Lung cancer mortality among Wismut employees who worked underground is 2.4 times higher than in the general population (Kreuzer et al. 2021). In purely mathematical terms, about 40 % of the 3,942 lung cancer deaths in the Wismut cohort can be attributed to occupational exposure to radon (Kreuzer et al., 2018). Most of these deaths involve individuals who were exposed to extremely high levels of radon in the early years of mining activity.

For radon exposure, the unit Working level month (WLM) is used for miners. The radon exposures of individuals workers in the Wismut cohort (about 8,000 individuals) range between 0 WLM and 3,224 WLM (280 WLM on average).

In the Wismut cohort, the risk of lung cancer mortality increases proportionally with increasing radon exposure (Walsh et al., 2010). A radon exposure of 2,000 WLM resulted in a fourfold increase in lung cancer risk compared with individuals not exposed to radon. However, this radon-related risk increase depends on additional factors such as the time since exposure, the age at exposure, and the rate of exposure. The risk of lung cancer as a result of radon decreases considerably the longer the exposure was in the past (about 60 % every 10 years). However, it remains significantly elevated even after 35 years. Age at exposure also plays a role. For younger workers, a comparably high radon exposure increases the risk of lung cancer more than for older workers. The increase in lung cancer risk per WLM is thus highest 5 to 14 years after exposure and among those under 55 years of age. In addition, there is a higher risk when the exposure is spread over a longer period of time than when it occurs over a shorter period of time.

A more recent evaluation of the risk of lung cancer from radon in the Wismut cohort focused on the low-dose range. It includes, among others, employees with a cumulative radon exposure of less than 50 or 100 WLM and those who started working at Wismut only after radon concentrations had been considerably reduced by various measures. Here, too, a significant correlation was found between lung cancer mortality and cumulative radon exposure (Kreuzer et al., 2018). Even in the low-dose range, the risk of lung cancer from radon depends on the time since exposure, age at exposure, and smoking behaviour. The lung cancer risks from smoking and radon don’t simply add up but rather multiply. In other words, when both risk factors are present, the risk of lung cancer increases particularly strongly.

These findings are relevant for calculating lifetime risks and association probabilities. The latter play an important role in procedures for recognising occupational diseases. The ProZES programme was developed on behalf of the BfS as a tool for this purpose. It calculates the probability that a disease was triggered by a previous occupational radiation exposure. For the lung cancer risk from radon, the results from the Wismut study were used in ProZES.

In addition to occupational radon exposure as a risk factor for lung cancer mortality, occupational exposure to fine quartz dust was also investigated (Sogl et al., 2012). Dust years (mg/m³ years) were used as the exposure unit; one dust year is defined as 1 mg/m³ of fine quartz dust over 220 working shifts of 8 hours each. The total exposure to fine quartz dust of the cohort members ranges from 0 to 56 dust years. A significant increase in the risk of lung cancer with the level of dust exposure was also found for fine quartz dust. If the additional risk factors radon and arsenic are taken into account in the analysis, no significant increase in lung cancer risk is observed up to a dust exposure of 10 dust years. From a total exposure to fine quartz dust of 10 dust years, the risk increases linearly by 6.1 % per dust year. The effects of radon and fine quartz dust seem to add up rather than multiply.

Leukaemia is one of the best-known long-term effects of radiation exposure. For a long time, the estimation of the risk of leukaemia as a result of radiation exposure was based almost exclusively on the study of the atomic bomb survivors of Hiroshima and Nagasaki. These individuals were essentially exposed to a high dose of loosely-ionising radiation on a one-off basis. The results of a study on nuclear workers (INWORKS study) also showed an increased risk of leukaemia in persons with a long-lasting relatively low radiation exposure (Leuraud et al. 2015). Here, too, the influence of loosely-ionising radiation was investigated.

The radiation exposure of the Wismut miners results primarily from exposure to radon and its decay products, which lead to internal radiation exposure when inhaled. The dose is predominantly due to alpha radiation, which is produced during the decay of the radioactive decay products (densely-ionising radiation). However, the miners were also subjected to external radiation exposure, especially external gamma radiation (loosely-ionising radiation). With an average value of just under 50 millisieverts, this external radiation exposure of the Wismut miners is in the low-dose range in contrast to radon exposure as a whole.

The Wismut study therefore offers the opportunity to investigate the influence of prolonged radiation exposure – both loosely-ionising radiation (as in the case of the atomic bomb survivors and the nuclear workers) and densely-ionising radiation – on the risk of leukaemia. So far, there are hardly any findings on the influence of densely-ionising radiation as occurs with exposure to radon and its decay products.

To investigate the risk of leukaemia in the Wismut cohort (Kreuzer et al., 2017), the dose to the red bone marrow resulting from both loosely-ionising radiation and densely-ionising radiation was estimated. Findings from several epidemiological studies suggest that the radiation-related risk of chronic lymphocytic leukaemia (CLL ) differs considerably from the radiation-related risk of other types of leukaemia. The risk for CLL was therefore evaluated separately.

In the Wismut cohort, the risk of death from leukaemia (excluding CLL ) increases with dose from both loosely-ionising and densely-ionising radiation; however, this increase is not significant. Looking at individual types of leukaemia, a significant correlation between the dose from loosely-ionising radiation and chronic myeloid leukaemia (CML) emerges. Likewise, a correlation between the dose from densely-ionising radiation and the occurrence of chronic and acute myeloid leukaemia overall is shown. There is no correlation between radiation exposure and the risk of dying from chronic lymphocytic leukaemia – neither for the dose from loosely-ionising radiation nor for the dose from densely-ionising radiation.

The aforementioned study on the cancer risk of nuclear workers (INWORKS study) also showed the most pronounced risk increase for CML depending on loosely-ionising radiation.

In the Wismut study, the risk of leukaemia increases considerably more with the dose from densely-ionising radiation, which is mainly due to radon and its decay products, than with the dose from loosely-ionising radiation. Whether this can also be observed in other studies remains to be seen.

The highest radiation dose from radon is received by the lungs; to a somewhat lesser extent, the throat/nose/throat area is also affected. Only a negligible part of the radon and its decay products enters the blood and thus the other organs.

Therefore, a relatively small risk increase – if any – for cancers outside the respiratory tract is to be expected. In order to be able to demonstrate a low existing risk in a statistically significant way, large observational studies on individuals with high radon exposure are needed. Previously published miner studies found no evidence of an increased risk of cancer outside the lungs from radon. However, these studies were too small in scope in order to be able to derive stable statements on this. The Wismut study is therefore also important for clarifying this question.

The previous evaluations of the Wismut cohort showed an increase in risk with increasing radon exposure for most of the sites outside the lungs where tumours can develop. However, after taking into account the additional exposure to dust, external gamma radiation, and long-lived radionuclides, the increases were no longer significant (Kreuzer et al., 2008, Walsh et al. 2010). Only for tumours of the upper respiratory tract (mouth, nose, throat, larynx, and trachea) was there a statistically significant correlation in the observation period 1946–2003 (177 deaths) (Kreuzer et al., 2010). After extending the observation period to 2008 (234 deaths), a risk increase was still found; however, it was no longer significant (Kreuzer et al., 2014).

In general, there was no significant correlation between the relevant organ dose of radiation (separated into alpha and non-alpha radiation) and mortality from stomach cancer (n = 592) (Kreuzer et al., 2012), liver cancer (n = 159) (Dufey et al., 2013) or kidney cancer (n = 174) (Drubay et al., 2014). The same applies to exposure to fine quartz dust or arsenic dust.

Until now, it was assumed that the damage caused by ionising radiation concerned mainly the risk of cancer. In the meantime, however, there is increasing evidence of an increased risk of cardiovascular diseases even in the low-dose range – for example, from the study of the atomic bomb survivors of Hiroshima and Nagasaki.

There are few and inconsistent findings from miner studies on this. Therefore, in the Wismut cohort, the risk of dying from a cardiovascular disease was also investigated as a function of the dose from external gamma radiation. The observation period up to the end of 2008 was taken into account (Kreuzer et al., 2013). For the miners exposed, the absorbed dose is 47 millisieverts on average with a maximum value of 909 millisieverts.

By the end of 2008, 9,039 deaths were attributed to cardiovascular diseases. No significant correlation was found here: Neither for all cardiovascular diseases taken together nor for the subgroup of ischaemic heart diseases (heart diseases caused by circulatory disorders; 4,613 deaths) was an increase in risk found with the total dose from external gamma radiation. In the group of people who died of stroke (2,073 deaths), an increase in risk of 44 % per sievert was observed; however, this was not significant.

Between 1946 and 2008, 975 people in the cohort died of silicosis. The mortality from silicosis increases strongly with cumulative exposure to fine quartz dust (Kreuzer et al, 2013, Kreuzer et al. 2021). For an exposure of more than 30 dust years, it is increased 90-fold compared with an exposure of less than 2 dust years. For other non-malignant respiratory diseases studied, including chronic obstructive pulmonary disease (COPD), there was no association with exposure to fine quartz dust or radon.