A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Followup: Does second-hand smoke cause heart disease?

July 7, 2000

Dear Cecil:

Does environmental tobacco smoke (ETS) seriously threaten the general public's health? The June 2 Straight Dope claimed this threat is "unproven at best." To your credit, you did concede that this smoke is a "danger to vulnerable folk such as asthmatics, children, and the elderly" and is "harmful, broadly speaking." You also found "impressive" the smoking opponents' list of official pronouncements and studies, 63 of which found "some evidence of harm from ETS."

So I wonder why the evidence in these 63 studies still left you suspecting the tobacco industry "may be right" in denying any link between passive smoking and lung cancer. The link may not be 100 percent, but it's there (except, perhaps, in most studies by authors with ties to the tobacco industry).

And what about the studies linking secondhand smoke to heart disease, hardening of the arteries, and stroke? A study of 32,000 nurses by the Harvard School of Public Health and Harvard Medical School found a 91 percent greater risk of heart disease for those regularly exposed to other people's smoke. According to the study's main author, "there may be up to 50,000 Americans dying of heart attacks from passive smoking each year."

Cecil replies:

Let me go through this a step at a time. First I'd better make a confession. I said the Journal of the American Medical Association published a review of 100 studies, 63 of which found ETS was harmful. Actually, the review looked at 106 studies and found evidence of harm in 67, or 63 percent. Not a huge difference, but one strives for perfect accuracy.

You ask "why the evidence in these [67] studies still left you suspecting the tobacco industry 'may be right' in denying any link between passive smoking and lung cancer." Let's not mix apples and oranges, bud. The 106 studies covered an assortment of ailments; only 27 had to do with lung cancer.

The article, "Why Review Articles on the Health Effects of Passive Smoking Reach Different Conclusions," is an interesting piece of work. The authors write, "Our a priori hypotheses were that review articles concluding that passive smoking is not harmful would tend to be poor in quality, published in non-peer-reviewed symposium proceedings, and written by investigators with tobacco industry affiliations." Sure enough, three-fourths of the studies concluding that ETS was not harmful were written by investigators with tobacco industry affiliations (although sometimes the affiliations were tenuous; e.g., participation in at least two industry-sponsored symposia). The authors found, however, that these studies weren't low quality and didn't appear mostly in non-peer-reviewed publications. In short, while the investigators were friendly with Big Tobacco, they weren't necessarily producing bad science.

Let's turn to the claim that ETS is linked with heart disease, and in particular to the study of 32,000 nurses, commonly known as the Kawachi study after its lead investigator. A number of studies in recent years have found a weak link between ETS and heart disease — on the order of 20 to 25 percent increased risk. These studies are free of the obvious statistical sleight of hand that marred the government report declaring ETS was carcinogenic. (For one thing, every study I saw used a 95 percent "confidence interval," a measure of statistical reliability, rather than the less rigorous 90 percent CI used in the government report.) Nonetheless, 20 to 25 percent is a small increase in epidemiological terms, and even some passive-smoke opponents concede that "confounding factors" may yet be found.

The Kawachi study illustrates the problems with this type of research. The study involved a subset of the participants in a larger long-term project called the Nurses' Health Study. In 1982 participants were asked, "Are you currently exposed to cigarette smoke from other people?" The question was vague, no follow-up questions were asked in later years, and no attempt was made to corroborate the responses with objective measures of ETS exposure. What's more, while 32,000 people participated in the study, only 135 of those exposed to passive smoke had coronary events. Granted, that was more than the 72 events expected based on the experience of the control group, but in both cases the actual quantity was so small that any number of factors might have skewed the results, as critics of the Kawachi study have pointed out. Kawachi himself, in a later review of workplace exposure to ETS (Environmental Health Perspectives, December 1999), concedes that while some association between ETS and cardiovascular disease was shown in five of six studies, "none of the relative risks was statistically significant." I don't claim ETS doesn't cause serious health problems, just that it hasn't been proven yet.

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