Oak Hall on the Pitt-Johnstown campus was evacuated late Saturday evening when campus police "determined that an unknown irritant was present in the air," a university news release said.
The incident occurred at 9:20 p.m., when police responded to a report of an individual experiencing difficulty breathing in Oak Hall.
Cambria County Emergency Management Services and Richland Fire Department were dispatched to the scene, and the building was ventilated.
All residents of Oak Hall were relocated to the Cambria Room in the Student Union and were accounted for, UPJ spokesman Robert Knipple said early Sunday.
County EMS officials employed a mass spectrometer, which detected no foreign matter in the air, Knipple said.
At midnight, all residents were told they could return to their rooms.
Knipple said 75 students were staying at Oak Hall.