ORLANDO, Fla. -- Room air conditioners appear to be less effective than central air in keeping harmful mold and bacteria from getting into homes and causing breathing problems in people with asthma or allergies.

University of Michigan researchers who studied 12 homes with either type of air conditioners found that higher levels of mold and bacteria were found in homes that had only room air conditioners.

The levels of the mold and bacteria found in these homes were comparable to the levels that would be expected in homes with no air conditioning, lead researcher Harriet A. Burge reported Tuesday at the American Academy of Allergy and Immunology annual meeting.

She said that while the study does favor the use of central air, particularly in homes of people with allergies or asthma, room air conditioners are not all bad.

"If you remove all the allergen sources in a room and keep the doors and windows closed, I think that is an effective way of using a room air conditioner," Ms. Burge said.

The study was part of a larger study of indoor pollutants on school-age children done in conjunction with Harvard University.

Twenty percent of the population has allergies, noted Dr. Stanley Wolf, an allergist from Silver Spring, Md., who moderated the session on indoor allergies.