Are active and passive smoking harmful? Determining causation.

Assessing the evidence regarding any causal question involves examining the strength of the studies conducted and applying a series of "diagnostic tests" for causation. We have reviewed the strength of the evidence incriminating smoking as a cause of lung cancer, and passive smoking as a cause of respiratory illness and decreased pulmonary function in children. There are eight prospective studies of smoking and lung cancer which have consistently shown a strong relationship. These studies have confirmed the temporality of the association and demonstrated a dose-response gradient. The studies addressing the effects of passive smoking in children are considerably weaker. Although they are consistent in suggesting increased infections for children less than one year of age, neither increased risk nor a dose-response gradient is consistently found in older children and the effect size, when present, is small. The rules for assessing causation applied here can be used to integrate new information concerning the health hazards of smoking.