A couple in Dartmouth, N.S., died of accidental carbon monoxide poisoning, Halifax police confirmed Friday afternoon.

A family member called an ambulance to the home on Marilyn Drive Friday morning. When paramedics couldn't gain entry to the building, they called police. Police in turn called the fire department. Firefighters found the couple dead.

Police Staff Sgt. Greg Mason said the Nova Scotia medical examiner's office concluded the man and woman died from accidental carbon monoxide poisoning. "Our thoughts are with the family and friends of this couple at this difficult time," he said.

When fire crews in full breathing apparatus entered the home, they found carbon monoxide readings of 300 parts per million when they're supposed to be zero, according to deputy fire chief Roy Hollett.

'Silent killer'

"We then looked for a source of the carbon monoxide," Hollett said. "They determined it was the furnace, which was in the basement. They went in, isolated the furnace, they shut the furnace off and then started to open windows and to ventilate the house."

The deputy fire chief said investigators remain unclear whether the exposure to the gas occurred in a short burst or whether there was a gradual buildup overnight, and it may require an autopsy report before that issue is clear.

"You hear it said that carbon monoxide is the silent killer, and it's true. You can't smell it, you can't see it, you can't even taste it in the air," he said.

"The reason it's so deadly is, it attaches to the hemoglobin in the blood more easily than oxygen. Carbon monoxide will ... overcome you and you will gradually become unconscious."

Call to install carbon-monoxide detectors

The couple, believed to be in their 50s, were pronounced dead at the scene. The home is a duplex and the other side of the structure was briefly evacuated of its residents as a precaution.

A fuel-safety officer from the provincial Labour Department is also investigating.

Hollett said elevated levels of carbon monoxide in a home can cause headaches, and prolonged exposure can render a person unconscious and eventually kill them.

He said the Dartmouth tragedy should serve as a reminder to homeowners to have their oil-fired furnaces inspected once a year and to install carbon monoxide detectors in their homes.