CHICAGO (AP) - Asthma cases could drop nearly 40 percent among U.S. youngsters under age 6 if susceptible children didn't have pets or other allergy triggers in their homes, researchers say.

Their study suggests that eliminating known household risks could prevent asthma in more than 500,000 children a year and underscores the important role environmental factors play in development of the disease.

``This could have a profound effect ... on the health of children,'' said Dr. Bruce Lanphear, associate professor of pediatrics at Children's Hospital Medical Center in Cincinnati.

His findings appear in the March issue of Pediatrics. They are based on an analysis of data on 8,257 children under 6 who participated in a national health survey between 1988 and 1994. About 6 percent had been diagnosed with asthma.

Children with pet allergies were 24 times more likely to have asthma than those without. Living in homes with smokers or where gas stoves were used for heat also were significant risk factors; such children were nearly twice as likely to have asthma.

``If residential exposures, including tobacco smoke and indoor allergens, were eliminated, and if these exposures are determined to cause asthma, which is the central hypothesis among experts, we would reduce asthma in this age group by 39 percent, or about 530,000 cases a year,'' Lanphear said.

The data he examined did not include information on exposure to dust mites or cockroaches, two other known household risk factors for asthma.

Dr. Gillian Shepherd, a New York allergist and member of the academy's board of directors, said the study underscores a consensus among asthma experts that ``we should be a lot more aggressive at trying to block young kids' exposure to allergic materials.''

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