Firefighters were called to a southwest neighbourhood after a 911 call from a teenage boy who became ill on the phone while telling the operator he had found his mother and sister unresponsive in their home.
Upon arrival, crews had to force their way into the residence where they found the boy, a female teen and a woman in her 40s who were taken to hospital in serious but stable condition.
A family of four, including a two-year-old girl and a nine-month-old boy, were found in one of two adjacent homes that had elevated levels of the colourless, odourless gas.
Both children were in stable, non-life-threatening condition when they were taken to hospital, while the other two people were treated at the scene.
The third unit was not occupied.
Earlier this week, a man died and seven other people were sent to hospital as a result of carbon monoxide from a vehicle left running in the garage of a Calgary townhouse development.
Police said the man's death was not considered suspicious.
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