Background A causal association between exposure to radon and its progeny and lung cancer is well established. However, a better knowledge on effect modification by time, age and exposure rate and on risk at low exposures or exposure rates is of high importance.

Method We analysed 58 974 male uranium miners of the German Wismut cohort with a mortality follow-up from 1946 to 2013 and a sub-cohort of 26 765 miners hired after 1960 characterised by radon values at low exposure-rates. Internal Poisson regression was applied to estimate the Excess Relative Risk (ERR) per unit of cumulative radon exposure in Working Level Months (WLM) with exponential time-related effect modifiers.

Results In the full cohort the crude ERR/WLM was 0.0019 (95% confidence interval (CI): 0.0017;0.0022) based on 3.947 lung cancer deaths. Age at median exposure, time since median exposure, and exposure-rate were strong effect modifiers. Taking them into account led to an estimate of 0.0067 (95%CI: 0.0052;0.0087) for an age at median exposure of 30 years, a time since median exposure of 20 years, and an exposure-rate of 3 WL (strong inverse exposure-rate effect). In the 1960+ sub-cohort, the crude ERR/WLM was 0.0111 (95%CI: 0.0064;0.0173) based on 495 lung cancer deaths. In this relatively young sub-cohort, time since median exposure was also a strong modifier, leading to an ERR/WLM of 0.0165 (95%CI: 0.0083;0.0247) at time since median exposure of 20 years.

Conclusion The present findings provide evidence for an increased lung cancer risk also at low exposures or exposure rates among miners.