The study, led by researchers at the Keck School of Medicine of the University of Southern California (USC), appears this week in the Online Early Edition of the Proceedings of the National Academy of Sciences.
The study found that the risk of asthma associated with traffic-related pollution was significantly higher for children of parents reporting high levels of stress. Stress, as well as low parental education, was also associated with larger effects of exposure to tobacco smoke during pregnancy.
"We found that it was children exposed to the combination of air pollution and life in a stressful environment who were at highest risk of developing asthma," says principal investigator Rob McConnell, M.D., professor of preventive medicine at the Keck School of Medicine of USC and Deputy Director of the Children's Environmental Health Center at USC.
The study drew upon data from the USC Children's Health Study, a longitudinal study of respiratory health among children in 13 southern California communities.
Researchers followed 2,497 children with no history of respiratory problems over three years, tracking whether they developed asthma starting in kindergarten or first grade.
hey also measured parental stress and parental education-as an indicator of socioeconomic status-using a questionnaire, and collected information on exposure to traffic-related pollution and whether the children had been exposed to tobacco smoke in utero.
The results showed that parental stress alone did not increase the risk that children would develop asthma. However, when children had a combination of parents with stressful lives and also lived near high levels of traffic-related pollution, their risk of asthma increased compared with children only exposed to pollution.
"Air pollution can promote inflammatory responses in the airways of the lung, which is a central feature of asthma," McConnell says.
"Stress may also have pro-inflammatory effects and this may help explain why the two exposures together were important," the expert added.
Children whose parents perceived their lives as unpredictable, uncontrollable, or overwhelming were susceptible to the effects of pollution, the authors noted.