Radon Gas and Passive Smoking
Smokers who are exposed to radon appear to be at even greater risk for lung cancer, because the effects of smoking and radon are more powerful when the two factors are combined.
The basic data for the European pooling study makes it impossible to exclude that such synergy effect is an explanation for the (very limited) increase in the risk from radon that was stated for non-smokers.
A study from 2001, which included 436 cases (never smokers who had lung cancer), and a control group (1649 never smokers) showed that exposure to radon increased the risk of lung cancer in never smokers.
But the group that had been exposed to passive smoking at home appeared to bear the entire risk increase, while those who were not exposed to passive smoking did not show any increased risk with increasing radon level.
This result needs confirmation by additional studies. Despite the startling results from 2001, new studies seem not to have been implemented.