Babies who sleep in rooms with a fan have a dramatically lower risk for sudden infant death syndrome, a new study shows.

The finding is the latest evidence to suggest that a baby’s sleep environment is a critical factor in the risk for SIDS, which is diagnosed when the sudden death of an infant can’t be explained by other health concerns.

Since 1992 the rate of SIDS deaths has dropped by 56 percent, from 1.2 deaths per 1,000 live births to about 1 death per 2,000 live births. The decline is linked to a national “Back to Sleep” campaign that promotes putting babies on their backs instead of the stomach, which has been shown to lower risk for sudden death. The American Academy of Pediatrics also recommends that parents avoid soft bedding, allow a baby to use a pacifier if he or she wants, and avoid overheating a baby’s room as ways to lower the risk for SIDS.

Despite the gains, SIDS continues to be the leading cause of death among infants in the first year of life, and researchers are looking for additional measures to lower risk.

The latest study compared the sleeping circumstances of 185 babies who died of SIDS with another 312 randomly selected babies who were matched by race, ethnicity, country of origin and age. The study, published today in The Archives of Pediatrics and Adolescent Medicine, found that simply having a fan in the room lowered a baby’s risk of SIDS by 72 percent. The gains were even greater if the baby was wrongly placed on its stomach or was sleeping in a warmer room, both of which are risk factors for SIDS.

“Even though we don’t know why certain babies are more susceptible, sleeping environment matters,’’ said Dr. De-Kun Li, reproductive and perinatal epidemiologist at Kaiser Permanente’s division of research in Oakland, Calif., and a co-author of the study.

While the study wasn’t designed to identify why fans make a difference, the theory is that by circulating the air, fans lower the risk of “rebreathing” by the baby. The rebreathing of exhaled carbon dioxide trapped near an infant’s airway has been suggested as a possible reason SIDS risk is higher when children sleep on their stomachs, in soft beds or without pacifiers.

Parents who worry that their child will be chilled by a fan should know that fans do not cool the air, they simply move air around. A baby will only feel a chill if he or she is perspiring, doctors say.

Parents who use fans in a child’s room should make sure to take normal safety precautions, keeping cords out of the way and making sure the fan can’t be knocked down by a toddler or pet.

Dr. Li said fan use can’t replace the other sleeping strategies for lowering SIDS, such as removing soft bedding and putting a baby on his or her back. He notes that the gains shown in the study were for the whole group, but that the benefit appeared to be smaller when parents followed all of the A.A.P. guidelines. However, even if a baby already is sleeping on the back, SIDS risk was lowered by about 16 percent, he said, although the trend was not statistically significant.

“If parents wanted to do more to reduce the baby’s SIDS risk, they can add a fan,’’ Dr. Li said.