Is the wool crib bumper safe for sensitive babies?
Can I customize the bumper?
Can I wash the bumper?
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The AAP says never use crib bumpers:
Crib bumpers (or bumper pads) may seem as though they can help protect babies from drafts and bumps, but they should not be used in cribs. There is no evidence that bumper pads can prevent serious injuries, and they pose a risk of suffocation, strangulation, or entrapment. In addition, older babies can use them for climbing out of the crib.
Source: Choosing a Crib
I originally didn't want to write this as a separate answer because swbarnes2's answer is perfect. However, I did want to add this extra info.
Without a crib bumper the worst-case nightmare scenario, that is almost impossible is a broken arm or leg without a crib bumper (and I have never heard of that happening, it would be a freak accident). With a crib bumber the worst case scenario is suffocation and I hear about that all the time.
The only time a crib bumper is appropriate is when the child's head can fit through the rails of the crib because of a deformity of the child. Even then, it's better to stick with other forms of sleep like maybe a flat crib matress on the floor instead of in a crib. This example child would be a very odd exception and not the rule. To be clear if there is no deformity, and the childs head fits through the slats then it's time to go get a new crib.
For most everyone the answer is no, crib bumpers are not safe, and should be avoided. There is a very small population of children that may need a crib bumper due to circumstances, but even then other sleeping setups should be attempted first.