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Three years ago, Lisa Smith got a call from her babysitter that every parent fears: something was wrong with her toddler — she wasn't waking up.

Smith then rushed to the hospital, where she was told her 1-year-old daughter, Mia, had died of positional asphyxia after taking a nap in her car seat. Somehow, the girl must have slumped down in the seat, restricting her airway.

"[It was the] worst call I’ve ever had in my life," Smith, of Aledo, Texas, recalled to WFAA.

Sharon Evans, a trauma injury prevention coordinator at Cook Children’s Hospital in Fort Worth, told the news station parents and other guardians should pay close attention when their children are in car seats. In particular, Evans stressed the dangers of sleeping in them.

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“There’s nothing about the car seat that’s designed to sleep. Of course, if the straps aren't tight, the child can kind of slump down,” Evans said, adding that straps can also pose a choking hazard.

According to a safe sleep brochure circulated by New York City Administration for Children's Services (ACS), a car seat, carrier, swing, or any type of bouncer should never be used as a designated sleeping spot, especially if the child is under 12 months old.

"Because babies don’t lie flat in car seats, they can suffocate when their heads (which are very heavy compared to the rest of their bodies), tip forward, blocking their airway," the ACS explains. "These products are not made for babies to sleep in and are dangerous because babies are not sleeping flat on their backs."

If your child does fall asleep in one of these products, the ACS recommends immediately moving them to a flat, firm surface.

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A July 2015 report from the Journal of Pediatrics found evidence of at least 47 deaths in children two years and under as a result of car seats — while used outside of a car — over a four-year period.

"The analysis showed that more than one-half of the deaths were from strangulation because of straps not being secured appropriately. The others were from unrecognized positional asphyxia," the report, which included data from the Consumer Product Safety Commission, stated.

How many parents do you think might not know this?” Smith asked WFAA, adding that the babysitter watching Mia at the time had ran a licensed home daycare in Iowa. “Probably a lot.”

Since Mia's death, Smith has been working nonstop to spread the word about the potential dangers of sleeping in car seats, particularly when they're used outside of vehicles. She did not reveal what kind of car seat the family had purchased.

“I walk around town and see people using a car seat on the seats at restaurants or putting them on the floor at tables,” Smith said.

“She was our first born, our only child, and she didn't deserve what happened to her. But other families don't deserve this to happen to them either," she added.