Absent that option, emergency rooms have taken up the slack. A recent study of 88 pediatric hospitals around the country found that 87 of them regularly board children and adolescents overnight in the E.R. On average, any given hospital saw four boarders per day, with an average stay of 48 hours.

“There is a pediatric pandemic of mental health boarding,” said Dr. JoAnna K. Leyenaar, a pediatrician at Dartmouth-Hitchcock Medical Center and the study’s lead author. In an interview, she extrapolated from her research and other data to estimate that at least 1,000 young people, and perhaps as many as 5,000, board each night in the nation’s 4,000 emergency departments.

“We have a national crisis,” Dr. Leyenaar said.

This trend runs far afoul of the recommended best practices established by the Joint Commission, a nonprofit organization that helps set national health care policy. According to the standard, adolescents who come to the E.R. for mental health reasons should stay there no longer than four hours, as an extended stay can risk patient safety, delay treatment and divert resources from other emergencies.

Yet in 2021, the average adolescent boarding in the E.R. at Boston Children’s Hospital spent nine days waiting for an inpatient bed, up from three and a half days in 2019; at Children’s Hospital Colorado in Aurora in 2021, the average wait was eight days, and at Connecticut Children’s Medical Center in Hartford, it was six.

Emergency-department boarding has risen at small, rural hospitals, too, with “no pediatric or mental health specialists,” said Dr. Christian Pulcini, a pediatrician in Vermont who has studied the trend in the state. “There is one clear conclusion,” he told the Vermont legislature recently. “The E.D. is not the appropriate setting for children to get comprehensive, acute mental health services.”