Past research shows that 145 of every 1,000 children ages 5 to 17 will develop asthma in their lifetime.
A child whose maternal grandmother smoked while pregnant may have double the risk of developing asthma, according to a new study.
Even if the child's mother did not smoke when pregnant, the youngster's risk for developing asthma was still 1.8 times greater if the grandmother smoked, according to the report published Monday in the April issue of the journal Chest.
If both mother and grandmother smoked while pregnant, the risk of the child developing asthma was 2.6 times greater than if neither mom nor grandma had smoked. If only the mother smoked while pregnant, the risk for childhood asthma was 1.5 times greater.
While considerable research has shown that exposure to tobacco smoke contributes to asthma in children and adults, only recently have scientists considered the impact on childhood asthma from maternal smoking -- and now a generation further back.
"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," said Dr. Frank Gilliland, senior author of the study and a professor of preventive medicine at the University of Southern California-Los Angeles.
"The findings suggest that smoking could have a longer-lasting impact on families' health than we had ever realized."
Statistics from the federal government show that the number of people with asthma in the United States has more than doubled in the past 20 years, with particularly notable increases seen among preschool children. The 2001 National Health Interview Survey shows that 145 of every 1,000 children ages 5 to 17 will develop asthma in their lifetime.
In the new study, researchers at the University of Southern California's Keck School of Medicine interviewed parents or guardians of 908 youngsters participating in a long-term children's-health study, which includes children and teens recruited in grades four, seven and 10.
Of this group, 338 had developed asthma within the first five years of life, and 570 were asthma-free. Detailed maternal and childhood smoking histories and other asthma risk-factor information was obtained during the interviews.
The scientists believe that asthma risk is increased for the grandchildren in at least two ways.
"We suspect that when a pregnant woman smokes, the tobacco might affect her fetus's DNA in the mitochondria [a structure within cells that provides energy and whose genetic information is passed only through the mother to offspring]. If it is a girl, it might also affect her future egg cells as well," Gilliland said.
"We speculate that the [genetic] damage that occurs affects the child's immune system and increases her susceptibility to asthma, which is then passed down to her children."
In addition to being passed down the maternal line, mitochondrial DNA evolves rapidly.
"These findings indicate there is much more we need to know about the harmful effects of exposure to tobacco products in the uterus," said Dr. Paul Kvale, president of the American College of Chest Physicians, which publishes the journal. "They also demonstrate how important smoking cessation is for both the person smoking and their families."