Changing exposure: passive smoking.

Major effects on health postulated to result from passive smoking are increased risk of lung cancer in spouses of smokers and increased respiratory conditions in children of parents who smoke. Prospective studies have added little to knowledge of the first, but could potentially differentiate between the several causal hypotheses for the second. A number of studies have collected respiratory symptoms on lung function data on several occasions, but few have analysed them in relation to changing exposure. Only sufficient data on children whose parents change their smoking habits considerably can differentiate between effects that are immediate, reversible and non-cumulative, irreversible effects on infants only, and cumulative effects.