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Wismut uranium miners cohort study
The Wismut cohort is one of the largest cohort studies of miners occupationally exposed to radon. The cohort comprises almost 59,000 male employees who were working in uranium ore mining in the former German Democratic Republic between 1946 and 1990. The Federal Office for Radiation Protection (BfS) has been conducting this study since 1993 to scientifically investigate the health effects of mining-related activities. The large scale, the long observation period and the wealth of information about different risk factors make this study unique. The cohort allows for the investigation of various research issues and previous results were published in many publications. On request, the BfS provides data to interested external scientists to investigate their own specific research questions.
Mining has a tradition over many centuries in Saxony and Thuringia. In the southern region of the Ore Mountains (Erzgebirge) mining yielded copper, nickel and tin, silver, cobalt and bismuth. After World War II, the production and processing of uranium took place on a major scale. Initially, uranium mining was by command of the soviet military administration because the uranium was required for the soviet A-bomb program. The whole mining operation was kept top secret from the beginning.
The official operating company behind the mining activities was the Soviet corporation with the code name "Wismut" (the German name for the chemical element bismuth) and later on the "Soviet-German-corporation (SDAG) Wismut".
During the whole period of operation, about 231,000 tons of uranium ore were extracted. Hence, the German Democratic Republic (GDR) was the third largest producer of uranium ore worldwide up to 1990. In the period from 1946 to 1989, about half a million people were employed in saxonian-thuringian uranium mining. After the German reunification active uranium mining ended.
Uranium extraction flourished at a time when effective radiation protection regulations did not exist. In addition to the hard working conditions underground, the miners were exposed to high levels of radiation, mainly due to the radioactive noble gas radon and its progeny especially during the first years of mining. Working conditions can be classified roughly in three time periods:
"The wild years" (1946 to 1954)
- many miners (about 100,000),
- high radiation exposures,
- no well-established radiation and labor protection,
- dry drilling with high dust exposure,
- no radon measurements,
- natural ventilation.
Transition period (1955 to 1970)
- 30,000 to 40,000 miners,
- broad range of exposures,
- first measures of radiation and labor protection,
- wet drilling
- radon measurements in objects and shafts,
- artificial ventilation due to mine fans.
Consolidation period (1971 to 1989)
Health effects of an employment at the Wismut company
In the period from 1952 to 1990 5,275 cases of lung cancer and 14,592 cases of silicosis were recognized as occupational disease. After the German reunification, further approximately 3,800 cases of lung cancer and approximately 2,500 cases of silicosis were acknowledged by the statutory accident insurance as occupational diseases until 2014. The investigation of health effects among Wismut employees is an essential task for radiation protection in Germany. Thus, possible future risks can be evaluated and new labour and radiation protection measures can be derived.
The Federal Office for Radiation Protection (BfS) conducts the German uranium miner cohort study with support of the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) since 1993. In close collaboration with the German Social Accident Insurance (DGUV), the BfS randomly selected a cohort of about 59,000 former employees of the Wismut Company. The working history of each of these individuals was reconstructed from corresponding employment records. By means of a job-exposure matrix the individual annual exposure to radiation was assessed. A mortality follow-up is conducted every five years to determine the vital status and - in case of death - the cause of death via registration offices and health authorities. Up to now the following follow-up periods have been completed:
- first follow-up with cut-off date Dec, 31st 1998
- second follow-up with cut-off date Dec, 31st 2003
- third follow-up with cut-off date Dec, 31st 2008
- fourth follow-up with cut-off date Dec, 31st 2013.
For each cohort member time, duration, type and place of work have been assessed. Subsequently, individual annual exposure values were determined for the different risk factors with a job-exposure-matrix. The most important risk factors are radon and its progeny and silica dust followed by long-lived radionuclides from uranium dust as well as external gamma radiation and arsenic.
In the cohort study mortality risk due to occupational radiation and dust exposure is investigated. For lung cancer, a positive relationship with radon exposure has already been demonstrated in other miner studies. Beyond that, several questions could not be satisfactorily answered in previous studies: This includes the risk of low radon exposures, the effect of a combined exposure to radon, dust and arsenic, or the question whether radon is also involved in the genesis of other types of cancer (for example, cancers of the nasopharyngeal region or leukemia) or of non-cancer diseases.
Currently, a worldwide "pooling" project called PUMA (Pooled Uranium Miners Analysis) is running, including miner cohorts from the EU, US and Canada.
Results of the cohort study
Lung cancershow / hide
In the Wismut cohort the lung cancer risk increases proportionally with increasing radon exposure. The exposure to radon and its progeny is expressed in so-called "working level months" (WLM). The total exposure in WLM is calculated as the product of the energy emitted by the short lived radon daughters—or, in more simple terms—the radon concentration in one liter of air and the time a miner has spent in the respective environment.
Assigned radon exposures of the Wismut employees ranged from 0 (about 8,000 persons) to 3,224 WLM (mean = 280 WLM). Compared to non-exposed subjects, an exposure of 2,000 WLM resulted in a four-fold increase in lung cancer risk. However, this increase is also influenced by other factors like time since exposure, attained age and level of exposure. The increase in lung cancer risk per WLM is highest 5 to 14 years after exposure and among persons under 55 years of age. With increasing time since exposure lung cancer risk decreases by about a factor of 2 every ten years, but remains significant even after 35 years. Furthermore, the risk decreases with increasing attained age by 30 % every ten years. In addition a higher risk was observed when exposure was protracted over a longer period of time.
An analysis of the risk of death from lung cancer in uranium miners hired in 1960 or later, and thus exposed to rather low levels of radon, found a statistical significant association between lung cancer mortality and cumulative exposure to radon even in those low levels of radon exposure (Kreuzer et al. 2015). The mean radon exposure for those miners was 17 WLM which is only one tenth of the mean radon exposure in the whole cohort.
Additionally, the role of occupational quartz fine dust exposure as a risk factor for lung cancer mortality was analyzed (Sogl et al., 2012). The unit of exposure is dust-years (mg/m³-years). One dust-year is defined as an exposure to 1 mg/m³ quartz fine dust for 220 shifts of 8 hours. Cumulative quartz fine dust exposures range between 0 and 56 dust-years. Again, a significant increase in lung cancer risk with increasing dust exposure was observed. After adjustment for radon and arsenic, up to 10 dust-years no statistically significant increase of risk was observed. Above 10 dust-years, the risk increased linearly by 6.1 % per dust-year. The combined effect of radon and quartz fine dust is rather additive than multiplicative.
Leukaemiashow / hide
Leukaemia is a well-known long-term effect of radiation exposure. For a long time, the estimation of the risk for leukaemia has almost completely been based on studies on atomic bomb survivors of Hiroshima and Nagasaki. These persons were primarily exposed to a unique high dose of low-LET radiation. Recently published results of a study of nuclear workers (INWORKS study) showed an elevated risk for leukemia in subjects with a relatively low long-term radiation exposure (Leuraud et al. 2015). Exposure to low-LET radiation was also investigated in this study.
Wismut uranium miners were mainly exposed to radon and its progeny, leading to an internal radiation exposure due to inhalation. The corresponding dose can mainly be attributed to alpha radiation from radioactive decay products (high-LET radiation). In addition, the uranium miners were exposed to external radiation, particularly due to gamma radiation (low-LET radiation). In contrast to the exposure to radon and its progeny, this external radiation exposure with a mean of almost 50 millisievert is located in the low-dose range.
Therefore, the Wismut study offers the opportunity to examine the impact of the long-term radiation exposure on leukaemia risk both for low-LET radiation (as for the atomic bomb survivors and the nuclear workers) and for high-LET radiation, where so far findings are rare.
To examine the leukaemia risk in the Wismut cohort (Kreuzer et al., 2017), the doses for the red bone marrow were estimated from low-LET as well as from high-LET radiation. Since there is evidence from several epidemiological studies that the radiation-related risk for chronic lymphatic leukaemia (CLL) differs from other radiation-related risks for other types of leukaemia, the risk of CLL was analyzed separately.
There was a positive non-significant dose-response for mortality from non-CLL in relation to low-LET and high-LET radiation. Assessing subtypes of leukaemia, a statistically significant excess risk was found for the subgroup chronic myeloid leukaemia in relation to low-LET radiation and for the subgroup myeloid leukaemia (combining chronic and acute myeloid leukemia) in relation to high-LET radiation. Results indicate no association of death from CLL with either type of radiation. The leukaemia mortality risk was higher for high-LET radiation arising from radon and its progeny than for low-LET radiation.
Extrapulmonary (non-lung) tumoursshow / hide
Radon is a radioactive noble-gas. The main radiation dose is received by the lung and to a minor degree by the nasopharyngeal region. Only a small fraction of radon enters the blood stream and thus reaches other organs. Therefore, it is plausible that the risk for tumours outside the respiratory tract is increased only very little if at all. Such low risks can only be detected in large observational studies with high radon exposures. Miner studies published up to now did not show any evidence for an increase in risk for tumours outside the lung due to radon. However, this may be due to the small size of these studies. The Wismut cohort might contribute substantially to answer this question.
Increases in risk with increasing radon exposure were found for several of the considered types of tumours. However, after adjustment for exposure to dust, external gamma radiation and long-lived radionuclides increases were no longer statistically significant (Kreuzer et al., 2008, Walsh et al. 2010). Only for tumours of the upper respiratory tract (mouth, nose, pharynx and larynx) a statistically significant increase in risk was found for the study period from 1946-2003 (177 cases) (Kreuzer et al., 2010). When follow-up was extended until the end of 2008 (234 cases), the increase in risk was no longer statistically significant (Kreuzer et al., 2014).
In general, neither mortality from stomach cancer (Kreuzer et al., 2012), nor mortality from liver cancer (Dufey et al., 2013) or from kidney cancer (n=174) (Drubay et al., 2014) was found to be associated statistically significantly with the respective organ dose from ionizing radiation (alpha and non-alpha radiation considered separately). The same is true for exposure to quartz fine dust or arsenic dust.
Cardiovascular Diseasesshow / hide
Until recently it was assumed that injuries due to ionizing radiation are mainly related to the risk of cancer. However, in the last years evidence for an increase in risk for cardiovascular diseases at low-dose exposures has been found for example, in the study on the atomic bomb survivors of Hiroshima and Nagasaki.
Evidence from miner studies is sparse and inconsistent. Therefore, the association between mortality due to cardiovascular diseases and external gamma radiation dose was investigated with data from the German uranium miner cohort including 9,039 cases of death due to cardiovascular diseases to the end of 2008 (Kreuzer et al., 2013). Among exposed miners the mean cumulative external gamma exposure is 47 millisieverts with a maximum of 909 millisieverts. In contrast to the lung, the relevant radiation dose here is mainly determined by gamma exposure.
Neither for the group of all cardiovascular diseases nor the subgroup of ischemic (caused by disturbed blood flow) or heart diseases (4,613 deaths) an increase in risk with gamma exposure has been observed. In the subgroup of cerebrovascular diseases the mortality risk is increased by 44 % per sievert. However, this increase is not statistically significant.
Silicosis and other non-malignant respiratory diseasesshow / hide
Within the Wismut cohort, 975 persons are known to have died from silicosis. Mortality from silicosis increases strongly with cumulative quartz fine dust exposure (Kreuzer et al, 2013). For exposures over 30 mg/m³-years a 90-fold increase is observed compared to exposures under 2 mg/m³-years. For other non-malignant respiratory diseases - including chronic obstructive pulmonary disease (COPD) – no significant association with quartz fine dust or radon exposure has been observed.
Currently, the association between the risk of death from lung cancer and radon exposure is being examined with the newest follow-up (1946-2013).
Besides, the Wismut cohort is part of a worldwide pooling called PUMA (Pooled Uranium Miners Analysis), including miner cohorts from the EU, US and Canada.
Analyses of data from female Wismut employees are planned. These have not been considered in the analyses up to now as only few of them had worked underground and were exposed to radiation.
The German Wismut uranium miner cohort offers the opportunity to gain new insights relevant to radiation protection and to broaden the scientific basis for the acknowledgment of occupational diseases.
Lung cancer risk among Wismut employees is clearly increased due to occupational radon exposure also with low radon exposure rates and due to exposure to quartz fine dust. The radon-related lung cancer risk depends on further effect-modifying factors such as time since exposure, age at exposure and exposure rate. Also a radiation-related increase in risk of death from leukaemia was observed. However, this increase was not statistically significant. For specific subtypes of leukaemia significant associations were found. Furthermore, a strong increase of mortality from silicosis with increasing exposure to silica dust was observed.
Regarding the other investigated causes of death and risk factors no statistically significant increase in risk has been observed. However, with extended follow-up further valuable findings concerning diseases that rarely occur in the cohort are expected.
State of 2018.02.02