CHICAGO » Lofty living may make babies vulnerable to sudden infant death syndrome, according to a Colorado study that found higher risks above 8,000 feet.

While the research shows that the SIDS rate in Colorado’s tall mountains is very low, it’s still two times greater than in the Denver area and other regions where the altitude is less than 6,000 feet. The results echo earlier research done in Austria’s Alps.

Mountain air has less oxygen than air at lower elevations, and conditions that reduce infants’ oxygen levels have previously been linked with SIDS. But exactly how mountain air might put babies at risk is uncertain and whether there is a solid connection requires more study, the researchers said.

Lead author Dr. David Katz, a University of Colorado heart specialist, emphasized that SIDS deaths are rare; only six occurred at Colorado’s highest altitudes during the six years studied. The rate at high altitudes was just 0.8 SIDS deaths per 1,000 births, versus 0.4 per 1,000 in the state’s regions with an altitude of less than 6,000 feet.

The study was published online Monday in Pediatrics. The authors examined Colorado birth certificate and death registries for 2007 to 2012.

SIDS kills about 2,000 U.S. infants each year, and is the leading cause of infant deaths after the first month.

The causes are unknown but certain conditions linked with reduced oxygen levels seem to increase risks, including brain-stem abnormalities, mothers smoking during and after pregnancy, respiratory infections and stomach sleeping.

SIDS used to be called crib death because infants were often found lifeless, lying face-down in their cribs. Public health efforts launched in the 1990s that emphasized placing babies to sleep on their backs dramatically reduced SIDS deaths.

Katz said to help protect their infants from SIDS, parents should focus on known risks. That advice includes no stomach sleeping or bed-sharing, avoiding soft bedding and pillows in cribs, and keeping infants away from cigarette smoke.