Fire officials say a carbon monoxide detector saved two families from harm Monday morning in northwest Calgary.
The CO detector went off in a duplex on the 1000 block of First Avenue N.W. just before 9 a.m.
Both sides of the building were evacuated, officials said.
Fire crews detected CO levels as high as 170 parts per million.
The fire department's policy is to evacuate when as little as 12.5 parts per million are detected in cases where children are present.
There was an infant in one of the homes.
The source of the gas is still under investigation.
Fire officials reminded Calgarians about the danger of carbon monoxide — a colourless, odourless, tasteless gas that can cause illness and death — and the importance of installing CO detectors.