Researchers at the Keck School of Medicine of the University of Southern California looked at the relationship between chronic cough, phlegm production or bronchitis and dog and cat ownership among 475 southern California children with asthma.

Children with dogs had significantly increased cough, phlegm production and bronchitis responses to the measured pollutants, including nitrogen dioxide, ozone, particulate matter and acid vapor. There were no increases of these symptoms in children who lived in homes without pets or who lived with only cats, the researchers found.

"Further work is needed to determine what it is about dogs that may increase an asthmatic child's response to air pollution," said lead author Dr. Rob McConnell of the Keck School of Medicine.

The findings are published in the online edition of Environmental Health Perspectives.