New York Mayor Michael Bloomberg says his ban on smoking in bars and restaurants, which took effect last month, will save "literally tens of thousands of lives." Anti-tobacco activist Stanton Glantz claims such bans cut heart attack rates in half.
In a political environment where such extravagant claims are credulously accepted, it's useful to be reminded that the scientific debate about the hazards of secondhand smoke is far from settled. A study in the May 17 issue of the British Medical Journal shows once again how tricky it is to measure the effects of environmental tobacco smoke (ETS).
UCLA epidemiologist James Enstrom and State University of New York epidemiologist Geoffrey Kabat analyzed data from an American Cancer Society study that tracked 118,000 Californians from 1959 until 1998. They focused on 35,561 subjects who had never smoked and whose spouses' smoking habits were determined through a questionnaire. As is typical of research in this area, Enstrom and Kabat used marriage to a smoker as an indicator of ETS exposure, subdividing subjects based on how much their spouses smoked.
"No significant associations were found for current or former exposure to environmental tobacco smoke," they report. "The results do not support a causal relation between environmental tobacco smoke and tobacco related mortality, although they do not rule out a small effect. . . . Given the limitations of the underlying data in this and other studies of environmental tobacco smoke and the small size of the risk, it seems premature to conclude that environmental tobacco smoke causes death from coronary heart disease and lung cancer."
Since anti-smoking activists and public health officials confidently assert annual death tolls from secondhand smoke of 50,000 or more, you may suspect that Enstrom and Kabat's findings are unusual. They are in fact similar to the results of most studies looking for a connection between ETS and lung cancer or heart disease. Such research typically finds small, statistically insignificant associations.
If you pool the data together, you can generate statistically significant results. But such meta-analyses tend to mask the weaknesses of the studies on which they're based.
As Enstrom and Kabat note, these weaknesses include the possible misclassification of current or former smokers as lifelong nonsmokers. Because smokers tend to marry smokers and are at an elevated risk of lung cancer and heart disease regardless of their ETS exposure, that kind of mistake could account for the observed associations.