Children have nicotine in their body even if parents smoke outdoors with the doors closed, researchers have found.
Closing the door does not protect children
Researchers from Linkoping University in Sweden found children whose parents smoked outside had twice as much nicotine in their body as children of non-smokers.
But they still had significantly less nicotine in their bodies than children who were directly exposed to smoke.
Experts said the best way to protect children was not to smoke at all.
Respiratory illness link
The researchers reviewed studies which looked at the effects of passive smoking.
One study tested the urine of 366 children aged two to three for cotinine, a chemical formed when the body breaks down nicotine.
The parents of 216 of 366 children aged 2-3 years used this method.
Children whose parents smoked outside had twice as much cotinine in their urine as the children of non-smokers.
But in homes where both adults smoked indoors, children had fifteen times higher levels of cotinine than children of non-smokers.
A separate study of parents of 1,600 children aged one to two found a correlation between passive smoking and respiratory illnesses in children.
The researchers also found parents know passive smoking can harm children, but do not know why.
They say relatively few people stop smoking when they have children, but many do change their smoking behaviour.
Public health researcher AnnaKarin Johansson, who carried out the review, said: "The care-providers tend to want to avoid pointing the finger, but perhaps they are sometimes too cautious."
Naj Dehlavi, a researcher with the campaign group Action on Smoking and Health, told BBC News Online: "17,000 children under the age of five are admitted to hospital every year in the UK due to passive smoking.
"This study shows that despite parents smoking outside for the sake of their children's health, inadvertent exposure still occurs.
"There really is no substitute for giving up smoking altogether - its a win-win situation for the whole family."