In studies of uranium miners, workers exposed to radon levels of 50 to 150 picocuries of radon per liter of air (2000–6000 Bq/m3) for about 10 years have shown an increased frequency of lung cancer.[49] Statistically significant excesses in lung cancer deaths were present after cumulative exposures of less than 50 WLM.[49] There is, however, unexplained heterogeneity in these results (whose confidence interval do not always overlap).[50] The size of the radon-related increase in lung cancer risk varied by more than an order of magnitude between the different studies.[51]