Studies have found a link between tobacco smoke and depression in children. For example boys with asthma who are exposed to environmental tobacco smoke have higher degrees of hyperactivity, aggression, depression and other behavioral problems. The researchers said that behavioral problems are increasing with higher smoking exposure levels, but they added that even low levels of tobacco smoke may be detrimental to behavior.

Kimberly Yolton, Ph.D., a researcher at the Children's Environmental Health Center at Cincinnati Children's, said: "These findings should encourage us to make stronger efforts to prevent childhood exposure to tobacco smoke, especially among higher risk populations, such as children with asthma." Among 220 children in the study, 61 percent were boys, 56 percent were African American and 77 percent had moderate to severe asthma, and the rest having mild asthma. Other than asthma, the children have no other health problems. They were exposed to at least five cigarettes a day.

Investigators also measured the cotinine levels in the children's blood. Cotinine is a byproduct, or metabolite, of nicotine and is often used as a biomarker to more accurately measure tobacco smoke exposure. At the end of the study, researchers found that though girls in the study were on average exposed to higher levels of tobacco smoke than boys, the exposure did not lead to an increase in behavioral problems among them, investigators said. But in boys, however, behavioral problems increased about two fold with each doubling in their tobacco smoke exposure.

Although no data exist to specifically explain why tobacco smoke causes behavioral problems in children with asthma, added Dr. Yolton, and they found only a bit of evidence that nicotine in tobacco smoke affects development and functioning of the nervous system, as well as child development and behavior.