"The size of these effects were between about one-third and one-half that seen in relation to active smoking, which seems disproportionately large, but fits with previous studies that have shown similar effects in relation to disease risk," said lead researcher Andrea Venn, an associate professor at the University of Nottingham's Division of Epidemiology and Public Health.

The findings are published in the Feb. 13 edition of Circulation: Journal of the American Heart Association.

For the study, Venn and co-author Dr. John Britton collected data on 7,599 adults who participated in the third National Health And Nutrition Examination Survey (NHANES III, 1988-94). None of these people had ever smoked.

Study participants had their blood levels of cotinine, an indicator of nicotine, measured. They also had their levels of fibrinogen, homocysteine and C-reactive protein measured, all markers of potential heart disease.

The researchers found that 18 percent of the participants had no detectable levels of cotinine. The rest had either low or high levels. Eighteen percent of those with low levels of cotinine and 56 percent of those with high levels said they lived with a smoker or were exposed to tobacco smoke at work.

The researchers also found that the low- and high-cotinine groups had significantly higher levels of fibrinogen and homocysteine, compared with those who had no detectable levels of cotinine. The increased fibrinogen and homocysteine levels were equivalent to about 30 percent to 45 percent of levels seen in active smokers.

"Furthermore, our study showed that these effects were not restricted to people exposed to high levels of secondhand smoke but were also evident in our low-exposure group, the majority of whom reported not living with a smoker and not being exposed at work," Venn said.

The findings suggest that secondhand smoke has a significant effect on susceptibility to cardiovascular disease, even at relatively low levels of exposure, Venn said. "Secondhand smoke is likely to be an important avoidable cause of cardiovascular disease in the population, and it is therefore important that measures are implemented to minimize the public's exposure to secondhand smoke," she said.

One heart expert thinks this study provides further evidence of the dangers of secondhand smoke.

"Contrary to what the tobacco companies will tell you, there is overwhelming evidence that even secondhand smoke is harmful," said Dr. Byron Lee, an associate professor of cardiology at the University of California, San Francisco.

Some research suggests that secondhand smoke inhaled by nonsmokers causes more harm than would be expected, Lee said. "This article helps us understand the mechanism behind this apparent effect, by showing us that passive smoke exposure can increase biomarkers of cardiovascular disease to levels seen in everyday smokers," he said.

In other research involving secondhand smoke, the Flight Attendant Medical Research Institute has awarded an $8.7 million grant to New York-Presbyterian Hospital/Weill Cornell Medical Center and other hospitals to look at the lingering effects of secondhand smoke among nonsmoking fight attendants and other service industry workers.

Until bans on in-flight smoking took effect in 1988, flight attendants were regularly exposed to secondhand smoke. Now, 5,000 of these men and women and others are being recruited for a series of clinical studies on the health effects of secondhand smoke on nonsmokers.