In those days, women were not heavy cigarette smokers, spent a large fraction of their lives at home and seldom migrated from one place to another. One of the weak points in this analysis is that ecological studies, on which Figure 1 is based, are susceptible to confounding.
Cigarette smoking is the most important cause of lung cancer.
Since data on cigarette sales in the USA are available only on a state-wide level, lung cancer rates were compared with the mean radon levels for each state.