The new study, published in the June issue of Pediatrics, also found that the cost of asthma as a result of these residential exposures in older children and adolescents was more than $400 million a year. Taken together with the previous study, the cost of asthma due to residential exposures among all children and adolescents exceeds $800 million a year. Asthma's medical costs included clinic and emergency department visits, hospital outpatient services, hospitalization, medications, loss of work as a result of school absence, and illness days.

"The elimination of residential risk factors, if causally associated with asthma, would have a profound effect on medical costs of asthma and, more importantly, on the health of children," says Bruce Lanphear, M.D., M.P.H., associate professor of pediatrics at Cincinnati Children's and the study's main author.

Asthma is the most common chronic illness of childhood, estimated to affect more than 4 million children in the United States. From 1980-93, the prevalence of asthma increased by 75 percent, primarily in children younger than 5.

Each year, asthma leads to more than 3 million clinic visits; 550,000 emergency visits; 150,000 hospitalizations; and more than 150 deaths in children younger than 15.

Researchers have identified several residential risk factors for childhood asthma, including allergies to pets, environmental tobacco smoke, dust mites and cockroaches. Because risk factors vary by geography, urbanization and poverty, the contribution of housing factors to asthma in children in the United States has been unclear.

Dr. Lanphear studied 5,384 children and adolescents who participated in a survey between 1988 and 1994. The children participated in the National Health and Nutrition Examination Survey III, a survey of the health and nutritional status of children and adults in the U.S. Overall, 11.4 percent of the children had doctor-diagnosed asthma. This extrapolates to 4.6 million children and adolescents in the United States with asthma.

"Pets appear to be the major risk factor," says Dr. Lanphear. "Children and adolescents who had a history of allergies to a pet were 2.4 times more likely to have doctor-diagnosed asthma. More than 330,000 excess cases of asthma were attributable to having a pet allergy. Parents need to consider carefully the risks and benefits of owning a pet.

The study is also the first to estimate on a national level the number of cases of asthma linked to allergic response to specific indoor allergens, such as dust mites and cockroaches. About 520,000 excess cases were due to dust mites, and about 375,000 to cockroaches.