Even children who don’t show any signs of respiratory problems may be negatively impacted by their parents’ second-hand smoke, a new study shows.

Children of smokers may be experiencing damaging changes in their airways that could lead to lung disease later in life even if they don’t show any symptoms, according to a study presented Sunday at the American Throracic Society 2007 International Conference.

"Everyone knows that children of smokers have more respiratory problems—more puffing, wheezing, cases of pneumonia—but until now we haven’t known if lung function is impaired in children of smokers who don’t have any respiratory complaints or diagnosed lung problems,” said study researcher Dr. Bert Arets, of University Medical Center Utrecht in the Netherlands, in a news release from the Thoracic Society.

The study looked at 244 children ages 4 to 12 without any history of lung or airway disease. They were divided into four groups according to the smoking pattern of their parents: never smokers, smoking after birth but not during pregnancy, during pregnancy but not after birth, and both before and after birth.

Researchers found children of smoking parents had significantly reduced lung function similar to that seen in smokers. Smoking after birth appeared to be more harmful than smoking during pregnancy alone.

And widespread indoor smoking bans may be forcing parents to smoke more often in their own homes, which ultimately could do more harm than good when it comes to children, Arets said.

"We may see an increase in diminished lung function in children of smokers because of this trend," he said.

The researchers are continuing their study and have expanded it to include 2,000 "healthy" children of smokers.