July 21 (HealthDay News) -- Air pollution from cars can increase a child's chances of developing asthma, but add parental stress and the odds for asthma get even higher, a new study finds.

For children exposed to smoking while still in the womb, another asthma risk, parental stress also increases the risk for asthma, the researchers noted.

"There is an association between air pollution and asthma, and it grows with increasing exposure to stress in the household," said lead researcher Ketan Shankardass, a postdoctoral research fellow at the Centre for Research on Inner City Health at The Li Ka Shing Knowledge Institute of St. Michael's Hospital in Toronto.

"The cause of asthma is still unknown," Shankardass said. "It's a major illness that affects a lot of people all around the world and we still don't really have a handle on what causes it so we can't control it very well. But this finding contributes to our understanding of that causal process."

The report is published in this week's online edition of the Proceedings of the National Academy of Sciences.

For the study, Shankardass and his colleagues collected data on 2,497 children in southern California. The children, aged 5 to 9, had no history of asthma or wheeze when the study started. Over three years, the researchers tracked whether or not the children developed asthma.

In addition, the researchers had the parents fill out a questionnaire that measured stress. The questionnaire asked the child's mother about whether she felt in control of her life and whether she felt she was able to handle problems or whether she had problems coping with her life, Shankardass said.

The study authors also collected data on the children's exposure to traffic-related pollution and whether the children were exposed to tobacco smoke before birth.

By itself, stress or socioeconomic status did not increase the risk of developing asthma, the researchers found.

However, when parental stress was combined with traffic pollution or exposure to smoking before birth, the risk of asthma increased more than it did for children exposed to pollution or smoke, but not stress.

Shankardass noted that exposure to traffic pollution and prenatal smoking as well as stress are more common in lower socioeconomic areas, which may help explain why asthma may disproportionately affect children of disadvantaged parents.

"For once, we may have a piece of the puzzle that would explain the social disparities in asthma," he said.

Dr. Clifford Bassett, medical director of Allergy and Asthma Care of New York, said it is not surprising that parental stress can have an impact on children and asthma.

"Stress does have an impact on the immune system. Clearly, the relationship between stress, tobacco and air pollution are all bad guys," Bassett said.

"There are many different variables -- behavioral, socioeconomic, environmental and physiologic -- that dictate whether a child will develop asthma," Bassett added. "There are a lot of biologic pathways that are involved in the relationship of asthma and stress and the immune system."

Bassett also thinks gauging household stress should be part of treating children with asthma.

SOURCES: Ketan Shankardass, Ph.D., postdoctoral research fellow, Centre for Research on Inner City Health, The Li Ka Shing Knowledge Institute of St. Michael's Hospital, Toronto; Clifford Bassett, M.D., fellow, American Academy of Allergy, Asthma and Immunology, and medical director, Allergy and Asthma Care of New York, New York City; July 20-24, 2009,  Proceedings of the National Academy of Sciences, online