(CNN) The parents of an 11-month-old boy were arrested after allegedly falling asleep while their son overdosed on fentanyl in the backseat of their car, according to Pennsylvania State Police.

Kristen Bristow and Charles Salzman Jr. were charged with aggravated assault and endangering the welfare of the child, police said in a statement released Thursday. Salzman told police the two had been cleaning out Bristow's car on Wednesday after allegedly using heroin at their Chester County home, according to the criminal complaint.

Salzman's mother came out of the house to check on them and noticed the child unconscious and slumped over while his parents were either asleep or not reacting, the complaint said.

The family called 911, and first responders administered three doses of Narcan to the child, police said. He was taken to the hospital, and he is expected to survive, Michael Noone, first assistant in the Chester County District Attorney's office, said.

Charles Salzman told police at the scene that he believed the child may have put a small bottle cap in his mouth, the complaint said, and that the cap had a white powdery substance consistent with heroin or fentanyl.

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