Scientists have produced strong evidence that passive smoking has a lasting impact on the respiratory systems of children.
A team from the US National Center for Environmental Health in Atlanta studied data from 3,400 children aged 3 to 16 years.
The researchers analysed the children's blood to determine the concentration of a substance called cotinine, produced when the body breaks down nicotine from tobacco.
The researchers found that passive smoking had the strongest impact on the youngest children.It was linked to a permanently increased risk of asthma and wheezing. However, older children exposed to tobacco smoke were more likely to have poor lung function.
The authors conclude that exposure to environmental smoke is an important and preventable cause of illness among children.