Children who do not show any signs of respiratory problems may still be experiencing damaging changes in their breathing process that could lead to lung disease later in life, according to a new study presented at the American Thoracic Society 2007 International Conference on Sunday.

"Everyone knows that children whose parents smoke have more respiratory problems -- more puffing, wheezing, cases of pneumonia -- but until now we haven't known if lung function is impaired in the children who don't have any respiratory complaints or diagnosed lung problems," researcher Bert Arets of University Medical Center Utrecht in the Netherlands said.

The study included 244 children from the age of 4 to 12 and without any history of lung or respiratory disease. They were divided into four groups according to the smoking pattern of their parents: persons who have never smoked, smoking after birth but not during pregnancy, during pregnancy but not after birth, and both before and after birth.

The researchers found that children of parents who smoke had significantly reduced lung function similar to that seen in smokers. Smoking after birth appeared to be more harmful than smoking during .

The researchers have now expanded their study to include 2,000 healthy children of the parents who smoke. Earlier, a Dutch study pointed that exposure to second hand smoke is harmful for underweight babies.

Full-term babies with a low birth weight (5.5 pounds) have a significantly increased risk of developing respiratory symptoms such as coughing, wheezing and pulmonary infections up to age of five. The risk is greater if these children are exposed to second hand smoke.

The association between birth weight and respiratory symptoms decreased after age five and was not significant by age seven, the researchers found.

Analyzing data on more than 3,600 babies, the researchers found that during the first seven years of life, almost 39% of them had at least one wheezing episode, close to 52% had cough at night and more than 37% had a lower respiratory infection.

Apart from that, a Finnish study says persistent smokers may have higher risk of becoming depressed in comparison to non smokers. Smokers who quit have an elevated risk of depressive symptoms in short run. However, in long run this risk declines to the level of never smokers. Both completely smoke-free life style and successful smoking cessation seem to protect from depressive symptoms.