A new study from researchers at Cincinnati Children's Hospital Medical Center finds that boys with asthma who have been exposed to tobacco smoke have elevated rates of hyperactivity, aggression, depression and other behavioral problems.

Researchers found an increase in behavioral problems with higher tobacco exposure levels, but said that even low levels of tobacco smoke may affect behavior among asthmatic boys. "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," said Kimberly Yolton, a researcher at the Children's Environmental Health Center at Cincinnati Children's and lead author of the study.

The study observed 220 children ages 6-12; 61 percent were boys and 77 percent had moderate to severe asthma, the rest having mild asthma. On average, girls in the study were exposed to higher levels of tobacco smoke than boys, but the exposure did not appear to increase behavioral problems among females in the study.

However, investigators said behavioral problems in boys increased about twofold with each doubling of tobacco-smoke exposure. The study authors said that more research is needed to explain the disparity between genders.

"The largest increase we observed was in overall behavioral problems," said Yolton, "but it was interesting that in addition to externalizing behaviors -- like hyperactivity and aggression -- we also saw an increase in internalizing behaviors, such as depression."

Yolton noted that few studies have linked childhood depression and tobacco smoke but that there is "quite a bit of evidence" that nicotine affects development and functioning of the nervous system, along with a child's development and behavior.

The study appears online in the Journal of Developmental and Behavioral Pediatrics.