http://www.firstcandle.org/expectantparents/exp_safeinfant.html
Tummy sleeping and soft, fluffy or loose bedding can cause a dangerous buildup of carbon dioxide (exhaled air) around a sleeping baby’s head and face. Experts feel that this “rebreathing” of exhaled air is a risk factor for Sudden Infant Death Syndrome (SIDS) and can trigger a vulnerable baby to die suddenly and unexpectedly. In order to provide the safest environment possible for your sleeping baby, First Candle/SIDS Alliance reminds parents and caregivers of safe infant bedding practices.