An article published on LiveScience.com details various findings and references to where children died as a result of being allowed to nap in a car seat, or similar device that were never meant to be used for this purpose.
In the study, researchers examined 47 cases of children under 2 years old who died while in a device that was designed for them to sit in, or in which they could be carried. Two thirds of the cases involved car seats, whereas the remaining deaths occurred in slings, swings, bouncers and strollers.
In many cases in the study, the researchers found that the child had been placed in the device to fall asleep.
The original study, published in The Journal of Pediatrics, with the original press release here; set out to identify the risks associated to and the mechanisms of injury or death sustained within the sitting devices in children aged 2 years old or younger. A total of 47 deaths were analyzed and the devices within which the death of the child was reported included car seats, strollers, bouncers, swings and slings. The majority of these reported deaths were in car seats, totalling 65,96% of the reported deaths. Data was retireved from the United States Consumer Product Safety Commission and was based upon deaths reported within the 2004 and 2008 time period.
Specific to the car seat death findings, 52% of the deaths in car seats were attributed to strangulation from straps and the remainining 48% were attributed to positional asphyxia.
The conclusion of the study provides:
Infants and children 2 years of age and younger should be properly restrained and not be left unsupervised in sitting and carrying devices. Car seats should not be used as sleeping areas outside of the vehicle, and children should never be in a car seat with unbuckled or partially buckled straps. Infants in slings should have their faces visible and above the edge of the sling, should not have their faces covered by fabric, and their chins should not be compressed into their chests.