The editorial Effect of passive smoking on health responds to a study by Dr James Enstrom of the University of California, Los Angeles and Professor Geoffrey Kabat of the State University of New York, also published in the BMJ today that suggests the link between environmental tobacco smoke and coronary heart disease and lung cancer may be weaker than generally believed.

However, Professor Davey Smith points to several difficulties in studies of passive smoking, such as problems with measurement imprecision, misclassification, confounding factors, and low statistical power, that can lead to the risks being distorted.

The American study Environmental tobacco smoke and tobacco related mortality in a prospective study of Californians, 1960-98 looked at 118,094 California adults enrolled in the American Cancer Society cancer prevention study in 1959, who were followed until 1998. Particular focus was on the 35,561 people who had never smoked but who had a spouse in the study with known smoking habits.

The authors found that exposure to environmental tobacco smoke, as estimated by smoking in spouses, was not significantly associated with death from coronary heart disease or lung cancer, although a small effect cannot be ruled out.

Professor Davey Smith said "Misclassification is a key issue in studies of passive smoking. It is not being married to a smoker - the indicator of exposure to environmental tobacco smoke used in the paper by Enstrom and Kabat -that leads to disease; rather, it is the inhalation of environmental tobacco smoke.

"As an indicator of exposure to environmental tobacco smoke the smoking status of spouses is a highly approximate measure. This will lead to the risk associated with environmental tobacco smoke being underestimated."

Despite this it is certain that this paper will be hailed as showing that the detrimental effect of passive smoking has been overstated, and controversy will continue, he concludes.