Has the health effect of passive smoking been overstated?

The link between environmental tobacco smoke and coronary heart disease and lung cancer may be considerably weaker than generally believed, conclude James Enstrom of the University of California, Los Angeles and Geoffrey Kabat of New Rochelle, New York, in this week’s BMJ.

This study will add to the already controversial debate on the health impact of passive smoking.

Their analysis involved 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 never smokers 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 at any time or at any level of exposure. As expected, active cigarette smoking was confirmed as a strong, dose related risk factor for coronary heart disease, lung cancer, and chronic obstructive pulmonary disease.

These findings suggest that environmental tobacco smoke could not plausibly cause a 30% increased risk of coronary heart disease, as is generally believed, although a small effect cannot be ruled out, say the authors.

Despite some limitations, this large study has several important strengths, add the authors. As such, it seems premature to conclude that environmental tobacco smoke causes death from coronary heart disease and lung cancer.

The impact of environmental tobacco smoke on health remains under dispute, writes Professor George Davey Smith in an accompanying editorial. He 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.

Despite suggesting that passive smoking is associated with an increased risk of respiratory disease, the study will be promoted as showing that the detrimental effect of passive smoking has been overstated, he concludes.