Asthma is one of the most common ailments of young childhood rates among children under age 5 have risen 160% from 1980 to 1994 in the U.S. But while the list of triggers that set off bouts of wheezing and shortness of breath (allergies, pollution or strenuous exercise, for example) are well known, it's still not clear exactly how the various factors that cause asthma including genes, environment and exposure to pollution contribute to children's chances of developing the disease.
Now researchers at the University of Southern California say they have a clearer understanding. In their study of nearly 2,500 non-asthmatic children ages 5 to 9 who lived in the area, researchers found that the level of stress reported by the children's parents had a significant impact on the kids' susceptibility to other common contributors to asthma namely exposure to pollution from traffic and secondhand smoke. Scientists found that children whose parents described themselves as stressed and anxious were 50% more likely to develop asthma than kids with non-stressed parents at least when these youngsters were also exposed to pollution in a high-traffic, urban setting.
Parental stress alone did not increase the children's risk of asthma, but the combination of living in a household with high stress levels and being exposed to pollutants from traffic in the environment was sufficient to trigger the disease. The study found similar results with exposure to tobacco smoke.
Studying a combination of factors can help explain why some kids are more likely to develop asthma than others, says Rob McConnell, lead author of the study, published in Proceedings of the National Academy of Sciences. "Childhood asthma is a complex disease and probably has many contributing causes," he says. "This study provides another clue as to what might be causing it."
The results also shed light on how risk factors like stress can increase the vulnerability of the respiratory system to environmental pollution or allergens. Because asthma involves inflammation in the airways in response to particulates that enter from the air, a separate factor that also increases the body's inflammatory response like stress can help create especially fertile conditions for asthma to develop. So a child who feels anxiety in response to parental stress, for example, may already have inflammation in his airways, which makes him more likely to develop asthma because of exposure to environmental pollutants.
"The message to take away is that various factors overlap to create asthma," says Dr. Leonard Bielroy, an allergy, asthma and immunology specialist in Springfield, N.J., who has studied some of the condition's intersecting risk factors. "And whether those factors are psychological or physical, the more they overlap, the greater the chance of developing asthma."
While McConnell and his group did not specifically measure stress levels in the study's participants it's difficult to get a reliable reading in such a large number of youngsters quickly other studies have shown that parental stress translates directly with more-anxious kids. McConnell hopes that these findings will spur additional studies to investigate why, for example, asthma rates tend to be higher among lower socioeconomic groups. "There are a lot of potential reasons why poverty might be associated with asthma," he says, including that poorer families tend to live in more highly polluted, densely packed urban areas.
But there might also be a psychological contribution, which his study has highlighted. "It's not poverty itself but something about poverty that increases the risk of asthma, and we are suggesting that stress is another exposure we ought to think about."