I watched a show on sids on one of those learning channels. They said there are 2 factors here.
Recycled carbon dioxide, ie pillow, blanket, cat, that causes suffocation is one.
But there is a small percentage of children whos brain does not have the mechanism that tells them to pull back from the carbon dioxide area and get fresh air.
It said that every child autopsied that died of sids, was missing that little portion of their brain.
Although it is incredibly rare, if you've ever seen your child pull her face out of a "uncomfortable" type oxygen zone, then you can rest easier.
Same applies to the ones who exhale and simply do not inhale again.
Lemme see if I can find it. It was years ago that I saw that show.