Children and teens who smoke cigarettes have nearly four times the risk of developing asthma in their teens compared to children and teens who do not smoke, researchers at the University of Southern California (USC) report.
Lead researcher Frank Gilliland, M.D., Ph.D., professor of preventive medicine at the Keck School of Medicine of USC, also found that children who were exposed to cigarette smoke in their mothers' wombs have even higher risks of developing asthma, almost nine times the risk of those who didn't smoke. Their results appear Nov. 15 in the American Journal of Respiratory and Critical Care Medicine.
"We've been studying this group of children long enough that now some of them have started smoking," Gilliland says. "We found that teens who started smoking have a four times higher risk of developing asthma compared to teens who don't smoke. But if those same teenagers were also exposed to tobacco smoke before they were born, they get more than a double whammy - nine times the risk of getting asthma."
While cigarette smoke is known to have negative effects on lungs, including inflammation and hyper-responsive airways, evidence linking smoking and the development of asthma has been mixed. Some of that difficulty lies in trying to separate out confounding variables in adult smokers over their lifetimes, Gilliland and his co-researchers say. By studying adolescents, who had a shorter history of smoking, the researchers were able to make a clearer connection.
"These findings suggest that the harmful effects of cigarette smoking are not limited to those who are long-term heavy smokers," says National Institute of Environmental Health Sciences Director David A. Schwartz, M.D. "The study results provide clear evidence of a link between short-term smoking and respiratory illness in adolescents and young adults."
The study draws upon data from the Children's Health Study (CHS), a longitudinal study of respiratory health among children in 12 southern California communities. During annual school visits the researchers collected data on demographic factors, medical histories, household exposures, cigarette smoking and newly diagnosed asthma. They followed a group of 2,609 children and adolescents who were between the ages of 8 to 15 at the start of the study and had no prior history of asthma or wheezing.
The majority of the children were non-Hispanic white or Hispanic white. About 14 percent of children reportedly were exposed to smoking before birth during their mother's pregnancy, and 17.5 percent of children were exposed to secondhand smoke.
During the course of the study, about 28 percent of children reported smoking at any time during their life, 13.8 percent reported smoking weekly, and 6.9 percent reported smoking regularly (at least seven cigarettes per week). Children who were exposed to smoke before birth were slightly more likely to become regular smokers.
The children were tracked an average of 6.3 years, and as long as 8 years, depending on their age at joining the study and whether they continued participating. Over that time, 255 new cases of asthma were reported, with a greater percentage of girls than boys being diagnosed.
The increased risk for newly diagnosed asthma among regular, frequent smokers was largest among children who had been exposed to maternal smoking during their gestation. Children exposed in utero and who became frequent regular smokers (seven or more cigarettes per week) had an 8.8-fold increased risk compared to unexposed nonsmokers. In contrast, children who were not exposed in utero showed a small and statistically non-significant 1.2-fold increase in risk from frequent regular smoking.
Interestingly, children who were regular smokers but who also had a history of allergies were not found to have an increased risk of asthma.
The link between regular smoking and asthma was not substantially affected by adjusting for family income and other demographic factors, birth weight, gestational age, physical activity levels, family history of asthma, pets, or exposure to pollutants or secondhand smoke.
"The clinical and public health implications for our findings are far-reaching," the authors conclude. "Effective tobacco control efforts focusing on the prevention of smoking in children, adolescents and women of childbearing age are urgently needed to reduce the number of these preventable cases of asthma." The authors suggested that teens might be motivated to refrain from taking up smoking if they understand that smoking can cause them to develop asthma and perhaps even force them to limit their activities.
Frank D. Gilliland, M.D., Ph.D., Talat Islam, M.B.B.S., M.S., Kiros Berhane, Ph.D., W. James Gauderman, Ph.D., Rob McConnell, M.D., Edward Avol, M.S., John M. Peters, M.D., Sc.D. Regular Smoking and Asthma Incidence in Adolescents. Am. J. Respir. Crit. Care Med. 2006.
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