SIDS and Carbon Dioxide Retention Around the Infant Due to Rebreathing of Exhaled Air Sudden Infant Death Syndrome (SIDS) is defined as the sudden death of an infant under 1 year old that is unexpected by history and unexplained after a thorough postmortem examination, including complete autopsy, investigation of scene of death and review of medical history. SIDS is the third leading cause of infant mortality in the United States, accounting for 8% of all infant deaths. It is the most common cause of infant mortality between 28 days and one year of age, accounting for 40-50% of all deaths. Risk factors for SIDS may be categorized in two ways: maternal/ antenatal and infant risk factors or modifiable and nonmodifiable risk factors. Using the former categorization: maternal and antenatal risk factors include: smoking, drug use, low socioeconomic status including inadequate prenatal care, younger maternal age, inadequate education single marital status, shorter interpregnancy intervals and intrauterine and fetal growth restriction; infant risk factors include: age (peak 2-4 months), male gender, race, growth failure, non use of pacifier, prematurity, prone (face down) and side sleep position, recent febrile illness, smoking exposure (prenatal and postnatal), soft sleeping surface and soft bedding, infant-mother or parent bed sharing, thermal stress/overheating and colder seasons and lack of central heating. The latter categorization is more practical by dividing the risk factors into modifiable and non-modifiable risk factors: The non-modifiable risk factors include: age, race, prematurity, male gender, genetic predisposition etc. The modifiable risk factors include: pregnancy related factors, cigarette smoking, drug use, prone sleep, soft sleep surfaces or bedding, overheating, bed sharing and lack of pacifier use. Infants who had an apparent life-threatening event (ALTE) are also at increased risk. ALTE is defined as a sudden, unexpected change in an infant that is frightening to the caregiver but does not lead to sudden death or persistent collapse. Sudden color change mainly cyanosis (turning blue) oroccasionally pallor (turning pale) is the most frequent observation which is typically associated with initial unresponsiveness to external stimulation or apparent apnea (stopped breathing). Genetic studies have identified multiple ways in which infants who died of SIDS differ from healthy infants and those dying of other causes. Genetic defects in the heart, brain, the autonomic nervous system, immunological and other systems have been associated with increased risk of SIDS. The actual risk of SIDS in individual infants is determined by complex interactions between genetic and environmental risk factors. Face down or nearly face down sleeping can result in episodes of airway obstruction, but healthy infants will arouse before these become life threatening. However, infants with a genetic predisposition resulting in insufficient arousal responsiveness would be at increased risk. Sleeping in the prone position has been implicated as a risk factor for SIDS. Declines of 50% or more in rates of SIDS in the United States and around the world have occurred in the past decade. This is primarily due to implementation of the "back to sleep" campaign whereby parents were encouraged to place infants in the supine (on their back) position before sleep. Presently, infants at the highest risk of SIDS are those who are usually placed in the supine position but were placed in the prone position for their last sleep (unaccustomed prone) or were found in the prone position (secondary prone). Soft sleeping surfaces or bedding, such as comforters, pillows, sheepskins, polystyrene bean pillows and older or softer mattresses are associated with increased risk of SIDS. Head and face covering by loose bedding, particularly heavy comforters, is also associated with increased risk. Combination of risk factors result in an even higher risk; for example, prone sleeping on soft bedding has been associated with a 20-fold increased risk of SIDS. One hypothesis that explains SIDS in infants found in the prone position or with their face covered is the rebreathing theory. According to the rebreathing hypothesis, some bedding materials may act to retard the dispersal of exhaled air, retaining the gases near a prone infant's mouth. With each subsequent breath, the infant takes in an air mixture containing more carbon dioxide and less oxygen, which is progressively less adequate to sustain life. Certain infants who remain face down on certain types of bedding for more than a short period of time might therefore become susceptible to induced asphyxiation.