A statement from York County says five inmates remained hospitalized as of Thursday morning. The remaining 44 have been returned to the York County Prison.
The women fell ill Wednesday evening in a prison dormitory.
Officials say a preliminary investigation indicates the deadly odorless and colorless gas may have come from the heating, ventilation and air conditioning system.
That system has been shut down. The county's statement says carbon monoxide levels have returned to normal.
About 215 women are incarcerated at the facility, 85 miles west of Philadelphia.
Prisoners living in the affected unit have been relocated to other areas in the facility.
Warden Mary Sabol tells the York Daily Record she and the prison board will discuss installing CO detectors.
