Abstract Reduced ventilation due to energy saving has focussed interest on a potential lung cancer risk from increased indoor concentrations of alpha-emitting radon and radon daughters, escaping from building materials and from the ground. Some preliminary studies now also indicate a hazard to be present as related to radon daughter exposure in homes. However, the indoor radon daughter levels have probably been continuously increasing for half a century, especially in colder climates, due to the introduction of central heating instead of stoves and open fire places, reducing thermal ventilation. Furthermore, in our time, many people have got additional exposure through extended indoor work time instead of earlier outdoor activities in farming etc. The steeply increasing lung cancer rates over the past decades as well as the various oddities affecting the relationship between smoking and lung cancer, e.g. the urban-rural difference in lung cancer risk, also after standardization for smoking, the influence of immigration on lung cancer morbidity as well as the varying rates over the world and other observations, would obtain simple explanations by taking radon daughter exposure into account in addition to smoking. Then, also some curious and hitherto unexplained “inverse” relationships between lung cancer and inhalation of cigarette smoke or bronchitis in air-polluted areas, respectively, would become understandable.