Babies should never be allowed to sleep in car safety seats when the seat is not in a car, researchers say after a new study showing a link to decreased oxygen levels.
Dr Christine McIntosh and her colleagues have also warned that babies' time in car safety seats should be kept to a minimum, after the latest findings in their long-running research.
The researchers, from Auckland University and the Cot Death Association, have previously shown a reduction in mild breathing problems in babies whose car safety seat was fitted with a simple foam insert with a slot for the back of the head.
Babies' heads are larger, relative to the body, than adults' heads, and protrude behind the line of the back.
The commercially available insert helps to keep the baby's body forward. It also allows the head to remain upright even when the infant is asleep, rather than slumping forward, which can obstruct breathing when the chin is on the chest and pushed back.
A larger study, published today in the prestigious United States journal Pediatrics, confirms that using the insert does reduce breathing risks for babies who were on average 8 days old, but it does not eliminate the seats' risks.
Comparing sleep studies in two groups each of 39 babies, the researchers found those whose car safety seats had the inserts had fewer instances of stopping breathing because of a blocked airway, and had less severe reductions in oxygen saturation levels than the non-insert group.
But there was no difference between the two groups in the number of moderate reductions in oxygen saturation levels per hour.
"Even reducing severity of the fall in oxygen levels is important and is a good indicator that the insert did help make babies safer," Dr McIntosh said.
"This study also highlights the importance of not using car seats as a place of sleep for infants. Sudden unexpected deaths in infants can occasionally occur in car seats [and] capsules."
In their journal paper, the researchers note car safety seats are essential for infants' safe transport in cars but they express concern at the reported high rates of infants spending more than 30 minutes a day in a car seat, "often for sleep out of the car as well as for transport".
"There is now compelling evidence that hypoxia [oxygen deficiency] is associated with behavioural problems and adverse effects on development and school performance.
"Furthermore, car seat use has been associated with apparent life-threatening events and with at least some otherwise unexplained sudden deaths in term infants.
"We believe that caregivers should be strongly advised that car seats should not be used for infant sleep outside the car."
Even when the insert is used, the researchers say, young infants should never be left unattended in car safety seats. "Car seat use should be restricted to the minimum time required for essential travel."