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.