Radon and Lung Cancer
The figure below shows maps of radon levels and lung cancers in the USA. The comparison of the two maps makes it clear that counties with higher radon levels generally do not have higher lung cancer rates but lower lung cancer rates. Whereas smoking prevalence, which can confound the results, is not considered in this comparison, the presentation "Radon Health Benefits Awareness Month" shows confounding by smoking cannot explain this observed reduction of lung cancers in higher radon level counties.
Radon level map from: http://www2.lbl.gov/Science-Articles/Archive/radon-risk-website.html
Lung, Trachea, bronchus, pleura cancer mortality map from: http://ratecalc.cancer.gov/ratecalc/
Higher radon counties (green, yellow, red) enclosed in green ovals correspond to mostly lower rates of lung cancer (blue). Higher lung cancer counties (red) enclosed in red ovals correspond mostly to lowest radon areas (blue).