“This certainly suggests that there is no safe level of exposure,” she tells WebMD. “We don’t know what this means for their future, but studies in adults suggest these changes may not be totally reversible.”

Cardiovascular Impact

Kallio and colleagues from Finland’s University of Turku measured levels of the nicotine marker cotinine in the blood of children between the ages of 8 and 11 to determine their exposure to environmental tobacco smoke.

They assessed blood vessel health using an ultrasound testing method previously used to measure artery function in adults.

Of the 402 children enrolled in the study, 229 showed no evidence of exposure to secondhand smoke, while 134 showed evidence of low exposure and 39 showed evidence of high exposure.

The study is published in the latest issue of the American Heart Association (AHA) publication Circulation.

“There is already a lot of evidence that secondhand smoke is harmful to children,” Kallio says. “Hopefully, parents who are still smoking around their children will stop.”

22 Million Kids Exposed

Almost 60% of children in the United States between the ages of 3 and 11 -- roughly 22 million kids -- are exposed to secondhand smoke, according to a 2006 report from the U.S. Surgeon General.

Infants exposed to environmental tobacco smoke are at increased risk for sudden infant death syndrome (SIDS), and children have an increased risk for respiratory infections and ear problems. Asthma symptoms in children can also be triggered by exposure to cigarette smoke.

While the long-term health consequences of tobacco smoke exposure early in life are not well understood, it is clear that continued exposure poses significant risks. And children whose parents smoke are more likely to become smokers themselves, AHA spokeswoman Martha Daviglus, MD, PhD, points out.