People exposed to second-hand smoke increase their risk of experiencing warning signs of cardiovascular disease, a new British study suggests.

The study, published Monday in Circulation, the journal of the American Heart Association, adds more weight to previous research linking passive smoking and heart disease, says the study's lead author, Dr. Andrea Venn of University of Nottingham in Britain.

Researchers examined records from America's third National Health And Nutrition Examination Survey (1988-94), measuring levels of cotinine, a byproduct of nicotine, in the blood of more than 7,500 people who had never smoked.

The study is the first to measure cotinine, which is considered a more reliable indicator of nicotine exposure than the approach taken in previous studies — asking participants to report their own exposure.

Just 18 per cent of those surveyed had no detectable levels of cotinine, while the remainder had high or low levels.

Eighteen per cent of those with low levels and 56 per cent with high levels said they had lived with a smoker at home or had been exposed to second-hand smoke at work.

The study went on to check if people exposed to second-hand smoke had increased levels of fibrinogen, homocysteine and C-reactive protein, all linked with cardiovascular disease.

"Researchers found the low- and high-cotinine groups had significantly higher levels of fibrinogen and homocysteine … than the 'no detectable' group," the AHA said.

"The increased levels of fibrinogen and homocysteine seen in relation to second-hand smoke exposure were equivalent to about 30 per cent to 45 per cent of those seen for active smoking," Venn said in a news release.

Fruit and veggies help lower risk

The researchers also investigated elevated C-reactive protein, another marker of cardiovascular disease, in participants with elevated cotinine levels. They found no significant association.

The findings changed little even after researchers adjusted for participants' self-reported fruit and vegetable consumption, linked to boosting cardiovascular health, and lifestyle factors such as physical activity, social class and obesity. 

Restricting analysis to those 70 or younger with no history of heart attack, heart failure or stroke also made little difference in the association between second-hand smoke exposure and elevated levels of fibrinogen and homocysteine.

"Our study shows that very low levels of exposure to second-hand smoke may be associated with appreciable increases in cardiovascular risk," Venn said. "While the cotinine levels were only about 0.1 per cent of those in active smokers, the apparent effects of passive smoking on the biomarkers were about one-third to one-half those for active smoking."