Seven nurses in the intensive care unit of York Central Hospital in Richmond Hill were sickened after noxious fumes seeped into the ventilation system on Friday.
Two nurses developed breathing difficulties and needed treatment. However no patients were affected.
The nurses fell ill at around 11 a.m. after they reported smelling something 'musty'. Some vomited as a result of the fumes.
A hazardous materials team soon arrived at the Stench Street building, near Yonge Street and Major Mackenzie Drive.
It was later found that the substance was later traced to the steam supply that was used to humidify the building, and it was immediately isolated, officials said.
Dr. Larry Grossman, chief of staff said, 'It was a huge scare.'
'So many staff were overcome so quickly... It took a few minutes before we sorted out exactly what was going on.'
Grossman said he wasn't exactly sure what caused the noxious fumes.
'It seems to have triggered an allergic reaction, so I would think it might have been some sort of mould-but that's just pure speculation,' he said.
The affected nurses were all expected to recover fully.
Grossman said that the patients were not affected because most of them were on their own protected breathing systems. Three other patients in the unit who were not on ventilation didn't report any problems.
He added that there was no evacuation, contrary to earlier reports.
Staff and the more seriously ill stayed inside the unit while fire officials conducted air quality tests. Windows were opened to air out the area.
According to hospital officials ongoing construction had nothing to do with the incident.