Smoking is part of a causal chain

Abstract
EDITOR,—Peter S Blair and colleagues suggest that over 60% of cases of the sudden infant death syndrome may be attributable to the effects of parental smoking.1 This depends on the assumption that the association described is causal. While smoking is undoubtedly harmful to babies, the magnitude of the risk is less clear. The close correlation between adverse socioeconomic circumstances and smoking and between risk of the sudden infant death syndrome and deprivation requires that the analysis should take careful account of potential confounding. …