Scientists from the University of Southern California are saying that smoking could have a longer-lasting impact on families' health than ever before realised.
In a survey by Frank Gilliland, professor of preventive medicine at the Keck School of Medicine, the parents or guardians of 908 children were interviewed and it was found that by the age of five, 338 children had been diagnosed with asthma, while the other 570 children were asthma-free.
The researchers say the children surveyed whose grandmothers smoked, but whose mothers did not, had double the normal risk of asthma and the effect of tobacco on the lungs can be passed down even if the child's mother is unaffected.
They say DNA passed down to the grandchild could be affected and children whose grandmothers smoked were more than twice as likely to develop asthma compared with children whose mothers and grandmothers did not.
If a child's mother did not smoke while she was pregnant - but the child's grandmother did - the child had 1.8 times the risk of developing asthma.
If both the mother and grandmother smoked while pregnant, a child was more than two-and-a-half times more likely to have the condition.
A woman who smokes during pregnancy could increase her grandchild's risk of developing asthma, researchers suggest.
University of Southern California scientists say the effect of tobacco on the lungs can be passed down even if the child's mother is unaffected.
The University of California team, said the explanation was intriguing and could be because when a pregnant woman smokes, chemicals from the tobacco may biologically damage her foetus and suggests that, if the child is a girl, her eggs may be affected, which will in turn put her future children at risk.
Another explanation could be that the foetus's mitochondrial DNA, which is uniquely passed down by the mother, may be damaged through subtle changes in which genes are turned on or off. These alterations decrease immune function and reduce the body's ability to rid itself of toxins, thereby increasing the risk of asthma in smokers' children and grandchildren.
Gilliland says that this is the first study to show that if a woman smokes while she is pregnant, both her children and grandchildren may be more likely to have asthma as a result and the findings suggest that smoking could have a longer-lasting impact on families' health than ever realised before.
Paul Kvale, president of the American College of Chest Physicians, says the results indicate that there is much more we need to know about the harmful effects of in utero exposure to tobacco products and shows how important "smoking cessation is for both the person smoking and their family members."
Previous studies have shown an association between wheezing illnesses in early childhood and maternal, and to a lesser extent paternal, smoking. says Professor Martyn Partridge, Chief Medical Advisor to Asthma UK, and that some that have then followed such associations through to older childhood have shown less definite association suggesting that some of the early life wheezing was not necessarily due to asthma.
A different methodology has been used in this study and it again suggests an increased risk of asthma with early life exposure to smoking in the womb.
Partridge says the suggestion of an association with grandmaternal smoking is intriguing and whilst the authors suggested explanations for this are very reasonable, confirmation of the association in other studies should be the next step. Further research is needed to confirm their findings.