We used three different approaches to evaluate the risk of leukemia and overall cancer in the vicinity of the Finnish nuclear power plants. The leukemia results were consistent for children and did not indicate an excess of the leukemia in the closest inhabited area (5–9.9-km zone) or a general trend in relation to distance from the two sites. A key limitation of our analyses was the small number of cases. Ecological and case–control analyses included 16–17 children with leukemia. However, there was no means to increase the number of cases as our study covered both Finnish NPPs with their entire period of operations. Even if the small sample size increases random error and decreases precision, from a public health perspective, the small number of cases is reassuring.
Similar results were obtained in ecological and cohort analyses of other childhood cancers. There was, however, some indication of an increased overall cancer incidence among boys in 1981–1990 around Loviisa site. This may well be a false-positive finding as the analyses of the three datasets were largely explorative and covered a number of alternative exposure metrics and significance tests.
Results for adults (ages 15 years and older) did not suggest an increase in leukemia and overall cancer incidence, or an association between residential distance from a power plant and leukemia.
We were not able to assess possible effect within the 5-km zone of the NPPs due to the small population size. The number of permanent inhabitants within the 5-km zone of both sites combined is approximately 100.
Ecological analysis suggests that leukemia and overall cancer incidence in the vicinity of planned NPPs in new sites is generally comparable to the rest of the country both in children and adults. There was, however, some indication for an increase in overall cancer incidence among adults in the vicinity of planned NPPs in 1974–2004.
Residential history instead of an address at one time-point was taken into account in the cohort and case–control data. In the cohort analysis, the zones (0–15 km and 15–50 km) around the NPPs were defined by the residence at the end of 1980 and 1990. People may thus have moved out from those zones thereafter, but were still included in the data. In the case–control study, cases were defined as subjects diagnosed with leukemia with residence in the municipalities adjacent to the NPPs. Residential history of these cases as well as their matched controls was constructed since the start-up of the NPPs until the index date. These datasets do not include children who have lived in the vicinity of NPPs and have moved elsewhere just before leukemia diagnosis, but this number is more likely to be very small.
Residential history data had some missing residential coordinates. In children, the percentages of missing residential coordinates were slightly higher in cases (8%) than in controls (4%), whereas in adults, the corresponding percentage was slightly higher in controls (11%) than in cases (6%). The sensitivity analyses showed, however, that the results and conclusions were robust to missing residential data and the assumptions made. In children, the residential history data were complete from 1990 onwards.
Parents’ radiation work (in children) and own history of radiation work (in adults) were taken into account in the analyses as a surrogate for radiation exposure. More detailed information on radiation exposure, cumulative dose, for example, could not be used because the principles for recording doses in the occupational exposure registry have not been consistent during the study period. Current practice to record all doses has been introduced gradually, and earlier doses below a certain time-varying limit may not have been recorded.
Clusters of childhood leukemia have been shown not only in the vicinity of nuclear installations [16
] but also in other locations [17
]. Incidence of childhood leukemia has been increasing since the end of 1970 in the developed countries [18
]. Childhood leukemia is a multi-factorial disease, and ionizing radiation is one of the few well-established risk factors [19
]. Less consistent evidence is available for the roles of non-ionizing radiation [22
], pesticides [23
], common infections, and population mixing [24
The number of residencies could be considered as a surrogate for common population mixing and common infections. Moving involves contact with new populations, with potential being introduced to new infectious agents. This could increase the risk of leukemia as childhood leukemia may be a rare result of a common infection, possibly related to age at contact or immune factors [24
]. The elevated risk of leukemia in children with two or more residencies is in accordance with this hypothesis. Further exploration of this issue in a larger material may be worthwhile.
The mean effective dose due to ionizing radiation in Finland is about 3.7 mSv annually [27
]. Major sources of radiation are indoor radon (2.0 mSv), medical examinations (0.50 mSv), terrestrial radiation, and building material (0.45 mSv), as well as cosmic radiation (0.33 mSv). Actually, the existing NPPs and their surroundings differ in this sense: Olkiluoto surroundings have lower terrestrial background dose and the same relates also to the radon doses in the dwellings of nearby towns compared with Loviisa surroundings. Environmental surveillance for radioactive releases has been conducted by STUK around the Finnish power plants since the start of their operations. Measurable radioactive releases have been rare [28
]. The highest mean doses to the most heavily exposed groups living in the vicinity of Finnish NPPs were estimated in the 1980s, and they were about 1/1,000 of the annual effective dose received from other sources [27
]. Currently, the mean doses to the most heavily exposed groups are 1/10,000 or less of the mean overall effective dose from all sources. Such low radiation doses are not expected to cause any observable increase in leukemia or other diseases.
This study showed no evidence of increased incidence of childhood leukemia around Finnish nuclear power plants. The main limitation was the small sample size owing to small population size in the areas surrounding the plants. Therefore, we could not meaningfully assess the risks within a 5-km zone around the power plants or leukemia at 0–4 years of age. This does not, however, detract from the reassuring findings from the public health perspective.