Researchers from the University of Nottingham, England say in a new study that even low levels of secondhand-smoke exposure can up the risk of heart disease, WebMD reported Feb. 12.

Researchers gauged secondhand-smoke exposure by measuring blood levels of cotinine, a nicotine metabolite, then judged subjects' heart-disease risk by testing for four markers for the disease: C-reactive protein, white blood cell count, fibrinogen, and homocysteine. The authors found that individuals with low to high exposure to secondhand smoke had significantly higher levels of fibrinogen and homocysteine than those who had no measurable exposure to secondhand smoke.

"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," said researcher Andrea Venn, Ph.D.

Venn added that secondhand smoke appeared to increase heart-disease risk about a third to half the level of active smoking.

The study is thought to be the first to measure such risk without relying on participants' recall of exposure to secondhand smoke.

The study appears in the online edition of the journal Circulation.