One reason child safety organizations recommend against crib bumpers is that they pose a risk of suffocation. Just like a pillow or thick blanket, crib bumper pads can restrict a baby's breathing if the bumper is up next to the baby's nose or mouth. Suffocation risk is greatest when babies are very young and unable to move themselves away from potential hazards.
A secondary risk with crib bumpers is strangulation. Babies can become entangled in the crib bumper or its ties, or can get between the bumper and the crib. A few crib bumpers have been recalled because of stitching or trim that can come loose. Those loose pieces also could cause injury.
Rebreathing of stale air is another concern with crib bumper pads. The bumper reduces the flow of fresh air around baby during sleep, particularly if his or her face is very close to the bumper. The American Academy of Pediatrics suggests that some infants, when they are overheated or lack sufficient oxygen during sleep, are unable to arouse themselves enough to prevent death. Re-breathing of stale air may be a contributing factor to SIDS.
In the official AAP policy statement on SIDS suggests that some babies are more prone to SIDS due to biological factors such as brainstem development or serotonin levels. However, the policy statement indicates that "more than one scenario of preexisting conditions and initiating events may lead to SIDS." It goes on to say that we cannot focus on only one potential cause for SIDS, because there isn't likely to be just one cause. We cannot know ahead of time whether a baby is predisposed to SIDS due to biological reasons. What we can do, and what AAP suggests, is to reduce all of the other environmental risk factors, including the use of crib bumpers.
Since many infant safety organizations now recommend that nothing be inside the crib at all, the safest route for parents and babies would be to remove crib bumper pads altogether. For day care centers, the push for increased sleep safety may soon result in states mandating that child care providers no longer use crib bumper pads. For parents who are still concerned about their child sticking arms and legs through the crib slats, and feel that they must use a bumper pad, there are mesh crib bumpers on the market today that allow more air to flow through the crib.
In addition to SIDS, another risk with the bumpers is the ability a child might have to use them as a ladder. Young babies and children have been known to step on the bumper to try to get out of the crib.