Spouses also tend to share other habits -- related to diet and exercise, for example -- that can contribute to illness. Since smokers tend to be less health-conscious and risk-averse than nonsmokers, their spouses might be more prone to disease even if ETS had no effect. Another interesting point raised by Enstrom and Kabat is the effect of being widowed, which happens more often to spouses of smokers and is independently associated with higher death rates.
Finally, there is the problem of publication bias: Studies that find an association are more likely to see print. Hence the link between ETS exposure and fatal disease could be even weaker than it looks in the published research.
The question is not whether tobacco smoke can be dangerous. At high enough levels, it clearly is, as Enstrom and Kabat's study confirms. "As expected," they write, "there was a strong, positive dose-response relation between active cigarette smoking and deaths from coronary heart disease, lung cancer, and chronic obstructive pulmonary disease."
The question is whether there's a threshold below which tobacco smoke has no measurable impact on mortality. Based on the epidemiological data, Enstrom and Kabat calculate that smoking one cigarette a day would be associated with something like a 20 percent increase in lung cancer risk (as opposed to an increase of roughly 1,000 percent for a pack a day) and virtually no increase in heart disease risk.
"It is generally considered that exposure to environmental tobacco smoke is roughly equivalent to smoking one cigarette per day," Enstrom and Kabat note. If so, a small increase in lung cancer risk is possible, but the commonly reported 30 percent increase in heart disease risk -- the purported cause of almost all the deaths attributed to secondhand smoke -- is highly implausible.
Tobaccophobes such as Michael Bloomberg and Stanton Glantz are determined to eliminate smoking from bars and restaurants regardless of what the evidence shows. But those of us who have more respect for the truth should not let them banish skepticism along with cigarette smoke.