Children whose grandmothers smoked during pregnancy are at increased risk for asthma, even if their own mothers did not smoke, a new study suggests.

It's known that smoking can cause changes in gene activity. The new study findings suggest that those changes can be passed down through more than one generation, the researchers said.

The study included more than 66,000 grandchildren and nearly 45,000 grandmothers in Sweden. Children whose grandmothers smoked while pregnant with daughters had a 10 percent to 22 percent increased risk of asthma, even if their own mothers did not smoke during pregnancy.

The study, to be presented Wednesday at a meeting of the European Respiratory Society in Amsterdam, may help explain why there has been a sharp rise in asthma rates in the last 50 years, the researchers said.

The study only found a link between grandmothers who smoked and possible asthma in their grandchildren, however. It did not prove cause and effect.

"We found that smoking in previous generations can influence the risk of asthma in subsequent generations. This may also be important in the transmission of other exposures and diseases," study co-author Caroline Lodge said in a European Lung Foundation news release. Lodge is a research fellow at the University of Melbourne, in Australia.

To understand more about the asthma epidemic, she added, researchers need to better understand how harmful exposures over a lifetime may influence disease risks in future generations.

"Additionally, researchers in this area need to be aware, when interpreting the asthma risk from current exposures and genetic predisposition, that individuals may carry an inherited, non-genetic risk from exposures in previous generations. This knowledge will help to clarify the findings concerning current risk factors in asthma research," Lodge explained.

The next step in the research is to assess asthma risk in grandchildren whose grandmothers smoked while pregnant with sons, the researchers said.

Studies presented at meetings are typically considered preliminary because they are not subject to the same scrutiny as published research.