The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has published a substantive series of reports concerning sources, effects, and risks of ionizing radiation. This article summarizes the highlights and conclusions from the most recent 1986 and 1988 reports. The present annual per person effective dose equivalent for the world's population is about 3 mSv. The majority of this (2.4 mSv) comes from natural background, and 0.4 to 1 mSv is from medical exposures. Other sources contribute less than 0.02 mSv annually. The worldwide collective effective dose equivalent annually is between 13 and 16 million person-Sv. The Committee assessed the collective effective dose equivalent to the population of the northern hemisphere from the reactor accident at Chernobyl and concluded that this is about 600,000 person-Sv. The Committee also reviewed risk estimates for radiation carcinogenesis which included the new Japanese dosimetry at Hiroshima and Nagasaki. These data indicate that risk coefficient estimates for high doses and high dose rate low-LET radiation in the Japanese population are approximately 3-10% Sv-1, depending on the projection model utilized. The Committee also indicated that, in calculation of such risks at low doses and low dose rates, a risk-reduction factor in the range of 2-10 may be considered.
An obvious correlation between the type of reaction manifested by peripheral blood lymphocytes to low dose irradiation in vitro (adaptive potential), the RBM cell composition (during the period of the major exposure), and the peripheral blood cell composition (at a late time period coincident with the studies of induced radioresistance) has been found in the Techa riverside residents in the later periods after the onset of a long-term low-dose rate radiation exposure (55-60 years later) within a range of individual red bone marrow doses from 0.01 to 1.79 Gy. The nature of these dependences observed in chronically exposed individuals differs from that revealed in the controls. It can be suggested based on the results of the study that the capacity for the adaptive response shown by peripheral blood lymphocytes donated by exposed persons in the remote period after exposure can be regarded as a biological marker of the functional state of the hemopoietic stem cell pool.
Studies of radiation-associated risks among workers chronically exposed to low doses of radiation are important, both to estimate risks directly and to assess the adequacy of extrapolations of risk estimates from high-dose studies. This paper presents results based on a cohort of 45,468 nuclear power industry workers from the Canadian National Dose Registry monitored for more than 1 year for chronic low-dose whole-body ionizing radiation exposures sometime between 1957 and 1994 (mean duration of monitoring = 7.4 years, mean cumulative equivalent dose = 13.5 mSv). The excess relative risks for leukemia [excluding chronic lymphocytic leukemia (CLL)] and for all solid cancers were 52.5 [95% confidence interval (CI): 0.205, 291] and 2.80 (95% CI: -0.038, 7.13) per sievert, respectively, both associations having P values close to 0.05. Relative risks by dose categories increased monotonically for leukemia excluding CLL but were less consistent for all solid cancers combined. Although the point estimates are higher than those found in other studies of whole-body irradiation, the difference could well be due to chance. Further follow-up of this cohort or the combination of results from multiple worker studies will produce more stable estimates and thus complement the risk estimates from higher-dose studies.
[Analysis of the epidemiological data concerning radiation carcinogenic effects and approaches to the low doses' upper limits determination in the aspect of a threshold of the unhealthy influences of ionizing radiation]
The analysis of the epidemiological data regarding cancer mortality in cohorts of Japanese A-bomb survivors and Chermobyl liquidators exposed to different doses suggests that there are good reasons for recognizing the threshold of the radiocarcinogenic effect in the region of about 200 Gy (mSv). The analysis of solid cancer mortality in Japanese cohort, which exceeded the expected one in a dose diapason of 5-200 mSv, revealed a (quasi) plateau in a dose-effect curve and led to the conclusion that the nature of the overshoot is non-radiogenic. The analysis of supposedly dose dependent leucosis incidence in the limited low dose diapason in the Chernobyl cohort showed that the real coefficient of the excess absolute or relative radiation risk could not be received in the case because the larger part curve was placed under the control level. In supporting the principle of single hit in a cell nucleus as a base of microdosimetric determination of low radiation doses, the approach to objective delimitation between low, intermediate and high doses regions has been proposed. The low doses upper limit of sparse ionizing radiation for cell nucleus of 8 microns in diameter has been evaluated as 0.65 mGy. It can serve for evaluation of the dose rate threshold regarding the safe chronic radiation levels in the environment.
Death records were used to analyze cancer mortality in the rural areas of the Kamensky District, Sverdlovsk Region, within the East-Urals radioactive track area. A study group showed a significant increase in cancer mortality as compared with a control group (65 of the 691 examinees; 90% confidence interval (CI) 18-144; the mean colonic radiation dose was 80 and 3 mGy in the study and control groups, respectively). The additional relative risk per colonic dose was 1.3 Gy(-1) (90% CI 0.36-2.9 Gy(-1)). The association of the additional relative risk with the age-related and time factors was studied and revealed.
A review and analysis of published information combined with the results of recent gamma ray surveys were used to determine the annual effective dose to Canadians from natural sources of radiation. The dose due to external radiation was determined from ground gamma ray surveys carried out in the cities of Toronto, Ottawa, Montreal and Winnipeg and was calculated to be 219 microSv. A compilation of airborne gamma ray data from Canada and the United States shows that there are large variations in external radiation with the highest annual outdoor level of 1424 microSv being found in northern Canada. The annual effective inhalation dose of 926 microSv from 222Rn and 220Rn was calculated from approximately 14,000 measurements across Canada. This value includes a contribution of 128 microSv from 222Rn in the outdoor air together with 6 microSv from long-lived uranium and thorium series radionuclides in dust particles. Based on published information, the annual effective dose due to internal radioactivity is 306 microSv. A program developed by the Federal Aviation Administration was used to calculate a population-weighted annual effective dose from cosmic radiation of 318 microSv. The total population-weighted average annual effective dose to Canadians from all sources of natural background radiation was calculated to be 1769 microSv but varies significantly from city to city, largely due to differences in the inhalation dose from 222Rn.
According to the conclusion of the International Programme on the Health Effects of the Chernobyl Accident (IPHECA) Haematology Pilot Project (1991-1995), there was no increase in the incidence of malignant disease in hematopoietic and lymphoid tissues after the Chernobyl accident. Nevertheless, since studies of A-bomb survivors indicate that the peak in morbidity may occur more than 10 years after radiation exposure, long-term studies of hemoblastoses and myelodysplastic syndromes are needed today. Study of these leukemias and lymphomas that are potentially induced by ionizing radiation must include both fundamental and applied approaches, i.e., A) epidemiological design; B) utilization of modern methods of diagnosis (cytomorphology, immunocytochemistry, cytogenetics); C) studies of gene mutations, mechanisms of apoptosis, and G1 delay; D) monitoring of oncogene and multidrug resistance gene expression, and E) tracking changes in cell-cell signaling in the bone marrow microenvironment.
Mutations in senataxin have been described recently in 24 cases of French-Canadian descent with ataxia-oculomotor apraxia 2. This recessive ataxia is associated with an elevation in alpha-fetoprotein as in ataxia-telangiectasia. Because ataxia-telangiectasia cells are highly radiosensitive, we used a colony survival assay to measure the radiosensitivity of lymphoblastoid cell lines derived from five French-Canadian patients with ataxia-oculomotor apraxia 2. Two were homozygous for the common French-Canadian L1976R SETX missense mutation; the three others were compound heterozygotes for the common mutation and three different missense mutations. Overall, lymphoblastoid cell lines derived from these cases did not show significant variation from a normal response to 1 Gray of ionizing radiation but the two patients who were homozygous for the common L1976R mutation fell in the intermediate or non-diagnostic range.