Yup, epidemiology of non-smoker lung cancer is an active area of research. (Epidemiology of smoker lung cancer is not; it's very well established.) I haven't se

Yup, epidemiology of non-smoker lung cancer is an active area of research. (Epidemiology of smoker lung cancer is not; it's very well established.) I haven't seen this old article, but it sounds perfectly right; the quoted result is probably due to diagnostic criteria. Lung cancer in never smokers (2009) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3170525/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3170525/ shows the field in confusion. Radon, indoor air pollution, second-hand smoking are named as possible sources, but it's still not satisfactory.