A woman who smokes during pregnancy could increase her grandchild's risk of developing asthma, researchers suggest.
Smoking while pregnant 'may affect future generations'
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.
Children surveyed whose grandmothers smoked, but whose mothers did not, had double the normal risk of asthma.
The researchers, writing in Chest, say DNA passed down to the grandchild could be affected.
The parents or guardians of 908 children were interviewed for the study.
By the age of five, 338 children had been diagnosed with asthma, while another 570 children were asthma-free.
Children whose grandmothers smoked were more than twice as likely to develop asthma compared with children whose mothers and grandmothers did not.
Even 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.
The team, from the University of California's Keck School of Medicine, said the explanation could be that when a pregnant woman smokes, chemicals from the tobacco may biologically damage her foetus.
They suggest that, if the child is a girl, her eggs may be affected, which will in turn put her future children at risk.
A second 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.
They speculate that these alterations decrease immune function and reduce the body's ability to rid itself of toxins, thereby increasing their risk of asthma in smokers' children and grandchildren.
Frank Gilliland, professor of preventive medicine at the Keck School of Medicine who led the research, said: "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.
"The findings suggest that smoking could have a longer-lasting impact on families' health than we had ever realised."
The scientists say further research is needed to confirm their findings.
Paul Kvale, president of the American College of Chest Physicians, which publishes Chest, said: "These findings indicate that there is much more we need to know about the harmful effects of in utero exposure to tobacco products and demonstrate how important smoking cessation is for both the person smoking and their family members."
Professor Martyn Partridge, Chief Medical Advisor to Asthma UK, said: "Several previous studies have shown an association between wheezing illnesses in early childhood and maternal, and to a lesser extent paternal, smoking.
"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.
"This study uses a different methodology and again suggests an increased risk of asthma with early life exposure to smoking in the womb.
"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."