The bottom two floors of Washington Square apartments were evacuated at about 2:45 a.m.
A generator in the apartment building lost power during severe storms Wednesday afternoon. The Mount Lebanon fire chief said about 18 residents had to leave the building, including some in wheelchairs.
"I heard more pounding and I got up and there were firemen in the hallway with masks on and they were saying that there was a high level of carbon monoxide. They came into the condo and checked the levels. They said it was okay. But just the idea of having carbon monoxide was a pretty scary thought," said resident M.A. Sinnhuber.
Evacuees were taken to the municipal building next door while firefighters ventilated the building. They were allowed back in their building shortly before 7 a.m.
Firefighters believe a generator, which switched on after the building's power went out, was emitting the fumes.