Firefighters spent several hours clearing out high carbon monoxide levels in two Calgary apartment buildings overnight.

Both incidents were caused by the building’s heating systems, officials said.

About 200 people were evacuated from a complex in Royal Oak in northwest Calgary late Monday night.

The building’s fresh air intakes had frosted over, said Acting Battalion Chief Ed Cornez. 

Emergency crews responded to a second call at about 3 a.m. MT Tuesday morning on Bannister Road in the southeast community of Midnapore.

In that case residents were able to stay in their units, said Cornez.

"They were able to isolate it to a heating unit, which they shut down and then they proceeded to ventilate the structure. While they were doing that they were checking the CO levels in the units of the apartments.  The CO did not migrate into the apartments at this time.”

People who live in apartments should have their own CO detectors, since most buildings don't have them, Cornez said.

"In parking structures, they are required to have a CO monitor, but in the living areas, they may or may not. So it depends on each apartment,” he said.

"In most cases, like the one in Royal Oak, it was a resident whose personal CO detector had gone off and that's who called us to go look at it."