CHICAGO (AP) — Lofty living may make babies vulnerable to sudden infant death syndrome, according to a Colorado study that found higher risks above 8,000 feet (2,400 meters). 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 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. Public health efforts launched in the 1990s that emphasized placing babies to sleep on their backs dramatically reduced SIDS deaths.