Scientists say they have confirmed a link between secondhand tobacco smoke and an increased risk of heart disease. The findings, reported in tomorrow's issue of the British Medical Journal, also may clear up a mystery about why secondhand smoke appears to be such a potent risk factor for the condition.
The mystery is that 20-cigarette-a-day smokers have a 78% increased risk of heart disease over the general population--a risk elevation that's only three times that of passive smokers, even though active smokers inhale roughly 100 times as much smoke. Some have suggested that poor diet or some other factor might account for the high disease rate among smokers' spouses, but the new study suggests that tobacco smoke alone is responsible for most of the effect.
Malcolm Law and his colleagues at the Wolfson Institute of Preventive Medicine in London performed a statistical analysis of data drawn from 19 studies on the effects of passive smoking on heart-disease rates. Law says he had expected to disprove the implausibly high risk for heart disease from passive smoke. "We thought it must be that they ate a different diet or some kind of error in the studies," he says. Instead, his group found that passive smoke caused about a 23% increased risk of heart disease, even when accounting for the small effects of differences in diet.
To try to explain the elevated risk, Law's group analyzed several studies of platelet aggregation, which showed that even a relatively small amount of smoke causes clotting factors in the blood to become significantly stickier. Based on a study showing the increase in heart-disease risk of different degrees of platelet aggregation, they showed that passive smoking causes enough aggregation to explain the heart disease risk. They suggest that the effect does not become worse with more smoke. Other changes in the blood that promote heart disease occur in a more gradual way with increasing smoke, say the authors, and those are responsible for the gradual increase in risk between one-cigarette and 20-cigarette-a-day smokers.
The paper "represents the most thorough review of the evidence that's yet come out," says Richard Peto, an epidemiology professor at Oxford University. He says the authors "propose a strong case" to explain the heart-disease data, suggesting that "the risks are much higher than one would have thought."