From what I can tell, they didn't. The theoretical basis for it is the Linear No-Threshold Model, which is, bluntly put, garbage. The empirical evidence seems t

From what I can tell, they didn't. The theoretical basis for it is the Linear No-Threshold Model, which is, bluntly put, garbage. The empirical evidence seems to come mostly from so-called case-control studies, which are, bluntly put, garbage. You conduct a case-control study as follows: 1) Select some number of lung cancer patients and a control group without lung cancer. 2) Try to establish why the cancer patients got cancer and the non-cancer group didn't, by retroactively estimating their smoking habits, radon exposure (from radon maps or retroactive measurements of places they lived at), as well as other factors. 3) Use statistics to try to disentangle suspected radon cases from smoking (90%) and other stuff. 4) Pretend that all this is not totally biased, sampling or otherwise.