Most parents don't smoke in front of their babies, but some may not realise the dangers that smoking presents to older children too

Amanda Sandford, Action on Smoking and Health

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.

In this way they were able to get a much more accurate measure of the amount of tobacco smoke to which the children had been exposed than simply relying on information from parents.

Youngest children

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, and to take time off school.

The authors conclude that exposure to environmental smoke is an important and preventable cause of illness among children.

Amanda Sandford, research manager for the anti-smoking charity Action on Smoking and Health, told BBC News Online that it was particularly important parents tried not to smoke in front of their children.

She said: "Most parents don't smoke in front of their babies, but some may not realise the dangers that smoking presents to older children too.

"Their lungs are still developing, and at that stage they are much more susceptible to infection."

The research is published in the journal Archives of Pediatrics & Adolescent Medicine.