 75-80% fall on euthyroid goiter, 0.2-3.7% - on nodal formations in dependence of age group, 0.32-1.7% - on autoimmune thyroidites, 0.1% - on hypothyrosis with clinical manifestation and 5.0-7.8% - on that with subclinical course. As a result of screening, 14 cases of thyroid cancer were first discovered and confirmed histologically thereafter.

In total, 48 cases of thyroid cancer have been verified presently in the Bryansk region among children and adolescents who were children at the moment of the ChAPS accident. 33 girls and 15 boys are among them (Table 8).

Nearly 50% of children and adolescents with diagnosed thyroid cancer have lived on areas with pollution level as 15 and more Ci/km² for ¹³⁷Cs, i.e. in the resettlement zones.

Medical-dosimetric investigation is carried out concerning all cases of thyroid cancer. It has been found that the reconstructed individual dose on thyroid due to radioactive iodine depends on the zone of living and ranges from 1 to 270 cGy. Dose dependence was discovered of thyroid cancer incidence on the density of soil pollution with ¹³⁷Cs. 11.9 cases per 100,000 of children population were revealed on territories with pollution density from 0.1 to 1.0 Ci/km², 18.5 cases - on the territories with 1.0-5.0 Ci/km², 29.7 cases - on the territories with 15 and more Ci/km² [17].

2.8 Malignant neoplasm on territories polluted as a result of the ChAPS accident

Moscow Research Oncological P. A. Gertsen Institute analysed the dynamics of onco-epidemiological situation in the six polluted regions by the Chernobyl accident: Bryansk, Kaluga, Orel, Tula, Ryazan, Kursk, in 1981-1994 period. Uninterrupted rise of malignant neoplasm incidence was registered in all territories and the whole Russian Federation as well. Since 1987, the levels of malignant neoplasm incidence were permanently higher in six mentioned regions than in Russian Federation (Table 9).

The structure of malignant neoplasm incidence did not change substantially in the post-accident period. On all areas, except for the Bryansk region, neoplasm of lungs, trachea, bronchi occupied the first place in the structure, and tumours of stomach - the second place. In the Bryansk region, the most frequent nosological form was cancer of stomach; and tumours of respiration system organs were on the second place. The malignant neoplasm of stomach, blood-forming and lymphatic tissue, thyroid gland, and larynx have a bigger specific weight in the morbidity structure of the Bryansk region than in that of Russian Federation. The share of malignant neoplasm of thyroid in morbidity structure of the Orel region is more significant than that in Russian Federation.

The dynamics of disease incidence among the population of studied territories with cancer of lungs, trachea, bronchi is characterized by positive trend. The highest levels of morbidity are characteristic of the Bryansk, Tula and Ryazan regions. The stomach cancer incidence which is higher than in Russian Federation was registered on radiocontaminated territories both in the pre- and the post-accident periods although the uninterrupted trend of decrease of this index is observed. In 1994, the Bryansk region occupied the second place in Russian Federation by the level of morbidity among men (index - 55.4; Russia - 40.3) and the third place among women. The incidence of malignant neoplasm of blood-forming and lymphatic tissue is higher in the Bryansk, Tula, Ryazan regions than that in Russian Federation. The highest indices in 1994 were registered in the Ryazan region: 18.0 for men and 13.5 for women. The rise of incidence of thyroid malignant neoplasm is observed on all radiocontaminated territories, and the pace of increase is much higher than in Russian Federation. The highest levels of thyroid cancer incidence in Russian Federation were registered in 1994 in the Bryansk, Orel and Ryazan regions; the indices constituted 7.8, 7.7, 7.7 per 100,000 of population respectively [18].

More than 10 years passed since the Chernobyl catastrophe. The biggest technogenic accident in mankind history attracted attention of the world society. However, the problem of assessment of total integral damages for life and health of the irradiated people is still extremely complicated. The negative influence of Chernobyl included the entire range of factors which are causing their mutual strengthening. In particular, neither theoretical models nor practical recommendations for the integral assessment of social and psycho-emotional factors by radiation catastrophes exist at present. From the other side, in order to rehabilitate efficiently the sufferers, it is needed to range and to determine objectively the contribution of both radiation and non-radiation components of influence. Therefore, it is of great practical value to continue many-years studies and to obtain new scientific data in the field of radiation epidemiology .