WEDNESDAY, Aug. 30 (HealthDay News) -- Fido may not be an asthmatic child's best friend, a new study suggests.

A team at the University of Southern California's Keck School of Medicine found that the presence of dogs in the home can worsen lung response to air pollution in children with asthma. This was not the case with cats, however.

The findings were published this week in the online edition of Environmental Health Perspectives.

The study of 475 southern California children with asthma examined the relationship between chronic cough, phlegm production or bronchitis and dog and/or cat ownership.

It found that children with dogs had significantly increased cough, phlegm production, and bronchitis responses when they were exposed to air pollutants such as nitrogen dioxide, ozone, particulate matter and acid vapor.

Children who lived in homes with no pets, or in homes with only cats, did not experience an increase in symptoms when exposed to the air pollution.

"Further work is needed to determine what it is about dogs that may increase an asthmatic child's response to air pollution," study author Dr. Rob McConnell, a professor of preventive medicine, said in a prepared statement.

"Cats are highly allergenic, and children with asthma are often allergic to cats. Therefore, if an allergen were enhancing the lung's response to air pollution, we'd be more likely to see an association with cats. But in this study, we see an effect of air pollution in homes with dogs, so we think endotoxin exposure is a more likely explanation for our results than allergen exposure," he said.

Endotoxin is a part of the cell wall of common bacteria in the environment. Inhaled endotoxin produces an inflammatory response in the lungs and may cause constriction of the airways in people with asthma, the researchers noted.

Previous research found that endotoxin exacerbated the inflammatory effect of diesel exhaust particulate, inhaled highway aerosols and ozone in the lungs of animals.