In a new study of young child deaths in sitting devices like car seats, swings or bouncers, most were due to asphyxiation by improper positioning or strangulation in straps.

Using these devices as directed and not as substitutes for a crib would reduce the risk of death, according to lead author Dr. Erich K. Batra of Penn State College of Medicine in Hershey, Pennsylvania.

“The overarching advice goes back to a more basic message of safe sleep,” Batra told Reuters Health. “In an infant, a safe sleep environment includes the ABCs: they sleep alone, not in bed between parents, on their backs, and in a crib or bassinet without any loose bedding.”

Infants who fall asleep in one of the other devices, like a car seat, should not be left unattended, he said. Infants should not be placed in these devices for sleep.

The researchers reviewed the reports of 47 deaths of children under two years old that happened in car seats, bouncers, swings, strollers or slings and were recorded by the U.S. Consumer Product Safety Commission between 2004 and 2008.

This is not a complete database of all such events, only those reported by consumers or manufacturers, so while they appear to be rare this study cannot assess how often they actually happen, Batra said.

Most deaths, 31 of 47, occurred in car seats. Five happened in slings, four each in swings and bouncers and three in strollers.

About half of deaths in car seats were due to strangulation by the straps, while the other half were caused by suffocation due to positioning, the authors reported in The Journal of Pediatrics.

Strap strangulation usually happens when the restraints are not fastened as directed, Batra said. Whenever a child is in a car seat, the harness should be secured.

“If people leave an older infant or young toddler in a car seat and undo the straps thinking that it makes them more comfortable, that’s a significant hazard,” he said.

“A child properly secured in a car seat is in very little risk of danger,” he said.

Dr. Shital N. Parikh, an orthopedic surgeon at Cincinnati Children’s Hospital Medical Center in Ohio, has studied the risk factors for injury in these devices in infants up to age one. He also found car seats to be the most common setting.

“The commonest mechanism of injury was infants falling from car seats when not used in the car, used in the home,” Parikh told Reuters Health. Often parents would bring the car seat in the house while the infant still slept, undo the straps and place it on an elevated surface, he said.

Even four-month-old babies are mobile enough to wiggle out of the top straps and fall, or topple the whole seat from an elevated surface, he said.

“These are very simple things, very basic things,” Parikh said. “The basic idea is that you use (the devices) for their intended purpose only. For infants, you should not use it to make them sleep or carry them around if it’s not intended for that.”

Most devices, used as directed, are relatively safe, Batra said, but he emphasized that babies in slings need to be “visible and kissable,” as a sling may put baby’s head in a hazardous position otherwise.

Some deaths occurred after as little as four or five minutes of being left alone.

“That is one of the things we need to draw attention to,” Batra said. Sometimes a few minutes unattended is all it takes.

“If your infant is sleeping and you’re not observing them, then they need to be in a safe sleeping environment,” adhering to the ABCs, he said.