• Clinical epidemiology
• tobacco and the lung
• asthma epidemiology

Pool and colleagues make an important point about exposure to tobacco smoke1 but we are uncertain of the relevance to our paper,2 which reported the effects of smoking by parents as a behaviour rather than source of passive exposure. While we cannot discount the possibility that children of smokers are more likely to become smokers as a consequence of passive smoke exposure, our interpretation is that it is the behaviour that is responsible.