Increase of Regional Total Cancer Incidence in North Sweden Due to theChernobyl Accident?
Martin Tondel
Division of Occupational and Environmental Medicine, Linköping University, Sweden
Abstract
Study objective. Is an increase of cancer incidence detectable in Sweden due the Chernobyl accident?
Design. The fallout of caesium-137 was studied in relation to the cancer incidence 1988-1996.
Setting. In Northern Sweden, affected by the Chernobyl accident in 1986, we categorised 450 parishes bycaesium-137 deposition: <3 (reference), 3-29, 30-39, 40-59, 60-79 and 80-120 kBq/m2.
Participants. All individuals 0-60 years of age and living in these parishes in 1986 were included i.e. 1
143 182 persons. In the follow-up 22 409 cancer cases were recorded in 1988-1996.
Main results. Controlling for age and various other factors, potentially to influence the result, the adjusted
relative risks for the exposure categories were 1.00 (reference <3 kBq/m2), 1.05, 1.03, 1.08, 1.10 and 1.21.
The excess relative risk was 0.11 per 100 kBq/m2 (95% CI 0.03;0.20). The cancer incidence rate
differences between 1988-1996 and the reference period 1986-1987 in each category were 30.3, 36.8, 42.0,

Figure 1. Parishes in the study area classified by ground deposition of caesium-137 andnumber of parishes in parentheses for each category. The map was originally produced by
the Geological Survey of Sweden on behalf of the Swedish Radiation Protection
Authority, here modified after permission from the latter.
- 147 -45.8, 50.1 and 56.4 per 100 000 persons. No clear excess occurred for leukaemia or thyroid cancer.
Conclusions. A slight exposure-related increase in total cancer incidence has occurred in Northern
Sweden after the Chernobyl accident.
Introduction
In Europe concerns about the consequences of the Chernobyl accident have focused on childhood
malignancies, especially leukaemia, assumed to have a short latency period after irradiation. Severalstudies have been performed outside the former Soviet Union, but none has shown any clear relationshipwith the fallout from the Chernobyl accident. However, children exposed during pregnancy have shown anincreased risk of leukaemia in Greece, Germany and Ukraine, but not in Belarus. A recent study fromUkraine reports an increase in adult leukaemia after exposure from Chernobyl radiation. In Belarus,
Ukraine and the western part of Russia, there has been a dramatic increase in thyroid cancer incidence inchildren related to the accident. In other parts of Europe a similar increase in thyroid cancer has been seenin adults.
Five percent of the released caesium-137 from the Chernobyl accident was deposited in Sweden due
to heavy rainfall on 28-29 April 1986, mainly in the eastern coastal regions from Umeå in the North toStockholm in the South. During the first weeks the main contributors to the radiation dose were shortlived nuclides replaced by long-lived caesium-134 and caesium-137 isotopes. The dose to the inhabitants,depending on place of residence, outdoor activity and dietary habits, ranged from 1-2 mSv to a maximumof about 4 mSv the first year.
Our investigation includes the population of seven counties with the highest fallout, all in Northern
Sweden. There were also unaffected areas in these counties serving as internal reference areas. The Southof Sweden was excluded as much less affected by the fallout, but also because of higher backgroundcancer incidence rates, especially in the largest cities.
Methods
Out of Sweden’s 21 counties, the seven counties included in this study were Norrbotten, Västerbotten,Jämtland, Västernorrland, Gävleborg, Västmanland and Uppsala County. These counties and some major
cities are shown in Figure 1. All individuals in the population registry who were 0-60 years old in 1986
and who had the same address both on 31 December 1985 and 31 December 1987 were included in thestudy. Cancer cases and deaths along with date of diagnosis were retrieved from the Swedish Cancer
Registry for 1986 to 1996. The follow-up period started on January 1, 1988 including 1 143 182 persons
with information on age, sex and parish of residence during the two preceding years. Number of cancer
cases during the follow-up 1988-1996 are given in Table 1.
By assignment with the Swedish Radiation Protection Authority, the Geological Survey of Sweden
had performed aerial gamma-measurements over the entire Sweden from May to October 1986 resulting
in a ground deposition map of caesium-137 in 12 different categories. Collapsing these categories, thepopulation in the 450 parishes, i.e., the smallest administrative units of these seven counties, wereclassified into six exposure categories: <3, 3-29, 30-39, 40-59, 60-79 and 80-120 kBq/m2 (Figure 1 andTable 1). The 117 parishes not affected (<3 kBq/m2) in these counties served as reference areas.
The relative risks were adjusted by population density, smoking habits, socio-economic factors andcancer incidence in the parishes before the accident (1986-1987). The risk difference was calculated as
the age adjusted cancer incidence during the follow-up (1988-1996) minus the cancer incidence in eachexposure category before the Chernobyl accident (1986-1987). Hence, the relative comparison inincidence is within the column and the difference is a horizontal comparison of incidence before and after
the Chernobyl accident.

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Table 1. Number of individuals and cancer cases by exposure category. Adjusted relative risks by
category in relation to the unexposed (reference category). Age standardised risk difference asincidence per 100 000 persons during the follow-up period 1988-1996, minus the incidence 19861987 for each exposure category, respectively. Statistical uncertainty expressed as confidence
interval (95% CI) i.e. a relative risk above 1.00 and a risk difference above 0.00 is statisticalsignificant, respectively.
ExposurekBq
Cs-137/m2

Population
1 January 1988

Number ofcancer cases
1988-1996<3359 509

6 691

3-29

527 812

10 37830-39

92 323

1 827

40-59

124 862

2 744

60-79

21 625401

80-12017 051

368

Total number

1 143 182

22 409Relative risk
(95 % CI)

Risk difference(95% CI)

1.00(reference)
1.05(0.99-1.11)
1.03(0.95-1.12)
1.08(0.94-1.23)
1.10(0.89-1.34)
1.21(0.98-1.49)

30.3(25.5-35.2)
36.8(32.6-41.0)
42.0(33.0-51.0)
45.8(37.9-53.4)
50.1(29.4-70.8)
56.4(33.9-78.9)

Results
The relative risk for all cancer sites showed a slight increase in all exposure categories using <3 kBq
caesium-137/m2 as the internal reference. As an average over the categories an excess relative risk of 0.11
(95% CI 0.03-0.20) per 100 kBq/m2 was calculated. Because of an ageing population it was necessary torestrict the population to 5-59 years of age in order to obtain the risk difference corresponding to 13 823
cancer cases for the follow-up period. The risk difference of 30.3 per 100 000 persons in the reference
category represents the underlying time trend i.e. the increase in cancer incidence not affected by theChernobyl fallout (Table 1). Neither the relative risks, nor the risk differences could be explained bydifferent smoking habits in the exposure categories. Radiosensitive neoplasms with assumed shortlatencies like leukaemia and thyroid cancer did not increase in relation to the radioactive fallout in North
Sweden after the Chernobyl accident.
Discussion
Before the study we realized that if there would be a effect on the cancer incidence due to the Chernobyl
accident it would only be a small such risk. Therefore we designed our study to be as sensitive as possible
to catch such an increase, if existing. As many exposed counties as possible were included, small
geographical areas (parishes) were used for assessing the exposure, we used a two year residence inclusion
criteria as a considerable part of the dose from the Chernobyl accident was achieved during that time andfinally we applied an age restriction i.e. an age span with expected low incidence of malignancies ingeneral. A better exposure assessment would have been obtained if we could have traced the addresses ofeach individual during the follow-up period. However, most likely the majority of the population has
continued to live in the same parish throughout the duration of the study.
Unless simply representing a chance phenomenon, the findings in our study are somewhat
unexpected indicating a possible cancer effect of the Chernobyl fallout in North Sweden despite a shortlatency period and low degree of exposure. This is the first study suggesting a possible increase in total- 149 -

cancer incidence after the Chernobyl accident outside the former Soviet Union, let alone only a marginally
increased risk. Out of the 22 409 cancer cases an estimated 849 cases could be attributed to the fallout
from the Chernobyl accident. However, no less than 494 of these cases are derived from the second
category 3-29 kBq/m2. A slightly different classification of the reference categories would therefore
dramatically influence the number of exposure-related cancer cases.
Using the estimated collective dose in Sweden of about 6 000 man-Sv during 50 years due toChernobyl contamination, and the risk estimates given by the International Commission on RadiologicalProtection, the number of expected extra cancer deaths could be calculated to 300. Given a true effect, ourstudy indicates that the risk from low dose irradiation might come earlier and be slightly higher than
predicted by the International Commission on Radiological Protection estimates. The official risk estimate
relies to a great extent on the follow-up of the atomic bomb survivors in Hiroshima and Nagasaki, but has
been questioned because only those alive in 1950 were included in the study, hence ignoring early cancer
cases.
A short latency period like in our study has been seen in other epidemiological studies on ionisingradiation. Our findings of an increase of total cancer incidence in Sweden soon after the Chernobyl
accident is therefore not a unique finding, but we have not been able to detect any specific cancer site
responsible for our findings. An interpretation could therefore be that the ionising radiation might exert alate stage general promoting effect on cancer.
A larger problem is that the exposure assessment was based on the ground deposition of caesium-137
from the Chernobyl accident, not taking into account any internal dose contribution through food andinhalation. This is especially important in regions with relatively low ground deposition where a highintake of wild berries, mushrooms, game meat et cetera would give a higher internal than external dosefrom caesium-137. Restrictions in food intake due to governmental regulation are likely to have led tolower doses, especially in higher exposed areas, hence lowered the radiation-induced cancer risks for thepopulation in comparison to what would otherwise have prevailed. Such information on an individuallevel is practical is impossible in a large population as in our study.
We could not find an association between the radiation from the Chernobyl accident on thyroid
cancer. Regarding the thyroid cancer the iodine status of the Swedish population is good in contrast tochildren in the former Soviet Union where a dramatic increase has been seen. This is probably making theSwedish population less sensitive in addition to that locally produced food plays a limited role in the areas
with high fallout in Sweden. It is also possible that the radiation levels were too low in Sweden to cause anincreased risk of thyroid cancer.
We were also unable to detect any clear increase in leukaemia during the follow-up in relation to theradiation from the Chernobyl accident. Similarly, not even in the most heavily polluted areas in the former
Soviet Union any clear increase of leukaemia has been observed as yet, except a recent report from
Ukraine. On the other hand, it has been suggested that the established association between ionisingradiation and leukaemia is unique for the relatively high, but short exposure to radiation after the atomic
bomb explosions and maybe not applicable in low dose studies.
The follow-up period is still rather short in our study and a longer study period is necessary for any
more definitive conclusions about a causal relationship between the radioactive fallout in Sweden and thecancer incidence. Should we be observing some late stage promotion effect on ongoing cancer
development in the population, there might even follow a decrease in the cancer incidence with an aboutnormal cumulative incidence over a longer period of time. This remark, however, is a speculative
conjecture to be addressed in future studies.
Reference
Tondel M, Hjalmarsson P, Hardell L, Carlsson G, Axelson O. 2004. Increase of regional total cancer incidence inNorth Sweden due to the Chernobyl accident? J Epidemiol Community Health 58:1011-1016.
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