Four people were taken to hospital early Sunday morning, and dozens of residents were evacuated from a four-storey apartment building due to high levels of carbon monoxide.
Firefighters were called to a residential building located at 140 Mann ave. shortly after midnight.
Fire officials say Enbridge Gas workers called Ottawa Fire Services after a resident had called the gas company, telling them she was feeling dizzy.
Firefighters met with Enbridge Technicians on site and confirmed high levels of carbon monoxide in a 4th floor unit, fire officials say.
Technical Standards and Safety Authority were notified for readings above 35 parts per million of carbon monoxide.
"We were getting (readings of) about 200 parts per million, that can lead to fatal situations," said Assistant Deputy Fire Chief Sean Tracey.
"There is a possibility that at the lower levels we were detecting initially, that if they had stayed in the units for a longer period of time without the notification, it could have possibly been a fatal situation."
Officials on site confirmed carbon monoxide readings in multiple units throughout the building.
Residents were evacuated for nearly two hours, and in some cases longer as fire and gas officials checked the building.
OC Transpo buses were on site to keep dozens of residents sheltered and warm.
The cause of the high readings has not yet been confirmed.
"We are still waiting for work from Enbridge and TSSA, but we believe it was a couple of cracked flues or cracked heating units in at least one of the units."
Ottawa paramedics say four patients were transported to hospital for carbon monoxide poisoning as a precaution; a 49-year-old man, a 35-year-old woman, as well as another female of unknown age, and a 2-year-old boy.
Paramedics say a six-year-old girl and a three-year-old girl were assessed on scene, but did not have to be taken to hospital.
Residents were allowed back in the building by about 3:30 a.m.
With files from CTV's Claudia Cautillo
