Wet drilling (click on the picture for a larger view).

Figure 2: Radiation was exclusively measured with the objective to locate uranium bonanzas.

Figure 3: Spraying installations for dust abatement during charging.

Figure 4: Mechanized ore drilling.

Mining has a century old tradition in Saxony and Thuringia. In the southern region of the Ore Mountains (Erzgebirge) mining yielded copper, nickel and tin, besides silver, cobalt and bismuth. After World War II, also the production and processing of uranium took place on a mayor scale. This occurred on orders of the soviet military administration who needed the uranium for its A bomb program. In the beginning, the whole operation was kept top secret. The official operating company behind the mining activities was the Soviet incorporation with the code name “Wismut” (the German name for the chemical element bismuth) and later on the “Soviet-German-incorporation” (SDAG) Wismut. Active uranium mining ended to 31 December 1990. During the whole period of production, about 231,000 tons of uranium ore were extracted, hence, up to 1990, the German Democratic Republic (GDR) was the third largest producer of uranium ore worldwide. In the period from 1946 to 1989, about half a million people were employed in saxonian-thuringian uranium mining.

Working conditions at Wismut

Uranium extraction flourished at a time when effective radiation protection regulation did not yet exist. In addition to the hard working conditions underground, the miners were exposed, especially during the first years, to high levels of radiation, mainly due to the radioactive noble gas radon and its progeny. Working conditions can be classified roughly in three time periods:

"The wild years" (1946 to 1954)

• many miners (about 100,000),
• high exposures,
• no radiation and labor protection,
• dry drilling with high dust loads,
• no radon measurements,
• natural ventilation.

Transition period (1955 to 1970)

• 30,000 to 40,000 miners,
• broad range of exposures,
• first measures of radiation protection,
• radon measurements,
• wet drilling.

Consolidation period (1971 to 1989)

• constant number of about 20,000 miners,
• radiation surveillance on an individual basis, low exposure,
• international standard in radiation protection; measurement of radon progeny.

Up to 1999, 7,695 cases of lung cancer among employees of the Wismut were recognized as being radiation induced for compensation and about 200 new cases show up every year since then. From this fact arises a central objective for the radiation protection in Germany in the following years. The health effects of an employment at the Wismut are to be documented and analyzed. From this information, eventual future risks may be inferred and working and radiation protection measures, also for other areas, should be derived. Hence the BfS, with support of the Federal Ministry for the Environment and of the European Union (EU), conducts since 1993 the German uranium miner cohort study in the framework of the "Alpha-Risk" project.

The cohort study

In close collaboration with the German Social Accident Insurance (DGUV), the BfS selected randomly a cohort of about 59,000 former employees of the Wismut company. For each of these individuals the working history was reconstructed from the records of the employer. By means of a job-exposure matrix the individual annual exposure to radiation was assessed. A mortality follow-up is conducted each five years to determine the vital status and in case of death the cause of death via registration offices and the health authorities. The first follow-up to the end of 1998 and the second follow-up to the end of 2003 are already completed. The third follow-up to the end of 2008 started with the beginning of 2009.

About 8,250 of the 59,000 cohort members were never exposed to radon although being employed at the Wismut company. These persons form a control group for the estimation of the spontaneous mortality of cancer. The aim is to investigate whether the likelihood of cancer mortality increases as a function of the exposure to radon and its progeny. For lung cancer, a positive relationship with radon exposure has already been demonstrated in other miner studies. However, the question about the risk of low radon exposures or about the effect of a combined exposure to dust and arsenic is still open. It is also not clear whether radon is also involved in the genesis of other types of cancer (for example, leukemia or cancers of the nasopharyngeal region) or of non-cancer diseases.

Results

Lung cancer

The analysis of the lung cancer risk (second follow-up until end of 2003) mainly concentrated on the re-evaluation of the established knowledge on the association of occupational radon exposure and lung cancer risk. This knowledge was based on the joint analysis of eleven miner cohorts from seven countries, for which the data had been pooled. These eleven cohorts taken together are with about 60 000 miners not appreciably larger than the German cohort on its own which comprises 59,000 persons. Hence the available data basis world wide on the knowledge about the effects of occupational radon exposure on health could nearly be doubled with the German cohort.

The estimation of the lung cancer risk per unit of exposure - for which “Working Level Months” (WLM) is in use in the case of radon exposure for miners - varies between the cohorts by more than a factor of 10. 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 which a miner has spent in the respective environment. The analyses done up to now show that the risk rises with increasing total exposure. However, at the same time the risk decreases with increasing time since exposure, with the risk being highest about five to 14 years after exposure and then decreases by about a factor of 2 for every period of ten years. Furthermore it decreases with increasing attained age. The risk is also a function—depending on the method of analysis—of either the duration of the exposure or of the exposure rate. The lower the exposure rate the higher the risk per WLM.

In comparison with the eleven cohorts, the German cohort is more homogeneous and therefore represents not only because of its size an excellent basis for testing present theories. Up to the end of 2003, 3,016 deaths from lung cancer have been observed in the German study. The total exposure to radon varies between 0 and more than 3,000 WLM. The German study shows that the lung cancer risk rises with increasing total exposure by about 0.19 per cent per WLM. The highest risk is observed about five to 15 years after exposure, with the risk decreasing with both time since exposure and age. However, the decrease of the risk with time is somewhat less pronounced than in the preceeding study. The risk also decreases with increasing exposure rate to radon although also this decrease is less pronounced than in the preceeding study. Taking all factors together (total exposure, time and age effects), the estimated risk to die from lung cancer due to radon exposure in the earlier analyses and from the analysis based on the German data is not significantly different. The corresponding results of the second follow-up were published in „Radiation Research“ in the year 2010 (see also Grosche et al., 2006).

Cardiovascular Diseases

Up to now it was assumed that stochastic somatic damages of ionizing radiation are mainly related to the risk of cancer. However in the meantime sporadic evidence for an increased risk of cardiovascular diseases arose, for example, in the study on the atomic bomb survivors of Hiroshima and Nagasaki. There are only a few systematic studies on the topic of cardiovascular diseases and radiation. Spare and inconsistent results have been presented in some miner studies. Therefore the dependence of the risk of death from cardiovascular diseases on exposure to radiation (radon, external gamma radiation and long-lived radionuclides) was studied within the German uranium miners cohort including 5,417 cases of death due to cardiovascular diseases in the follow-up period to the end of 1998. With respect to all three radiation sources no increased risk for cardiovascular diseases was found. This was also true for the sub-group of cardial diseases and cerebrovascular diseases. The relevant results on this topic were published in Journal "Radiation Environment and Biophysics" in 2006.

In the second follow-up until the end of 2003, up to now only the relation between radon exposure and mortality from cardiovascular diseases has been studied (also separately for cardiac and cerebrovascular diseases) (Kreuzer et al, 2009c). While 7,395 deaths due to cardiovascular diseases were taken into account, again no increase of risk due to radon exposure was found.

Extrapulmonary (non-lung) tumors

Radon is a radioactive noble-gas. The main dose of radiation is received by the lung and to a minor degree the nasopharyngeal region, while only a small fraction of radon enters the blood stream and thus reaches other organs. Therefore, one has to assume - if at all - only a relatively small increase of the risk for tumors outside the respiratory tract. To detect a small risk with statistical significance one needs large observational studies of highly radon exposed individuals. The miner studies published up to now did not show evidence for an increased risk of non-lung tumors due to radon. However, the size of these studies was too small to allow for a stable risk estimates for non-lung tumors due to radon. Therefore the Wismut study is of high importance, here.

Overall a total of 3,355 deaths from non-lung cancers occurred in the Wismut cohort in the follow-up period up to 2003. A statistically significant increase of the risk for all non-lung cancers combined with increasing radon exposure was observed. The overall increase of the risk is with 0.014 per cent per WLM considerably lower than that for lung cancer with 0.19 per cent per WLM. In a separate analysis of the different tumor localizations a statistically significant increased risk was found for the tumours of the upper respiratory tract (mouth, nose, pharynx and larynx). The increase of risk is 0.062 per cent per WLM; about a third of the risk increase for lung cancer. Also for the mayority of the other tumor localizations, an increased risk due to radon exposure was found, however, in no case was it statistically significant.

Taken together the results of the Wismut study give indications for an increased risk for non-respiratory-tract tumors due to radon, especially for the cancers of the upper airways. However, it cannot be excluded that the observed increased risks are due to chance or confounding. The respective results were published in 2009 in "Radiation and Environmental Biophysics" and 2008 in the "British Journal of Cancer".