CHICAGO — Using a fan to circulate air seemed to lower the risk of sudden infant death syndrome in a study of nearly 500 babies, researchers reported Monday.

Placing babies on their backs to sleep is the best advice for preventing SIDS, a still mysterious cause of death. Experts also recommend a firm mattress, removing toys and pillows from cribs and keeping infants from getting too warm.

Such practices helped slash U.S. SIDS deaths by more than half over a decade to about 2,100 in 2003. But SIDS remains the leading cause of death in infants ages 1 month to 1 year.

"The baby's sleeping environment really matters," said study senior author Dr. De-Kun Li of the Kaiser Permanente Division of Research in Oakland, Calif. "This seems to suggest that by improving room ventilation we can further reduce risk."

SIDS is the sudden death of an otherwise healthy infant that can't be attributed to any other cause. These babies may have brain abnormalities that prevent them from gasping and waking when they don't get enough oxygen.

The new study, published in October's Archives of Pediatrics & Adolescent Medicine, offers another way to make sure babies get enough air.

More research is needed, said Dr. Fern Hauck of the University of Virginia in Charlottesville, but she said that because fan use is in line with theories, it may be worth considering.

"This is the first study that we know of that has looked at this issue," said Hauck, a member of the American Academy of Pediatrics SIDS task force.

Researchers interviewed mothers of 185 infants who died from SIDS and mothers of 312 infants of similar race and age. Moms answered dozens of questions about their baby's sleeping environment.

Researchers took into account other risk factors and found that fan use was associated with a 72 percent lower risk of SIDS. Only 3 percent of the babies who died had a fan on in the room during their last sleep, the mothers reported. That compared to 12 percent of the babies who lived.

Using a fan reduced risk most for babies in poor sleeping environments.

The study involved infants in 11 California counties. It was supported by grants from the National Institutes of Health.