Even low-level indirect exposure to cigarette smoke was associated with a significant rise in heart disease risk in the research conducted by researchers from the U.K.'s University of Nottingham.
The study is the first to directly measure secondhand smoke exposure through levels of a nicotine byproduct in the blood. Previous studies have relied on participants' recall of exposure.
Compared with people in the study with no detectable exposures to nicotine, those with low- and high exposure levels also had significantly higher levels of two important markers of heart disease risk.
"These findings suggest that secondhand smoke exposure has a clinically important effect on susceptibility to heart disease, even at relatively low levels of exposure, and they highlight the importance of minimizing the public's exposure to secondhand smoke," researcher Andrea Venn, Ph.D., tells WebMD.
A Known Risk Factor
Smoking is a well-established risk factor for cardiovascular disease. Past research shows that the risk associated with secondhand smoke exposure is about a third of that seen in active smokers.
In an effort to further clarify this risk, Venn and colleague John Britton, M.D., examined established blood biomarkers of cigarette smoke exposure and heart disease risk in 7,599 adults who had never smoked. The participants were enrolled in the Third National Health and Nutrition Examination Survey (NHANES III), conducted in the United States.
Sixty-eight percent of those evaluated were women, and the median age of the study participants was 38.
Blood sample analysis included measurements of cotinine, the nicotine byproduct used to measure nicotine exposure.
The researchers also examined whether nicotine exposure was associated with markers of increased heart disease risk. They did this by measuring blood levels of four markers of disease — C-reactive protein, white blood cell count, fibrinogen, and homocysteine.
Slightly less than one out of five study participants had no detectable cotinine in their blood samples, while the rest had low or high levels.
Eighteen percent of those with low blood levels of cotinine and 56 percent of those with high cotinine levels reported living with a smoker or being exposed to tobacco smoke at work.
Having low and high levels of cotinine was associated with a significant increase in two of the four heart disease markers, fibrinogen and homocysteine (fibrinogen is a blood clotting factor. Homocysteine is an amino acid and high levels have been linked to risk for heart, stroke, and blood vessel disease).
The association persisted even after the researchers controlled for other risk factors, including obesity, diet, physical activity level, and socioeconomic status.
No significant association was seen between secondhand smoke exposure and C-reactive protein or white blood cell count.
While the nicotine exposure levels seen in the exposed nonsmokers were far lower than those reported for active smokers, the apparent impact of secondhand smoke on heart disease risk was a third to half that associated with active smoking, Venn noted.
"Our study shows that very low levels of exposure to secondhand smoke may be associated with appreciable increases in cardiovascular risk," she says.
Researchers from the University of California San Francisco, came to a similar conclusion in a study presented at an American Heart Association meeting last spring.
James Lightwood, Ph.D., and colleagues concluded that eliminating secondhand smoke exposure would prevent more than 228,000 new cases of heart disease and 119,000 heart disease-related deaths by 2030.
The researchers developed a model to estimate the impact of secondhand smoke. They reported that 292,500 heart attacks could be prevented during th same period if exposure to secondhand smoke were eliminated.
SOURCES: Venn, A. Circulation: Journal of the American Heart Association, Feb. 27, 2007; online edition. Andrea Venn, Ph.D., associate professor in epidemiology, University of Nottingham, Nottingham, U.K. WebMD Medical News: Cut Secondhand Smoke, Cut Heart Death.
By Salynn Boyles
Reviewed by Louise Chang