CINCINNATI, Dec. 5  -- Boys with asthma exposed to secondhand smoke have higher degrees of hyperactivity, aggression, depression and other behavioral issues, U.S. researchers say.

Lead author Kimberly Yolton of the Children's Environmental Health Center at Cincinnati Children's Hospital says of the 220 children in the study ages 6-12, 61 percent were boys, 56 percent were African-American and 77 percent had moderate to severe asthma, with the rest having mild asthma.

The researchers compared cotinine levels in the blood of the children -- a byproduct of nicotine that accurately measures tobacco smoke exposure -- to behavioral patterns observed in the children.

However, although girls in the study were on average exposed to higher levels of tobacco smoke -- an average of 13 cigarettes a day -- than boys, the exposure didn't lead to an increase in behavioral problems among them, Yolton says. In boys, behavioral problems increased about two fold with each doubling in their tobacco smoke exposure, Yolton adds.

"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," Yolton says in a statement. (UPI)